Eutectic Bonding

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.

It offers better thermal conduction compared to the epoxies and other fluid bonding solutions.
The bonded die is instantly fixed which eliminates any chances of unwanted repositioning.
Does not require any curing operations which reduces processing requirements.
Does not produce any gas which could adversely affect the strength of the bond.

Eutectic Bonding Work Flow

Pre-Treatment

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.

Pick & Place / Heating

Typically for eutectic bonding, the substrate/package is placed on a heated stage which is initially heated to a temperature just below the eutectic temperature of the alloy. The eutectic preform is then placed on the substrate and the stage temperature is increased above the eutectic temperature to melt the preform. The die is then placed on the preform and mechanical force is applied on it to aid attachment. The thermal energy should be applied to the substrate carefully as excessive heat can damage the die or surrounding components on a substrate. Similarly, excessive mechanical force can either cause the preform to squeeze out of the bonding area or induce micro-cracks in the die.

Scrubbing

The die may also be scrubbed on the surface of the substrate/package to ensure the preform properly wets the die and the substrate/package surface. Scrubbing also ensures that the bond is free from any voids by forcing out any air trapped in the melted preform. Scrubbing requires die collects to minimize any potential drift. Amplitude, speed and frequency are the key scrubbing parameters, their choice depends on the die size and the proximity of the die to the adjacent components. When the preform melts, the metals start mixing and diffusing into each other until a saturation composition. The temperature and mechanical force should be applied uniformly to the die for uniform bonding.

Cooling

The die, substrate/package and eutectic preform assembly is then cooled, as the temperature drops below the eutectic temperature the alloy solidifies forming a continuous bond.

Eutectic Bonding Parameters

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.

Eutectic Bonding at our Facility

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.