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Wire Bonding Cycle

 

The wire bonding cycle using copper wire is almost the same as the gold wire as it forms the ball bond, loop, and stitch sequence. The introduction of forming gas during the free-air-ball formation for copper wire is the only difference in the process. Forming gas consist of 95% Nitrogen to prevent copper wire from oxidation and 5% Hydrogen for flammability enhancement to create concentric FAB during EFO (electronic flame off) firing. Highly oxidized copper free-air-balls are basically harder and more difficult to bond on sensitive silicon technology. In addition, the forming gas helps to inhibit the oxidation of copper wire once exposed to ambient temperature of wire bonders’ heater block.

1. Free Air Ball  --------------------------------------------------------------------------------------------------------------------
  
To achieve consistency of the free air ball size, it requires consistent tail length after second bond formation, and consistent electronic flame-off (EFO) firing.
  Gold Wire
After the second bond application, the capillary lifts up with its tail protruding outside the capillary tip. This action would then enable the electronic flame off (EFO) to be activated to form the gold FAB and then the bondhead goes to the reset position for the next bonding cycle. Thus, the FAB is already formed before the next bonding cycle begins.
  Copper Wire
To form a concentric copper free-air-ball, a “Copper Kit” is a must have to ensure continuous controlled flow of forming gas towards the tip of the capillary and torch electrode (EFO Wand) to prevent copper from oxidation. After the stitch bond application, the capillary lifts up with its tail protruding outside the capillary tip then the bondhead goes to the reset position with the absence of the FAB. The copper bonding cycle starts with a formation of the FAB.
 
2. Free-Air Ball is centered inside the capillary chamfer area  ------------------------------------------------------
  A wire tensioner is used to ensure that the free air ball is up and at the center of capillary face prior to being lowered onto the die bond area. If this condition is not met, there is a chance of producing an irregular ball bond deformation commonly known as “golf club ball bond”.
     
3. Ball Bond Formation  ----------------------------------------------------------------------------------------------------------
  The capillary is lowered with the free air ball at its tips’ center, and initial ball deformation is made by the application of impact force.

The application of the ultrasonic energy, force, temperature and time enabled the initial ball to be deformed further to the geometrical shape of inside chamfer, chamfer angle and the hole.
     
4. Capillary forms the loop and then stitch bond  -----------------------------------------------------------------------
 
After the ball bonding, the capillary raises, looping takes place as the capillary travels at the same time from the first position of the ball bond to the direction of the second bond to form the stitch.

The looping can be varied to a different modes depending upon the device / package type. Achieving low-loop, long lead bonding is no more a problem because of the programmable looping algorithm that optimizes its formation for each different lead length.
     
5. Tail formation after the stitch bond  --------------------------------------------------------------------------------------
  Once the capillary reaches the targeted second bond position, the stitch is then formed with similar factors applied during the first bond. The capillary deformed the wire against the lead or substrate producing a wedge-shaped impression.
 
     
6. Capillary lifts and forms a tail  ---------------------------------------------------------------------------------------------
  It is important to note that a certain amount of tail bond is left to allow pulling of the wire out of the capillary after the stitch bond application in preparation for the next free-air-ball formation.

 

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