[Buriedcode]

Hi,

New to the forum, so be gentle.  I am not a chemistry student but did study it at A-level (in the UK) and this is a purely academic question - out of interest rather than any 'need'.

I have used an electroless tin-plating solution for tin-plating the copper on circuit boards.  The product has a quoted shelf life of '6 months', when made up (aqueous).  It has been used occassionally for roughly two years, with inevitably decreasing results.  I have purchased another 'mix' to make up a small batch, however, I am curious as to the chemistry involved here, specifically 'what is left' in the old solution.

From the datasheet of the product, 90g to make up 1 litre of solution contains, (by weight):
Thiourea 27%
Stannous chloride (tin (II) chloride)15%

Now, obviously every time I plate copper by immersing it in this solution, I am removing tin from it, so of course it will 'run out'.  However, there is a notiably large clump of insoluble crystals left which I have yet to remove - the bottle is opaque plastic, so I cannot see' these crystals, jsut hear the lump rattling about.

I'm assuming the Thiourea is used to allow the plating to be done without electrical current, I cannot understand if this is 'used up', or if it simple has to be in the solution for the electroless plating to occur.  It still smells strongly of it (smells like cats wee..no joke).  But the tin chloride level must be low - if at all - since I've been plating with it.

My questions are:
1. What do you think this 'solid clump' could be?  There are only two components in the solution (not including water) and it has been exposed to low temperatures, as well as air for brief periods of time.

2. Although in terms of time/money/effort, it is not worth it.  Would it be possible to 'rejuvinate' (horrible word I know) this to a usable original solution by using tin metal, along with perhaps eletrolysis? 

If the Thiourea is used in the plating reaction, then I cannot replace this, so its useless, and I'll throw it.  However, if I can introduce, or increase the level of stnnous chloride back to its original concentration, then perhaps I can use this solution again.  The two formulas from wikipedia that interested me are:

SnCl2 (aq) + H2O (l) is in equilibrium with Sn(OH)Cl (s) + HCl (aq)

With Sn(OH)Cl being a possible candidate for 'the lump'.  he pesence of HCL would decrease the pH, and I can test for that.  If that is true I could remove the Sn(OH)Cl and perhaps use the small ammount of HCL prouced in the solution with tin metal to create tin(II) chloride in the solution.. (please point out the flaws in this logic)

the second equation is:
6 SnCl2 (aq) + O2 (g) + 2 H2O (l) to 2 SnCl4 (aq) + 4 Sn(OH)Cl (s)

Which brings tin(IV)chloride into the mix.  I wonder if I could reduce (or is it oxidise..) the SnCL4 back to SnCl2 using eletrolysis?  Either way, I am going to need to introduce some tin into the solution to replace the lost ammount.

Sorry for the long winded question. As I said, it is purely out of curiosity, I can easily 'buy another batch', but I like to know whats going on

Buriedcode.

[Nobby]

What you are talking about is an electroless immersion tin bath. This normally works at higher temperature about 60 ° C. Of course if you use it you have to replenish sometimes tin and also thio urea. But this should written in an datasheet of this bath. Tin-IV is lost, this cannot be converted in this solution because it forms unsoluoble  SnO₂.

And careful thio urea is poisson and carcenogene.