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January 2002

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 18 Jan 2002 22:19:30 EST
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Hi Don,
You write:
>Before you hit the slopes - two last questions.
>In point #2, do you mean exactly what is written: that the non-functional
>pads reduce the possibility of hole wall separation?  Or do you mean that
>the presence of additional anchors reduces the propagation or magnitude
>of separation when it occurs?
>How does the presence of additional pads reduce the possibility of inner
>layer separation?  My understanding is that the adhesion of b-stage to
>c-stage is greater than the adhesion of polymer to copper.
A: I mean that the non-functional pads prevent the occurrance, not the
propagation or magnitude, of hole wall separation. You need to understand
what the mechanism causing hole wall separation is. Hole wall separation has
little to do with adhesion. Hole wall separation is caused by the PCB resin
expanding towards the hole center during heating. The higher the resin
content (prepreg layer has mor than the laminate layer) the more this
expansion. The larger the drilled hole diameter, the thinner the plated
copper barrel, the longer the compressed cylinder (PCB thickness), the fewer
cylinder reinforcement ribs (read: functional and non-functional lands), the
easier the expanding resin is capable of buckling the PTH hole wall inwards.
Too much inwards movement will cause plastic deformation of the copper
cylinder. Thus, on cooling the resin returns to its former volume and
position; so will the PTH wall if the copper barrel is only elastically
deformed. However, if the copper barrel is only plastically deformed, it will
stay where it is--voila, you have hole wall separation.

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