Please consider attending this seminar: MATERIALS ENGINEERING SEMINAR "Phase Transformations in Multicomponent Systems for Solder Joint Reliability" By Alyssa Yaeger Purdue MSE Ph.D. Final Exam Advisors: Professor Carol Handwerker and Professor John Blendell ABSTRACT Solder joints serve as the physical and electrical connection between surface mounted components and circuit boards, and are expected to maintain that connection under stress. Shear stresses due to coefficient of thermal expansion (CTE) mismatch between component and circuit board are common during use, and can lead to fracture of the solder joint. The cracks begin near the solder-substrate interface and propagate through the joint, either through the bulk of the solder or along the interfaces of brittle phases. Therefore, the morphology of the interfacial intermetallic compounds (IMCs) can influence the crack path and whether the failure is ductile or brittle, and thus will determine the lifetime and reliability of the solder joint. Gold embrittlement has been a soldering issue for decades, although it is a broad category which indicates brittle failure due to the presence of gold, typically forming AuSn4. Mitigation of this has included the reduction of Au in the solder joint and the introduction of Cu to destabilize AuSn4-type intermetallics with the formation of (Cu,Ni,Au)6Sn5. A limit of 3wt% Au per solder joint attempted to mitigate this issue, but the methods of gold removal are not precise, and 3% is not a hard limit below which gold embrittlement does not occur. Nickel is able to substitute for Au in AuSn4, forming (Au,Ni)Sn4 in higher volumes at lower gold concentrations, making the establishment of a limit for Au in the joint more difficult. Two methods of establishing these limits are presented in this thesis: isothermal annealing, and thermal cycling. Microstructures evolve over time, even during periods of storage prior to use. Isothermal annealing above storage temperature and below the solidus promotes faster diffusion than seen in practice, due to the temperature dependence of solid state diffusion. This is also known as accelerated aging, as the time and temperature during annealing can be correlated to a different time and temperature during storage. Isothermal annealing allows potential issues, such as gold embrittlement, to be identified more quickly and prior to production, and can give an estimate of product lifetime. For Au-containing solder, Au from the bulk diffuses to the Ni interface to form (Au,Ni)Sn4 during annealing, which causes brittle failure when it forms a thick, continuous layer. Thermal cycling is another method of accelerated testing which can determine the limits of gold embrittlement. Solder joints undergo thermal cycling during product use, and this method increases the cycle frequency to induce earlier failure. This failure is determined by an increase in electrical resistance across solder joints. Failed solder joints can then be cross sectioned to examine the microstructure and crack path to determine cause of failure. Leadless ceramic chip carriers are stiff components with a low standoff height which tends to concentrate thermal expansion mismatch induced stresses in the solder and have thick Au coating, which together make them a good candidate for studies of gold embrittlement. In this thesis, it was determined that these solder joints failed in a ductile manner, determined more by the component characteristics and not as a result of gold embrittlement. A potential mitigation method for gold embrittlement in eutectic Sn-Pb solder joints is also presented in this thesis. Because one cause of gold embrittlement includes the thick, continuous layer of (Au,Ni)Sn4,it has been proposed that breakup of this layer can increase ductility and solder joint lifetime. This can be accomplished with a secondary reflow, or melting, of the solder, using the high solubility of Au in molten solder to destabilize the (Au,Ni)Sn4 and return the solder joint to a more ductile microstructure. The thermodynamics of and mechanisms leading to the destabilization of (Au,Ni)Sn4 are quantified as a function of Au concentration in the solder. The relationship between the solder compositions and microstructure, including interfacial intermetallics, and the fracture behavior of the resulting joints provides insight into the appropriate limits of Au in Sn-Pb solder joints to reduce Au embrittlement. . Date: Monday, November 22, 2021 Time: 9:00 A.M. Place: HAMP 2118