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74 The PCB Magazine • June 2016 Introduction The proper development of the primary photoresist is critical to the overall success of the imaging process and in turn the processes that follow—either etching to form innerlayers or the electroplating processes on outer layers. In this step, the unexposed photoresist (after re- sist lamination and exposure) is washed away via the developing process. Further, the qual- ity of the electroplated trace, pad and via are influenced by the developing process. Why is this an issue? Well, for the electrolytic copper to adhere to a developed surface it must be free of resist residues and the adhesion promoters in the unexposed resist. This means that the unex- posed resist gets solubilized by the action of the chemicals in the developing operation. Typical developing chemistry for aqueous photoresists is made up of either solutions of so- dium carbonate or potassium carbonate. Equa- tion 1 below illustrates how an aqueous resist is developed. (Potassium carbonate is used to show how the reaction proceeds.) With either potassium or sodium carbonate mixed with wa- ter, the process occurs as follows: (Equation 1) Unexposed resist gets solubilized (devel- oped) when the carboxylic acid groups of the resist binder react with base to form soluble so- dium or potassium salts of the binder. Equation (1) shows two bases, K 2 CO 3 and KOH, which get consumed during development. Equation (2) illustrates the consumption of the stronger base KOH (1). (Equation 2) by Michael Carano RBP CHEMICAL TECHNOLOGY Primary Imaging for Pattern Plating, Part 2: Development TROUBLE IN YOUR TANK

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