A process for forming a solder bump on an under bump metal structure in the manufacture of a microelectronic device comprising exposing the under bump metal structure to an electrolytic bath comprising a source of Sn.sup.2+ ions, a source of Ag.sup.+ ions, a thiourea compound and/or a quaternary ammonium surfactant; and supplying an external source of electrons to the electrolytic bath to deposit a Sn--Ag alloy onto the under bump metal structure.