Screaming Circuits: May 2008

Footprint Dimension Mismatch

Here's another simple case of part to pad dimensional mismatch. The part is an asynchronous SRAM, but that's not important. What's important is this is another case of "close, but no cigar."


The part has an 0.8mm center to center lead spacing. That's a little more than ten pie mils, or 31.496063 mils. That would be okay, except that the footprint used has 30 mil center to center spacing. That might work on a smaller part, like a 16 lead jobby, but for a 44 pin component, it's just not a good fit. Over the run of the full length of the side of the part, you end up with 22 mils misalignment. That's almost a whole lead space off.

In this case, we made it work, but - and you've heard this from me before - in a production environment, this would be bad. All of the tolerance is used up in the misalignment so there really isn't room for normal manufacturing gives and takes. You must find or make a part library component that exactly matches the part.

Duane Benson
The nearer your dimension, the more you're Slip slidin' away

Memorial Day Closure

Memorial_day100 We will be closed on Monday, May 26th in observance of Memorial Day. This day will not count toward your turn time. That means that if we receive your kit for a 24 hour turn job in at 8:00am on Friday, your job will ship out on Tuesday, the 27th. Thank you for your patience.

For future reference, we will also be closed on the following holidays later this year:

  • Independence day, Friday, July 4th
  • Labor Day, Monday, September 1
  • Thanksgiving, Thursday and Friday, November 27 and 28th
  • Christmas, Thursday and Friday, December 25th & 26th

Happy holidays!

Halogen Free PCB

80pxdibromine2ddimensions Halogen free PCBs still aren't that common and all of the bugs aren't worked out of the systems yet, but they are starting to show up here and there. I suspect that like RoHS a few years back, we will be seeing more and more as time goes by.

If you do feel the need to try out a Halogen free PCB, our experience here suggests that they are more sensitive to moisture than standard FR-4 and are a bit more brittle. That might be an issue when de-panelizing boards or when putting a lot of mechanical stress on a pcb.

An article in "Future Circuits International" suggests that some of the replacement flame retardant chemicals may have other environmental risks too, but I guess that seems to be the case with just about everything we pull up these days to replace something that is a known hazard.

Anybody with some good practical experiance here, feel free to chime in.

Duane Benson
No matter where you go, there you are

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