HOMEFAQsBOOKS JOBS: Help WantedSuggestions you are here: Hotline/Forum => Letter 44983
Alkaline Tin Bismuth Plating
+++++++
Q. My question involves alkaline tin plating alloyed with 0.3 % to 3.0% bismuth. The bath is a 320 gallon potassium stannate process. Tin metal 5.3 - 10.6 oz/gal, free KOH 2 - 8 oz/gal, soluble bismuth 20 - 40 ppm, temperature 170 F, eductors for mild solution circulation, cathode bar agitation, and stainless steel anodes. I plate at 10 - 20 ASF to 0.00015 inch minimum, with 0.0006 inch minimum copper underplate. My coating composition must be IAW ASTM B545 [link is to spec at TechStreet] except 0.3 to 3.0 wt.% bismuth is co-deposited with the tin. My problem is roughness. This is a new bath make up. Analytical analysis shows my tin metal at 5.8 oz/gal, free KOH at 2.5 oz/gal, and Bi at 20 ppm. The roughness looks to be dark, almost black, and is more likely to be found in HCD, and shelf areas. I'm thinking we plated at too high a current density and polarized the anodes, and started producing stannic tin. I can see a few very small black specs circulating in the solution. The deposit looks good with the exception of the roughness, and Bi in the deposit is approximately 1.0 %. I've added about 1.5 gallons H202 to oxidize the tin. I did this at 120 F (temperature too high ?). Am I on the right track here, or should I be looking elsewhere? Parts look good out of the copper. Thanks for any and all replies.
Stewart HollowaySr. Materials & Process Eng. - Cedar Rapids, Iowa, USA
January 24, 2012
Q. I am interested to know the method of analyzing the solution of Sn-Bi plating Bath. I am particular about finding out content of Bismuth Nitrate (less than 0.5 gm/l) in the solution. Expecting to get answer from plating experts I am posting my question in this forum. Thank you.
Amit k Singh- Hyderabad, AP, India