Reliability of Doped SnAgCu Solder Alloys with Various Surface Finishes Under Realistic Service Conditions
Type of DegreePhD Dissertation
Industrial and Systems Engineering
MetadataShow full item record
The electronic packaging industry has moved from eutectic Sn-Pb solder materials to near-eutectic SnAgCu (SAC) solder materials in the past decade because of the increasing awareness of health and safety concerns associated with the use of Lead (Pb). The reliability performances for SAC solder materials under thermal and isothermal aging conditions have been extensively studied and results show aging is universally detrimental to the solder joints reliability, which leads to a large degradation of mechanical properties including tensile strength, shear strength, and fatigue behaviors. Moreover, solder joints in realistic applications are typically exposed to cyclic loading, either through thermal cycling or mechanical cycling. Numerous solutions have been proposed to mitigate the aging-induced or cyclic-induced mechanical properties degradation. One of the possible solutions is to develop “next generation” solder alloys with additional elements mixed with SAC solder alloys. This process is called solder doping. Common solder dopants include nickel (Ni), bismuth (Bi), antimony (Sb), and indium (In). Solder doping was observed to have a substantial influence on mechanical properties of solder joints, such as an increase of solder ultimate tensile strength and shear strength. However, as bulk sample has been used in decades in testing the mechanical properties of solder materials, studies have revealed that mechanical properties exacted from bulk sample may not be the real reflection of solder joints implemented in the realistic application, since the solder joint was proved to demonstrate a much more complex structure than bulk sample. Therefore, in this study, solder joints assembled on the PCB were used directly as test specimens, where several important factors that may have critical impacts on solder joint reliability can be tested, for example, intermetallic compound (IMC) and surface plating (surface finish). The purpose of this dissertation is to study several impact factors (aging, surface finish, solder doping, IMC) on the reliability of solder joints under realistic applications. In this study, 5 commercially available doped solder alloys including SAC305 (Sn-3.0%Ag-0.5%Cu) and SAC105 (Sn-1.0%Ag-0.5%Cu) solder alloys were used as test samples. Three types of surface finishes were applied on the test vehicles, which were Organic Solderability Preservative (OSP), Immersion Silver (ImAg), and Electroless Nickle Immersion Gold (ENIG). The first part of the research investigates the effect of long-term room temperature aging on the fatigue behaviors of SAC305, SAC105, and various doped solder joints. Prior to the assessment, all the specimens are exposed to room temperature (25°C) for 4 years. Test specimens were then subjected to cyclic stress-strain loading under these two aging conditions, 0 aging and 4 years of aging. Failure data is carefully collected and analyzed using a Weibull distribution to study the effect of room temperature aging on the reliability of solder alloys. Results show that 4 years’ room temperature aging leads to a significant degradation on the fatigue properties (fatigue life and energy dissipation per cycle) and solder alloy doped with Bi exhibit better fatigue resistance than SAC solder alloys. In the second part of this research, different surface finishes and solder doping alternatives are studied under shear testing to examine their impacts on the shear properties of solder joints under realistic applications. The solder joint under each condition is tested using an innovative test schematic (cyclic shear testing) and ultimate shear strength and shear energy (area under the shear curve) are recorded within each condition. Results demonstrate that solder joint with higher Bi and Ag content demonstrates larger shear strength compared to SAC305 solder joint. The OSP surface finish demonstrates the largest shear strength, followed by ImAg, with ENIG demonstrating the least shear strength, regardless of solder alloy types. Ductile failure is the most prevalent failure mechanism during the shear testing, however, brittle failure is detected as well where the fracture occurred at the interfacial area between the Ni layer and bulk solder. Finally, the effects of surface finish and doped solder alloy are investigated under cyclic shear testing to study the interaction effects of both impact factors on the reliability of solder joint under realistic applications. Results reveal that solder alloys with high Ag and Bi content demonstrate higher strength, therefore, the fatigue resistance under cyclic shear testing is improved. However, the same doping option also increases the brittleness of the solder alloy, where increases the risk of brittle failure, especially when combing with ENIG surface finish.