Die bonding techniques and methods

Die Bonding is the process of attaching the semiconductor die either to its package or to some substrate. The process starts with picking the target die from wafer or waffle tray as shown in figure 1. The most common method is to push the target die from the tape with a pin.  The tape can also be drawn away from the die by vacuum.  The released die is generally picked by a vacuum tool and aligned to a target pad on the carrier or substrate, and then permanently attached, using one of several die bonding techniques.

A few important things that are needed for better die bonding are:

• It should exhibits better thermal conductivity to dissipate the heat generated from the die.
• There should be perfect contact required between the chip and substrate without any voids.
• The bond should be made very carefully, so it doesn’t destroy the die/chip.
• The bond should stand extreme temperatures without any degradation.

The choice of die bonding process depends upon package sealing strategy, operating conditions and environmental and reliability requirements. The die bonding can be generally accomplished by distinct types of attachments.

This paper describes the following die bonding techniques:

• Eutectic
• Epoxy
• Solder attach

Eutectic bonding
Eutectic bonding describes a die bonding technique with an intermediate metal layer (Au/Al) that can produce a eutectic system. A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients. The fact that the eutectic temperature can be much lower than the melting temperature of the two or more pure elements can be important in eutectic bonding.

The most important parameters for eutectic bonding are:

• bonding temperature
• bonding duration
• tool pressure

A eutectic bond is formed by melting a preform consisting of a mixture or alloy of two or more dissimilar metals in the joint between the die and substrate. The preform has a melting point which is lower than the melting point of its base materials as shown in figure2 . Consider a typical example of preform composed of gold and silicon. The melting point of gold is 1640°C, and silicon is 1414°C. However, when the preform is made up from the materials which are the combination of these two, the melting point becomes much lower than the actual melting point of the materials i.e. approximately 360° C. In this method a layer of gold metal is pre-deposited on the backside of the die. By heating the package above the eutectic temperature and placing die on it, a bond is formed between the die and package as depicted in figure 3.



Epoxy bonding
An epoxy bond is formed by attaching the die to the substrate with the use of epoxy glue. A drop of epoxy is dispensed on the package and the die placed on top of it. The package needs to be heated at an elevated temperature to cure the epoxy properly. This process uses adhesives such as polyimide, epoxy and silver-filled glass as die attach material to mount the die on the die pad. The mass of epoxy climbing the edges of the die is known as the die attach fillet. Excess of die attach fillet results in the die attach contamination of the die surface and little amount used may result in die lifting or die cracking. Epoxy adhesives are electrical insulators and have poor thermal conductivity. To improve the electrical conductivity, epoxy or polyimides are filled with the gold and silver material. In order to achieve a lower value of thermal resistance ceramic particles like SiC (Silicon carbide, compound of silicon and carbon) and BeO (Beryllium oxide, an inorganic compound) are added. Epoxy bonding is mainly preferred due to its major pros like low curing temperature, can be used for wide range of die sizes and can be reworked easily.


Solder Attach
Solder attach is the most preferably type of die bonding because of the better thermal conductivity of the solder material .As there is extreme variation of temperature on die during its operation, solder attach is used as the important concept to dissipate heat generated from the power device efficiently. By solder attach generally it is referred as soft-solder attach. Soft soldering are low melting binaries and ternaries metallic compositions.

The following steps were involved for solder attach technique:

1. Some initial solder alloy to be pre-plated over the die metallization and the substrate metallization.
2. The components are fluxed, placed together and reflowed.
3. Flux must be cleaned by cleaning mechanism before the device is encapsulated
4. In a fluxless solder wire process, a wire is fed into in-line system where it contacts the pre-heated lead frame which melts the solder and then the solder is formed in desired shape.

Solder attach provides good mechanical strength, high thermal conductivity and good electrical conductivity.

Conclusion
Die bonding is a phenomenon to connect die to the package for communication to the outside world. Fabrication labs and semiconductor manufacturers are looking towards adapting newer technologies of die bonding for high reliability in high pin counts.  Various innovations are going on to make die bonding more stable and more cost effective for better yield.

References
http://en.wikipedia.org/wiki/Eutectic_bonding
http://en.wikipedia.org/wiki/Eutectic_system

Authors
Gurinder Singh Baghria: Working at Freescale Semiconductors, India as Design Engineer and 2 years of experience in Physical Design activities.
Naveen Kumar: Working at Freescale Semiconductors, India as Senior Design Engineer and 4 years of experience in Physical Design activities.
Deepak Sharma: Working at Freescale Semiconductors, India as Senior Design Engineer and 8 years of experience in Physical Design, Analog Layout Design and Standard Cell Library Design.
Sachin Kalra: Working at Freescale Semiconductors, India as Senior Design Engineer and 6 years of experience in Physical Design, Analog Layout Design and Standard Cell Library Design.

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