Conformal Coating Bubbles and Peeling: Common Causes and How to Fix Them

Once a board has been coated, the two problems that give process engineers the most trouble are bubbles and peeling. Bubbles may look cosmetic, but if one bridges two solder joints or traces, moisture and contaminants have a path in; IPC-A-610 treats bubbles or voids that bridge adjacent conductors as a defect for all classes. Peeling is even more direct: once the coating separates from the board, that area is no longer protected.

The good news is that most of these defects are not caused by the material itself. Usually one link in the chain — film thickness, curing, cleaning or application technique — is off. This article covers the common causes, what to check on the line first, and when it makes sense to switch materials.

First, identify the defect

Defects that all look like “bad coating” can have completely different causes. Matching the symptom first makes the root cause much faster to find:

Symptom	Usually found	Most likely cause
Small bubbles or pinholes in the coating	Thicker areas, around components	Trapped solvent, curing too fast, air introduced while mixing
Coating beads up and exposes the substrate (dewetting, fisheyes)	Where there is flux residue or fingerprints	Surface contamination prevents wetting
Coating lifts in sheets and can be peeled off	Board edges, large copper areas, certain component surfaces	Poor adhesion: insufficient cleaning, incomplete cure, thermal stress
Coating cracks, then delaminates	After thermal cycling	Film too thick or material too hard to absorb expansion and contraction

Tip: Inspecting under UV-A light after coating is the fastest way to see bubbles, missed areas and the extent of any peeling. 419D, 4223F, 422B and the SL 1307 FLZ series all contain a fluorescent tracer, so coated areas glow and defects stand out immediately.

Common causes of bubbles and how to fix them

1. The coat is too thick and traps solvent
This is the most common cause. Solvent-based conformal coatings form a film as the solvent evaporates. If a coat is applied too thick, the surface dries and skins over first, the solvent underneath cannot escape, and it bubbles out during curing.

Fix: apply several thin coats and allow enough flash-off time between them. IPC recommends dry-film thicknesses of 30–130 µm for acrylic, polyurethane and epoxy, and 50–210 µm for silicone. To reach the upper end of the range, two or three thin coats are far more consistent than one thick coat.

2. Curing too fast or ramping too quickly
Sending the board straight into a hot oven after coating makes the surface skin over instantly, again trapping solvent and air.

Fix: let the coating flash off at room temperature first so most of the solvent evaporates, then move it to the oven, and ramp the oven temperature in stages rather than going straight to the maximum. Follow each product’s technical data sheet (TDS) for flash-off times and cure conditions.

3. Wrong viscosity, or air introduced during mixing or thinning
If viscosity is too high, the coating does not level well and bubbles cannot rise out. Stirring too vigorously or using the coating right after thinning drives air straight into the film.

Fix: adjust viscosity with the thinner and ratio recommended by the manufacturer, and let the coating stand after mixing so bubbles can rise out before use. Products with freely adjustable viscosity, such as SL 1307 FLZ and SL 1400 ECO-FLZ, are easier to match to spraying, dipping and other processes.

4. Air trapped under components and in holes
Under BGAs and connectors, in vias and in the gaps between components and the board, coating flowing in pushes air out, which then forms bubbles at the surface.

Fix: when dip coating, slow the immersion and withdrawal speed and adjust the board angle so air has time to escape; mask connectors and holes that should not be coated.

5. Moisture in the board
If boards are not fully dried after cleaning, or sit too long in a humid environment, moisture absorbed by the laminate and components is released during curing.

Fix: pre-bake boards before coating to remove moisture, and shorten the wait between drying and coating.

6. Application technique
Spraying at too high a pressure or too close to the board, or brushing back and forth repeatedly, easily works air into the coating.

Fix: adjust spray pressure and distance; when brushing, stroke in one direction and avoid going back and forth. If bubbles keep recurring on a production line, consider a material designed for automated spraying, such as 4223F.

Common causes of peeling and poor adhesion

1. Surface contamination (most common)
Flux residue, finger oils, release agents and silicone oil all prevent the coating from wetting the board. Mild cases show up as dewetting and fisheyes; severe cases are a general loss of adhesion where the coating peels off at the slightest touch.

Fix: clean and dry boards thoroughly before coating, and have operators wear gloves. On lines using no-clean flux, the residue is not always compatible with the coating — whenever you change flux or coating, run a trial and check adhesion first.

2. Moving to the next process before the coating is fully cured
Dry to the touch is not the same as fully cured. Assembling, packing or handling too early can scratch, dent or even tear the coating.

Fix: go by the full-cure conditions on the TDS, not the tack-free time. For lines short on capacity, choose a fast-curing product: 419D can move to the next process after 10 minutes, and 4223F fully cures in 2 hours at 100°C.

3. Thermal stress: film too thick or material too hard
During thermal cycling, the board, components and coating expand and contract at different rates. If the film is too thick or the material too hard, stress concentrates at component leads and corners; fine cracks appear first, and the coating then delaminates from the cracks.

Fix: keep film thickness within the recommended range, and if the product goes through large temperature swings, choose a more flexible chemistry. For how the four main chemistries compare on flexibility, see How to Choose a Conformal Coating.

4. Surfaces that are hard to adhere to
Some solder masks, plastic parts and surface finishes have low surface energy, so the coating does not adhere well.

Fix: whenever you change laminate or solder mask, or introduce a new component, run an adhesion test before going into mass production.

How do you confirm the fix worked?

After adjusting the process, confirm the result with the following methods rather than relying on visual inspection alone:

• UV light inspection: confirm full coverage with no missed areas or new bubbles.
• Film thickness measurement: confirm every area is within the recommended range, especially around components and at board edges.
• Adhesion test: the cross-hatch test (see ASTM D3359) is common — cut a grid into the coating with a blade, pull it with tape and check how much coating comes off.
• Environmental testing: for automotive or outdoor products, rerun the customer-specified damp heat, thermal cycling or salt spray tests after the fix. For tests commonly required in automotive, see Automotive Conformal Coating and Salt Spray Testing.

When should you switch materials instead of tweaking the process?

If film thickness, cure conditions and cleaning have all been checked and the defect keeps coming back, the material may simply not be compatible with your process conditions. In the following cases, it is worth considering a different conformal coating:

• Limited oven temperature or line time: choose a product that dries quickly at room temperature or cures at low temperature.
• The product needs rework: choose an easily removed acrylic, such as 419D or SL 1307 FLZ.
• Chemicals or corrosive gases in the operating environment: choose a polyurethane with better chemical resistance, such as SL 1400 ECO-FLZ or 4223F.
• The product goes through severe thermal cycling: choose a more flexible chemistry and recheck film thickness.

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FAQ

Q: Are a few small bubbles in conformal coating acceptable?
It depends on where they are and on your customer’s acceptance criteria. Under IPC-A-610, bubbles or voids that bridge adjacent conductors or exposed conductive surfaces are not acceptable; small bubbles that do not bridge conductors are judged by product class and customer requirements. Confirm the acceptance criteria with your customer first.

Q: Can boards that have already bubbled or peeled be repaired locally?
In most cases, yes: remove the coating completely from the defective area, clean it, then recoat locally. The removal method depends on the chemistry — for example, 419D and 422B can be removed with 4352 thinner/stripper, SL 1307 FLZ with the matching thinner V 1307 FLZ, and SL 1301 ECO-FLZ and SL 1400 ECO-FLZ by soldering through or mechanical removal.

Q: Same process, but bubbles increase when the season changes. Why?
The most common reason is a change in ambient humidity and temperature. In high humidity, boards absorb moisture more easily; at higher temperatures, solvent evaporates faster and the surface skins over more readily. Start by checking your pre-bake conditions and flash-off time.

Q: Can I send you photos of the boards and our process conditions?
Yes. Send us photos of the defect along with your application method, film thickness and cure conditions, and our technical team will help identify the cause and, if needed, provide samples of other products for a trial.

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