Merging modern software development with electrons and metal
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Posts from — March 2009

Making a Single Prototype PCB

I recommend Sierra Proto Express for making automation PCBs.  So far, I’ve had five different boards made at Sierra, including a larger 4 layer board, and have been very happy with the results.  Their pricing is competitive, the quality is excellent, and there are fewer limitations (e.g. drill sizes, non-plated holes) than many of their competitors.

However, when I am paying the bill, Sierra is not a good choice when I only want 1 board made or have a small board.  At Sierra, 1 PCB might cost about $85, and 3 PCBs about $110.  For small quantities, the cost does not vary much with board size (for example, a 6 sq in board costs the same as a 18 sq in board).

I haven’t found a great solution yet. I want all my boards to have top and bottom soldermasks and a top silkscreen, so “bare bones” boards (no soldermask or silkscreen) isn’t an option.  Besides, I don’t think the bare bones savings are large enough.

I do plan on making multiples of some personal boards (mainly some CANOpen-related boards), but for some boards, such as the FP-SMC-1, I only want one board (unless, of course, I can get multiple quality boards done for the same price as the cheapest price for one quality board).

The best solution so far is Batch PCB, which is run by SparkFun; the PCBs are made in China by Gold Phoenix.  Batch PCB charges $2.50 per square inch plus $10 for 2 layer PCBs; typically delivery is supposed to be 2-3 weeks (longer than I  like, but I can live with it for my personal boards).  The FP-SMC-1 PCB would cost about $40 at Batch ((4.32 * 2.83 * 2.50) + 10 = 40.56).

Another choice would be to use Gold Phoenix directly and panelize (combine multiple boards into one order).  Gold Phoenix only makes sense for orders of 155 sq in or more.  They will panelize for an extra fee ($30 IIRC).  So I am going to finish up some other PCBs first, then decide how to get all the boards made.  Given the boards’ design specifics, it may not make sense to panelize, but at least I want to check and see if this is a better option.

March 18, 2009   No Comments

FP-SMC-1 PCB Layout Is Done

3D Model Top View

3D Model Top View

It’s designed!  After taking way too much time, I have finally finished the layout for my FP-SMC-1 board.  Here’s the proof: above is a 3-D model of the board; below are pictures of the layout and the board model from the bottom.

PCB Layout

PCB Layout

PCB Bottom View

PCB Bottom View

The board hasn’t been built yet, because I wanted to model it first, and I haven’t found a good place to get one-off prototype PCBs made.

Creating the board model was challenging, but worth it, because it gives me more confidence that my board layout is correct.  I use the 3-D model to check:

  1. That my PCB footprints are correct.  Look at the picture of the board bottom, and notice how all the pins line up with the holes.  (This check relies on correct 3-D models from the manufacturer.  If you create 3-D models yourself, it’s possible for you to make a mistake in the model, but the manufacturer’s models should be correct).
  2. That my board dimensions are correct and the PCB will fit into the holder.
  3. That my component layout makes sense.  Look at the far left of the board top view picture, and notice that there is clearance between the connectors and the little plastic tabs on the PCB holder that stick out into the board area.

I will go over making the model in detail later, but what I did was roughly:

  1. In Eagle PCB board layout, turn on only the dimension, via, and pad layers (layers 17, 18, and 20)
  2. In Eagle PCB board layout, use a ULP to create DXF of the pads, vias, and dimensions.
    1. Eagle includes DXF.ULP to create DXFs.  However, this file does not produce DXFs that can be used to extrude a through-hole board.  To get DXFs that CoCreate can extrude, I had to modify DXF.ULP and then delete and re-create the board outline using DoubleCAD.
  3. Import the DXF file into CoCreate and extrude it to 0.062″.
  4. Import models of all the components (fortuneately, all the parts have STEP models available from the manufacturer).
  5. Assemble (using mates) all the components onto the PCB
  6. Assemble the PCB holder
  7. Mate the PCB to the PCB holder.

It sounds so easy, but mechanical CAD software has a high learning curve, just like PCB software.  However, if you’re a software guy, don’t be scared — I’m primarily a software guy, and if I can figure out how to make a PCB and then model it, then you can too.

I was originally planning on doing a series of blog posts on the FP-SMC-1 PCB covering just the automation-related aspects of the project, and not covering the details of using the tools (such as Eagle PCB).  However, after I looked at various Eagle PCB tutorials, I decided that none of them explained Eagle the way I think it should be explained.  I did not find any tutorials on making mechanical 3-D models.

So I have decided to write up a lengthy tutorial on how to design and make the FP-SMC-1 board.  The tutorial will be on my Trac site, since I think Trac is better suited for a lengthy tutorial, but I will blog here about my progess (hint: don’t expect the tutorial to be done quickly).

March 18, 2009   3 Comments