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Designing PCBs

Page history last edited by Kris 11 years, 6 months ago

Desinging PCBs

The good news is that it’s entirely possible to make PCBs using only free software!

Designing your board in Eagle:

This guide assumes you have already designed up your schematic and layed out a board in Eagle. For an introduction on how to do this check out the SparkFun eagle tutorials listed at the bottom of the page.

There are extensive amounts of free resources available to make using Eagle easier, such as parts libraries, user code and custom design rule checks. Check out some of the links listed in the appendix.

Generating Code for the CNC mill

The CNC mill requires G-code which contains the toolpath of all operations to be done in manufacturing the board. The following works through the steps of creating the g-code.

Installing PCB-Gcode

You can download it directly from the manufacturer’s website here, but for best results grab the ULP.zip file attached below to get PCB-gcode with sane settings based on the space’s mill. PCB-Gcode is written in User Language Program, a C like language for creating custom addons to the software.

Note: It’s a good idea to put addon ULP files in a separate folder and set up eagle to point to that directory too. That way if you upgrade eagle you don’t have to worry about losing your settings.

Add the routing path to the edge of the board

Eagle knows what the PCB traces and hole sizes of the components are, however you need to define the size of the board and tell eagle which parts should be routed free at the end of the operations.

Use the ‘wire’ tool and make sure it’s drawing on layer ’46 – Milling’, with a width of 1mm (or whatever routing bit you choose)
Draw the outline of the board, taking care to remember the tool width
Feel free to add chamfers and remove 90* corners from the board, the mill can cut most shapes easily

Ready to Generate G-code

Generate G-code

Use ‘File-->Run’ and then select ‘pcb-gcode-setup.ulp’
Look at the settings and confirm that you are satisfied with the parameters (if you’ve downloaded the space’s ULP.zip then this should be fine)
Click ‘accept and make my board’

This generates the following files for both the bottom and top layer
The etch files contain the toolpath for the PCB traces. The drill files are for holes in the board, and the mill files are for routing the board free after completion. Be careful not to confuse the bottom and top layers.

You can now take the G-code files and run them on the CNC as described in Cutting PCBs.

Visualising code (optional)

As an optional intermediate step, you can use software (such as NC plot) to visualize the toolpaths before loading them into Mach3 and cutting.

The toolpaths for a milled board

Tips for Milled boards

Make sure the traces are wide enough. 16 thou or larger is a good size. (Note: confusingly a ‘mil’ is not anything metric. It’s 1/1000th of an inch! Just call it a ‘thou’ to avoid confusion)
Avoid 90* angles in your traces where possible. Put nice 45* points in to avoid the copper lifting.
Make sure vias are reasonably large. 0.5-0.7mm or larger with big pads.

Next steps, having a PCB manufactured professionally

Most fab houses accept ‘Gerber’ files for board manufacture (http://en.wikipedia.org/wiki/Gerber_Format).

Steps to do:

Check out these tutorials on how to do it:

http://www.sparkfun.com/tutorials/115 Better PCBs in eagle
http://www.sparkfun.com/tutorials/111 Common mistakes and tricks
http://www.alternatezone.com/electronics/files/PCBDesignTutorialRevA.pdf Absolutely mandatory guide by David Jones on PCB manufacture
http://hackaday.com/2009/01/15/how-to-prepare-your-eagle-designs-for-manufacture/

Make sure the Electrical Rule Check (ERC) and Design Rule Check (DRC) pass.
Check your board is working (make a prototype and test it exhaustively)
Make the gerber files (instructions below)
Check the gerber files with a 3rd party gerber viewer like ViewPlot (http://www.viewplot.com/). This way you can see exactly what the fab house will see, and if it matches your vision.
Send them off

Resources for PCB manufacture

Eagle Tutorials

Sparkfun’s tutorials are first rate:

Getting started with Eagle and capturing a schematic:

http://www.sparkfun.com/tutorials/108

The basics of laying out a PCB with Eagle:

http://www.sparkfun.com/tutorials/109

How to create a custom footprint and part in Eagle:

http://www.sparkfun.com/tutorials/110

‘Better PCBs in Eagle’: http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=115

A great guide for the beginner is here:

Get your PCB's made:

http://dangerousprototypes.com/docs/Get_your_PCBs_made

Eagle Resources & Addons

Eagle Cad itself (used in freeware mode allows up to 100x80mm PCBs)

http://www.cadsoft.de/download.htm

PCB g-code, which allows you to make CNC capable g-code from your eagle files:

http://pcbgcode.org/read.php?12,361

SparkFun Part Libary:

http://www.opencircuits.com/SFE_Footprint_Library_Eagle

SparkFun Eagle keyboard shortcuts (put them in the SCR directory):

http://www.opencircuits.com/SFE_Eagle_Shortcuts

LadyAda’s Eagle Part Library:

http://www.ladyada.net/library/pcb/eaglelibrary.html

LadyAda has an arduino shield dummy, very useful to make a quick shield

http://ladyada.net/make/pshield/download.html

These guys have a good library:

http://www.microbuilder.eu/home.aspx

Expert system for choosing parts:

http://www.ladyada.net/wiki/partselector

If you have some EAGLE files with components you want to use in your own designs, you can extract the components into a library by running the user script called ‘exp-project-lbr.ulp’.