1

Computer-Aided Printed Circuit DesignFor the electronics enthusiast

Bill Pollack, bill@arcstarter.comJanuary, 2008

2

Computer-Aided Printed Circuit Design

● Overview:– Real-world sample designs– Brief discussion of available professional tools– Discussion of available FREE tools!– Demo of how to design a PCB using FREE software– Questions/Answers !

● Please jump in, interrupt if you can't hear me!

3

Computer-aided Printed Circuit Design

● Why try a custom PCB for your next project vs. 'rat's - nest' construction?– More professional appearance– Permits smaller packaging– Automated check of actual vs desired copper– Quicker to assemble

● Possible CON's:– Learning curve– Expense/ delivery time

BUT - It's cheaper and faster than you might think!

4

Don't let this happen to you!● How are you going to fix this?

5

Create test fixtures and jigswith a professional appearance

6

Create test fixtures and jigswith a professional appearance

7

Create test fixtures and jigswith a professional appearance

8

Would you rather assemble 1000 of this?

9

Or this?

10

A point-to-point wiring disaster!

11

Junction PCBoardAchieves 325 connections

12

Junction board installed!Increased reliability, decreased assembly time

13

Computer-aided Printed Circuit Design

● The 'Good Old days' of at-home PCB fab:– Create tape transparency, or directly draw w/ resist

marker onto solid copper clad board– Etch w/ Ferric Chloride etc– Hope to achieve even etch on both sides– Drill holes (hope to achieve good alignment)– Disposal of spent etchant, no soldermask, no

silkscreen legend, the general mess, and one-at-a-time aspects make this a real pain

14

Computer-aided Printed Circuit Design

● PCB costs, quantities and turn-around times have greatly improved over the last 10 years

● Year ~1995 - $1500 for NRE, 6-8 week turn● Year 2008:

– No NRE or tooling charges– Qty 5 2-sided boards including silk screen legend,

solder mask– 5-day delivery for $80!

● Bare bargains - 2-sided boards copper only (no legend or soldermask) Qty 10 for $49!

● How is this possible? 'Commodity' aspect - major board houses selling off otherwise unused panel periphery to the small user at incremental rates

15

Computer-aided Printed Circuit Design

● Modern tools enable PCB prototyping:● Use CAD program to design circuit

– CAD program may permit design rule check● Are parts too close? Copper too close/ proper width?● Traces or vias under head of a mounting screw?

– CAD program may ensure match between schematic and copper pattern - ie copper matches schematic

– Extract image files “Gerbers” and E-submit files + Credit Card number to internet fabhouse

– Wait for FedEx truck to deliver final professional boards!

16

Computer-aided Printed Circuit Design

● Which CAD tools to use? Depends on size and complexity of the project...

● Bad news - Very complex jobs will require very expensive software– 30x30 pin BGA FPGA part on 21 layer board -900

pin per component! $25,000 for the software!– Automatic, interactive controlled Z or T-delay is

possible● Good news - Most amateur projects are

possible using inexpensive or free tools– Including up to 16 layers, 500-1000 signals

17

PCB CAD software packages

● 'Professional' packages - would include:– OrCAD, ProTel, MentorGraphics PADS

● Cost $6K - $12K and up PER SEAT● Capable of multideveloper, system level project

– Including wiring harnesses, multiple PCBs

18

PCB CAD software packages● FREE tools are available:

– ExpressPCB www.expresspcb.com● Proprietary data format, must use their board service.

– May be OK for very simple projects - $53 for qty 3 2.5 x 3 boards, no soldermask or Silkscreen

● Limited design rule checking– Eagle Layout: www.cadsoftusa.com

● Standard Gerber image files - use any PCboard fab shop● Good set of design rule checking● 2-layer copper only ● Includes soldermask and silkscreen ● Size limitation ( 4 x 3.2 inch)● Single page schematic only● Autorouter with programmable weights

19

PCB CAD software packages

● Eagle Professional – ~$1200 for full install– Unlimited board, schem size– Layers up to 16– Unlimited schematic size– Programming language– External scripting– Intermediate price & performance options, student

user option– No support for multi-board projects (system) or for

team development

20

PCB layers● Top - white epoxy legend

indicates parts placement, polarity

● Green soldermask - isolates bare copper (device pad) from insulated copper (trace)

● Top copper traces● Fiberglass substrate● Plated-through holes

connect layers● Bottom copper● Bottom soldermask

● (demo layer control)

21

Basics of PCB CAD design

● Three steps:● (1) Create schematic

– Emphasis on flow of signals, logical function– Select part values: 10Kohm, 0.1uF, NE555– No emphasis on eventual physical placement– May use either 'wire' or 'named' signal description

● You don't have to draw a spaghetti bowl ● (2) Create /utilize parts library

– Use libraries already available– Modify an available 'close fit' into your component

22

Basics of PCB CAD design

● (3) Create Board layout– Choose device packages

● TO-92, TO3, SMD etc– Use AutoRouter!

● Autorouter can connect nearly ALL signals, given a good set of design rules

– Use Design Rule Check to detect proximity issues– Ground/ power planes (if needed)– Physical location of all components and connectors

● Put the parts where YOU want them● Let router run the copper to match

– Silkscreen naming nomenclature– Revision level, identifying text, mounting holes

23

Three aspects of a 'component'● Schematic symbol

● Resistor, capacitor, FET, relay etc● Multi-gate part such as 74LS04 hex-inverter etc● Power pins, common VCC pins, NC pins etc

● Physical Package● Surface Mount vs through-hole,TO-92 vs TO-5 etc. One

symbol may exist in multiple packages!● Various resistor wattages - same schematic symbol,

different physical shape● Connecting symbol pins to any/all package pins

● Device suffix may control package type

24

Demo circuit - relay and driver

● Create schematic● Set down parts

– Relay– 2N2222– 1K resistors– DB-9 connector

25

Demo circuit - relay and driver(contd)

● Show 'air wires'– Represent connections on the

schematic.– Will be converted into copper

● Show relocation of components

● Show modification of schematic

● Show package selection

26

Demo circuit - relay and driver(contd)

● Auto-router - will apply copper to execute schematic

● Can change package types– TO3 vs TO-92 vs SOT23 etc

● Can pour ground planes

● Can control copper widths– Power traces can automatically be

heavier than signal traces

● (demo addition of PCB mounting hole and use of Design Rule check /autoroute to reposition traces around the mounting hole)

27

Demo Circuit - tachometer

● This board contains 240 wires 64 LEDs on 4x5 inch● Routed in 35 seconds!● Show schematic● Auto-create board● Run positioning script● Route

28

Demo circuit via script - Humongous 7-segment LED sign

● 11x17 inch● 294 LEDs, 386 wires!● Run schematic generating script● Auto-create basic PCB● Run placement script● Auto-Route!

– Time: Any guesses?

29

Demo circuit via script - Humongous 7-segment LED sign

● (Route time is three seconds to place 294– LEDs, 386 wires!)

30

How do I solder those small parts?

● Don't panic!● Buy a small chisel-tip soldering iron

– Suggest 1/16th wide chisel tip● Fine, curved-tip tweezers to hold parts● Fine pitch SMD flood-solderable using special flux● Drag-soldering demo (video)

"0805" chip resistors are trivial– Tin one pad, place part, solder one pad,then solder

next pad!● SOIC - 0.050 inch pitch - trivial● 0.031 pitch is not tough with flood soldering● Personal best is QSOP - 0.025 inch pitch

– Clearance is 0.012 inch!

31

Questions?

●www.polycase.com injection molded enclosures with inexpensive customizations

●www.howardelectronics.com - sells JBC soldering/desoldering equipment

●www.pcbfabexpress.com - fast turn small quantity PCBs

●www.cadsoftusa.com - Eagle PCB CAD software

●Gerber file viewer- "Viewmate" from www.pentalogix.com

●I can recommend several PCB assembly shops if you are looking for completed/assembled product. Please contact me for more information. Email address is on the first page. Thank you!