Fatigue crack-growth behavior of Sn-Ag-Cu and Sn-Ag-Cu-Bi lead-free solders

Abstract

INTRODUCTIONLead-free solder alloys are intended to completely substitute for traditional Pb-Sn compositions in conventional processes in the future. Among them, tinsilver eutectic solder is an attractive candidate alloy for meeting the requirements for demanding hightemperature service environments, such as automotive applications. 1 However, Sn-Ag eutectic has a higher melting temperature and poorer wettability compared to Sn-Pb eutectic. 2 To improve the properties and decrease the melting temperature, some additional elements, such as Bi and Cu, are added to the Sn-Ag eutectic.As surface-mounted devices have been used under thermomechanical-cycling conditions, mechanical deformation and cracking could develop in solder joints. Therefore, knowledge about their mechanical properties, in particular, fatigue deformation and fracture is required to ensure the reliability of the joints in electronic devices. 1,3-7 Although Bi additions can decrease the melting temperature, degradation of mechanical properties and fatigue resistance by over-addition of Bi above 3-5% has been reported. 5,6 Kariya and Otsuka 6 investigated the effect of Bi on the isothermal fatigue behavior of the Sn-Ag solder. They indicated that the fatigue life drastically decreased with increasing content of Bi, and this was attributed to the decrease of ductility. McCabe and Fine 7 investigated the creep properties of precipitation-strengthened Sn-based alloys and found that the additions of very small particles improved the creep resistance at moderate to high stress levels but could possibly adversely affect the diffusional-creep resistance of the materials at low stresses. However, very little is known about the fatigue crack-growth behavior of the Sn-Ag-Cu-Bi alloys.Because fatigue is a process of crack initiation and propagation, studies on fatigue-crack growth are essential for fully understanding the fatigue behavior of the solders. The results of the studies on Fatigue crack-growth behavior and mechanical properties of Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders have been investigated at room temperature (20°C). The tensile strength and hardness of the solders increased with increasing Bi content. However, the yield strengths of Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders were nearly similar, but the 3Bi solder exhibited the lowest ductility. Fatigue crack-growth behavior of the solders was dominantly cycle dependent in the range of stress ratios from 0.1-0.7 at a frequency of 10 Hz, except for the Sn-3Ag-0.5Cu solder tested at a stress ratio of 0.7. Mixed intergranular/transgranular crack propagation was observed for the Sn-3Ag-0.5Cu solder tested at the stress ratio of 0.7, indicating the importance of creep in crack growth. The Sn-3Ag-0.5Cu-1Bi and Sn-3Ag-0.5Cu-3Bi solders had higher resistance to time-dependent crack growth, resulting from the strengthening effect of the Bi constituent. It appears that the addition of Bi above a certain concentration is harmful to the mechanical properties of Sn3Ag-0.5Cu.

Influence of the Substrate on the Creep of SN Solder Joints

“…[1] Given the technological need, there is an increasing body of published research on the creep of high-Sn alloys. [2][3][4][5][6][7] However, most of this work describes the creep of the alloys in bulk form. Solders in microelectronic joints may have microstructures and, hence, mechanical properties that differ significantly from those found in bulk samples.…”

A review of photovoltaic module technologies for increased performance in tropical climate

“…This will increase the operational reliability of PV module interconnections especially in the tropics. It has also been reported that interconnection degradation contributes to increase in series resistance of PV modules which decreases their power output [126], [137], [138], [141], [145]. …”

“…Currently, there is a shift from lead-based alloys to lead-free alloys because of the environmental impacts of lead, globally [144], [65], [148], [149]. More and more lead-free solders are being developed to substitute lead based solders [141], [142], [150]- [152]. The tin-silver-copper (SnAgCu or SAC) solder alloys have been reported to be the best alternative for eutectic SnPb solder in the PV industry [144], [153].…”

Creep-Fatigue Crack Growth in Sn-3.0Ag-0.5Cu Solder

“…Since the use of lead solders in electronic devices is strictly restricted by the RoHS and WEEE legislation, most of lead solders have been replacing with lead-free solders. For seeking suitable lead-free solders, mechanical properties of lead-free solders have been actively investigated (3)- (12) as well as the other performance of the solders.…”