Etched copper foil conductor circuitPhotoimagiable overlay CL ink printingDryingPattern printingDevelopmentSinteringCopper foil circuitCL resin coatingPattern printingDevelopment & printingFigure 4 The processing pro-cess flow of liquid photoima-giable overlay CLFigure 3 Processing process of photoimagiable overlay CL64At that time, Du Pont (US) had already commercialized dry film type photoimagiable overlay CL for rigid circuits, and it was commonly believed that it would be indisputably possible to re-alize CL materials for flexible circuits by improving those models. Du Pont pushed ahead with the development of photoimagiable overlay CL material based on heat resistant acrylic resin with accomplishment as a solder mask for rigid circuits instead of polyimide resin, the specialty of the corporate. The creation of photoimagiable overlay CL for flexible circuits could hardly be re-garded as a business success unfortunately. Mechanical prop-erties such as flexibility were less desirable when compared with standard polyimide film-based CL, not to mention the fact that neither solder resistance nor plating resistance were up to standard. In addition, the need for a vacuum laminator in the lamination process was another barrier for flexible circuit man-ufacturers. Furthermore, as significant cost reduction could not be achieved, market share was limited.On the other hand, Japan Polytech Co., Ltd. (a Japanese resist ink manufacturer) kept spending time and effort develop-ing liquid photoimagiable overlay CL materials for flexible circuits based on its extensive experience with resist inks. The material was epoxy resin-based while urethane resin was added to in-crease flexibility. The CL material was in the form of ink, so flex-ible circuit manufacturers could use screen printing to coat the entire circuit surface with the ink. The completion of the drying process was followed by printing and development to form the resist pattern. The final step in the process was heat treatment (Figure 4). At the initial status, many remained skeptical to the at-tempt, eventually it has been adapted by a large number of flexi-ble circuit manufacturers with notable growth in its market share because of its well-balanced, all rounded features along with its low manufacturing cost. Since then, the liquid printing type has become the de facto standard for photoimagiable overlay The CL of flexible circuits remains as a major issue today. Aiming for a breakthrough, many material and flexible circuit manufacturers strived to develop new technologies while we still have not met any strong possibilities. This time, we have taken a deep look at CL among many possible materials for flexible circuits. It would be great if you could have some basic concepts that the material is still low in terms of its completeness when compared with CCL, and the practical application of new materials is still a bit far ahead. Next time, we are going to look at some new materials as current possible candidates for materials for high-performance flexible circuits, including CL.CL in the flexible circuit industry. Many material manufacturers developed CL with this concept and joined in the competition.Polytech Corp.’s liquid photoima-giable overlay CL concept has been welcomed by many users, but it failed to replace or take the spot-light from CL. The major-ity of commercial flexible circuit applications con-tinued to rely on polyimide film-based CL even nowadays as it’s reliable and promising. We can thus conclude that the image of polyimide films was reliable that trust is successfully built. It’s not based on scientific evi-dence but “trust” in products and services is not something that could be easily changed in a short period of time.Nippon 40. Photoimagiable overlay CLSolder masks for rigid circuits were originally made by screen printing epoxy resin while we witnessed a shift to surface mounting of components that fine pattern formation with a pho-toimagiable overlay solder mask material has become common. The same concept has been applied on flexible circuits as well. If photoimagiable overlay CL is put to practical use, high-density surface mounting of chip components on flexible circuits should become feasible. Next, automation of the CL processing pro-cess and that of the CL laminating process (which is a labor-in-tensive process) proceeded so productivity sharply increased (Figure 3). Yet, increasing flexibility to conventional solder masks has long been a difficult “goal” all in all.
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