Stress Action
The repeated physical distortion experienced by electronic components results from the different thermal expansion rates of bonded materials during temperature fluctuations. This phenomenon, known as thermal cyclic mechanical strain, puts high stress on solder joints that connect silicon packages to plastic boards. The strain accumulates with each temperature change, eventually leading to structural degradation.
Deformation Physics
When the operating environment heats up, the silicon die, the fiberglass substrate, and the metal solder expand at different rates. The resulting thermal expansion coefficient mismatch forces the solder joints to flex and shear to accommodate the dimensional differences. Over many cycles, this continuous flexing causes plastic deformation and microstructural coarsening in the solder alloy.
Micro-cracks begin to form at the joint boundaries where the stress concentration is highest. These cracks grow larger until they completely disrupt the electrical connection.
System Lifetime
Designers use specific material pairings and compliant lead geometries to reduce the impact of these thermal forces. Choosing substrates with a low thermal expansion coefficient matches the behavior of the silicon components more closely. Underfills are also applied to distribute the mechanical load across the entire component footprint instead of concentrating it on the solder spheres.
These mitigation techniques prolong the service life of high-density electronic assemblies.
