Brazing & Soldering Today

JULY 2012 50 High-temperature solder alloys are extensively used in dieattach, power semiconductor, and optical device packaging, flip-chip packaging, etc. Current industry standard solders for these applications are mainly high-lead solders (90–97 wt-% Pb) and gold-based eutectic solder alloys such as the 80Au20Sn solder. The die-attach process involves connecting the silicon die or chips to a lead frame or other substrate with the use of adhesive bonding or solder joining. Soldering is a preferred method for die attach to a lead frame for power devices because of the higher current-carrying capability and better thermal conductivity of a solder alloy than an adhesive. The latter feature proves to be beneficial in dissipating the heat generated by the device. The solders used for die attach usually have a liquidus temperature of 280°C or higher to allow subsequent mounting of the packaged devices on printed circuit boards, which is done with eutectic SnPb or lead-free SnAgCu (SAC) solders by reflow soldering at a temperature of 200° to 250°C. The most widely used solders for die attach are the highPb alloys, e.g., 95Pb5Sn, 88Pb10Sn2Ag, and 92.5Pb5Sn 2.5Ag. However, Pb is poisonous, and is banned in many applications. Although the high-Pb solder alloys for the first level packaging applications are exempted from the current Restriction of Hazardous Substances (RoHS) regulations because of the lack of a reliable replacement for them, the conversion to Pb-free materials in these areas will eventually be implemented. The Pb-free eutectic Au-Sn (280°C), Au-Si (363°C), and Au-Ge (356°C) alloys can be used as die-attach solders, but the cost is too high. Although other hightemperature lead-free solders in the SnSb, Bi-Ag, Zn-Sn, and Zn-Al systems are also known to be candidates, each has its own drawbacks (Refs. 1, 2). For example, the solidus temperatures of Sn-Sb and Zn-Sn alloys are too low. The Zn-Al alloys are highly corrosive and easily oxidize, while the Bi-Ag alloys have brittleness and low thermal and electrical conductivity issues. In view of the foregoing, it would be desirable to develop an alternative Pbfree soldering material and process for high-temperature soldering applications, especially for die attach in power semiconductor packaging. In this work, based on the principles of transient liquid phase (TLP) bonding (Refs. 3–6), a special laminate composite preform has been developed for hightemperature, Pb-free soldering applications, where a melting temperature of 280°C or higher is required. The composite preform is composed of a highmelting, ductile metal core layer and a low-melting solder coating layer at both sides of the core layer. During soldering, the liquid solder layer reacts with the core metal and the substrate materials to form high-melting intermetallic comA Composite Solder Alloy Preform for High­Temperature Lead­Free Soldering Applications