Comparison of Thermal Cycling Induced Mechanical Property Evolutions in Bulk Solder and Solder Joints

Abstract

During the service life of electronic packages, solder joints undergo thermal cycling which occurs between low to high temperature extreme. Also, solder joints can experience same exposure during accelerated thermal cycling testing which is being used for characterizing thermal-mechanical fatigue behavior. Mechanical behavior of solder joints evolves due to this variation in temperature and microstructure evolution of solder materials. In addition, during dwell at high temperature extreme, thermal aging phenomena occurs in the solder material which causes the microstructural evolution and leads to the material property degradation. Additional aging effects may occur during ramping between low to high temperature extremes in thermal cycling. In literature, many studies showed the evolution of mechanical properties of solder materials under isothermal aging whereas studies on evolution under thermal cycling exposure is limited. In our prior study, evolution of mechanical behavior of SAC305 lead-free solder material under different thermal cycling profiles have been reported for up to 5 days of thermal cycling. Severe mechanical properties degradation was observed, particularly for the thermal cycles with slow ramping rate and dwell period. In the present study, mechanical behavior evolution of small solder balls under stress free condition in different thermal cyclic loading for up to 50 days has been investigated. Particularly, the evolutions of mechanical behavior in both bulk SAC305 miniature solder bar samples and small SAC305 solder balls under stress free condition have been investigated for several thermal cycling profiles, and then the results were compared.The bulk solder specimens for uniaxial tensile testing were prepared by reflow solidification technique in rectangular glass tubes with a controlled temperature profile. On the other hand, BGA solder balls were ground and polished to find out single grain by optical microscopy for avoiding grain orientation effect in nanoindentation technique. Afterwards, both types of specimens were placed into the thermal cycling chamber and thermally cycled between -40 C to +125 °C under a stress-free condition (no load) in several thermal cycling profiles. The thermal cycling profiles were: (1) 150 minutes cycles with 45 minutes ramps and 30 minutes dwells, (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps, (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramp only), and (4) Isothermal aging at high temperature extreme (no cycle). After each thermal cycling exposure, mechanical properties evolution of uniaxial tensile test specimens was presented in terms of effective elastic modulus (E), yield strength (YS), and ultimate tensile strength (UTS). For the BGA solder balls, the evolution of mechanical properties was measured using nanoindentation and presented in terms of E, YS, and hardness. Finally, the mechanical properties evolution of both bulk solder material and BGA solder balls were compared.