(2) Process. In order to have a cleansing effect on a metal surface, the
flux used must melt at or below the fusing point of the solder, and it must prevent
any oxidation by excluding the air during the soldering process. Different metals
solder better with different fluxes: therefore, the manufacturer's recommendations
should be followed whenever possible.
(3) Liquid flux. Liquid fluxes are usually salts of acids in solution; for
example: zinc chloride dissolved in water. All liquid fluxes, with the exception
of rosin dissolved in alcohol, are corrosive. The residue of the salt solution
left on the work after soldering must be thoroughly removed to prevent
deterioration of the soldered joint. Liquid rosin flux is noncorrosive, and its
residue need not be removed.
(4) Paste flux. Soldering pastes may be acid- or nonacid-type fluxes. They
consist of grease or wax containing an acid or nonacid flux. Acid pastes are
corrosive, and their residue must be removed. Nonacid pastes, such as petrolatum
(e.g., vaseline) mixed with rosin, are noncorrosive but should also have their
residue removed to prevent dust and dirt from collecting on the finished work.
This is particularly important in soldering electrical connections, because the
dust and paste mixture may form a short circuit. In all work in which pastes are
used as fluxes, the joints should be thoroughly cleaned to remove any excess grease.
b. Preparation of zinc chloride. Zinc chloride flux is made from hydrochloric
(muriatic) acid by placing the acid in an earthenware container and adding a few
pieces of zinc to "cut" it. This causes a vigorous chemical reaction. Enough zinc
should be added to "kill" the acid; that is, until all reactions have stopped. The
reaction produces explosive hydrogen gas; therefore, the preparation of the
solution should be done outdoors. After the solution has been strained through
several thicknesses of cloth, it is ready for use. Hydrochloric acid may be used,
without adding zinc, as a cleaning agent on heavily oxidized surfaces. When using
hydrochloric acid on zinc and galvanized iron, the zinc metal or the zinc in the
galvanized coating combines with the acid to form zinc chloride flux.
31. SOLDERING IRONS. Soldering irons are heated by one of two methods
-externally or internally. The externally heated iron may be heated by a
blowtorch, bunsen burner, portable fire pot, etc. The internally heated iron is
heated by an electrical heating element.
a. Flame-heated iron (nonelectric). The ordinary flame-heated soldering iron
consists of a tip made of solid copper 3 or 4 inches long, usually octagonal in
cross section with a tapered point on one end and a steel rod to which a suitable
wooden handle is fastened (fig 22). Soldering irons are rated according to weight.
Selecting the proper size or weight iron is important. A heavy iron will hold heat
longer than a light one; however, a medium or lightweight iron is easier to handle
and is more suitable for small jobs. Beginners should start by using a medium
weight iron. The lightweight flame-heated iron is generally used by radiator
repairmen, electricians, etc, who require only a little solder at a time and an
easily manipulated tool that retains sufficient heat for a short period. This type
of soldering tool, however, requires repeated heating over an open flame.
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