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Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on Nov. 10, 2009. "June 2009." "In partial fulfillment of the requirements for the degree [of] Master of Science in Mechanical Engineering." "Presented to the faculty of California Polytechnic State University, San Luis Obispo." Major professor: John Ridgely, Ph.D. Includes bibliographical references (p. 91-97).

¡@¡@Wire bonding has been used in integrated circuit packaging for many years which has been more full-grown than other bonding methods, and gold wire has been the preferred choice. Because of the rising price of gold every year, copper wire has been increasingly used to replace gold wire. ¡@¡@The main focus of this paper is to simulate 3D copper-Al pad thermosonic wire bonding stage by using 3D finite element method. Firstly, the differences between mechanical analysis (the thermal effect was not considered) and thermo-mechanical coupling analysis from both impact stage and ultrasonic vibration stage, respectively, were compared. Secondly, the differences between copper thermosonic wire bonding analysis and gold thermosonic wire bonding analysis were discussed. Finally, the effects of Al pad thickness variation on the copper thermosonic wire bonding analysis were studied. ¡@¡@Results showed that, due to the mechanical properties will be decreased by thermal effects caused from temperature increasing, the obtained effective stress and efective strain of thermo-mechanical coupling analysis were less than the results obtained from mechanical analysis. The pad plastic defomation in copper thermosonic wire bonding is more critical than gold thermosonic wire bonding. Therefore, copper thermosonic wire bonding will lead to serious pad splashing. Also, quantity of the decreasing of pad plastic deformation was limited by increasing the pad thickness.

Wire bonding has been used in integrated circuit packaging for many decades because of its high reliability and performance. The most common metal used has been gold, but with the surge in commodity prices of gold in recent years, copper wire is now used to altered gold wire for cost saving. Many challenges have to be solved to meet its application requirement; coating is one of the applications. In this study, a 3D coated copper wire and coated Al pad is built by finite element method to simulate ultrasonic bonding and thermosonic bonding. To consider the effect of coated material to stress and strain field on ultrasonic bonding and the effect of coated material to temperature field on thermosonic bonding. Then use the Taguchi experiment method to discuss the effect on Cu-Ball and Al pad under different coated material and thickness combination. The results show that with coated material on Al pad or copper wire could reduce more than 48% of effective plastic strain after the bonding process, it obviously reduce the Al splash phenomenon in copper wire bonding. But the coated material such like palladium and nickel which have lower thermal conductivity would resist the heat transfer. And the Taguchi experiment method shows that the most effective way to reduce the effective stress during impact stage and ultrasonic vibration stage is to increase the thickness of palladium and nickel respectively, and when the thickness of coated material Au reached 0.01£gm could increase the temperature of Cu-Ball and Al pad mostly.

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To improve the wire bondability, interfacial adhesion and popcorn cracking resistance in the packaging processing of IC and MEMS chips, this thesis utilized oxygen and helium plasmas to modify and clean the surface of metal pads. The influences of the plasma cleaning time, metal pad materials and wire bonding time/temperature/power on the strength of wire bonding were investigated. Two different wire materials (Al wire with 32 £gm in diameter and Au wire with 25 £gm in diameter) were bonded on the surface of Al, Au and Pt metal pads using a commercial ultrasonic wire bonder (SPB-U688), respectively. The pull strength detection of the implemented micro joints is characterized by an accurate pull strength testing system (Dage SERIES-4000). Based on hundred measurement results, this research has three conclusions described as follows. (I) The pull strength of Au pad is higher than that of Al and Pt pads no matter with the plasma cleaning process or not. The maximum pull strength (12.286 g) can be achieved as the surface of Au pad was modified by the helium plasma for 180 seconds. (II) Helium plasma cleaned wafer can obtain larger improvement of pull strength than that of the oxygen plasma under the same plasma time. However, this result can not be concluded in Al and Pt pads. (III) The optimized wire bonding time/power of the Au, Al and Pt pads are 0.07 s/2.1, 0.05 s/0.6 W and 0.03 s/2.7 W, respectively.

Ball bonding with gold wire has been the preferred choice to connect semiconductor chip and a lead frame. Recently, copper wires have been increasingly used to replace gold wires because of the rising price of gold. However, copper is harder than gold and has the tendency to induce the damage of bond pad or other underlying layers. Herein, Al pad material has to be changed from bulk to single crystal with (100) surface orientation in order to improve bonding reliability. Firstly, finite element method was adopted to simulate 3D wire bonding. Also, from the impact of gold wire bonding, the stress concentration was found on pad and underlying layers due to the higher elastic modulus of bulk Al. During copper ball impact, there is not only the serious stress concentration at pad, but also a pad splash due to the insufficient strength of bulk Al, even though bulk Al has a lower elastic modulus. Secondly, material properties of Al(100) were obtained by uniaxial tensile tests at constant speed. With molecular dynamics method, the incorporated result showed that Al(100) has the lower elastic modulus and higher yield strength than those of bulk material. Finally, single crystal Al(100) was used, instead of bulk material, to carry out copper ball impact process by using multi-scale simulation. Al(100) material is able to transform impact energy into the resilience of strain energy effectively owing to its high yield stress and low elastic modulus. Results show that the application of Al(100) material reduces the effects of stress concentration and pad ¡§splashing¡¨ successfully during copper ball impact process.

The goal of this thesis is to improve the understanding of the stitch bonding process in microelectronic wire bonding. In particular, it focuses on investigating the effect of the process parameters bonding force, scrub amplitude, and skid on experimental bond quality responses, including qualitative (non-sticking, sticking, and tail-lifting) and quantitative (stitch pull force, tail pull force). In addition to the experimental work, a finite element (FE) model is developed for the stitch bonding process using ABAQUS software, and compared with the experimental observations. For the first set of experiments, the stitch bonding is performed with a 18 ??m diameter Pd coated Cu (PCC) wire on a ???low bondability??? Au/Ni/Pd plated quad-flat non-lead (QFN) substrate. Results showed that a high bonding force, a high scrub amplitude, and a positive skid provoke the sticking of the stitch bond and reducing the chance of non-sticking observation. However, such parameters also increase the chance of tail-lifting. As a trade-off for a low bondability substrate, a process parameter combination containing a high bonding force and a high scrub amplitude and a negative skid could ensure a strong enough stitch bonding process with low chance of tail-lifting. For the second set of experiments, the stitch bonding is performed with a 18 ??m diameter uncoated Cu wire on a ???high bondability??? Ag plated QFN substrate. Statistical analysis of stitch and tail pull force showed that the skid and scrub parameters have a more significant influence than bonding force. A positive skid can degrade the stitch pull force, while enhancing the tail pull force. A high scrub amplitude is found to degrade both the stitch and the tail pull forces. The bonding force is shown to improve the stitch and tail pull forces slightly. Performing an optimization, process parameters of 70 gf (687 mN) bonding force, 3 ??m scrub amplitude, and zero skid result in acceptable stitch and tail pull forces, along with a reliable stitch bond appearance (low peeling and shallow capillary tool impression). The influence of the process parameters is significantly different depending on if bonding on low or high bondability substrates. For example, a positive skid increases the chances of sticking and tail-lifting on low bondability substrate, but it decreases the tail pull force and increases the tail pull force for high bondability substrate. This indicates that finding a general experimental rule for understanding the effect of process parameters on the stitch bond quality is difficult if not impossible. In other words, instead of general rule, it is more likely to find individual rules for specific individual applications. To improve the understanding of stitch bonding a three dimensional (3D) dynamic explicit FE model is developed in ABAQUS. The model components and boundary conditions are constructed and applied to reflect the experimental conditions. The bonding force, scrub, and skid are successfully implemented into the model. Mass scaling is applied carefully to save calculation time while ensuring there are no artificial effects of inertia. The model is able to render the conventional responses reported in the past including stress and strain distributions. However, these conventional outputs were not sufficient to provide a correlation between model and experiment. Therefore, new candidate responses were developed and extracted from the numerical results. The new responses are based on accepted welding mechanisms. One of the mechanisms is interfacial cleaning by frictional energy which is beneficial for bonding. Thus the friction energy accumulated during the simulated bond duration is extracted as a candidate response. For classical cold welding processes, the interfacial surface expansion is a key mechanism, as it opens up cracks in the surface contamination and oxide layers and thereby generates paths to bring the fresh metals together under pressure. Therefore, candidate responses related to surface expansion at the contact interface are extracted from the model. The complete set of new responses extracted from the numerical model includes contact areas, surface expansion per areas, frictional energy, and combination of frictional energy combined with surface expansions per areas. In addition the bond interface is divided into ???wedge??? and ???tail??? regions. The model is run for the same DOE cells as used in the first set of experiments and candidate responses are extracted and compared with the experimental observations. By ranking the correlation coefficients of each individual candidate responses, for the first time correlations that are relatively strong are found between a numerical response and experimental observations of stitch bonding. Responses that have correlation coefficients of 0.79 and 0.85 were found for wedge sticking and tail-lifting, respectively. Such relatively strong correlation indicates that the friction enhanced cleaning and the surface expansion mechanisms are proper theories for the current stitch bonding system. These theories can be used for developing similar models for other types of the solid-state bonding processes. Based on the best candidate responses, a procedure to determine numerical process windows is demonstrated for a specific application. Such a window defines the parameter ranges which result in an acceptable stitch bonding process and is an excellent indication of how suitable a process is for mass production. Depending on the application, materials, geometries, and tools, the FE model and process window procedure allow a variety of numerical process windows to be produced and compared.

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