Epoxy Potting & Casting
Potting and casting compounds are poured around live components, so the colour paste must be genuinely solvent-free and electrically clean. Any solvent left in a thick casting has nowhere to go: it plasticises the cured resin, forms voids and can attack the component it was meant to protect, which is why this application is the least forgiving of carrier choices. Colour also has to serve inspection, because a cast block that is too dark hides voids and misaligned parts that would otherwise be rejected before shipment.
Key Requirements in This Application
Genuine solvent-free formulation
A casting may be several centimetres thick, and solvent cannot escape from the interior. The paste must contribute no volatile solvent, so that the cured block is free of voids, shrinkage cavities and soft centres.
Low ionic and extractable content
Encapsulated circuits are sensitive to ionic contamination and to free amines or acids that corrode metallisation. The paste must be formulated so that it does not raise conductivity or leave corrosive residues at the interface.
No interference with cure
Pigments and carriers can retard or accelerate the epoxy-amine reaction, or consume part of the hardener. The stoichiometry has to remain valid after the paste is added, otherwise the casting stays under-cured and soft.
Viscosity matched to the filling method
Gravity casting, vacuum casting and pressure pot filling all need a different flow. A paste that thickens the mix beyond the working window will leave air pockets around fine leads and connectors.
Thermal cycling and colour consistency
Encapsulated parts see repeated thermal excursions, and identification colours must remain stable. The paste must not bleed, migrate or change shade after cure and ageing.
Filler compatibility
Many encapsulants contain silica or alumina to control expansion and heat transfer. The colour paste has to disperse within the filled system without disturbing filler packing, settling ahead of the filler, or raising viscosity beyond the point where the mould can still be filled around fine geometry.
Inspection contrast
Automatic optical inspection and manual visual checks rely on contrast between the encapsulant and the components beneath. The colour must be dark enough to look consistent but not so opaque that voids, delamination or misplaced parts become invisible after casting.
Colour stability under thermal load
Encapsulated power components dissipate heat, so the cured resin runs warmer than ambient for much of its life. The colour must hold under that continuous thermal load rather than drifting toward yellow or brown, and the paste must not contain a component that oxidises or migrates at the service temperature and discolours the block after months in the field. Because the casting is opaque, a shade change cannot be corrected once cured, so stability has to be established on aged samples before the formulation is released.
Recommended DENSON Series
Solvent-free epoxy colour pastes designed for electronic-grade epoxy systems, so they add colour without introducing the solvent that would be trapped in a thick casting. Because the carrier is itself epoxy-compatible, it becomes part of the cured matrix rather than an additive that has to be tolerated.
View seriesTechnical Considerations
- Recalculate the mix ratio when paste is added to a two-pack system, because the paste becomes part of the resin side and shifts stoichiometry.
- Degas after mixing and before casting; the paste raises viscosity and air is harder to release once the fill has started.
- Keep pigment loading as low as the colour allows, since every percent of paste displaces resin and changes both viscosity and cured hardness.
- Control exotherm in thick sections by casting in stages or limiting batch size rather than by adding solvent to thin the mix.
- Confirm that fillers such as silica and the colour paste do not compete for hardener or settle at different rates in the pot.
- Check flow around fine leads at the lowest casting temperature used in production, because viscosity rises sharply as the resin cools toward the end of pot life.
Testing and Compliance
Electrical cleanliness
Ionic content and extract conductivity are checked on the cured resin to confirm that the paste does not introduce conductive species into the encapsulation.
Cure and mechanical integrity
Hardness development, glass transition behaviour and the absence of voids are assessed on cast blocks, and thermal cycling is used to expose cracking or delamination at the component interface.
Dispersion
Fineness by GB/T 6753.1, viscosity by GB/T 9751 and particle size distribution to ISO 13320 are used to keep the paste uniform, because agglomerates in a casting act as stress raisers.
Thermal cycling with continuity
Assemblies are cycled between low and high temperature limits while electrical continuity is monitored, which exposes opens caused by differential expansion between the encapsulant, the paste and the component body.
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