The MM01019 is an ERSIN? 0.381mm 60/40 grade Multicore Solder Wire, made of Sn and Pb alloy. Flux-cored solder wire is universally made by an extrusion process followed by drawing through dies progressively reducing the wire diameter. As solder is opaque and usually contains lead, it is not possible to see and test inside the wire to check for flux presence.