Fluxless process of fabricating In-Au joints on copper substrates

Based on the oxidation-free fluxless bonding technology, we have developed a bonding process to manufacture In-Au joints on copper substrates. 4 mm×4 mm Si blank dice and 6 mm×6 mm copper substrates are used. The dice are deposited with indium-rich Au/In/Cr multilayer structure in a single high vacuum cycle to prevent oxidation. Right after deposition, the outer Au layer interacts with the In layer to form AuIn₂ intermetallic compound. This compound is quite stable and thus can protect the In layer against oxygen penetration when it is exposed to ambient. On the other hand, it can easily be dissolved by the molten In during the bonding process. The substrate is deposited with Cr and Au. The dice are bonded to the substrates at 180°C in inert environment. Nearly void-free joints have been obtained as examined by a 75 MHz Scanning Acoustic Microscope (SAM). Cross sections of several samples are studied using SEM and EDX to identify the microstructure and composition of the joints. Shear test has been performed according to MIL-STD-883C. All the well-bonded devices meet the shear test force requirement. Despite the large mismatch on the thermal expansion coefficient between silicon and copper, no die cracking is observed on the 30 samples produced. To assess further endurance, two samples underwent thermal cycling test between -50 and 120°C for 20 cycles, SAM examination indicates that the joints incur little degradation after the test. This bonding method requires neither flux nor scrubbing action. It is thus particularly attractive for bonding devices that cannot be exposed to flux