Inhibition from the substrate or module
Some materials used in the construction of the substrate or module housing
can potentially cause inhibition. In most cases it isn’t the module that
causes the problem but rather residual mold release, cutting oil, etc. If
the encapsulant or gel being used is fairly viscous, the contaminants causing
the inhibition may not diffuse very far into the material and the effects on
the cure may be spotty, uneven, or only show up at the interface.
Inhibition from the dispensing equipment
Some materials used in the construction of dispense equipment can
potentially leach contaminants into the encapsulant and cause inhibition.
Most often seen are sulfur compounds used in the vulcanization of rubber
tubing, hoses and o-rings. Leaching is a gradual process that may
take a very mild form that is often unnoticed while running. During
shutdowns, when material may stay in contact with the hose or seal, the
inhibition on restart is often more severe, and as a result, the encapsulant or
gel does not cure. If the uncured encapsulant or gel is fairly viscous,
the contaminants causing the inhibition may not diffuse or mix very far into
the material and the effects on the cure may be spotty or uneven.
These same considerations apply to other tools used in the processing of the
materials. Equipment such as stirrers, funnels and mixing containers will
sometimes pick up and retain residual catalysts. They should be scrupulously
cleaned before being used with addition cure materials.
Inhibition from individual components in module
Unlike other causes of inhibition, this is often restricted to an isolated
component or area of the module. This problem is usually seen as uncured
or poorly cured encapsulant in locations where it contacts a specific
component. Generally, the bulk of the encapsulant is cured properly. Cleaning
prior to applying the encapsulant may help, but often the inhibiting material
is being leached from the component during cure.
Inhibition from the surrounding environment
When ovens have been used to cure other types of materials, harmful vapors
can sometimes remain which poison the cure of the encapsulant or gel.
Some materials used in the construction of ovens or other equipment in the area
where the encapsulant is curing can potentially transfer contaminants into the
encapsulant or gel and cause inhibition. Most common are sulfur compounds
used in the vulcanization of rubber tubing and hoses or vapors from other
processes such as curing of epoxies. Contaminant transfer is a gradual
process that may take a very mild form that is often unnoticed while running.
During startup or shutdowns, when air circulation is lower, the material
is exposed to the contaminant longer and the inhibition on restart is often
more severe, sometimes to the extent that the encapsulant or gel does not cure
or has a wet surface.
Excess or unconverted flux on the board or module
Flux that has not been heated sufficiently to convert into its inert form
may contain cure-inhibiting elements. This is most often seen at the
surface of a circuit board as an uncured wet layer of encapsulant or gel, or in
extreme cases, as entirely uncured material. In mild cases, the
encapsulant or gel may appear to be cured, but may not have proper adhesion or
may even peel off. The poor cure at the surface will prevent proper
adhesion from developing.
Even if there is no inhibition, some solder fluxes leave powdery residues,
particularly when an excess of flux is used. Anything that interferes
with the encapsulant or gels ability to contact the surfaces of the module or
components can interfere with its ability to gain adhesion. Remember that
some of the encapsulants will not gain adhesion to most common surfaces without
the use of primers and that this may be the cause of any lack of adhesion that
you are observe.