Abstract: Disclosed is an aqueous alloy plating solution for surface treatment of a modular PCB. The plating solution comprises 1-30 wt % of an organic acid having at least one sulfonic acid group (—SO3H), 0.1-20 wt % of a complexing agent, 0.1-15 wt % of a thio-compound having at least one —S—, 0.05-5 wt % of a water soluble gold compound, 0.001-1 wt % of a water soluble silver compound and 0.1-10 wt % of a sequestering agent, based upon the weight of the plating solution. According to this invention, all plating properties required for the modular PCB are obtained through a single plating process, instead of the conventional double plating process.