Conformal coating in electronics – application methods

Introduction

Conformal coating is a widely used method of protecting electronic circuits against moisture, contamination, and corrosion. It also reduces the risk of shorts caused by tin whiskers, and with the right material selection it improves vibration resistance as well.

The functions and types of conformal coatings were covered in the previous article Conformal coating in electronics - what you need to know. In this article I present the application methods.

Enjoy the read :)

Before you apply the coating

Before applying a conformal coating, the following aspects need to be addressed first:

• Adhesion to the substrate. Ensuring good adhesion of the coating to the substrate (to the PCB and the components) is a key aspect. Flux residues, salts, and other surface contamination reduce that adhesion, so minimizing contamination is very important. Sometimes dewetting of the coating occurs because the "surface free energy" (SFE) of the solder mask is too low.
• Ionic cleanliness. Some residues can lead to electrochemical migration (ECM) under the coating, commonly referred to as "dendrites" or "corrosion". This is why the high cleanliness level and a very low level of absorbed moisture shall be ensured. Ideally, the boards are cleaned and then dried before the coating is applied - although such a comprehensive approach is typically used in high-reliability applications.
• Masking. Keep-out areas such as connectors, test points, adjustment components, and so on should be clearly defined in the assembly documentation. Ideally, masking would not be necessary at all, but that depends on the board design and the application method. Masking is therefore a frequently used operation, and it carries various quality risks: missing or incorrectly positioned masking, the effect of residues on ionic cleanliness, contamination left behind after mask removal, mechanical damage to the PCB and components during mask removal, and so on.
• Ambient conditions during coating. Temperature and relative humidity in the coating and curing area are a very important aspect. Monitoring and controlling these parameters is therefore an essential element of a professional coating process.

Conformal coating application methods

A conformal coating can be applied to an electronic assembly using a variety of methods, from the simplest manual ones through to advanced automated systems that dispense the coating in a tightly defined manner. Below is a list of typical application methods:

Brushing (manual)

This is the simplest and cheapest method, suitable for rework, prototypes, and very short runs. The advantage is the low cost of tooling; the drawbacks are that it is labor-intensive, with large variation in the applied layer thickness, streaks, bubbles, and sometimes contamination from the brush... and so on.

Spraying (manual)

A faster method than brushing, but still dependent on operator work. The advantage is the low cost of tooling; the drawbacks are: a non-repeatable process, wide thickness variation, shadowing behind tall components (you have to spray at an angle and in several passes), and operator exposure to coating vapors (occupational health and safety). In addition, the spray method wastes a large share of the material, which never lands on the board.

Dipping

The board is immersed in and withdrawn from a tank of coating material. The immersion and withdrawal speeds should be tightly controlled (automatically). Advantages: excellent coverage of both sides of the PCB, no gaps, and high repeatability as long as the material viscosity is stable. Typical problems are: tight viscosity control of the material is mandatory, because solvent evaporation changes the coating thickness "from batch to batch". Another difficulty is the need for leak-tight masking of areas that shall not be coated during the immersion, as well as uneven coating run-down at the PCB edge.

Selective

Applying the coating selectively, that is, to defined areas of the PCB. This is achieved with an automated system fitted with an appropriate material dispensing valve. Various technical solutions are used, particularly in terms of valves and nozzles. The advantages are: high application accuracy, minimal material waste, and very good throughput. The drawback is the higher up-front investment in a good-quality machine. A very important aspect is matching the valve/nozzle correctly to the material and to the PCB requirements. Typical solutions:

• Micro spray (also called ultra-fine spray, precision low-pressure spray valve, select spray). The valve atomizes the coating and sprays it onto the board. Low pressure and a low material flow rate are used.
• Film curtain (film coating, film valve). The material leaves the nozzle as a thin curtain (wall) of coating. Nozzle selection determines the optimum curtain width and coating thickness. In addition, closed-loop control is now well developed, using a laser to measure the curtain width and adjusting the coating pressure/flow. This keeps the curtain width constant, which means greater accuracy in the position and thickness of the applied coating.
• Needle (needle dispense, needle valve). A method intended for difficult locations - essentially "pouring the coating" wherever high accuracy, a small quantity of material, hard-to-reach areas, dispensing of high-viscosity coating, masking, gel dispensing, and so on are required.
• Swirl (swirl valve/mode). The material is dispensed by means of a rotating swirl of material. This solution is more accurate than needle dispensing, cuts off the coating cleanly, and does not cause splashing.
• Jetting (jetting valve). The material is ejected - "shot" - from the nozzle by a fast mechanical/piezoelectric mechanism. This generates a series of "micro-doses" of material that leave the nozzle in a controlled manner, without atomization (no spray mist). With this solution the nozzle can sit higher above the board, and there is little risk of touching the PCB or the components. This method allows very accurate delivery of the coating into hard-to-reach areas while at the same time avoiding areas that must remain free of coating.

Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) is the process used to apply parylene. It consists of three main stages:

• Sublimation. The raw material (di-para-xylylene dimer) in powder form is placed in a low-pressure chamber and heated to a temperature that allows the material to pass from the solid to the gaseous state (sublimation). The temperature range for this process is 70-200°C, typically 150-175°C.
• Pyrolysis. The resulting gas is heated further, to approximately 650-700°C, which breaks the dimers down into smaller molecules - para-xylylene monomers.
• Polymerization, deposition. The monomer (in gaseous form) is introduced into a vacuum chamber holding the electronic assemblies. The temperature in this chamber is in the typical room-temperature range. Polymerization takes place in this chamber, forming poly(para-xylylene). The material deposits evenly over every accessible surface of the board, on component leads, on vertical walls, and so on. The polymer deposition rate is typically 1-5µm per hour, so a 25µm layer, for example, requires anywhere from several to well over ten hours of process time.

Summary

Conformal coatings make it possible to increase the reliability of electronic assemblies. Success depends on the right material selection, surface preparation (cleanliness, working conditions), and the use of an optimal application method.

For high-quality production, the focus should be on reliable, precise coating application systems. Particular attention should be paid to a well-matched valve type and to appropriate process control methods.