10 Common Customer Complaints About Conformal Coating and How to Prevent Them

The Complaint: "Some areas of the PCB have thick, gloopy coating, while others are so thin you can barely see it. It looks unprofessional and we're worried about protection gaps."

Why It Happens: Uneven thickness is often a marriage of equipment, technique, and board design. Manual spraying, for example, relies heavily on operator skill—holding the spray gun too close creates buildup, while moving too fast leaves thin spots. Automated systems aren't immune either: clogged nozzles, inconsistent pressure, or outdated programming can cause uneven coverage. Complex PCB layouts with tall components or tight spaces also play a role; shadowing from capacitors or connectors can block coating, leaving adjacent areas overexposed.

The Complaint: "Tiny bubbles are forming in the coating, and some have pinholes that go all the way through. We can't risk moisture seeping in during use."

Why It Happens: Bubbles and pinholes are usually linked to moisture or trapped air. If the PCB isn't properly dried before coating, moisture trapped under the surface can evaporate during curing, creating bubbles. Fast-drying solvents in the coating can also cause problems—if the solvent evaporates too quickly, it leaves behind tiny voids (pinholes). Contamination is another culprit: dust or oil on the PCB surface prevents the coating from adhering, trapping air pockets.

The Complaint: "The coating is peeling off in sheets, especially around the edges of components. It's like it's not even sticking to the PCB!"

Why It Happens: Peeling coating is a adhesion failure, and the root cause often traces back to surface preparation or coating compatibility. Oils from fingerprints, flux residues, or even mold release agents from PCB manufacturing can create a barrier between the coating and the board. Using the wrong coating type for the PCB's substrate (e.g., acrylic on polyimide) is another common mistake—different materials have different bonding properties. Finally, under-curing or over-curing the coating can weaken adhesion: under-cured coating remains tacky and never fully bonds, while over-cured coating becomes brittle and prone to cracking.

The Complaint: "We found spots under the microcontrollers and along the PCB edges with no coating at all. How did that happen?"

Why It Happens: Missed areas, or "shadowing," occur when components block the coating from reaching the PCB surface. Tall components like transformers or connectors cast shadows, while tight spaces between ICs and resistors trap air, preventing coating from flowing in. Manual application is particularly vulnerable—operators may overlook small areas or struggle to reach under components. Even automated systems can miss spots if the programming doesn't account for the board's 3D geometry.

Another culprit is improper masking. If tape or plugs covering connectors aren't applied correctly, coating can seep under them, but more often, over-masking leaves adjacent areas unprotected. For example, masking a USB port too generously might block coating from the surrounding traces.

The Complaint: "After a few months in the field, the coating turned yellow. It doesn't affect performance yet, but customers are complaining about the appearance."

Why It Happens: Yellowing is often a sign of coating degradation, triggered by environmental factors or material quality. UV exposure is a common culprit—standard acrylic coatings, for example, are prone to yellowing when exposed to sunlight or fluorescent lighting over time. High temperatures can also break down coating polymers, leading to discoloration. Low-quality coatings with impurities or unstable pigments are more likely to yellow, even in controlled environments.

Discoloration isn't just cosmetic. In severe cases, it can indicate chemical breakdown, reducing the coating's ability to protect the PCB from moisture or corrosion.

The Complaint: "We need to ｒｅｐｌａｃｅ a faulty resistor, but the coating is so tough we can't remove it without damaging the surrounding components. Rework is taking twice as long as it should."

Why It Happens: Rework challenges stem from coating type and application thickness. Epoxy or polyurethane coatings, known for their durability, are also the hardest to remove—they require harsh solvents or mechanical scraping, which risks damaging traces or components. Over-applying coating exacerbates the problem; thick layers take longer to strip and increase the chance of residue buildup. Even peelable coatings, designed for easy removal, can fail if they're under-cured or applied unevenly.

Another issue is a lack of rework procedures. Without clear guidelines, technicians may use excessive heat or incorrect solvents, worsening the problem.

The Complaint: "There are tiny particles trapped in the coating—dust, hairs, even what looks like metal shavings. It's unacceptably unprofessional."

Why It Happens: Contamination is a product of poor environmental control and equipment maintenance. Dust in the coating booth, for example, can land on wet coating and become trapped as it cures. Dirty spray nozzles or hoses may shed debris into the coating material. Even something as simple as an open coating container can collect airborne particles. In some cases, contamination comes from the PCB itself—loose solder balls, fiberglass from the PCB substrate, or leftover flux residue can mix with the coating.

Contamination isn't just unsightly. Particles can create weak spots in the coating, or if conductive (like metal shavings), they may cause short circuits.

The Complaint: "The coating feels sticky weeks after application, or it's so brittle it cracks when we bend the PCB. What did we do wrong?"

Why It Happens: Curing is a delicate balance—too little heat or time, and the coating remains tacky (under-cured); too much, and it becomes brittle (over-cured). Oven temperature inconsistencies are a major cause: hot spots can over-cure some areas, while cool spots leave others under-cured. For UV-cured coatings, inadequate light intensity or exposure time can leave the coating soft. Humidity also plays a role—high moisture in the air can interfere with chemical curing reactions, leading to incomplete hardening.

Under-cured coating attracts dust and is prone to damage, while over-cured coating loses flexibility, cracking under thermal expansion or vibration.

The Complaint: "Our PCBs passed functional tests before coating, but after coating, some sensors are giving erratic readings. The coating must be causing interference."

Why It Happens: Conformal coating is designed to be electrically insulating, but in rare cases, it can affect performance. Thick coating on high-frequency traces (e.g., RF antennas) can change impedance, leading to signal loss. Conductive particles in contaminated coating may create tiny pathways between traces, causing crosstalk or short circuits. Even air bubbles can act as dielectric anomalies, altering capacitance in sensitive circuits.

Another issue is coating on gold fingers or contact points. If masking fails, coating can insulate these areas, preventing proper connections with mating components.

The Complaint: "We're spending 20% of our coating budget on rework—stripping bad coating, reapplying, and retesting. It's eating into our profits."

Why It Happens: Rework costs balloon when issues are caught late in the process. For example, discovering bubbles after curing means stripping the coating, cleaning the board, and reapplying—a time-consuming process that risks damaging components. Scrap occurs when rework isn't feasible (e.g., over-cured coating that cracks during removal). Root causes include inconsistent processes, untrained operators, outdated equipment, and lack of in-process inspection. Without real-time checks, a small mistake (e.g., a clogged nozzle) can ruin an entire batch before it's caught.