Electrodepositable resists

Electrodeposition, also called electrocoating, is a process by which organic materials are coated from aqueous suspension, or solution, onto a conductive substrate under the influence of electricity. The process utilizes direct current for depositing resins, so that predominantly electrophoretic processes operate. Electrophoresis in this context is understood to mean the migration of colloidal or suspended particles in an electric field. The particles migrate, according to their charge, to the anode (anaphoresis) or to the cathode (cataphoresis). 

Although the principle of electrophoresis has been known since 1809, from the work of Reuss, it has remained confined to a very few areas of application in medical, analytical and other technological fields. The process of electrophoretically depositing paints and lacquers could only be applied industrially when new ionizable paints and resins were developed that could be diluted with water and deposited from an aqueous medium under the influence of an electric current, similarly to the electrodeposition of metals (although the electrodeposition of organic material is much more complex). It was not possible to electrodeposit conventional organic-based paints, since these did not form ions, and known water-soluble paints that could be applied by conventional immersion or spraying techniques were too expensive. 

The new technique gained industrial significance when the Ford Motor Co. [1] elaborated a method for prime-coating metal automobile bodies. Following several years research to produce inexpensive, safe, water-soluble electrodepositable (ED) paints, the first production facility opened in 1963. The superior coating performance, uniformity on complex surfaces, freedom from pinholes, efficient use of paint solids, reduced solvent emission and reduced overall costs led to rapid worldwide market penetration in the automobile and other sheet-metal 

ED paints may be thermally or photochemically cured for improved performance. None the less, it was some time before serious attempts were made to use ED photocurable films as resists for metal patterning. It had been foreseen that dry-film photoresists, which have been the mainstay of the printed circuit board inner-layer fabrication process for the last two decades, would soon reach their resolution limit and that a process that coated much thinner layers of resist would take over. ED resists that were capable of coating layers up to five times thinner than dry film seemed the natural successors. In 1986 the Rohm and Haas Co. [2] issued a patent describing a photoresist composition for cataphoretic deposition onto copper during the process of forming a printed circuit board. Many other patents in this field, describing both cataphoretic and anaphoretic deposition of a wide variety of resins, have been issued since then. 

With their attractive properties, these resists have the potential to displace both solvent-based liquid resists and dry film photoresists in most present-day applications involving conductive substrates. 

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Manufacture of Printed Wiring Boards. Printed wiring boards, or printed circuit boards, are usually thin flat panels than contain one or multiple layers of thin copper patterns that interconnect the various electronic components (e.g. integrated circuit chips, connectors, resistors) that are attached to the boards. These panels are present in almost every consumer electronic product and automobile sold today. The various photopolymer products used to manufacture the printed wiring boards include film resists, electroless plating resists (23), liquid resists, electrodeposited resists (24), solder masks (25), laser exposed photoresists (26), flexible photoimageable permanent coatings (27) and polyimide interlayer insulator films (28). Another new use of photopolymer chemistry is the selective formation of conductive patterns in polymers (29). 

Hydrophobic solvents (or plasticizers) are often added to electrodepositable resists to lower the Tg of the polymer, enabling electrodeposition to take place at low temperatures and allowing easier control of the film thickness. The coated resist can also flow better (coalesce) during baking, when a plasticizer is present, to give a more compact, defect-free surface. 

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