Hi Gaby! The Achilles heel of all tin finishes - immersion tin included -
has been its thermal degradation characteristics. Thermal excursions cause
two degradation mechanisms: 1) the growth of surface oxidation - as the SnO
oxide transforms to SnO2 oxide the solderability goes downhill very fast;
2) the growth of copper/tin intermetallic phases - if the copper/tin
intermetallic phases (especially the Cu3Sn phase but also the Cu6Sn5 phase)
reach the surface and become oxidized then the solderability goes to zero.
Remember - the more active the flux system you use the less impact both
these mechanism have in your production process.  What differentiates the
various available immersion tin chemistries from each other is how each of
the individual chemistry suppliers formulates their immersion tin bath to
account for these two degradation mechanisms.  I agree with you on
procurement - if you need an immersion tin finish which will withstand
multiple thermal excursions then you should expect to pay more and have
fewer potential chemistry supplier choices. If you only need one thermal
excursion pass then more immersion tin possibilities exist. The needs of
the process should be matched to the characteristics of the surface finish
- and both the pwb fabricator and pwb user need to work together to meet
those goals. Enough said - I'm off to find a Diet Coke for supper!

Dave




Gaby Bogdan <[log in to unmask]> on 11/25/2002 06:19:51 PM

To:    "TechNet E-Mail Forum." <[log in to unmask]>,
       [log in to unmask]
cc:

Subject:    Re: [TN] Immersion Tin


Dave,
What is the cause of "degradation"?
Do you mean that this process is not the same from one supplier to the
other
? And  even more- that the procuring department should buy boards from
different suppliers according to their proven performance for each
finishing
process?
By the way- that's what we try to do.
Gaby
----- Original Message -----
From: "Dave Hillman" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, November 25, 2002 11:53 PM
Subject: Re: [TN] Immersion Tin


> Hi Yannick! Not all immersion tin chemistries are created equal - some
are
> much more robust in terms of thermal excursion degradation than others. I
> recommend you conduct a solderability test per JSTD-003 to determine if
> your solderability has gone south or if you are having a solder process
> issue. Good Luck.
>
> Dave
>
>
>
>
> Yannick Brisson <[log in to unmask]>@ipc.org> on 11/25/2002 07:03:35 AM
>
> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please
respond
>        to Yannick Brisson <[log in to unmask]>
>
> Sent by:    TechNet <[log in to unmask]>
>
>
> To:    [log in to unmask]
> cc:
>
> Subject:    [TN] Immersion Tin
>
>
> Hi,
>
>   I made a board wich I plated with immersion Tin, I tought it should be
> the
> same result as we had with tin lead but we ran into a weird problem.  We
> use
> a 63/37  no clean paste and on the Board with Immersion Tin the solder
just
> make a ball ON the pad...we ran the same profile with a board Tin-Lead
and
> we get almost a perfect solder joint.  Because it's immersion tin should
we
> put our temperature higher?
>
>   We try to solder with a iron and the paste can hardly wet all the pad
the
> fisrt time..if we do it a first time, cleand the pad and do it again it's
> more easy.  If someone have work with it can give me some trick I would
> appreciate.
>
> Thanks You
>
> Yannick Brisson
> ============================
> Technicien Ingénierie de Production
> Technician Production ingineering
> M2S Électronique
> 2855, Rue de Celles
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