Centroid file part rotation

[mjt]

Hi all,

When sending a PCB out for pick-and-place assembly, assemblers often ask for a centroid file, listing the X-Y positions of surface mount parts. It also includes a rotation.

As far as I can tell, with my CAD software, the rotation it puts in the centroid file is relative to how the footprint was drawn. So a diode footprint with the cathode on the left and not rotated when placed, and a footprint with the cathode on the right then rotated 180° during board layout, would have different rotations in the centroid file. Parts like polarised capacitors are the same way.

Obviously, an assembly company *could* be comparing visually to the silkscreen, but that sounds like an error-prone procedure.

So how do PCB assembly companies know which way round to put diodes? Is there a standard somewhere dictating which way around I should be drawing my footprints?

Thanks!

[Styno]

You’re looking for IPC landpattern standard class A (or B) standard (IPC-7351). See e.g. https://blogs.mentor.com/tom-hausherr/blog/tag/ipc-standards/

[mjt]

Thanks Styno!

Now to figure out how to get a copy of IPC-7351 in a sensible format....

[SMTech]

In the ideal world yes, the package as defined in the designers CAD and as defined on the pick and place will both follow the IPC standard. However it is really not something you can rely on, on our Essemtec machine all the predefined "SO" packages are rotated 270 degrees from the IPC spec and that means so are all the packages I have subsequently created using them as starting points. Equally I sometimes even encounter chip type devices that are defined 90 degrees from the spec and sometimes even not using the component centres. So I do in fact have to use the assembly layer/silkscreen to verify what the orientation should actually be. However it is only necessary to check rotation for each specified package, not every device, then the software can create a rule that knows that every other device in that package will need rotating X degrees to match the machines definition. On Essemtec machines you typically create a rule file for every customer/datasource  and that way the more often you encounter data from the same client the less work you need to do checking all the rotations.

[Styno]

Thanks SMTech for bringing some real world experience in this discussion. My machines will hopefully arrive this week. I'm curious how proprietary the machine library will be.

@mjt: for most packages you will be able to find IPC definitions using Google. The blog I linked already lists a whole bunch, e.g


And in this document (PDF).