preferably 0 2 to 50 g l an acid substance can be incorporated into the tin silver alloy plating bath in order to control the ph of the bath on acidic side the acid substances are preferably alkanesulfonic acids having 1 to 3 carbon atoms such as methanesulfonic acid ethanesulfonic acid and hydroxypropanesulfonic acid benzenesulfonic acids and phenolsulfonic acids having 6 or 7 carbon atoms such as sulfosalicylic acid and cresolsulfonic acid alkanolsulfonic acids such as isopropanolsulfonic acid and sulfamic acid these acids are usable either singly or in the form of a mixture of two or more of them although the acid concentration in the plating bath is not particularly limited so far as tin and silver are soluble therein it is preferably 10 to 500 g l more preferably 50 to 400 g l in particular the acid concentration in the plating bath is controlled so that the ph of the bath can be kept not higher than 2 preferably not higher than 1 the plating bath of the present invention can comprise the above described indispensable components and the balance of water and if necessary additives such as a brightening agent and lubricating agent the brightening agent may be any of those used for brightening tin and silver such as nonionic surfactants anionic surfactants synthetic polymers e g pvp peg and pva amines e g hexamethylenetetramine and triethanolamine ketones e g benzalacetone and acetophenone aliphatic aldehydes e g formalin and valeric aldehyde aromatic aldehydes e g.