Preventing the Presence of Voids in Aerospace Potting Applications

Author : Paul Whitehead, Intertronics

05 July 2023

Figure 1: An example of the presence of voids in a potting compound
Figure 1: An example of the presence of voids in a potting compound

Trapped air or voids can cause failures in aerospace electronic assemblies. Voids can subvert the purpose of the potting or encapsulating polymer, by compromising thermal conductivity and heat dissipation.

They can also affect electrical insulation properties. Voids and delamination provide pathways for moisture or other contaminants and weaken its physical structure, allowing greater susceptibility to damage or cracking. The following article explains how aerospace manufacturers can mitigate the introduction of voids during the processing of potting compounds.

Formulators of potting materials for aerospace electronics applications are careful not to supply products with entrapped air. Consequently, entrapped air usually represents a processing issue.

Processing issues 
Correct storage is important in ensuring that compounds do not absorb moisture. Some chemistries are hygroscopic and readily absorb moisture from the air, which reacts to form a gas. They are packaged in airtight containers and/or with desiccants. Taking care in handling the materials in accordance with the manufacturer’s recommendations can prevent problems occurring later.

Non-contact methods of mixing, such as planetary centrifugal mixers or automated metering and mixing systems, typically introduce less air than other mixing methods. If using metering and mixing equipment, it is important to ensure this is regularly maintained and introducing air during replenishing should be avoided. If air inclusion from mixing proves unavoidable, then removal via degassing should be considered. 

Low viscosity materials may be able to adequately degas themselves. For best results, a container with a large surface area and shallow depth should be used. Any bubbles that rise to the top should be scooped off. For higher viscosity products, or where production time doesn’t allow for unaided degassing, vacuum can expedite the process. If the mixed working life is sufficient, vacuum chambers may be employed for degassing purposes. Using excessive vacuum should be avoided, as some constituents may be volatile and will evaporate at lower pressures, thereby altering the material’s formulation.

Centrifugal force can also be used to degas the material before application. In practice, this is often seen when using a planetary centrifugal mixer, which mixes and degasses plural component materials of multiple viscosities. The non-contact mixing action (revolution and rotation) does not add air, instead it tends to remove it. An additional ‘defoaming’ mode applies a more direct centrifugal force to the mixed materials, separating out the air. Care must be taken not to separate any fillers or heavier constituents via centrifugal action. 

For critical applications, these planetary centrifugal mixers can be configured so mixing happens under vacuum. 

Application methods
Once the compounds are mixed, it is important not to introduce or entrap air during application. Techniques to eliminate or minimise air bubbles in the completed assembly include pouring or injecting in stages, driving out air rather than trapping it under components. Tilting the assembly, filling from the bottom and perhaps preheating to reduce viscosity and surface tension can help. 

In critical applications that are geometrically challenging, potting can be done under vacuum conditions, or the potted unit can be vacuum degassed before cure. Another tactic is to place potted parts onto a vibrating table, helping to release trapped air.

Pressurised feed systems can also saturate the material with dissolved air, which can result in bubbles later on. It is good practice to depressurise systems when inactive, and to use only the required pressure. Alternatively, bubble formation can be avoided by having a barrier between the compressed air and the potting compound. 

Contamination and cure
The uncured potting compound or encapsulant can react with contaminants, potentially causing a lack of adhesion and subsequent delamination, or the formulation of gas bubbles - both of which cause voids. Contamination issues can be resolved by using cleaned and dried assemblies.

Shrinkage of the compound during cure may create voids in parts of the assembly. Again, applying the materials in stages can minimise this by allowing the materials to fill any gaps left by shrinkage. 

Conclusion
Mitigating the presence of voids in PCBs is essential if maximum aerospace assembly performance is to be attained. By carefully considering each stage of the potting process, manufacturers can prevent voids occurring, and create parts with reliability and longevity.


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