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66 The PCB Magazine • May 2016 board just prior to pattern plating. In the case of pattern plating, the resist laminated onto to a surface that has either been processed with conventional electroless copper or with one of several alternatives (carbon based, conductive polymer, etc.). The lamination step is designed to achieve intimate contact between the resist and the copper surface as a protective coating against plating or etching chemical attack. By utilizing heat and pressure, contact between the resist and the copper is achieved. Essentially the dry film resist flows and is able to conform to the surface. This is explained as follows: "Flow is achieved by lowering the resist vis- cosity and applying a pressure differential for a certain amount of time. Viscosity is then low- ered by heating the resist. A pressure differential can be created by pneumatic, hydraulic, or me- chanical (springs) pressure on the lamination hot rolls of a hot roll laminator or by applying a vacuum to a vacuum laminator [1] ." There are several concerns that need to be pointed out in the primary resist lamination process. First consider that the printed circuit boards have been processed through wet pro- cesses (either conventional electroless copper or direct metallization) prior to resist lamination. In order to effect optimum adhesion of the resist, the boards must be free as pos- sible from moisture (particu- larly in small vias) and on the panel surface. Secondly, the copper surface must be free of oxides. Oxidation on the copper surface may increase the chance of resist lock-in and subsequently prevent proper development and/or resist removal. With respect moisture in the holes, this author highly recommends a horizontal dryer or vertical box system that sufficiently removes the moisture. In or- der to prevent oxidation af- ter metallization, de-ionized water rinsing followed by anti-tarnish (benzotriazole- or citric acid-based) are beneficial in preventing oxidation. Some type of cleaning and surface struc- turing is required in virtually every step of the printed circuit manufacturing process, from preparing the raw laminate for etch or plating resist to final assembly board cleaning before shipment. Many cleaning procedures are inte- gral within certain manufacturing processes, such as plating and innerlayer imaging. Through mechanical, chemical, or electro- chemical processing, the copper surface's chem- ical composition and topography are prepared for optimal dry film adhesion and subsequent clean release. Processes will vary, depending on the type of copper surface to be prepared (ven- dor copper, electroless, electroplated). Surface preparation processes typically roughen the sur- face to increase film contact area and remove chemical impurities and anti-tarnish coatings, which could interfere with film adhesion. The mechanism of resist adhesion is a function of the contact area of the copper surface, resist film thickness and flow characteristics and lam- ination variables. In general, surface preparation is done to assure good adhesion of metal, dielectric, pho- toresist, or soldermask to the prepared surface, primary imaGinG for pattern platinG, part 1 Figure 1: Possible failure modes due to poor surface preparation (Source-IPC 740).