Earl, there's a big motivation to use solder mask defined pads when you're
squeezing lines in between pads because you don't have to account much for
soldermask tolerance.

With non-soldermask defined pads, you have to consider where your soldermask
will wind up (usually a 2 mil tolerance). Soldermask shouldn't be on the
pads, and it shouldn't shift so much that you have an exposed line next to a
solder ball pad. For this reason, it was the original design recommendation
(in 1990) for boards using BGAs.

Once the reliability data got out, non-soldermask defined pads became the
preferred option.

Terri

-----Original Message-----
From: Earl Moon [mailto:[log in to unmask]]
Sent: Thursday, May 23, 2002 1:05 PM
To: [log in to unmask]; Houston, Terri
Subject: Re: BGA Solderability


Totally agree with you on SMDP's. Did much work on this in earlier days and
concur about stress. Question I always ask is why, but for this situation
though not needed as such, does anyone use solder mask defined pads for
anything?

Earl

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