ABSTRACTWith the ever-increasing drive for miniatunsation, microjoining is critical to the successfulperformance of assemblies in a wide variety of industries. These include medical products,electronics, telecommunications, defence, automotive, aerospace, and power generation, andcover a wide range of microjoining applications. The increasing complexity andminiaturisation of devices used by these industries demands sophisticated joining processeswith low defect rates as they strive to decrease the size of their assemblies whilst increasingperformance.This article defines microjoining, the processes commonly used and some of the issuesconcerned with joining very small components. In recognition that worldwide there is stronggrowth in the requirement for miniaturised assemblies by these many diverse industries,CSIRO in Adelaide, South Australia commenced a new Microjoining initiative. This articlethen describes some of the work that has so far been conducted.
1. INTRODUCTIONAs microtechnology advances the need for miniaturised components is rapidly increasing.The field of microjoining has come into existence because of this driving force. Althoughthere is no dictionary definition of microjoining, there is consensus that joining sheetmaterials of less than 0.5 mm in thickness, or tubular materials of less than 1 mm in diameter,constitutes microjoining.Microjoining is a growth area and critical to the successful performance of many componentsin a wide variety of industries. These include medical products, telecommunications,electronics, defence, automotive, aerospace and power generation, and the components covera wide range of microjoining applications by welding1 brazing, soldering, or adhesivebonding.Environmental concerns such as solvent and lead use are also driving research in this area.The increasing complexity and miniaturisation of devices used by these industries demandssophisticated joining processes and low defect rates as manufacturers strive to decrease thesize of their assemblies whilst increasing performance and decreasing cost. Microjoining hasemerged as a major part of micro-assembly and micro-mechanics technologies and isreceiving increasing interest1.As well as problems in joining together components, there are additional concerns with thesheer scale of some of these components. With pieces smaller than the width of a human hairbeing common the use of microscopes as a joining aid is frequent.
2. CSIRO Microjoining FacilityRecognising the value of a designated Microjoining facility, CSIRO Microjoining wasinitiated in the later half of 2000. A laboratory area was prepared as a nominally clean workplace, fully air conditioned with a HEPA filter extraction system. Investment in equipmentand resources was planned to allow the group to develop, compete with, and complement
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
other microjoining facilities or research organizations.A multi-disciplinary team, including scientists, engineers and technicians covering the fieldsof metallurgy, welding, brazing and soldering, power beams and electrical design wasassembled. This was necessary to support the diverse fields in which projects would bedeveloped. Recruitment of staff was completed in January 2001 and since then the team hascontinued to develop its capabilities and conduct market research.The Microjoining team is presently establishing itself as a resource in Australia, and iscurrently the only one in Australia. It has affiliations with WTIA (The Welding TechnologyInstitute of Australia), and also with other microjoining establishments around the world.Some of the recent projects are described in more detail in Section 6.
3. MICROJOINING TECHNOLOGYThe techniques available for microjoining are much the same as in otherjoining industrieswith similarly controlled variables such as voltage, current and travel speed. Joiningprocesses can be divided into four main groups dependent upon how the heat is applied andthe effect of the heat.
3.1 Fusion WeldingIn this method a heat source is used to melt the joint and form a bridge between thecomponents. In microjoining, close control of the energy is required if damage to the workpiece is to be avoided. The processes that lend themselves most readily to microjoining are:Gas Tungsten Arc (GTA), plasma, and power beams (laser and electron beam). Theseprocesses offer high precision and fine control.2
3.1.1 Arc WeldingWith the advent of transistorised power sources that can give regulated and controllableoutputs of less than 1 amp, arc welding has become an important joining process utilising aversatile and inexpensive power source. Of the available methods of arc welding twoprocesses stand out: GTA and Plasma welding. By pulsing the current in either of theseprocesses the heat input can be further lowered, but this is at the expense of an increasednumber of welding variables. The peak pulse controls the penetration whilst the backgroundpulse allows solidification without extinguishing the arc. Tooling is extremely important withthese processes. Failure to utilise this correctly will result in joint opening and burn-through.When joints are correctly designed and appropriate tooling is used excellent joints areproduced.
3.1.2 Power Beam WeldingLaser (LBW) and electron beam welding (EBW) processes have long been identified as beingexcellent microjoining tools, capable of very fine control over power and positioning. Theycan focus a high energy beam onto a very small spot size allowing deep penetration with littledistortion.EBW is almost always conducted under a vacuum unlike LBW which utilises an inert gasatmosphere to protect the joint area. For this reason EBW provides probably the highestquality joints but at the expense of throughput due to pumping time. The size of the vacuumchamber also limits the size of the component.In addition in EBW, as the beam focus diameter tends to be of similar dimension to the depthof penetration, and hence not very deeply penetrating. For this reason the process is more
Proc. SPIE Vol. 4592526
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
likely to be conduction limited welding rather than deeply penetrating, which limits some ofthe attraction of the process4.Lasers however can focus their beam energy down to very small sizes, typically of the orderof tens of microns. LBW therefore offers greater advantages for microjoining where lowpenetration is required5. The laser energy can also be pulsed to reduce thermal input andhence distortion.Lasers can be used for similar applications to arc and electron beam welding, however due tothe decreasing cost of lasers they are continually finding new application areas. Lasers arealso finding use in electronics manufacture where they can be used to reduce the solderingtime and hence heat input6 7
3.2 Solid State Bonding
Here there is no melting of material. Joints are made by plastic flow occurring at the interfaceto bring the components into intimate contact and thereby form a bond. Many differentprocesses are available, but those used in microjoining tend to be based upon ultrasonicvibration or friction welding to join dissimilar materials. Diffusion bonding either in theliquid or solid phase is another process that has also been successfully used on a microjoiningscale.
3.2. 1 Ultrasonic BondingUltrasonic bonding is a solid phase joining process that relies upon displacing the interfacialoxides and contaminants whilst at the same time using pressure to form a bond. There is aslight temperature rise during bonding.Ultrasonic bonding is probably the most important process in the electronics industry where itis used as a means of first level interconnect between semiconductor chips and package pin-outs. In this process there is a high frequency vibration with a low pressure applied to causethe plastic flow required.There are two variants of ultrasonic wire bonding —ball bonding and wedge bonding8. Wedgebonding is primarily performed using aluminium wire. The wire is wedge bonded at onepoint using ultrasonic energy, then drawn out in a loop then similarly wedge bonded at theother end. It is usually performed at ambient temperature, unlike ball bonding which ischaracterised as a thermosonic process, meaning heat (typically 150°C) is applied during thebonding process.Today the most used method is ball bonding with gold wire. This process works by forming asmall ball on the end of the wire. This ball is bonded as the first joint, then the wire is drawnout in an arc before attaching this as a wedge bond. Unlike wedge bonding, ball bonding hasthe advantage of being able to be drawn out in any direction.
3.2.2 Diffusion BondingDiffusion bonds are made by using intermediate heat (typically 0.4 of melting temperature)for a period of time, and cause minimal distortion to components. Dissimilar metal joints canalso be made this way without intermetallic compounds being formed.During the time at temperature diffusion occurs between the surfaces until a joint is formed.There are two types of diffusion bonding: solid phase and liquid phase. During liquid phasediffusion bonding a low melting point eutectic is formed by diffusion between the componentparts.
Proc. SPIE Vol. 4592 527
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
3.2.3 Friction WeldingFriction Welding may be used to join together a wide range of materials including non—metals, however the geometry of components is fairly limiting. In the microjoining field ithas been used to join components to heat sinks, specifically in the electronics industry whereit has been used to directly attach aluminium heat sinks to alumina substrates5. It is alsocommonly used for attaching tubes or rods to bulk or sheet components.
3.3 Soldering I BrazingDuring soldering or brazing, a third lower melting point material is drawn into the gapbetween the components by capillary action. There is no melting of the componentsthemselves. The difference between soldering and brazing is defined by the melting point ofthe filler metal, with brazing considered to be occurring at temperatures greater than 450°C.9The gap through which the solder or braze alloy is drawn should be small so that maximumstrength is imparted to the joint. Soldered and brazed joints gain their strength from thewetting of the parent metals by the molten solder or braze metal. Any contamination wouldcause a reduction in wetting and consequent lowering ofjoint strength.Both processes may be used to join similar or dissimilar materials by selection of the correctfiller alloy, or by using coatings. Brazing is also finding use as a means ofjoining metals tonon—metals.Soldering is one of the primary joining processes used by the electronics and electricalindustries, and may be used in the manufacture of such products as printed circuit boards,control systems or audio systems. Solderjoints have to provide electrical, thermal, andmechanical functions.Traditionally solders containing lead have been used in various applications. However,concerns about toxicity and health hazards means that there is a drive to develop and use leadfree solders. This may cause problems with the traditional soldering methods and newtechniques may be required.
3.4 Adhesive BondingAdhesive bonding relies upon the attractive forces between the molecules at the surface of theadhesive and those of the surfaces to be joined. The larger the molecules the better theadhesion, which is why organic adhesives are commonly used.The liquid adhesive is used to wet the surfaces to be bonded. This is then cured, or hardened,to form a solid bond. This curing can be aided by applying temperatures of approximately150°C. Like brazing or soldering, the thickness of the adhesive should be minimal to providea strong bond.Recent developments with adhesives has increased the scope for their use. By doping theadhesive with metal (typically silver) it is possible to make them electrically or thermallyconductive'0. The development of thermally conductive adhesives has aided heat dissipationin electronic devices, thereby increasing component lifetime, and electrically conductiveadhesives are being considered as replacements for solders as they have the potential toremove lead from the assembly. However it is not just a case of simply changing toelectrically conductive adhesives as various other factors, such as electrical conductivity,lifetime, component finishes and design issues would need further assessment.
Proc. SPIE Vol. 4592528
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
4. DESIGN FOR MICROJOININGLike any joining process there are many important considerations to take into account duringselection, but process control becomes critical when joining small components. Notsurprisingly different processes may often be selected for the same application. Considerationwould then need to be given to such factors as capital expenditure, staff skills, and ease ofmechanisation or automation. The major considerations are:
S heat input. fit-up. consistency I repeatability. other factors
4.1 Heat InputWith such thin and delicate components, precise control over the welding parameters must beexercised. Too much power will result in over-penetration and potential damage to delicatecomponents. Often the welding is conducted near to components that may be damaged by theheat from welding. Very tight control of the heat input is required in this case.When welding metals of high thermal conductivity to metals of low thermal conductivity, orthick to thin metals, correct joint design and heat sinks are needed to achieve a thermalbalance.
4.2 Fit-upIt is not uncommon to have to join together components that are smaller than human hairs orthinner than sheets of paper. This makes them extremely difficult to handle and assemble sothat they fit together well enough to allow joining. Accurate fixturing and handling devicesare almost mandatory for components of this size. These add to the cost ofjoint production.Consistency in part machining during mass production will also be essential if the maximumbenefit is to be obtained from these fixtures.
4.3 Consistency I repeatabilityRepeatability of the joining process is essential for microjoining. With the advent ofadvanced electronics, motion controllers, and monitoring systems it is possible to accuratelycontrol the joining process. Once a process is developed no changes to parameters should bemade without consideration of the effect on the rest of the process.
4.4 Other Factors
As with any application, there are often situations where two or three processes will work fora given material or design', and a further assessment must be made as to which would be theoptimum depending upon various factors that must be considered for any joining process:
• investment in capital equipment• production rate required• degree of automation required• level of training of operator.
Proc. SPIE Vol. 4592 529
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
5. TYPICAL APPLICATIONS
5.1 Electronics
A very high percentage of manufactured goods rely upon electronic circuit boards andcircuitry. Traditionally eutectic or near eutectic lead tin solder is used in the manufacture ofthese components. There are different ways of applying heat to melt the solder but themethods used almost universally are reflow ovens and wave soldering machines. Ascomponents become more complex, more delicate, and packing density increases, there is ahigher probability of joining related problems.
5.2 Controls and Sensors
With the current trend in computer control and remote sensing, there is a need to manufactureever smaller and more intelligent sensors and actuators.Sensors are devices that detect and monitor changes or variations in an object or processrelative to a reference or standard. Depending on their intended function, sensors recognizevariations in weight, vibration, motion, pressure, colour, heat, light, magnetism, chemistry,etc12.With modern, rapidly developing technology, new sensors and instruments are continuouslybeing developed. Their components are miniaturized, sensitive, and precisely arranged in avery small and compact assembly. Most often they utilize micro-electronics, and to protecttheir components from the elements of the user environment, sensor devices are usuallysealed. This protects devices from contamination, moisture, impact, abrasion, heat,magnetism, etc. By doing this, the sensor is manufactured to give consistent and reliableperformance throughout the life of the product.As mechanical assembly (screws, gaskets, clamps, etc.) causes time related failures, weldingis often the preferred joining process for both intermediate and final assemblies. Whenhermetic sealing is required, welding is mandatory13. Materials used to manufacture sensorsvary, but include aluminium, stainless steel, titanium or nickel super alloys depending uponthe potential environment.
5.3 Bio-medical
With the recent rapid advances in medical technology there are a variety of intrusiveprocedures used in the medical industry today requiring tools, instruments, sensors andcomponents in materials that are inert with respect to reactions with the body. Due to theintrusive nature of surgery these are usually as small as possible.Product reliability is one of the major concerns with the medical industry, and anymanufacturing process including joining must be able to satisfy stringent standards to ensurethat in-service failure will not occur14.In addition to components used internally, a large number of microjoining applications can befound in support equipment such as automated drug dispensers.
5.4 JewelleryThere is a small amount of joining required in the jewellery industry, mainly to join preciousmetals for the fashion industry. Traditionally mechanical fixing, adhesives, brazing andsoldering have been used in the jewellery industry'5.Joining is also used to provide a colour contrast by application of a different metal slurry in arecess followed by flowing at temperature.
Proc. SPIE Vol. 4592530
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
6. CSIRO MICROJOINING PROJECTSA literature survey and thorough market analysis was carried out. From this a number ofpossible areas for more detailed examination were identified and projects initiated withAustralian companies.
6. 1 Welding thin sheet material
A project to improve upon a currently used manufacturing method was initiated with atelecommunications company. They currently used small stainless steel cans to protect adelicate component, and these are made from machined bar stock material.Analysis of the requirements showed that welding of thin stainless steel sheet could provide aless expensive method to produce the protective container. The requirements of the joint werethat it was hermetically sealed, and the weld did not stand proud of the outer surfaces, toallow insertion into another assembly.Low heat input welding processes, laser and micro-plasma, were examined, and trialsconducted on 0.2 mm thick type 304 stainless steel that replicated the material and thicknessof the components.Initial trials showed that welding material of this thickness would present problems for manyof the conventional welding processes due to the lack of rigidity of the components givingunacceptable distortion. Close fitting fixtures were required, both to aid rigidity and toremove heat from the joint area. Failure to have such fixturing resulted in opening of the jointahead of the weld, and consequent failure to close the joint or weld drop through as shown inFigures 1 and 2.Both the micro-plasma and the laser were able to produce joints satisfying the mechanicaland physical requirements, however since the laser is a lower heat input process, it wasselected for further work.A joining process was developed that used the CSIRO 500 W pulsed NdYAG laser. Thewelding of the components was completed successfully as shown in Figure 3. Current work isaimed at determining the internal temperatures that would be experienced by componentryinside the assembly during welding, however control of this should be accomplished byselection of optimum travel speed with respect to pulse frequency.
6.2 Welding fine diameter dissimilar metal wires
A company contacted CSIRO with a requirement to find a way to join together lengths of0.05 mm diameter Electrolytic Tough Pitch copper wire to 0.35 mm diameter stainless steelwire for a medical application. The copper wire was covered with a 0.006 mm thick enamelcoating with a melting point of approximately 160°C.The requirements of the joint were that it would provide an electrical contact and have a jointstrength sufficient that it could withstand the rigours of handling prior to encapsulating in asealant for insertion and withdrawal from the human body. Figure 4 shows the thickness ofthe copper wire in relation to a human hair.The joining process needed to be able to supply a controlled, small amount of energy, asfailure to control the heat input would result in damage or even vaporisation of thecomponents.Trials were conducted at CSIRO and overseas. These early trials soon revealed that laserwelding at CSIRO appeared to give the greatest chance of manufacturing repeatable joints,and was a process that could easily be assimilated into a production environment. Furtherwork concentrated upon this process only.
Proc. SPIE Vol. 4592 531
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
A single laser pulse could attach the copper wire to the stainless steel wire as shown in Figure5, however there was found to be too much expulsion of material, resulting in a reduced crosssectional area of the weld. It was felt that there was too much reduction in area to carry eventhe low current required.The enamel coating on the copper wire proved troublesome, however a technique wasestablished where the energy from the laser beam was initially used to strip this layer prior towelding. The entire welding process including layer removal to welding is shown in Figure 6.Following the work by CSIRO and with work being conducted elsewhere supporting thefindings it appears that the company is on the verge of producing a new medical componentthat has the potential to revolutionise certain medical procedures.
7. THE FUTURE FOR MICROJOININGMicrojoining technology has advanced significantly in recent years, and equipment andprocesses are available to join very small or thin components. To continue developing insome of the new technology fields, be they electronics, biomedical, instrumentation or sensor,microjoining capabilities also need to advance, and take advantage of other scientificdevelopments like robotics and automated systems to aid repeatability.There will be changes imposed upon industry due to environment concerns that will affectboth manufacture and end of life. Some of the materials used are potentially toxic and afteruse there is concern that when disposed of in landfill sites they could leach out andcontaminate the land or water.Continued research is essential to ensure progress of microjoining technology into the future.CSIRO and WTIA are working in collaboration under the Ozweld banner, where the businessknowledge and contacts of WTIA is being utilised and combined with the technical andscientific abilities of CSIRO. This is part of the SMART Technet Project initiated to improvethe competitiveness of Australian industry through the use of the latest technology.
8. REFERENCES
1 S Dunkerton and N Stockham: TWI Bulletin Jan 19912 CC Otter et al: TWI Technology Briefing 710, Aug 20003 KJ Ely: Proc Taiwan mt. Welding Conf. 1998, Sep 7-8, Taipei, Taiwan, pp 43 1-4394 KI Johnson: TWI Document "Introduction to Microjoining", 19855 SB Dunkerton: Welding Review International, May 1992, pp696 E Russell: IPC APEX Proceedings, Paper P-AD/3-17 F Legewie et al: G-ICALEO, 1998, pp 438 B Hueners: Electronic Packaging and Production, Oct. 1998, pp 559 AWS Soldering Handbook, 3rd Edition10 G Thomas: TWI Bulletin March 2000, pp 2511 A Cullinson: Welding Journal, May 1996, pp 2912 J Goward: TWI Bulletin May/June 199913 Private Communication with Davidson Pty Ltd14 N Stockham: Microtechnology for Surgical Tools and Implants15 N Stockham: Welding and Joining, May 1997, pp14-16
Proc. SPIE Vol. 4592532
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
Figure 1 . Weld closure failure due to poorly fixtured components x 6
Figure 2. Weld slump due to excessive heat x 40
Figure 3. Cross section of welded joint made using pulsed laser x 40
Proc. SPIE Vol. 4592 533
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
:%a¾'
. ::::::::.:. . .•:•..:.:.:..•::.• . Tae
. . •:.::•:•.•.: . .
0 t% a 4t:itS It .
Figure 5. Laser welded 0.05 mm diameter copper wire to stainless steel wire x 200
Figure 4. Comparison of human hair (left) to 0.05 mm copper wire (right)
Proc. SPIE Vol. 4592534
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
Further cleaning using the laser
x 200
Figure 6. Completed weld of 0.05 mm thick copper wire to stainless steel wire x 200
Cleaning of the enamel layer using a laser
x 200
Proc. SPIE Vol. 4592 535
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 02/10/2014 Terms of Use: http://spiedl.org/terms
![Page 1: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/1.jpg)
![Page 2: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/2.jpg)
![Page 3: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/3.jpg)
![Page 4: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/4.jpg)
![Page 5: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/5.jpg)
![Page 6: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/6.jpg)
![Page 7: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/7.jpg)
![Page 8: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/8.jpg)
![Page 9: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/9.jpg)
![Page 10: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/10.jpg)
![Page 11: SPIE Proceedings [SPIE International Symposium on Microelectronics and MEMS - Adelaide, Australia (Monday 17 December 2001)] Device and Process Technologies for MEMS and Microelectronics](https://reader031.documents.pub/reader031/viewer/2022020300/5750964f1a28abbf6bc975db/html5/thumbnails/11.jpg)
