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December 2004

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Sat, 4 Dec 2004 10:36:29 EST
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TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
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From:
Werner Engelmaier <[log in to unmask]>
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Hi Swati,
Hole wall separation and resin recession occur during cooling from soldering 
process temperatures. During reflow soldering of the assembly, one of the 
hottest part of the assembly is the PCB. The PCB resin is significantly above its 
glass transition temperature, and has a very high CTE. This expansion is 
constrained/resisted by the PTV copper barrel. If the barrel plating is thin and/or 
the attached inner-layer pads are spaced too far apart [particularly at 
prepreg layers], the barrel walls will deform inwards. Some of this deformation is 
elastic, but can be plastic as well. This plastic deformation of the barrel 
wall prevents the copper barrel to resume its original geometry on cooling when 
the resin shrinks back to its original volume. Thus, the copper stays where it 
is, the resin returns to its original geometry--violá, "hole wall separation 
and resin recession."
You can prevent hole wall separation and resin recession by reducing the 
soldering temperatures (not a good idea if you want to have good solder 
joints--and it will be getting worse with this lead-free insanity), plate thicker copper 
barrel walls, use non-functional inner-layer pads, avoid thick prepreg 
layers, have a low resin content in your lay-up.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com


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