I’ve written about ambiguity a few times before, like this post about fiducials. But I’m not talking about the PCB today. I’m talking parts. More specifically, I’m talking about silkscreen markings for your parts on the PCB.
Diodes have a lot of opportunity for ambiguity, as you can read here. There are many ways to mark parts, but fewer ways to clearly mark them. Take a typical electrolytic capacitor. It can be through-hole, SMT metal can, tantalum, or a few other form factors. The capacitor manufacturers aren’t doing any of us any favors insofar as “markation” is concerned.
Check out the image at the right. Yikes! In all cases shown here, I’ve oriented positive on the left, which, according to IPC is pin 1. This is also the zero degree rotation for the centroid value. But, isn’t it nice of those component manufacturers to put the identification bar on the positive side for tantalum capacitors and on the negative side for metal can electrolytics? Not!
So, how should you mark this in the silkscreen on your PCB? For an electrolytic capacitor, the best approach is to mark the positive sided with a plus (+) sign. If you mark pin 1 with the number 1, it can easily be mistaken for the minus sign. If you mark the negative side with a minus sign, it can easily be mistaken for pin 1.
For a metal can capacitor, it is also acceptable to put the notched outline in silkscreen. We still recommend that you place the plus (+) sign on there too.
I’m just positive I put the negative right on the left
I’ve added a couple of fields in my centroid x/y output, one to indicate if a component is polarised, and the other with a small comment on what that polarisation is. For example:
C1 CAPMP3216N 23.40 56.50 90 “Polarised” “+ marked”
Plus I keep all markings consistant, so an electrolytic SMT capacitor would have the + marked in silkscreen on the board plus on the assembly diagram.