Page 128 - Wire Bonding in Microelectronics

106 Cha pte r F o u r

Table 4-1 was summarized from extensive data from several
sources to show the bond characteristics and values obtained by this
method. Shear force, SS values, and intermetallic percentages are com-
pared for bonds intended for a range of fine-pitch processes. The cor-
relation between the amount of intermetallic, or metal disruption, on
the ball and the shear strength has been established [4-26, 4-27], and,
thus, the actual amount of intermetallic is only determined occasion-
ally (e.g., for a major change in metallization or Au wire dopant).
One would assume that a simple square-law equation (related to
the bonded-ball area) using the data from Table 4-1 could be used to
predict the expected shear force of fine pitch bonds, knowing the actual
welded area (percentage of intermetallics). This could then be related
back to Fig. 4-17. However, the actual curve-fitting equation using data
from Table 4-1 turned out to be less than square law. ∗ It is
SF = 0.024 D 1.78 (4-7)
(welded area dia.)
where SF is the shear force in gf, and D is the equivalent diameter of the
actual welded area in micrometers. The correlation coefficient is: r =
2
0.999. This equation can be used on fine-pitch bonds (≤100 µm) to deter-
mine the equivalent welded diameter (and from that the percent that are
intermetallics) or the shear force if the percentage of intermetallics is
known. Also, using the data from Table 4-1, one can project that, for a
60-µm pitch process, the effective bonded-ball diameter would be 34 µm
(for 80% welded area and represents an actual diameter of 38 µm) and
the shear force would be 13.4 gf. From Fig. 4-17 it would be nearer 12 gf,
not a bad approximation. (See Chap. 9 for further fine pitch details.)
Using the actual measured bonded-ball diameter (in micrometers),
the equation becomes:

SF = 0.023 D 1.75 (4-7a)
(ball dia.)
2
where r = 0.98. Equation (4-7a) assumes ∼80% welded area under the
bonds, and the diameter is measured optically as the outer perimeter.
This equation is easier to use than Eq. (4-7), since only the measured
diameter of the bonded ball is required. However, since the welded
area under production bonds normally varies, results on any experi-
ment may also vary. Still, this equation is an adequate approximation
for most finer-pitch (≤70 µm) situations (see testing of fine pitch
bonds, Chap. 9).

∗ Since the actual bonded area was used, a square-law behavior related to the
in creased cross-section should have been observed. The reasons for fine-pitch
bonded balls producing a shear-force-to-bonded-area relationship different from
larger diameter bonds are not understood, and more study is indicated. Very fine
pitch bonds have been observed to have a larger percentage of welded area (shear
2
strengths consistently >7 gf/mil than equivalent larger diameter balls).