Die Attach Process

Die Attach (also known as Die Mount or Die Bond) is the process of attaching the silicon chip to the die pad or die cavity of the support structure (e.g., the leadframe) of the semiconductor package. There are two common die attach processes, i.e., adhesive die attach and eutectic die attach.  Both of these processes use special die attach equipment and die attach tools to mount the die.

Adhesive Die Attach

Adhesive die attach uses adhesives such as polyimide, epoxy and silver-filled glass as die attach material to mount the die on the die pad or cavity. The adhesive is first dispensed in controlled amounts on the die pad or cavity. The die for mounting is then ejected from the wafer by one or more ejector needles.

 

Fig. 1.  Two examples of die attach machines

While being ejected, a pick-and-place tool commonly known as a 'collet' then retrieves the die from the wafer tape and positions it on the adhesive. All of the above steps are done by special die attach equipment or 'die bonders' (see Fig. 1).

The mass of epoxy climbing the edges of the die is known as the die attach fillet.  Excessive die attach fillet may lead to die attach contamination of the die surface.  Too little of it may lead to die lifting or die cracking.

Another critical aspect of adhesive die attach is the ejection of the die from the wafer tape during the pick-and-place system's retrieval operation. The use of inappropriate or worn-out ejector needle and improper ejection parameter settings can cause die backside tool marks or microcracks that can eventually lead to die cracking.

 

Fig. 2.  Photo showing the D/A adhesive as the grainy material between the die and the die pad

See also: Die Attach Failures

Eutectic Die Attach

Eutectic die attach, which is commonly employed in hermetic packages, uses a eutectic alloy to attach the die to the cavity. A eutectic alloy is an alloy with the lowest melting point possible for the metals combined  in the alloy. The Au-Si eutectic alloy is the most commonly used die attach alloy in semiconductor packaging.

A gold preform is placed on top of the cavity while the package is being heated. When the die is mounted over this gold preform, Si from the die backside diffuses into the gold preform, forming Au-Si alloy. As more Si diffuses into the gold preform, the Si-to-Au ratio of the alloy increases, until such time that the eutectic ratio is achieved. The Au-Si eutectic alloy has 2.85% of Si and melts at about 363 degrees C. Thus, the die attach temperature must be reasonably higher than this temperature to achieve the eutectic melting point. At this point, the alloy melts, attaching the die to the cavity. 

To optimize the die attachment, the operator 'scrubs' the die into the eutectic alloy for even distribution of the die attach alloy. Eventually the diffusion of silicon atoms into the gold preform exceeds the eutectic limit, and the die attach alloy begins to solidify once again.  The package is then allowed to cool down to completely solidify the eutectic alloy and complete the die attach process.

Aside from the Au-Si alloy, semiconductor assembly may employ other metal alloys for eutectic die attach.   Table 1 lists some of the other alloys used in eutectic die attach preforms.

Table 1.  Compositions and Melting Points of some Eutectic Die Attach Preforms   

Effects of  Die Attach Voids

Regardless of die attach process, the presence of voids in the die attach material affects the quality and reliability of the device itself. Large die attach voids result in low shear strength and low thermal/electrical conductivity, and produce large die stresses that may lead to die cracking.  Small voids provide sufficient shear strength and electrical/thermal conductivity, while 'cushioning' large dice from stresses.  Total absence of voids may mean high strength, but it may also induce large dice to crack. The strength of die attachment is measured using the die shear test.

Front-End Assembly Links:  Wafer Backgrind;  Die Preparation;  Die Attach;  Wirebonding;  Die Overcoat

Back-End Assembly Links:  Molding;  Sealing;  Marking;  DTFS;  Leadfinish          

See Also:  Die Shear Testing;  Die Attach Tools;  Die Attach Materials;  Die Attach Failure Mechanisms; 

IC Manufacturing;  Assembly Equipment

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