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October 1997

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Subject:
From:
Eddie Brunker <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Tue, 21 Oct 1997 10:34:17 +0100
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text/plain
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Werner,
Thanks for your response,
The thinking behind the resist contolling the lateral spread of spheres in
BGA lands has developed as follows:-
The collapse of eutectic solder spheres means the stand off height of the
device is reduced. In the earlier days of BGA (2 years ago) as we introduced
a device the general thinking coming out of "American BGA groups" was the
height of the standoff could be controlled by the diameter of the copper
pad. A small diameter copper pad could contain the lateral spread and thus
maintain a reasonable standoff.
The practice of this I found to be untrue as test microsections showed that
the solder spread beyond the edge of the copper pad. In various examinations
I found that what did control the spread in certain cases was the solder
resist. This is perfectly reasonable to assume since the inherent quality of
the material is to do just that. One of the biggest BGA processor
manufacturers now call for the lands to be defined in the mask(resist). I
assume that this is their reason for doing so. I have seen your dimensions
in action already on a large BGA processor and the balls do spread to the
edge of the resist. Have you seen any situations where the solder sphere
continued it's lateral progress beyond the resist's edge under the weight of
the device?

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