Mail Archives: geda-user/2019/03/21/07:04:25
With decreasing SMD pin pitch sizes and ever more pinless (QFN)
housings, I increasingly run into problems with short circuits or open
pins, at least with manual assembly. With more complex PCB's, I
sometimes get 20%-30% with solder problems.
The main problem seems to be an excess of solder paste ending up on the
SMD pads. One cause I found is pre-tinned pads, which tend to slightly
raise the stencil. This is solved easily by requesting gold plated
surface finishing, so that the stencil lies as much as possible flush to
the surface when applying the solder paste.
But even then, and with the thinnest (100 um) stainless steel stencils,
I occasionally get shorts.
(Open pins result when a largish QFN housing has a central ground pad
that accumulates more solder paste relative to some of the small pins,
making the device and thus those smaller pins float above the paste.
This too could probably be fixed by making the central aperture smaller,
although this seems mostly a matter of being very careful about pressure
when applying the paste, and of course keeping the stencil absolutely
clean.)
After some searching, I found that decreasing the size of the openings
in the stencil and thus the amount of solder paste ending up on the PCB
appears to be a common way to prevent shorts of this nature.
Can anyone confirm that this is indeed a good idea? It would probably
mean that I'd need to make two copies of each layout: one with the
desired PCB pad sizes, and one with reduced pad (and thus aperture)
sizes for stencil production. And what reduction percentage would be
recommended? At several dozen dollars plus at least a week delivery time
per stencil, I don't want to mess around too much with trial-and-error.
Or is it a matter of (still) better workmanship on my behalf? I'm
already using a stereoscopic magnifying headset, I have quite a steady
hand, and the solder paste usually looks quite good right after
application (and if it doesn't, I simply scrape it off, clean the PCB,
and do it again).
Any thoughts on this are appreciated!
Thanks in advance, best regards,
Richard
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