Fluxless Tin Bonding Processes with Intermetallic Study and Aluminum Circuit Board Technology
This research starts with developing fluxless bonding process using electroplated Sn solder between Si chip and Al substrate. The joint thickness was controlled either by bonding pressure or by incorporating Cu spacers. Cu6Sn5 and Cu3Sn IMCs are observed at Sn/Cu interface. A new two-step anodization process is also introduced to grow high quality alumina on Al boards and coat thick Cu layer over the alumina to produce a Cu/alumina/Al insulated metal substrate (IMS) structure. The measured resistance and breakdown voltage of the as-formed 50 µm anodic alumina layer is larger than 40 MΩ and 600 VDC, respectively. To test the reliability of the boards, they were put through 500 cycles of thermal cycling test and high temperature storage test. To ensure its compatibility with soldering operations, Cu substrates were bonded to the Al boards using a fluxless tin process.
The fluxless bonding process is also applied to bond Si chips to low carbon steel substrates electroplated with Ni with desired thickness. Ni3Sn4 is the only IMC formed at the Sn/Ni interface. In order to compare the result to the case without using underbump metallurgy. Sn was electroplated over low carbon steel directly and then Si was bonded to the low carbon steel substrate. Sn solder layer was bonded to Fe substrate by forming FeSn2 at the Sn/Fe boundary. These results suggest that Sn-Fe reaction system should be another promising bonding pair candidate that could mitigate excessive IMC thickness. The liquid Sn/solid Fe reaction couples were fabricated and annealed at different temperatures. The growth kinetics of FeSn2 was modeled by parabolic law and empirical power law.
We finally move to develop a fluxless bonding process with suppressed IMC formation. According to Cr-Sn phase diagram, there is no IMC formation in Cr/Sn system. Cr is thus selected as the barrier metal. Several bonding experiments have been performed and the preliminary results show that high quality Sn joints could be produced with little IMC formation before high temperature aging.