Screaming Circuits: Ambiguity


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?

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A registration wall and an ambiguous pin 1 indicator - a double whammy

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