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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