There are a few tin plating baths around, but the one often
used for PCBs is the tin(II) sulfate sulfuric acid bath. Its
a tin only plating , i.e. it cannot plate tin/lead alloy
like fluroborate type baths can do.
All tin plating baths require platting additives to produce
smooth bright deposits. Without additives the tin will form
crystals and long thin spikes around the edges of the PCB.
The additives are available from electroplating suppliers.
The manufacture of the additives will specify the correct
bath chemistry and conditions. You will have to ask for
plating additives for a sulfuric acid based tin plating
bath. I am using a set of additives from an Australian
company called R.O.HULL & CO Australian Pty, Ltd
The product is called HULLPLATE SN
There are three additives. Two are "once off" additives for
bath makeup, and the other is brightener which requires
regular addition, depending on how much tin plating is being
done. The supplier charged me total of AUD$90 for 1 liter
of each additive. Shelf life on the additives is indefinite.
Bath composition;
range optimum
tin metal 7.5 to 22 g/l 15 g/L
sulfuric acid 140 to 220 g/L 170 g/L
HULLPLATE SN #1 2% vol.
HULLPLATE SN #2 0.5% vol.
HULLPLATE SN (brightener) 0.4% vol.
operating conditions:
temperature 10 - 30 °C
cathode current density 0.5 - 3 A/dm^2 1 A/dm^2
anode current density 0 - 3 A/dm^2
notes:
Dissolved tin metal is in the form of stannous sulfate,
SnSO4. 1g of SnSO4 = 0.55g of tin
Stannous sulfate is usually sold in a solution with sulfuric
acid. If you cannot buy small quantities then you can easily
prepare it from electrolytically dissolving tin anodes
inside your plating tank. All it requires is a ceramic
garden pot and your plating tank. I can give those details
later if you wish. I wrote a web page on how to do it along
time ago but never put it online.
All stannous solutions are readily oxidized by atmospheric
oxygen, so solutions should never be air agitated. The
stannous ion forms insoluble stannic compounds [tin(IV)]
which makes the solution a milky yellow. Some oxidation is
unavoidable so all baths turn milky yellow after a few
weeks. A lid should be kept on the tank when not used. I've
had my bath 2+ years and its still works perfectly ok.
Adam
Stefan Trethan wrote:
> Now, tell me who is right?
>
>
> Marvin Dickens says electroplating tin chemicals are dangerous,
> Adam says no.
>
>
> I didn't think I was kidding.
> You know my knoweledge is very poor about chemistry
> but i really thought electroLESS plating is much more complicated
> and needs much more dangerous chemicals.
>
> I did electroplating (ok, only "copper plating" with coppersulphate)
> once and thought i understand the process.
>
>
> Adam, you are right with "connect every track first", this
> would be needed if you etch first.
>
> (I was told this is how they make the gold plating on ISA/PCI and similar
> board edge connectors.
> I was told the pcb is initially a bit longer and all pins are shorted
> on this extension. after gold plating this small piece is simply cut off.
> (you can actually see the ends of the tracks running down to the edge at
> some cards))
>
> BUT if you plate your board BEFORE etching you don't need this.
> simply make a NEGATIVE film/exposing/developing and then electroplate.
> the tin should only be deposited where there is no resist.
>
> then strip off the resist and etch.
> (some etchants don't etch tin, but you can also mask with gold)
>
>
> I'm not sure if this would result in a reasonably thick tin plating.
> or if the surface of the tin would be "ugly" black (this happens if you dip
> solder
> in hcl/h2o2 etchant but i think sulphuric acid works)
>
>
> I would appreciate to know which "salt" is needed for tin plating.
> as sayd i used copper sulphate once for copper plating, but have no idea
> which salt of tin there is.
> Also i wonder which "organic additives" there are. had no idea.
>
>
> st
>
> gold plating would also be interesting - but the electroplating liquids for
> gold plating
> i saw at a electronics supplier were VERY expensive.
>
>
>
>
> On Fri, 14 Nov 2003 12:14:45 +1100, Adam Seychell
> <adam_seychell@...> wrote:
>
>
>>An acid sulfate electroplating tin bath is very safe, as it only contains
>>sulfuric acid and tin(II) sulfate, plus some proprietary non-toxic
>>organic plating additives. Let me know if you want the exact bath makeup
>>described on the additive data sheet
>>
>>There many other tin electroplating baths, which as you say contain nasty
>>stuff. This is the reason I chose the sulfuric acid tin(II) sulfate bath
>>for plating my PCBs. But of course electroplating is not suitable for
>>your application unless you can connect every PCB trace to the negative
>>terminal of the power supply.
>>
>>Electroplating solutions are almost everlasting, so there is much less
>>disposal than immersion tin. Electroplating also is highly predictable
>>and reliable because the quantity of tin deposited is directly related to
>>the total electrical charge from the power supply. The finish of
>>electroplated tin is much easier to control than immersion tin, making it
>>possible to obtain bright mirror like finish. In addition, the chemical
>>running costs are only tiny fraction, even considering initial bath
>>makeup.
>>Use immersion only when you cannot make electrical contact with the
>>metal.
>>
>>
>>
>
>
>
>
>
>
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