Effect of Au Content on Microstructural Evolution of SnAgCu Solder Joints That Undergo Isothermal Aging and Reliability Testing

Author

Patrick J. Hyland, California Polytechnic State University, San Luis ObispoFollow

DOI: https://doi.org/10.15368/theses.2011.163
Available at: https://digitalcommons.calpoly.edu/theses/605

Date of Award

8-2011

Degree Name

MS in Engineering - Materials Engineering

Department/Program

Materials Engineering

Advisor

Linda Vanasupa

Abstract

Electronics, especially, printed circuit boards (PCBs) are a widespread technology. Metal coatings or “surface finishes” are often added to PCB board pads and component leads during manufacturing to improve their performance. Electroplated nickel/gold over copper is a popular surface finish for printed circuit boards and component leads. The presence of gold in solder joints, however, is known to have detrimental effects referred to as gold embrittlement. It is generally understood that tin-lead solder joints with less than 3 weight percent (wt%) of gold will not experience reliability issues. The acceptable level of gold in lead-free solder joints, however, is less well understood, as the technology is younger. The purpose of this study was to investigate the effect of gold content on the microstructural evolution of SnAgCu solder joints. Three integrated circuit packages with various thicknesses of gold coatings were assembled on boards that were made with thin (flash) or thick gold over nickel coatings. The boards were divided into three groups based on the isothermal aging they underwent: 0 days, 30 days, or 56 days of aging at 125 °C. Thirty four of the forty boards then underwent mechanical reliability testing. Components were cross-sectioned and polished. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS/EDX) were used to characterize the morphology and elemental composition of the solder joints and any intermetallic compounds (IMCs) that formed. The growth of bulk and interfacial layer IMCs in each package/board system was studied. In thick gold boards, AuSn₄ particles observed in the bulk solder grew larger over time, absorbed Ni, and migrated to the component and board interfaces. (Cu_1-p-qAu_pNi_q)₆Sn₅ and (Au_1-xNi_x)Sn₄ IMCs were found at most board and component interfaces after aging. It was observed that most fractures occurred in or along the (Au_1-xNi_x)Sn₄ IMCs. Cracks were observed within IMC particles in the bulk solder, along the boardside and component side interfaces, and in the bulk solder traveling toward voids. Components with joint Au contents higher than 10 wt% had unacceptably poor reliability. The conclusion of this work is that gold content of SAC305 solder joints on boards with Au over Ni surface finishes should be kept below 3 wt% to conservatively minimize the risk of creating a microstructure that has poor reliability.