WCA July 2011

other than the casting process. One is called “contamination” breaking, when foreign materials are found on the fracture surface, and the other is called “pressed hollow” breaking, when no foreign materials are found. Additionally, “split end” breaking is considered to be caused by surface flaws, and “tension” breaking is caused by applying a stress stronger than the wire strength 5 . 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.

Nothing

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%))

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

Frequency

3.1 Effect of inclusions inside a wire

D

/D

i

o

Most inclusions in wires are hard materials. The results of the analysis of inclusions on the fracture sur- faces 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.

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

and fewer inclusions in bars and wires brought about by effective material cleaning and because of the improvement of drawing conditions. However, the finer the diameter of a wire, the higher the frequency of occurrence of wire breakage during drawing. There are several causes of wire breaking ( Figure 1 ). Two types of breaking are caused by inclusions. One occurs when inclusions are found on the fracture surface, and the other, called “hollow end” breaking, occurs when traces of concave inclusions are found but the inclusions themselves are not found. Two types of breaking are caused by foreign materials introduced during a process

66

Wire & Cable ASIA – September/October 2007 July/August 2011