Reduction of Lead Free Solder Aging Effects Using Doped SAC Alloys

Abstract

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. Large degradations that occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging have been demonstrated in the past several years. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging are relatively small in conventional Sn-Pb solders. In an attempt to reduce the aging induced degradation of the material behavior of SAC solders, several doped SAC-X alloys have been explored and studied. The doped materials are lead free SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ge, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, Fe, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and this approach has been extended to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. In this research, four popular doped lead free solder alloys, including SACX0307 (SAC-X, where X is 0.1%Bi), SAC-Zn (0.21%Zn), SN100C (0.05%Ni + 0.01%Ge) and SN96CI (0.05%Co), have been scrutinized. Also, the enhancement of aging resistance of doped lead free solders has been explored when compared to corresponding reference SAC alloys (SAC105, SAC205, SAC3595, Sn-0.7Cu, SAC3810). The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0-12 months) at room temperature (25 oC), and several elevated temperatures (50, 75, 100, and 125 oC). For all of the solders, variations of the mechanical and creep properties (effective modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. The doped SAC-X alloys illustrated reduced degradations with aging for all of the aging temperatures considered. Also, the stress-strain and creep mechanical properties of doped solders are better than those of reference solders after short durations of aging. After long term aging, doped solder alloys were found to have more stable behaviors than those of the standard SAC alloys. A parallel microstructure study has shown that aging effects have significant influence on phase coarsening and degradation, grain/sub-grain growth and internal residual stress relaxation. However, the changes in microstructure have been demonstrated to be smaller in doped solder materials when compared to non-doped solders after severe aging.