Cracks in the potting compound: causes and material selection
A cracked potting compound loses its protective function – moisture and other substances penetrate, and the mechanical support is compromised. Cracks almost always have one of a few well-understood causes. Those who know these causes choose the right material and avoid the problem from the outset.
To the point
Cracks arise from mechanical stresses that the material cannot withstand: temperature shock, differential thermal expansion, shrinkage during curing, or embrittlement in cold temperatures. The most effective countermeasure is an elastic material – usually silicone; where epoxy is necessary, a tough-elastic modified type with a controlled layer thickness.
The four main causes
Different thermal expansion
Potting compound, components, circuit board, and housing expand at different rates. Each temperature cycle creates stresses. A rigid material transmits these stresses directly and tears at notches or component edges; an elastic material absorbs them.
Temperature shock
Rapid temperature changes generate brief periods of high stress peaks. Brittle, highly filled materials or those operated below their glass transition temperature are then more likely to crack.
Curing shrinkage
Some systems shrink during curing. If the potting process is obstructed (undercuts, embedded rigid parts), internal stress builds up, which is later released as a crack. Silicones with very low shrinkage are advantageous here.
Embrittlement in the cold
Below the glass transition temperature, a material becomes brittle. Epoxies often have their glass transition temperatures within their operating range; silicones, on the other hand, remain elastic well below zero degrees Celsius.
| cause of crack | Countermeasure |
|---|---|
| CTE difference / temperature change | elastic silicone, low tension |
| Temperature shock | Use flexible material; avoid nicks/sharp edges |
| Curing shrinkage | low-shrinkage system, pouring in layers |
| cold embrittlement | Material with deep glass transition (silicone) |
Material selection and products
Where temperature fluctuations and mechanical stress are prevalent, an elastic silicone such as SILISIL RTV MF-Flex 20, PC-Flex 20 , or – for sensitive, tightly packed assemblies – a soft MD-Soft 10 is the crack-resistant choice. If epoxy is necessary for mechanical reasons, choose a tough-elastic type and limit the layer thickness and curing temperature (see Epoxy Resin Basics). Related failure modes are discussed in Avoiding Potting Defects, the Role of Thermal Expansion in Windings, and its Contribution to Inductive Components.
Frequently Asked Questions
Why does my epoxy potting crack in the cold? The operating temperature is probably below the glass transition temperature – the material becomes brittle. An elastic silicone remains flexible in the cold.
Silicone instead of epoxy – will I lose strength? Yes, in mechanical stiffness, but you gain resistance to cracking under temperature changes. The choice depends on the dominant requirement.
Cracks despite correct mixing? Then the cause is usually shrinkage, CTE differences or temperature shock – not the dosage.
Consultation and samples
Tell us about the structure, temperature range, and geometry – we'll recommend a crack-resistant material and provide a sample. Contact us or write to info@silitech.ch.
