EoW May 2011

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. 3.1 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 3 FEM analysis of drawing of a wire with inclusions or foreign materials

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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

D

/D

i

o

▲ ▲ Figure 4 : Frequency of wire breaking vs D i /D o

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.

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EuroWire – May 2011