2001 Volume 19 Issue 3 Pages 521-536
Thermal cycling of an electronic assembly produces repeated inelastic strains in solder joints, which may cause thermal fatigue cracks to initiate and grow in a solder. The stress-strain relationship, creep deformation and stress relaxation in 37Pb63Sn, 50Pb50Sn and 90Pb10Sn were measured at various temperatures. Micro-crack initiation and growth behavior in plain specimens of the solders were observed under low-cycle fatigue and creep-fatigue at elevated temperatures. Even at room temperature, strain rate dependency of stress, creep deformation and stress relaxation in the solders were noticeable. Therefore, elements joined by solder are considered expanding freely at elevated temperatures, which results in a large shear strain in solders. Micro-cracks initiated at an early stage and their growth rates dominated the fatigue life of the specimen. The fatigue crack growth rate in 50Pb5OSn was much lower than in 90Pb10Sn and 37Pb63Sn. A slow tensile and fast compressive strain-wave caused a higher crack growth rate than a slow tensile and slow compressive one at elevated temperatures. Cracks 0.1 mm long existed at from 1/20 to 1/10 of the failure life of plain specimens. Above this size the fatigue crack growth rate had a good relationship with the strain intensity factor in different strain ranges. The strain intensity factors of cracks in die bondings were calculated using a low elastic modulus value for the solder. Also, the fatigue crack growth behaviors were calculated using the strain intensity factor values and fatigue crack growth rate of the solders at the maximum temperature of thermal cycling. This simulation gave a good approximation to the measured crack behavior in the thermal fatigue of the die bondings.