Degradation Process
Electronic assemblies operate under cyclic thermal loads that lead to the progressive degradation of materials and solder joints over time. This thermal wear-out is the primary mechanism of long-term failure in boards exposed to high-power environments or harsh outdoor conditions. The mismatch in the coefficient of thermal expansion between different board materials causes stress to build up during thermal cycles.
This continuous stress eventually cracks the solder joints and delaminates the laminate layers.
Material Fatigue
The rate of material degradation increases with higher temperatures and larger thermal fluctuations. Solder joints undergo creep and fatigue as the board cycles between hot and cold states. This behavior is especially pronounced in lead-free solder alloys like SAC305, which are stiffer than traditional leaded alloys.
To prevent early failure, designers must choose substrate materials with a high glass transition temperature. These materials maintain their physical structure at higher temperatures.
Lifetime Assessment
Engineers evaluate this failure mechanism by subjecting test boards to accelerated thermal cycling in environmental chambers. This testing simulates years of operational use in a few weeks. The resistance of the daisy-chained test joints is monitored continuously during the test.
A sudden increase in resistance indicates the onset of wear-out.
