I’m finalizing my very first pcb (not without any uncertainties ^^) and a bit lost regarding the pos file and more specifically the rotation value. (I’m planning the send gerber, bom and pos files to jlcpcb to use their assembly service).
I saw that the pos file generated is describing the rotation of the footprint.
It means that the footprint must first be created in the correct orientation.
But how to know the default orientation (rotation 0)? I’m not finding that info in any datasheets.
I googled it and found some person that are saying that it depends on the tape, other says that it must follow some IPC standards with pin 1 top left, other says it depends on the manufacturer machines.
Personally I just tried to match what the datasheets were saying. I’ll explain the situation with 2 of the sensors I’m using: Bosch BME688 and Osram TSL2591.
so my footprint looks like:
(pin 1 is not top left, and I have no idea what the tape orientation is, the part is already in jlcpcb, never saw the tape)
but jlcpcb has a footprint (for easyEDA), so I suppose the footprint must no be matching the datasheet… I’m probably wrong in my way of creating footprints then…
So I created the footprint, in the same direction as jlcpcb did:
But I think it’s stupid to do it like that “because they did it like that”
I would like to understand.
- Is there a general rule to know what is the “default” orientation of a part ?
- If not, is there a way to find that info somewhere ?
- Is it correct to follow pin number from datasheet ? Like pin 8 top left in the case of bme688.
Short answer is the CPL file describes the position and orientation on the board according to a standard. But unfortunately the fabricator and tape supplier may be using a different standard. So that’s why JLCPCB provides you with a preview of the parts on the board as they will be assembled and you should modify your CPL file accordingly (usually rotate by 180°) until it is correct. If it’s uncorrected on the submission they will notify you and ask you to confirm their fix.
There is no general fix. People have even gone to the extent of storing the discrepancies on a part number basis in a local database when dealing with a fab.
You can read my experience with one job here, near the bottom: Costing assembly service for my PCB | Ken Yap | Hackaday.io
You can see the orientation of the osram on page 21 of the datasheet.
Anyway, the assemblers I work with do not trust too much that information. They want the orientation marked on the silkscreen or fab layer layers.
Thanks for those informations. I already tried with the part placement preview but unfortunately they don’t have any 3D model for that part so I can’t visualize how it will be.
That’s why I posted here because I’m really not sure how I can ensure it’s the correct orientation.
Looks like the 3D model is missing from their image. Contact them and ask them to check for you.
Good point, I missed that page apparently Thanks!
I already added a point for point 1 on the silkscreen, do you think it is what manufacturer will follow ? If there is a mismatch in the orientation given in the pos file and what the silkscreen indicate, which one would be used ?
Do you think that pin 1 from datasheet is something I can rely on ? Because if the pin 1 from my footprint is not the pin 1 for the manufacturer I’ll also have a problem ^^
I’m asking that for bme688 because I find it weird to have pin numbered starting with 8 going downward in the first column of pads then going upward in the second column.
I just sent them an email, will see what they suggest. Thank you
I’ve had that situation with JLCPCB (no some of the 3D models in their viewer). Asked the support chat what to do. They said they will fix it and send me a proof to accept (this is what I choose at placing the order) and they did. So don’t worry too much. Just make sure you see the components placement before they go ahead with manufacturing.
@greg_m has answered the question. I have no experience with jlcpcb
Well Bosch acknowledges in their datasheet that their numbering is not typical:
The pin numbering of BME688 is performed in the untypical clockwise direction when seen in top view and counter-clockwise when seen in bottom view.
So make sure your pad numbering is also untypical. If necessary modify an exisitng footprint to match, or your circuit will not work.
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