Achieving Void-Free Potting: Kohesi Bond’s Tips for Eliminating Air Bubbles

Picture a high-voltage automotive inverter operating at full load. 

A single 500-µm air pocket remains hidden between two power traces, which is an insignificant defect at first glance. But under a few kilovolts of stress, the electric field crowds into the void where air’s dielectric strength (~3 kV/mm) collapses compared to epoxy’s typical 18–22 kV/mm. 

Within microseconds, partial discharge sparks, generating ozone and reactive species. The void carbonises, expands, and ultimately triggers catastrophic insulation breakdown. 

One bubble. One failure. The entire system is down.

This is what’s at stake with void-free potting.

Potting protects assemblies from moisture, vibration, ionic contamination, and mechanical fatigue. But trapped air undermines every one of these defenses. 

Voids become moisture pathways, weaken adhesion, distort thermal conduction, and in high-voltage assemblies, this creates microcavities where discharge can initiate. 

In thermal environments, voids act as insulators, raising junction temperatures and accelerating degradation. In medical devices subjected to autoclave or EtO cycles, gas expansion from trapped air initiates cracks and undermines seal integrity.

Here’s the challenge: even sub-millimetre voids compromise reliability.

At Kohesi Bond, we engineer epoxy potting compounds and application workflows designed specifically to eliminate bubble formation and ensure consistent, void-free encapsulation across electronics, aerospace, and medical systems.

A] Common Causes of Air Bubbles in Potting Compounds

1. Mixing-Induced Air Entrapment

There’s a paradox inherent to two-part epoxies: mixing initiates cure, but mixing also creates the burden of bubbles that engineers must eliminate. Manual stirring folds air into viscous matrices, whereas high-shear mixers (>1,000 RPM) create turbulent vortices that disperse air as microbubbles.

What the data consistently shows is bubble size and its likelihood of escaping, which depends strongly on mixing dynamics. While hand mixing introduces larger bubbles (0.5–3 mm) that rise quickly, high-shear dispersers form 10–100 µm microbubbles with negligible rise velocity. 

Based on Stokes’ Law:

v = 2gr²(ρ_liquid − ρ_gas) / 9η

Understanding the equation:

v   = Velocity at which the bubble rises to the surface

g   = Acceleration due to gravity

r   = Radius of the bubble

ρ   = Difference in density between the liquid epoxy and the gas

η   = Viscosity of the epoxy

Why it matters:

Viscosity (η) appears in the denominator, which means that as viscosity increases, the bubble’s rise velocity drops sharply. 

This explains why a small 50 µm microbubble in a high-viscosity, 5,000 cP resin moves so slowly that it often cannot reach the surface before the epoxy hardens.

Filled systems (40–60% loading) reach viscosities of 10,000–50,000 cP, maximising air entrapment unless countered through degassing or low-viscosity formulation design, which highlights the importance of two-component epoxy mixing.

2. Pouring and Dispensing Techniques

Fast pouring from height entrains air at the liquid-air interface. Turbulent flow through narrow nozzles creates shear-induced cavitation. Top-down pouring traps air beneath components, especially in recesses, under chip bodies, and between dense leads.

Bottom-up filling demonstrates the opposite behaviour, as resin displaces air ahead of the rising front, enabling escape instead of entrapment. 

Incorrect needle geometry or excessive dispense pressure further nucleates bubbles, while poor tip-to-substrate control allows air to re-enter the bead.

3. Equipment and Process Limitations

Uncalibrated pumping systems produce pulsation, generating negative pressure zones where air infiltrates. Static mixers with insufficient elements yield incomplete component blending and localised cure variations that release gas during crosslinking. 

Applying resin too late in the pot life, after the viscosity rise begins, traps bubbles that would otherwise escape during the early, low-viscosity phase.

4. Environmental Factors

Viscosity is highly temperature-dependent, often doubling with a 10°C decrease due to Arrhenius-type behaviour (activation energies ~40–60 kJ/mol). High viscosity slows bubble rise exponentially.

Moisture-sensitive urethanes react with atmospheric humidity to generate CO₂ bubbles. Barometric pressure shifts alter dissolved gas content; materials mixed at low pressure may release bubbles at sea level.

5. Kohesi Bond’s Insight

This is where formulation chemistry meets process engineering. 

Kohesi Bond’s epoxy solutions leverage controlled rheology: low-viscosity grades (500–2,000 cP) for minimal air incorporation and shear-thinning thixotropic systems that flow during dispensing but recover viscosity afterward, enabling bubble escape during the gel-time window.

B] Risks of Voids in Sensitive Applications

1. Electrical and Thermal Reliability Issues

Let’s quantify the electrical risk. 

Cured epoxy has εr ~3.5–4.5; air has εr ~1.0. 

Electric fields concentrate in low-permittivity regions. A void within a high-voltage insulation barrier experiences amplified field intensity:

E_void ≈ (ε_epoxy / ε_air) × E_bulk

Understanding the variables:

ε  = Permittivity (dielectric constant) of the materials

E_bulk = Overall electric field applied to the system

Why it matters:

Because epoxy has a much higher permittivity than air (approximately 4.0 versus 1.0), the electric field concentrates inside the air bubble. 

As a result, the electrical stress within the bubble is roughly four times higher than in the surrounding material. 

This is why even very small voids can initiate sparks (partial discharge) and eventually lead to system failure.

For example, a bulk field of ~2 kV/mm across epoxy results in ~8 kV/mm inside a 1 mm air void, approaching or exceeding air breakdown limits. This highlights the severe consequences of epoxy curing without bubbles.

Thermally, air’s conductivity (~0.026 W/m·K) is dramatically lower than epoxy (~0.17–0.25 W/m·K) and orders of magnitude below filled formulations (1–3 W/m·K). 

Even tiny voids can create hot spots. Given the Arrhenius rule of reliability, failure rates double for every 10°C rise, while voids directly reduce system life.

2. Mechanical and Structural Weakness

Voids act as crack initiators.

A spherical void generates stress concentration factors (Kt ≈ 2) even under mild mechanical load. Under thermal cycling, CTE mismatch amplifies local strain. Voids also act as structural weak points where cracks can initiate. 

In fracture mechanics, the stress intensity factor (K) is calculated using:

K = σ × √(π × a)

Understanding the variables:

σ   = Mechanical stress applied to the part

a   = Size of the void or crack

Why it matters:

As the size of the void (a) increases, the stress concentration at its edges rises. 

When this stress intensity exceeds the material’s fracture toughness (K_IC), the epoxy will crack. 

This is why eliminating even sub-millimetre voids is critical for components exposed to vibration or thermal shock.

Industry Examples

• Automotive ECUs: Voids accelerate corrosion and dielectric drift during −40°C to +125°C cycling. Moisture diffuses through voids, leading to leakage currents and premature field failures.
• Medical implants: Repeated sterilisation (autoclave, gamma, EtO) expands gas within voids, initiating microcracks that compromise sealing and sensor accuracy.
• Aerospace electronics: Low pressure at altitude causes dissolved gases to outgas and voids to expand. In a vacuum, gas escape can contaminate optics or destabilise attitude control systems.
  
  

3. Kohesi Bond’s Reliability Focus

Our epoxy systems inherit aerospace-grade reliability assumptions: failure is unacceptable. 

We validate long-term performance through accelerated ageing, thermal shock, vibration testing, and high-voltage endurance to ensure void-free potting integrity under extreme conditions.