Bond Lifting

Bond lifting refers to any of several phenomena in which a wire bond that connects the device to the outside world becomes detached from its position, resulting in loss or degradation of electrical and mechanical connection between that bond and its bonding site. 

In this context, a bond may be one that attaches to a bond pad of the die (also referred to as the first bond) or one that attaches to a lead or post of the package (also referred to as the second bond).  First bonds are usually in the form of gold ball bonds or aluminum wedge bonds, while second bonds are usually gold or aluminum crescent bonds (also known as 'fishtail' bonds).

Ball bond lifting, or simply ball lifting, is the detachment of a ball bond from the bond pad of a semiconductor device.   It can be due to a variety of factors.  Poor wire bond equipment set-up and bond pad surface contamination are primary causes of ball lifting.  Poor set-up includes improper wirebond parameter settings, unstable workpiece holders, and worn-out wirebonding tools.  These result in poor initial welding and inadequate intermetallic  formation between the bond pad and the ball.

Ball lifting can also be due to contaminants on the bond pad, which act as barriers between the ball and the bond pad.  Common contaminants that inhibit good bonding include unetched glass, unremoved photoresist, and Si saw dust.  Resin bleed-out from the die attach material can also impede good bonding and result in ball lifting. Halides such as Cl on the bond pad can trigger corrosion, which is again another source of ball lifting.

A disturbed or uneven bond pad surface also inhibits bonding. Excessive probe digging results in aluminum heaps and an exposed substrate or barrier metal area, which prevent good intermetallic formation. Silicon nodules on the surface of bond pads can also result in poor ball bonding.

Lifted balls may also result from excessive interdiffusion between the bond pad and ball bond metals. Kirkendall voiding, which is the formation of voids underneath the ball bond due to excessive diffusion of Al from the bond pad to the Au ball bond to form purple plague, is an example of this mechanism.  The reflow of thermoplastic die attach material at the bonding temperature also results in ball lifting, because it allows movement of the die during the thermosonic bonding itself. 

Cratering, which is considered to be a different failure mechanism, can also manifest as a lifted ball, with the Si underneath the bond pad coming off with the bond.  Excessive probing and overbonding are common causes of cratering. Similarly, bond pad peel-off, or the mechanism wherein the bond pad metal peels off from the barrier metal or substrate, can result in ball lifting.