Authors: JP Tucker DK Chan G Subbarayan CA Handwerker
Publish Date: 2011/12/07
Volume: 41, Issue: 3, Pages: 596-610
Abstract
During the transition from Pbcontaining solders to Pbfree solders joints composed of a mixture of SnPb and SnAgCu often result from either mixed assemblies or rework Comprehensive characterization of the mechanical behavior of these mixed solder alloys resulting in a deformationally complete constitutive description is necessary to predict failure of mixed alloy solder joints Three alloys with 1 wt 5 wt and 20 wt Pb were selected so as to represent reasonable ranges of Pb contamination expected from different 63Sn37Pb components mixed with Sn30Ag05Cu Creep and displacementcontrolled tests were performed on specially designed assemblies at temperatures of 25°C 75°C and 125°C using a double lap shear test setup that ensures a nearly homogeneous state of plastic strain at the joint interface The observed changes in creep and tensile behavior with Pb additions were related to phase equilibria and microstructure differences observed through differential scanning calorimetric and scanning electron microscopic crosssectional analysis As Pb content increased the steadystate creep strain rates increased and primary creep decreased Even 1 wt Pb addition was sufficient to induce substantially large creep strains relative to the Sn30Ag05Cu alloy We describe ratedependent constitutive models for Pbcontaminated SnAgCu solder alloys ranging from the traditional timehardening creep model to the viscoplastic Anand model We illustrate the utility of these constitutive models by examining the inelastic response of a chipscale package CSP under thermomechanical loading through finiteelement analysis The models predict that as Pb content increases total inelastic dissipation decreases
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