Electronics > Manufacturing & Assembly

Solder paste coverage for large pads?

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d1wang:
I have searched the Web for information on this. I haven't found any formula or a ballpark target of the solder paste volume. I would appreciate it if someone can provide a formula/number that people can use as starting point.

The idea here is to have some acceptable result (without the added cost of via plugging). The following comes from TI's application note:


--- Quote ---Another solder-mask tenting technique is to tent from the top of the board. The via solder-mask diameter must be 0,1 mm larger than the via hole diameter when tenting. Another variation is to create a crosshatch pattern of solder mask to create a predictable area of coverage. Trials have shown that via tenting from the top is less likely to produce random voids between the exposed pad and the PCB pad.

--- End quote ---

Does anyone have experience with via-tenting on exposed pad?

I haven't read anything that mentioned height variation. How big of a problem is the height variation?

ar__systems:
I did vias on the exposed pad and masked via on the bottom side. Works fine. I notice that at least some of lead free pastes don't really flow that much (or at all, really). In particular GC10 practically does not flow. So sucking solder into via might be much less of an issue than you think.

d1wang:
Good to know. What's the drill size of your via hole? Do you know if the stencil apertures avoided covering paste on the via holes?

mrpackethead:
tenting the bottom side is not a good idea. It can result in outgassing.. which can do all sorts of nasty things when you reflow.. the gas thats trapped in the via will expand when heated.

I have good results with not tenting either side,  I put my solder paste in squares to avoid the holes.. and with GC10 it really doe'stn seem to move that much, and i don't get wicking.




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