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February 1997

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Tue, 11 Feb 1997 23:58:57 -0500 (EST)
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I am not aware of any specific studies about solder joint integrity as it
relates to the amount of solder mask encroachment onto the SMY pads; however,
a number of comparies have done studies about the thickness of solder mask as
it relates to good solder joints (DuPont, Morton, Ciba-Geigy, etc).
Basically, with solder mask that is higher than the surface mount pads ( for
example, 2.0 mil high SMT pads being surrounded byu 3.0 mil high solder
mask), you can have a starving effect when stencil screening the solder paste
onto the pads (ie, low solder paste deposit or no solder paste at all) on the
component side and/or a shadowing effect when wave soldering the SMT
components on the solder side. Both of these conditions could lead to poor
solder joint integrity.

When solder mask does encroach onto the SMT pads, it must obviously be higher
than the pads and could cause solder joint problems. My question is:
Is the solder mask encroachment caused by the design of the board or by poor
registration?

I have seen designs where the solder mask does come up onto the land areas of
plated through holes (by design) and many designs allow solder mask to
completely cover and fill via holes. I'm not sure if I've seen designs where
mask is allowed to come up onto SMT pads.

Larry Fisher
Dexter Electronic Materials
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