In addition, the differences in CTE mismatch between small caps and
larger caps is so small as to be neglible. You are still "anchoring" the
caps by allowing large fillets around the three sides of the endcaps,
with little solder underneath. If the cap is a MLCCC with a ceramic body
material, then there is nothing to absorb the cyclic strains. Hence
there are going to be either the solder joint cracks, or the capacitor
itself will crack. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Thursday, October 19, 2006 7:07 AM
To: [log in to unmask]
Subject: Re: [TN] Tombstoning and nitrogen

Hi David,
What happens with all chip components and large pads is, that most of
the solder goes for the fillets and very little winds up between the
chip and the PCB pad. Thus, the stand-off height, one of the prime
reliability parameters, is very small and you get crack initiation
underneath the chip very early. What the fillets do in this case is
provide functional life because of the larger volume through which the
cracks need to propagate for functional failure. Out in the fillets the
cyclic strains are smaller than underneath the chip, because of a much
larger 'effective' solder joint thickness.
However, you would be much better of if the solder were underneath the
chip instead of the in the fillet, thus preventing any premature crack
initiation in the first place.

Werner

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