Vectronics VEC-1500K - Manual (Page 12)

7
Base Metal
Solder
Base Metal
Solder "wets" surface.
Metallic oxide
Solder
retracts from
A. Cohesive forces pull solder into a ball. B. Adhesive forces spread solder out.
Oxide-free
surface.
Flux is a specially-formulated chemical agent that removes oxide to expose the
base metal underneath. Once oxide is removed, the surface becomes chemically
"active" (Figure-B). This means the molecular bonding sites are restored and the
surface is again free to combine with other metals. When liquidous solder is
applied to an activated surface, powerful molecular forces take over--pulling the
solder downward and forcing it outward to cover the area in a process called
adhesion. When solder adheres to a chemically active surface, we say it wets the
surface. Complete wetting is essential for good solder connections.
In addition to removing oxides, flux has a second job--to form a protective
coating
over the newly-activated metal. Although flux is a semi-solid at room
temperature, it melts and spreads well below the melting temperature of solder.
This allows it to flow ahead of liquidous solder--activating the surface and
locking out air to prevent re-contamination.
Finally, it's important to remember that flux isn't a "cleaning agent". Flux
removes oxides through chemical action and floats them off the surface in a
chemical suspension. If metal is dirty, greasy, or contaminated in other ways, it
should be cleaned prior to applying flux. Also, note that flux residue contains
the oxides it has removed after the solder connection is made. In some
applications, these deposits may need to be removed through cleaning.
Common Types of Flux
Fluxes fall into two general categories--inorganic and organic. Inorganic, or
"acid-core" types, are normally used for plumbing and are far too corrosive for
electronic applications! Most electronic solder fluxes are organic. Organic flux
falls into three classes: rosin, water-soluble organic, and solvent soluble
organic.
Of those groups, rosin flux is most common. Rosin is a natural
substance produced by pine trees that contains abietic (a-bee-tic) acid. Rosin
fluxes are classified by their degree of chemical activity and residue
conductivity. Some rosins are mild and poorly-conductive, while others are very
aggressive and more conductive. The two most-common rosin fluxes are RMA
and RA:
RMA Flux: " Rosin--mildly activated" flux (RMA) is a good choice for
assembling products made from highly solderable metals. The cleaning action of
RMA is adequate for hand assembly in a well-controlled manufacturing
environment, but generally insufficient for general bench and field work where