Ambiguity

P3281577 smIt's pretty important to have unambiguous polarity markings and pin one markings printed on your PCB. In theory, for SMT parts, it really shouldn't matter; the centroid would take care of the placement orientation. But, you may have noticed that it's not a perfect world. It took me a while to figure that out, but I have finally concluded such.

It's not uncommon for the CAD library part to have the wrong zero degree rotation orientation The IPC specified location for pin one orientation Quad and BGA for square chips like QFPs, QFNs and BGAs is either the upper left or middle top. Check out our Centroid guide for more detail. If it's wrong in CAD, the centroid will be wrong as will everything downstream. That's why markings on the board are still important.

What do you do if your part is ambiguous though? This particular chip has three markings that could be interpreted as pin one indicators. At first glance, I'd assume it's the dot in the center top. It would match with the text. However, there is a white dot in the lower left that could be pin one indicator which would mean, in this case, the CAD library component had the incorrect zero rotation orientation.

Datasheets aren't always easy to find. This one is behind a registration wall. If you have a part like this, it's really helpful if you include some documentation (in electronic form) clarifying. I found the datasheet for this particular part and was able to confirm that it is correct as placed with pin one down in the lower left (90 degrees).

Duane Benson
Via via in the board,
what's the top on my PCB?

Connectors Kill

Lot's of types of components can cause footprint woes. QFNs have their center pad issues. BGAs have escape via issues. But the most common footprint issues seem to be with connectors. At least with chips Connector footprint 2smand discrete silicon and passive components most manufacturers pretty much follow IPC standard footprints. Sometimes they'll create new ones for smaller parts, but generally they still stay reasonably close to in line.

Connector footprint 1smConnectors are another story though. I'm not sure any manufacturer follows anything close to a standard. This pair of ethernet jacks is a good example. Often the actual pin layout will match, but the mounting will vary widely. I've seen it on ethernet, mini-USB, micro-USB and even the old, old RS232 connector.

It gets more frustrating when they're almost the same. We see that a lot; the layout will almost, but not quite match a footprint in the library. The bottom line is never take a connector footprint for granted. Always double check before getting your boards fabbed.

Duane Benson
Carburetors man. That's what life is all about.

More CAD footprint woes

AT this point, I really shouldn't call them "woes." More like business as usual. I'm talking about the need to make custom footprints, or at leas modify footprints. Back in the old days, the only thing needed to make footprints was some copper pds, maybe plated through, maybe not. It was pretty rare to even need to make a custom footprint. Other than the occasional odd switch or relay, it was all done.

I really need to just get over it though. On the one hand, it seems like none-productive time; like I should be able to get right to schematicing and layouting. On the other hand, It's so common, I just need to see it as no different than any other routing task.

Starting at the top of my BOM, I have:

  • An MCU in QFN format - I modified a symbol and added a custom paste layer to the copper land
  • Two SOIC Mosfet drivers - I modified the symbol on an existing footprint
  • Some Mosfets in a PowerQFN package - Made a complete custom footprint
  • A Mosfet in SOT-23 package - Who hoo! I found a workable part in the library
  • Some Power Schottky diodes - custom copper land

Custom footprints

I have another Schottky, some TVS diodes, LEDs and a bunch of passives that came straight out of the library. It's certainly not everything that needs footprint work, but with so many variations of the more complex parts these days, it safe to assume that any SMT project will require a fair amount of library work. It's just the way it is.

Duane Benson
It's a pain but at least it's not as bad as 11811 has it

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