Some great comments on solder mask defined pads for BGA.

First if people are trying to overcome lifting of pads during rework using this
technique rework operators need re training.

Over lapping resist pads is poor practice, it goes against the grain of basic
stencil printing paste, i e the stencil seals with the pad during printing, or
do you want to increase you under stencil cleaning.

Most of the photographs that are seen showing cracking or crack initiation on
BGA joints at the solder to pad interface are on thick masks >0.002" or on dry
film used on the BGA components this is were the failures occur. Some of the
photographs published in articles even show the failure at the BGA pads stating
they are the board sites.

I have conducted tests and had failures due to mask expansion, when a second
party reviewed the photographs he made the mistake of thinking the failure was
at the PCB pad turnthe picture up side down it is easy.

If you are interested I will be conducting workshops at Nepcon West which
include BGA, BGA Design and board requirments which are little different that
for fine pitch board. After all a board with BGA will also include some fine
pitch !!!

Bob Willis
SMART Group UK
2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England.
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Home Page: http://