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2 Journal of Achievements in Materials and Manufacturing Engineering

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

Matahir M, Chin L T, Tan K. S., Olofinjana A. O.

Journal of Achievements in Materials and Manufacturing Engineering

2011

Vol. 46, nr 1

50--56

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.

Achieving combined high strength and high conductivity in re-processed Cu-Cr alloy

Olofinjanaa A. O., Tan K. S.

Journal of Achievements in Materials and Manufacturing Engineering

2009

Vol. 35, nr 1

14-20

achieve the optimum combination for strength and conductivity in copper based alloys. However, precipitation strengthened Cu- alloys are limited to very dilute concentration thereby limiting the volume proportion hardening precipitates. In this work, we report the investigation of the reprocessing of higher Cr concentration Cu- based alloys via rapid solidification. Design/methodology/approach: The ingot alloys with Cr content up to 10 wt.% were prepared via semi-chilling of small rods before been cast into ribbon using chill block melt spinner. Thermal aging studies followed by conductivity and microhardness tests were performed to follow the HSHC properties. Findings: It is found that the rapid solidification in the as-cast ribbon imposed combined solution extension and ultra-refinement of Cr rich phases. X-ray diffraction evidences suggest that the solid solution extension was up to 6wt%Cr. Lattice parameters determined confirmed the many folds extension of solid solution of Cr in Cu. Thermal aging studies of the cast ribbons indicated that peak aging treatments occurred in about twenty minutes. Peak aged hardness ranged from about 200 to well over 300Hv. The maximum peak aged hardness of 380Hv was obtained for alloy containing 6wt.%Cr but with conductivity of about 50%IACS. The best combined strength/conductivity was obtained for 4wt.%Cr alloy with hardness of 350HV and conductivity of 80% IACS. The high strengths observed are attributed to the increased volume proportion of semi-coherent Cr rich nano-sized precipitates that evolved from the supersaturated solid solution of Cu-Cr that was achieved from the high cooling rates imposed by the ribbon casting process Research limitations/implications: The rapid overaging of the high Cr concentration Cu-Cr alloy is still a cause for concern in optimising the process for reaching peak HSHC properties. It is still important to investigate a microstructural design to slow or severely restrict the overaging process. Originality/value: The optimum HSHC property reported here is a rare combination of high strength (>350Hv ~ 900MPa) and conductivity (50 – 80% IACS) found in metallic alloys.