Figure 4 A sample of the COF circuit for LCD driver modulesFigure 5 COF circuit with 25 μm pitch (fixed on the conductor base)64The starting material for COF circuits is single-sided and adhesiveless copper clad laminate (CCL). Since the circuit for-mation is a detailed and delicate process, the copper foil used as the conductor must be as thin as under 9 μm. The majority of LCD production has been taken up by Korean manufac-turers hence the standard specification has been decided by Samsung Electronics, the largest Korean manufacturer, with a 9μm copper conductor layer formed on a 38μm PI film. In the 21st century, when Samsung Electronics played a pioneering role in the FPD market, there was a momentum of transfer of the production of driver modules to local manufacturers in Ja-pan so local tape manufacturers (TAB & COF manufacturers) swiftly lost their market share, finally led to divestment or dives-titure. Still, material manufacturers managed to maintain their business as suppliers for a while. The specification of “sputter-ing + electroplating” was adopted in CCL production since a thin conductor layer was required as the starting material for the COF circuit. In an earlier stage, manufacturers attempted to sputter metallic copper directly onto the surface of the PI film while its adhesion strength was a disappointment. There-fore, they changed to sputter nickel or a nickel-chromium alloy into a thin seed layer (less than 0.1 μm) before sputtering the layered copper to a predetermined thickness in an electrolytic plating process.These attempts differed from traditional CCL manufactur-ing processes. They generated buzz and attracted a number of new CCL manufacturers to join in the competition. Manu-facturers of adhesiveless CCL using the casting method have taken various approaches to grow their business as the market was large enough to accommodate but it’s difficult to assert they succeeded in the COF circuit market.On the other hand, for small to medium-sized displays, only one or two high-density flexible circuits are brought in to cov-er not only the driver but also other wiring instead of having multiple tape circuits to cover just the driver circuit. The circuit density could be high (although it’s not comparable with tape circuits) since it connects directly to the display- the driver chip is also installed directly which makes dimensionally stable PI film essential. The largest use of small to medium displays is in mobile phones, especially smartphones with an annual production reaching billions of units, by far the largest in all electronic devices. The amount of PI film consumed in this production process is amazingly high and the popularity of phone makers became the determining factor affecting the sales of PI films. The spread of other portable devices including game con-soles has been remarkable in recent years as well, though the penetration rate was not as high as smartphones. New models surface one after another. Most of them are equipped with small to medium displays, so they also contribute to the consumption of dimensionally stable grades of PI film (Fig. 6). 56. Flexible circuit for small and medium displaysWithout a shadow of doubt, the new PI film-consuming COF circuit was primarily produced as FPD driver modules. Previ-ously in the last issue, we touched on the difficulties of covering the driver circuit with a single flexible circuit in larger displays, so the final adoption was to divide the tape circuit into smaller units and for arrangement on the tape circuit. A large display like a television could consume more than 20 tape circuits. Nowadays, more than 200 million flat-screen TVs are man-ufactured globally every year, so an extremely large amount of dimensional-accurate PI film is required. This is closely fol-lowed by displays for desktop computers, laptops, and tablets with a total of approx. 500 million pieces are manufactured every single year, and a considerable amount of tape circuits are consumed in these products.
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