Abstract
Active solder alloy 92Sn3Ag3Ti was used to join Al2O3 ceramic on cast A356 aluminum alloy substrate using an ultrasonic assisted soldering process. Due to the large discrepancy in thermal shrinkage coefficients between materials, large thermal stresses occur during cooling down to room temperature. Further cooling to −78°C and −196°C was used to inflict additional thermal stress in the samples. Shear strength, image analysis of fractured surfaces and microstructure analysis were performed to study the influence of cooling. Ceramic samples with different surface finish were used to investigate the influence of the contact between ceramic and aluminum alloy substrate on the fracture behavior. Two types of failure were observed, a detachment of the solder alloy from the ceramic surface and a cohesive failure through the solder layer. The results show that cooling the joined sample to −78°C does not significantly affect the bonding strength of the samples. However, samples that were rapidly cooled to −196°C showed reduced shear strengths even when the amount of cohesive failure area was taken into account. Many newly formed cracks in the Sn matrix were observed in the joint after the cooling which were identified as strength-determining defects.
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