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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 resist lamination and exposure) is washed away via the developing process. Further, the quality 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 unexposed 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 sodium carbonate or potassium carbonate.
Editor's Note: This article originally appeared in the June 2016 issue of The PCB Magazine.