60 / 108
EuroWire – May 2011
58
technical article
Types of wire breakage that occur when
gold bonding wires are drawn were
examined.
Figure 2
shows the frequency of
occurrence of each type of wire breaking,
with the diameters of the drawn wires
divided into three groups, 100-350μm,
50-99μm and 20-49μm
4
.
Figure 1
and
Figure 2
show that surface
flaws, foreign materials and the occurrence
of excessive drawing stress caused by
seizing are causes of wire breaking.
However, many of the causes of wire
breaking for wires with a diameter of less
than 50μm are speculated to be due to
inclusions inside wires that form during
casting
5
.
Considering the above it can be concluded
that for wires to be resistant to breaking
during drawing they should have no
surface flaws and any inclusions be as
small and as few as possible.
3 FEM analysis of
drawing of a wire
with inclusions or
foreign materials
3.1 Effect of inclusions inside a wire
Most inclusions in wires are hard
materials. The results of the analysis of
inclusions on the fracture surfaces of
wires by Energy Dispersive Spectrometer
(EDS) showed that many of them were
A1
2
O
3
or SiO
2
, and the rest were foreign
materials formed by abrasion of the die
and equipment. Therefore, assuming that
there are inclusions of alumina and foreign
materials inside wires, an FEM analysis of
wire drawing was carried out. The ratio of
inclusion size to wire diameter, Di/Do, was
set to vary from 0.3 to 0.7. The material
constants and drawing condition for FEM
are shown in
Table 1
.
The changes in drawing stress on the
inner side of the die were examined by
FEM analysis, using wires with various
sizes of inclusions. The results are shown
in
Figure 3
. It was found that drawing
stress moves rapidly upwards when an
inclusion passes through the die. It can be
seen that the higher the ratio of inclusion
size to wire diameter, Di/Do, the more the
drawing stress rises. In the case of a wire
with an inclusion for which Di/Do is 0.7,
the drawing stress reaches the strength of
the wire. This means that there is a high
probability that the wire will break. Taking
the safety factor into account, it is thought
that there is a danger that the wire breaks
when Di/Do is higher than 0.4.
The sizes of inclusions were measured at
the fracture surface when gold wires with
the diameters of 20-50μm were drawn.
Figure 4
shows the frequency of wire
breaking for different values of Di/Do. This
figure suggests that there is a danger of
wire breakage if Di/Do is 0.3 or higher, and
the highest frequency of wire breakage is
when Di/Do is approximately 0.7.
3.2 Effect of foreign particles
In some cases foreign materials are present
on the wire surface during drawing, or
foreign materials may enter the die via
the lubricant. These foreign materials are
mainly formed by erosion of the wire or
of the die or equipment, or may originate
from dust in the air. Depending on the
shape and hardness of the foreign material
wire breakage, as shown in
Figure 5
, may
occur. As an example,
Figure 6
shows
scanning electron microscope (SEM)
images and EDS photographs of a wire
after drawing with foreign materials on
the wire surface. The material of the wire is
austenite stainless steel.
Analysis by EDS revealed that the foreign
material was composed of iron carbide,
which included a scarce Ni component.
Non-dimensional wiredrawing stress
Non-dimensional drawing length
▲
▲
Figure 3
:
Variation of drawing stress when inclusion passes through die (examined by FEM(R/P=10%))
Frequency
▲
▲
Figure 4
:
Frequency of wire breaking vs D
i
/D
o
D
i
/D
o
Nothing