Thermal Conductive Bonding, Inc.
Bonding with Indium solder has been an industry standard for decades. It has relatively good thermal and electrical properties and is malleable enough to absorb some stress associated with the CTE mismatch in materials. The melting temperature of indium is 156.6C, which is suitable for most applications.
TCB utilizes backside metallization of the substrates to ensure superior adhesion and a void/defect free bond. Our three-layer process includes an adhesion layer, barrier layer, and wetting layer. The adhesion layer is chosen to optimally adhere to the substrate with a strength greater than that of indium. The second layer prevents diffusion of the indium through the films during the molten state, thereby acting as a barrier layer to prevent unwanted alloying with the indium from occurring. Finally, the third layer is the wetting, or diffusion layer, which easily reacts with indium and creates a uniform adherence of the solder to the substrate.
The challenge with bonding with this material is that the zero-stress level of the assembly will be at the solidification point of 156.6C. Having said this, the materials are both heated to above the melting point of indium and then put together and cooled down to ambient temperatures. During the heat up process, these materials will expand at different amounts, which change the geometries of the materials.
When cooled, the materials will attempt to revert to their original shape. The indium will absorb some of this stress, but much will still be applied to the target material. There are techniques that TCB employs to manage this stress to achieve a flat bonded assembly and reduce residual stresses in the substrate materials being joined.