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Influence of thermal ageing on process-induced voids growth and shear strength for selected lead free solder thermal interface materials
Keywords: lead-free, solder die attach, thermal ageing
In flip-chip packaging technology, thermal performance can be improved by attaching heat spreader to the backside of the heat generating silicon die via thermal interface material. Solder thermal interface materials (STIMs) are often preferred to their polymer-based counterparts because they relatively offer higher thermal conductivities. Recent transition to lead (Pb)-free solder materials has necessitated the extensive examination of various Pb-free solders in order to fully understand their potentials as thermal interface materials especially for high temperature applications. This is because the applications of these solders usually experience elevated temperature for a long period of time during service life. Hence, high temperature storage (thermal ageing) is one of the reliability tests employed to simulate the effects of such high temperature exposure over an extended period of time. In this study, Pb-free solder preforms with compositions of 96.5Sn3Ag0.5Cu, 99.99In, 97In3Ag and 58Bi42Sn were used for silicon die attachment on copper heat spreader. The void growth and shear strength of the solder joints after thermal ageing at 125oC for 50 hours, 100 hours, 200 hours and 300 hours were evaluated. The four studied STIMs show reduction in voids with thermal ageing. 96.5Sn3Ag0.5Cu alloy and 99.99In are shown to be relatively more stable throughout the ageing time. The shear strength of 58Bi42Sn is seen to rise with increase in thermal ageing time. 97In3Ag showed a random trend in shear strength during thermal ageing. The results also indicate that samples with lower void content mostly resulted in higher shear strength and vice versa. These results are expected to add to the current state of knowledge as regards Pb-free STIMs reliability at temperature applications of up to 125oC.
Kenny C. Otiaba,
University of Greenwich
Chatham Maritime, Kent

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