An innovative composite solder preform for TLP bonding — Microstructure and properties of die attach joints

A transient liquid phase (TLP) bonding process using an innovative composite solder preform has been developed for die attachment bonding applications. The composite preform is composed of a high-melting, ductile Ag core layer and a low-melting solder coating at both sides of the Ag layer. Unlike the conventional TLP bonding process, the solidification reaction in the present process takes place as a result of intermetallics formation. During TLP bonding, the Ag layer and the liquid solder layer together with the substrate metals react to form high-melting intermetallic compound phases (IMCs) to consume the low-melting phase rapidly. The resultant TLP joint consists of a remaining Ag layer sandwiched by the IMCs layers at both sides. The Ag layer improves the mechanical, thermal and electrical properties of the TLP joint. The microstructures, mechanical properties and reliability of the solder joints made with a SnAg/Ag/SnAg composite preform are investigated. Results show that the TLP bonding with the composite preform is a promising joining technology for die attachment in high temperature power electronics. The intermetallic compound phases formed in the TLP bonded joints are mainly the Ag-Sn ζ phase, Cu₃Sn and Cu₆Sn₅ phases when a Cu substrate is used. The composite preform TLP joint has a remelt temperature higher than 450 °C. An appropriate low pressure is needed to help ensure close contact between the composite preform and the substrates, and thus to reduce voiding in the joint. Temperature cycling test results indicate that the TLP die attach joints have a characteristic life of 5016 cycles when tested at a cycling temperature range from -55°C to 125°C.