Conformal coating

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Conformal coating material is a thin polymeric film which conforms to the contours of a printed circuit board to protect the board's components. Typically applied at 25-250 μm [1] (micrometers) thickness, it is applied to electronic circuitry to protect against moisture, dust, chemicals, and temperature extremes.

Printed circuit board Board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

Contents

Coatings can be applied in a number of ways, including brushing, spraying, dispensing and dip coating. Furthermore, a number of materials can be used as a conformal coating, such as acrylics, silicones, urethanes and parlyene. Each has their own characteristics, making them preferred for certain environments and manufacturing scenarios. Most circuit board assembly firms coat assemblies with a layer of transparent conformal coating, which is lighter and easier to inspect than potting. [2]

Potting (electronics)

In electronics, potting is a process of filling a complete electronic assembly with a solid or gelatinous compound for resistance to shock and vibration, and for exclusion of moisture and corrosive agents. Thermosetting plastics or silicone rubber gels are often used, though epoxy resins are also very common. Many sites recommend using a potting product to protect sensitive electronic components from impact, vibration, and loose wires.

Reasons for use

Conformal coatings are used to protect electronic components from the environmental factors they are exposed to. Examples of these factors include moisture, dust, salt, chemicals, temperature changes and mechanical abrasion. Successful conformal coating will prevent the board from corroding. [1] More recently, conformal coatings are being used to reduce the formation of whiskers, [3] and can also prevent current bleed between closely positioned components.

Whisker (metallurgy) Phenomenon in electrical devices

Metal whiskering is a phenomenon which occurs in electrical devices when metals form long whisker-like projections over time. Tin whiskers were noticed and documented in the vacuum tube era of electronics early in the 20th century in equipment that used pure, or almost pure, tin solder in their production. It was noticed that small metal hairs or tendrils grew between metal solder pads causing short circuits. Metal whiskers form in the presence of compressive stress. Zinc, cadmium, and even lead whiskers have been documented. Many techniques are used to mitigate the problem including changes to the annealing process, addition of elements like copper and nickel, and the inclusion of conformal coatings. Traditionally, lead was added to slow down whisker growth in tin-based solders.

Conformal coatings are breathable, allowing trapped moisture in electronic boards to escape while maintaining protection from contamination. These coatings are not sealants, and prolonged exposure to vapors will cause transmission and degradation to occur. There are typically four classes of conformal coatings: Acrylic, Urethane, Silicone, and Varnish. While each has its own specific physical and chemical properties each are able to perform the following functions:

Applications

Precision analog circuitry may suffer degraded accuracy if insulating surfaces become contaminated with ionic substances such as fingerprint residues, which can become weakly conductive in the presence of moisture. (The classic symptom of micro-contamination on an analog circuit board is sudden changes in performance at high humidity, for example when a technician breathes on it). A suitably chosen material coating can reduce the effects of mechanical stress and vibrations on the circuit and its ability to perform in extreme temperatures.

Analogue electronics are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represents the signal. The word analogue is derived from the Greek word ανάλογος (analogos) meaning "proportional".

Ionic compound chemical compound involving ionic bonding

In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as the sodium (Na+) and chloride (Cl) in sodium chloride, or polyatomic species such as the ammonium (NH+
4
) and carbonate (CO2−
3
) ions in ammonium carbonate. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in a crystalline structure.

For example, in a chip-on-board assembly process, a silicon die is mounted on the board with an adhesive or a soldering process, then electrically connected by wire bonding, typically with .001-inch-diameter gold or aluminum wire. The chip and the wire are delicate, so they are encapsulated in a version of conformal coating called "glob top." This prevents accidental contact from damaging the wires or the chip. Another use of conformal coating [5] is to increase the voltage rating of a dense circuit assembly. An insulating coating can withstand a much stronger electric field than air, particularly at high altitude.

Die (integrated circuit) an unpackaged integrated circuit

A die, in the context of integrated circuits, is a small block of semiconducting material on which a given functional circuit is fabricated. Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon (EGS) or other semiconductor through processes such as photolithography. The wafer is cut (diced) into many pieces, each containing one copy of the circuit. Each of these pieces is called a die.

Soldering

Soldering is a process in which two or more items are joined together by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering. In the past, nearly all solders contained lead, but environmental and health concerns have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.

Wire bonding

Wire bonding is the method of making interconnections (ATJ) between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one printed circuit board (PCB) to another. Wire bonding is generally considered the most cost-effective and flexible interconnect technology and is used to assemble the vast majority of semiconductor packages. Wire bonding can be used at frequencies above 100 GHz.

With the exception of parylene, most organic coatings are readily penetrated by water molecules. A coating preserves the performance of electronics primarily by preventing ionizable contaminants such as salts from reaching circuit nodes, and combining there with water to form a microscopically thin electrolyte film. For this reason, coating is far more effective if all surface contamination is removed first, using a highly repeatable industrial process such as vapor degreasing or semi-aqueous washing. Extreme cleanliness also improves adhesion. Pinholes defeat the purpose of the coating, because a contaminant film would make contact with circuit nodes and form undesired conductive paths.

Coating methods

The coating material can be applied by various methods, including brushing, spraying, dipping or selectively coating by robots. Different methods of curing and drying are available depending on the conformal coating material. Nearly all modern conformal coatings contain a fluorescent dye to aid in coating coverage inspection. [6]

Brush coating

This works by flow coating the material onto the board and is suitable for low volume application, finishing and repair. The finish tends to be cosmetically inferior and can be subject to many defects such as bubbles. [7] The coating also tends to be thicker unless skilled operators apply the coating. [8]

Spray application coating

Conformal Coating Spray booth Spray booth designed for application of conformal coatings, lacquers and RFI shielding paints.png
Conformal Coating Spray booth

This coating can be completed with a spray aerosol or dedicated spray booth with spray gun and is suitable for low and medium volume processing. [9] The quality of the surface finish can be superior to all other methods when a skilled operator completes the process, provided that the circuit board is clean and the coating has no adhesion problems. The coating application may be limited due to 3D effects. Masking requirements are more of a shield nature rather than a barrier, since there is less penetration. The lack of penetration can be a problem where the coating is desired to penetrate beneath devices.

Spray application can be one of the most cost-effective ways of applying conformal coating, as it can be done on the bench top for small rework and repair jobs. This method can be done in spray booths for medium scale production. [8]

One of the key attributes of atomised spraying is giving excellent tip coverage to components. When conformal coatings are applied to a PCB they have a tendency to slump. The first layer of a coating can give a thin edge on the corner of components. This can be improved with a second coat by double dipping or brushing, but this is a repeat process and may not be acceptable. To eliminate this problem atomised spraying can be used.

Conformal coating dipping

Conformal Coating Dip System DS101 Dip Coating System for application of conformal coatings.jpg
Conformal Coating Dip System

This coating is a highly repeatable process. If the printed circuit board (PCB) is designed correctly, it can be the highest volume technique. [9] The coating penetrates everywhere, including beneath devices, hence masking must be perfect to prevent leakage. Therefore, many PCBs are unsuitable for dipping due to design.

The issue of thin tip coverage where the material slumps around sharp edges can be a problem, especially in a condensing atmosphere. This tip coverage effect can be eliminated by either double dipping the PCB or using several thin layers of atomised spraying to achieve good coverage without exceeding coating thickness recommendations. A combination of the two techniques may also be used.

Selective coating by machine

This method is the best choice for high volume applications. It is a fast and accurate way of applying the coating to the exact areas of the board where it is required. [10]

It works by using a needle and atomised spray applicator, non-atomised spray or ultrasonic valve technologies that can move above the circuit board and dispense / spray the coating material in select areas. Flow rates and material viscosity are programmed into the computer system controlling the applicator so that the desired coating thickness is maintained. [11] This method is effective for large volumes, provided that the PCBs are designed for the method. There are limitations in the select coat process [12] like the other processes, such as capillary effects around low profile connectors which suck up the coating accidentally. A skilled operator is required.

The process quality of dip or dam-and-fill coating and non-atomised spray technology can be improved by applying then releasing a vacuum while the assembly is submerged in the liquid resin. This forces the liquid resin into all crevices, eliminating uncoated surfaces in interior cavities.

The differences in application methods can be seen in a comparison presentation. [13] Choice of method is dependent on the complexity of the substrate to be coated, the required coating performance, and the throughput requirements.

Curing and drying

Solvent and water-based conformal coatings

For standard solvent-based acrylics, air drying (film forming) is the normal process except where speed is essential. Then heat curing can be used, using batch or inline ovens with conveyors and using typical cure profiles. [14] [15]

Water-based conformal coatings can be treated in the same manner, but with more care in the heat application due to longer drying times.

UV conformal coatings

UV Inline Conveyor for curing conformal coatings UV200 UV Inline Conveyor for curing conformal coatings, lacquers and adhesives.jpg
UV Inline Conveyor for curing conformal coatings

UV curing of conformal coatings is becoming important for high volume users in fields such as automotive and consumer electronics. [16]

This increase in the popularity of UV curable conformal coatings is due to its rapid cure speed, ease of processing, environmental friendliness and thermal cycling resistance. [17]

UV conformal coatings can be cured with arc and microwave lamps.

Thickness and measurement

Coating material (after curing) should have a thickness of 30–130 μm (0.0012–0.0051 in) when using acrylic resin, epoxy resin, or urethane resin. For silicone resin, the coating thickness recommended by the IPC standards is 50–210 μm (0.0020–0.0083 in).

There are several methods for measuring coating thickness, and they fall into two categories: wet film and dry film.

Wet film conformal coating measurement

Wet film gauge for Conformal Coating Thickness Measurement Wet film gauge for Conformal Coating Thickness Measurement.png
Wet film gauge for Conformal Coating Thickness Measurement

The wet film method ensures quality control while the coating is still wet.

Applying too much coating can be expensive. Also, wet film measurements are useful for conformal coatings where the dry film thickness can only be measured destructively or where over-application of conformal coating is a problem.

The wet film gauges are applied to the wet conformal coating; the teeth indicate the coating thickness. The dry film thickness can then be calculated from the measurement.

Dry film conformal coating thickness measurement

Dry film Conformal Coating Thickness Measurement Positester conformal coating thickness measurement system.jpg
Dry film Conformal Coating Thickness Measurement

An alternative to wet film measurement is by using using eddy currents. The system works by placing the test head on the surface of the conformal coating. The measurement is almost instantaneous and provides an immediate repeatable result for thickness measurement.

Test coupons are the ideal method for measuring coating thickness, and can be archived as a physical record. Apply the coating to test coupons at the same time as the circuit boards provides a permanent record of coating thickness.

Thicker coatings or better-applied coatings may be required when liquid water is present due to possible pinhole formation in the coating [7] or when the coating is too thin on sharp edges of components due to poor application. This is considered a defect and can be eliminated with appropriate steps and training. These techniques effectively "pot" or "conform" to components by completely covering them.[ citation needed ]

Conformal coating inspection

Conformal Coating Inspection Booth IB101 Inspection Booth for Inspecting Conformal Coatings on Circuit Boards.jpg
Conformal Coating Inspection Booth
Conformal Coating AOI AOI image OK for conformal coating inspection.png
Conformal Coating AOI

Traditionally, conformal coating inspection has been done manually. A typical situation is an inspector sitting in a booth, examining each PCB under a high intensity long wave UV lamp. The inspector checks for proper workmanship and that standards are met.

Recent developments in conformal coating automated optical inspection (AOI) have begun to address these manual processes and issues. Automated Inspection Systems can be camera- or scanner-based, hence the technology can be matched to the project.

Conformal coating selection

The selection of conformal coating material needs to be done carefully, and in relation to the application method. [18] [19] Incorrect selection can affect long term reliability of the circuit board, and can cause processing and cost problems.

The most common[ citation needed ] standards for conformal coating are IPC A-610 [20] and IPC-CC-830. [21] These standards list indications of good and bad coverage and describe various failure mechanisms such as dewetting [22] and orange peel. [23]

Another type of coating called parylene is applied with a vacuum deposition process at ambient temperature. Film coatings from 0.100 to 76 μm can be applied in a single operation. The advantage of parylene coatings is that they cover hidden surfaces and other areas where spray and needle application are not possible. Coating thickness is uniform, even on irregular surfaces. Desired contact points such as battery contacts or connectors must be covered with an air-tight mask to prevent the parylene from coating the contacts. Applying parylene is a batch process which does not lend itself to high volume processing. The cost per PCB can be high due to high capital investment and the cost per batch.

Coating chemistries

There are many chemistries of conformal coatings available. It is important to choose a coating chemistry meeting the application needs. Below are five common attributes for each coating chemistry. [24] [25]

Acrylic
Epoxy
Polyurethane
Silicones
Fluorinated or non Fluorinated - Poly-Para-Xylylene (Parylene)
Amorphous Fluoropolymer

The basics of conformal coating processing are found in a presentation available at: [26]

Material considerations

Selecting the correct coating material is one of the process engineer's most critical decisions. This criteria includes: [27]

Answers will determine the suitability of a particular material, be it acrylic, polyurethane, silicone, epoxy, etc. Process, production and commercial issues will then enter the equation:

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Silicone polymers that include any inert, synthetic compound made up of repeating units of siloxane, which is a chain of alternating silicon atoms and oxygen atoms, frequently combined with carbon and/or hydrogen

Silicones, also known as polysiloxanes, are polymers that include any synthetic compound made up of repeating units of siloxane, which is a chain of alternating silicon atoms and oxygen atoms, combined with carbon, hydrogen, and sometimes other elements. They are typically heat-resistant and either liquid or rubber-like, and are used in sealants, adhesives, lubricants, medicine, cooking utensils, and thermal and electrical insulation. Some common forms include silicone oil, silicone grease, silicone rubber, silicone resin, and silicone caulk.

Epoxy family of polymer

Epoxy is either any of the basic components or the cured end products of epoxy resins, as well as a colloquial name for the epoxide functional group. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups.

Varnish transparent, hard, protective finish or film used in painting

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Thermosetting polymer polymer material that irreversibly cures

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Metallizing

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Parylene chemical compound

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Industrial oven

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A thermoset polymer matrix is a synthetic polymer reinforcement first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the space shuttle. In polymer matrix composites, polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements.

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Titanium is often used in medical and military applications because of its strength, weight, and corrosion resistance characteristics. In implantable medical devices, titanium is used because of its biocompatibility and its passive, stable oxide layer. Also, titanium allergies are rare and in those cases mitigations like parylene coating are used. In the aerospace industry titanium is often bonded to save cost, touch times, and the need for mechanical fasteners. In the past, Russian submarines hulls were completely made of titanium because the non-magnetic nature of the material went undetected by the defense technology at that time. This article will discuss surface preparation for adhesive bonding to titanium. There is not a single solution for all applications. For example, etchant and chemical methods are not biocompatible and cannot be human used in blood and tissue contact. Mechanical surface roughness techniques like sanding and laser roughening may make the surface brittle and create micro-hardness regions that would not be suitable for cyclic loading found in military applications. Air oxidation at high temperatures will produce a crystalline oxide layer at a lower investment cost but the increased temperatures can deform precision parts. The type of adhesive, thermosetting or thermoplastic, and curing methods are also factors in titanium bonding because of the adhesive's interaction with the treated oxide layer. Surface treatments can also be combined. For example, a grit blast process can be followed by a chemical etch and a primer application.

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