The effects of aging on the cyclic stress-strain behavior and hysteresis loop evolution of lead free solders

Abstract

Solder joints in electronic assemblies are typically subjected to thermal cycling, either in actual application or in accelerated life testing used for qualification. Mismatches in the thermal expansion coefficients of the assembly materials leads to the solder joints being subjected to cyclic (positive/negative) mechanical strains and stresses. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure. While the effects of aging on solder constitutive behavior (stress-strain and creep) have been examined in some detail, there have been no prior studies on the effects of aging on solder failure and fatigue behavior.In this investigation, we have examined the effects of several parameters (aging, temperature, strain/stress limits, and solder alloy composition) on the cyclic stress-strain behavior of lead free solders. Uniaxial SAC lead free solder specimens were subjected to cyclic (tension/compression) mechanical loading. Samples were cyclically loaded under both strain control (constant positive and negative strain limits) and stress control (constant positive and negative stress limits). The hysteresis loop size (area) was calculated from the measured cyclic stress-strain curves for a given solder alloy and temperature. This area represents the strain energy density dissipated per cycle, which can be typically correlated to the damage accumulation in the joint.Most tests in this investigation were performed with SAC105 solder alloy.However, the effect of solder composition was examined in a limited way by testing four SAC alloys (SAC105, SAC205, SAC305, SAC405) with varying silver content (1-4%) under strain controlled cycling. In addition, the effect of the testing temperature has also been studied by performing cyclic testing of SAC405 samples at four different temperatures (25, 50, 75, and 100 o C).Prior to cyclic loading, the specimens in this study were aged (preconditioned) at 125 o C for various aging times (0-6 months). From the recorded cyclic stress-strain curves, we have been able to characterize and empirically model the evolution of the solder hysteresis loops with aging. Similar to solder stress-strain and creep behaviors, there is a strong effect of aging on the hysteresis loop size (and thus the rate of damage accumulation) in the solder specimens. The observed degradations in the fatigue/cyclic behavior of the lead free solders are highly accelerated for lower silver content alloys (e.g., SAC105), and for aging and testing at higher temperatures. In our current work, we are also subjecting aged solder samples to cyclic loading until failure occurs.Our ultimate goal is to understand the effects of aging on the thermomechanical fatigue life.

Effects of moisture exposure on the mechanical behavior of flip chip underfills in microelectronic packaging

“…The glass transition region for T > +100 C is clearly evident. In our prior work [24][25]32], we have found that an empirical four-parameter hyperbolic tangent model can be used to accurately model the observed nonlinear stress-strain data at any temperature. The general representation of this relation is…”

Evolution of the tension/compression and shear cyclic stress-strain behavior of lead-free solder subjected to isothermal aging

“…The excellent representation provided by the elastic-plastic empirical model suggests that it indeed provides a mathematical description of a suitable "average" stress-strain curve for a set of experimental curves measured under fixed test conditions. We have extensive stress-strain data for SAC lead free and Sn-Pb Solders [32][33][34][35][36][37], and have demonstrated the model in eq. (3) works extremely well for all alloys.…”

“…These visible reductions in cross-sectional area lead to nonuniform stress-states in the specimen and drops in the applied loading near the end of the stress-strain curve. Although, several different empirical models can be used to represent the observed stress-strain data for solder, we have chosen to use four parameter hyperbolic tangent model [37] )…”

Nanomechanical characterization of SAC solder joints - reduction of aging effects using microalloy additions

“…Mustafa, et al [7] have studied the aging-induced changes occurring in the cyclic stress-strain behavior of lead free SAC solders for both tension/compression and shear loadings. Using the developed approaches, the influence of aging on the fatigue life of lead free alloys was also examined [8].…”

“…Aging effects on the constitutive and failure behaviors of lead free solders have been studied extensively by the authors and their coworkers [2][3][4][5][6][7][8][9][10][11][12][13][14]. In early investigations, the mechanical properties and creep behavior of SAC alloys were shown to be severely degraded by prior exposure to room temperature (25 C) and elevated temperature (50, 75, 100, and 125 C) aging [2][3][4][5][6].…”