Local CTE Mismatch in SM Leaded Packages: A Potential Reliability Concern

The thermal expansion mismatch between Surface Mount (SM) components and the substrate to which they are attached, referred to as global mismatch, has been identified as a major reliability concern. This is widely recognized for leadless devices and leaded packages with 'stiff' leads. Compliant leads take up the global expansion mismatch and eliminate the associated failure mode. However, a more subtle mechanism takes places that raises another reliability concern. Thermal expansion mismatches across the solder joint interfaces, referred to as local mismatches, also create thermomechanical strains, and thus fatigue damage, in solder joints of leaded devices.