Die Cracking; Die Cracks

Die Cracking

Die cracking is the occurrence of fracture(s) in or on any part of the die of a semiconductor device. Die cracks may be due to a variety of causes, but they usually originate from die attach problems and/or mechanical stresses on the package that get transmitted to the die.

Most die cracks that are traceable to the die attach process are caused by imperfections in the die attach material, such as voids and incomplete fillet formation. Excessive voids in the die attach material act as stress concentrators that can exert large flexural stresses on the die when the device is subjected to thermo-mechanical stresses, resulting in cracks at the backside of the die. These cracks can propagate upward to the active circuitry, and subsequently to the surface of the die.

Units using eutectic die attach are very vulnerable to die cracking caused by voids, although this phenomenon is also encountered in other die attach materials. During thermal stressing, eutectic die attach material expands more rapidly than the silicon die, exerting stresses on the die that tend to split its backside.

Imperfections in the die itself can also result in cracks. Damage or defects at the backside of the die also act as stress concentrators, and can serve as crack initiation points once the package is subjected to thermo-mechanical stresses. These die backside defects include micro-cracks and chip-outs caused by back-grinding and wafer saw. Backside micro-cracks caused by ejection needles used for ejecting die from the wafer tape during the die attach process have also been known to cause cracks.

Figure 1. Photo of a Die Crack

Damage on the die surface can also cause die cracks. Improper equipment set-up can cause probe needles, die overcoat dispense tools, etc. to land on the die and fracture it. Excessive wirebonding force and energy can also cause cratering, or silicon fracture under the bond pad.

Excessive mechanical stresses on the package can also be transmitted down to the die, causing it to crack. Even in relatively thick packages like SOICs, these stresses may be transmitted down to the die without damaging the package itself, making the problem even less detectable. The common sources of these mechanical stresses are the mechanical deflash, trim, and form processes, especially if the package nests employed by the steps contain debris or particles that can act as fulcrums for aggravating the stresses.

Die Cracking may be accelerated by SHRT, Temp Cycle, and Thermal Shock.

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