Effects of thiourea and polyoxyethylene lauryl ether on electrodeposition of Sn-Ag-Cu alloy as a Pb-free solder

Sn-Pb alloy plating is widely used in conjunction with Sn-Pb solder as a solderable lead coating in the assembly of electronics. However, the use of Pb is restricted because of toxicity. Therefore, there is need for development of lead-free alloy plating and solder. The electrodeposition of the eutectic Sn-Ag-Cu alloy (Ag: 3.5 wt %, Cu: 0.75 wt %) as a Pb-free solder in an H sub 2 SO sub 4 plating bath containing thiourea and polyoxyethylene lauryl ether (POELE) as additives was examined. The addition of thiourea was necessary for the stabilization of the plating bath containing all metal cations Ag exp + , Cu exp 2+ , and Sn exp 2+ . Both POELE and thiourea produced alloy films with a smooth surface and a homogeneous distribution. Thiourea adsorbed on Ag, Cu, and Sn and suppressed the subsequent codeposition, while POELE adsorbed only on Sn, leading to the suppression of the subsequent Sn deposition. These effects resulted in the codeposition of the Sn-Ag-Cu alloy with a eutectic composition and smooth surface. The melting point of the electrodeposited Sn-Ag-Cu alloy was 216 deg C, which is consistent with the phase diagram of Sn-Ag-Cu alloy and satisfies the industrial temperature requirements for Pb-free solders.