Device holeSemiconductor chipMicro bump arraySemiconductor chipCopper foil conductorFlying leadBase filmBase filmFigure 2 The implementation of TAB circuit Figure 3 The implementation of COF circuit Figure 1 Flying leads for high-density TAB circuits(The white part; circuit pitch = 40 μm)semiconductor chipsemiconductor chip PI film manufacturers all aimed to develop a product that matches the desirable dimensional stability and heat resis-tance of Upilex S. The Du Pont Group has developed and launched “Kapton E” and “Kapton EN” with new heat-resistant grades to replace the conventional and general-purpose grade “Kapton H” and “Kapton HN”. On the other hand, Kaneka also developed and marketed “Apical NPI’’ to replace “Apical A H”, the previous general product. “Apical AH” was, in fact, the very first PI film product of Kaneka. It was very much similar to “Kapton H” in terms of molecular structure and manufacturing conditions, but NPI found success in synthesizing a PI film with a unique molecular structure. When compared with the previous general items, Kapton E, EH, and Apical NPI had significant improvement in their main mechanical properties. The vision (the color and the texture) was almost the same as the previous general items though the new creation came with a lower hygroscopicity. It was rather difficult to distinguish merely through a shoulder check. Yet, they are still far behind Upilex S when we do a thorough comparison from a macro point of view particularly in terms of their chemical resistance- Upilex S was insoluble in most organic solvents, acids, and alkalis, whereas those new inventions by Kapton and Apical were highly soluble in high pH alkaline solutions. That is to say, the newcomers failed to take over TAB in display driver modules although this recognizable alkali-soluble feature brought about enormous changes for tape circuit manufacturers later.bend the conductor leads and the connection would com-pletely collapse. Under such undesirable circumstances, the focus shift-ed to the low stability of the flying lead structure, and materi-al manufacturers, circuit manufacturers, and implementation manufacturers worked hand in hand to develop a bonding method without flying lead, which was the COF (Figures 2 & 3). Different manufacturers adopted different ways to do COF implementations but they all stayed away from the flying lead structure. In other words, the structure of the COF circuit as a flexible circuit is the same as that of those typical single-sid-ed flexible circuits, except for the higher circuit density of the former(Figures 4 and 5). It also implies that the manufactur-ing process was simplified, the process yield has increased, and manufacturing costs could be restrained. Manufacturing COF circuits requires no major changes in the TAB circuit pro-duction line or any additional investment for TAB circuit man-ufacturers. Nevertheless, the burden was, after all, shifted to semiconductor chip manufacturers as they had to work on the system implementation. 6354. Aiming at a higher dimensional stabilityThe TAB circuits market expanded tremendously in the 1980s. The general trend in the electronics market was to go for light-er, thinner, and smaller circuits, and the entire industry was heading for higher density and higher multilayer printed cir-cuits, including flexible circuits.55. Creation of the COF Circuit When the TAB circuit reached its peak, LCD driver module manufacturers gradually started to feel they were at their lim-it in the pursuit of the miniaturization of flexible circuits. The problem was especially notable in flying leads. When the cir-cuit pitch went under 40 μm, not only the throughput yield for circuit manufacturers but also the first pass yield for module manufacturers experienced a significant drop (Figure 1). When the pitch of the circuit reached 40 μm, the conductor could become just 20 μm wide, which was less than one-fifth of the thickness of our eyelash. Even a slight blow of air would thus
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