Hot oil reflow used to be one of the cheapest PTH
finish's. Boards suffered the handicap of having solder covering the entire
copper track area. This tended to result in both poor solder resist adhesion,
and either deformed of removed solder resist after wave soldering. As a
Tin Lead etch resist layer could be used over the copper track
re-flowing was a very efficient and cheap solution. [ The current
normal procedure is to plate a thin sacrificial layer of Tin instead,
strip if off apply solder mask, and then a final finish.]
The process is carried out by first coating the panel with an active
flux, and the immersing in very hot oil for 20-30 seconds. Very often a
second slightly cooler oil tank would then be used which would gently
cool the solder below its melting point, thereby enhancing it's appearance.
This is the cheapest finish for PTH circuit boards. After the
circuits have been etched they are fed through a couple of tanks
containing oil which is hot enough to fuse the previously plated tin
lead. The main drawback with this finish is that all the tracks remain
covered with solder, which can cause surface deformities during wave
soldering.
It's closely related to Infra
red and Hot Air Reflow the only real difference is the heat source. All
of which will shortly on exist in the history books when the Lead ban if
brought into force.
