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

Abstract

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 time and lower cost performance. On the contrary, compression attach process is a simple surface mounting with high cost performance. 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 is not fully established yet. As no metallurgical connection exists, reliability of One-step depends on binding force, generated by material's shrinkage. Therefore reliability of One-step 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 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 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.