White Tin, Why Are You Black?
Allan Wilcox, RD Chemical Company
Every immersion Tin plater faces the same problem with their immersion Tin bath, it ultimately begins to plate black Tin. And until now, there has been no way to predict when the bath would begin to plate black Tin. Now there is a way, but before we discuss the method, let us begin by discussing WHY the Tin plates black.
When Tin immersion plates on to the Copper, two atoms of Copper dissolve for every atom of Tin that plates out. The Copper Metal goes into solution as “Copper One” (also known as Cuprous Ions, or Cu+). Copper in this form is well tolerated by most immersion Tin plating baths, and at reasonable levels has no effect. However, Cuprous can react with atmospheric Oxygen to go to “Copper Two” (also known as Cupric Ions or Cu+²), this is the form of Copper found in most electroplating baths, and is blue in color. Cupric Ions react with Tin metal, and plates out. This Copper plating on to the Tin is the black deposit on top of the Tin.
RD Chemical has developed a spectrophotometric method to measure Cupric Ions in ANY immersion Tin (or in other words, we added a sample of the bath to a dye, which changes color when it reacts with Cupric Ions). In theory, you could actually use your eye to estimate color as an indication of Cupric Ions, (see photo) but a spectrophotometer will give you an exact reading.
To predict when Tin Bath will plate black, measure the Cupric level in the bath just when plating starts to darken and use this measure of “Copper Two” in your Tin Bath to predict when it is time for a new bath. The maximum amount of Cupric that your Tin bath can tolerate is a function of specific bath chemistry, and varies with different chemical suppliers. If an absolute standard is desired, 1000 ppm of Copper can be achieved by adding 15 mls of standard Copper Sulfate solution to a liter of bath.
This measure should be done carefully, under conditions described below, as Cuprous Ions in the bath sample can change rapidly to Cupric Ions, thus conditions used by the method are designed to slow conversion of Cuprous to Cupric during the test. With our immersion Tin plating bath, we have been able to get to 1400 ppm (1.4 grams/liter) Cupric Copper before the plating darkens.
In order to understand the Zincon-Cupric Method it is useful to discuss the science behind spectrophotometric methods. The critical detail is that spectrophotometric methods measure of the amount of a specific metal ion in a solution based on the ability of a dye to form a “complex” with the metal ion of interest, causing a change in color of the dye.
The Zincon-Cupric method uses Zincon, to selectively form a complex with Cupric ions. Whereas Cupric ions form a stable, bright blue complex with Zincon, Cuprous and Tin ions do not form a complex with Zincon. The selectivity of Zincon for Cupric and change in color is only part of what makes this method possible.
The amount of light absorbed by a solution is directly proportional to the amount of material in solution. So, as the amount of cupric increases in solutions of Zincon, more Zincon-Cupric complex is formed, and the solution color changes from orange-red to bright blue (Figure 1).
The properties of Zincon provide a method to measure how much Cupric is in the Tin Bath. The method uses a spectrophotometer to measure how much light is absorbed at a specific wavelength of light.
Zincon-Cupric Method Specifics
The spectrum of uncomplexed Zincon in the Indicator Solution used in the Zincon-Cupric Method is shown in Figure 2 (Red Line). In the absence of Cupric, the wavelength of maximum absorbance by Zincon is 490 nanometers (nm).
When a bath of ChemPosit™ Sn (RD-51) immersion Tin is doped with Cupric Ions and the bath analyzed by the method, the complex formed between Zincon and Cupric absorbs most strongly at 605 nm (Figure 2, Blue Line).
The maximum allowable amount of Cupric in a Tin Bath can be measured and specified by the Zincon-Cupric Method. The method is very simple, consisting of adding a sample of the bath to an Indicator solution, and measuring the absorbance at 605 nm with a spectrophotometer.
- 1.Prepare the Zincon-Cupric Indicator Solution (1.0 Liter). To approx. 500 mls of deionized water add: 100 mls Cupric Inhibitor (RDZ-1924)*** 100 mls Zincon Indicator concentrate (RDZ-1923)***
- 2.Add a sample (1.0 mL) of the Tin Bath to the Indicator Solution.
- 3.Adust pH to 10.4 with concentrated ammonia, then dilute to 1.0 liter with deionized water
- 4.Allow solution to sit for 20 minutes for reaction to complete
- 5.Read the absorbance at 605 nm by a spectrophotometer.
The Zincon Indicator Concentrate (RDZ-1923) is 0.23 grams per liter Zincon indicator dissolved in 400 grams/liter Triethanolamine** solution. The Cupric Inhibitor (RDZ-1924) contains Thiourea (76 g/L) and Sodium Bisulfite (10 g/L) in deionized water.
In the absolute sense, no calculations are necessary to monitor the build up of Cupric with the Zincon-Cupric Method. Simply recording the absorbance at 605 nm is sufficient to establish the Zincon-Cupric Method for specific bath conditions. The maximum absorbance limit is obtained when the bath begins to plate out Black Tin.
We have described a convenient method useful for measuring the amount of Cupric in a Tin Bath. The method is very simple to implement, and involves adding a sample of the Tin Bath to an Indicator Solution and measuring the absorbance with a spectrophotometer.
And the GOOD news:
RD Chemical will supply the reagents required for this test at no charge to any company requesting them. Companies not on the North American continent are asked to pay freight charges.
** For those attempting to make their own reagents, be aware, conventional commercialTriethanolamine is actually 80% Triethanolamine + 20% Diethanolamine. The presence of Diethanolamine causes the Zincon indicator to become unstable. We are using a special grade of pure Triethanolamine available commercially diluted to 85% with water.
Figure 1 Solutions of Zincon with Increasing Concentration of Cupric.