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Square pads standard?

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BertX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BertX Quote  Post ReplyReply Direct Link To This Post Topic: Square pads standard?
    Posted: 18 Aug 2022 at 1:40am
I have been looking for this for a while now. 

Opinions seem to be divided on this. 

But does the IPC say anything on the use of square pads? 

I was always taught at school it denotes the positive lead or pin 1. 

So that would mean the anode of a diode or capacitor, but it seems most use it to indicate the cathode. 

So is there a standard for this?

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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2022 at 8:18am
IPC has no guidelines for using Square Pad Shape to indicate Pin 1 for through-hole footprints. 

It started back in the 1970's with Bishop Graphic decals for DIP14's for hand taping and just migrated into CAD in the 1980's. 50 years ago silkscreen was rare so we used copper shapes to indicate Pin 1.
 
It's a defacto standard that is totally up to the end user to define how they want to indicate the location of Pin 1. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BertX Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2022 at 10:24am
Thank you. If I may ask, does the standard say how to mark anodes/cathodes or is this also up to the user?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2022 at 11:04am
Anode/Cathode polarity markings are defined by each PCB designer in cooperation with their assembly shops recommendations. 

Assembly shops should have their own DFA (Design for Assembly) handbook, but it may not contain polarity marking guidelines. 

There are some recommendations coming from IPC-7352 which should be released by the end of the year. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote feynman Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2022 at 12:19pm
Diodes are a real pain in the ass for assemblers. Diode orientation should be made ridiculously clear:
- Draw a diode symbol or a triangle on the silkscreen (if possible).
- Draw a diode symbol on your assembly drawing.
- Name the pins "A" and "C" instead of "+", "-", "1" or "2".

If something can be remotely considered "industry standard" with diodes than it's the cathode being the "special" pin that is marked one way or the other.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2022 at 12:26pm
For Diodes, the package polarity is normally on the Cathode. 

For LEDs, the package polarity is normally on the Anode. 

Footprint Expert allows the user to manually insert polarity markings that stay save in the FPX library. 

Here are some of the shapes and the user defines the size, location, shape, rotation and layer. 


 
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