If the potting stays sticky or liquid only where it touches certain parts, such as a wire, a gasket or the board surface, a contaminant is blocking the cure. This is called cure inhibition. If the whole batch is soft, the cause is usually the mix: the wrong ratio, parts not stirred before use, or poor mixing. A cold workshop can also make the cure very slow.
First, find which problem you have
| What you see | Most likely cause |
|---|---|
| A sticky or liquid layer only where the potting touches certain parts; the rest cures normally | Cure inhibition (this guide) |
| Soft or sticky everywhere, or in streaks | Wrong ratio, parts not stirred, or poor mixing. See how to mix and cure 1:1 potting. |
| Cures everywhere, but very slowly | Low temperature. Heat cure at 80 °C. |
Why inhibition happens
Two-part 1:1 silicone potting compounds cure through an addition reaction driven by a platinum catalyst. Tiny amounts of certain chemicals bind to the catalyst and stop the reaction at the surface they touch. The potting a few millimetres away cures normally, so the problem only shows up at that surface.
Common causes on an electronics line
| Source | Where it comes from |
|---|---|
| Sulfur | Latex gloves, sulfur-cured rubber (rubber bands and some EPDM or NBR gaskets and O-rings), some tapes, modelling clay |
| Tin and other metal compounds | Residue from tin-catalysed silicones and many one-part RTVs, including on shared tools and cups; ionic compounds of tin, lead, mercury and arsenic |
| Nitrogen compounds (amines) | Epoxy hardeners, some polyurethanes and adhesives, some solder flux residues |
| Phosphorus compounds | Some additives, plasticisers and flame retardants |
| PVC | Plasticisers and stabilisers in PVC wire insulation, PVC tape and vinyl gloves |
| Other | Unsaturated compounds, such as some vinyl and alkyne compounds, and cleaning solvents such as alcohols that have not fully evaporated |
A simple patch test
Test any new material before production, and run this test whenever a cure problem appears.
- Mix a small batch, for example 10 g of Part A and 10 g of Part B.
- Put a small blob on each material you suspect, such as a piece of wire, a gasket, a scrap of flux-covered board or a glove fingertip. Also put one on clean glass or aluminium as a control.
- Cure everything with your normal schedule.
- Peel each blob back. If the control has cured but a sample is sticky or liquid underneath, that material is causing the problem.
How to fix it
- Change your gloves. Use nitrile gloves, not latex or vinyl.
- Keep tools separate. Use clean mixing cups and sticks that are never shared with RTV, epoxy or polyurethane.
- Clean the board. Wipe off flux and resin residue, and let any cleaning solvent evaporate completely before potting. Low-lead solder reduces the risk.
- Replace the problem material. For example, use non-PVC sleeving, or silicone gaskets or ones your supplier confirms are sulfur-free.
- Seal the surface. If a part cannot be changed, coat it first with a compatible barrier such as a conformal coating, and patch-test the coating itself.
- Let RTV cure fully before potting near it.
- Use heat. Curing at 80 °C can overcome mild inhibition, but it will not fix heavy contamination.
If a part has already inhibited
Remove all the uncured material at the affected surface, clean the surface, fix the cause and pot again. Pouring fresh silicone over a sticky layer does not cure it, and the new layer may inhibit too.
What does not work: adding extra Part B, waiting longer, or adding more heat when the contamination is heavy.
Common questions
Will the rest of the potting still protect the board?
Not fully. Where a layer stays uncured, the potting does not seal against that component, and moisture can travel along the gap. Re-pot parts that matter.
Is AceAdhesive potting more sensitive than other brands?
No. All platinum-cure (addition-cure) silicones are sensitive to the same kinds of contaminants. Our data sheets list them so you can avoid them from the start.
Can you help us find the cause?
Yes. Tell us which materials touch the potting, such as wires, gaskets, flux and coatings, and we will suggest what to test first. You can also request a free sample and run the patch test above on your own boards.
