A Study on the Thermal Reliability of Cu/SnAg Double-Bump Flip-Chip Assemblies on Organic Substrates

Abstract

The Cu/SnAg double-bump structure is a promising candidate for fine-pitch flip-chip applications. In this study, the interfacial reactions of Cu (60 mu m)/SnAg (20 mu m) double-bump flip chip assemblies with a 100 mu m pitch were investigated. Two types of thermal treatments, multiple reflows and thermal aging, were performed to evaluate the thermal reliability of Cu/SnAg flip-chip assemblies on organic printed circuit boards (PCBs). After these thermal treatments, the resulting intermetallic compounds (IMCs) were identified with scanning electron microscopy (SEM), and the contact resistance was measured using a daisy-chain and a four-point Kelvin structure. Several types of intermetallic compounds form at the Cu column/SnAg solder interface and the SnAg solder/Ni pad interface. In the case of flip-chip samples reflowed at 250 degrees C and 280 degrees C, Cu(6)Sn(5) and (Cu, Ni)(6)Sn(5) IMCs were found at the Cu/SnAg and SnAg/Ni interfaces, respectively. In addition, an abnormal Ag(3)Sn phase was detected inside the SnAg solder. However, no changes were found in the electrical contact resistance in spite of severe IMC formation in the SnAg solder after five reflows. In thermally aged flip-chip samples, Cu(6)Sn(5) and Cu(3)Sn IMCs were found at the Cu/SnAg interface, and (Cu, Ni)(6)Sn(5) IMCs were found at the SnAg/Ni interface. However, Ag(3)Sn IMCs were not observed, even for longer aging times and higher temperatures. The growth of Cu(3)Sn IMCs at the Cu/SnAg interface was found to lead to the formation of Kirkendall voids inside the Cu(3)Sn IMCs and linked voids within the Cu(3)Sn/Cu column interfaces. These voids became more evident when the aging time and temperature increased. The contact resistance was found to be nearly unchanged after 2000 h at 125 degrees C, but increases slightly at 150 degrees C, and a number of Cu/SnAg joints failed after 2000 h. This failure was caused by a reduction in the contact area due to the formation of Kirkendall and linked voids at the Cu column/Cu(3)Sn IMC interface.