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The interfacial reactions of solder joints between the Sn-4Ag-0.5Cu solder ball and the Sn-7Zn-A1 (30 ppm) presoldered paste were investigated in a wafer level chip scale package (WLCSP). After appropriate surface mount technology (SMT) reflow process on the printed circuit board (PCB) with organic solderability preservative (Cu/OSP) and Cu/Ni/Au surface finish, samples were subjected to 150°C high-temperature storage (HTS), 1,000 h aging. Sequentially, the cross-sectional analysis is scrutinized using a scanning electron microscope (SEM)/energy-dispersive spectrometer (EDS) and energy probe microanalysis (EPMA) to observe the metallurgical evolution in the interface and solder buck itself. It was found that Zn-enriched intermetallic compounds (IMCs) without Sn were formed and migrated from the presolder paste region into the solder after reflow and 150°C HTS test. [ABSTRACT FROM AUTHOR]

The interfacial reactions of solder joints between Sn-4Ag-0.5Cu solder ball and a couple of presoldered pastes (Sn-7Zn-Al(30ppm) and Sn-3Ag-0.5Cu) were investigated in wafer-level chip-scale package (WLCSP). After appropriate surface mount technology reflow processes on printed circuit boards with a Cu/OSP (organic solderability preservative) surface finish, samples were subjected to 150°C high-temperature storage (HTS) for 1,000 h of aging or 1,000 cycles of a thermal cycling test (TCT). Sequentially, cross-section analysis is scrutinized by scanning electron microscopy/energy dispersive spectrometry and energy probe microanalysis to observe metallurgical evolution in the interface and solder buck itself. It was found that the degradation of the joint shear strength after TCT is more pronounced than that of the shear strength after HTS. Fracture surface analyses of the shear tests show that the degradation of the joint strength for HTS is solely due to the influence of the interfacial IMC grain growth, while the shear strength degradation for TCT is mainly due to the coefficient thermal expansion mismatch from the thermal cycling at the chip-solder interface and can lead to the occurrence of the crack. [ABSTRACT FROM AUTHOR]