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Conformal Coating

There are many different facets to conformal coating, from equipment and applications to the different types of coatings. Conformal coating is a protective non-conductive dielectric layer that is applied onto the printed circuit board assembly to protect the electronic assembly from damage due to contamination, salt spray, moisture, fungus, dust and corrosion caused by harsh or extreme environments. Conformal coatings are usually used in products that are made for use in outdoor environments where heat and moisture are prevalent. Coating also prevents damage from rough handling, installation, and reduction of mechanical and thermal stresses.

Why Conformal Coating?

Conformal Coating prolongs the life of the product during its operation. At the same time, it helps to increase the dielectric strength between conductors enabling the design of the PCB to be more compact and small. It also acts to protect circuitry and components from abrasion and solvents.

After the coating process is complete the coating is clearly visible as a clear and shiny material. Some coatings are hard, while others have a slightly rubbery texture. Most coatings include a marker that appears greenish white when viewed under UV light. This marker enables easy inspection of the coating thoroughness checking during production.

In the past, coatings were only applied to military and life/medical products as the cost and the process was significantly higher per unit. In recent years the development of materials and new processes has enabled a larger variety of items to be coated, including a wide array of consumer electronics products. As prices continue to drop, conformal coating will become increasingly common for circuitry and electronic components, especially as these items continue to shrink in size and dimension.

Conformal coating started as a simple process performed on electronic substrates in need of extra protection from external elements, with little attention paid to quality factors beyond adequate component coverage. The increased capability of semiconductor assemblies to perform complex tasks in automotive applications, traffic control, signage, outdoor surveillance, and mission-critical elements has increased the demand for conformal coating. Because device failure could have dire consequences, the quality of coating material application is critical. New equipment and processes are in place to accommodate the conformal coating requirements of these emerging technologies. A smooth transition to automated precision conformal coating can be achieved through an analysis of the product being coated and the desired result, the coating material used, the process selected, and of the economic costs over a period of time.

Different conformal coating types, such as acrylic conformal coating, urethane conformal coating, and silicone conformal coatings are frequently used in conjunction with different conformal coating systems to apply the coatings in a quick, easy manner for larger scale production.

Thickness

Conformal coating thickness can be critical to the proper function of a PWA. If a coating is too thin, proper coverage is impossible; if a coating is too thick, it may create excessive stresses on solder joints and components (particularly glass-bodied components). Controlling coating thickness is of special importance with rigid coating materials (e.g., epoxies and some of the urethanes) because the residual stresses associated with an excessively thick application of these materials are much greater than with flexible coating materials (such as silicones and some urethanes).

Conformal coating thickness requirements are in accordance with IPC J-STD-001 for the following coating types:

Type AR Acrylic Resin 0.00118 to 0.00512 in
Type ER Epoxy Resin 0.00118 to 0.00512 in
Type UR Urethane Resin 0.00118 to 0.00512 in
Type SR Silicone Resin 0.00197 to 0.00827 in
Type XY Parylene Resin 0.000394 to 0.00197 in

Additionally conformal coating thickness is measured on a flat, unencumbered, cured surface of the printed circuit assembly or a coupon that had been process with the assembly. Coupons may be of the same type of material as the printed board or may be of a nonporous material such as metal or glass.

Conformal coating thickness/coverage shall have the following characteristics to ensure that coating thickness specifications are upheld:

  • Be completely cured and homogeneous.
  • Cover only those areas specified on the assembly drawing/documentation.
  • Be free of blisters, breaks that could affect the operations of the assembly or sealing properties for the conformal coating.
  • Be free of cracks, crazing, voids, bubbles, mealing, peeling, wrinkles or foreign material which expose component conductors, printed circuit conductors, or other conductors and/or violates design electrical clearance.

Repairs and Rework
Performed per IPC-7711 and IPC-7721 Specifications.

We hope this is a good description of the different types of Conformal Coating. If you have further questions please contact us and we'll be a good resource for you.

Parylene Coating Applications

Cleanliness Testing

It is well known in the electronics manufacturing industry that cleanliness of a Printed Wiring Board (PWB) is crucial to the assembly’s performance and reliability. Contamination of circuit boards can bring about severe degradation of insulation resistance and dielectric strength. Cleanliness of completed circuit boards is, therefore, of vital interest. Monitoring and quantifying the degree of cleanliness is necessary in order to ensure that the final cleanliness of an assembly is acceptable.

Harmful residues and contaminants are separated into two main categories: ionic and nonionic. Ionic residue can be described as residue that contains molecules or atoms which are conductive when in solution. With the addition of moisture ionic residues can disassociate into either negatively or positively charged species and increase the overall conductivity of the solution. Some of the most common sources of ionic residue include:

  • Plating chemistries
  • Flux activators
  • Perspiration
  • Ionic surfactants

Nonionic residues are not conductive and are usually organic species that can remain on the PWB after board fabrication or assembly. These include:

  • Resin
  • Oils
  • Greases
  • Hand lotion
  • Silicone

While both ionic and nonionic contamination can impact the operation and reliability of the device on which they are present, the effects of ionic contamination is of greater interest to most PWB manufacturers. A higher figure of failures is associated with ionic contamination than its nonionic counterpart. Corrosion and dendrite growth are the two most common failure modes resulting from ionic contamination. Either can lead to shorts and opens in an electronic circuit, particularly in fine pitch applications or assemblies with high component packing density.

Nonionic (nonpolar) residue will lead to unwanted impedance because of its insulative properties. This is an issue particularly where plug-in contacts or connectors are utilized. In addition to acting as an unwanted insulator, non ion contamination on an assembly can cause poor adhesion of solder mask and conformal coating, physical interference with moving parts, encapsulation of ionic contaminants, and retention of foreign debris.

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Conformal Coating Applications

Conformal coatings are very thin layers of material designed to protect the surface of a printed circuit board. These layers are applied onto the circuit board or the substrate and act as a protective coating from harsh elements. Conformal coating is used when the finished product containing the printed circuit board would be exposed to a harsh environment. These environments may include heat, chemicals, moisture or any environment that could damage the mechanics of a printed circuit or the substrate.

First application is the Dipping Process: this can be done typically by automated equipment or manual application. PCBs are typically hung by an arm and then lowered in a dip tank containing the coating with an immersion rate determined by the population density of the PCB to be dipped. Some advantages of this process is coating penetration under components, coating thickness is assured, can be a fast process time, and this process has a low skill process in application. Some disadvantages of dip coating is thin tip coverage, inconsistent coating thickness, and rather intensive masking is required.

Spray Coating either from an aerosol can or spray gun application is the second most widely used and accepted application of liquid conformal coatings. In this process the material is diluted and sprayed with multiple passes each at a 45-degree angle with multiple coats to achieve desired thickness required. Advantages for spray coating are high volume capable of 1000’s of boards per week, reduced masking in comparison to the dip method, better tip and edge coverage, along with a more uniform thickness. Some disadvantages is multiple passes are needed to achieve desired thickness and there is less penetration of coating under components in comparison to “dipping.”

Brush Coating application is commonly used with a low volume production run. During this process it is important to keep the brush loaded with coating and let it flow; do not paint the assembly. Some advantages to this process is the extremely low cost and that it is well suited for low volume applications as well as for repair and rework of an assembly. Disadvantages are it can be slow on labor intensive on large board, which also makes for a questionable finish for the assembly.

Selective Spray coating application is very common amongst automotive and other high volume application in which board design and layout does not change much of long periods of time. For this application, selective spay maximizes throughput by minimizing the amount of masking as well as providing and highly controlled process creating precise repeatability. However there are some disadvantages to this process. Boards may still require masking, and it does require a skilled operator to program and operate this machine. Programming is time consuming as well in addition to the high cost of the spray machines.

Parylene Coating Requirements

Conformal Coating Removal

It is important to consider how the choice of a conformal coating material affects the rework and repair issues. The need for rework or repair of a conformal coating can arise at any time after completion of an assembly due to a variety of process/product requirements or component replacement issues. Hence, rework of conformal coatings needs to be addressed up front when choosing a coating chemistry.

Thermal
The thermal removal method using a soldering iron is the least recommended method. Most conformal coatings require a very high temperature and long exposure times. These, in turn, can cause discoloration, leave residues and adversely affect solder joints and other materials/components used in the fabrication of assemblies.

The process must be monitored to ensure that excessive temperatures do not cause delamination, lifting pads or overheat surrounding temperature sensitive devices. Extreme caution must be taken when burning rt conformal coating because some coatings emit toxic vapors which are hazardous.

Chemical
Chemical methods are the most popular for the removal of conformal coatings. As long as the solvents used do not adversely affect the PWB or components and there are no environmental issues this technique works well. However, there is no one perfect solvent for all applications and in some cases it may be difficult to find a suitable solvent.

The following sections discuss the chemical removal methods for various types of coating:

Urethane
There are several solvents which provide a wide range of speed and selectivity that can be matched to a specific application. These solvents include:

  • Methanol base/alkaline activator solvents which provide a range in the dissolution power and selectivity
  • Ethylene glycol ether base/alkaline activator solvents which are relatively the fastest and least selective

Silicone
Methylene chloride based systems are very effective in removing silicone conformal coatings. Several hydrocarbon-based solvents are also used as alternatives. While not as fast as the methylene chloride, the hydrocarbon based solvents are more selective, and where not contaminated by water, will not attack epoxy-glass PCBs, components, metals and plastics.

Acrylic
The chemical removal of acrylic coatings was done in the past with highly volatile and flammable solvents such as methylene chloride, trichloroethane or ketones. A relatively safe alternative based on butyrolactone has been used for the removal of most of the acrylic coatings.

Epoxy
The complete removal of epoxy coatings for repair is nearly impossible by chemical means. The solvent can’t discriminate between the epoxy coating, epoxy-glass PCB and any epoxy-coated or potted components. However, if done carefully spot removal may be accomplished by the application of methylene chloride and an acid activator with a cotton tipped swab.

Repairs and Rework
Performed per IPC-7711 and IPC-7721 Specifications.

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Masking Boots

Diamond-MT can provide both standard and custom boot solutions for a customer depending on their needs. We have a wide range of custom boots and designs in stock or ready to manufacture. We can quickly match these to your needs or help develop a custom solution through our design services to help you get the protection you need in your conformal coating process.

Masking boots are an integral part of a full masking solution in coating applications for printed circuit cards, whether this is for a liquid conformal coating process or a vapor deposition process such as parylene. Many components and parts on a printed circuit board must remain uncoated from conformal coating. The purpose of the masking boots is to provide easy to use covers instead of alternative materials such as masking tape and tape dots to prevent coatings from going in the wrong areas. Conformal coating masking boots come in all shapes and forms to protect different types of components for all conformal coating application techniques.

The purpose of masking prior to conformal coating an assembly is to keep the coating from being applied to areas that would not be able to perform properly with it. Masking is often the most critical and thus the most meticulous and time consuming aspect of the conformal coating process. Failure to properly mask could result in improper coating hindering the performance of the technology, or worse, completely ruining the assembly. Our technicians are specially trained to ensure proper masking of your assemblies so you have peace of mind that your product will be returned to you without issues, imperfections or defects.

The masking process begins with proper, effective communication with our customers. Detailed drawings are obtained that identify the “coating-free” areas. If these areas are not properly identified early on the in procedure it can lead to improper coating and increased cost. If you are not familiar with the masking or conformal coating process we are willing to work with you, following standardized and customer driven specifications to develop a masking process to suit your needs.

The masking process continues with our team developing working instructions for our production personnel to follow. Once production has completed masking your assemblies, our quality assurance team performs 100% inspection to ensure that the end result is complete compliance with your coating requirements.

Parylene Removal

The typical boots can be divided into two types: A and C type Cups.

A Cups are boots which cover vertical connectors and components and are the simplest form. These include covers for d type and molex connectors.

C cups are boots which cover horizontal connectors and wrap around the component from both sides of the board in general. these tend to be more complex in design although we can provide both standard and custom boots solutions for a customer depending on their needs.

A Cup

A Cup

A Cup

C Cup

C Cup

C Cup

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Diamond Mt provides a full range of conformal coating application equipment and conformal coating systems for those looking to bring the conformal coating process in-house. Please explore our list of products below to find the perfect piece of equipment for your specific needs!

Diamond MT can help you discover the best equipment solutions for your specific needs. We work with you to ensure that you are getting the most cost effective solution for your product, whether that is parylene coating solutions, bringing equipment in house, or another type of conformal coating.

Automated Conformal Coating Equipment:

Selective Coating is a more recent development among conformal coating methods. Selective coating combines the advantages of several coating methods into a single coating method. There are several types of coating applicators. Spray coating is an atomized process that can achieve a thinner film pass; however, if selectivity is a requirement, the applicator usually has to move more slowly. The finish is a feathered or fuzzy coating on the edges.

Conformal Coating Equipment (Dip):

Automated Dip Coating is one of the most efficient methods for application of conformal coating and is excellent for all volume production whether large or small. The process of dipping a circuit board in a conformal coating material contained in a tank ensures complete coverage, including underneath components and around difficult later 3D boards and this is no over spray or material wastage.

Conformal Coating Equipment (Semi-Automated):

We utilize a variety of HVLP spray guns for our hand spraying process, in addition to multiple spray booths. This method is ideal for customers with small to medium quantities. Additionally this method is preferred with customers who have tall components or a geographical layout that does not lend itself to a programmable conformal coating method.

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Parylene Solutions for Every Industry

There are many different industries that conformal coating plays a critical role in.

Parylene Solutions for Aerospace
Defense / Aerospace
Parylene Solutions for Electronics
Electronics
Parylene Solutions for Automotive
Automotive
Parylene Solutions for LEDs
LEDs
Parylene Solutions for Medical
Medical
Parylene Solutions for MEMs
MEMs

Download Our Parylene 101 Guide: Coating Facts Sheet

Download our Parylene 101 Guide