Deformation Behavior of Sn-3.8Ag-0.7Cu Solder at Intermediate Strain Rates: Effect of Microstructure and Test Conditions

Abstract

When mobile electronic devices drop during service, solder interconnects are loaded under intermediate to high strain rates. Therefore, the strain response of solders at elevated strain rates is critical to reliability prediction. This paper presents the plastic flow behavior of Sn-3.8Ag-0.7Cu solder under compression over strain rates ranging from 0.1 s -1 to 30 s -1 at several different temperatures and under various aging conditions. Both yield strength and work hardening rate were observed to increase substantially with increasing strain rate, with the strain rate sensitivity at higher temperatures being greater. Empirical expressions capturing the strain rate and temperature dependence of the yield and work hardening parameters are presented. Microstructural observations revealed greater strain localization following testing at higher strain rates, with the development of distinct flow patterns and localized kinking of dendrites due to twin formation. Low-temperature aging (35°C) appeared to enhance yield strength slightly relative to the as-reflowed condition, but decreased the work hardening rate. With aging at higher temperatures (100°C and 180°C), and commensurate coarsening of the precipitate structure, both yield strength and work hardening rate decreased dramatically.

Incorporation of Interfacial Intermetallic Morphology in Fracture Mechanism Map for Sn-Ag-Cu Solder Joints

“…4,12,15 These strain concentrations arise due to misfit strains induced via elastic-plastic mismatch between the solder and the IMC layer. 10,12,13 In addition, strain localization within the solder immediately adjacent to and parallel to the interface can cause stress concentrations in the scallops, enhancing the propensity for IMC fracture. 10 Therefore, a rougher IMC layer forces crack propagation either along the solder-IMC interface via microvoid nucleation and growth or through the IMC layer and/or the IMC scallops; all of these lead to low fracture toughness.…”

“…The definition of the effective yield strength used for the FMMs is consistent with a number of reports showing a power-law dependence of solder yield strength on strain rate. 13,14 The formulation of t eff , on the other hand, was first proposed in Ref. 11, and the functional dependence of the fracture path on t eff was qualitatively validated based on observations on the solder-Cu system.…”

On the Mutual Effect of Viscoplasticity and Interfacial Damage Progression in Interfacial Fracture of Lead-Free Solder Joints

“…2 Failure of the solder joint can mainly take place in two different modes: first, due to viscoplastic deformation accumulation and final failure in the bulk solder, and second, interfacial damage and crack growth causing fracture along the interface of the solder and the substrate. [4][5][6][7][8][9] Several experimental and theoretical investigations on the creep properties and viscoplastic nature of lead-free solders have led to good insight into this subject. 4,5,[10][11][12][13] However, comparison of the experimental results obtained in various studies shows considerable disagreement in several cases.…”

“…[4][5][6][7][8][9] Several experimental and theoretical investigations on the creep properties and viscoplastic nature of lead-free solders have led to good insight into this subject. 4,5,[10][11][12][13] However, comparison of the experimental results obtained in various studies shows considerable disagreement in several cases. Indeed, a review of identified constitutive parameters related to the hyperbolic sine model in different studies confirms the existence of wide scatter in the creep property database of the solders.…”

“…9,[14][15][16] In addition to the solder strength and IMC thickness, some other factors such as the geometry of the joint, working temperature, microstructural evolution, roughness of the interface, and the amount of voids can affect the extent of the failure mode. 5,15,[17][18][19] The mechanics of the interfacial failure of solder joints were considered in several studies. 14,[20][21][22][23][24][25][26][27] Hayes et al 14 experimentally estimated the mode I fracture toughness of solder joints by performing high-strain-rate fracture tests on modified compact tension specimens.…”