Eutectic bonding refers to attaching a die to a package or a substrate using a metal alloy as an intermediate layer to form a continuous bond. A eutectic bond is formed when the metal alloy in the melted state forms atomic contact with the die and the substrate. The alloy metals diffuse into the surface layers of the die and the substrate to form a continuous bond.
The eutectic bonding process is normally assisted by application of mechanical force on the die when the alloy is in the liquid form. It may also be assisted by a scrubbing motion of the die on the substrate/package. Metal alloys can achieve a range of melting temperatures depending on the proportions of the constituent metals in the alloy. At a specific proportion of the constituent metals the alloy achieves the lowest melting temperature known as the eutectic temperature. At this temperature the alloy melts as a whole; eutectic temperature is lower than the individual melting temperatures of the alloy constituents. Once the bond is formed the alloy is allowed to cool which immediately fixes the die on the substrate/package. Commonly used eutectic alloys consist of gold-silicon, aluminium-silicon, gold-indium and copper-tin.
Eutectic bond is highly suitable for applications which require high thermal conductivity and reliability. It is specifically crucial for communication devices which require high data rates and low noise signals. RF amplifiers and power devices are examples which make use of eutectic bonding. Eutectic bonding offers the following advantage over other die attach processes.
Eutectic bonding may require pre-treatment of the bonding surfaces to remove any oxide layer and other impurities that could reduce the wettability of the surfaces and affect the adhesion. The bonding surfaces can be treated with either plasma or solvents to remove the unwanted materials. After the surface pre-treatment, the surfaces are either made to contact immediately or the bonding process is carried out in the presence of an inert gas such as nitrogen to avoid regeneration of the unwanted intermediate layers.
It is the force applied on the die head to hold the die during the bonding process.
The initial temperature is the temperature just below the eutectic temperature and the increased temperature is the temperature above the eutectic temperature at which the die is bonded.
The time duration of application of mechanical force and the increased temperature during the bonding process.
The rate of flow of the hot inert gas affects the rate at which the preform melts and flows.
The Advanced Packaging Facility is equipped with a semi-automatic Tresky 3002 FC1-XP die bonder which is capable of making eutectic bonds. In addition to the pick and place, the die bonder has a heated stage, a temperature control unit, a eutectic gas unit and a scrubbing head. The pick and place is used for placing the die and the eutectic preform on precise locations of a substrate. The heated stage and the temperature control unit can control the temperature to appropriately melt the eutectic preform. The eutectic gas unit allows the eutectic bonding process to take place in an inert gas environment to prevent oxidation of the metal surfaces. The scrubbing head can be used to make void free bonds. The facility also has access to plasma cleaning and a range of cleaning solvents for pre-treatment of the surfaces to be bonded.