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Passives and Via In Pad

Are 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 certainly 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...

Posted on November 1, 2007 in Via
in Pad

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