Properties of Ternary Sn-Ag-Bi Solder Alloys f Part l- Thermal Properties and Microstructural Analysis

摘要

Bismuth additions of 1/100 to 10/100 were made to the 96.5Sn-3.5Ag (wt./100) alloy in an effort to develop a Sn-Ag-Bi ternary composition. A DSC evaluation of the melting properties of the 91.84Sn-3.33Ag-4.83Bi composition suggested the appearance of metastable, short-range order in the atomic structure as a result of low temperature, thermal aging. More extensive solid-state aging studies on 91.84Sn-3.33Ag-4.83Bi/Cu couples resulted in the growth of an intermetallic compound layer at the solder/substrate interface comprised of Cu_3Sn and the Cu_6Sn_5 sub-layers. The growth kinetics of the total layer thickness (x) as a function of solid-state aging time (t) and temperature (T) were represented by the following expression: x - x_o = A t_n exp(-Q/RT) where x_o = 0.57 x 10_6 m; A = 6.22 x 10--3 m/s~n; n = 0.46 ± 0.15; and Q = 49± l8 kJ mol. TEM analysis of the 91.84Sn-3.33Ag-4.83Bi composition indicated that solid-solution and precipitation strengthening mechanisms were a likely conse- quence of the Bi additions. Contact angle measurements, Cu/solder/Cu solder joint shear strength tests, and microhardness evaluations were also performed on the Sn-Ag-Bi alloys; those results are reported in Part II.