Microstructural effect on the creep strength of a Sn-3.5%Ag solder alloy

Abstract

INTRODUCTIONAs discussed by Morris and Reynold, 1 there is a large possibility that important mechanical properties, such as the creep strength of eutectic solder alloys, can change by orders of magnitude if the microstructure is altered. Morris et al. have performed vigorous research work 2-5 on the effect of microstructure, particularly the grain size effect, on the mechanical properties of Sn-Pb eutectic solder alloys and solder joints. Their studies show that, as the cooling rate during solidification increases, the microstructure of the solder alloys changes from the lamellar type typical of slow cooling to a more equiaxed and refined structure. 4 Such microstructure induces the superplastic creep, 2,5 reduces the tensile strength at relatively slow strain rate and high temperature, 3 and increases the fatigue cycle at 50% load drop by enhancing the grain boundary deformation mechanism. 4 The decreasing of yield strength and Vickers hardness or the increasing of fatigue life with the reducing of grain size caused by the increased cooling rate during solidification are also reported by Rack and Maurin, 6 Sutliff and Notis, 7 and Wild, 8 respectively. These studies, however, have been made on the very-fine-grained alloys, where superplasticity occurs.On the other hand, there are few studies that investigated the microstructural effect on the mechanical properties of Pb-free solder alloys. Recently, Kim et al. 9 investigated the effect of microstructure on the ultimate tensile strength (UTS), 0.2% proof strength, and elongation of Sn-3.0 to 3.9%Ag-0.5 to 0.7%Cu alloys at room temperature. The microstructure was altered by changing the cooling rate during solidification of the alloys. Kim et al. concluded that slow cooling decreased the UTS and 0.2% proof strength because of the coarsened eutectic structure and the formation of large primary Ag 3 Sn precipitates.However, to the authors' knowledge there are no studies on the effect of microstructure on the creep strength of Sn-3.5%Ag alloy. In the present research, the effect of microstructure on the creep-rupture strength of Sn-3.5%Ag alloy has been investigated after altering the microstructure by adopting different solidification and cooling rates of the alloy cast into the mold.The effect of microstructure obtained by rapid or slow solidification and cooling of a Sn-3.5%Ag lead-free solder alloy on the creep strength has been investigated. The rapidly cooled alloy showed that the microstructure consisted of the primarily crystallized Sn phase and the quasi-eutectic phase, where fine Ag 3 Sn particles dispersed in the Sn matrix. In the slowly cooled alloy, large platelets of Ag 3 Sn were formed sparsely in the Sn matrix. A difference of about 2.5 orders of magnitude in the cooling rate translates to about 1.5 orders of magnitude in the creep-rupture time. Accordingly, fine particle dispersion of Ag 3 Sn is considered to be very beneficial for the restraining of creep deformation, that is, for the decreasing of creep rate of the Sn-3.5%Ag alloy, compared with...

In Situ Observation of Small-Scale Deformation in a Lead-Free Solder Alloy

“…This shows that IMC particles at grain boundaries can severely limit boundary behaviors (GBS/GBD), as pointed out by previous studies. 18,19 The optical microscopy observations of the eutectic area yield similar results. In Fig.…”

“…17 Such a fine dispersion of hard spherical IMC particles can enhance the creep resistance of the alloy by hindering the gliding of dislocations (dispersion hardening). 5,18 In order to observe the local deformation mechanism(s) of the SAC alloy, Vickers indentation was performed. Figure 4a shows an SEM image of a Vickers indent in the primary b-Sn region with ledges forming around it.…”

“…dispersion hardening. 5,18,19 Some zigzag paths can be identified on Fig. 9b (illustrated by the arrows).…”

Tensile creep and microstructural characterization of bulk Sn3.9Ag0.6Cu lead-free solder

“…1-3 A number of papers have been published on Sn-Ag binary-alloy creep behavior measured with bulk samples. [4][5][6][7][8][9][10] For example, Huang et al 3 has shown that steady-state creep rates are controlled by dislocation-pipe diffusion in the tin matrix in a Sn-3.5Ag alloy. Choi et al showed that aging affects the secondary creep rates.…”

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimen

“…The present authors have also performed research on the creep and rupture behaviors of Cu wire/solder-joint specimens of Sn-3.5% Ag and Sn-0.5% Cu alloys and compared the results with those of the test specimens cut from the same alloy ingots. [5][6][7] …”