Investigation on the Mechanical Behavior Evolution Occurring in Lead Free Solder Joints Exposed to Thermal Cycling

In electronic packages, solder joints are frequently exposed to thermal cycling environment where temperature variations occur from very low to high temperature. These exposures can occur in real life applications as well as in accelerated thermal cycling tests used for the characterization of thermal-mechanical fatigue behavior. Due to temperature variations and CTE mismatches of the assembly materials, cyclic temperature leads to damage accumulation due to shear fatigue and material property evolves in the solder joints. In addition, the thermal cycling dwell periods at the high temperature extremes will cause thermal aging phenomena in the solder material. This leads to microstructural evolution and material property degradation. Further aging effects can occur during the ramp periods between the low and high temperature extremes of the cycling.While changes in solder materials during aging have been examined in detail in prior studies, there have been limited studies examining material evolution occurring during thermal cycling. In a previous study of the authors, mechanical behavior evolutions of SAC305 lead-free solder material under several different thermal cycling profiles have been reported. The results demonstrated severe degradations in the mechanical properties, especially for thermal cycles with the long ramp and dwell periods. In our other recent work, evolution of the mechanical behavior of real solder joints has been investigated. In the current investigation, these prior studies have been extended. In particular, the mechanical behavior evolutions in both bulk SAC305 solder samples and SAC305 solder joints have been investigated under the same slow thermal cycling profile, and then the results were compared.In the first part of this study, miniature bulk solder uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile. After reflow solidification, the samples were placed into the environmental chamber and thermally cycled between -40 C to +125
^o
C under a stress-free condition (no load). The thermal cycle consisted of 150 minutes cycles with 45 minutes ramps and 30 minutes dwells. The test specimens were separated into groups that were subjected to various durations of cycling (e.g. 0, 10, 50, 100, 250 cycles, etc.). After the environmental exposures, stress-strain curves of the cycled uniaxial samples were recorded, and then the mechanical properties were measured including the effective elastic modulus (E), yield stress (YS), ultimate tensile strength (UTS). The evolutions of the mechanical properties were characterized as a function of number of applied thermal cycles.In the second part of this study, the evolution of the mechanical behavior in thermally cycled BGA solder joints was studied using nanoindentation. PBGA solder joint strip specimens were first prepared by cross sectioning BGA assemblies followed by surface polishing to facilitate nanoindentation testing. Single grain solder joints were tested since they had large regions of solder material with equivalent mechanical behavior, which could then be indented several times after various durations of cycling. After preparation, the solder joint strip samples were thermally cycled using the same thermal cycling profile as the bulk samples. At various points in the cycling, the package was taken out from the chamber, and nanoindentation was performed to obtain the modulus and hardness. This allowed for investigation of the evolution of the mechanical properties of the SAC305 solder joints with the duration of thermal cycling.The results for the thermally cycled bulk samples showed that the detrimental effects of aging are accelerated in a thermal cycling environment. Similar degradations were found in the BGA solder joints subjected to thermal cycling. The degradation for both bulk samples and solder joints showed exponential variation with number of cycles. However, the degradation rates were higher in the bulk solder samples relative to those in the real solder joints. For example, the effective elastic modulus and yield stress reduced by 69% and 43%, respectively, for the bulk samples; whereas for the real solder joints, these values both reduced by 26%.