When packaging a diode laser, choosing the appropriate solder preform for the die-bonding process is the critical first step for ensuring long term reliability, since the solder joint serves as the primary thermal conductor for heat dissipation. As a result, soft solders such as InSn are generally preferred because the flexibly of the solder helps to mitigate the stress caused by the mismatch of coefficient of thermal expansion (CTE) between the chip and the submount. An additional benefit of utilizing InSn solder is the relatively low melting point (157oC) which reduces the thermal load on the diode during the bonding process. While soft solder can suffer from greater thermal-fatigue and creep-rupture than hard solders such as AuSn, the risk of cracks in the die or detachment of the die during thermal cycling because of CTE mismatch often out-way any advantages of AuSn.
After die-bonding, the most critical step in the diode packaging process is wire bonding. In this process, microscopically thin wires are attached to the top side of the die and the submount to allow current to be passed through the diode. For traditional high-current wire bonding alloyed type electrodes are used, but when used on laser diodes they tend to induce bulk defects in the diode structure due to metal diffusion into the diode. Therefore, to maximize the lifetime of a laser diode, you are much better off utilizing a higher number of pure gold wire bonds which are capable of carrying the same current load but are more chemically compatible with the die significantly reducing the probability of metal diffusion. If there is no other option than to use alloy electrodes, then it is best to use Schottky-type electrodes which contain gold and therefore, tend to cause fewer bulk defects than traditional alloys.