Investigations on Zn-Al-Ge alloys as high temperature die attach material

Zn-Al based alloys have been suggested as high temperature lead-free solder for Si die attach by different researchers. But, so far, these alloys have not been tested in the industrial environment. In this study, Zn-Al-Ge alloy solder wire was used for Si die attachment on Cu substrate in an automatic die attach machine. The back side of the die was metalized with Ti/Ni/Ag layers. Die attachment was performed in a forming gas environment at temperatures ranging from 370°C to 390°C. Interfacial structure at the Si die and Cu substrate interfaces, and the bulk solder microstructure were investigated using an optical microscope, scanning electron microscope (SEM), energy dispersive x-ray (EDX) and electron probe microanalyzer (EPMA). Wetting characteristics and die shear strength were also studied. Cross sectional microstructural investigation revealed that as many as three intermetallic (IMCs) layers form at the solder/copper interface. A CuZn intermetallic layer forms close to copper, a scallop shaped CuZn
₄
forms at the solder side, while Cu
₅
Zn
₈
forms at the middle. At the Si die-solder interface, with Ti/Ni/Ag metallization, no IMC layer was detected. Microstructural evidence suggests that Ni layer did not react with the Zn-Al-Ge solder. The microstructure of the bulk solder consists of mainly two phases: one with a lighter contrast and the other one being darker under the SEM. The phase with lighter contrast is a zinc rich phase containing an average of about 70.9% Zn. The darker one is found to be an aluminum rich phase containing an average of about 54.9% Al. In addition, some particles with brighter appearance are observed at different places in the bulk solder. These particles are rich in zinc and their composition closely resembles that of scallop type CuZn
₄
IMC which forms at the solder side of the copper-solder interface. It is suggested that these particles are CuZn
₄
IMC which have been spalled from the interface during soldering. Wetting of Zn-Al-Ge solder on Cu substrate was found to be lower at 370°C as compared with that at 390°C. Voids were found in the solder which were more in number at higher die attach temperatures. Die shear strength was found to be higher at the die attached temperature of 390°C (22.3 MPa) as compared with those obtained at 370-380°C (15.5-17.4 MPa). Die shear strength of standard Pb-Sn solder was also measured for comparison and was found to be 28.2 MPa. The microstructure and properties of the solder joint are discussed.