Microstructural Evolution and Interfacial Reaction Mechanisms of Sn-9Zn/Cu Solder Joints during Thermal Aging

With the implementation of the EU RoHS directive, the electronics manufacturing industry has accelerated the development of lead-free solders. Sn-9Zn alloy has emerged as one of the preferred mid-temperature solders due to its melting point being close to that of the traditional Sn-37Pb alloy, as well as its cost-effectiveness and excellent resistance to electromigration. However, the interfacial reaction mechanism between Sn-9Zn solder and Cu pads remain poorly understood, especially regarding their long-term reliability under elevated temperatures. This study focuses on the interfacial microstructural evolution of Sn-9Zn/ Cu solder joints under high-temperature service conditions. Accelerated aging tests were conducted at 100℃ and 150℃. The interfacial morphology and elemental distribution were characterized using field-emission scanning electron microscopy (FESEM) and electron probe microanalysis (EPMA) . Results show that at 100℃, the Cu5Zn8 intermetallic compound (IMC) gradually thickens and remains stable. In contrast, at 150℃, the Cu5Zn8layer decomposed, resulting in interfacial voids that enable direct reaction between the solder and Cu substrate. This leads to the formation of pit-like Cu6 (Sn, Zn) 5 phases within the substrate, which could critically compromise the reliability of the solder joint