(A) study on the bending properties of ultra-fine pitch chip-on-flex (COF) packages using anchoring polymer layer (APL) anisotropic conductive films (ACFs)

Abstract

In this study, anisotropic conductive films (ACFs) composed of polymer adhesive resin and conductive particles have been used as interconnection materials for ultra fine pitch Chip-on-Flex (COF) packages due to their flexibility, stable electrical properties, and bending reliability. To realize an efficient ultra fine pitch COF packages, new anchoring polymer layer (APL) was recently introduced. However, the chip flexibility in terms of the ACFs materials after APL layer integration needs lot of improvement. Herein we study the order to obtain more flexible and reliable COF packages using APL ACFs interconnection materials, the effects of the geometry and surface defects on Si chips flexibility, the effects of the ACFs materials properties on the COF bending reliability were investigated. In part 2, the silicon properties were studied. The geometric and surface defects effects on static bending properties of Si chips were demonstrated. 3 kinds of Si chips were used in this study which were respectively 16mm x 16mm, 16mm x 8mm and 16mm x 1.5mm. The geometric effect on flexibility which determined by bending radius of silicon was tested by the 4 point bending method. The surface morphology and roughness was also calculated by the AFM analysis. As a result, through the simulation by FEA, it was found that the bending radius was the same as a theoretical value in terms of different geometric silicon chip, but for the experimental result, the bending radius was decreased when the silicon changed from square shape to rectangular shape, the surface defects which influence the flexibility of silicon chip was demonstrated by Sopori etch method. It found that the rectangular shape chip had the better flexibility comparing with the square one presumably doe to the less amount of surface defects resulting in high static bending strength. In part 3, the most flexible geometric Si chip was applied to the COF packages using APL ACFs as the interconnection materials. It was found that the PAN APL ACFs showed better ACFs joint properties and dynamic bending properties comparing with the PVDF APL ACFs due to its higher interface adhesion property. In addition, the strain applied to the PAN APL ACFs joints during the bending test was 1.35% which was quite smaller than that of the PVDF’s, 2.01% resulting in better bending reliability too. Not only the polymer materials used in APL structure had the effect on the interface adhesion, but also there was NCF thickness effect. The thickest NCF remaining of PAN APL ACFs at 0.5Mpa bonding pressure had thicker NCF thickness and the highest adhesion strength resulting in the best dynamic bending reliability. In part 4, the effects of the APL ACFs modulus on the COF dynamic bending reliability was also studied. The 2% silica fillers were added into NCF to improve the APL ACFs modulus and the O2 plasma treatment was performed to the APL layer to improve the adhesion property. As a result, the higher modulus of APL ACFs resulting in the better dynamic bending reliability due to the less applied strain at the COF joints which was analyzed by the FEM simulation. Finally, comparing with the modulus effect, the major effect on dynamic bending reliability was interface adhesion effect as a function of contact resistance variation and the slope of contact resistance versus bending cycles.