Following Part 1 in my April column, I have received some queries regarding testing results in relation to the understanding of reliability and/or solder joint behavior. Part 2 will answer some queries and provide a few examples on fatigue test results with relation to the damage mechanism of solder joints. Fatigue is one of the most likely culprits for material failure under cyclic or repeated stress conditions, regardless of metals, polymers, or ceramics. In-circuit functions and external environments create temperature fluctuations and mechanical stresses during the service life of a product, which inevitably impose cyclic strains on solder joints. Consequently, fatigue is deemed as an inherent phenomenon in solder joint assemblies. Fatigue life is generally measured by a low-cycle isothermal fatigue mode and/or a ther-momechanical mode as imposed by temperature-cycling conditions.
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