Hi Darrelt,
What happened to your BGA is pretty obvious. Just consider what is physically
happening when you hit iot with the freeze spray.
You hit the BGA with the freeze spry from the top, I am sure. This quickly
cools the top layer of the BGA, the rest of the BGA to a lesser degree. That
sets up differential thermal shrinking (the opposite of thermal expansion on
heating), which causes the BGA to warp like a potatoe chip with the top
surface being concave. This puts tremendous tensile stresses on the solder
joints, as well as on other parts in the BGA depending on construction. On
top of which, the PCB does not cool very much at all (the BGA shields it from
the freeze spray) and sets up shear stresses between the BGA and the PCB.
I would not at all be surprised if failure analysis shows fractured solder
joints. likely in combination of soldering pads ripped out of the polymer
matrix.

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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