1. A unit of gold bonding wire-1

Mr. Joe:

Do you know how fine a bonding wire is?

Miss Lisa:

Well …although I saw one in the last Bonding-lab and have come to understand that it is a fine wire, I can’t imagine its actual size.

Prof. Birdie:

Lisa, the units for manufacturing of wire such as a size, length, and weight, have a very important meaning. Learning the actual scale of bonding wire is also important. Joe, why don’t we start with the unit used for gold bonding wire this time?

Mr. Joe:

Ok then, first of all I would like to talk about the size. Generally speaking of wire, the most well-used ones are made from steel and copper, but bonding wire is made from gold, aluminum and copper. Although steel and copper wires are bunched up several wires together to be used at mot of the cases, but bonding wire is used as a single wire to connect IC chip and lead frame at all time. The diameter will be called the size of a gold bonding wire. A diameter of 20 to 40 micrometers (micrometer grade, μm) is used on the average.

Miss Lisa:

Is the wire diameter variable?

Mr. Joe:

Well, actually it is not. The size of bonding wire is always decided by use. For instance, sometimes it would be 20μm or 40μm depending on what the uses are. But it dose not mean that the wire has a variety of diameter within the same wire.

Prof. Birdie:

A micrometer is 1/1,000,000 of a meter in size. It seems that there are also some people who have gotten used to calling it a micron, since it was the term used a couple of years ago.

Miss Lisa:

Anyway, you mean it is extremely fine, isn’t it?

Mr. Joe:

Generally, it is said that the diameter of a typical person’s hair is about 80μm, so that the size of bonding wire is less than half of this value. Furthermore, seemingly the white corpuscles in blood are 10μm to 20μm in diameter.

Miss Lisa:

I don’t understand which is small or large anymore.

Mr. Joe:

By the way, we’ll state briefly that a mil is 1/1,000 of an inch, and is equal to about 25.4μm.

2. A unit of gold bonding wire-2

Prof. Birdie:

Ok, Joe. Why don’t we stop talking about the diameter and size of bonding wire, and move onto the next topic?

Mr. Joe:

I think this will be easier to understand than the last topic, Lisa. Although the length of a bonding wire is also based on the application, 3000m is most commonly used. A bonding wire is wound on to a spool (which is also called a bobbin or a reel) and is shipped out on it.

Miss Lisa:

3,000m is about the same length as my jogging trails.

Prof. Birdie:

In other words, if we talk about a golf course, 3km is only a half round of a course or a little longer. However, it is more common to use yards in golf in Japan. 1 yard is about 0.9m.

Mr. Joe:

I think I run more than that. Ok, back to the subject. I would like to talk about the weight at the end of this session. Although I have talked about diameter and length, a bonding wire is in addition described by weight. Since the density of gold is 19300kg/m³, a 3km gold bonding wire weighs about 24g in the case of a diameter of 23μm.

Miss Lisa:

Ok…….

Prof. Birdie:

You sound confused. Let me then give you some additional information. Assuming the same diameter and length, the weight is about 11g for copper, about 9.8g for iron and 3.4g for aluminum.

Miss Lisa:

It certainly reminds me how heavy gold is.

Mr. Joe:

Alright, shall we now discuss the manufacturing process? The character of gold always changes with purity. Usually, 24 carats for things said to be made of pure gold corresponds to a purity of 99.99% or more. Although the purity is 99.99% or more in the case of a typical gold bonding wire, a bonding wire with suitable physical properties can be made by adding various elements into gold of 99.999% or more purity.

Prof. Birdie:

18 carats corresponds to a purity of 75%; however, most gold bonding wires are of such high purity that they can be called pure gold.

Miss Lisa:

Even if a wire is rated at 18 carats, which has the appearance of the gold that we are accustomed to, it is very different.

3. Elasticity and plasticity

Mr. Joe:

Can you imagine how bonding wire is made with a size finer than that of human hair, Lisa?

Miss Lisa:

Is it cut like thin wheat noodles, taken out like jelly, or lengthened like starch syrup?

Mr. Joe:

That is an interesting way of thinking. It is all based on food. However, your idea is close.

Prof. Birdie:

Suppose that there are two balls: a ball made of rubber and a ball made of iron. Although a rubber ball can be changed simply by hand, an iron ball is hard and is impossible to reshape by hand. Even if the rubber ball changes a little, it returns to the original form. Even if one applies a load on a certain object and transfers energy to it, if the load is removed, the character in which the object returns to its original form exhibits “elasticity.”

Prof. Birdie:

However, I think that you know intuitively that an iron ball does not return to its previous shape as in the case of a rubber ball, even though an iron ball can be changed in someway or the other. Thus, the character in which an object, after being changed by adding load, does not return to its original form and size is called “plasticity.” Depending on how rubber is smashed, it either does or does not return to its original form. It is very difficult to carry out plastic deformation.

Miss Lisa:

Even if I don’t know a complicated concept, but I can understand it somehow.

Prof. Birdie:

Since the back ground is deeper from the academic point of view, we will proceed to the next step. Gold will be processed into long fine wires with this plasticity. Let us return to our discussion regarding the iron ball for a while. What would you do if you wanted to change an iron ball?

Miss Lisa:

Since such a ball seems to be very hard, it is impossible to change its shape by hand; it may be done using a bat or pliers, but I’m not sure.

Prof. Birdie:

Is that right? To change an object, it is necessary to apply a load exceeding a physical-property-based value called the “yield point” of the object. Furthermore, a tool that is harder than the target material is needed. Generally, in processing based on the plasticity of the material, the shape of the tool that is harder than the material to be changed can be thought of as being transferred onto the material.

4. Melting and Rolling Process

Mr. Joe:

It is getting off the track. Gold with a purity of 99.999% or more is not suitable for use as a bonding wire. Then, a melting process is carried out to form an ingot, in which gold of high purity is melted and mixed with various elements; this ingot serves as the base material of a bonding wire. The characteristic of the material will be decided almost completely by this process. The purity of this ingot as stated previously is 99.99%. The purity is still high but the character differs from that of 99.999% pure gold considerably.

Prof. Birdie:

Although melting processes, involving elements such as iron and aluminum, are performed under atmospheric conditions, when melting gold for bonding wires, a vacuum is used first and the gold melts in inert gas, such as nitrogen gas, in many cases. Although gold itself does not oxidize, is adopted to prevent the oxidization of the element to be mixed. Because the melting point of gold is about 1,064°C, oxygen activity will increase if at least gold is melted at a high temperature under atmospheric conditions.

Miss Lisa:

The temperature required to melt this ingot seems to be higher than that used to make gratin, since the temperature used for baking cakes is 180℃.

Prof. Birdie:

Hahaha, this is beyond your imagination, isn’t it? But the idea is very simple. Just think of an ingot as a huge block.

Mr. Joe:

First, the diameter of the ingot is gradually reduced by the processing method called rolling. The ingot is crushed by the tool that is harder than the ingot material; the tool is called a caliber roll. To lengthen the material without cutting or destroying it, the volume of the material before processing is kept constant, and only its form changes.

Prof. Birdie:

Since plastic processing enables us to transform the material with high yield and allows us to process it fast than other processing methods, it is an indispensable technology for various material processing techniques.

5. Drawing Process

Mr. Joe:

However, there is a limit in making the material thin by rolling; beyond a certain size, a different processing method called drawing is necessary. The material is made finer using tools called dies, after which a wire is fabricated. A very hard alloy and diamond are used for the dies.

Miss Lisa:

Is a diamond used? What a waste! I want it….

Prof. Birdie:

I bet you do. It is more convenient to use a diamond than other materials, since it is very hard and does not easily wear out; thus, it is a reasonable choice. Moreover, it is also an important tool for realizing smoothness on the surface of a wire, which is indispensable to a bonding wire.

Mr. Joe:

Let’s explain in more detail the processing method called drawing. We can easily understand the underlying principle by comparing the form of a die to that of sandglass. Let’s say the narrowed part of glass is the die and the sand is the gold. The sand accumulates at the narrowed part and falls gradually to the bottom. Thus, for a die, the diameter is made reduced, and the material is drawn to form a long fine wire.

Prof. Birdie:

In this drawing process, the key factors to be determined are the shape of the die, its diameter, and the quality of the die material. Drawing speed, lubricant type, intermediate annealing, and equipment, among others are important factors too. At the beginning, Lisa asked if the resulting wire diameter is variable. This drawing process is actually an excellent method of maintaining the cross section of the gold formed consistent, enabling us to pass gold through the die at a speed of several hundreds of meters per minute.

Miss Lisa:

The lecture is getting more complicated to understand. It seems so deep.

Mr. Joe:

Shall we call it a day?

Prof. Birdie:

Finally, do you have any idea when it was that man first started forming with gold?

Miss Lisa:

I guess it was at the time of the Egyptian civilization because gold was used for ancient accessories.

Prof. Birdie:

According to literature, gold had been lengthened and cut out from around the 30th century, from B.C. 20, gold had been drawn out by hand through a small hole; it is said that a gold wire had already been made by that time, which is 5000 years ago. It was around B.C. 1340 when the famous Tutankhamen reigned, so it was much long ago than that period.

Mr. Joe:

And we are still making gold wires even to this day.

Miss Lisa:

That’s fascinating history. I understand now to some degree what kind of material a gold bonding wire is from the talk on units of measurement to the drawing process. I look forward to the next lecture!!