PCB Components That Must Not Be Coated with Conformal Coating

Conformal coating is a common form of PCB protection that is used to significantly improve the PCB's resistance to moisture, salt spray, chemical corrosion and dust. It is a protective insulating film that protects common electronic components and circuit structures from harsh working conditions. But this protective insulation is harmful when used in certain functional parts. One of the typical mistakes in PCB manufacturing is considering that the full board coating would provide better protection. In reality, for coatings that do not need to be targeted, the lack of masking may result in product failures which are not understood, even after the conformal coating has been completed. This article explains some of the components that must be avoided for coating and why this is a recommended practice in the industry.

Connectors

Connectors have the highest priority among the parts that should not be coated with conformal coatings because they are the parts that actually depend on the physical contact between them and must provide a low-resistance connection. In normal circumstances, the resistance of the connector contact points is only a few tens of milliohms, so that the circuit can be conducted well. After conformal coating gets into the contact area, pin holes, mating area, etc., a thin insulating layer is created and thereby the contact resistance is artificially increased. Partial coating residue may be removed during the first plug-and-pull but this is not reliable. Residual coating will result in oxidation, surface contamination, and contact instability under repeated temperature and humidity changes over the course of time, during mechanical vibration. This is particularly important for automotive electronics and industrial control equipment which have continuous vibrations. All connector pins, jacks, contact areas and mating surfaces need to be well masked, and not just based on a temporary electrical connectivity.

Relays and Switches

Another example of non-coating elements that are essential because of their precision mechanical moving part is relays and all other types of switches. A relay is composed of internal contact points, armatures, springs, and transmission moving components. Conformal coating that has been infiltrated will cause a change in the mechanical motion properties of these components. Most of all, contaminated contact points exhibit reduced contact resistance, poor arc suppression and poor operational reliability. The designed creepage distance and the insulation clearance are also broken and it will make the circuit unsafe if the high voltage relay is improperly coated. The best practice is to coat the normal area of the PCB around the relay and leave all the moving and contact part of a relay completely uncoated, so as to maintain its switching accuracy and service life.

Buttons and Potentiometers

Mechanical parts, such as tactile buttons, potentiometers, and DIP switches that are adjusted by hand are very susceptible to contamination from conformal coating. Potentiometers rely on a sliding contact to make a smooth adjustment in resistance and to produce an analog output. When the seep is coated into the internal sliding structure, the surface of the seep is contaminated by an invisible insulating layer. This results in intermittent and onerous component burn-outs like resistance jumps, poor contact, increased circuit noise, drifting ADC readings in analog circuits or other unobvious faults. These frequency of failure, non-permanent failures are hard to detect and debug, hence it is crucial that strict masking is done for adjustable mechanical components.

Heat Sinks and Power Devices

Full coating is not feasible with power devices and heat dissipating components as thermal resistance would be an issue. MOSFETs, LDOs, DC-DC chips, power inductors, and rectifier diodes continuously dissipate heat during operation with exposed copper foils, thermal pads and thermal vias for heat dissipation. Conformal coating is very low in thermal conductivity, when compared with metal and copper, which also means that the overall thermal resistance is higher. For example, if a power device has a power loss of 2W, with 30℃/W thermal resistance, the temperature rise will be 60℃. When coated, thermal resistance is raised to 40℃/W, temperature rise will rise to 80℃ which is a 20℃ sharp lift of junction temperature. Overheating for longer periods can cause a component to become aged and circuit instability. So there is no blind full covering coating for all kinds of heat-dissipation structures of power devices.

Optoelectronic Components

Conformal coating is unsuitable for optoelectronic components because of the critical requirements on optical performance. The LEDs, photoelectric sensors, infrared receivers, photosensitive elements and camera modules are calibrated from fixed light transmittance and reflectance parameters. The refractive index and the light transmission state of the optical window surface will be changed by the conformal coating, and this will cause the original optical path to change. This causes the output signals to vary between different sets of sensors, the brightness of the LEDs and the overall accuracy of the detection. Optoelectronic components have to be coated according to the official coating specifications of component manufacturers, not with default coating operations as in the case of electronic parts.

Scientific Coating Partition Principles for PCBs

In essence, a well-designed conformal coating is a design that provides protection in areas where it is needed, not everywhere. PCB areas are further classified into three categories: Mandatory coating areas such as exposed copper foils, solder joints and conventional resistors and capacitors; Forbidden coating areas including connector contacts, mechanical moving parts, test points, and optical windows which are interpreted according to the datasheet of the component; Conditional coating areas such as power devices, relays and crystal oscillators, which are interpreted using the datasheet of the component. The key to excellent PCB protection design is to clarify the boundaries of the coating when designing a PCB, and to comprehensively consider the protective performance and functional reliability of the components.