Hi Peter,
In a message dated 10/21/97 8:34:40, you wrote:
>Just a quick note to let you know how much I appreciate your always polite
and >to-the-point answers to questions put up on this forum.  More often than
not they >are the gems that make it worthwhile to wade through all the s... at
some >moments. I don't know whether you ever get any recognition for this, but
in my >eyes you certainly deserve a lot of it. Keep on 'doing' it !
>By the way, is there any scientific reason why soldermask-defined lands
should >give more stress in the solder joints ?

Thanks for the kudos. I did get a Special Recognition Award for my TechNet
efforts at IPCWorks'97.
Yes, there is a fundamental reasons why SMD solder joints perform less well
than NSMD solder joints. From a purely geometric point of view, the solder
joint geometries, even for NSMD, are less than optimum. An optimum solder
joint geometry would be a slight hour-glass. The reason is, that because of
local expansion mismatches between the solder and the surfaces soldered to,
there is always a higher loading condition near the interfaces; a slight hour-
glass simply provides a larger crossectional area and thus evens the loading
along the whole solder joint height and thus the failure probabilities. By
having the barrel shaped solder joints of the BGAs the larger crossections
near the center of the solder joint 'column' resist deformation more, thus
forcing the smaller crossections near the interfaces to deform more and thus
sustain larger fatigue damage. By a necking down of the already smaller
crossections near the interfaces due to the restrictions of the solder mask,
as well as a stress concentration effect due to the non-continuous surface
geometry, this effect is significantly heightened.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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