Fluxless flip–chip SnAu solder interconnect on thin Si wafers and Cu laminated polyimide films

Initial success of new fluxless flip–chip interconnect technique is reported. Non-eutectic Sn-rich SnAu solder bumps are used to connect thin silicon chip to glass substrate and Cu laminated polyimide film, respectively. Fluxless process development on tin-rich alloys is challenging because tin atoms get oxidized easily. The solder bumps of SnAu multilayer composite with overall composition of 95 at.% Sn and 5 at.% Au are fabricated by vacuum deposition on thin silicon wafers to prevent solder oxidation. The bonding is performed at 250 °C in hydrogen environment without using any flux. The diameter of the bump is 200 μm and height is 20 μm. Nearly void-free solder joints with randomly distributed AuSn4 intermetallic compound (IMC) are produced using this fluxless bonding process. Continuous layers of Cu3Sn and Cu6Sn5 instead of commonly known scallop-type Cu6Sn5 are formed between laminated Cu layer and the Sn-rich non-eutectic AuSn solder. The re-melting temperature of solder joints is measured to be 220 °C. The bonding process is entirely flux-free and thus is particularly valuable for many devices that cannot take any flux, such as biomedical, photonic, MEMS, and sensor devices.