Encapsulants

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.