Research On Thermosonic Bonding Mechanism Based On Interfacial Deformation And Microcontact

The thermo-sonic bonding was born in 1970s, which is one of the most important micro-joining ways in the electronic packaging. The abundant process technologies and experience about parameter selection have been accumulated from the long-time practical operation. However, the obtained knowledge can not meet the requirements from the modern electronic packaging with the shrinking wafer line dimensions when it tends to be the robust, stable wire-bonding processes capable of making reliable interconnections. The primary problem is that there is still not mathematic model about the thermo-sonic bonding which can provide the base for the improvement of bonding process and the innovation of bond machine.The bond strength is one of the critical factors affecting the interconnection reliability, and the research on the bond strength is always the focus of bonding mechanisms. Today, some key problems, e.g. what is the principle of interfacial joining, how to affect the joining does the ultrasound, what is the effect of complex contact phenomena on the joining from high-frequency ultrasonic vibration, are still not solved. In the present study, the constitutive model considering the ultrasonic softening mechanism, the dynamic friction model of bond interfaces, microcontact model of rough surfaces, and the void shrinkage model are established by combining the basic theories such as material science, contact mechanics and atomic diffusion, and the experiments including the thermosonic bonding test, bond shear test and micro pattern observation of bond interface. The interfacial deformation, sliding contact and the formation and increasing of real contact are investigated to offer the necessary inputs to the calculation of bond strength. Finally, the calculation method of bond strength is presented, which provide the basic theories for the improvement of bond process and the innovation of bonder machine. Above all, the research fruits in this dissertation have very important theory value and practice significance.1. Research on basic scientific problems of thermosonic bondingThe basic scientific problems of thermosonic bonding include the material constitutive model considering the ultrasonic softening mechanism and the complex interfacial contact phenomena from high-frequency ultrasonic vibration.(1) Material constitutive model considering the ultrasonic softening mechanism The material mechanical characteristics are the base for studying the interfacial plastic deformation and microcontact phenomena. The development of bonding process makes the trouble that there exist errors when the Takahashi constitutive model is adopted to simulate the thermosonic bonding. In addition, the ultrasonic energy also produces more difference between the material deformation in thermosonic bonding and the general deformation. According to experimental data, the Takahashi model was improved to make it possible to be used in the analysis of thermosonic bonding. The constitutive model considering ultrasonic softening mechanism was finally presented by combining the bonding test and theoretical research, which was also proved by comparing the results of numerical study and experiments.(2) Sliding friction and local microslip of bond interfaceThe dynamic interfacial friction model was firstly established based on the energy transferring through the bond interface. It is found that the"stick-sliding"and microslip are the two basic forms of interfacial contact phenomena from high-frequency ultrasonic vibration. The matching of preload to ultrasonic power can be used to determine whether the"stick- -sliding"or micro-slip exists. The equation of temperature rise from the sliding friction and the energy dissipation from the microslip are deduced. The exponential function relationship between the energy dissipation from microslip and initial bond strength is discovered. The microslip theory is the proper interpretation for the phenomena that the joining prefers to exist at the edge of contact area.2. Research on essential factors affecting bond strengthThe preliminary relationship equation between interfacial plastic deformation and bond strength and the interfacial microcontact model are established. The interfacial deformation and microcontact phenomena are regarded as the two essential factors affecting bond strength.(1) Bond interfacial deformation and it relation to bond strengthThe 3D finite element model coupled temperature-displacement was established based on the constitutive equation considering ultrasonic softening mechanism. The interfacial stress and temperature distributions are studied. The cratering and pad metal peeling at the edge of contact area was found in the SEM image of bond interface, which reason is that there exists a peak of compressive stress at the region. The basic equation of bond strength was introduced and deduced to make it possible to be used in the analysis of thermosonic bonding. The microslip theory has to be adopted to modify the basic equation because the strong non-uniform distribution of interfacial deformation. The revised equation can be used to calculate the bond strength but the calculation results are all higher than the real ones. (4) Formation of interfacial real contact and its increasing.The discontinuities of contact area and pressure functions between the elastic and plastic stages in the Horng microcontact model were modified. The revised model was used to solve the initial contact area of bond interface. It is found that the real contact area increases linearly with the increasing of preload but the contact ratio increase will decrease after it exists the maximum. The real contact area increases with the increasing of ultrasonic power while the contact ratio always decreases. Based on the proper simplifications of void geometry and its shrinkage process, the velocity equations from creep and interfacial diffusion mechanisms were deduced, and thus the increasing of real contact area can be solved by these equations. It was found that the stress gradient in the tip of void increases with the increasing of preload, and then the role of interfacial diffusion increases. The role of creep mechanism also increases with the preload or ultrasonic power. So the increasing value of real joining area becomes higher when the preload or ultrasonic power increases.3. Calculation method of thermosonic bond strength based on essential factors and its verificationThe relationship equation between interfacial deformation and bond strength was modified by the contact ratio. So the calculation mehtod including two essential factors, i.e. interfacial deformation and microcontact, was presented. The thermosonic bonding tests were carried out to obtain the effects of bond parameters on bond strength and intermetallic coverage ratio. The calculation method was verified by comparing the calculation and experimental results with different bond parameters.In this study, the basic scientific problems such as the bond material characteristics and interfacial deformation and microcontact were all investigated. The deformation behavior of bond material under the thermal, force and ultrasonic conditions and the relationship between the interfacial complex contact phenomena and bond strength were discovered. The interfacial deformation, the formation and increasing of real contact area in bond interface were studied, and thus the theoretical calculation method could be presented and was verified by the experimental results. So the thermo- sonic bonding mechanism was clarified.