Dear Steve,

I do have information on solder mask sidewall profiles.  The ability to get
a straight sidewall depends on many factors including; LPI mask type, LPI
thickness, tack drying conditions, Exposure equipment (collimated or
non-collimated), exposure lamp intensity, and developing conditions.

Most board fabricators need to "overdevelop" the LPI solder mask to ensure
that the LPI is completely developed out of holes.  This typically creates
the undercut situation.  It is important to note that undercut is typically
caused by underexposing or overdeveloping.  You can also create an
"overhang" on the sidewall by overexposing the solder mask.  Sometimes this
"overhang" is mistaken for undercut.

With BGA boards, I have seen solder mask thicknesses of 0.5 mil  to 2 mils.
Are you specifying a certain solder mask thickness?
Your chances of a straight sidewall will be much better at 0.5 mil versus
2.0 mils.

With BGA boards, it is also important to note whether the board fabricator
is gold plating before or after solder mask.  If the board fabricator is
applying solder mask after gold plating, the tendency is too develop very
aggressively so that the gold surface is free of mask residues for gold wire
bonding.  This may result in undercut if the exposure energy is not
increased to compensate for the aggressive developing.

Steve, I would be happy to provide you with cross section photos showing
various sidewall profiles.  Please contact me at 702-885-9959.


Best regards,

David Rund
Taiyo America
-----Original Message-----
From: Joy, Stephen C <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, June 16, 1998 9:37 AM
Subject: [TN] Solder mask profile information


>     We have been having some problems with a BGA device where we see
>     ball fatigue (solder ball cracks) after temp cycling. We think it
>     may be related to severe undercut of the solder mask.
>
>     Does anyone have information on different solder mask foot
>     profiles? How difficult is it to get a straight sidewall on
>     solder masks?
>
>     Are there any that are much better than others for this
>     attribute? What would be the trade offs?
>
>     You can contact me off line if you prefer.
>
>     Steve Joy
>     503-696-2239
>     [log in to unmask]
>
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