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.

             

 

Fig 1. Photo of a lifted ball bond

Fig 2. Photos of bond pads w/ contamination that prevented good intermetallics and led to ball lifting  

      

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.

         

Fig 3. Photo of a bond pad crater

Fig 4. Photo of a bond pad metal peel-off that led to ball lifting

       

Wedge lifting is the detachment of a wedge bond from the bond pad or bonding post,  or the crescent bond from the leadframe bonding finger. Like ball lifting, it can be due to a variety of factors, primarily poor wirebonder set-up and bond pad surface contamination.  Poor set-up includes improper parameter settings, unstable workpiece holders, and worn-out tools.  These result in poor bonding between the bond pad, post, or finger and the wedge.

      

Wedge lifting can also be due to contaminants on the bond pad, post, or bonding finger.  Contaminants act as barriers between the wedge and the bonding area. Common contaminants that inhibit good bonding include unetched glass, unremoved photoresist, and Si saw dust. Halides such as Cl on the bond pad can also trigger corrosion, which is another cause of wedge lifting. Silicon nodules on the surface of bond pads with no barrier metallization underneath can also result in poor ball bonding.

  

Sub-bond pad cratering, which is considered to be a different failure mechanism, can also manifest as a lifted wedge, with the Si underneath the bond pad coming off with the bond.  Similarly, bond pad peel-off from the barrier metal can result in wedge lifting. Wedge lifting due to metallization peeloff from the bonding post and fingers are likewise possible.  Studies have also shown that excessive probing damage on the bond pad can cause wedge lifting.

       

See also:  WirebondingCommon Causes of Wirebonding FailuresBall Lifting FA Flow;

Package FailuresFailure AnalysisReliability Models

 

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