Eutectic die bonding, sometimes known as eutectic die attach or fluxless eutectic solder attach, is a process that forms high thermally and electrically conductive bonds that are often needed for the densely packed circuits in today’s dies. It is a highly controlled die attach process for high reliability, high accuracy requirement devices.
Eutectic solder is unique in that it is an alloy that has a melting point which is lower than the melting point of the metals that make up the alloy. These melting points are low enough for attachment processes but raise significantly after attachment for thermal stability.
An example of this is AuSn, which reflows at 280 Celsius, but as the alloy absorbs gold from the surfaces of the substrate and die, the melting point rises closer to the melting point of pure gold, and the solder solidifies. Due to the unique nature of eutectic solders, strong, extremely low void bonds can be formed without the use of flux. The solders are usually either predeposited onto either the die or substrate, or are presented as a thin cut sheet of alloy, usually called a preform.