Constitutive Behavior of Sn3.8Ag0.7Cu and Sn1.0Ag0.5Cu Alloys at Creep and Low Strain Rate Regimes

Abstract

Constitutive models for SnAgCu solder alloys are of great interest at the present. Commonly, constitutive models that have been successfully used in the past for Sn-Pb solders are used to describe the behavior of SnAgCu solder alloys. Two issues in the modeling of lead-free solders demand careful attention: 1) Lead-free solders show significantly different creep strain evolution with time, stress and temperature, and the assumption of evolution to steady state creep nearly instantaneously may not be valid in SnAgCu alloys and 2) Models derived from bulk sample test data may not be reliable when predicting deformation behavior at the solder interconnection level for lead-free solders due to the differences in the inherent microstructures at these different scales. In addition, the building of valid constitutive models from test data derived from tests on solder joints must de-convolute the effects of joint geometry and its influence on stress heterogeneity. Such issues have often received insufficient attention in prior constitutive modeling efforts. In this study all of the above issues are addressed in developing constitutive models of Sn3.8Ag0.7Cu and Sn1.0Ag0.5Cu solder alloys, which represent the extremes of Ag composition that have been mooted at the present time. The results of monotonic testing are reported for strain rates ranging from 4.02E-6 to 2.40E-3 s 1 . The creep behavior at stress levels ranging from 7.8 to 52 MPa is also described. Both types of tests were performed at temperatures of 25 C, 75 C and 125 C. The popular Anand model and the classical time-hardening creep model are fit to the data, and the experimentally obtained model parameters are reported. The test data are compared against other reported data in the literature and conclusions are drawn on the plausible sources of error in the data reported in the prior literature.

Constitutive Behavior of Mixed Sn-Pb/Sn-3.0Ag-0.5Cu Solder Alloys

“…In previous work by (among others) Darveaux and Bannerji 1 (on Sn-Pb), Bhate et al 2 (on Sn-3.8Ag-0.7Cu and Sn-1.0Ag-0.5Cu), and Mysore et al 3 (on Sn-3.0Ag-0.5Cu), constitutive model fits from such experimental data were presented. Towashirporn et al 4 utilized the constitutive model together with a novel failure theory to develop experimentally validated fatigue crack propagation simulations in these ductile alloys undergoing significant plastic deformation.…”

Maximum-Entropy Principle for Modeling Damage and Fracture in Solder Joints

“…The necessity of measuring the displacement at the solder interconnect was demonstrated in Ref. 23. The microscale mechanical tester is shown in Fig.…”

“…This profiles mimics the types of strains imposed on solder joints by the substrate and printed circuit board during accelerated thermal cycling. Accurate constitutive behavior of the alloy was available from prior testing of identical samples on an Instron 5848 microtester, 23 and therefore, the total cyclic inelastic dissipation over the course of testing was determined using the constitutive model.…”

“…A time-hardening creep model described in Ref. 23 was used in the finiteelement models to capture the rate-dependent behavior. The use of the time-hardening creep model is not common, but is critical since it captures the time-evolving primary creep behavior that is observed to be of significant duration in Sn-Ag-Cu alloys.…”

“…The rate-independent material parameters were also input to the finite-element model based on experimentally obtained data reported in Ref. 23. The die was given the thermomechanical properties of silicon.…”