Logo

Capillary Profiles Affecting Ball Bond

Hole Size (H) is determined based on the Wire Diameter (WD) to be used in a given application. Typically, the ratio is around 1.2X to 1.5X of the WD. A smaller hole size ratio is necessary for ultra-fine pitch application to compensate for the smaller chamfer diameter requirement.



Chamfer Diameter (CD)  is determined based on the target Mashed Ball Diameter (MBD). Normally, the MBD is restricted by the bond pad-opening dimension.



Chamfer Angle (CA) provides a certain amount of squash out in the formation of MBD. It also controls Free Air Ball (FAB) centering during impact. Typical chamfer angle is 90°


Free Air Ball (FAB)
The combination and interaction of the hole size, chamfer diameter, chamfer angle, and inner chamfer determine the total amount of volume necessary to form the ball bond. The total volume of FAB must be greater than the volume created by the above combination so that enough gold material is squashed out of the chamfer area to form the desired MBD.
 

 

Typical Capillary Hole Size Selection Based on Wire Diameter
The proper selection of hole size for a given wire diameter is vital in the design of the capillary. This applies not only for fine pitch application but also for standard designs.  Table 1  summarizes the recommended combination, which would provide better control and consistent looping profile.
 

Table 1
Given Wire Diameter
(in µm)
Hole Size (in µm)
12 / .0005  15 / .0006 - 16 / .00063
15 / .0006 18 / .0007 - 21 / .0008
18 / .0007 21 / .0008 - 25 / .0010
 20 / .0008 25 / .0010 - 28 / .0011
 23 / .0009  28 / .0011 - 30 / .0012
 25 / .0010  33 / .0013 - 38 / .0015
 28 / .0011  35 / .0014 - 38 / .0015
 30 / .0012  38 / .0015 - 41 / .0016
 33 / .0013  43 / .0017 - 46 / .0018
 38 / .0015  51 / .0020 - 56 / .0022
 51 / .0020  64 / .0025 - 68 / .0027
 64 / .0025  75 / .0030 - 90 / .0035
 75 / .0030  90 / .0035 - 100 / .0039
 100 / .0039 127 / .0050
 127 / .0050  178 / .0070

 

Ball Bond


In ultra-fine pitch ball bonding, the consistency of the mashed ball diameter (MBD), looping, and stitch bonds are essentially required in order to define a robust process.
 
The following considerations are important to produce a consistent MBD:

1.  Consistent and symmetrical free-air-ball (FAB) is important to produce a consistent MBD.

Consistent & Symmetrical FAB UnSymmetrical FAB UnSymmetrical MBD from unsymmetrical FAB

2.  Correct capillary design considering the hole size, chamfer diameter, chamfer angle, wire diameter, targeted MBD, and mashed ball height (MBH).


FABØ3 = 1.5H2(H-WD) + (CD3-H3)/4tan(0.5CA) + 1.5MBD2(MBH)

 
3.  Controlled impact or initial force is needed for better control and consistent ball height
 


                                                                                                               Uncontrolled MBD & MBH due to high impact force
 
4.  Optimized bond force, and bond power - for wider process window

 
  Contour Plot (Ball Shear Stress N/mm2)
Z=211.474+26.525*x+0.911*y-0.729*x*x-0.021*x*y-8.763e-4*y*y
Star Point
 

Consistency of the Small Ball Bond Deformation
The continuous growth in the development of new packaging technology has posed a greater challenge for wire bonding process to optimize the ball and the stitch bonds. Maintaining consistency in the formation of bonds is the key to success. To attain a consistent small ball bond deformation, the following are essential consideration:

  • Optimum capillary design selected - typically, the hole size, chamfer diameter, and chamfer angle are the major dimensions in consideration. A 900 chamfer angle (CA) as a standard; given a hole size (H)= WD + 8µm as the minimum; and chamfer diameter (CD)= H + 10µm as the minimum.
  • Consistent free air ball and wire diameter aspect ratio- around 1.6 to 1.7x WD range.
  • Consistent tail length protruding outside the capillary tip after the second bond
  • Consistent electronic flame-off firing to form the free-air ball.
  • Maintaining adequate gap between the tail and the EFO wand to prevent shorting or open wire problems.

Consistent EFO firing to form consistent free-air ball Consistent tail bond after stitch bond

 
Copyright by SPT-Group http://www.smallprecisiontools.com