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Crack Behavior in Lead Free Solder Joints During Isothermal and Thermal Cycling
Keywords: Crack behavior , lead free solder, recrystallization
The behavior of lead free solder is fundamentally different from that of SnPb, and the absence of long term field experience requires special care to ensure that accelerated tests are in fact relevant to field conditions. Relevance is of course easiest to maintain by keeping the test parameters close to service conditions but this may easily slow down the test to an unacceptable degree. An ongoing effort aims to remedy this by the prediction of failure in a test at a much earlier stage. The proposed approach relies on the interruption of cycling long before failure, measuring the distribution of fatigue cracks in the lead free solder joints, and predicting the number of cycles to failure. The establishment and validation of such an approach requires a systematic study of crack evolution and correlations with the associated solder microstructure. The evolution of damage in, and eventual failure of, SnAgCu solder joints under a cyclic load tends to fall into two somewhat different categories. Thermal cycling commonly leads to recrystallization of the Sn and the formation of a network of grain boundaries across the high strain region in a joint followed by intergranular crack growth. Highly accelerated isothermal cycling, on the other hand, often leads to limited or almost no recrystallization in front of the crack tip as it progresses through the original Sn grain structure. This obviously depends on a large number of factors still under investigation. Preliminary results for Ball grid Array (BGA) assemblies suggested that the number of cycles to failure could be predicted after less than 10% of a typical thermal cycling test. The applicability to isothermal cycling is more complex and may depend on specifics of the assembly design.
Awni Qasaimeh, Student
Binghamton University
Binghamton, NY

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