Two identical wound cores, one potted and one varnished, can differ by 15 K in hot-spot temperature and by a factor of three in humidity life. The encapsulation step looks like a finishing detail; in practice it is a thermal and reliability design decision that should be made before the bobbin is tooled.
1. The four options
| Method | What it gives | What it costs | Typical use |
|---|---|---|---|
| Dip varnish | Mechanical bonding of windings, basic moisture protection | Low; thin coating, minimal thermal penalty | Open-frame inductors, chokes, low-voltage transformers |
| Vacuum impregnation | Voids filled in the winding, good corona resistance | Moderate; needs vacuum equipment | High-voltage and high-frequency transformers |
| Full encapsulation (potting) | Mechanical robustness, sealed against moisture and dust | High; adds mass, cure time, rework difficulty, thermal resistance | Automotive, outdoor, telecom, welding |
| Conformal coating | Surface protection only | Low | PCBA-level protection, not a substitute for impregnation |
2. Thermal consequence: potting is not automatically better
A potting compound with thermal conductivity around 0.5–1.5 W/m·K conducts heat better than air (0.026 W/m·K), but only if it is void-free. A potted part with air bubbles at the core-to-compound interface can run hotter than the same part in free air, because the bubbles insulate and the compound blocks convection at the same time.
Practical rules: use vacuum potting for anything above 10 W dissipation, measure the hot spot after potting rather than assuming an improvement, and remember that the compound also increases thermal mass — which slows warm-up but lengthens cool-down and can shift thermal cycling behaviour.
3. Electrical consequence: dielectric strength
Impregnation raises the partial-discharge inception voltage in the air gaps inside a winding. Unimpregnated windings in high-voltage or high-dv/dt applications can develop corona at a few hundred volts of inter-layer stress, which erodes insulation over time. Vacuum impregnation with a suitable resin is the standard mitigation for isolation transformers rated above roughly 1 kV.
4. Mechanical consequence: vibration and magnetostriction
Loose winding layers move under magnetostrictive force and produce audible noise; they also fret against insulation, which is a long-term wear failure. Varnish bonds the layers and damps the movement. For a transformer that must be quiet — medical, laboratory, audio, residential — vacuum impregnation is often more effective than a larger core.
5. Choosing the compound
- Epoxy: hardest, best mechanical and chemical resistance, highest thermal expansion mismatch with copper. Good for ruggedised and outdoor parts.
- Polyurethane: softer, better thermal shock tolerance, more flexible. Common for automotive and potted chokes.
- Silicone: widest temperature range, best flexibility, lowest hardness. Used where thermal cycling is severe and mechanical support comes from the housing.
All of them must be checked against the insulation class: a Class F winding inside a compound rated to 130 °C has effectively been derated to Class B.
6. Process control that actually matters
- Preheat the part before potting — a cold part traps moisture and creates voids.
- Degas the compound and use vacuum potting for void-free fill.
- Control the cure profile: a fast cure creates exotherm and shrinkage voids.
- Inspect by weight and by X-ray or cross-section on the first-off samples.
- Re-qualify hipot and partial discharge after potting, because the process can damage a marginal winding.
Shaanxi SiRui Technology winds and encapsulates Magnetic Core series assemblies in-house, with vacuum impregnation and vacuum potting available as standard process steps rather than subcontract operations — which keeps the thermal and dielectric results inside one qualification report.
Discuss your encapsulation requirement at sales@sirui-tech.com.