Modified flip-chip attach process using high performance non-flow underfill paste

Controlled collapse chip connection (C4) and Gold to gold interconnection (GGI) are the typical processes of the flip chip interconnection. In these processes, solder balls or gold bumps formed on the IC chip and the circuit on the interposer substrate are metallurgically connected with each other. Next the metal-metal connection is encapsulated with the underfill material. These flip chip surface mounting processes give excellent contact reliability. However, two separate steps are required at the underfilling process, such as "Encapsulation" and "Curing". Accordingly, it takes relatively longer process time and lower cost performance. On the contrary, "One-step compression attach process" is a simple surface mounting process with high cost performance. "One-step process" utilizes the thermal shrinkage of the connecting materials and no metallurgical connection is attained. Typical materials used for these processes are ACF (anisotropic conductive film), ACP (anisotropic conductive paste), NCP (non conductive paste) and NCF (non conductive film). Unfortunately, reliability of this process is not fully established yet. As no metallurgical connection exists, reliability of One-step process depends on binding force, generated by material's shrinkage. Therefore reliability of One-step process is considered to be inferior to the underfill encapsulation (Two-step) process, especially under the severe level reliability test condition. We investigated both processes and then developed "Modified one step compression attach process" for Chip On Film (COF) applications. This process achieved metallurgical connection, using non-flow underfill. This means, metal-metal interconnection, encapsulation and cure of the underfill are processed in a single step. This "Modified one step compression attach process" used "ESPANEX" as flexible printed circuit board (FPC). "ESPANEX" is adhesive-less Copper Clad Laminate material which has good heat resistance and dimensional stability. New grade of the non-flow underfill material, "ESAREX" is developed for this process. "ESAREX" has high adhesive strength and long pot life at room temperature. As combined of these two materials, interconnection between gold and tin, encapsulation and curing of non-flow underfill are processed in a single step. We achieved excellent reliability results in the thermal cycle, pressure cooker (PCT) and the high temperature and high humidity (HHT) tests, using "Modified one step compression attach process".