Die-Bonding of LED Chips on Ag/Cu Substrate Using Sn/Zn/Bi/Sn and Sn/Bi/Zn/Bi/Sn Bonding Systems

Two multilayer bonding structures have been designed to die-bond light-emitting diode (LED) chips on Ag/Cu thermal substrate, viz. Sn/ZnBi/Sn bilayer solder structure and Sn/BiZnBi/Sn sandwich solder structure. Both multilayer bonding structures successfully achieved LED chip die-attachment on Ag/Cu thermal substrate at relatively low temperature of 150°C. However, voids formed more seriously at the bonding interface for the Sn/ZnBi/Sn bilayer structure. On the other hand, little voiding was seen at the bonding interface for the Sn/BiZnBi/Sn sandwich structure. The average shear strength of the Sn/ZnBi/Sn bilayer solder structure and Sn/BiZnBi/Sn sandwich solder structure was 25 MPa and 40 MPa, respectively. We believe that the improved shear strength results for the sandwich solder structure compared with the bilayer solder structure are mainly due to less voiding at the bonding interface, which weakens the interface joint shear strength. Also, the intermetallic compounds (IMCs) jointing region at the joint interface of the sandwich solder structure was larger than at the joint interface of the bilayer solder structure. We believe that the IMC jointing at the interface could improve the die-bonding strength, while the Zn content in the bonding structure promoted voiding at the bonding interface for both solder structures. Moreover, the Zn content in the bonding structure slightly reduced the IMC joint region at the bonding interface for both solder structures.