Rajen S. Sidhu

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

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, GC, 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 GC. GC 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 GC. 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.

High Strain Rate Fracture Behavior of Sn-Ag-Cu Solder Joints on Cu Substrates

During service, micro-cracks form inside solder joints, making a microelectronic package prone to failure particularly during a drop. Hence, the understanding of the fracture behavior of solder joints under drop conditions, synonymously at high strain rates and in mixed mode, is critically important. This study reports: (i) the effects of processing conditions (reflow parameters and aging) on the microstructure and fracture behavior of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates, and (ii) the effects of the loading conditions (strain rate and loading angle) on the fracture toughness of these joints, especially at high strain rates. A methodology for calculating critical energy release rate, GC , was employed to quantify the fracture toughness of the joints. Two parameters, (i) effective thickness of the interfacial intermetallic compounds (IMC) layer, which is proportional to the product of the thickness and the roughness of the IMC layer, and (ii) yield strength of the solder, which depends on the solder microstructure and the loading rate, were identified as the dominant quantities affecting the fracture behavior of the solder joints. The fracture toughness of the solder joint decreased with an increase in the effective thickness of the IMC layer and the yield strength of the solder. A 2-dimensional fracture mechanism map with the effective thickness of the IMC layer and the yield strength of the solder as two axes and the fracture toughness as well as the fraction of different fracture paths as contour-lines was prepared. Trends in the fracture toughness of the solder joints and their correlation with the fracture modes are explained using the fracture mechanism map.Copyright

Fracture mechanism map for the fracture of microelectronic Pb-free solder joints under dynamic loading conditions

Solder joints, which serve as mechanical and electrical interconnects in a package, are particularly prone to failure during a drop. Hence, the fracture behavior of solders at high strain rates and in mixed mode is a critically important design parameter. This study reports the effects of (a) loading conditions (strain rate and loading angle), (b) reflow parameters (dwell time and cooling rate), and (c) post-reflow aging on the mixed mode fracture toughness of a lead-free solder (Sn-3.8%Ag-0.7%Cu)/Cu joint. A methodology based on the calculation of critical energy release rate, GC, which is equal to the fracture toughness of a material under limited plasticity condition, was employed. An increase in the strain rate results in limited plasticity ahead of the crack tip leading to a reduction in the fracture toughness of the solder joints. Fracture toughness also decreases with increasing mode-mixity (up to a loading angle of 75°). A slower cooling rate increases the fracture toughness whereas a longer dwell time adversely affects it. Also, aged samples show higher GC value. A fracture mechanism map is developed to describe the correlation between the yield strength of the solder, which depends on the solder microstructure and the loading rate, the IMC morphology, which depends on the reflow conditions and aging, and the fracture toughness of the solder joint.