TANAKA TECHNOLOGIES

Bonding Lab Process of gold bonding wire, vol.2

You will find our explanation on mechanical properties which are the most popular physical properties of bonding wire with focusing on tensile test carried out at the annealing process.

July 2009

As a series of gold bonding wire manufacturing process, the annealing process and the winding process are explained here as Vol.2. You will find our explanation on mechanical properties which are the most popular physical properties of bonding wire with focusing on tensile test carried out at the annealing process.

Mr.Aki電子チャンDr.Yama

1. Tensile Test Property & Type of wire

Miss Bondy:
I have learned up to “Drawing process" in Bonding-lab Lecture 2.
Dr. Yama:
Yes, Okay. Let's continue. In today's lecture, we will focus on mechanical property which is the most typical physical properties of gold bonding wire. Mr. Aki, please.
Instructor Mr.Aki:
Hi, Bondy. The next process of drawing is Annealing process (Heat treatment process) as shown in Fig.1. At annealing process, heat will be applied to the wire to obtain the proper mechanical properties.

Fig.1

Miss Bondy:
Mechanical Properties.....what exactly does mechanical property mean?
Instructor Mr.Aki:
It refers to one of the characteristics of gold bonding wire that consists of Breaking Load and Elongation. These 2 characteristics are measured by tensilon tester. In tensilon measurement, wire is secured at both ends by fixed wire length with constant pulling speed. The breaking load is measured at the maximum load applied and elongation is measured based on the wire elongated from the start until the wire breaks.
Miss Bondy:
Oh, I Understand. B.L. and El stated in the products guide are referring to Breaking Load and Elongation.
Instructor Mr.Aki:
Fig. 2 shows the IC manufacturing process. Breaking load and elongation are key physical properties which have close relationship to the wire bonding operation and molding process. The comparison data of wire types and their breaking load is shown in Fig. 3. Although all types are used for semiconductor assembly, each type is designed to perform well in different package applications.

Fig.2

Fig.3

Miss Bondy:
Oh I see. By comparing the breaking load, it is easy to understand the difference of each wire type.

2. Tensilon tester and heat treatment condition.

Miss Bondy:
By the way, what is the relationship between breaking load and elongation ?
Instructor Mr.Aki:
Good question! Generally, the mechanical properties of breaking load and elongation are controlled by the temperature of heat treatment. If the temperature is increased, the value of breaking load will be decreased (Blue line) but the value of elongation will be increased (Red line).

Fig.4

Miss Bondy:
In Fig 4, the red line exhibits the behavior of elongation when heat is applied.
Dr. Yama:
Under American Society of Testing and Material standard specification, ASTM, the elongation of Au bonding wire is generally between about 1~10% range, even though there are a few exceptional cases.
With improper heat treatment condition, such as too low temperature, it will cause the Au bonding wire in bad loop formation else if the heat treatment temperature is too high, breaking load will be too low.
Instructor Mr.Aki:
Fig.5 shows the crystal structure of bonding wire cross section. L1~L6 complied with lines shown in Fig.4.

Fig.5

Miss Bondy:
Oh, I Understand! When the heat treatment temperature increased, the crystal structure of the Au bonding wire will become larger.
Instructor Mr.Aki:
At annealing process, the stress accumulated in the crystals through the previous mechanical fabrication processes is relieved by heat, after that, recrystallization will take place.
Miss Bondy:
Oh! The taste would be much changed by having different temperature at the annealing process, right?
Instructor Mr.Aki:
Right! It is similar to cooking but the heat treatment condition is an important recipe for bonding wire.

3. Method of Tensilon testing

Instructor Mr.Aki:
Next, I would like to further explain about the method of tensilon testing. Fig 6 is the equipment used for tensilon testing, called Tensilon tester. As per earlier explanation, with the fixed length of wire and constant pulling speed, load value being given to the wire piece is measured in real time.

Fig.6

Miss Bondy:
How are the breaking load and elongation measured?
Instructor Mr.Aki:
The value of breaking load and elongation are obtained from graph, called Stress-Strain diagram. As Stress is load per unit cross-section area and Strain may be considered as elongation, breaking load measured from y-axis and elongation measured from x-axis is often named as Stress-Strain diagram as well in bonding wire industry.

Fig.7 Video.1

Miss Bondy:
I see! Breaking point that in Fig.7 shows elongation is associated with x-axis and the breaking load is associated with y-axis. It can be confirmed the moment it breaks as you can see in Video.1.

4. S-S Diagram explanation

Instructor Mr.Aki:
What we can find from the Stress-Strain diagram is not only mechanical properties at the breaking point.

Fig.8

Miss Bondy:
That is true. In Fig.8, it shows two different curves between Au Wire and Fe Wire.
Instructor Mr.Aki:
Generally, for the metal of steel, it clearly exhibits the yield point in the tensile testing and based on this yield point, it can draw a clear boundary between elastic deformation and plastic deformation.
Miss Bondy:
I have learnt elastic deformation and plastic deformation in Bonding-lab Lecture 2. However, are there elastic deformation and plastic deformation also in Au ?
Instructor Mr.Aki:
There are a lot of metals such as Au, Cu, Al and so on, that cannot clearly distinguish the elastic deformation and plastic deformation due to the yield point is not clearly exhibited. For those metals, proof strength which is similar to yield point is used to draw the boundary of the elastic deformation and plastic deformation. In Fig.9, a tangent line, L1, is drawn from the starting of Stress-Strain curve. Then, a parallel line based on tangent line, L1, will be drawn at 0.2% of elongation. The intersection of L2 line and Stress-Strain curve at P1 is called proof strength.

Fig.9

Miss Bondy:
Why do we use 0.2%?
Dr. Yama:
Because the yield point of steel is normally occurred at about 0.2% of its elongation. Therefore, for other metals, 0.2% of its elongation is used to determine the proof strength.

Information: Calculation of Young's modulus

Young's modulus (Elastic modulus) E is measured according to obey Hooke's law. If Y axis is tensile load, X axis is elongation in tensile tests; this can be experimentally determined from the slope (L1) of initial liner created during tensile test. Generally, Young's modulus of gold bonding wire is about 88GPa.
E = (Tensile Stress:σ) / (Tensile Strain:ε) = σ/ε
E = (Force per unit area) / (Elongate per unit length) = (F/A) / (ΔL/L0)
* F:tensile force, A:cross-section area,
* ΔL:force exerted which the length of the object by F, L0:measurement length

Miss Bondy:
Mr. Aki! My brain is getting overheated!

5. We will explain up to here !

Dr. Yama:
Okay, we shall stop here. The next process after annealing is winding process.
Instructor Mr.Aki:
In winding process, the prescribed wire length shall be wound on the spool like in Fig.1. Generally, winding length per spool is ranging from 100m~10,000m but subjected to customer's requirement. Winding may sound simple but Au bonding wire can be broken with so small as a few grams of load. However, pulling load is essential to wind the wire properly. Otherwise, it may be loosened and entangled during the transportation. Once it happens, the wire can not be used at our customers. You know, the winding is absolutely delicate process.

Fig.1

Miss Bondy:
Oh ! Really. Please share with me more interesting topics next time. Thank you, Mr. Aki!, Dr. Yama!