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Verifying PCB Footprints

A great way to waste time and money on Printed Circuit Boards (PCBs) is to create them with the wrong footprints.  It’s worth spending the time to verify before ordering.

The footprint is what a part looks like on the PCB: the holes, the pads, the silkscreen, etc.  PCB design software typically comes with footprint libraries, but some people (including myself and the guys at Sparkfun) prefer to do their own.

It’s easy to make a mistake when creating your own footprints.  You should still check all footprints, because the creator could have made a mistake or it could have been designed for a different part (for example, not all DB9F right angle through hole connectors have the same footprint).

There are two ways of verifying a part’s PCB footprint:

Model the PCB using a MCAD (mechanical CAD) program

  1. You have to create a PCB first that uses the footprint.
  2. You need a 3D model of the part, preferably from the manufacturer.
  3. You need a suitable MCAD program.  Links to some free personal use possibilities (such as CoCreate PE, PowerSHAPE-e, and Medusa4 Personal) are here.
  4. I discussed how I modeled the FP-SMC-1 here.

Mock the PCB using a life size printout

  1. You have to have the parts you are going to verify.  I like to have the parts first, anyway, since I like to see what the look like and how they work before I use them in a design.
  2. You have to create a PCB first that uses the footprint.
  3. You then print the footprint at life size (1:1 scale), with the pads, holes, and (optional) silkscreen showing.  You should check that the printout really is life size (printers aren’t perfect).
  4. Cut out the PCB, and then mount the parts onto the paper.  I find it’s easier to punch through hole parts through the paper when it’s backed by something like foam.
  5. Check the footprint with the parts mounted and removed: holes in right places, pads line up, silkscreen is visible, etc.
Parts inserted into paper PCB printout

Parts inserted into paper PCB printout

Pictured above is a printout of the FP-SMC-1 PCB layout with the parts inserted.  You can see that the text is readable, and silkscreen outlines appear correct, and such.

Paper PCB printout after parts inserted

Paper PCB printout after parts inserted

Pictured above is a view of the FP-SMC-1 layout printout after I removed the parts.  You can see, especially in the larger version, that all the holes (made by the connectors’ pins) line up with the layout’s holes.  (Click on the picture to see the full size version).

In some ways, it’s easier to use the second method.  MCAD programs are fun, but they do have a substantial learning curve; I’ve found mating parts is often very challenging.  But it can also be challenging to poke parts through paper accurately, and I haven’t tried the second method with surface mount parts yet.

4 comments

1 Alex { 01.05.10 at 10:46 pm }

It took me an expensive mistake before I thought to do the printout trick, lol!

I have yet to try 3D cad for PCB design though. I use it a lot for mechanical assembly, and then 2D cad for things like electrical enclosure layout fit testing. Maybe my next project.

2 Tony { 01.06.10 at 9:42 pm }

I’ve done the 3D CAD method for a while — at work, it’s simple: I just give our ME a DXF of the layout, and all the part models, and he puts it all together, in SolidWorks, much faster than I could. But I’m enjoying learning how to do it by myself.

We do a rough model of the components in 3D to make sure everything should fit, then let our assembly tech put it all together — he’s good at figuring out the best layout. Then we re-do our CAD model to match his arrangement. And it’s always good to leave a little extra space, for extra requirements that customers want added…

3 Rob LaMoreaux { 02.23.10 at 1:11 pm }

I do all that inside Altium Design. I add the Step model to the footprint, or create the body using their simple 3d bodies and then I can render the board and be sure everything fits. I can also use Alibre of such to create new step models.

4 Tony { 02.23.10 at 5:28 pm }

That’s nice to know.

But Altium is about $4,000 for commercial users, which is way too much for my budget (or my company’s budget). I’m pretty sure some other PCB EDA tools have similar 3D capabilities, but I haven’t had time to do the required research — and unfortunately won’t for quite a while.

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