[pcb layout] square pad conventions

[gaussmarkov]

chris/stobiepole recently pointed out to me that the capacitors in eagle libraries are marked in reverse of a convention:

the square pad is for the positive lead of a polarized capacitor

this is what tonepad and geofex both use.  ggg and amz-fx do not seem to pay attention to this convention.  other useful physical characteristics to note are that the negative lead is usually the one marked on an electrolytic and also usually has the shorter lead.  so that longer lead of an electrolytic capacitor goes into the hole with the square pad.

i am going to fix this in all of my eagle libraries and, eventually, my layouts.  and now i am curious about other square pad conventions that it is helpful to know.  two conventions seem to be that the square pad denotes

• pin 1 or
• the cathode

the pin 1 convention seems natural and that is how i marked my IC's pads without knowing that there was a convention.  but you don't see that very often for pots and it seems like a good idea, when the pads for a pot are adjacent.

the cathode convention implies that the shorter lead and/or the banded end of a diode gets the square hole.  this does not seem to be as widely used in stompbox land as for capacitor pads.  is this not such a widely accepted convention?  with respect to lead length and polarity, it seems to be the opposite of the polarized cap rule.

if there is widespread agreement on these things, this would be a good FAQ/Wiki item.  and we could get our layouts synched up on this matter.

cheers, paul

[R.G.]

All of my layouts have the square pad as pin 1 and the positive lead on caps. It's the negative lead on LEDs (Ack! what an inconsistency) and I don't think I use it anywhere else except for places where a square pad is called for by the part having a slot or rectangular lead, that kind of thing. I don't mark polarity with pads on diodes at all. Both pads of a diode are round.

The problem you're facing is that even if you get one corner of the world all the same, the rest of the world is going to go on using triangular pads to mark pin 2, that kind of thing. A huge effort for consistency here will only frustrate you when you read some other layouts.

[gaussmarkov]

All of my layouts have the square pad as pin 1 and the positive lead on caps. It's the negative lead on LEDs (Ack! what an inconsistency) and I don't think I use it anywhere else except for places where a square pad is called for by the part having a slot or rectangular lead, that kind of thing. I don't mark polarity with pads on diodes at all. Both pads of a diode are round.

The problem you're facing is that even if you get one corner of the world all the same, the rest of the world is going to go on using triangular pads to mark pin 2, that kind of thing. A huge effort for consistency here will only frustrate you when you read some other layouts.

thanks, R.G.  no point in fighting gravity.     so this is good to know.