• Getting Custom PCBs for Prototyping

    Monday, December 2, 2013

    12/2/2013— Jia Huang

    When we first started making hardware, we knew we needed custom printed circuit boards, but didn’t exactly know how to go about getting them. I thought I’d just share some of the tools and resources we use for getting PCBs made.

    PCB files

    Most manufacturers will ask for Gerber files. These are the files that specify how each layer of the PCB should be made. These layers are then stacked together to form the PCB. From top to bottom, Gerbers are usually split into the following files:

    • Top silk - the (usually) white informational text on PCBs
    • Top paste - this is used if you have a stencil for the PCB. The top paste is laser cut out of a stencil and then put over the PCB and solder paste is put on top. Components are placed on top of the solder. The paste layer is useful if you have a bunch to make and don’t want to apply all the solder yourself.
    • Top soldermask - this goes over the copper layers
    • Top copper - the top signal layer
    • Inner layers (if you have any) - inner layers are for ground, power, or extra signal layers.
    • Bottom copper - bottom signal layer
    • Bottom soldermask - same as the top soldermask
    • Bottom paste - same as the top paste
    • Bottom silk - same as the top silk

    Additionally, if you have any holes, you’ll need to have a drill file.

    The files for the accelerometer look like this:

    Note that this doesn’t include the drill file even though the accelerometer has one. The bottom paste also isn’t included since we don’t have any components that need to be mounted from the bottom.

    We use Diptrace for schematic & pcb layout. Then I use the MCN Gerber viewer to double check files before sending them out. As far as I know, MCN is the only native OSX gerber viewer. MCN has some issues displaying drill files so I also use gerbv to double check. Eric uses Graphicode on Windows.

    Manufacturing

    We spent a while looking at different ways to source the custom printed circuit boards we needed. Different fabrication houses tend to specialize in different things, and vary in:

    • lead time (a few days to a few weeks)
    • number of boards needed
    • quality

    The price for the same board differs depending on the manufacturer and their setup. Depending on what we need to do (prototype, test run, side project), we’ll use different manufacturers. Since most of our modules go through multiple revisions, it’s not uncommon for us to use different fab houses at different stages of development for the same module.

    Here are some of the PCB manufacturers we’ve used so far:

    AP Circuits

    Lead time: 3 days

    Number of boards: 2 or 4 boards. While you can order a lot more, we only do 2-4. Anything higher and we tend to use Seeed or Silver Circuits.

    Quality: Electrical testing isn’t included, and sometimes we’ve had soldermasks shift. But for the turnaround time and price, it’s a pretty good trade off.

    If you want to forgo the silkscreen and the soldermask, AP circuits can make the boards in 1 day. AP Circuits also has very good engineering support. They’ve caught more than a few of my mistakes.

    Seeed Studio

    Lead time: 2.5 weeks

    Number of boards: 10+

    Quality: There’s free electrical testing included, and Seeed usually makes extra boards in case some fail.

    Seeed also allows different colored PCBs at low volumes as well as great pricing for 4 layer boards. In addition, they also have a stencil service which is useful if your boards have a lot of pads.

    Seeed also has an assembly service for 100+ units.

    Silver Circuits

    Lead time: 1.5 weeks

    Number of boards: 4+

    Quality: There’s an option to include electrical testing, but I’ve never had any boards that were defective.

    Silver Circuits also offers 1oz and 2oz copper layers at similar costs which is useful for our RFID module.

    OSH Park

    Lead time: 3 weeks

    Number of boards: 3+. Must be a multiple of 3.

    Quality: Never have had any issues with quality.

    OSH park has automated design rule checks that it runs through as soon as you submit your files. Additionally it takes Eagle files directly as well as Gerbers.

    Gold Phoenix PCB

    Disclaimer: I haven’t used Gold Phoenix for Tessel, but I’ve used them before for other boards so I thought I might as well include them in this list.

    Lead time: 2-3 weeks

    Number of boards: 20+. They can do any number, but it’s probably not worth it unless you’re doing at least 20 boards.

    Quality: Optional electrical testing, though I’ve never had any issues with getting working boards.

    Like Seeed, Gold Phoenix also has assembly services though I’ve never used them for that.

    -Jia
    jia@technical.io

    #jia huang #tessel #technical machine #hardware #pcb #manufacturing #gerber files #seeed #seeedstudio #gold phoenix pcb #osh park #ap circuits #silver circuit #Electrical Engineering

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