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Flip-Chip Package Reliability Using Copper Micro Bumps on Substrates
Keywords: Flip-chip, Microcontact, Structural analysis
Flip-chip packaging technology has been the main packaging method for high performance devices. Any device that has high interconnect-to-area ratio and needs to distribute the interconnects on the whole surface has to use a flip-chip package. With short interconnect lengths, flip-chip package provides superior electrical performance and smaller overall package size. For many decades, the C4 process has served as the main interconnect method in the flip-chip package. But with increasing interconnect density, the solder bump based C4 process is facing challenges in terms of reducing pitch and underfill process. In this paper, a method is presented that allows for smaller pitch while maintaining larger gap between the device and the substrate for easier underfill process. This is achieved by using copper micro bumps on the substrate. These micro bumps, or microcontacts, are joined to the device through solder paste to form the interconnects. These microcontacts allow for finer pitch and offer sufficient gap between the device and the substrate for the underfill process. The biggest potential issue with these microcontacts is the reliability of the formed interconnects. There are many design parameters that critically effect the life of the interconnect and eventual failure under various external loading conditions such as thermal cycling, drop, vibration, etc. Detailed structural analysis is done to determine the stress state in the interconnect as a function of its dimensions, materials and the loading conditions. The analysis is primarily computational through the use of commercial finite element software. The reliability of the microcontact interconnects is compared against the traditional C4 ones in terms of design and reliability. This study will be able to show whether the microcontacts are a viable alternative for the future of flip-chip packaging or the C4 technology is still the best option.
Baharah Banijamali, Sr. Mechanical Engineer
Tessera, Inc.
San Jose, CA

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