Thermal stress in ball array always exists (epoxy encapsulant between solder bumps reduces it a bit) and is annoying when there's a big temperature difference like in the article on Electronics Cooling where tests are made between 20°C and 150°C. Dilatation effects are very small (some microns or less, depends of length and temperature, look at their tests with a ~5µm per fringe) which is at least one order magnitude less than a geometrical defect on a concave IHS. Don't forget that on their tests, there's no IHS on the die, so no pressure load to constraint the dilatation effect.
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