Electronics > Projects, Designs, and Technical Stuff
Via in pad
OM222O:
I have mostly used it with 1206 , 0805 and SOIC or QFP packages. Others like
SOP or SOT are usually way too fine pitch to actually use via in pad.
I actually never had issues with too little solder or unreliable joints. I order from jlcpcb with HASL which I assume fills the vias beforehand? If solder sucking was an issue I assume you can increase the paste "extension" on the pads which have vias in them, since a bigger stencil cutout means more solder for the pad?
Also no issues with tombstoning.
Unixon:
--- Quote from: Siwastaja on July 25, 2020, 11:27:34 am ---Sorry but this advice is incorrect. HASL sometimes gets some solder into the vias, but not reliably.
(I have made exactly that mistake and seen the results so this is why I know.)
--- End quote ---
Oh, thanks for correction, I should have said "probably safe", HASL is not that much reliable especially for small vias.
ogden:
--- Quote from: OM222O on July 25, 2020, 09:38:42 am ---I've been using the "via in pad" pretty extensively for different things, such as: reinforcing the screw mounting holes, connecting signals or power / gnd to chips while using the underside as a bus, etc.
--- End quote ---
Via-stitching of ground planes, transmission lines, mechanical holes, also thermal pad vias shall not be confused with via-in-pad. Thermal pads have through vias for a good reason. When you manufacture make products for yourself or "use at your own risk" market - everything that works for you is fine. When you work for volume consumer/industrial market - use only plugged+plated via-in-pad or better avoid if possible because such adds to the cost. Further reading1 reading2
I wonder - anyone here with experience using blind vias as "lower cost" via-in-pad?
T3sl4co1l:
--- Quote from: Siwastaja on July 25, 2020, 10:18:24 am ---Laser drilled 0.1mm microvias are OK, they do not suck significant amounts of solder.
If you have 0.2mm drill available at your fab you are using, that's usually OK as well, but maybe a bit iffy.
--- End quote ---
Particularly on lead-free, which flows much more slowly than leaded solder. I've seen QFN pads that didn't wick with this combination, 0.3mm even. (Some vias do wick visibly, and moreso the larger the hole. It's not a perfectly repeatable thing, sometimes none do, sometimes a few...)
By the way, do not use tented vias, in an effort to block the flow of solder into the hole -- this backfires, quite literally: the solder flux (and other residues from PCB fab) outgasses through the open end, causing voids in your solder joint. Either leave the via open both sides, or tent/cap it both sides.
A tented via touching a pad, however, will neither void the solder joint, nor steal solder. This can help squeeze out precious fractions of a mm in a design.
--- Quote ---Copper-filled holes are ultimate, but expensive. The pads look completely normal and flat. No solder sucking, and massively good electrical and thermal conductivity. They conduct a lot of heat during soldering, though, which you must take into account, balancing the pads.
--- End quote ---
Yup. There's also filled and capped vias. Expensive -- there are additional steps to mask, paste and plate only the target holes, by hand as I understand it. The result is fantastic, not as conductive as copper-filled of course, but the vias are completely flat and no solder wicks away.
Speaking of laser drilled, they often plate shut anyway. You might see a little dimple on the board, not planar like filled-and-capped, but hardly anything to worry about. HDI has this between every layer pair, so you don't have vias blocking traces or components on the opposite side, it's a routing free-for-all! You can easily route fine pitch BGAs.
Tim
Someone:
--- Quote from: ogden on July 25, 2020, 12:46:52 pm ---I wonder - anyone here with experience using blind vias as "lower cost" via-in-pad?
--- End quote ---
I have only ever seen that on the "not recommended" list, for the same reasoning as tented vias; as discussed by T3sl4co1l above.
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