Effect of Ni metallization on interfacial reactions and die attach properties of Zn-Al-Mg-Ga high temperature lead-free solder

Interfacial reactions during Si die attachment with Zn-Al-Mg-Ga high-temperature lead (Pb)-free solder on bare Cu lead-frame (Tamac 4) and Ni metallized Cu lead-frame were investigated using an optical microscope, scanning electron microscope (SEM) and energy dispersive x-ray (EDX). Die attachment was performed in an automatic die attach machine in a forming gas environment at temperature 380°C. The back side of the die was metallized with Ti/Ni/Ag layers. Comparative studies of die attach properties such as wetting, void and die shear strength on bare and Ni metallized Cu lead-frame was made. Cross sectional microstructural investigation revealed that as many as three intermetallic compound (IMCs) layers form at the solder/lead-frame interface for bare Cu lead-frame. A CuZn intermetallic layer forms close to copper, a scallop shaped CuZn₄ forms at the solder side, while Cu₅Zn₈ forms at the middle. At the interface with Si die, IMC layer could not be detected by SEM. With Ni metallized Cu lead-frame, no IMC layer was observed at the solder/lead-frame as well as Si die/solder interface by SEM. Wetting on Ni metallized Cu lead-frame was found to be lower as compared to that at bare Cu lead-frame. Die shear strength was found to be higher on bare Cu lead-frame (24.2 MPa) as compared to Ni metallized Cu lead-frame (20.5 MPa). Die shear strength of standard Pb-5Sn solder was also measured for comparison and found to be 28.2 MPa.