Computer-Aided Printed Circuit Design. For the electronics enthusiast Bill Pollack, January, 2008

Transcription

1 Computer-Aided Printed Circuit Design For the electronics enthusiast Bill Pollack, January,

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! 2

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! 3

4 Don't let this happen to you! How are you going to fix this? 4

5 Create test fixtures and jigs with a professional appearance 5

6 Create test fixtures and jigs with a professional appearance 6

7 Create test fixtures and jigs with a professional appearance 7

8 Would you rather assemble 1000 of this? 8

9 Or this? 9

10 A point-to-point wiring disaster! 10

11 Junction PCBoard Achieves 325 connections 11

12 Junction board installed! Increased reliability, decreased assembly time 12

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-atime aspects make this a real pain 13

14 Computer-aided Printed Circuit Design PCB costs, quantities and turn-around times have greatly improved over the last 10 years Year ~ $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 14 panel board houses selling off otherwise unused 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 15 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, signals 16

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 17

18 PCB CAD software packages FREE tools are available: ExpressPCB Proprietary data format, must use their board service. May be OK for very simple projects - $53 for qty x 3 boards, no soldermask or Silkscreen Limited design rule checking Eagle Layout: 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 18 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 19 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 20 (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 your21component

22 Basics of PCB CAD design (3) Create Board layout Choose device packages 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 TO-92, TO3, SMD etc Put the parts where YOU want them Let router run the copper to match Silkscreen naming nomenclature 22 Revision level, identifying text, mounting holes

23 Three aspects of a 'component' Schematic symbol Physical Package Resistor, capacitor, FET, relay etc Multi-gate part such as 74LS04 hex-inverter etc Power pins, common VCC pins, NC pins etc 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 23

24 Demo circuit - relay and driver Create schematic Set down parts Relay 2N2222 1K resistors DB-9 connector 24

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 25

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) 26

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 27

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? 28

29 Demo circuit via script Humongous 7-segment LED sign (Route time is three seconds to place 294 LEDs, 386 wires!) 29

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 inch pitch - trivial pitch is not tough with flood soldering Personal best is QSOP inch pitch Clearance is inch! 30

31 Questions? injection molded enclosures with inexpensive customizations - sells JBC soldering/desoldering equipment - fast turn small quantity PCBs - Eagle PCB CAD software Gerber file viewer- "Viewmate" from I can recommend several PCB assembly shops if you are looking for completed/assembled product. Please contact me for more information. address is on the first page. 31 Thank you!