Solder paste opening for fiducials on PCB - Page 1
Forum section: electronics / general

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: October 15, 2019, 12:06:50 pm
content: Friends,What is your opinion on having paste opening for the fiducials on the PCB. My assembly house insists on having it open so that the stencil can be aligned over the PCB with ease.But most of the fiducial footprints available on KiCad, Altium etc have the paste layer closed on the fiducials.What is your take on this?

author: SMTech
author_rank: Frequent Contributor
timestamp: October 15, 2019, 12:30:05 pm
content: Well a printed fiducial is without doubt going to be less well defined for the Pick&Place than your proper shiny PCB fiducial so this seems like an odd insistence. If the printer is purely based on a lookdown camera is can just as easily align the board using any other aperture on the stencil and matching that to a PCB feature.To my knowledge it is more common for printers to have a lookup & lookdown camera view and so fiducial marks are often 1/2 etched into the stencil underside. I know Speedprint use look down cameras and they categorically can align without a fiducial at all if needs be.

author: Fire Doger
author_rank: Regular Contributor
timestamp: October 15, 2019, 12:46:07 pm
content: Having reference on the far corners of the panel will definitely help stencil alignment.It's not your problem, maybe their pnp doesn't have any problem reading them after or they clean the paste from them.Do they ask for every fiducial (included local ones) or just at the corners?

author: voltsandjolts
author_rank: Unknown
timestamp: October 15, 2019, 01:51:27 pm
content: I think you have misunderstood their request.They will likely be asking for an opening in the soldermask (for solder resist coating), not the pastemask (for solder application).

author: jmelson
author_rank: Super Contributor
timestamp: October 15, 2019, 07:53:24 pm
content: Quote from: EEEnthusiast on October 15, 2019, 12:06:50 pmThe only way I can see this working is if their stencil printer can guarantee no paste goes on the fiducial, or they have a kapton tape or something covering that aperture.Then, the alignment could see through the tape, but paste will not get applied to the fiducial.I know the fiducial sensor on my Philips CSM84 requires a pretty shiny fiducial.JonJon

author: jmelson
author_rank: Super Contributor
timestamp: October 15, 2019, 07:54:29 pm
content: Quote from: voltsandjolts on October 15, 2019, 01:51:27 pmRIGHT! You want extra pull-back from the fiducials, so there is nothing shiny near them to confuse the sensor.Jon

author: DerekG
author_rank: Frequent Contributor
timestamp: October 15, 2019, 11:30:14 pm
content: Quote from: voltsandjolts on October 15, 2019, 01:51:27 pmvoltsandjolts is correct.The industry standard is to place a 1.0mm round fiducial near each corner - but not evenly spaced from each corner as this picks up reversed panels/boards more easily.The industry standard is then to open the solder mask by a further 1.0mm around the fiducial (ie the solder mask opening would be 2.0mm in diameter).If you do the above, pretty much all the assembly shops will be more than happy

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: October 16, 2019, 03:39:13 am
content: Thanks for the replies. I double checked with them about the opening on the paste. They confirmed that they need an opening on the stencil. The volumes are low and they are doing a manual alignment of the stencil with the board and hence they need to see through the stencil. Probably they will mask out the stencil using tape for this purpose. I will visit them to see how they do that.

author: SMTech
author_rank: Frequent Contributor
timestamp: October 16, 2019, 07:20:13 am
content: I have a manual printer for low volumes... I use the apertures, as do all the demo videos you might choose to watch for semi auto printers like a Go23 etc aperture alignment has its weaknesses, relying on a 1mm+ feature will only amplifiy them.

author: mrpackethead
author_rank: Super Contributor
timestamp: October 22, 2019, 08:52:10 am
content: I make octagonal shaped fids, that are made exclusively for this purpose. ( open ) These are different fids from the ones that the pnp uses for alignment. It works well, i'm not concerned that paste gets put on these pads, as they are always on a tooling strip.