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Mechanical strength and its variability in Bi- modified Sn-Ag-Cu solder alloy
Journal article   Open access   Peer reviewed

Mechanical strength and its variability in Bi- modified Sn-Ag-Cu solder alloy

M Matahir, L T Chin, K S Tan and Ayodele Olofinjana
Journal of Achievements in Materials and Manufacturing Engineering, Vol.46(1), pp.50-56
2011
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Abstract

Materials Engineering reliability assessment Pb-free solders
Purpose: Many previous work on Sn rich Pb-free soldering had focused on the evolution, morphology and the role of interfacial intermetallic compounds (IMC) layers of Cu3Sn and Cu6Sn5 on the mechanical integrity of soldered joints. However recent studies had shown that under static shearing stress, more fracture failures had been found to occur through the solder and thus indicate the significance of solder microstructures in the joint integrity. In this work, we investigated the effect of Bi substitution for Sn on the shear strength of solder joints of near eutectic SAC alloy Sn3.5Ag0.9Cu. Design/methodology/approach: Ingot alloy were prepared from pure elements and their melting characteristics were followed with thermal analyses. Copper plates were soldered together in lap joints that were subjected to shear testing in the as-soldered conditions. The microstructures were followed by SEM and EDS. Findings: Results show that failures occurred in quasi-brittle manner, with large variability. The ternary SAC alloy had average shear strength of 30 MPa better than binary eutectic Sn/Cu. Small Bi substitution of Sn up to 2 wt% lead to increased average shear strengths with maximum strengths of about 50 MPa recorded for compositions with Bi content of 0.5 to 1.5 wt%. Bi substitutions beyond 2wt% gave substantially lower strength values. The application of Weibull criteria suggest untypical high variability in strength with Webuill moduli less than 10. Higher variability in shear strengths were found in compositions containing more than 2 wt.% Bi. Research limitations/implications: Micro-structural evidence suggest that the role of Bi in increasing strength may be related to the high solubility of Bi in Sn and this would have provided some solution hardening effect. Higher Bi content however, lead to the formation Bi rich phases in the microstructure and this would have affected the mechanics of deformation thus leading to generally lower strength values and much higher variability in measurements. Originality/value: This paper clarifies the role of Bi substitution in improving the mechanical properties and reliability of soldered joints with unleaded solders.

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