AN OBSTACLE-CONTROLLED CREEP MODEL FOR SN-PB AND SN-BASED LEAD-FREE SOLDERSAuthor: Jean-Paul Clech
Company: EPSI Inc.
Date Published: 9/26/2004 Conference: SMTA International
Lead-free, Sn-based solders are amenable to such models since dispersed intermetallics, grain boundaries, the lattice of the tin matrix itself and other dislocations, are all obstacles that impede dislocation motions. The basic single-cell, obstacle-controlled creep model, after Frost and Ashby (1982), features a stress-dependent activation energy and an athermal flow strength parameter which represents the maximum flow strength of the material at the absolute zero (0 Kelvin). Depending on how many creep mechanisms have been identified, the solder creep model is formulated as a single- or double-cell model. In the latter case, creep rates for the two mechanisms are additive. For each alloy, one set of steady-state creep measurements is used to determine the scaling constants and the physical parameters of the model (4 or 8 constants in total for the single- or two-cell models, respectively). Each model is then tested against independent test results (as many as nine test cases for the Sn3.5Ag alloy).
The obstacle-controlled, solder creep models allow for the bridging of tension, compression and shear test results as well as that of creep, strength and stress relaxation data, often without, and sometimes with the use of a simple, multiplicative calibration factor. The models also allow for the prediction of solder joint stress/strain measurements during thermal cycling of soldered assemblies. The need for calibration factors suggests that creep models derived from a given mechanical test, and specimen type or size, should not be used without justification in the stress/strain analysis of soldered assemblies. Model calibration and validation is a critical step in the application of creep models to stress analysis or reliability models.
The use of obstacle-controlled creep models resolves many anomalies observed in the classical analysis of leadfree solder creep data, including activation energies and power-law exponents that were found to be stress- and/or temperature-dependent. In accord with experts who warned against the use of power-law and hyperbolic sine creep models for engineering metals, we recommend caution when using such models for stress or reliability analysis of lead-free assemblies. As demonstrated by bouncing the models in this paper against many independent datasets, obstacle-controlled creep models offer a promising alternative.
Using the two-cell creep models, creep contour charts were generated to quantify the contribution of competing creep mechanisms to the total creep rates. The patterns of creep contour lines are quite different for Sn37Pb and Sn3.8Ag0.7Cu, a reflection of vastly different creep mechanisms. The Sn3.Ag0.7Cu creep contour chart suggests a transition from one mechanism to another that is highly temperature related. The transition occurs at about 75°C, in agreement with microstructural and creep rate analysis conducted by Vianco et al. (2004) on Sn3.9Ag0.6Cu solder.
Key words: SnPb, SnBi, SnAg, SnCu, SnAgCu, lead-free solder, obstacle-controlled creep models, stress dependent activation energy, creep mechanism contour charts.
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