Genny:

They say you learn more of a subject when you try to teach it than any other
way, and if so, I may be one of the smarter guys on the subject of Hot Oil
Reflow, AKA, Fusing.

Reflow or Fusing is NOT Hydrosqueegee...which came later.

The process started out in the early days of PCB production, and involved
dipping the PCB just before application of the solder mask into a bath of oil
heated to 400+ F.  The Tin/Lead deposit which had been plated on the boards
as an etch resist, like pure Tin is used today, would then melt, or "reflow"
or "fuse", in about 30-60 seconds.  The board was then pulled out, rinsed
off, and Soldermask applied.

This fusing process turned the plated Tin/Lead, (which is really discrete
chrystals of Tin and Lead, and not really a true alloy) into a true alloy,
and thus it became very resistant to tarnish and oxidation, and had a VERY
long shelf life, fully equal to HAL today.

Believe it or not, the oil originally used was peanut oil, but this required
solvents to rinse off, and thus was later changed to a detergent type oil
that rinsed (slowly) in water, with A WHOLE LOT OF FOAMING....

Later a process-specific oil was developed that rinsed easily, with no
foaming.

I cannot discuss the flatness of the deposit, because nobody knew about
"flatpacks" in those days, and thus did not care.

The whole process of leaving the plated Tin/Lead deposit on the board under
the soldermask was dumped because the solder would flow during assembly, and
short out when the PCB designers began to crowd the terrain with tight
spacing of circuitry.

This process was also done (later on) in high volume shops by an IR
(Infrared) conveyorized machine.

Rudy Sedlak
RD Chemical Company

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