Class I Creep Deformation of Sn–Ag–Cu Containing Solid Solution Elements and Its Effect on Thermal Fatigue Life of Solder Joints

Abstract

The creep deformation mechanism of SnAgCu alloy doped with Bi and Sb (SACBiSb) alloys is theoretically and experimentally analyzed in order to clarify the effect of solid solution additives in SnAgCu alloys. The theoretical prediction results and test results are found to mostly agree with each other. The breakaway stress which is the stress at the transition from Class II to Class I in particular is finely reproduced in the theoretical prediction, with a stress of 25 MPa found both by the prediction and test results. In the stress range where the stress is higher than the breakaway stress, the creep strength of SACBiSb is higher than that of SnAgCu, while it was predicted that the creep strength of SnAgCu would be superior to that of SACBiSb in the range of stress lower than the breakaway stress. The thermal fatigue life of SACBiSb was predicted to be longer than that of SnAgCu in the temperature profile mainly used above the breakaway stress. However, in the temperature profile mainly used in the low-stress range, a reversal of the creep strength between SACBiSb and SnAgCu was predicted to occur and the loss of the superiority of SACBiSb in the thermal fatigue life was also predicted.

Effect of Bi, Sb, and Ti on Microstructure and Mechanical Properties of SAC105 Alloys

“…The dissolving of Bi or Sb into the Sn matrix could induce the lattice distortion in the crystalline structure which could act as an obstacle to impede the movement of dislocations. The impeding force could be represented using the following equations [34]:…”

“…The dissolving of Bi or Sb into the Sn matrix could induce the lattice distortion in the crystalline structure which could act as an obstacle to impede the movement of dislocations. The impeding force could be represented using the following equations [ 34 ]: where is the misfit strain, is the solute atomic radius, is the atomic radius in the matrix phase, is the maximum force of the obstacle acting on a dislocation, is the modulus of rigidity, and is the Burgers vector.…”

Microstructures and shear properties of Sb and In strengthened Sn5Bi/Cu joints

“…Masaki Moriuchi et al [20] rst theoretically investigated the solution strengthening mechanism of SAC doped with Bi and Sb. In the solution strengthening mechanism (we suppose that it is precipitate strengthening here), the solute atmospheres that are formed around solute atoms distort the crystalline structure and become obstacles to dislocations.…”

“…where, is the critical shear stress, C is the concentration of the solute atoms, w is a parameter that describes the range to which the interaction between the dislocations and the solute atoms extends, which is approximately 5b [20]. Thus, the critical resolved shear stress caused by In addition can be further expressed by Eq.…”

“…During thermomechanical fatigue, the β-Sn matrix recrystallization and IMC particles coarsening have tendency to occur at the locations where the strain is concentrated [18]. Thus, introducing solid solution strengthening into precipitation hardening tin based alloys can effectively improve the high temperature resistance of the β-Sn matrix [20,21]. For β-Sn matrix, solid solution strengthening can be achieved by Bi, Sb, In without obvious oxidation [4,15,22,23].…”