I found the following information from the Yahoo Group Homebrew_PCBs.

In the September 1971 issue of "Ham Radio Magazine" Larry Hutchinson had an article "Practical Photofabrication of Printed-circuit boards". In that article he had an electroless tin-plating bath.

Formula

• 0.5g Stannous Chloride
•  2.0g Thiourea
•  3.0g Sulfamic Acid
• 100.0 mL Distilled water

To 100ml distilled water add 500 milligrams Stannous Chloride, 2 grams Thiourea, and 3 grams of Sulfamic acid. The chemicals are all powders so they keep but the bath is unstable, so only mix enough for one days job. All you need is enough to cover the board. Action takes 10 to 20 minutes. The article suggests a few mg of Alizarin or Alizarin Red S can be added as a brightner.

The poster said, :"The Alizarin is very expensive and I could not see much difference in the brightness, I abandon the Alizarin when I ran out. Don't remember any odors, but with all chemicals you should take proper care and dispose properely."

So decided to try it.

Here is a video of the Stannous Chloride, Thiourea, Sulfamic Acid solution.

Other comments

• Sulfamic Acid (H3NS03) is correct for this formula. I have also seen formulas that use Sulfuric acid and suspect they work in a similar way. The Sulfamic acid is a white powder so none of the chemicals in the formula are liquid.
• I don't think the Diethyl Thiourea is the correct chemincal but its been a long time since chemistry 101. The articel lists Thiourea which is I think is CAS 62-56-6 (CH4 N2 S). Diethylthiourea is different CAS 105-55-5 ( (c2H5NH)2 CS ).
• Anyone contemplating the use of the above recipe should first read the MSDS on thiourea:  http://www.sciencestuff.com/msds/C2854.html

Al followup poster said:

I got my chemicals back in 1972 from my college chemistry department to do the boards for my 8 digit frequency counter senior project. I did a lot of borads back in those days but not much in the past 20 years untill recently. Recently a did a few small boards and I still had the plating chemicals from 1972 which still worked fine. The Stannous Chloride looked a little funky but still gave
good results.

I have seen all the chemicals on E-bay but the Stannous Chloride seems to be a
little harder to find.

As for measurements the article provided a much more accurate formula indicated
below. (LOL)
Stannous Chloride 1/4 teaspoon
Thiourea 1/2 teaspoon
Sulfamic Acid 1/2 teaspoon
Water 1/3 Cup
I didn't have a scale so I used the teaspoon method and then only to a very crude approximate. I found it was not all that critical.

The formula also called for an optional brightener but I never found that it did anything and I stoped using it after I ran out. The brightner was 5mg of Alizarin or Alizarin red S. The stuff must have been expensive because the chem department would only give me a very small amount. It didn't seem to disolve and just floated on the top of the solution.

It woked quite well and really helped out with the soldering, not tried it with any SMT and solder paste though.

 Another Formula

Another formula I haven't followed up on is here: http://www.finishing.com/0200-0399/260.shtml