Polyimide filmAdhesive Release sheetWindow drilling (drilling)Conductor circuitHeat pressEtched copper foil circuitScreen printing with an insulating protective layerFigure 1 Standard CL film processFigure 2 Standard liquid CL insulation layer screening printingThe role of CL in flexible circuits is to protect copper conduc-tors from mechanical stress, thermal stress such as soldering, as well as chemical stress such as plating while the main con-structions are those with polyimide film and those with liquid resist similar to rigid circuits. The usage of CL films made up a large portion while the basic structure and processing process were the same as those used in the early days. As shown in fig-ure 1, material manufacturers provides the basic structure of CL film in the form of polyimide film coated with a layer of adhesive and laminated with a layer of release sheet. The adhesive is a thermosetting plastic, such as epoxy resin, supplied in a semi-cured state. The polyimide film used for CL film is basically the same as the base material.Nevertheless, there are two fundamental problems in CL films. The first one is that the heat resistance of the adhesive is significantly lower than that of polyimide film. The adhesive layer then become the very first layer to be damaged in thermal pro-cesses such as soldering. The second problem is the difficulties in automizing the complicated processing process of the film materials so productivity is incredibly low. After all, the CL film processing process has to rely on human-wave tactics while the proportion of the total production cost remains large. Therefore, not only flexible circuit manufacturers in Japan but also many major manufacturers in the US, Korea, and Taiwan established large factories for mass-production in regions with a relatively lower labor cost, such as Thailand, China, and Vietnam.Manufacturers spent a huge amount of time and effort right from the very beginning investigating the possibility to replace film-like CL adhesive with a highly heat-resistant thermoplastic polyimide resin. In fact, the three leading polyimide film man-ufacturers mentioned earlier tried combining their exclusive thermoplastic polyimide resins for the commercialization of their unique polyimide-only products. Regrettably, CL films with poly-imide resin adhesives couldn’t fit in any application in a practical scale- the operational cost was high and bonding temperature exceeds 300°C, which was also indisputably high. * I worked at Nippon Steel Chemical for three years from 1988 to 1991, devoted much time and effort to build the business model framework for “ESPANEX”, the casting CCL. Now, Nippon Steel Chemical no longer exist as a business organization that the “ESPANEX” project was taken over by NIPPON STEEL Chemical & Material Co., Ltd. In this article series, we have chosen to use the corporate name “NSCC” as it was the historical business entity for “ESPANEX”. 6339. Liquid CLLiquid solder mask is applied to the surface of rigid printed cir-cuits. Screen mask printing was the mainstream in the old days but thanks to the size reduction of mounted parts, fine window drilling patterns, and the emphasis on high resolution, photo-lithography gradually replaced screen printing. Attempts were made in a very much similar way to apply liquid CL to flexible circuits. There were high hopes on the reduction of both material and processing cost if liquid material could be used as CL. The first attempt was on the direct application of rigid circuit solder mask while unfortunately, solder mask was too hard and fragile for rigid circuits so it cracked during mechanical bending that it could not be adapted at all. Ink manufacturers then developed flexible materials based on epoxy resin that screen-printing could become feasible (Figure 2). The attempt was a success to a certain extent that UL was also attained for its heat resistance for soldering and flame retardance so it achieved quite a large percentage of the total revenue in the market. Its mechanical performance is, however, obviously not comparable with CL film. Also, its market share remained low and disappointing for its narrow application scope. Nippon Steel Chemical Co., Ltd. (NSCC)* developed and marketed a polyimide-based ink for screen-printing as a CL material for “ESPANEX”, an adhesive-less CCL. It functioned very well as a flexible circuit so manu-facturers in both Japan and the western countries have actually switched to adapt it although NSCC finally decided to focus on their main business on laminate products and terminated the production before a full-scale launch.
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