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A microstructurally-based computational simulation is presented that predicts the behavior and lifetime of solder interconnects for electronic applications. This finite element simulation is based on an internal state variable constitutive model that captures both creep and plasticity, and accounts for microstructural evolution. The basis of the microstructural evolution is a simple model that captures the grain size and microstructural defects in the solder. The mechanical behavior of the solder is incorporated into the model in the form of time-dependent, viscoplastic equations derived from experimental creep tests. The unique aspect of this methodology is that the constants in the constitutive relations of the model are determined from experimental tests. This paper presents the constitutive relations and the experimental means by which the constants in the equations are determined. The fatigue lifetime of the solder interconnects is predicted using a damage paraneter (or grain size) that is an output of the computer simulation. This damage parameter methodology is discussed and experimentally validated. I NTRO DU CTlO Ndirectly related to the lifetime of the solder interconnects in the package. The solder interconnect is no longer simply an electrical conductor but is also the structural material that holds the package together. The importance of the reliability of solder interconnects increases as the trend in the electronics industry moves toward surface mount technology, smaller joints, and finer pitch. One of the serious challenges to solder joint reliability is failures that are a result of thermomechanical fatigue. In an electronic package, the solder is typically constrained between two materials with different coefficients of thermal expansion. Fatigue failures originate as cyclic strain is applied to solder joints when the package is exposed to thermal fluctuations caused by either ambient temperature changes or by heat dissipation from the integrated circuit devices in the package. A thermal fatigue failure of a solder interconnect is shown in Figure 1. The long term reliability of electronic systems is oftenThe solder typically used for electronic applications is the near-eutectic 60Sn40Pb alloy. The metallurgy and time-strain-temperature behavior of this solder alloy is Hli l!MSTf?lf3IJTION OF THIS DOCUMENT I S U N L I M m surprisingly complex and is n predicting the behavior and In the as-soldered condition, the Sn-Pb alloy has a two phase microstructure. This structure is metastable due to the high interfacial energy that results from the large number of phase boundaries present. Under thermomechanical fatigue conditions, this microstructure evolves into a heterogeneously coarsened band where all subsequently applied strain is concentrated. The evolution of this heterogeneous coarsening is shown in Figure 2 where the initial, as-solidified, microstructure can be compared to the heterogeneously coarsened structure in a 60Sn-40Pb solder joint. It is clearly important that the life predi...

An expression for the coarsening rate of the Pb-rich phase particles was determined through isothermal aging experiments and comparative literature data as:where LOand k are the initial and final mean Pb-rich particle diameters, respectively (mm); T is temperature ("K); t is time (s); and dy/dt is the strain rate (s-l). The phase coarsening behavior showed good agreement with previous literature data from isothermal aging experiments. The power-law exponent, p, for the Pb-rich phase size coarsening kinetics:~~-~o~= t increased from a value of 3.3 at the low aging temperature regime (70°C-100°C) to a value of 5.1 at the high temperature regime (135°C-170"C), suggesting that the number of short-circuit diffusion paths had increased with further aging. This expression provides an important basis for the rnicrostructurally-based, constitutive equation used in the visco-plastic model for TNIF in Sn-Pb solder. The revised visco-plastic model was exercised using a through-hole solder joint configuration. Initial data indicate a satisfactory compatibility between the constitutive equation and the coarsening expression.

No abstract

This paper summarizes the results of a program to construct an internal variable viscoplastic damage model to characterize 95.5Sn–3.9Ag–0.6Cu (wt.%) lead-free solder under cyclic thermomechanical loading conditions. A unified model is enhanced to account for a deteriorating microstructure through the use of an isotropic damage evolution equation. Model predictions versus experimental data are given for constant strain-rate tests that were conducted at strain rates of 4.2×10−5s−1 and 8.3×10−4s−1 over a temperature range from −25°Cto160°C; cyclic shear tests; and elevated-temperature creep tests. A description is given of how this work supports larger ongoing efforts to develop a predictive capability in materials aging and reliability, and solder interconnect reliability.