Common Causes of Wire Bonding Failures

Wire bond failures comprise a major concern of any semiconductor manufacturing company. Common causes of wire bond failures include the following:

1) Voiding in the Bonds

Atomic interdiffusion between different metals is a natural phenomenon in a wirebond metallurgical system. If left unchecked, however, this can lead to voids in the bond that can result in significant degradation of the bond's mechanical strength and electrical conductivity.

Voiding is generally caused by unequal diffusivities exhibited by the different metals used in the wire bond, a phenomenon known as 'Kirkendall Effect.' In gold ball bonding, for example, the rates of diffusion of gold atoms from the gold ball into the aluminum bond pad and the aluminum atoms from the bond pad into the gold ball bond are unequal. Voiding failures from such interdiffusion process can be accelerated by long exposure to high temperatures and the presence of contaminants.

Halogen contaminants can also cause voiding failures. For instance, aluminum bromide formed from free bromine can volatilize, creating voids within the bonds.

2) Presence of Contaminants

The presence of halogen contaminants on the bond pads can cause the bond pads to corrode in the presence of moisture. Corrosion per se is a major cause of bond failure as the bond and wire are eaten away. The formation of corrosion byproducts are harmful too, especially if already present at the time of bonding, since these can impede the sticking of the bonds onto the bond pads. The presence of other types of contaminants on the bond pad such as unetched glass or silicon dust also impede proper bond formation between the wire and the bond pad.

Contaminants on the lead fingers where second bonds are formed likewise cause weak bonds, or even non-sticking. Such contaminants include residual plating bath components as well as metallic impurities. Organic contaminants in raw leadframes are a common issue too.

3) Looping Problems

Correct wire looping is important during wirebonding. Lack of adequate wire looping can result in excessive stresses at the bond neck or heel, which can lead to neck and heel breaks when the device is subjected to thermo-mechanical stresses. Excessive wire looping, on the other hand, can result in sagging wires and wire sweeping, both of which can cause wire shorting. Voiding is generally caused by unequal diffusivities exhibited by the different metals used in the wire bond, a phenomenon known as 'Kirkendall Effect.'. In gold ball bonding, for example, the rates of diffusion of gold atoms from the gold ball into the aluminum bond pad and the aluminum atoms from the bond pad into the gold ball bond are unequal. Voiding failures from such interdiffusion process can be accelerated by long exposure to high temperatures and the presence of contaminants.

4) Bond Placement/Geometry Problems

The bond must be placed well within the bond pad. A bond that is partially positioned outside the open window of the bond pad can result in weak bonding or, worse, shorting with an active metal or another bond. Inferior bond geometry as characterized by under- or over-sized bonds and/or incorrect aspect ratio can also lead to weak bonds.

5) Bonding Site/Substrate Issues

Aside from surface contamination, there are other wire bonding site or substrate problems that can lead to bonding failures. Common bonding site/substrate issues include: 1) excessive probe digging on bond pads; 2) lifting of the bond pad metal; 3) voids in the silver plating of the lead fingers; 4) silicon nodules on the bond pad; and 5) silicon damage beneath the bond pad which can lead to cratering (cratering, by the way, is generally attributed to fractures caused by overbonding).

6) Equipment-related Problems

Equipment-related issues that can cause wirebond failures include: 1) incorrect parameter settings; 2) incorrect equipment set-up; 3) calibration issues; 4) dirty, damaged, or worn-out capillaries/bonding tools; 5) excessive vibrations; 6) reverse motion/looping control problems.

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