Printed Circuit Boards The Design Process

What is the Output? Not just a PCB Engineering Drawing Set Design Specification Schematics PCB Artwork Fab Drawing Bill of Materials Part Drawings Procurement Specifications Firmware Assembly Drawings Test Specifications Minimum Set to Produce PCB PCB Artwork Fab Drawing

PCB Artwork Negatives that will be used to produce layers of the PCB Currently created from Gerber files Gerber Photoplotter RS-274X Gerber format is current standard also known as X-Gerber or Extended Gerber Produced via EDA (Electronic Design Automation) Software

Typical set of RS-274X file names .GBL - Gerber Bottom Layer .GTL - Gerber Top Layer .GBS - Gerber Bottom Solder Resist .GTS - Gerber Top Solder Resist .GBO - Gerber Bottom Overlay .GTO - Gerber Top Overlay .GBP - Gerber Bottom Paste .GTP - Gerber Top Paste .GKO - Gerber Keep-Out Layer .GM1 - Gerber Mechanical 1 .GM2 - Gerber Mechanical 2 .GPT - Gerber Top Pad Master .GPB - Gerber Bottom Pad Master

NC Drill File The other type of file NC Drill file provides X and Y locations along with size for all holes Plated through holes vs uplated holes

Eagle Extensions *.cmp Top, Via, Pad Component side *.ly2 Route2, Via, Pad Inner signal layer *.ly3 Route3, Via, Pad Inner signal layer *.ly4 $User1 Inner supply layer ... ... *.sol Bot, Via, Pad Solder side *.plc tPl, Dim, tName, Silkscreen comp. side *.pls bPl, Dim, bName, Silkscreen solder side *.stc tStop Solder stop mask comp. side *.sts bStop Solder stop mask sold. side *.drd Drills, Holes Drill data for NC drill st.

Design Process in Practice Create Parts Create Schematic Run ERC (Electrical Rules Check) Create Board Place Components Route Traces Run DRC (Design Rules Check) Produce Gerbers and Drill Data

Design Rules Manufacturer Specific Advanced Assembly Rules 4 Mil Line Width/Spacing, 1-24 Layers, .02-.15 thick 2-3 Mil Line Width/Spacing, 25-38 Layers, .01-.25 thick Minimum Drill Size = 15 to 1 Aspect Ratio Minimum Pad Size over finished hole Internal .012/External .008 Recommendations (http://www.aapcb.com/PCBAssemblyTips.asp) Component to Component 10mil Component to Board Edge 125mils Part Polarity/Pin 1 Markers Diodes/LEDs mark cathode (-) end with a bar Capacitors mark positive end – use square hole Number pins/pads as the manufacturer does

Ogres, Onions, and Parfaits Parts, Schematics, and PCBs are like Ogres They have layers Pretty Much Automatic but Pay Attention to them

Creating Parts in Eagle Parts are created and stored in a library Difficult to move from one library to another Parts consist of Package: The footprint in the layout Symbol: The drawing for the schematic Device: Assigns a symbol to a package

Symbol Pins Outline “Wires” on 94 Symbols layer Visible attribute Pad – Display pad name from Package Pin – Displays pin name from Symbol Both – Displays both pin and pad names Direction should match pin’s function ERC issue Outline “Wires” on 94 Symbols layer Name “>NAME” on 95 Names layer Value “>VALUE” on 96 Values layer Instead of “>VALUE” may use name of device i.e. TSP78514, Teensy++

Package Package name should reflect package type DIP14, TO220-5, LCC20 Pads – type ‘1’ including ‘s in text box then place pads - pads will be numbered sequentially Pin 1’s pad should be square or otherwise identified Package outline on 21 tPlace layer Name “>NAME” on 25 tNames layer Value “>VALUE” on 27 tValues layes

Package cont. Critical Dimensions Start by placing Pin 1’s pad Hole to hole spacing Pad to pad spacing in surface mount devices Package size is non-critical Start by placing Pin 1’s pad Place other pads, holes, and outlines in reference to Pin 1

Device Add a Symbol (Library editor calls this Add a Part) by clicking on two input AND Add a Package by clicking on the New button Connect the Symbol’s Pins to the Package’s Pads by clicking on the Connect button Opens a new dialog box Click on Pin and Pad to be connected then Click on Connect Repeat Click on Prefix button to assign a prefix IC, R, C, CONN, J, etc…

Save Library Perhaps you shouldn’t have waited until now to save your work

Schematic Entry Schematic captures the electrical connections of your design Schematic should flow from left to right Frames make your schematic look professional Decoupling Capacitors Bulk where power connects to board At least one per IC

Schematic Entry Design Flow Place parts Define electrical connections Run ERC

Eagle Schematic Tool Bar Info Move Group Cut Delete Name Smash Bend a Line Bus Place Net Junction ERC Copy Rotate – also right click when moving or placing Change Paste Place Part Value Text Electrical Connection Label Net or Bus

PCB Design Define board outline and other mechanicals Place parts Mounting holes Place parts Use Air Wires/Ratsnest to help place Autoroute Add finishing touches Name of Board Board Revision Assembly Revision Helpful silkscreen legends DRC

PCB Design cont. Edit/Net classes Allows different widths and hole sizes for named nets

Eagle Board Tool Bar Rip up Route Autorouter DRC Route Manually

Eagle Window Switch to Board Command Line CAM Processor Info Bar

Creating Gerbers and Drill File Create Drill File File/Run drillcfg.ulp create .drl file A list of all drill sizes needed Use CAM Processor to Create more Drill files File/Open/Job – choose excellon.cam Click on Process Job button Creates .dri and .drd files Use CAM Processor to Create Gerbers File/Open/Job – choose gerb274x.cam Check Tabs for proper layer setup Creates .cmp, .gpi, .plc, .sol, .stc, .sts, .crc, etc… Might have to add cream component

View Gerbers Before submitting your files to a board house view them in a Gerber file viewer Viewplot www.viewplot.com