Don,
I was interested in your question about tin chlorides and FTIR, so I did
some digging.  As has been pointed out, SnCl4 itself is a liquid.  It is
tetrahedral like methane and has four principle vibrations, all active in
terms of Raman spectroscopy, but only two active in infrared spectroscopy.
The vibrations occur at 368, 106, 403 and 131 cm-1, with only the latter two
being the ones active in IR.

SnCl4 reacts with water to form cis-[SnCl4(H2O)2]linked with other less
tightly held water molecules.

SnCl6-2 is octahedral and again has four vibrations - three Raman active
(314, 235 & 157) and only one IR active (313 cm-1).

So, it is a fair bet that all Sn-Cl vibrations occur at less than 500 cm-1.
If your spectrometer optics or the discs or cells you are using are made of
NaCl, then the low frequency cutoff is about 650 cm-1 and you will never see
any vibrations of bonds between atoms as massive as tin and chlorine.  Even
with KBr optics the cutoff is around 400 cm-1, so you are going to still be
out of luck.  

With regards to Raman Spectroscopy, the issue is whether there are multiple
organic impurities mixed in with the inorganics that will result in enough
fluorescence that your spectrum of any SnX species is swamped by the
fluorescence signal.

Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Don McFarland
Sent: Thursday, August 04, 2011 8:22 AM
To: [log in to unmask]
Subject: Re: [TN] Removing SnCl2

Sn(iv) would make more sense because this stuff seems impervious to anything
we've thrown at it. 

We have traced it back and found that this residue is visually present on
the bare ENIG pcb. We have the sem, edx ad ftir tests being done on the bare
board to compare residual signature to post-assembly residues. The
interesting this was our FTIR only showed FR-4, but nothing else. Should
ftir detect these Sn residues if present and adequate sample captured? I am
wondering if this is a laminate curing issue and not a contamination issue,
since our data points arent seeing the chemistry.

The SEM/edx found minute flakes of Sn that appear to attach to Cu
contaminate found on bare pcb. These embedded flakes are only visible at
1000x and are small at that...just a bit out of the allowed mag for fod
inspection. 



Sent from my iPhone

On Aug 3, 2011, at 4:24 PM, R Sedlak <[log in to unmask]> wrote:

> Don:  I think I qualify as a "real Chemist", or at least I have a lot of
people fooled...
> Actually, SnCl2 is easy to remove, but it is highly unlikely your deposit
is SnCl2, but is much more likely to be SnCl4 (Stannic Chloride) which is
rather more difficult to remove...
> I would recommend either a Caustic cleaner, preferably with some chelate,
if your substrate can tolerate that.
> If not, probably the only other cleaner which will remove Stannic Chloride
is a dip in an Ammonium Bifluoride solution, say, 5-10%.   Stannic salts are
highly insoluble, and hydrolyze easily to Stannic oxide, which needs either
strong alkali, or HF to remove.
> Wish I had better news.
> Rudy Sedlak
> RD Chemical Company
> 
> --- On Wed, 8/3/11, Don McFarland <[log in to unmask]> wrote:
> 
> From: Don McFarland <[log in to unmask]>
> Subject: [TN] Removing SnCl2
> To: [log in to unmask]
> Date: Wednesday, August 3, 2011, 10:49 AM
> 
> Any ideas on methods to remove a tin chloride residue that was found on a
FR-4 substrate after wave solder. We have done some good homework to find
the source, but we are looking for good and proven methods to remove the
residues from affected assemblies.
> 
> Any input is welcome. 
> 
> Regards, 
> 
> Don McFarland
> 
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