Solder microstructures

The 150 °C (302 °F) 24 h anneaUng treatment was more effective at stabilizing the solder microstructure over the entire test temperature regime. A relatively good correlation was obtained with a single sinh law equation for dddt, (Eq 23) ... 

Tin-Lead and Lead-Free Solder Microstructure after Reflow Cycles... 

The characteristics of the nonequilibrium, Pb-Sn solder microstructure are threefold. First, the compositions of the phases are not accurately represented by the phase diagram boundary lines. Second, the relative quantities of the phases are not accurately described by the lever rule. Third, the spatial distribution of phases in the microstructure are sensitive to the cooling rate. For example, in terms of individual phase compositions, excessive Sn may be retained in the Pb-rich a phase, causing a supersaturation condition. In effect, the phase boundary line between the a single-phase field and the (a -I- p) two-phase field is shifted to the right. (A more detailed discussion of approximating a nonequilibrium microstructure from the equilibrium phase diagram can be found in Refs. 5 and 7.) A consequence to the solder microstructure caused by a supersaturated Pb-rich phase is the precipitation of Sn particles in the Pb-rich phase. 

FIG. 4 Optical micrograph of the 15Pb-8 5Sn solder microstructure. The cooling rate used to produce this micro structure was 10°C/min. (Courtesy of Sandia National Laboratories.)... 

Recent advances in the development of TMF predictive models have included the incorporation of solder microstructure as a state variable in the constitutive equation [92,93]. A contour map (Fig. 30) shows the Pb-rich phase coarsening predicted in a leadless chip solder joint subjected to six thermal cycles having temperature limits of —50° and 80°C, ramp rates of 6°C/ sec, and dwell times of 10 min at both limits. The microstructure-based, viscoplastic constitutive equation for Pb-Sn solder improves the accuracy of solder fatigue strain predictions throughout the interconnection geometry. It provides a real-time adjustment of the local solder mechanical properties resulting from local changes in the solder microstructure (i.e., the Pb-rich phase). 

Vianco, P.T. Burchett, S.N. Neilsen, M.K. Rejent, J.A. Erear, D.R. Coarsening of the Sn-Pb solder microstructure in constitutive model-based predictions of solder joint thermal mechanical fatigue. J. Electron. Mater. 1999, 28, 1288-1294. 

Sn-Ag-Cu solder/Cu and Sn-Ag-Cu solder/Ni interfaces and the associated evolution of the solder microstructure. J. Electron. Mater. 2001, 30, 1157. 

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