Meaning
Incremental buildup of deformation in material joints subjected to cyclic temperature changes drives structural fatigue. Thermal strain accumulation occurs in microelectronic packages due to the different expansion coefficients of the joined materials during operation. As the device heats and cools, the solder joints absorb the resulting displacement through elastic and plastic deformation.
This progressive accumulation of damage weakens the material and leads to eventual electrical open circuits.
Fatigue Mechanism
Each thermal cycle adds an increment of plastic strain to the material structure. Over hundreds of cycles, these individual increments sum to a significant total strain that exceeds the ductility limit of the solder. The metal begins to undergo microstructural damage, which starts as grain boundary sliding and advances to micro-void coalescence.
Eventually, these voids coalesce into macro-cracks that propagate across the joint. The rate of this deterioration depends on the temperature range and the dwell time at peak temperatures.
Stress Relaxation
At high temperatures, the accumulated strain does not remain constant but relaxes through creep processes. Creep converts the high elastic stresses into additional plastic strain, further damaging the solder joint. This behavior means that longer dwell times at temperature extremes can accelerate the damage accumulation.
Numerical simulations must account for this time-dependent relaxation to predict the physical life of the assembly correctly.
Reliability Evaluation
Thermal cycling chambers test assemblies under accelerated conditions to measure this damage rate. This testing helps validate the fatigue models used during the initial design phase of the product.
