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This paper describes the approach used by the authors to select the flux materials for a simple water‐soluble solder paste flux formulation, what those materials were, and how they interacted to give the correct properties. Consistency of formulation and performance are discussed with emphasis on the need for adequate process parameter control as with any formulation. With this water‐soluble paste formulation various circuit card designs were successfully built possessing 25‐mil pitch, and larger, components. The cards were cleaned in aqueous cleaners and passed IBM standard insulation resistance testing.