Ultra-fine trench circuit on polymer film

As electric equipment decreases in size and height and increases in functionalities from day to day, high-density, fine circuits are required more than ever before. According to ITRS, its requirement for trace width/spacing of a printed circuit board will reach less than 10 μm/10 μm in 2014. However, it is difficult to fabricate such fine circuits on an organic substrate using current processes such as a semiadditive process and a subtractive process. So several new processes are suggested for fabricating such fine circuits [1]. This paper focuses on the first demonstration of an ultrafine circuit with a new process, both of which we developed. The circuit fabricated with our new process has trench-shaped traces buried in the insulating layer. The aspect ratio of the trace cross-section is more than 1, namely, the thickness of the trace is greater than the width. The minimum width of the conductor is 2 μm, the minimum spacing between the conductors is 2 μm and the maximum aspect ratio is more than 3. The fabrication process consists of trench-forming on a surface of polymer film and trench-filling by copper plating. Owing to a buried conductor structure, the adhesion of the trace to the polymer film is fairly good without roughening of the surface. No delamination occurred while bending of film and even after a thermal shock test. The adhesion and electrical continuity of the trace was verified by a 3-cycle-reflow test performed after the circuit board was let stand in 85 °C, 85 %Rh for 168 hours, and a thermal shock test in temperatures between -65 °C and 125 °C for 1000 cycles. No visual failure was observed, and the variation of continuity resistance was small. The surface smoothness of the trace is an advantage of this trench circuit for high-speed signal transmission. The trench circuit also shows good insulation and high resistance to electrochemical migration due to a buried trace structure. The insulation resistance between the 5 μm-wide comb traces spaced every 5 μm kept higher than 10M ohms for 336 hours in an unsaturated pressure cooker bias test under the conditions of 135 °C, 85 %Rh, 5V. Furthermore, micro via holes were able to be formed in the insulation layer simultaneously with the trench circuit. Manufacturing of multi-layer test vehicle with this trench circuit and via is finishing.