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November 2001

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 10 Nov 2001 17:23:29 +0200
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Hi, all!

Please let me set the record straight.

1) Hot oil reflow is NOT the same as hot-oil levelling

2) Hot oil reflow consists of reflowing an electrolytic tin/lead
plating, typically about 30 um thickness. It melts the plated metals
into solder and leaves a good, thick solder coat, just like IR reflow,
except smoother and shinier. The process was usually a three bath affair
with simple immersion in oil at, say, 140°C preheat for 10 secs, 230°C
fusion for about 3 seconds (more for thick boards) and 190°C quench for
a couple of seconds, so that the solder "froze" very rapidly on
extraction.

3) Hot oil levelling (such as the Hydrosqueegee) consists of wave
soldering or dip soldering the bare board and then removing the excess
solder by jets of hot oil. This is the process that, badly adjusted,
just left the intermetallics but, correctly adjusted, gave a result
quite similar to HASL, except that the coating was slightly more uniform
because the interface between the surface tension of two liquids is
better than between the molten solder and air, which tends to form a
bumpier meniscus.

4) In both cases, 3 types of oil were used. The initial one was Shell
Peblum A, which was a mineral oil designed originally for levelling the
tin on sheet steel used for making tin cans. For reasons of a) a
relatively short lifetime b) a fairly high cost, some intrepid
adventurers replaced it with peanut oil. Then some of the flux
manufacturers brought out special water-soluble "oils", such as Lonco
Fusecote 240. These were, in fact, blends of polyglycols and glycol
ethers. They had a lifetime in the machine of about 3 - 4 times that of
the mineral Peblum oil at about the same price and much reduced
operating costs (water clean, instead of solvent cleaning was also
cheaper).

5) All three oils were smelly, unless good ventilation and quite messy,
which was their downfall, in both processes. There was also worry about
loss of SIR with the water-soluble oils, although this was more worry
than fact. They also fell into some disrepute by a number of fires,
especially with peanut oil which has a flash point under the melting
point of solder. I don't believe there was a fire with the water-soluble
oils which had a TAG CC flash point of about 260°C, dropping to about
250°C after about three weeks of 1 shift use.

6) Hot-oil reflow was more popular than levelling as it was a very
simple process which was exceedingly easy to control and the fusion
temperature, unlike IR, was asymptotic. It was also popular for 3D flex
circuits which were simply impossible to reflow with IR.

For the anecdote, something like the Hydrosqueegee, but more powerful,
is coming back into the thinking within Europe in preparation for WEEE.
By putting in an end-of-life assembled board, all the solder can be
"washed" off and recovered for recycling. Roughly 95% of the lead and
85% of the tin can be recovered (more tin remains on the intermetallic
residues).

Hope this trip down 30 years of memory lane will explain.

Brian

Genny Gibbard wrote:
>
> You learn something new (or new to you) every day.
> I was just told of a board finishing process called Hot Oil Solder
> Levelling.  It was portrayed as a very flat, nice finish.  I was also told
> that it is old technology, which may be why I am not familiar with it.  It
> is only within the last year that I have really gotten involved with
> specifying board finishes.  I've been searching on the internet to learn
> more and am a bit confused.
> My questions:
> Is Hot Oil Reflow and Hot Oil Surface Levelling the same thing?  If not,
> what is the difference?
> What are advantages/disadvantages to this over HASL, or any of the newer
> so-called 'flat' finishes needed for fine pitch?
>
> Thanks.
>
> Genny Gibbard (mailto:[log in to unmask])
>
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