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Ball Bond Reliability

The area of the bond pad directly affects the effective ball shear readings taken from the bond units. Considering the fine-pitch & ultra fine-pitch application, one can expect much lower set of ball shear readings for 70um bond pad size as compared with the standard devices greater than 125um. The maximum value the mashed ball diameter (MBD) can go, is much dependent on the area of the bond pad. Typically, the MBD is less than 5 to 10um of the bond pad size opening depending upon the amount of FAB and the capillary tip design used. However, the effective ball contact area is described as ball contact diameter (BCD), which is less than the diameter of the MBD – the area where the actual intermetallic formation exists between the bonded ball and the bond pad surface.
Ball Shear Stress


Since the Ball Shear Reading (BSR) will be obviously low as the area of the contact decreases, it is important to consider the normalized BSR instead or it is referred to as the Ball Shear Stress (BSS) measured in N/mm2. The formula to use is:
Ball Shear Stress, BSR Ball Shear Stress  =  BSR / Pi BCD2 / 4
 

Other factors of consideration, which directly affect the ball shear readings, are the following:

  • The manner the ball is sheared by the tool with respect to the ball height – typically set at 3 – 5um range – from the bond pad surface. 
  • The tip size of the ball shear tool must not interfere with the adjacent bonds – considering the bond pad pitch – during the actual testing. 
  • Ball shear tester equipment must be free from any influence of vibration in its surroundings. 
  • The ball shear tool speed setting is recommended at 200um/sec. 
  • Different failure mode of ball shear test.

 

Ball Shear Stress, Mode 1

Ball Shear Stress, Mode 2

Ball Shear Stress, Mode 3

Shear mode 1 – Ball lift

Shear mode 2 – Aluminum shear

Shear mode 3 – Bond shear

Ball Shear Stress, Mode 4

Ball Shear Stress, Mode 5

Ball Shear Stress, Mode 6

Shear mode 4 – Ball shear

Shear mode 5 – Pad peeling

Shear mode 6 – Cratering

Ball Shear Stress, Mode 7

 

  

Shear mode 7 - Wire shear

 

 

 
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