App. Note 34 - Solder-flux & Resins
What problems can be caused by solder-flux reside?
The removal of solder-flux from printed circuit boards is essentially
one of washing the board with either an organic-solvent or water based
cleaning solution. With the increasing limitations on organic-solvent-systems
that are imposed by environmental considerations, the water-based-systems
are starting to dominate flux-removal processes. Whichever process is used,
it is generally involves a dissolving/dilution/flushing process where the
flux is dissolved and dispersed within the flushing-solvent through the
action of one or more surface-active agents.
But so long as electrical components and connectors are made of more
than one part, there will be minute gaps between these constituent parts,
or even between the components and the circuit-board itself. These gaps
often form traps for some the flux-bearing cleaning agent resulting in
small residuals of flux being left in place when the solvent/water evaporates.
It is not too uncommon to find connectors and/or switches where these
minute flux residues are either are waiting to migrate (the moment a vehicle
for their movement is introduced) or have migrated onto the contact surfaces.
The problem caused by this condition can be identified where the initial
treatment does not solve the problem, and the treatment must be repeated
in order to get the contact to operate reliably.
The mechanism should be obvious. If the treatment involves the minimization
of the amount of solvent, it may simply soften and mobilize some of the
residual flux, re-depositing it on the surface of the contact where it
will cause a problem once more,
Thus, some contact faults can be cured by a thorough cleaning using
an aggressive solvent or solvent/surfactant involving the massive dilution
of any flux residues. But this process may require solvents that are environmentally
Much the same mechanisms can exist with airborne contaminants such as
wood resins, or even the tar and nicotine from cigarette smoke. The condition
can be aggravated by the placement of the connectors in the path of the
airflow used to cool the electronic equipment. In addition, certain types
of closed-back-end push-button switches have developed a bad reputation
based on their "inhalation" of contaminants, because of the pumping-action
that occurs when they are released.
Can the use of Stabilants solve these problems?
Yes. Once it is recognized that a contaminant-migration condition exists
there are three alternatives that can be used.
The first involves use of substantially more of the Stabilant than
would normally be required, in order to flush the contamination from the
contact area. Besides being wasteful in those cases where the flushing
action is not needed, there are some resinous deposits which are very slow
The second alternative requires a repetition of the Stabilant treatment
some hours or days later. The initial treatment with Stabilants will
soften and lift contaminants which would not be removed except by the use
of aggressive solvents or cleaners. The additional Stabilant treatment
will normally carry this softened residue away, resulting in reliable operation
of the contacts as well as protection from further occurrence or airborne-contaminant-based
The third alternative requires a thorough pre-cleaning of the connector
with an aggressive solvent which will remove all traces of the flux or
resinous contaminant. The use of Stabilants will then provide long-term
protection against contaminants as well as the other conditions which cause
These connector-failure-modes are, fortunately, rather uncommon. Proper
record keeping can be of great benefit in identification of these anomalies.
Once a consistent pattern of failure is found involving this type of contamination,
it's usually simple to select of one of the alternatives as a dedicated