Fracture of Sn-Ag-Cu Solder Joints on Cu Substrates: I. Effects of Loading and Processing Conditions

Abstract

During service, microcracks form inside solder joints, making microelectronic packages highly prone to failure on dropping. Hence, the fracture behavior of solder joints under drop conditions at high strain rates and under mixed-mode conditions is a critically important design consideration for robust joints. This study reports on the effects of joint processing and loading conditions on the microstructure and fracture response of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates. The impact of parameters which control the microstructure (reflow condition, aging) as well as loading conditions (strain rate and loading angle) are explicitly studied. A methodology based on the calculation of the critical energy release rate, G C , using compact mixed-mode (CMM) samples was developed to quantify the fracture toughness of the joints under conditions of adhesive (i.e., interface-related) fracture. In general, higher strain rate and increased mode-mixity resulted in decreased G C . G C also decreased with increasing dwell time at reflow temperature, which produced a thicker intermetallic layer at the solder-substrate interface. Softer solders, produced by slower cooling following reflow, or post-reflow aging, showed enhanced G C . The sensitivity of the fracture toughness to all of the aforementioned parameters reduced with an increase in the mode-mixity. Fracture mechanisms, elucidating the effects of the loading conditions and process parameters, are briefly highlighted.

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.…”

“…Several reports [4][5][6][7][8][9][10][11][12] addressing the fracture behavior of solder joints are available in the literature. However, until recently, [10][11][12] mixed-mode fracture of solder joints under high-strain-rate conditions, which is applicable to failure of electronic solders due to accidental drops, had not been studied.…”

“…However, until recently, [10][11][12] mixed-mode fracture of solder joints under high-strain-rate conditions, which is applicable to failure of electronic solders due to accidental drops, had not been studied. Recent work by the present authors reported on the development of fracture mechanism maps (FMMs) for Sn-Ag-Cu solder joints on Cu substrates, 11 where the various operative fracture mechanisms are represented in a two-dimensional space described by an effective solder yield strength on the ordinate and an effective interfacial intermetallic compound (IMC) thickness on the abscissa.…”

“…In previous work, [10][11][12] we presented detailed analysis of the fracture behavior of Sn-3.8Ag-0.7Cu (SAC387) solder joints attached to Cu substrates under both quasistatic and dynamic loading conditions (strain range of 0.01 s À1 to 100 s…”

“…For the same IMC thickness, a smoother IMC layer, such as that obtained after aging, increases G C . The lower G C for thick and rough IMCs is attributable to the 12 The interfacial crack was produced by depositing a 500 nm thick film of Al, which is not wetted by the solder. Details are available in Ref.…”