The condition of the gold pads sounds like the "Black pad" problem exhibited
by circuit boards. The lack of nickel is odd because I thought this
phenomenon was related to Phosphor-Nickel oxidation in the plating
process...
Just a thought,
Jeffrey A. McGlaughlin CID
Battelle Memorial Institute
Columbus Ohio
[log in to unmask]
-----Original Message-----
From: Marsico, James [mailto:[log in to unmask]]
Sent: Friday, October 19, 2001 7:31 AM
To: [log in to unmask]
Subject: Re: [TN] Vapor Phase Soldering material
BTW, the reason for my inquiry is that we are using Fluorinert as a medium
for a liquid burn-in chamber. We're building a multi-chip module in a 655
BGA ceramic package. These components require 160 hrs of burn-in at 125 C,
under power (MIL 883 requirement). The test fixture consists of BGA sockets
(clam-shell with gold plated copper pins) soldered to a test board. The
liquid is used to prevent oxidation of the solder terminations during
burn-in, since the solder balls are not attached yet. The solder
termination base material is tungsten which is nickel and gold plated. We
recently had a batch, or two, of components which exhibited damage to the
solder terminations... anywhere from flaking plating with black metalization
underneath to completely black, almost burnt-looking pads. Attempts at
cleaning these pads (warm HCL) were fruitless. Surface analysis of the pads
where metal was found showed gold, copper and molybdenum, but no nickel.
(Where the molybdenum came from is a mystery.) At this stage of the game
these components are probably worth a couple of thousand dollars, so you can
see the reason for trying to get to the root cause. Someone had mentioned
that Fluorinert breaks down and forms acid in the presence of water, so
everyone is pointing to this as the cause... I'm not so sure.
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
-----Original Message-----
From: Brian Ellis [SMTP:[log in to unmask]]
Sent: Friday, October 19, 2001 5:25 AM
To: [log in to unmask]
Subject: Re: [TN] Vapor Phase Soldering material
Jim,
This surprises me and I'd like to know more. In practice, in VP
soldering, I doubt whether there is any great significance. Firstly,
the
bulk of the vapour is anaerobic, with just the top interface in
contact
with the air. This air will be heated and therefore the RH, even in
humid climates, will become extremely small. Then PFCs are
demonstrably
resistant to hydrolysis in the presence of free H and OH radicals
(this
is why they have atmospheric residence times of thousands of years):
there is simply not enough energy available to break the F-C bonds.
Naturally, Mr Arrhenius will say that, at 200°C, the energy will be
much
higher so there will be more reactivity, but it would require temps
exceeding 300°C for it to become significant. Notwithstanding, I
believe
that we are talking about 1 molecule in 10^n where n is a very large
integer that will decompose at soldering temperature under practical
conditions in a given time, unless there is something new that I
haven't
heard about (always possible, of course).
I just looked at Fluorinert data sheets on the 3M web site (requires
some searching!) and there is no mention of a hydrolytic reaction in
anything I could find.
Brian
"Marsico, James" wrote:
>
> Brian,
> I'm getting conflicting comments from someone from 3M who is
claiming that
> Fluorinert will decompose at temperatures of 200C and above and
this is not
> a good thing in the presence of water (usually available as
moisture in the
> air). Any comments?
>
> Jim Marsico
> Senior Engineer
> Production Engineering
> EDO Electronics Systems Group
> [log in to unmask] <mailto:[log in to unmask]>
> 631-595-5879
>
> -----Original Message-----
> From: Brian Ellis [SMTP:[log in to unmask]]
> Sent: Wednesday, October 17, 2001 2:38 AM
> To: [log in to unmask]
> Subject: Re: [TN] Vapor Phase Soldering material
>
> Back in the days? Vapour-phase soldering is still done!
The great
> advantages of perfluorocarbons over other halocarbons are
that a)
> they
> are chemically extremely stable because the C-F covalent
bond
> requires
> much energy to break them; b) they are virtually non-toxic
(for the
> same
> reason: they do not break down in the body). In fact they
can be
> used to
> replace blood in the body for short periods as they do
allow oxygen
> to
> be dissolved and some "sportsmen" have even injected PFCs
into the
> bloodstream to improve their endurance performance by a
higher
> oxygenation; c) they are very inert chemically; d) they
withstand
> high
> temperatures; e) they are NOT ozone-depleting. Teflon is a
solid
> PFC.
> The disadvantages are a) that they are very expensive; b)
they are
> EXTREMELY global-warming (typically 1 kg of PFC is
equivalent to 10
> tonnes of carbon dioxide which is roughly equivalent to
half the
> emissions of a medium car during its lifetime); c) their
stability
> (~10,000 years atmospheric residence time) is such that
end-of-life
> disposal is very difficult and costly.
>
> The fluids, be they pure PFCs or PFEs, used for soldering
are
> sufficiently stable at 210 - 260°C that there is little
significant
> decomposition. Water hardly enters into the equation
because it is
> boiled off instantaneously, even if there be some
condensation on
> the
> cooling coils. Theoretically, there may be some hydrogen
fluoride
> generated (no hydrofluoric acid), but I believe that the
quantities
> would be really negligible, in practice. With the presence
of other
> organics from the flux residues, I would personally be
more
> concerned
> with trifluoroacetic acid, another nasty, but I have not
heard of
> any
> cases. Carbonyl fluoride is also not to be excluded, also
very
> toxic.
> There is another fluoro-compound, whose name escapes me
for the
> moment,
> which did cause some concern in the early days, which is
toxic in
> the
> ppb rnnge. However, in practice, I have never heard of any
of these
> toxic substances being present in sufficient quantity to
be of
> practical
> concern.
>
> The real crunch comes when there is a secondary blanket,
used in the
> early days to reduce losses of the primary fluid. This was
initially
> always a pure CFC-113 (e.g., Freon TF, Arklone P etc.).
Later,
> because
> of the ozone depletion potential of CFC-113 being as high
as 0.8, a
> lower BP PFC was also proposed. CFC-113 does decompose at
the vapour
> temperature of the primary fluid, so that the interface
zone between
> the
> two vapours was always a hotbed of chemical reactions.
This
> certainly
> produced, especially in the presence of water, whose BP
was higher
> than
> that of the secondary fluid, a whole panoply of acids and
other
> toxic
> substances. For this reason, these machines had both a
molecular
> sieve
> to de-water the condensed vapours, but also a filter to
remove the
> acids, otherwise the stainless steel tank corroded into
holes in a
> matter of months. It is possibly this that you are
thinking of. The
> most
> important decomposition product was hydrogen chloride,
which becomes
> hydrochloric acid in the presence of water, but there was
also
> phosgene,
> hydrogen fluoride, trichloroacetic acid and goodness knows
what
> else. I
> feel reasonably certain (abstraction of any environmental
effects),
> if
> vapour phase soldering with a CFC-113 secondary blanket
were
> invented
> for the first time today, it would never be allowed into a
workshop
> because of the H&S aspects.
>
> Hope this makes it clear.
>
> Brian
>
> "Marsico, James" wrote:
> >
> > Back in the days of vapor phase soldering, I seem to
recall that
> > hydrofluoric acid was a bi-product of the fluorinert
mixed with
> water, or
> > was it the Freon mixed with the fluorinert? Could
anyone
> elaborate?
> >
> > Jim Marsico
> > Senior Engineer
> > Production Engineering
> > EDO Electronics Systems Group
> > [log in to unmask] <mailto:[log in to unmask]>
> > 631-595-5879
> >
> >
>
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