Theoretical and numerical analysis of the effect of constant velocity on thermosonic bond strength
Introduction
Wire bonding is one of indispensable micro-joining techniques to the electronic packaging. There are three methods of wire bonding, namely, thermocompression, ultrasonic, and thermosonic, and the last is used for the majority of interconnections to integrated circuits because of the nature of the process: self-cleaning, low temperature, high yield rate and flexibility. The operating parameters of thermosonic wire bonding can be varied to obtain the optimum bond strength. The constant velocity is the speed at which the capillary impacts the bond pad, which is an important bond parameter used by assembly engineers to assess the size of the bonded ball. The constant velocity also affects strongly the bond strength.
Some investigations [1], [2], [3], [4] have been carried out to study the effects of bonding parameters including the constant velocity on bond strength. Their studies gave many valuable conclusions. However, it is believed that the basic mechanism of thermosonic bonding is not well understood [5]. It is very important to understand fundamental mechanism of bonding parameters effect on bond strength today, especially when finer pitch bonding becomes the primary trend. Traditionally, experimental method was adopted to examine the effects of these variables on bond strength. However, it is very difficult to investigate the bonding mechanism by experimental method because the bonding is a micro and transient process. More recently, numerical studies [6], [7] have also been used to analyze the ultrasonic bonding process. However, there are two difficulties in investigating the effect of bonding parameters on bond strength by FE method. The one is that there is no definite relationship between the bond strength and the results obtained directly by FE analysis such as stress and plastic strain. The other is that the numerical implementation of the ultrasonic vibration requires advanced numerical techniques, such as contact area searching and boundary welding, which makes the calculation more difficult to perform.
Takahashi and Inoue [8], [9] gave an excellent simulation of wire deformation in thermocompression bonding by using FE method. They investigated the interfacial deformation between pad and wire and the effect of the pad thickness, the pad hardness, and the tool shape on the interfacial deformation. Their study offered a good idea that the effect of bonding parameters on bond strength could be found by FE method. Yeh and Lai [10] investigated transient mechanical responses of the Cu/Low-K structure during the impact stage of the wire bonding, and researched the effect of constant velocity on the stress distributions of pad and Cu/Low-K structure. Liu et al. [11] also performed a transient non-linear dynamic method to model thermosonic bonding. His simulation included the ultrasonic transient dynamic bonding process and the stress wave transferred to bond pad device and silicon in the 1st bond. All these previous studies provide solid foundation for further investigation of bond strength mechanism by FE method.
All these consideration lead to strong interest on thermosonic bond strength mechanism by FE method. The purpose of this study is to present a new analysis method of thermosonic bond strength. The effective relationship between bond strength and strain or stress is detected firstly according to theoretical equations in solid state welding. Then the compete process of thermosonic bonding is simulated in the proper boundary conditions. Furthermore, the effect of constant velocity on bond strength is studied using the present method, which is also verified by the experimental data.
Section snippets
Theoretical equations of bond strength
Thermosonic bonding belongs to solid state welding. The bond surfaces are usually over-laid by cover layers or contaminant films. Quick reciprocating motion and bonding load help disburse contaminates on the contact surfaces and the oxide layers of the bonding material, force the raw surfaces of the bonding material together, and also form a strong bond with the help of the thermal energy.
The thermosonic bond can be initiated and established in the areas of the contacting interface surfaces
Finite element model
The actual thermosonic bonding process is rather complex because of such factors as the tool shape and the input of ultrasonic vibration, but it is possible to simulate the process by simplifying the boundary condition. In the present study, the following considerations are included:
- (1)
The free air ball (FAB) and pad are regarded as rate dependent elastic plastic material. The capillary is considered as a rigid body due to its great hardness and Young’s modulus.
- (2)
The contact intermetallic effect and
Effective normal pressure
Fig. 8 shows the fluctuations of bonding force with the change of bonding time in different constant velocities. Table 2 shows the statistical variables of bonding force during the bonding. It indicates that not only the average value of bonding force but also the values of SF and ΔF increase with the increasing of the constant velocity. That is to say, the oscillation of bonding force is larger, which does not facilitate the control of bonding force during the ultrasonic stage. Fig. 9 shows
Conclusions
In the present study, a transient non-linear dynamic finite element framework was developed, and the strain rate sensitivity of the gold ball and pad was considered. The whole simulation included the impact and ultrasonic stages. The proper contact conditions and the vertical position curves of capillary were applied to the model based on the experimental results. We offered some related theoretical equations in solid state welding for the establishment of relationship between bond strength and
Acknowledgement
The authors would like to acknowledge the supports of National Natural Science Foundation of China through Grant NSFC 50390060 and 973 Program 2006CB705400.
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