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Third edition, April 2004Design version 5.0, April 2004ISBN 91-973125-0-9
1. Laminate ........................................................................................... 23Material Description .............................................................................................. 23Survey of Resin Systems ......................................................................................... 24Copper Foils .......................................................................................................... 24Base Material ......................................................................................................... 25Laminate Thickness (including copper foil) ........................................................... 26Panel size ................................................................................................................ 26Green PCBs ........................................................................................................... 27Most Common Type of Laminate .......................................................................... 27
2. Hole Drilling .................................................................................... 28Purpose of Plated-Through-Holes .......................................................................... 28Process ................................................................................................................... 28Conventional Drilling of Large Holes ≥ 0.5 mm (20 mils) ..................................... 29Drilling of Small Holes from 0.25 to 0.5 mm (10 to 20 mils) ................................ 29Position of Holes .................................................................................................... 30Oversize of Drilled Holes ....................................................................................... 30Aspect Ratio ........................................................................................................... 31Locating the Panels on the Table of the Drilling Machine ...................................... 32Formation of Microvia Holes < 0.25 mm (10 mils) ................................................ 33Laser Drilling: From 0.05 to 0.2 mm (2 to 8 mils) ................................................. 33Mask Imaging Drilling ........................................................................................... 34Conformal Mask Drilling....................................................................................... 34Contact Mask Drilling ........................................................................................... 35Prevalence of Lasers ................................................................................................ 35Photodefinition ...................................................................................................... 36Plasma Etching ...................................................................................................... 36
3. Electroless Copper ............................................................................ 37Purpose of Electroless Copper Process .................................................................... 37Process ................................................................................................................... 37Deposit Thickness .................................................................................................. 37Alternate Metallization Methods ............................................................................ 38
4. Image Transfer .................................................................................. 39Introduction .......................................................................................................... 39Purpose of Image Transfer (Double-Sided Boards) ................................................. 39
4 A. Lamination ....................................................................................... 41Purpose of Lamination ........................................................................................... 41Process ................................................................................................................... 41
4 B. Registration ...................................................................................... 43Purpose of Registration .......................................................................................... 43Principles ............................................................................................................... 43Optical Registration to the PTH Pattern Using Diazo Film ................................... 43Optical Registration to Registration Holes in the Panel Area .................................. 44Pin Registration to Registration Holes Drilled or Punched in the Panel Area ......... 44Combination of Optical Registration and Pin Registration .................................... 45Automatic Registration .......................................................................................... 45
4 C. Exposure ........................................................................................... 46Purpose of Exposure ............................................................................................... 46Process ................................................................................................................... 46
4 D. Developing ....................................................................................... 47Purpose of Developing ........................................................................................... 47Process ................................................................................................................... 47Plotting the Film .................................................................................................... 47Phototool (Film) Development .............................................................................. 48Inspection and Touch-up ....................................................................................... 48Coating of the Phototool ....................................................................................... 48Exposure of the Plating Resist ................................................................................ 49
4 E. Laser Direct Imaging ........................................................................ 50
5. Electroplating ................................................................................... 54Purpose of Electroplating ....................................................................................... 54Principle of the Electrolytic Process ........................................................................ 54
5 A. Copper Plating ................................................................................. 55Purpose of Copper Plating ..................................................................................... 55Process ................................................................................................................... 55"Throwing power" ................................................................................................. 55Ductility ................................................................................................................ 56Plating Problems .................................................................................................... 56Pattern Plating vs. Panel Plating ............................................................................. 57Copper Plating Specifications................................................................................. 57
10. Tin/Lead Stripping or Tin Stripping ................................................. 80Purpose of Tin/Lead Stripping or Tin Stripping ..................................................... 80Process ................................................................................................................... 80
11. Solder Masks (Photopolymer) ........................................................... 81Purpose of Photopolymer Solder Masks ................................................................. 81Types of Solder Masks ............................................................................................ 81Process ................................................................................................................... 82Thickness of Dry-Film Solder Masks ..................................................................... 82Clearance around Solder Pads ................................................................................ 83Coverage of Spaces between Parallel Tracks ............................................................ 84Intentional Coverage of Via Holes ......................................................................... 84
11 A. Lamination of Dry Film .................................................................... 85Purpose of Lamination ........................................................................................... 85Process ................................................................................................................... 85
11 B. Coating with Liquid Film ................................................................. 86Purpose of Coating ................................................................................................ 86Process ................................................................................................................... 86Curtain Coating ..................................................................................................... 86Screen-Printing ...................................................................................................... 86Hole Plugging ........................................................................................................ 87
11 C. Registration ...................................................................................... 88Purpose of Registration .......................................................................................... 88Process ................................................................................................................... 88Optical Registration to Pad Pattern by Using Diazo Film ....................................... 88Optical Registration to Registration Pads in the Panel Area .................................... 88Pin Registration to Registration Holes Drilled or Punched in the Panel Area ......... 88Combination of Optical Registration and Pin Registration .................................... 89Automatic Registration .......................................................................................... 89
11 D. Exposure ........................................................................................... 90Purpose of Exposure ............................................................................................... 90Process ................................................................................................................... 90
11 E. Development and Post-Curing .......................................................... 91Purpose of Development ........................................................................................ 91Process ................................................................................................................... 91
12 A. Solder Coating and Hot-Air Leveling (HAL) .................................... 93Purpose of Solder Coating and Hot-Air Leveling. .................................................. 93Process ................................................................................................................... 93Solder Coating and Hot-Air Leveling Problems ..................................................... 94Dwell Time in Solder Pot ....................................................................................... 94Speed of Withdrawal .............................................................................................. 95Adjustment of Air Knives ....................................................................................... 95Webbing ................................................................................................................ 95Solder Sagging ....................................................................................................... 95Intermetallic Compounds ...................................................................................... 96Copper Contamination .......................................................................................... 96
12 B. Electroless Nickel and Immersion Gold (Ni/Au) ............................... 97Purpose of Electroless Nickel and Immersion Gold (Ni/Au) ................................... 97Process ................................................................................................................... 98Advantages of Boards with Nickel/Gold ................................................................. 99Flat Surface ............................................................................................................ 99Low Defect Rate .................................................................................................... 99Solderability ........................................................................................................... 99Stressing of Boards ................................................................................................. 99Dimensional Stability ............................................................................................. 99Contamination of Board Surface ............................................................................ 99Fiducials ................................................................................................................. 99Shelf Life .............................................................................................................. 100Keyboard Contacts ............................................................................................... 100Disadvantages of Boards with Nickel/Gold .......................................................... 100In-House Deposition Lines .................................................................................. 100Price Conditions .................................................................................................. 100Solder Mask Adhesion.......................................................................................... 100Problem Areas ...................................................................................................... 100Embrittlement of Solder Fillets ............................................................................ 100Gold Contamination of Solder Bath .................................................................... 100Ultra-High Frequency Problems........................................................................... 100
12 C. Organic Solderability Preservatives (OSP) ...................................... 101Purpose of Organic Solderability Preservatives (OSP)........................................... 101Development of OSP ........................................................................................... 101Process ................................................................................................................. 101Problems with OSP-Coated Boards ...................................................................... 101
14. Machining (Contouring) ................................................................. 106Purpose of Machining (Contouring) .................................................................... 106Process ................................................................................................................. 106Slot Drilling ......................................................................................................... 107Drilling of Large Holes ........................................................................................ 108Recess Milling ...................................................................................................... 108Determination of Contour ................................................................................... 108Dimensioning Based on a Reference System......................................................... 108Dimensioning Based on Customer's Router Data ................................................ 110Dimensioning Based on Corner Marks ................................................................ 110Machining Tolerances .......................................................................................... 110Panelization ......................................................................................................... 111Depanelization ..................................................................................................... 112
15. Tooling Holes by Tenting ............................................................... 113Purpose of Tenting ............................................................................................... 113Background.......................................................................................................... 113Process ................................................................................................................. 113Design Considerations ......................................................................................... 114
16. Gold-Plated Edge Connectors ......................................................... 115Purpose of Gold-Plated Edge Connectors ............................................................ 115Background.......................................................................................................... 115Process ................................................................................................................. 115Thickness Distribution of Gold ........................................................................... 118Machining of Edge Connectors ............................................................................ 119Edge Connector Slots ........................................................................................... 119Bevelling of Connector Edge ................................................................................ 119
21. Image Transfer ................................................................................ 130Purpose of Image Transfer .................................................................................... 130Process ................................................................................................................. 130Screen-Printing .................................................................................................... 130Dry-Film Photoresist ............................................................................................ 130Selecting the Type of Etch Resist .......................................................................... 131Pattern Density .................................................................................................... 131Panel Size ............................................................................................................. 131Number of Boards ............................................................................................... 131
22. Etching ........................................................................................... 132Purpose of Etching ............................................................................................... 132Process ................................................................................................................. 132
32. Innerlayer Laminates ...................................................................... 150Purpose of Innerlayer Laminates .......................................................................... 150Process ................................................................................................................. 150Double-Sided Innerlayer Laminates Etched on One Side Only ............................ 150Double-Sided Innerlayer Laminates Etched on Both Sides ................................... 151Double-Sided Innerlayer Laminates with Buried Via Holes .................................. 151Innerlayer and Outerlayer Laminates with Blind Via Holes .................................. 152Conventional Drilling .......................................................................................... 152Laser Drilled Microvia Holes ............................................................................... 155
33. Prepreg............................................................................................ 157Purpose of Prepreg ............................................................................................... 157Process ................................................................................................................. 157Selecting Prepreg Thicknesses ............................................................................... 157Resin Filling of Openings in Ground and Voltage Planes ..................................... 158Lamination Voids ................................................................................................. 158Hole Wall Pull-Away ............................................................................................ 158Barrel Cracking and Deformation of Pads ............................................................ 159
PCB Specification - Example A. Dimensions in inches, µinches or mils. .............. 218PCB Specification - Example B. Dimensions in mm or µm. ................................. 219
Part 1. In Search of a Candidate ................................................................. 22170.1 Sources of Information ............................................................................... 22170.2 Request for Quotation ................................................................................ 22270.3 PCB Buyer’s Specification ........................................................................... 22270.4 Additional Information .............................................................................. 22370.5 Evaluation of Quotations ............................................................................ 22370.6 Evaluation Criteria ..................................................................................... 22570.7 Test of PCB Samples .................................................................................. 22570.8 Quality Assurance Requirements ................................................................ 22670.9 Vendor Survey/Plant Audit ......................................................................... 228
Part 2. Factory Inspection .......................................................................... 22871.1 Reception and Opening Conference ........................................................... 22871.2 General Impression of Plant Tour ............................................................... 23071.3 Quality Department ................................................................................... 23271.4 Preproduction .............................................................................................. 23271.5 Drilling ...................................................................................................... 23371.6 Process Control .......................................................................................... 23371.7 Statistical Process Control ........................................................................... 236
Part 3. Plant Audit Plan .............................................................................. 237
Part 4. The Final Choice............................................................................. 25073.1 Criteria of Choice ........................................................................................ 250
Chapter 8 ........................................................................................ 253Purpose ................................................................................................................ 254Procedures............................................................................................................ 254Complexity of SMT Boards versus HMT boards ................................................. 25580.1 Conductor Width ....................................................................................... 25680.2 Spacing Between Parallel Conductors ......................................................... 25980.3 Width of Annular Ring .............................................................................. 26080.4 Hole Diameter ............................................................................................ 26280.5 Aspect Ratio ............................................................................................... 26280.6 Diameter Tolerance..................................................................................... 26380.7 Solder Masks .............................................................................................. 26480.8 Handling Faults .......................................................................................... 26580.9 Multilayer Boards ....................................................................................... 26680.10Warp and Twist .......................................................................................... 26780.11 Displacement of Holes .............................................................................. 26880.12 Pattern Displacement on SMT Boards ...................................................... 26980.13 Machining the Contour ............................................................................ 269
Chapter 9 ........................................................................................ 270Appendix 1. Laminates ..................................................................................... 271Appendix 2A. Examples of Build-up of Multilayer Boards. 4-L, 6-L, 8-L, in inch 272Appendix 2A. Examples of Build-up of Multilayer Boards. 10-L, 12-L, in inch.... 273Appendix 2B. Examples of Build-up of Multilayer Boards. 4-L, 6-L, 8-L, in mm. 274Appendix 2B. Examples of Build-up of Multilayer Boards. 10-L, 12-L, in mm .... 275Appendix 3. Used Websafe Colors - Test Page ................................................... 276
In our discussions with PCB manufacturers worldwide, we discovered, quite surprisingly, that many ofthe layouts they receive from their customers are of unacceptable quality. To prevent this and other
manufacturing-related problems, it is imperative that all persons involved in PCB fabrication - from PCBdesigners and circuit engineers and those working in the fabrication shop - have sufficient knowledge of thedifferent process steps in order to achieve total quality.
The purpose of this material is to give a general overview of what takes place in a PCBmanufacturer´s plant. This material has been written for in-house education/training. Almost every processdescription includes a picture of a PCB cross-section. As the process moves forward, each part of the processis added to the preceding picture.
The original manuscript and drawings have been prepared by:
The complete course material was prepared in digital form on Apple Macintosh and is availableon a CD-ROM so that the pages can be viewed on a computer screen and color printouts can be made bythe customer. Printouts can be made from any ink-jet printer, laser printer or from a color laser copier. Thecomplete CD-ROM contains a text- and figure -section with 275 pages in color. Each figure and table isincluded in a separate section 285 pages long for printing out in color as overhead transparencies or slides.
Purpose of Solder Coating and Hot-Air Leveling.To deposit an even layer of solder (tin/lead) on all pattern surfaces appearing as bare copper, i.e., solderpads not covered by the solder mask and the hole barrels of plated-through holes.
ProcessAfter pickling (a light etching), the board (a) is clamped in the clamp fixture (b) of the solder coatingmachine, ( Pictures A and B of Figure 12A-2). After fluxing and preheating, the board is immersed in asolder pot (c) for a specified dwell time. The molten solder, which has an alloy composition of 60/40 or63/37 (tin/lead), wets the exposed copper areas, including the edges of the tracks and pads. Edge coverage,unlike that of ordinary reflowed boards, does not depend on the amount of tin/lead on the surface becausethe edges are in direct contact with the molten solder.
It should be noted that the board receives a certain thermal shock when immersed in the solderpot. If the copper in the through-plated hole is not sufficiently ductile, barrel or corner cracking can occur.
During withdrawal from the solder pot, the board passes between a pair of air knives (d). The hotair jets (e) produced by the air knives level the solder on both sides of the board by removing the excesssolder. After cooling, the board is automatically passed to a cleaning station where the flux residues arecleaned off.
It is possible to equip an ordinary hot-air leveling machine with rollers just below the air knivesso that most of the excess solder is removed by the squeezing action of the rollers before the final levelingtakes place.
Fig. 12A-1
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
Horizontal hot-air leveling machines have been on the market for several years. The advantageis that the pads become more flat than in the case of vertical hot-air leveling machines. In production, theadvantage is a continuous workflow because of the conveyorization of the horizontal machine.
When the machine is properly adjusted, there is no perceptible difference between solder holesthat have been tin/lead plated and reflowed or solder coated and hot-air leveled. The advantage of the soldercoating and hot-air leveling process is that there is no tin/lead on tracks or ground planes to cause wrinklingor flaking of the solder mask during machine soldering.
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
Solder Coating and Hot-Air Leveling ProblemsSome PCB manufacturers claim that the solder coating method yields greater hole-to-hole uniformity thandoes the ordinary tin/lead plating with reflowing. This applies, however, only to the tin/lead alloycomposition that naturally will be the alloy composition of the solder pot. This statement should be viewedsceptically because adjustment and maintenance of the machine are fairly critical.
Dwell Time in Solder PotThe dwell time shall be appropriate for the board. If it is too long, blistering can occur. If it is too short,dewetting can take place since the flux has insufficient time to remove possible oxides. The optimum dwelltime depends to a certain degree on the nature of the board including the:
◆ ratio between the copper area and the board area◆ board thickness◆ type of board, such as a double-sided PTH board or a multilayer board
Speed of WithdrawalAfter solder coating, the board is passed between air knives where the speed of withdrawal plays animportant part. Insufficient speed causes excessive removal of tin/lead, and under worst-case conditionsonly a very thin coating, similar to that of hydrosqueegee-treated boards, is left. Because of the very thinlayer, the formation of intermetallic tin/copper compounds can create solderability problems later.
Excessive speed leaves too much tin/lead on the board with a risk of blocked holes. Amanufacturer of solder coating equipment warrants that no more than 0.1% of the holes will be blockedprovided the ratio between the board thickness and the hole diameter is kept less than 2. This assumes,however, that the machine is adjusted correctly.
Adjustment of Air KnivesAll air knife adjustments have to be correct. This applies particularly to the:
◆ air pressure◆ clearance between the air knives and the board◆ offset between the air knives
Incorrect adjustments can cause the following faults:
◆ a thin tin/lead layer with the risk of solderability problems◆ a thick tin/lead layer with some solder holes too narrow, or even totally closed◆ an unacceptable front-to-back variation in the tin/lead thickness
WebbingIf the solder pot level is too low, flux can accumulate on the surface of the molten tin/lead and become tar-like because of evaporation of the solvent. The flux can now become intermixed with the tin/lead, whichis circulated just as in a wave soldering machine, or it can stick to the surface when the board is withdrawnfrom the solder pot. Small tin/lead drops can form during the leveling, and when these drops are trappedby the sticky, tar-like flux, they can possibly solder to the pattern and create short-circuits.
Solder SaggingWhen the board is withdrawn from the solder pot and leveled, the molten tin/lead tends to run downwards.This so-called solder sagging can form uneven surfaces of the solder pads, which can be very problematicwith SMT boards. Great care must be shown when adjusting the machine to minimize solder sagging asmuch as possible.
Measurements of the evenness of solder coated and hot-air leveled pads show that there is a fairlybig difference in solder thickness over a pad. This is illustrated by Figure 12A-3. At one end of the pad,the thickness can be as little as 1 or 2 µm (0.04 to 0.08 mil) and at the other end as much as 40 to 50 µm(1.6 to 2 mils), although the illustration only shows about 25 µm (1 mil).
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
Intermetallic CompoundsAs discussed in Section 8, "Coverage of Hole Knee", intermetallic compounds such as Cu
3Sn and Cu
5Sn
6are formed at the interface between tin/lead and copper with a thickness of 1 µm (0.04 mil). This could beeven more dependant upon the temperature and time conditions of the solder coating and hot-air levelingprocess and in the customer´s stock room. Cu
5Sn
6 crystals can grow up through the thin tin/lead layer to
the surface and become oxidized, which causes reduced solderability. The formation of intermetalliccompounds is shown in Figure 12A-3.
Copper ContaminationIf the tin/lead in the solder coating machine is contaminated with more than 0.5 % copper, a tin/coppereutecticum can form and create solderability problems with the finished boards. The cause of thiscontamination is copper dissolving from the copper surface of the boards. This is why the copper contentsof the solder pot must be analysed on a regular basis.
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
12 B. Electroless Nickel and Immersion Gold (Ni/Au)
Purpose of Electroless Nickel and Immersion Gold (Ni/Au)To selectively deposit an even layer of nickel and gold on all pattern surfaces appearing as bare copper (seeFigure 12B-1). The deposition usually consists of 4 to 6 µm (0.16 to 0.24 mil) electroless nickel and 0.05to 0.15 µm (0.002 to 0.006 mil) of immersion gold. The nickel should have a content of 8-10 % phosphorus.
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
ProcessElectroless nickel is deposited by an autocatalytic process based on a precious metal catalyst and an internalreducing agent. The deposition takes place as long as the chemistry is active. In general, the deposit is stoppedat a thickness of 5 µm (0.2 mil) with the deposition rate being about 5 µm (0.2 mil) in 15 to 20 min. Theminimum thickness required to be an effective barrier for copper migration from the underlying copper is2.5 µm (0.1 mil.).
Since nickel is a fairly active metal, which quickly oxidizes so that it becomes difficult to solder tothe nickel surface after a short period, the nickel layer must be coated with immersion gold.
Immersion gold deposition follows the nickel deposition and is based on an exchange reactionwhere the nickel on the surface is replaced with a thin layer of gold. This reaction stops automatically whenall of the exposed nickel at the surface is replaced with gold. In other words, the process is self-limiting ata gold thickness of 0.2 µm (8 µin.). In order to protect the nickel, the minimum layer thickness of gold isabout 0.025 µm (1 µin.).
These processes are carried out by immersing the boards in tanks where the non-electrolyticdeposition takes place. It should be noted that in many articles the word "plating" is used instead of"deposition", but in its deeper sense, plating means an electrolytic process (see Section 5 about electro-plating).
There are two alternative deposition methods. With one method, nickel/gold is depositedselectively – in plated through holes and on pads not covered by the solder mask, which has been appliedbefore the nickel/gold process. This requires a minimum usage of gold and is the most common execution.
Nickel/gold can also be deposited all over the board – on all tracks and pads, after which the soldermask is applied. This method is more expensive and is primarily used in cases where the boards are exposedto very harsh environments because all tracks are encapsulated by nickel/gold. Keep in mind that soldermasks are more or less permeable so that bare copper under the solder mask can be attacked.
Some PCB manufacturers, often those located in the Far East, use the so-called flash gold, whichis actually a very thin layer of gold on nickel. The layer thickness is so low [0.01 µm (0.4 µin.) or less] thatit does not offer any protection to the underlying nickel, which soon becomes oxidized. In other cases, itturns out that the nickel/gold is electrolytically plated, which means that it has been used as an etch resist,leaving the edges of tracks and pads in bare copper. Sometimes, it is very difficult to clarify what the PCBmanufacturer means when specifying nickel/gold.
CHAPTER 1. HOW TO PRODUCE PLATED - THROUGH - HOLE BOARDS
Advantages of Boards with Nickel/GoldFlat SurfaceThe most important feature is that the surface of all pads is perfectly flat, corresponding to the underlyingcopper surface, with all pad and track edges covered by nickel/gold. See Figure 12B-3.
Low Defect RateAn important reason for choosing nickel/gold surface protection is a highly reduced failure rate duringassembly and soldering compared with solder-coated and hot-air leveled boards. It is especially true of fine-line boards with a component pitch of 0.5 mm (20 mils) or less. This is illustrated by an investigation madeby Philips Telecom a couple of years ago. Figure 12B-4, which actually is a learning curve, shows acomparison of the yields achieved with solder coating and hot-air leveled boards and nickel/gold boards.
SolderabilitySolderability is high but the soldering time is a little longer (about 5 sec.) compared with wave soldering (3sec.).
Stressing of BoardsBecause the boards have not been exposed to high temperatures, no stressing of the plated-through holeswill occur. Under unfortunate conditions, this could otherwise lead to barrel and innerlayer cracks. Anotheradvantage of the low-temperature processes is that no delamination of the laminate will take place.
Dimensional StabilitySince the boards are not subjected to temperatures above 90° C (194° F) during manufacture, thedimensional stability is high. This is of great importance when screen-printing solder paste on fine-line SMTboards because a better fit between the stencil and the pattern is achieved than in the case of solder coatedand hot-air leveled boards.
Contamination of Board SurfaceSince there is no flux residue on the board surface as there is with solder-coated and hot-air leveled boards,surface contamination is considerably lower. Measurements recently published indicate a 4.5 µg NaCl/sq.cm (29 µg NaCl/sq.in.) contamination of solder coated and hot-air leveled boards and just 1.5 µg NaCl/sq.cm (9.6 µg NaCl/sq.in.) contamination of nickel/gold boards.
FiducialsFiducials, also called optical targets, achieve a better definition because of the thin nickel/gold layer.
Nonplated-through-hole boards are usually single-sided, but in some cases, it is necessary to usedouble-sided boards although competition does not allow the use of plated-through holes. The descriptionsgiven in Sections 21 to 27 are based on single-sided boards. However, double-sided boards can be producedby transferring the image to both sides of the board in close registration.
When double-sided nonplated-through-hole boards are used, it is frequently necessary to establishvia connections. The simplest way is a clinched jumper wire soldered to the pads on both sides of the board.See Picture A of Figure 20-2. It is not good practice to utilize component leads as via connections.
Another approach is to use eyelets, which are hollow solder or tin plated copper rivets. Eyelets canbe funnel-flanged and soldered in place on both sides of the board. See Picture B of Figure 20-2. Or, theycan be flat-flanged and fused (reflowed) on both sides of the boards. See Picture C of Figure 20-2. Thisrequires, however, that the pads be tin/lead covered.
Fig. 20-2
A B C
CHAPTER 2. HOW TO PRODUCE NONPLATED - THROUGH - HOLE BOARDS
A more modern way of establishing via connections is to use PTF (polymer thick film) technology.It has been used for many years in the manufacture of high-temperature thick-film hybrid circuits, and hasalso found applications in the PCB marketplace. PTF technology can be used for producing tracks, straps,resistors and printed-through holes on laminates for PCBs at a low costs.
The technology of producing double-sided boards with PTF printed-through holes instead ofordinary plated-through holes is addressed in the following. This technology is well suited for massproduction since it is based on a number of screen-printing, drying and curing processes for PTF paste.
PTF paste consists of a conductive element, usually silver particles, polymer organics and a solvent,which provides the necessary viscosity for screen-printing.
As Figure 20-3A shows, silver paste is screen-printed on the copper foil of the top (side A ) of theboard. By drawing a vacuum, the silver paste is pulled down through the hole until it covers about 2/3 to3/4 of the hole wall whereupon it is dried. The board is then turned upside down. See Figure 20-3B. Silverpaste is screen-printed on the copper foil of side B, and vacuum is drawn to pull the silver paste down intothe hole so that it overlaps the first applied layer of silver paste. After drying and curing, a printed-throughsilver hole is created. The hole offers a good electrical connection from side to side. It should be noted,however, that such holes shall never be used as component mounting holes – only as via holes. If solder froma wave soldering process comes into contact with the silver, it will dissolve the silver. This phenomenon,which is called leaching, is well known from bipolar SMT components such as chip resistors where a nickelor palladium barrier under the silver is required.
CHAPTER 2. HOW TO PRODUCE NONPLATED - THROUGH - HOLE BOARDS
Printed-through holes must, therefore, be protected by screen-printing a solder mask across theholes. In some cases,they must be further protected by screen-printing dots on top of the holes, e.g., whenscreen-printing the component notation.
It should also be noted that silver has a tendency to migrate, which means that silver ions movebetween two neighboring tracks under the presence of an electric field and humidity. This can cause aleakage current or even a short-circuit. A protective layer such as solder mask can prevent such unfortunateproblems.
20. LaminateLaminate Characteristics and Applications
Information on laminate thickness, copper foil thickness and temperature conditions can be found in Tables20-1, 20-2 and 20-3.
Phenolic Paper, FR-2This laminate consists of plies of paper impregnated with a phenolic-resin binder. See Figure 20-4. It hasgood electrical and physical properties, although its dimensional stability is not very high. It can be punchedat room temperatures. FR-2 is the least-expensive laminate and is typically used for single-sided boards. Itis primarily utilized in non-demanding applications such as radios, TV sets, calculators, toys and electronicgames.
Epoxy Paper, FR-3As with FR-2, this laminate consists of plies of paper, but the paper is impregnated with an epoxy-resinbinder. See Figure 20-4. This laminate is quite similar to FR-2 but because of the epoxy resin, the electricaland physical properties are somewhat higher. Like FR-2, it can be punched at room temperature. It is usedin consumer products, computers, radios and TV sets.
CHAPTER 2. HOW TO PRODUCE NONPLATED - THROUGH - HOLE BOARDS
Purpose of Innerlayer LaminatesTo form the inner circuits of the multilayer board.
ProcessThe most predominant and simple types of innerlayer laminates are produced in very much the same wayas nonplated-through-hole boards. The main difference is that innerlayer laminates are very thin andinclined to flexing during manufacturing, such as during etching, unless special precautions are taken. Ingeneral, innerlayer laminates are not drilled until lamination has been completed.
Innerlayer laminates with via holes (buried via holes and blind via holes) are produced by meansof more complicated processes involving through-plating of the via holes. Therefore, the via holes have tobe drilled prior to the plating and lamination processes.
Innerlayer laminates are divided in the following groups:
◆ double-sided innerlayer laminates etched on one side only◆ double-sided innerlayer laminates etched on both sides◆ double-sided innerlayer laminates with buried via holes◆ innerlayer and outerlayer laminate with blind via holes
Double-Sided Innerlayer Laminates Etched on One Side OnlyInnerlayer laminates are produced as bare-copper boards, but the side to remain unetched must be totallycovered with etch resist to protect the copper foil against etching. The innerlayer laminates AB and CDshown in Figure 32-1 are the same as used in Picture A of 30-3, Section 30. Hole drilling, through-holeplating and etching of the outerlayer circuits are carried out after the board has been laminated.
Double-Sided Innerlayer Laminates Etched on Both SidesInnerlayer laminates are produced as bare-copper boards. The innerlayer laminate shown in Figure 32-2 isthe same as used in Picture B of Figure 30-3, Section 30. After lamination, the board is finished as anordinary plated-through-hole board.
Fig. 32-2
B
C
Fig. 32-3
CHAPTER 3. HOW TO PRODUCE MULTILAYER BOARDS
Double-Sided Innerlayer Laminates with Buried Via HolesIf the circuits on each side of a thin innerlayer laminate need side-to-side interconnections, the mostcommon course of action would be to use plated-through holes, possibly as small via holes, with a diameterof 0.4 mm (16 mils) or less. Plated-through holes, however, occupy valuable real estate on all innerlayersand outerlayers, and make routing the circuitry more difficult. Buried via holes interconnect the two sidesof the thin innerlayer laminate only and do not occupy real estate on the other layers of the board. See Figure32-3. Buried via holes, however, signify that the innerlayer laminate cannot be produced as a nonplated-through-hole board. This is reflected in a somewhat higher price.
Since the innerlayer laminate is very thin, it is possible to drill small holes – 0.3 mm (12 mils) –and still achieve a low aspect ratio, which ensures acceptable plating conditions (see Section 2).
Innerlayer laminates with buried via holes are produced in the same way as bare-copper plated-through-hole boards, see Section 10. When innerlayer laminates with buried via holes are laminated, thevia holes are filled with epoxy resin originating from the prepreg sheets. The via hole should be 100% filledwith resin, and the thickness of the copper wall should not be less than 13 µm (0.5 mil).
Innerlayer and Outerlayer Laminates with Blind Via HolesBlind via holes interconnect an outerlayer circuit with the adjacent innerlayer circuit. Blind via holes are veryuseful in SMT circuits since they are closed, meaning that the solder cannot be drained away from the solderpads during soldering. This means that blind via holes can be placed within the footprint pads and notoutside. With this method, great real estate savings can be achieved. Blind via holes, however, are quiteexpensive because of the complicated processes. There are several methods of producing blind via holes.
Conventional Drilling The first method utilizes typical PCB manufacturing equipment. The thin laminate used for the outerlayerand the adjacent innerlayer is drilled and electroless plated, just like an ordinary plated-through-hole board.The image of the innerlayer is now transferred in the usual way, whereupon the thin laminate is electroplatedwith copper and tin/lead or tin. This means that the innerlayer is pattern plated, whereas the outerlayer ispanel plated since no image has been transferred to this side.
After etching and stripping the tin/lead or tin, the thin laminate is ready for lamination. A thinlaminate is shown in Figure 32-4.
During lamination, all blind via holes will be filled with epoxy resin from the prepreg sheets. Toavoid epoxy bleeding over the surface, it is common practice during the lamination to cover the surface withteflon foil or something similar so that the via holes are more or less sealed. After brushing the surface toremove all possible traces of epoxy on the copper surface, the board is now processed as a normal plated-through-hole boards. See Figure 32-5.
Because of the panel plating of the outerlayer, the copper layer is 25 to 30 µm (10 to 12 mils) thickerthan usual. Unless an ultrathin copper foil is used, such as a 9 µm (0.25 oz.) foil, serious undercutting cantake place when etching the board.
Blind via holes are often specified with a finished diameter of 0.20 or 0.25 mm (8 or 10 mils), whichmeans that the holes can be drilled by means of 0.3 mm (12 mils) drill bits. These via holes are often locatedin footprint pads.
Figure 32-5 shows that the blind via hole is closed by a "copper lid". In order to avoid solderingproblems, some PCB customers set requirements to the flatness of the copper lid. A possible depression shallnot be more than 25 µm (1 mil), but often nothing about projections is specified. Projections should beconsidered most problematic when soldering SMT components, such as BGAs.
Figure 32-6 shows an example of a closed blind via hole with a virtually flat surface of the copperlid. The via hole should be 100% filled with resin, and the thickness of the copper wall should not be lessthan 13 µm (0.5 mil).
The other method is based on a drilling machine that can lower and raise the drill bit in a veryprecise way. Thin laminates are produced as double-sided innerlayer laminates etched on one side only, -with unetched copper on the outerlayers. Following lamination, all holes to be plated-through are drilled.The blind via holes, however, are drilled just through the copper pads of the adjacent innerlayer, whichrequires precision drilling in depth. See Figure 32-7. The board is now processed as a normal plated-through-hole board. Although the via holes are not going through the entire board, all through-platingprocesses are possible since the depth of the holes is rather limited, e.g. 0.2 to 0.3 mm (8 to 12 mils) in depth.The insulation from the bottom of the drilled hole to the underlying copper layer, should not be less than0.25 mm (10 mils).
Laser Drilled Microvia HolesThe majority of multilayer boards with microvia holes have the microvia holes formed by laser drilling ratherthan by photoforming. Laser drilling techniques are described in Section 2.
One of the big problems is the capacity of the laser drilling machine. With multilayer boards formobile phones, the number of holes is very high. A batch of boards may contain 30 million 0.1 mm ( 4 mils)microvia holes, for which reason drilling capacity is extremely important. As an example, some laser drillingmachines can drill 10,000 holes in 75 seconds; to which is added loading and unloading of the individualpanels – additional time of 2 x 10 seconds – for a total of 95 seconds. So, drilling 30 million microvia holeswill take about 80 hours.
The shape of a laser drilled hole is shown in Figure 32-8a. If not cleaned sufficiently after drilling,some resin residues may remain at the bottom of the hole as shown in Figure 32-8b. Although it is difficultto measure, the diameter (d) of the dome consisting of the resin residues should not be more than 25 to 30%of the bottom diameter (D). According to some specifications, the bottom diameter should not be less than50 µm (2 mils), and the thickness of the copper wall should not be less than 13 µm (0.5 mil). Figure 32-8cshows a microvia hole that has been closed so that there is a void in the via hole. This is not permitted becausethere is bare copper in the cavity and also possibly some electrolyte.
The build-up of the board´s outerlayer, including the dielectric material between the outerlayerand the adjacent innerlayer, is based on RCC (a registered trademark of Allied Signal Laminate SystemsInc.), meaning resin coated copper foil. In some cases, it is called RCF meaning resin coated foil. It consistsof a layer of resin supported on electrodeposited copper foil. As shown in Figure 32-9, the resin containstwo layers: one layer of C-stage epoxy resin, which is fully cured, and another layer of B-stage epoxy coating,which is partially cured. There is no glass reinforcement so that the formation of microvia holes can be madeeasily by means of laser or plasma processes. The underlying layers are made of FR-4 (see Figure 32-8).
The thickness of the copper foil is usually 18 µm (0.7 mil), and the resin layers can be obtainedin different thicknesses: for the B-stage and C-stage from 25 µm (1 mil) to 35 µm (1.4 mils).
Another approach is to use Thermount (a registered trademark of DuPont) laminates and prepregsheets based on epoxy and nonwowen aramid reinforcement.
IntroductionThis chapter addresses the production of flexible circuits and flex/rigid circuits. Most of the manufacturingprocesses, i.e. imaging, plating and etching, are quite similar to those used when producing plated-throughboards. However, there are differences due to the thin and floppy materials used in flexible circuits.Additionally, special processes are necessary for producing and laminating the coverlays and for contouringthe flexible circuits.
A breakdown of a flexible circuit shows that it consists of three basic elements: copper foil,dielectric material and adhesive.
The following sections describe, in detail, how to produce flexible circuits and flex/rigid circuits,including two examples of how flexible circuits can be used as interconnects as shown in Figure 50-00.
Purpose of Flexible Circuits◆ to provide interconnections between printed circuit boards and/or other components.◆ to serve as three-dimensional substrates for the mounting of SMT components, e.g., in
photographic and video cameras.◆ to establish interconnections capable of withstanding dynamic flexing.◆ to form part of flex/rigid circuit boards.
Basic Types of Flexible CircuitThere are five basic types of flexible circuits as described below:
1. Single-Sided Flexible CircuitsThis is the simplest type, and consists of a thin and flexible base material to which a copper foil is laminatedby means of an adhesive. The finished circuit is frequently provided with a covercoat bonded to the copperside by means of an adhesive. See Figure 50-1.
Holes for components or connector pins are drilled or punched in the flexible circuit to providenonplated- through holes. Holes in the covercoat are drilled or punched before bonding the covercoat to theflexible circuit. See Figure 50-2.
2. Double-Sided Flexible CircuitsAs the name suggests, the circuit consists of a thin and flexible base material with copper foil laminated toeach side. The outer sides of the finished circuits are frequently provided with covercoats bonded to the outersides (copper). See Figure 50-3.
Plated-through holes in double-sided flexible circuits are usually drilled, instead of punched. SeeFigure 50-4.
Fig. 50-3
Polyimide
Polyimide
Polyimide
Adhesive
Adhesive
Adhesive
Adhesive
Copper Foil
Copper Foil
Covercoat
Covercoat
FlexibleCircuit
Fig. 50-4
Adhesive
Polyimide
Adhesive
Fig. 50-5
CovercoatPolyimideAdhesiveAdhesive
Polyimide
AdhesiveAdhesive
PolyimideCovercoat
Usually the flexible circuits are provided with a covercoat on both sides as shown in Figure 50-5.
3. Multilayer Flexible CircuitsA multilayer flexible circuit consists of a number of thin and flexible base laminates and copper foilslaminated together by means of adhesive, in very much the same way as rigid multilayer boards. Also it iscommon practice to bond covercoats to the outer sides (copper). See Figure 50-6. Plated-through holes canbe provided in virtually the same way as in double-sided flexible circuits.
4. Flex/Rigid CircuitsA flex/rigid circuit is a combination of rigid boards and flexible circuits, the latter creating flexible inter-connects between the rigid boards to which they are laminated by means of bond plies. See Figure 50-7.
The flexible circuit is manufactured separately and bonded to the rigid boards, see Figure 50-8,either symmetrically, i.e., in the middle of the rigid boards, or asymmetrically, i.e., to the outer side of therigid boards to be interconnected.
The previous chapters addressed how to manufacture PCBs and gave a certain understanding of theprocesses necessary. Quite another problem is to present the board data in such a way that the PCBmanufacturer can quote and produce the boards without error. The challenge is to present all of the primarymanufacturing data in a complete and unambiguous way.
This chapter shows a specification form that has been used for several years with good results. Thisform makes it possible to send a request for quotation (RFQ) to a PCB manufacturer and receive a quotethat is correct within a few percentage points with respect to the final quote based on the real Gerber data.The form can also serve as a checklist for the PCB designer by assuring him/her that all of the importantinformation has been included in the PCB specification. The form can be downloaded into your system andfilled in line-by-line. Parameters not necessary for a particular job can be deleted so that the printout onlyreflects the relevant data. The form is shown on the last pages of this chapter. The lines are numbered andthe text below corresponds with those numbers.
(1 - 7) Main Data and PricingThe board should be defined by company name (1) and part number (2). Lot size and annual requirements(3) are important for the pricing, not least when determining the test fixture to be used: Either a dedicatedor a universal test fixture.
The layer count (4), board size (5), thickness (6) and board shape (7) should be given. For PCBswith cutouts so big that two boards can be "wrapped" into each other, a note referring to a sketch couldbe useful.
(8) PanelizationIt is also important to state if the board should be panelized, and if so, the overall dimension of the deliverypanel (8a) and the number of boards per delivery panel (8b).
Panelization has a certain impact on pricing. The customer`s panelization choice is not alwayseconomical because it can lead to wasting laminate. It is better to discuss the panelization beforehand inorder to obtain the best possible fit to the size of the production panels. For reasons of fitting into theassembly line and/or the test equipment, the only thing the customer should state is the maximum widthof the delivery panel. It is very seldom that the length of the delivery panel poses any problems.
There might be a problem if one or several of the PCBs on the delivery panel is/are defective.Willthe entire delivery panel be rejected, or can one or two defective PCBs be accepted provided they be clearlymarked, and the pick-and-place machine be able to ignore the defective PCBs ?
It is important to set a maximum limit on the acceptable number of defective PCBs per batch, e.g.5% of the total number of panels, and a maximum of one or two PCBs per panel. This can be stated in avendor agreement.
A separate panel drawing indicating the layout and the width of the milled grooves separating theindividual boards, as well as the breakaway areas, should be supplied (see Item 23e).
(9) Board Build-UpIn many cases, the build-up is of no special importance, so it is advantageous to let the PCB manufacturerchoose the build-up. Examples are given in Appendix 2A and Appendix 2B.
(10) Boards with Controlled ImpedanceIn cases where the multilayer board has controlled impedances, the build-up should be specified in termsof layer distance and the dielectric constant of the prepreg and the hard cores – possibly with an indicationof brand and type. It can be expedient to supply a cross-sectional drawing.
(11) Test CouponVery few customers demand a co-delivered test coupon. The usefulness of a test coupon can be discussedbetween both parties. A test coupon is usually located outside the board where the plating conditions arebetter than within the board area. A disadvantage is that more copper will be deposited in the PTHs andgive a false impression of the general plating quality.
If a test coupon shows good quality, it is not tantamount to a perfect board. However, if it showsdrilling and/or plating problems, the board could be of inferior quality.
(12) Machining (Contouring)The usual type of contouring is routing (milling), but V-cutting, sometimes called scoring, or punchingcan be specified. V-cutting is cheaper than routing but leaves tiny glass splinters along the edges so thatsanding can be necessary.
(13) Laminate Type and UL FlammabilityThe most common laminate is FR-4 with a UL flammability rating of 94V-0. The PCB manufacturernormally carries this base laminate, with the highest flammability rating, in stock. There is no real savingsby specifying a lower class, e.g. 94V-1, which often has to be purchased in smaller quantities and, therefore,at higher prices. If the customer has specified 94V-1, the PCB manufacturer may chose to use 94V-0laminates, but this just gives the customer a better board.
Some users require "green" PCBs, i.e., PCBs that can be incinerated after their life cycle withoutdeveloping toxic fumes. In such cases, it is necessary to specify FR-4 laminates that do not contain PBB(polybrominated biphenyls) or PBBO (polybrominated biphenylene oxides).
(14) Warp & TwistThe usual value is 1%, but in certain cases, such as when soldering BGAs, it is advantageous to specify a lowervalue of 0.5%. Not all PCB manufacturers, however, will commit themselves to guarantee such low warpand twist values .
Some PCB manufacturers press finished warped/twisted boards under heat, but this is a short-termcure. After soldering, most of the original warp/twist will return.
IntroductionFor a company manufacturing electronic equipment, it is risky to depend on just one PCB manufacturingsource. Something unforeseen could happen, e.g. a strike, or worse, a fire to interrupt production. Suchincidents can stop deliveries for long periods, and it is a well-known fact that it is impossible to transfer evenas few as 25 different PCBs to another PCB manufacturer, and have the deliveries resumed within a fewweeks.
Therefore, it is important that the PCB manufacturer and the new customer are thoroughlyknowledgeable about each other’s capabilities and expectations to ensure compatibility and cooperation.There can easily be differences in the perception of quality, or the documentation package can seeminadequate to the PCB manufacturer, thus requiring additional communication with subsequent adjust-ments, and inevitably, this takes time.
Another important advantage of having a number of regular PCB suppliers is a broader spectrumof PCB technology at the customer’s disposal. This can be very significant in times of great technologicalchanges, especially when introducing SMT, HDI (High Density Interconnect), or laser drilled boards.
Thus, the conclusion is that any electronics company should always utilize a number of approvedand regular PCB vendors. The purpose of this chapter is to describe how to find a suitable number ofqualified PCB manufacturing candidates, and how to best choose between them.
Part 1. In Search of a Candidate
70.1 Sources of InformationThere is a variety of information available to obtain names and addresses of prospective PCB suppliers.
70.2 Request for QuotationTo a PCB buyer, a PCB manufacturer is characterized by price, delivery time, and the quality of the boardsdelivered. To evaluate these parameters, a request for quotation, RFQ, should be sent to selected candidatecompanies. Special attention should be given to:
◆ Pattern density◆ Minimum conductor width◆ Minimum conductor spacing◆ Minimum annular ring◆ Minimum hole diameter◆ Minimum pad clearance in solder and insulation masks◆ Build-up and number of layers in multilayer boards◆ Blind, buried and stacked microvias◆ Accuracy of pattern position◆ Solderability protection◆ Gold plating of edge connectors◆ Carbon printing of keyboard contacts◆ Type of solder masks
The request for quotation should require the following information:◆ Prices, when
● batch sizes = X, Y, Z● annual consumption = W
◆ Delivery times for● first orders● repeat orders
◆ Production costs:● set-up charges● electrical test charges● blanking tools, if any
70.3 PCB Buyer’s SpecificationIt is very important that the prospective PCB suppliers be informed about the PCB buyer’s technical andquality expectations. The easiest way is to send a PCB specification that should:
◆ Be self-contained, unambiguous and result-oriented◆ State technical and quality requirements◆ Avoid cosmetic criteria in favor of functional criteria
An important feature is that the PCB specification addresses the finished result, and not themanufacturing processes to be used. Only in very special cases should the manufacturing processes bespecified, but it should be noted that the PCB buyer participates in the responsibility of the finished product.
70.4 Additional InformationBesides the plain commercial data requested, other information is needed in order to properly assess the PCBmanufacturers selected for quotation. The request for quotation should, therefore, pose a number ofquestions derived from the following survey:
◆ Company size and turnover◆ Number of employees◆ Ownership◆ Use of sub-contractors◆ Experience within export trade◆ Product range /capability◆ Capacity (sq.m or sq.ft per annum)◆ Approvals (e.g., ISO9002, ISO14000, QS9000 and UL)◆ Reference list◆ Prototype service
70.5 Evaluation of QuotationsAssuming that the PCB buyer has sent his request for quotation to a number of PCB manufacturers, it isimportant to compare the quotations received. Immediate questions are:
◆ What was the response time?◆ Does the structure of the quotation follow the request for quotation?◆ Does the quotation answer all questions asked?
The evaluation of the prices quoted is of great importance since this starts the real separation ofthe quotations. It is not easy to compare all the prices by simply setting up tables, so a graphicalrepresentation as shown in Figure 70-1, is recommended. At a glance it is then possible to survey all thequotations, including the quantity discounts offered.
◆ To provide the PCB designer an overview of the manufacturing difficulty factors andrejection rates (expressed by percentages) for the parameters which, based on the design valueschosen, are crucial to the PCB manufacturer.
◆ To influence the PCB designer to design boards with the lowest manufacturing difficulty factorpossible to achieve:● lowest rejection rate● timely delivery● lowest price
ProceduresFour difficulty factors (DF) are used when describing the influence of the parameter on question upon themanufacturing conditions. Rejection rates, expressed by percentages, are assigned to the difficulty factors,but since the manufacturing conditions vary from one PCB manufacturers to another, some variation inthe rejection rates should be anticipated. The intention is not to add, for a certain board, the rejection ratescorresponding to the various parameters in order to reach a worst-case value. The rejection rates serveexclusively to show the PCB designer the possible consequences by choosing difficulty factors above 1.
DF Characterization
1. Difficulty factor 1 is fully mastered by all PCB manufacturers, and thus it is not necessaryto employ special measures to achieve a low rejection rate. The rejection rate will usually beclose to zero.
2. This difficulty class is mastered by nearly all PCB manufacturers, but a stricter processcontrol than in the case of difficulty factor 1 is required. The rejection rate will normally besomewhat higher than for difficulty factor 1.
3. In this case, the PCB designer sets such high demands to the PCB manufacturer’s capabilitythat not all PCB manufacturers are able to manufacture the boards with a sufficiently highyield. Manufacturing places very high demands on process control.
4. Difficulty factor 4 is only mastered by relatively few PCB manufacturers, and as aprincipal rule, the PCB designer should refrain from designing PCB of this difficulty class.
It should be noted that during the practical design phase different difficulty factors can occurwithin the very same board. However, it is not the intention to characterize the board on the basis of thehighest difficulty factor. The following discussion of the difficulty factors versus the design values, or thedesign requirements, serves to give the PCB designer a quantitative understanding of the technical contentsof the difficulty factors in relation to the various parameters. It is not possible to state concrete guidelineson how the board price is calculated by the PCB manufacturer in relation to the difficulty factors. It is quiteobvious, however, the less the difficulty factor, the lower the price.
Complexity of SMT Boards versus HMT boardsThe complexity of SMT boards (Surface Mount Technology) boards of today is much greater than in thecase of the older HMT (Hole Mount Technology) boards in common use not so many years ago.
The basis of calculating a complexity factor is a board pattern representing the technology of anaverage HMT PCB. The complexity factor of such a reference board can be expressed by the followingparameters:
Reference HMT board SMT boardmm inch mm inch
Board thickness 1.6 0.062 1.6 0.062Board area, BA 100 x 150 4 x 6 100 x 150 4 x 6Layer count, LC 2 2 6 6Hole count, HC 500 500 6000 6000Hole diameter, min. HD 0.8 0.032 0.25 0.010Line spacing, min., LS 0.3 0.012 0.15 0.006Line width, min., LW 0.3 0.012 0.15 0.006Annular ring width, min., ARW 0.3 0.012 0.15 0.006Aspect ratio, AR 2 2 6.4 6.4Solder mask clearance, min., SMC 0.3 0.012 0.10 0.004
The complexity factor (CF) of the SMT board can now be calculated by means of the followingformula, which equals 1 for the reference board when inserting in the formula the values stated for thereference board.
BA LC HC 0.8 0.3 0.3 0.3 AR 0.3 CF = ———- x —— x —— x —— x —— x —— x —— x —— x ——
100 x 150 2 500 HD LS LW ARW 2 SMC
BA LC HC 0.032 0.012 0.012 0.012 AR 0.012 CF = ——— x —— x —— x —— x —— x —— x —— x —— x ———
4 x 6 2 500 HD LS LW ARW 2 SMC
When inserting the values stated for the HMT reference board, the result is 1
When inserting the values stated for the SMT board, the result is 8847. This figure should beinterpreted only as an indication of how difficult it is to design, manufacture and inspect fairly complicatedSMT boards, even when the density is rather limited as in this case. And it should not be interpreted as anindication of how much longer it takes to design a 6-layer SMT board compared with a double-sided board.It should also be remembered that modern designs are most often created as computer aided designs, whichcertainly reduce the design time.
No wonder that it is difficult to design, manufacture and inspect fine-line SMT boards.
80.1 Conductor WidthIn particular, there are three conditions affecting the production difficulty factor and thereby the rejectionrate of conductors:
◆ the thickness of copper to be etched away◆ the etch factor (depth-wise etching divided by side-wise etching)◆ the imaging quality, especially in the case of very narrow conductors
The difficulty factors and the rejection rates are stated in table 80.1 below.
Outer Layers, Plated (PTH and MLB)Conductors having a width w > 0.30 mm (12 mil) pose no particular manufacturing problems, but thenarrower the width, the larger the demand for good control of the imaging and the etching.
The 75% rule applies to all widths in the above table provided a suitable thickness of the copperfoil has been chosen, assuming an etch factor very close to 1, and pattern plating.
The 75% rule states that the remaining width of a conductor shall not be less than 75% of theconductor’s nominal width (w), i.e., 75% of the design value. With an etch factor f, the undercut per sideequals the copper foil thickness divided by f. The remaining conductor width r, expressed in percentage, istherefore:
w – 2 x f x copper foil thicknessr = —————————————— x 100%
w
Parameter Design Value DF Reject. Comments
Conductor width, w mm (mil) - % Copper Foilµm (mil) µm (mil)
For the conductor widths w and the copper foil thicknesses stated below, the formula gives thefollowing remaining conductor width r. See Table 80-2.
The 75% rule can be met by choosing the proper copper foil thickness. The PCB manufacturerusually chooses to etch a little more (a little longer) than required by the copper foil thickness, partly toremove the last traces of the treatment of the copper foil, and partly to ensure that copper specks are notfound anywhere on the board. The narrow conductors, particularly, will be subject to a perceptiblereduction of the remaining width. Because of light creeping during the imaging, a further reduction in theremaining conductor width takes place, again most pronounced along the narrowest conductors. Therefore,the remaining conductor width of 84% to 92% cannot be fully achieved, unless the PCB manufacturercompensates for pattern shrinkage.
The fact that the difficulty factor increases with decreasing conductor width is partly due to theabove conditions, and partly to possible irregularities along the conductor sides due to imaging or plating.
Outer Layers, Nonplated (Non-PTH)In the case of nonplated-through boards, etching is the predominant process for developing the conductors.When using a 35 or 70 µm (1.4 mil or 2.8 mil) copper foil, conductor widths of 0.30 and 0.50 µm (1.2 and2 mil) do not pose any manufacturing problem. However, narrower conductors, even when using 17.5 µm( 0.7 mil) copper foil, pose manufacturing problems, among other things, because of the stricterrequirements on the imaging and the etching.
For reasons of mechanical strength, it is not advisable to use a copper foil thinner than 17.5 µm(0.7 mil) for nonplated-through boards.
For the conductor widths stated in Table 80.3 below, with the addition of a few supplementarywidths, the remaining conductor width r can now be found, assuming that the etch factor is 1 as before.
Conductor width, w Copper Foil Remaining cond.mm (mil) µm (mil) width, r
0.30 (12) ≤ 35 ≤ (1.4) 77
0.20 (8) ≤17.5 ≤ (0.7) 83
0.15 (6) ≤ 9 < (0.35) 88
0.12 (5) ≤ 5 < (0.2) 92
Table 80-2
Conductor width, w Copper Foil Remaining cond.mm (mil) µm (mil) width, r
