Kyoung-sei Choi

Fine-pitch, cost effective flip chip package development: TAB-BGA

This paper presents a cost effective fine-pitch flip chip solution to meet the increasing demand for the high performance small form factor packages. Tape Automated Bonding (TAB)-BGA package utilizes thermo-compression to bond a flip chip onto a film based substrate instead of reflow soldering. This approach allows for fine-pitch connection that can save significant substrate area, which can be especially cost effective for some package designs that can reduce the conductive layer count from 2 to 1 [1]. For second level interconnects, typical BGA approach was taken for easy adoption. Successfully implementing TAB-BGA has several technical challenges. Applying underfill under the fine-pitch flip chip without creating voids can be difficult. One way to overcome this challenge is by using no filler underfill and overmold type flip chip; however, that can potentially lead to high water absorption rate and delamination issues. Furthermore, board level reliability, especially temperature cycle, can have high failure rate for this type of single layer package structure [2]. To overcome these challenges, flexible PI film based substrate has been used to reduce the residual stress within the package. In addition, Epoxy Molding Compound (EMC) with low modulus at high temperature and solder resist stress buffer layer have been applied to improve the package and board level reliability.

27.3: Improvement of Electrical Stability of Poly-Si TFTs Using Vertical a-Si Offset

Polycrystalline silicon (poly-Si) thin film transistors (TFTs) employing vertical amorphous silicon (a-Si) offsets have been fabricated without additional photolithography processes. The a-Si offset has been formed utilizing the poly-Si grain growth blocking effect by thin native oxide film during the excimer laser recrystallization of a-Si. The ON current degradation of the new device after 4 hours electrical stress, was reduced by at least 5 times compared with conventional poly-Si TFTs.