David,
You can do this if you are using a water soluble paste and wash the
assembly. You should not consider doing it if you are using a no-clean
process. The reason is that some paste invariably does not agglomerate
into the solder joint during the reflow process if it is not printed
onto the metal pad, but spatters leaving small solder balls. Even if you
use a water wash process it may still not be a good idea. Typically a
.006" or .007" thick deposit of paste is sufficient if the pad is the
same or almost the same size as the component endcap. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Greig
Sent: Thursday, October 19, 2006 1:41 PM
To: [log in to unmask]
Subject: Re: [TN] Tombstoning and nitrogen

Hi Werner

Many thanks for an insightful answer.

Might it be possible to gain an increased thickness of solder between a
chip components metallization and the PCB by designing with a pad that
matches the underside metallisation and having a stencil aperture that
is larger than the PCB pad (perhaps not larger at the 'toe' for
tombstoning reasons).

Best Regards

David Greig

  _____

From: [log in to unmask] [mailto:[log in to unmask]]
Sent: 19 October 2006 13:07
To: [log in to unmask]; [log in to unmask]; [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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