Thermo-mechanical damage accumulation during power cycling of lead-free surface mount solder joints

It is well known that in surface mount technology (SMT), thermal strains in electronic assemblies are induced in the solder joints by the mismatch between the coefficients of thermal expansion (CTE) of the components, substrate and solder, both during their processing and in service. Therefore, thermo-mechanical damage is likely to occur in the solder and the principle reliability hazard in SMT assemblies is the resulting fatigue cracking of the solder fillet, caused by cyclic thermal stresses. These stresses may be caused by both cyclic variations in power dissipation within equipment and by external environmental temperature changes. Most work reported to date has focused on the effects of environmental temperature changes, although for many types of equipment power cycling may result in significant stresses. The present paper describes the experimental determination of the actual temperature distribution in a chip resistor assembly when it is powered. The paper also discusses the significance of such experimentally determined non-uniform temperature distributions in electronic assemblies to fatigue damage accumulation due to both power cycling and to cyclic variations in the ambient temperature whilst the chip resistor is powered. This fatigue damage accumulation study is carried out using finite element analysis.