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.
Eutectic solder attach can be aided through the use of custom temperature profiles based on the specific application. Settting specific ramp rates and temperatures is key to achieving the best bond for various assemblies. Ensuring that the environment is inert with the use of Nitrogen gas to prevent metal oxides from forming is also important, along with removing existing oxides with a low percentage of Hydrogen gas added in.
The presence of voids increases the die operating temperature by inhibiting efficient heat transfer through the thermal interface material (TIM). Voids may be created by surface oxides, trapped gas and poor wetting. Our vacuum reflow systems and their processes minimize voids through the precise control of vacuum levels, positive pressures and thermal profiles. While our die bonders can incorporate techniques such as scrubbing to reduce voids caused through trapped gas.
Resources for Eutectic Die Bonding: