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.

Common Die Attach-related Failure Mechanisms:

Die Lifting - detachment of the die from the die pad or cavity. Common Causes: contamination on the die pad or cavity, die backside contamination, excessive die attach voids, incomplete die attach coverage, inadequate die attach curing

Die Cracking - occurrence of fracture anywhere in the die. Common Causes in the context of Die Attach: excessive die attach voids, die overhang or insufficient die attach coverage, insufficient bond line thickness, excessive die ejection force on the wafer tape, absence of die attach voids

Adhesive Shorting - electrical shorting between exposed metal lines, bond pads, bonds, or wires as a result of adhesive accidentally dripping on the surface of the die (sometimes called 'epoxy on die').  Common Causes: incorrect die attach material viscosity, incorrect adhesive dispensation

Bond Lifting - lifting of the first or second bond from the die or leadfinger, respectively. From the die attach process point of view, this is often due to resin bleeding of the die attach material into the bond pads or leadfingers, inhibiting good intermetallic formation. See also Wirebonding.

Die Scratching - inducement of any mechanical damage on the die, as when an operator scratches a die with tweezers due to mishandling. Common Causes: insufficient operator training, worn-out or contaminated pick-and-place tool, disorderly workplace, use of improper tools

Die Metallization Smearing - depression or deformation of any metal line on the die surface. Common Causes: dirty or worn-out die attach pick-and-place tool, wafer mishandling