Quantification of creep strain distribution in small crept lead-free in-situ composite and non composite solder joints

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2000-06, Vol.285 (1), p.25-34
• Main Authors: McDougall, J, Choi, S, Bieler, T.R, Subramanian, K.N, Lucas, J.P
• Format: Article
• Language: English
• Subjects: Applied sciences; Composite and non composite solder joints; Creep; Creep strain distribution; Exact sciences and technology; Friction-type joining (riveting, screwing, clamping, bending); Joining, thermal cutting: metallurgical aspects; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Small crept lead-free
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/S0921-5093(00)00661-4

Single shear lap creep specimens with a 1 mm 2 cross sectional area (similar in size to solder joints used in electronic packaging) were developed using lead free solders to examine the effect of in situ composite microstructures on the creep resistance and damage accumulation processes at temperatures between 25 and 150°C. Local strain measurements were made optically by following the change in shape of a scratch. Average creep strain measurements on non-composite solder joints were similar to comparable data from solder joint specimens in the literature. The distribution of strain across the specimen was measured in detail to determine how strain instabilities develop near interfaces, or near the center of the solder joint. The evolution of strain is quantified across the joint and related to microstructural features such as voids and reinforcements. Room temperature creep resistance is far superior in the composite, but at elevated temperatures, both types of joints have similar creep resistance. By analysis of the local strain history, a criteria for damage initiation is identified in terms of the local strain where the onset of tertiary creep is first observed, and subsequently accelerates heterogeneous straining in the joint. The in situ composite solders cause a more homogeneous strain evolution compared to the non-composite solder, but local tertiary creep commences at a lower strain than in the non-composite joint, but only, at lower temperatures and higher strain-rates. The impact of these different constitutive behavior and damage accumulation conditions is discussed as it pertains to improving the long term reliability of a solder joint.