Bonding of ceramics using reactive NanoFoil®

A new bonding technique with reactive NanoFoils® developed by Indium Corp. was presented recently. These 40 μm thick foils consist of aluminum - nickel multilayers, which react in a short but extensive exothermal heat pulse after an ignition. Additionally, these foils are sometimes covered with 5 μm tin layers on each side, in order to improve bonding results. The foils act as local heat source for the bonding process. The short reaction time reduces the thermal stress to the package, what offers the possibility to join materials with different thermal expansions. In an evolutionary approach the tensile strength of the bond was tested. Therefor alumina ceramics and low temperature co-fired ceramics (LTCC) with a silver-based metallization were bonded together. The NanoFoil® was activated by an electrical discharge. Also a LTCC package with integrated activation methods was build to make allowance for the fact, that the NanoFoils® are difficult to access during the bond process. Using NanoFoils® without tin layers resulted in micro cracks within the reacted foil. The additional tin layers melted and filled the micro cracks. The bond strength of the joints was tested with good results. Additionally, the thermal performance of the bonds was investigated. Therefore, a small thermal test chip with an integrated heater was bonded on a metalized alumina using NanoFoils® with tin layers. The generated heat was measured with IR spectroscopy. To compare the thermal performance with common die attach processes the test chip was also soldered and glued on the alumina carrier. It could be shown, that the bond had a good thermal performance, which was comparable to the performance of the classic bonds.