Pick and place file direction Ambiguity

[Mahsheed]

Hi,
I am curious how the AD determines the rotation angle ( for directional components i.e. LEDs and diodes). I followed the videos on Youtube, but my concern is that we draw schematic symbols and marks on pcb footprints ( on any layer) for our ease. I mean which pin of LED is to be placed on Cathode, how AD takes this information? As we drew schematic symbol according to our own choice and then mapped the pcb footprint according to this. But how come our schematic symbol actually corresponds to pick and place file rotation?
Secondly I was wondering, which origin is taken as reference for PCBA? as X and Y coordinates are mapped according to our origin ( whose information is not provided in Pick and Place file). Thanks in advance.

[Paul van Avesaath]

to you first question.. you schematic symbols only connect to you pcb through their unique ID, this has nothing to do with rotation what so ever.. you can define the pin name to be whatever you want and as long as that pin name is corresponding to the name of your footprint pad it wil match them.. but i think what you want to ask is how a pick and place manchine knows the orientation of the comopnent it gets in its feeder.. that is done by the operator..and according to your footprint he will match it up accordingly..

when you make markings on you pcb on the toplayer or top overlay the AOI (automated opticla inspection) can check these as wel as the operater. so it not just for our ease..

as mentioned the origin of a footprint is user depended so for 99% of the time you will choose the center of the footprint.. for connectors you can go with pin1 for instance..
again here the operator of the SMT pick and place matches up your footprint origin with the design.. this will be checked always before the machine is set loose on your pcb.


in 99% of the usecases the operator will flag an issue before it becomes a problem..and in most cases they fix it themselves.

[Mahsheed]

Thank you very much Paul van Avesaath. Your answer is so satisfying. Also, does this implies the (0,0) coordinates of the pcb does not matter. right?

[robertferanec]

There is a standard what specifies how footprints should be normally oriented (google for "ipc diode footprint rotation"), check for example this: https://ohm.bu.edu/~pbohn/__Engineer...0Libraries.pdf

Of course, a good assembly house still should double check everything - when they are preparing for pick and place, they usually check the orientation with the marking on assembly drawing and PCB.

PS: file from the link above is attached

[Mahsheed]

There is a standard what specifies how footprints should be normally oriented

Does that employ that Altium will give rotation values of diodes according to the pin defined as 1 ( considering pin 1 cathode automatically )

[robertferanec]

I believe, if you draw the footprints as they are shown in the document, the rotation is 0 (so for example for rotation 0 the cathode is the left, for rotation 180 cathode would be on the right - this rotation has nothing to do with pin name or pin number).

[Mahsheed]

Yes, But how I map the footprint on pcb layout changes the rotation. For example, the attached picture shows rotation of components in my pick and place file. Now, does this rotation corresponds to standards shown in the file you mentioned before?

[robertferanec]

does this rotation corresponds to standards shown in the file you mentioned before

I believe, the rotation is relative to your footprint

[Mahsheed]

I believe, the rotation is relative to your footprint

I am totally confused. I am thinking of two directions now.

1) It is totally dependent on the operator to set the ICs according to Top overlay directions.
2) Pins of the footprints should be designed according to the IPC file, so that it gives a reference point to the Pick and Place machine.

Which one is correct?

[robertferanec]

Both are correct Standards can make the life of the operator easier but not everyone follows standards, so operator of a pick and place machine should still double check rotation based on manufacturing documentation. If you provide documentation (e.g. assembly drawing), the manufacturer should follow it .. that is how the board should be assembled and if board is not assembled according to documentation, it is the assembly house fault.

I believe the whole assembly process is much more complex - e.g. they need to be sure the rails and trails are placed in correct direction (so components have correct rotation, etc.) ... in some rails the components are place one way, in other the other way ... I don't think, that machine itself is going to somehow figure out correct rotation of a component without confirmation be the operator.

[Mahsheed]

Perfect. Thank you so much for such a satisfying answer. One last query, Usually the PCBA manufacturers ask for pick and place file and BOM list only. Then should I assume that the operator will follow the top overlay and bottom overlay for directions ( since they do not ask for assembly drawings).

[robertferanec]

If they do not need assembly drawing, they will be probably looking on the marks on PCB. But I have seen so many mistakes when components were fitted in wrong rotation when assembly drawing was not supplied (e.g. very often 4 pin crystals were fitted wrong way or headers were fitted wrong way), that I always provide assembly drawing with clear pin 1 marking - that helps a lot.

[Mahsheed]

Ok thank you very much robertferanec. Your Answers really helped me. Thanks a lot.