Kester 44 Rosin Flux is an activated rosin formula for use in flux-cored solder wire. 44 has virtually dominated the field of activated rosin core solders for well over four decades. An outstanding performance feature of this flux is the “instant-action” wetting behavior. The high mobility and fast-spreading action of this flux results in more reliable production line soldering. 44 is classified as ROM1 per J-STD-004.