Semiconductor Packaging:  Die Attach Process (Page 2)

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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.