PCB Design

The the prototyping is done, putting the design proper PCB board makes it more rigid and possible to offer unassembled kits. The process is mostly straight forward but the design process to lay out the board requires some time to get right.

Eagle will be used here, it's cross platform and great part libraries.

Setup

• Eagle parts libraries:
  • SparkFun Eagle Library
    • Includes most of the parts SparkFun offer ont their website
  • Adafruit Eagle Library
    • Ladyada has made a library for most of the parts used in their tutorials
• CAM, Computer Aided Manufacturing, to create processing files for the fab house.
  • SparkFun sfe-gerb274x.cam CAM - Excellent generator of the necessary gerber and drill files, picks the right layers
  • Creates the proper gerber files for manufacturing:
    • glt - Top copper layer, main traces
    • gbl - Bottom copper layer, same bottom
    • gto - Top silkscreen, all text, lines, illustrations can be painted on top of the soldermask
    • gbo - Bottom silkscreen, same bottom
    • gts - Top soldermask, where to put tin/gold plated pads and mask all other green/other color
    • gbs - Bottom soldermask, same bottom
    • gtp - Top solder paste stensil, only needed for assembly of parts to the board
    • txt - Drill coordinates, for vias and mounting holes

Good practices

Some of these guidelines can be used in the Design Rules property window in Eagle. The PCB manufacturer normally provide a list of minimum requirements for their equipment/method.

Design rules:

• Clearances:
  • Width: 10 mils / 0.254 mm - spacing between traces, ground pour, pads, vias. Also same signal smd components, pads, vias
• Distances:
  • Board edge copper isolation: 12-50 mils / 0.3048-1.27 mm - for clean routing cut or v-score and avoiding shortcircuitings
  • Drill and hole distance: 8 mils - minimum distance between drill holes
• Sizes:
  • Signal trace width: 12-16 mils / 0.254-0.4064 mm - enough distance to avoid shortcircuiting of traces after fab
  • Power trace width: 16-24 mils / 0.4064-0.6096 mm - allow large currents to flow in vcc and ground traces
  • Via and hole drill size: 20 mils / 0.508 mm - minimum hole size, restricted by fab, use larger for better via connections
• Restring for pads, vias:
  • Percentage of drill size: 25% - enough surface to add solder
  • Min. width: 12mils - minimum restring size
  • Max. width: 20mils - maximum restring size
• Masks:
  • Stop soldermask: 4 mils / 100% / 4 mils - Bleed a litt over the pad or via to make sure it's properly tinned
  • Soldermask limit: 32 mils - little higher than the largest via to cover them with the mask instead of open tin/gold plating, aka. tented

Other:

• Labels and text:
  • Add silkscreen labels to all connections and interfacing points
  • Add a revision and date label
  • Use vektor text, size 0.05 mils, and bold ratio 15%
• Ground pour:
  • Add a final rectangular polygon plane over the entire design and click Ratsnest - this helps to avoid board warping, better ground and signal conditions
  • To only see the board without the pour, use Ripup on the polygon

PCB Manufacturers

A great place to start is Ladyada's PCB cost calculator. It lets you compare different manufacturers by cost per square inch. Note: Prices from 2007.

• BatchPCB - Easy and cheap for small boards

BatchPCB order

In mid-December 2010 I tried BatchPCB, a SparkFun service, to do a sample board. The result was satisfactory. I ordered 3 pieces but got 6 back, bonus!

Timeline:

• 2010-12-16 07:43:39 Submitted, passed, paid and pending
• 2010-12-17 07:12:18 Processing
• 2011-01-03 10:39:40 Shipped
• 2011-01-13 14:39:00 Arrived

This was during the xmas holidays and shipped from US to Norway, a normal turnaround would probably be 3 weeks.

Board config

• FR4 - Stands for Flame Retardant and 4 means woven glass reinforced epoxy resin
  • Standard 1.5 mm thickness

Tools

• Eagle - Connect schematic and design PCB board layout
• Viewplot - View the files coming out of Eagle for verification