Longitudinal thermosonic bonding for flip-chip assembly

Thermosonic flip chip bonding is a solderless technology for flip chip assembly. Its mechanism is the same as that of thermocompression bonding, but the bonding temperature, pressure and time are reduced due to the introduction of ultrasonic energy. This technology can be applied to low-cost flip chip manufacturing and prototyping because of its compatibility with wire bonding and high throughput rates. However, existing thermosonic flip chip bonding systems are not stable for high-I/O assemblies. A longitudinal thermosonic bonding system has been developed in this study to solve this stability problem. The horn is used as the bonding tool, providing longitudinal rather than transverse waves to soften the connection joints. The system is much simpler and stiffer than the existing transverse system. The feasibility of this technology has been demonstrated by the successful mechanical bonding tests and the assembly of a functional memory module.; In addition to the technology development, this thesis work also addressed basic issues of ultrasonic energy transmission, force distribution, and bonding time associated with the longitudinal bonding. In particular, the effects of a polymer layer on the system stability were studied in detail.; An important tool for the study was a finite element model developed to calculate the distribution of ultrasonic vibration amplitude. This model was validated by experimental amplitude measurement and was used to determine the design guidelines for large size, high-I/O flip chip assemblies.; A self-planarization concept was developed to solve a stability problem with a non-planar contact between the tool and the chip. A layer of polymer material was used to redistribute the ultrasonic vibration amplitude across the area array bumps. Both modeling and experimental studies have proven that this polymer layer is effective in solving the planarity problem. Although polymer absorbs a large portion of the ultrasonic energy, it assures a uniform amplitude distribution needed for a stable bonding process. Based on the modeling study, a polymer layer with 350{dollar}mu{dollar}m thickness and 2GPa elastic modulus is recommended for a 8mm x 8mm, 1024-I/O assembly. The use of a thick polymer layer can also increase the bonding force window with a large planarity angle.; A high-speed video system was used to study the deformation time of the thermosonic bonding process. The entire deformation process lasts for about 15msec. Such a long period can not be explained by a finite element analysis based on a time independent constitutive equation. Instead, the viscoplastic behavior of the gold joint should be included. The deformation period measured also suggested that the optimum bonding time can be much shorter than the 500msec being used.; In summary, this thesis work proposed, demonstrated, and studied a new longitudinal thermosonic flip chip bonding system with the following contributions: (1) demonstrated a novel bonding system, (2) demonstrated the thermosonic flip chip bonding for a 200-I/O assembly, (3) developed both modeling and experimental techniques to understand the bonding process, (4) determined the design guidelines for high I/O, large size flip chip assemblies, (5) developed a self-planarization technology using polymer, and (6) improved understanding of the bonding process through the use of a high-speed video system.