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March 2003

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 31 Mar 2003 00:30:12 EST
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Hi Peter,
What I am saying is, that "barrel cracking is now the more common problem in
small-diameter holes, that the "non-functional pad" issue has fallen off in
importance and that n-f pads do not contribute to barrel cracking."
Small-diameter holes are more prone to barrel cracking, which is caused by
the z-axis expansion of the resin. Thin, non-uniform and low-ductility Cu
plating, as well as high resin content and low Tg, and of course PCB
thickness and Tmax all contribute to the possibility of barrel cracking. The
presence of n-f lands replaces some of the resin, which has some but not a
significant positive effect.
Larger diameter holes are more prone to inner-layer [post] separation. Thin
Cu plating, as well as high resin content and low Tg, and of course PCB
thickness and Tmax all contribute to the possibility of ILS; further bad or
thin Cu foil as well as low quality wet processing also are an issue. The
absence of n-f lands increases resin recession, but not necessarily ILS.
>How then to support hole plating and prevent its deformation through resin
recession and/or >hole pull-away?
****You got it backwards--barrel hole wall is deformed during resin
expansion; when the resin cools and returns to its initial volume, the
plastically deformed Cu cannot follow and 'resin recession' or 'hole
pull-away' results.
>Or are these defects not significant in purely interconnect via holes?
**** 'Resin recession' or 'hole pull-away' are not real reliability issues
either for vias or PTHs.
>Is removal of n-f pads to prevent barrels cracking better than keeping them
to give support
>to the barrel plating?
****As I said, n-f pads do not significantly affect barrel cracking one way
or the other. If processing problems are not an issue, I would keep the n-f
pads, however.
>Do you know what, if any, correlation there is between plating thickness,
>hole diameter, aspect ratio, number of layers and the point where barrel
>cracking becomes more prevalent and hole pull-away/resin recession becomes
>less significant?
****Aspect ratio is a 'red flag' indicator, not a real parameter. However,
for barrel cracking the correlation between plating thickness, hole diameter,
PCB thickness, resin Tg and T-excursion exists and has been modeled. For ILS
the correlation is qualitative, but not quanitative.
I had hoped to deal with some of these issues in workshops in Singapore,
which because of the SARS issue I had to postpone.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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