Material Strain
A mechanical tension arises within composite structures when they are exposed to simultaneous temperature changes and moisture absorption. This hygrothermal stress is a major cause of delamination and solder joint failure in electronic packages. The different expansion rates of plastic mold compounds and silicon dies generate internal forces during thermal cycles.
These forces can cause physical separation at the material boundaries.
Moisture Swelling
Plastic materials expand when they absorb atmospheric moisture, while silicon and metal components remain dimensionally stable. This differential expansion creates localized shear stresses at the interfaces when the temperature changes. The magnitude of the stress depends on the moisture content and the temperature range.
Stress Verification
Reliability labs use highly accelerated stress testing to evaluate the resistance of assemblies to these combined environmental conditions. Devices are exposed to high humidity and elevated temperatures before being subjected to rapid thermal cycling. The failure rate under these conditions determines the lifetime of the assembly.
Acoustic microscopy is used to inspect the internal interfaces for signs of delamination after testing.
Failure Prevention
Designers select mold compounds with low moisture absorption properties and matched thermal expansion coefficients to minimize stress. Baking the components before assembly removes trapped moisture and reduces the risk of internal damage during reflow. Maintaining controlled storage environments also protects the parts.
