When we talk about off-eutectic alloys, we must take a step back and talk about what makes a eutectic alloy. Each solder alloy has both a solidus (not melted) and liquidus (completely melted) temperature. When these temperatures are the same, the alloy is eutectic. For example, 80Au20Sn is a eutectic alloy with a melting temperature of 280°C. When these temperatures are different, the alloy is off-eutectic, or non-eutectic. Going back to the AuSn example, when we slightly change the composition of the 80Au20Sn to 79Au21Sn, 78Au22Sn, or even 75Au25Sn, these alloys will have different solidus and liquidus temperatures. This creates a plastic, pasty, or I’ve even heard it called molten, range. The way it was first described to me from a colleague was to think about if you put some ice cubes in a glass and set it outside on a sunny day. The glass starts out having solely ice in it, then there will be both ice and water coexisting in the glass, until finally, the glass will be completely water. The mixture of ice and water would be considered the plastic range of an off-eutectic alloy!
With Gallium Nitride (GaN) dies becoming popular in high frequency, high power, high-reliability RF power amplifier devices for 5G and other critical military and aerospace wireless communications, Indium Corporation was presented with some challenges. Some GaN dies have a thick gold plating to prevent oxidation. It’s extremely crucial that the solder joint maintain the eutectic composition of 80Au20Sn to benefit from all the unique properties of the alloy, mainly the high joint strength. However, if an 80Au20Sn preform were used as the soldering material, the solder would begin to absorb some of the gold from the plating during reflow, skewing the final solder joint composition away from 80Au20Sn and directly weakening the solder joint strength.
This is where developing an off-eutectic alloy provided us with some great opportunities in GaN die-attach applications! By lowering the gold content in the AuSn alloy, there is room for the solder to absorb some of the gold from the plating, and end up with a final joint composition of the eutectic 80Au20Sn. The thicker the gold plating on the die, the lower the gold content the AuSn solder preform must be, and voilà: back to a strong and reliable solder joint! Indium Corporation’s off-eutectic preforms ensure strong solder joints in a plethora of device and package hardware. Our off-eutectic AuSn preforms are currently available now in 79/21, 78/22, and 75/25 compositions, with a wide range of X, Y, and Z dimensions available, unique to any die size.
For clarity, H2O is not a metal alloy and therefore cannot have eutectic/off-eutectic properties. It was used as an example only