Thermal Mismatch
Cyclic mechanical strain resulting from differential thermal expansion between dissimilar materials drives structural degradation in electronic packaging. Circuit board assemblies suffer thermo mechanical fatigue when temperature changes induce shear stresses across solder joints connecting components to printed substrates. Printed circuit substrates expand at higher rates than ceramic integrated circuit packages during thermal operation.
This expansion mismatch concentrates cyclic shear strain within the solder interconnects holding component terminations. Over repeated operational temperature cycles, accumulated creep strain causes microstructural coarsening and intermetallic fracture in the solder matrix.
Crack Initiation
Repeated thermal expansion cycles induce localized plastic deformation along intermetallic boundary layers in solder joints. Experiencing thermo mechanical fatigue causes fine surface microcracks to initiate at outer component corners where thermal shear strain reaches maximum values. Continued temperature cycling propagates these microcracks through the solder bulk until complete electrical open circuits occur.
Strain range, peak dwell time, thermal ramp rate and temperature delta determine the rate of fatigue damage propagation.
Fatigue Limit
Material selection and physical layout geometry dictate the fatigue endurance limits of surface mount assemblies. Matching coefficient of thermal expansion values between components and substrates reduces cyclic strain magnitude. Underfill encapsulants distribute shear forces away from individual solder joints to extend assembly operational lifetime.
