Die Lifting

  

Die lifting is the disbonding or detachment of the die from its die pad or die cavity. A die that undergoes die lifting is commonly referred to as a 'lifted die'

          

Die lifting mechanisms may be classified into three types: 1) die lifting caused by a fracture within the die attach material itself (cohesion failure); 2) die lifting due to delamination between the die backside and the die attach material (adhesion failure); or 3) die lifting due to delamination between the die attach material and the die pad or cavity (adhesion failure). Determining which of these mechanisms is predominant in a die lifting issue is important in preventing its recurrence.

    

Excessive voids, insufficient fillet formation, and inadequate bond line thickness lower the fracture strength of the die attach material, which can lead to its cohesion failure once the unit is subjected to thermo-mechanical stresses. When this happens, the die attach material fractures in the middle and results in die lifting, leaving die attach material still sticking on both the die backside and the die pad. The degradation of the mechanical strength of the die attach material can also be due to: 1) contamination; 2) chemical degradation with time; and 3) chemical degradation from external factors, e.g., moisture, temperature, etc.

 

      

Figure 1. SEM photo of a lifted die

caused by insufficient die attach fillet

          

Adhesion failures can also be caused by the aforementioned issues, i.e., excessive die attach voids, insufficient fillet formation, inadequate bond line thickness, and die attach material problems. However, they are also frequently encountered when contaminants are present on the attachment surface.  Thus, contaminants on the die backside can lead to die attach-to-die delamination, while contaminants on the die pad can lead to die attach-to-die pad delamination. Either way, the resulting delamination can lead to die lifting. Eutectic die attach delaminations may also be due to inadequate scrubbing, incorrect preform size, and improper equipment settings.

          

Inadequate die attach fillet formation and excessive die attach voids act as stress concentrators that can also result in contiguous cracks at the backside of the die, especially in units that use eutectic die attach. These cracks can propagate to a point wherein the upper part of the die is separated from the bottom part. If the bottom part of the die is still attached to the die attach system, then this, technically, is still a die cracking  problem (not die lifting), although extreme cases indeed give the impression that the die has lifted off from its resting place.

    

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

      

See also:   Package Failure MechanismsDie Crack FA FlowDie AttachFailure Analysis

     

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