using pcb component hole as fiducial?

[Jim Freeman]

Has anyone had experience using a component hole as a fiducial? Do you have the top vision pipeline for finding it? Also how do you  retrieve the x y center of the found hole?

[Marek T.]

Bad experience. Holes are drilled not always well "synchronized" with the pads, so basing on it is not good idea. Better use pads of tht pins not the holes.
But no problem to find it if you insist..., use pipeline of nozzle calibration included as default by Michael.

[John Plocher]

There are two related problems with drilled holes based on how the board is placed into the drilling machine:

1) the per-board registration on the holes might not be reliably consistent between boards, and

2) the relationship between the holes and the pads is not predictable - ie, the drill hole center line cross might not be the same as the pad's center line cross.

I've had boards where the drill holes were off center "north/west" on the top left of a board and off center "south/west" on the bottom right - the board was slightly skewed clockwise and offset on the drill machine.  Other boards differed within a batch, as they presumably were cut out of larger boards drilled on different machines/runs - the relationship between board edges and drill holes is not always as precise as one could hope.  As for silk screen on some, I've seen graffiti on a warehouse with better fidelity!

All these boil down to poor registration tolerances in the low cost fast as possible board fab lines we use :-)    To be fair, if you are placing 0805's or larger, the holes-for-registration may be good enough; my experience says you will end up being frustrated when you move to smaller components and higher pitch devices.

On the other hand, the fab process for pads and etching on a board is very precise, which is why Fids/Marks are often specified as copper circles with concentric copper rings, with no solder mask or silk or drill activity to muck them up.  

  -John

On Sun, Mar 31, 2019 at 3:31 AM fish <lefish...@gmail.com> wrote:

Last batch it worked for me with enig finished pcb's and a dark black carton below pcb's to get needed contrast.

[Michael Anton]

This is also why I could never understand why people make stencil printers that use pins to align holes punched into the stencil, with holes on the PCB.  The registration of hole and copper is just not good enough, especially for fine pitch parts.

[Brynn Rogers]

talking with a PnP house, their stencil printers align optically using the same fiducials as the PnP machines.   So the pro's don't make the mistake of aligning to a drilled hole...

[Michael Anton]

Yeah, I knew the pros do optical alignment, but I've seen amateur solutions that use holes, which I could never understand (other than that then no adjusters are required to move the stencil or board around).  I align mine manually to the copper lands, which works fine down to 0.5mm pitch or so.  Probably having a camera above the stencil would make this easier, as it is important to be looking straight down, but so far I just use my eyes.

[SMdude]

I place fiducials on the panel boarder in 3 corners and then get my stencils made with the fiducials cut out on the boarder.

Then when I mount in the printer and align them everything in between is lined up too.

It is always a good idea to sprinkle fiducials about the board. I have them as a base schematic, so anything I design from new already has some fids.

Probably wouldn't matter so much to use holes to align if you were doing 1206 and SO pitch ic's.

[Brynn Rogers]

Actually, if you think about it the only place to register the stencils is it's holes- the only layer it has are holes and they are on the paste layer, which your fiducial generally does not have paste on the board. So they must be doing something tricky to align the paste layer on the stencil to something (copper layer?) on the board, because the fiducials do not have any feature on the paste layer.