PCB Assembly Services - Screaming Circuits: Passives and Via In Pad


Passives and Via In Pad

Small_fillet_passive_via_in_padAre you tired of this topic yet? Myself, I'm a bit scatterbrained so I can pretty much talk about the same thing forever. And, I can repeat myself and not even feeling like I'm being redundant or repeating myself. I'm not sure what that little dip into my personal philosophy has to do with the trials of one tasked with a complex layout, but I certainlySkewed_passive_via_in_pad_2 know what via in pad has to do with it.

Most of our via-in-pad articles relate to QFNs and BGA type devices, but via-in-pad can also impact passives. It can cause tombstoning, part misalignment and open solder joints. Here's a couple of examples where the open via didn't cause an electrical open - both have good electrical connection, for now - but there is almost no fillet and very little solder under the part. These will work at the initial turn-on but after a few thermal cycles and maybe some vibration or rough handling, they are very susceptible to cracks in the solder joint on the via side.

If there are other parts really close by, the crooked part causes a shorting risk as well. A misalignment like that happens for the same reason that tombstoning can: Capillary action from the via sucks solder away from the pad and The via sinks heat away so the solder on the other pad melts first. Then the surface tension of the liquid solder can pop up or move the part around like this.

Duane Benson
305 down, 60 to go...


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