Electroplating baths of silver and tin alloys

1. A tin/silver alloy electroplating bath comprising metal ions consisting of tin ions and silver ions, one or more compounds having a formula:

X—S—Y  (I)

wherein X and Y may be substituted or unsubstituted phenol groups, HO—R— or —R′S—R″—OH with the proviso that when X and Y are the same they are substituted or unsubstituted phenol groups, and wherein R, R′ and R″ are the same or different and are linear or branched alkylene radicals having 1 to 20 carbon atoms; and one or more mercaptotetrazole compounds having a formula:

wherein M is hydrogen, NH 4 , sodium or potassium and R 1 is substituted or unsubstituted, linear or branched (C 2 -C 20 ) alkyl, or substituted or unsubstituted (C 6 -C 10 )aryl, wherein the tin/silver electroplating bath is free of flavone compounds.

2. The electroplating bath of claim 1 , wherein a ratio of silver ions to tin ions ranges from 1-12.

3. The electroplating bath of claim 2 , wherein the ratio of silver ions to tin ions ranges from 1-6.

4. The electroplating bath of claim 1 , wherein X and Y are different and are HO—R— or —R′—S—R″—OH.

5. The electroplating bath of claim 1 , wherein a ratio of the compound of formula (III) to the compound of formula (I) is from 3-300.

6. A method of electroplating tin/silver alloy comprising:

a) contacting a substrate with a tin/silver electroplating bath comprising metal ions consisting of tin ions and silver ions, one or more compounds having a formula:

X—S—Y

wherein X and Y may be substituted or unsubstituted phenol group, HO—R— or —R'S—R″—OH with the proviso that when X and Y are the same they are substituted or unsubstituted phenol groups, and wherein R, R′ and R″ are the same or different and are linear or branched alkylene radicals having 1 to 20 carbon atoms; and one or more mercaptotetrazole compounds having a formula:

wherein M is hydrogen, NH 4 , sodium or potassium and R 1 is substituted or unsubstituted, linear or branched (C 2 -C 20 ) alkyl, or substituted or unsubstituted (C 6 -C 10 )aryl, wherein the tin/silver electroplating bath is free of flavone compounds; and

b) electroplating a silver and tin alloy on the substrate.

7. A method of electroplating a tin/silver alloy comprising:

a) providing a semiconductor die having a plurality of interconnect bump pads;

b) forming a seed layer over the interconnect bump pads;

c) contacting the semiconductor die with a tin/silver alloy electroplating bath comprising metal ions consisting of tin ions and silver ions, one or more compounds having a formula:

X—S—Y

wherein X and Y may be substituted or unsubstituted phenol group, HO—R— or —R′—S—R″—OH with the proviso that when X and Y are the same they are substituted or unsubstituted phenol groups, and wherein R, R′ and R″ are the same or different and are linear or branched alkylene radicals having 1 to 20 carbon atoms; and one or more mercaptotetrazole compounds having a formula:

wherein M is hydrogen, NH 4 , sodium or potassium and R 1 is substituted or unsubstituted, linear or branched (C 2 -C 20 ) alkyl, or substituted or unsubstituted (C 6 -C 10 )aryl, wherein the tin/silver alloy electroplating bath is free of flavone compounds;

d) electroplating silver and tin alloy interconnect bumps over the interconnect bump pads; and

e) reflowing the silver and tin alloy interconnect bumps.

8. The method of claim 7 , wherein the one or more mercaptotetrazoles are chosen from 1-(2-diethylaminoethyl)-5-mercapto-1,2,3,4-tetrazole, 1-(3-ureidophenyl)-5-mercaptotetrazole, 1-((3-N-ethyl oxalamido)phenyl)-5-mercaptotetrazole, 1-(4-acetamidophenyl)-5-mercapto-tetrazole and 1-(4-carboxyphenyl)-5-mercaptotetrazole.