Light Pipe Design Guide

Everything You Need to Know About Light Pipes

What Are Light Pipes?

Light pipes (also called light guides or light tubes) are components that carry light energy  including UV, infrared, visual light, laser, and more in a controlled area and defined size from point A to point B with high efficiency.

How Do Light Pipes Work?

A light pipe is placed very close to an LED light source on one end of a PCB board. Its optical-grade materials help carry the rays of light to the desired destination, typically at the user interface of a device.

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Light pipes transmit roughly 80 to 90 percent of the light from the LED source, depending on design and spacing. The closer the light pipes are to the LED(s), the more efficiently they can transfer light.

Of course, the light pipe(s) used will need to be rated for the same amount of heat that is produced by the LED or light source, so that must be a design consideration as well that can dictate the minimum allowable distance between the light pipe and the light source.

Why Use Light Pipes?

Depending on the equipment and the design of the user interface of a device, a product can require more than one and up to many indicators. For designs that require multiple indicators, light pipes are an ideal solution due to their low cost, excellent visual communication, and design flexibility.

Light pipes offer many advantages in efficiently transporting light in a defined area. But they can also transport the energy of electromagnetic waves, such as UV, infrared (IR), visual light, laser, and other types.

Illuminated Component Terminology

Light pipe - the term “light pipe” can also be called a “light guide” or “light tube.” To make things more confusing, they’re sometimes called “litepipes”, too. A light pipe is a rod made of optical acrylic or polycarbonate that transmits light from a light source, typically an LED, at the circuit board to the panel of the device. These light pipes are ideal for carrying the light in a small space, like a status indicator or control panel.

Light diffuser - light diffusers are used to spread light from an LED source over a defined area but also reduce the brightness of the light it transmits. Therefore, light diffusers are typically made of opaque materials. Light diffusers can be used in conjunction with light pipes or light guides. They also increase the viewing angle and are ideal for soft illumination of defined areas.

For this guide, we will be focusing on the fundamentals of LED light pipes, or litepipes, if you prefer that spelling.

LED Light Pipe Design Benefits

We touched on how beneficial light pipes can be — yet we can’t mention it enough. They deliver a host of benefits for designers and OEMs, such as:

Greater flexibility and control over - by adjusting the length, angle, and spacing, designers can get the exact visual indication and uniform light visibility they desire with the right light pipe design.

Minimized light leaks - when excess light shines through a device without illuminating the indicator properly, it leads to wasted light and provides a poor user experience in the final device. Selecting the right light pipe and optimal placement between the LED and panel delivers optimal illumination without losing light.

Easy installation - no matter the type of light pipe, the installation can be literally a snap with a snap-fit or press-fit light pipe.

Production efficiencies - when a device has multiple panel mount indicators (PMIs), the production costs can really add up. Light pipes are a great alternative that brings down the cost without skimping on reliability, design flexibility, or visual indication.

Easily customizable solutions - light simulations and ray tracing with advanced software help design teams develop the ideal indication solution that can achieve all of the manufacturer’s goals

Reliable, uniform performance - when combined with LEDs, light pipes of various lengths deliver a consistent and uniform illumination that can last the life of the device

ESD protection - light pipes can prevent electrostatic discharge that can damage the device by isolating the PCB or circuit board from the user interface

Light Pipe Characteristics & Design Guidelines

From one end to the other, there are several physical features of a light pipe that can impact performance and visibility.

At one end, you have the input, or where the LED fits inside the light pipe. On the other hand, you have the output or exit surface, where the light travels to the indicator light or user interface.

The geometry or shape of the light pipe itself is another variable. The shapes and materials selected are just a few factors that can determine how much light makes it to the end where the interface is located.

As a general rule, light pipes include the following attributes:

• Rounded corners - leveraging rounded edges inside the light pipe minimizes the amount of light that can escape before shining outside of the exit surface. A minimum radius of 0.5mm is recommended.
• Smooth, flat, or concave contoured surface - whatever matches the surface of the LED needs to be happening on the interior of the light pipe on the side where the LED is inserted (the input surface).
• Diffused exit surface - using a diffused surface where the light is visible at the exit will provide a uniform illuminated surface.
• Smooth, mirrored surface - to avoid light loss, a smooth or mirrored surface can be added to the light pipe’s exterior.
• Light-reflecting paint - for extra credit (and brightness), an optional coat of paint can be applied to the outside of the light pipe to aid in amplifying the light.

The physical features of your light pipe are only a fraction of the design decisions that will need to be made during the design phase. Throughout this guide, we’ll cover additional considerations and the science behind them when choosing the ideal light pipe for your needs.

Light Pipes Vary in More than Just Color

Like with any new product design, the decisions made early in the process can shape some of the constraints that must be accommodated later in the process, from the size of the PCB to the amount of space on a panel. These decisions may not seem like that big a deal, but when light indication or user interface decisions are made early-on, the manufacturers  and ultimately the end users win.

Light pipes are available in a variety of styles and can suit a host of LED mounting options.

To really maximize efficiencies in your product design, you need to know the pros and cons of the different types of light pipes.

They typically fall into two major categories: rigid and flexible.

Rigid Light Pipe Design Guidelines

Rigid light pipes are meant to be used in applications where the light doesn’t need to bend around a corner or angle. Therefore, these do not bend like flexible light pipes. They’re made of polycarbonate rods that transfer light from the PCB to the user interface.

Rigid light pipes are meant to be used in devices where the light only needs to travel a short distance from the PCB and panel, compared to flexible light pipes that transmit light longer distances... Although rigid light pipes do not bend themselves, they can still move light around right angles and tight corners.

Benefits of Using Panel Mount Indicators There are many pros to using panel mount indicators instead of light pipes to communicate device status, including:

Pros of rigid light pipes:

• Ideal for vertical or right-angle orientation
• Great for board-mount, press-fit, and surface-mount LEDs
• Bend light around a fixed corner
• Available in right angle, 1-position, 2-position, and 3-position

Cons of rigid light pipes:

• Designed to transmit light short distances
• Length cannot exceed:
• Press fit: 1.20”
• Custom design: 1.5”
• Standard design: 2”

One Additional Design Consideration for Rigid Light Pipes — Face Shape

The surface, or face, of a rigid light pipe is the acrylic or plastic piece that is visible to the end-user through the panel. These exterior surfaces are more important than they seem, as various properties can improve the performance of the light pipe.

Working environment, application, device design, and even light pipe length and shape should dictate which face shape would be ideal for a specific product. For example:

• Longer light pipes perform best with smooth light pipe faces or surfaces.
• Scattered or frosted textures, like a Fresnel lens, help diffuse or soften the light.

Common shapes for rigid light pipe faces include:

• Dome - provides the widest viewing angle and best visibility from different working positions.
• Semi-Dome - splits the difference between dome and flat shapes, delivering a diffused light output while maintaining a wide viewing angle.
• Flat - in applications where rays must travel longer distances through the light pipe, a flat face is the perfect solution. Additional elements, such as a Fresnel design, can help concentrate the beam.

Light modeling is essential to find the ideal specifications for the desired output.
To accommodate a range of applications and needs, VCC provides standard and custom rigid light pipes with dome, semi-dome, and Fresnel lenses.

Flexible Light Pipe Design Guidelines

Flexible light pipes are the exact opposite of rigid light pipes. They are made of a polycarbonate fiber that is designed to be extremely flexible. These bendable properties make it possible to transmit light around corners and tight spaces without losing much energy.

An ideal solution for a range of applications, flexible light pipes attach to the PCB and carry the light from board-mounted LEDs to the control panel while remaining secure and providing ESD protection.

Pros of flexible light pipes:

• Carry light around curves
• Carry light around tight spaces
• Carry light longer distances
• Can bend up to 30 degrees
• Provide greater control over light optics
• Greater design flexibility

Cons of flexible light pipes:

• Typically cost slightly more than rigid light pipes
• No length limits per se, but the longer the light needs to travel, the more will be lost along the way
• An accessory is required to keep the flexible light pipe end close to the light source

What Are Light Pipes Made Of?

Light pipes can be made with glass as well as a variety of synthetic materials with their own pros and cons.

The two most popular materials by far are optical acrylic and polycarbonate. Both of these materials are lightweight yet durable. They also possess the optical properties manufacturers require for a variety of applications.

Acrylic Light Pipes

Compared to polycarbonate, acrylic is hands down superior when it comes to optical properties. It’s easy to mold, naturally UV stable for indoor use, and transmits light better than any plastic material available.

So why wouldn’t you always use acrylic as your go-to light pipe material? There may be instances where you need a more durable light pipe that still performs well but also provides more resistance to heat. That’s where polycarbonate comes in.

Polycarbonate Light Pipes

Polycarbonate light pipes provide outstanding light transmission while being more suitable for higher-temp operations. Polycarbonate is also available in UV-stabilized grades that are ideal for outdoor use.

So, how do you choose the best light pipe for your application?

Start with your product and application requirements, as these will generally lead you to a more obvious choice:

• Type of light being used
• Flammability requirements
• Outdoor usage
• Vibration and moisture needs
• Operating temperature range
• Board, indicator, and indicator orientation being used

For help identifying the best light pipe material for your application, contact VCC.

Many Mounting Types to Work With

If flexible and rigid were the only options, your options might be limited. Light pipes are designed to accommodate a wide range of mounting types.

In addition to the rigid vs. flexible decision, there are several other considerations when choosing a light pipe for your design. Your power source and overall design footprint can dictate which mounting type you should use with your light pipe.

Panel Mount Light Pipes

As the name implies, panel mount light pipes are those that are attached directly to the control panel or user interface.

They are attached to the panel in two ways:

• Crushable ribs that “crimp” closed
• A plastic or metal retainer ring to seal the exit surface

Board-Mount Light Pipes

On the same end where the LED is attached to the board, the light pipe can also be attached to the PCB or surface. The LED fits inside the light pipe and is also attached to the PCB.
oard-mount light pipes can feature a rigid or right-angle design.

Rigid light pipes do not bend like their flexible counterparts.

When 90-degree angle alignment to the LED or PCB board is crucial, right-angle light pipes are mounted to the board directly over the LED. Right-angle light pipes are available in multiple board-mount options, including:

• 1-position
• 2-position
• 3-position
• 4-position

Board-mount light pipes are designed for streamlined production and assembly. The PCB board can be printed in one line, the panel in another, and then they can be machine-assembled, saving time and costs vs. manual labor.

For high-vibration applications, the panel can also be secured to a wall for additional protection.

Panel Mount / Board Mount Light Pipess

If your design includes a board- or panel-mount light pipe, you’ve got additional options to work with.

Flexible light pipes are also flexible in how they’re mounted — they work well with both panel and board mount designs. These light pipe types are ideal for devices where light needs to bend around corners and other obstacles.

An LED adapter attaches the LED and the flexible light pipe to the panel or board.

So, when you’re specifying light pipes, there are several decisions to make right off the bat. Fortunately, some of them will be made for you as other options are eliminated.

• Rigid or flexible?
• Panel mount or board mount?
• Right angle or not?

Light Pipes Work with Many LED Mounting Types

But, the decisions need to drill down even further to which types of LED mounting you will be using in your design:

Through-hole LEDs
Flexible light pipes work well with through-hole LEDs and can be mounted from the panel or the back of the device.

Right-angle through-hole LEDs
Right-angle through-hole LEDs work well with flexible light pipes that are mounted to the PC board or panel.

Surface Mount (SMD) LEDs
For LEDs that are surface mounted to the panel, flexible and rigid light pipes are options available.

Right-angle Surface Mount LEDs
Flexible right-angle light pipes that are mounted to the PC board are ideal for right-angle surface mount LEDs.

Light Pipes in Real-world Applications

Rigid LED light pipes are ideal for several industries and applications, including audio equipment, portable devices, and even mission-critical aviation equipment.

Personal Aviation Jetpack
When you’re several hundred feet in the air, there’s no time to question your aviation system’s status. See how a board-mounted light pipe solution enables clear and reliable communication day or night, even at a 140-degree viewing angle.

Portable Device Charger
Powering up to 20 devices at once requires reliable and intuitive communication. Light pipes like the LFC Series provide a soft, uniform glow and a wide viewing angle of 120 degrees for across-the-room indication.

Audio Equipment
Handmade audio equipment like mixers, preamps, compressors, and mic presses need reliable indication for adjusting levels of various inputs to create distortion and signature sound. High-intensity LEDs pair well with rigid light pipes with crushable ribs for easy, secure installation.

Flexible light pipes are perfect for transmitting light around tight corners and curves.
And their only real limitation is your imagination. Here’s a creative use of a flexible light pipe:

Smart Parking Meter
From hostile weather and the elements to vandalism, parking meters can really take a beating. But they must also reduce glare for day and night operation, especially when parking tickets are at stake. This flexible light pipe solution provides NEMA 4 and IP67 Ratings to protect from shock, vibration, and even ESD protection.

A Word About Application Environment

For light pipes that will be operated in outdoor or high-vibration environments, there are several ways to help protect and secure them for continuous reliable operation.

When light pipes will be used outdoors, they must include the appropriate level of moisture protection for their applications as well as UV stability.

Both rigid and flexible light pipes can be moisture-sealed for use in outdoor and harsh environments.

From crushable ribs and gaskets to retainer rings, there are several options to help seal in moisture and attain the IP or NEMA rating of the light pipe you need.

In addition to moisture, some applications may require shock and vibration resistance as well.

Some light pipes, like the FLXR Series litepipe, protect from all the above, and have electrostatic shock discharge (ESD) protection.

What to Consider When Choosing a Light Pipe

1. Product Design

• How many indicators are needed?
• How far apart will the power source and LED be?
• How will the light pipe be mounted?
• How far does the light need to travel? Around curves and angles?
• How much room is on board or panel?

2. Application Environment

It may seem obvious, but the environment plays a huge role in machine reliability and operation. Before landing on a light pipe, ask yourself, “Where will the device be used?”

Outdoor and High-moisture Environments

For outdoor and high-moisture environments, there are several ruggedized light pipe solutions available.

Look for products with Ingress Protection (IP) and NEMA ratings that align with your product’s needs.

For more Luminous Translucent Resin bord for Lamp Postinformation, please contact us. We will provide professional answers.

Here are some examples of ruggedized light pipe solutions:

• FLXR Series
• LSS Series
• LMS Series
• LCS Series

We also offer custom silicone light pipes, which provide the ideal IP and NEMA ratings to meet your design requirements.

High-Vibration Environments

For industrial and other high-vibration environments, additional hardware may be needed to secure the light pipe in place.

Flexible light pipes can be secured with either a grommet retainer or a spring retainer, depending on their length.

• Grommet retainer - for light pipes that are .250” to .500” long
• Spring retainer - for light pipes that are .500” to 1.000” long

High-vibration application in action:

Programmable Power Unit

DC power supplies demand safe and reliable operation on job sites. This low-profile, rigid light pipe solution delivered maximum brightness and minimal light loss.

Storage and Operating Temperatures

Two other key considerations for the environment of your device are the storage and operating temperatures. While the storage and operating temps will vary by device, some good rules of thumb are:

• Storage temperature – as a general rule, polycarbonate and acrylic light pipes can be stored at temperatures ranging from -100℃ to 25℃ with minimal degradation.The lower the temperature and humidity, the less degradation of the device.
• Operating temperature – this will depend on how much strain is put on the light pipe in its environment. If there is very little stress, the operating temperature can range from -100℃ to 100℃ for acrylic and -100℃ to 125℃ for polycarbonate.

Be sure to review your product’s data sheet carefully, as each light pipe will have its own properties.

3. Light Pipe Position

Of course, light pipe position can impact the light properties. To minimize light loss, a general rule is to use an LED with a narrow viewing angle of 160 degrees or less and position the light pipe and LED no more than 0.05” apart.

There’s a little more to it than that to get the best product for your design. That’s where light simulations come in. Contact our team to run a simulation and determine the ideal light pipe for your design.

And don’t forget the constraints you may have based on the type of light pipe you use:

Rigid light pipe design constraints

• Can’t bend
• Length limits:
  -Press fit: 1.20”
  -Custom: 1.5”
  -Standard: up to 2” with shorter versions available

Flexible light pipe design constraints

• Bends up to 30 degrees

Refractive Index: Medium Matters

Light doesn’t always behave the same way, which makes light pipe design that much more complex. Why?

Let’s go back to physics class and learn about refraction.

Refraction is defined as the change in direction of a wave from one medium to another or from a gradual change in the medium itself.

So, in addition to all the other variables that go into light pipe design (size, shape, brightness, distance to LED, viewing angle, and material used, to name a few), designers must also consider the Refractive Index.

The Refractive Index is used to determine how fast light travels through a medium.

Calculated as n = c/v, c is the speed of the light, and v is the velocity of the light in that specific medium.

This number represents how much of the light is bent or refracted when it hits the medium. It also calculates how much light is reflected when reaching the medium.

In a vacuum, light breaks at a rate of 1.0. But no one designs light pipes to be used in this type of vacuum, so additional consideration must be given to optimize performance.

Plus, any number above 1.0 shows the decreased speed at which it travels compared to in a vacuum.

Sample Refractive Indices:

• In air, the rate is 1.003
• In water, the rate is 1.33
• And in acrylic — which is what most light pipes are made of — it’s ~1.49.

Yet, the Refractive Index isn’t the only number that needs to come into play when it comes to how a surface affects light.

There’s also total internal reflection (TIR), which is the angle at which light is reflected when none of it shines through to a surface exit. Instead, it is reflected from one surface to another where it is visible.

TIR is a phenomenon that occurs when the angle of incidence is greater than a certain limiting angle, referred to as the critical angle.Why Does TIR Matter in Light Pipe Design?

For clear polycarbonate, the critical angle is 39, and for acrylic, it’s 42.

Understanding how the light reacts in different scenarios will lead to better, more optimized light pipe performance.

For example, when using a right-angle light pipe, here’s how TIR would affect the specifications:

When designing light pipes, most people assume light behaves like electric current and ignore TIR.

Because light has unique properties, designing a light pipe like the right-angle illustration shown above will result in significant light loss at the TIR. To avoid light loss, incorporate the critical angle to create a geometry to guide the light around the critical angle without having the light escape. This concept also applies to light pipes with round corners (or any geometry your design includes).

To minimize the amount of light loss, designers can use an LED with a narrow viewing angle instead of one with a wide viewing angle to help close the distance between the light pipe and LED where light could escape. The wider the gap between the light pipe and the LED, the more room for light to escape before it enters the light pipe and travels to the exit point. There will always be some level of light that gets lost traveling from where the LED is located and the exit point where the end-user sees the light.

Good, Bad, and Ugly: Light Pipes Are Everywhere

Light pipes are used in many devices and applications because of their design flexibility. However, not all devices use optimized light pipe design. Notice any similarities?

Devices with good light pipe design have the following traits:

• Uniform illumination
• No hotspots
• Ideal visibility for application

Devices with less-than-ideal light pipe designs yield bad/ugly results:

• Hot spots
• Light loss
• Difficult to see in the operating environment

Custom LED Light Pipe Design

Even with all of the design options available in standard flexible and rigid light pipes, there’s still plenty of need for a custom light pipe solution. Having a custom light pipe designed for your application may seem expensive, but you may be surprised at how affordable a custom design can be, often ranging from $3,000 to $15,000 (not including tooling costs).

Before you enlist the help of an engineer for a custom light pipe, you’ll need an idea of the following:

• Number of colors needed in LED
• Number of indicators on device
• Specifics on working environment
• Desired illumination intensity
• Desired viewing angle
• How far away the light indicator needs to be viewed
• Do you need help with design and production, or just design?
• Estimated annual usage of device (EAU)
• Budget and timing
• Access to design files and lighting specs (existing designs only)

How Does VCC’s Custom Light Pipe Design Process Work?

To ensure your light pipe is the perfect fit for the application, environment, and device, our engineering team is made up of seasoned professionals across multiple disciplines, including:

• Mechanical engineering
• Optical engineering
• Electrical engineering
• By combining design know-how with state-of-the-art technology, VCC can easily develop a custom light pipe that meets or exceeds your optical and design requirements.

How Does VCC’s Custom Light Pipe Design Process Work?

VCC’s custom light pipe services can optimize your design while reducing costs and maximizing efficiencies. Here’s how the design process works:

Step 1: Engineering Drawing
A team of engineers collaborates with OEMs and designers to establish goals and optical requirements. VCC works closely with you to prepare detailed drawings, parts lists, and circuit schematics.

Step 2: Light Simulation
VCC offers a full suite of custom light pipe design and testing services. Using the latest light modeling software, our team can run light simulations, such as ray tracing, to track the path of light through the light pipe to devise the right solution. The design is then revised and optimized until the ideal optical requirements are met.

Step 3: Light Pipe Prototype
VCC helps you cut development times using rapid prototyping techniques for plastic components. We use a 3D printer to prototype the light pipe and further test it prior to tooling the custom solution, ensuring the optimal viewing angle and lighting properties are achieved.

Step 4: Tooling & Production
When the light pipe prototype is approved, it’s time to move into tooling. Because the 3D prototype we provide is 90% accurate to the finished product, you can feel confident investing in the tooling after you’ve validated the design. Once tooling is complete, we fabricate and assemble our components in our state-of-the-art facility in Mexico, ensuring you receive the utmost level of quality, plus the many advantages nearshore manufacturing provides.
The whole process can take as little as 2 weeks or as long as 12 — it all depends on the complexity of the design.

3D Printing Brings Prototypes to Life Faster

Over the last few years, 3D printing has changed the game in light pipe prototyping, allowing some manufacturers to use in-house printers to dramatically reduce production times. Shaving days or weeks off of a timeline is a huge benefit for designers and OEMs, but 3D printing can also provide a less common advantage: completing small light pipe runs to be used in actual products.

VCC’s in-house 3D printer can print light pipes using photopolymer resin with different additives then hand polish them for use in products. These in-house printed pipes can match approximately 82-95% of the light characteristics delivered by going the injection molding route. In this case, doing a small pilot run could give OEMs and designers the quantity they need to verify the concept of their final product.

In this case, doing a small pilot run could give OEMs and designers the quantity they need to verify the concept of their final product.

For example, even complex custom light pipe concepts like this non-surgical medical device can be validated quickly and efficiently with our in-house 3D printing capabilities.

The Last Word on Light Pipes

A well-designed human-machine interface (HMI) can deliver faster throughput and reduced downtime. Custom, flexible — and even rigid — light pipes open up a wide range of design possibilities by providing greater flexibility and control.

Making indicator decisions early on in the design process can help you maximize efficiencies and get exactly what you want in the finished design. This includes visibility of status indicators from distances that help the end user maintain safe and reliable operation.

To learn more, check out the VCC’s custom light pipe solutions or explore our robust light pipe portfolio.

Need Help with a Custom Light Pipe?

Contact our experts

With the increased popularity of miniaturized electronics and circuitry, conformal coating use has skyrocketed to solidify its relevance in a wealth of modern PCB-related applications . Choosing the ideal type of coating and application methods for your electronics is crucial. However, processing the vast amount of information online can often present a daunting task.

Well, not anymore!

In this article, you’ll be given all the information that you need to identify the ideal conformal coating for your application’s requirements. If you are searching for something specific, feel free to use the index for a more selective approach. Otherwise, this article is helpful both for beginners who seek to understand conformal coating methodology and use, and for seasoned applicators and businesses who wish to confirm their knowledge-base and procedural legitimacy. You can also check out our selection of conformal coatings here.

Types of Conformal Coating
Application Methods
Thickness Measurement
Curing Methods
Removal Methods 
Certifications
Regulatory Considerations

What is Conformal Coating

Conformal coating is a special polymeric film forming product that protects circuit boards, components, and other electronic devices from adverse environmental conditions. These coatings ‘conform’ to inherent irregularities in both the structure and environment of the PCB. They provide increased dielectric resistance, operational integrity, and protection from corrosive atmospheres, humidity, heat, fungus, and airborne contamination such as dirt and dust.

What are the Types of Conformal Coating

There are several options for coating technologies, and the best option for your particular application should depend primarily on your level of necessary protection. The application method and the ease of rework are also important factors, but should generally be considered secondary to the necessary protective performance.

Traditional Conformal Coatings

What we call “traditional” conformal coatings are 1-part systems that have a resin base and can be diluted with either solvent or (in rare cases) water. Traditional coatings are semi-permeable, which is why they are not fully hermetic nor do they seal the coated electronics. They provide resistance to environmental exposure, which increases PCB durability while keeping application and repair processes in practice. However, they are NOT fully water-proof.

The following categories are based on the basic resin of each coating. The chemical composition of each conformal coating determines its major attributes and functions. Choosing the proper conformal coating for your application is determined by the operational requirements of your electronics.

• Acrylic Resin (AR) – Acrylic conformal coating provides fair elasticity and general protection. Acrylic conformal coating is recognized for its high dielectric strength, and fair moisture and abrasion resistance. What generally distinguishes acrylic coating from other resins is its facility for removal. Acrylic coatings are easily and quickly removed by a variety of solvents, often without requiring agitation. This makes rework and even field repair very practical and economical. On the other hand, acrylic coatings do not protect against solvents and solvent vapors, which could result in less than ideal performance for an application that involves something like pumping equipment. Acrylic coatings can be considered basic, entry-level protection, because they are economical and protect against a broad-level of contamination. However, they are not the best-in-class for any characteristic except possibly dielectric strength.
• Silicone Resin (SR) – Silicone conformal coating provides excellent protection in a very wide temperature range. SR provides good chemical resistance, moisture, and salt spray resistance, and is very flexible. Silicone conformal coating isn’t abrasion resistant because of its rubbery nature, but this property does make it resilient against vibrational stresses. Silicone coatings are commonly used in high-humidity environments. Special formulations that can coat LED lights without color shift or reduction of intensity are available, and make SR conformal coatings a popular choice for applications such as outdoor signs. Removal can be challenging, requiring specialized solvents, long soak time, and agitation from a brush or an ultrasonic bath.
• Urethane (Polyurethane) Resin (UR) – Urethane conformal coating is known for its excellent moisture and chemical resistance. It is also very abrasion resistant. Combining those factors with its solvent resistance results in a conformal coating that is very difficult to remove. Like silicone, full removal generally requires special solvents, long soak time, and agitation with a brush or an ultrasonic bath. Urethane conformal coating is commonly specified for aerospace applications where exposure to fuel vapors is a common concern.

The rest of this article is concerned mainly with what we call “traditional” conformal coatings, but we’ll first cover other coating types to provide a complete picture of the options available.

• Epoxy Conformal Coating - Epoxy resins (ER) are usually available as two‐part compounds and create a very hard coating. Epoxy conformal coatings provide very good humidity resistance and are not generally permeable, unlike traditional conformal coatings. They also have high abrasion and chemical resistance. Typically, they are very difficult to remove once cured and are not as flexible as the other materials. Epoxy coatings are common in potting compounds, which in contrast to conformal coatings, completely cover the electronics in a solid and level layer of material.
• Parylene Conformal Coating - Parylene conformal coatings are a unique type of coating applied by vapor phase deposition. They provide excellent dielectric strength and superior resistance to moisture, solvents, and extreme temperatures. Because of the vapor deposition method, parylene coatings can be applied thinly and still provide excellent circuit board protection. However, removal for rework is very difficult, requiring abrasion techniques, and without access to vapor phase deposition equipment, recoating with parylene is impossible.
• Thin Film /“Nano” Coatings - A coating is dissolved in a fluorocarbon‐based carrier solvent and applied with a spray or dip method to create a very thin coat, although not at a nanometer scale as the nickname suggests. They are commonly used to provide a minimal amount of hydrophobicity, which may prevent losses from very quick exposure to water. This type of coating does not offer the level of  surface protection that  other coating methods do.

How To Apply Conformal Coating

Once the type of coating is selected, the next question is how to apply the conformal coating. This decision should be based on the following variables:

• Production throughput requirements – The necessary prep work, the speed of the coating process, and how quickly the boards can be handled after the coating process.
• Board design requirements – Connector-laden designs, solvent-sensitive components, and other issues impact your decision.
• Equipment requirements – If a coating is only sporadically required, tying up capital and floor space with additional equipment may not make sense.
• Pre-coating processing – Some processes require masking or taping before coating in order to prevent coating of unwanted surfaces.
• Quality requirements – Mission critical electronics that require a high degree of repeatability and reliability will generally require more automated application methods.

The following are the application methods for traditional conformal coatings:

• Manual spraying - Conformal coating can be applied with an aerosol can or handheld spray gun. It is generally used for low volume production when capital equipment is not available. This method can be time-consuming because areas not requiring coating need to be masked. Also, quality and consistency of outcome are operator-dependent, so variations are common from board to board. 
• Automated spraying – A programmed spray system that moves the board on a conveyor under an alternative spray head that applies a conformal coating.
• Selective coating - An automated conformal coating process that uses programmable robotic spray nozzles to apply the conformal coating to very specific areas on the circuit board. This process is used in high volume processes and can eliminate the need for masking. An applicator may have a built-in UV lamp to cure coating immediately after it is applied.

Photo courtesy of PVA

• Dipping - The circuit board is first immersed, then withdrawn from the conformal coating solution. Immersion speed, withdrawal speed, immersion time, and viscosity determine the resulting film formation. It is a common conformal coating technique for high volume processing. A great deal of masking is generally required before the coating process. Dipping is only practical when coating on both sides of the board is acceptable.
  
  

• Brushing - Brushing is a simple application technique used mainly in repair and rework applications. The conformal coating is applied with a brush to specific areas on the board. It is a low-cost method but it requires a lot of manual labor and is highly variable depending on operator proficiency and consistency This method is best suited for small production runs.
  
  

How To Measure Conformal Coating Thickness

Conformal coatings are usually applied as very thin coatings, providing the maximum amount of protection possible while still using the thinnest amount of material. The thinness of the coatings minimize heat entrapment, unnecessary additional weight, and a variety of other concerns. Common thickness with most conformal coatings is anywhere between 1 to 5 mils (25 to 127 microns) with some coatings applied at an even thinner level. Anything greater than this thickness is usually an encapsulate or a potting compound, which typically provides more mass and thickness to protect the boards.

There are three primary ways to measure the thickness of a conformal coating.

• Wet film thickness gauge - Wet film thickness can be measured directly by using the appropriate gauge. These gauges incorporate a series of notches and teeth, each tooth having a known and calibrated length. The gauge is placed directly onto the wet film for the film measurement. See http://www.geionline.com/wet-film-gauge. This measurement is then multiplied by the percent of solids in the coating to calculate the approximate dry coating thickness.

• Micrometer - Micrometer thickness measurements are taken on the board (or on a test panel) at several locations before and after coating takes place. The cured coating thickness is subtracted from the uncoated measurements and divided by 2, providing the thickness on one side of the board. The standard deviation of the measurements is then calculated to determine the uniformity of the coating.  Micrometer measurements are best taken on harder coatings that do not deform under pressure.
• Eddy current probes – The Eddy current measurement of conformal coating thickness uses a test probe that directly measures the thickness of a coating by creating an oscillating electromagnetic field. The thickness measurements are both non-destructive and very accurate, but can be limited depending on the availability of a metal backplane or metal under the coating, and the direct contact available of the test sample. Without metal below the test area, no measurements will be made, and if the probe does not fit flat on the test area, readings will be inaccurate.
• Ultrasonic thickness gauge – This type of gauge measures coating thickness using ultrasonic waves. It has an edge over the eddy current probes because it does not need a metal backplane. Thickness is determined by the amount of time the sound takes to travel from the transducer, through the coating, onto the surface of the board, and then back through the coating to the transducer. A conductive medium, like propylene glycol or water, is needed to provide good contact with the surface. This is generally considered a non-destructive test unless there is a concern with the conductive medium affecting the coating.

  Curing Methods

While the curing mechanism is not a primary criterion when selecting a coating, it has a direct impact on the type of application method that will be feasible, and the throughput that can be expected. Some mechanisms are relatively infallible, while others are very complex and leave room for application errors when used in an uncontrolled process.

• Evaporative Curing Mechanism - The liquid carrier evaporates, leaving just the coating resin. Although very simple in theory, circuit boards usually need to be dipped at least two times to build up an adequate coating on the edges of their components. Whether the liquid carrier is solvent or water‐based, humidity affects application parameters. Solvent systems tend to be easy to process, and provide consistent coverage due to good wetting, and fast cure times. However, solvents are often flammable, so adequate ventilation and fume extraction methods are required. Using water as a carrier can eliminate the flammability concern, although these coatings tend to take much longer to cure, and can be very sensitive to ambient humidity.
• Moisture Curing - Primarily found in silicone and some urethane systems. These materials will react with ambient moisture to form the polymer coating. This type of curing mechanism is often coupled with an evaporative curing. As carrier solvents evaporate, moisture reacts with resin to initiate final curing.
• Heat Curing - Heat curing mechanisms can be used with one or multicomponent systems, as a secondary curing mechanism for UV curing, moisture curing, or evaporative curing. The addition of heat will cause the system to polymerize or speed the curing of the system. This can be advantageous when one curing mechanism is insufficient to gain the curing properties required or expected. However, thermal sensitivity of circuit boards and components must be taken into consideration when curing at high temperatures.
• UV Curing- Coatings that are cured using ultraviolet light offer very fast throughputs. They are 100% solid systems with no carrier solvents. UV curing occurs in the production line, so a secondary curing mechanism is needed under the components and in shadowy areas. UV cured coatings are more difficult to repair and rework and require UV curing equipment and UV radiation protection for workers.

How to Removal Conformal Coating

On occasion, it is necessary to remove a conformal coating from the circuit board to replace damaged components or perform other reworking procedures. The methods and materials used to remove coatings are determined by both the coating resins and the size of the area, which can impact the time required for removal.

The basic methods as cited by IPC are:

• Solvent Removal – Most conformal coatings are susceptible to solvent removal; however, it must be determined if the solvent will damage parts or components on the circuit board. Acrylics are the most sensitive to solvents hence their easy removal. On the other hand, epoxies, urethanes, and silicones are the least sensitive. Parylene cannot be removed with a solvent.
• Peeling – Some conformal coatings can be peeled from the circuit board. This is mainly a characteristic of some silicone conformal coatings and some flexible conformal coatings.
• Thermal/Burn‐through – A common technique of coating removal is to simply burn through the coating with a soldering iron as the board is reworked. This method works well with most forms of conformal coatings.
• Microblasting – Micro blasting removes the conformal coating by using a concentrated mix of soft abrasive and compressed air to abrade the coating. The process can be used to remove small areas of the conformal coating. It is most commonly used when removing Parylene and epoxy coatings.
• Grinding/Scraping – In this method, the conformal coating is removed by abrading the circuit board. This method is more effective with harder conformal coatings, such as parylene, epoxy and polyurethane. This method is only used as a method of last resort, as serious damage can be incurred.

If all you are doing is replacing a component or working on an isolated area, it is common to simply burn through the coating with a soldering iron. In cases when this is aesthetically unacceptable, contamination is a concern, or components are densely spaced, coating removers are available in pen packaging.

Certifications

Certifications are an important way to distinguish general purpose varnishes and shellacs from engineered coatings designed specifically for PCB protection. Although there are dozens of user and industry specifications, the two major certifications are IPC-CC-830B and UL746E. When selecting a coating, look for the availability of 3rd party test documentation, rather than coatings with the claim that “they meet the requirements”.  Both standards use the UL94 standard to judge flammability, with a V-0 rating signifying the lowest flammability potential.

IPC-CC-830B / MIL-I-C

This standard originated with the military standard MIL-I-C, which became obsolete in . The civilian version IPC-CC-830B is nearly identical, so it is generally understood that if a board passes the IPC spec it will also pass the MIL spec., and vice versa. IPC-CC-830B is a battery of tests, some are pass-fail and others provide data that can be referenced and compared to:

• Appearance
• Insulation resistance
• UV fluorescence
• Fungus resistance
• Flexibility
• Flammability
• Moisture and insulation resistance
• Thermal shock
• Hydrolytic stability 

UL746E

Underwriters Laboratories (UL) is considered a credible and reliable safety certification body worldwide, and UL certification is commonly required for consumer goods. UL746E tests for the electrical safety and flammable safety of  coated electronics. For electrical safety, there is a battery of tests similar to IPC-CC-830B, but with a cycling current load to constantly measure the failure of the isolative properties of the coating. The flammability test uses the UL94 standard like IPC-CC-830B, which involves attempting to light the cured coating with an open flame and observing the sustainability of the flame.

Once a coating has passed the UL746E standard, it can be registered with UL and assigned a registration number. Products certified and registered to UL746E standards can include the UL symbol (which looks like a backward “UR”). To maintain the registration, a coating must be retested annually.

Coatings can, and often are, tested to standards that only represent a portion of the whole standard. In the case of UL94, this is helpful when flammability is the main concern. Some specialty coatings may not be tested to the entire IPC-CC-830B or UL746E standards because they may fail parts of the test. These failures may be due to the nature of the product and the coating’s necessary applications, and are not always a reflection of the quality of the product. For example, some coatings intended to coat LEDs leave out the UV indicator to prevent color shift, but this automatically would cause disqualification under IPC-CC-830B. In other words, it is impossible by definition to pass IPC-CC-830B and have optical clarity in the UV part of the spectrum.

Regulatory Considerations

Safety and environmental considerations should always play a part in chemical selection and process design, but different regulatory bodies make this an even more challenging feat, as requirements must be interpreted and matched with product specifications.

OSHA (Occupational Safety and Health Administration) - In the US, the OSHA has overriding authority over worker safety concerns. Many coatings are very flammable, and many emit fumes that have a high level of toxicity. Close attention needs to be paid to ventilation (explosion-proof when dealing with flammable fumes) and the appropriate PPE (personal protection equipment) to keep operator exposure down below the appropriate safety threshold. Flammability may be difficult to avoid without exploring more specific water-based coating materials. Newer coatings have been introduced that do not include HAPs (hazardous air pollutants – a government classification of particularly toxic chemicals) like toluene, xylene or methyl ethyl ketone (MEK). The Global Harmonized System (GHS – with those red diamond symbols) needs to be followed for labeling, which is generally taken care of by the manufacturer. Make sure safety data sheets (SDS) are readily available to operators, as they should be for any hazardous chemical in a facility.

EPA (United States Environmental Protection Agency) – In the US, the EPA requirements must be followed at the national and regional level. The EPA, following the Montreal Protocol treaty, enforced restrictions on ozone-depleting chemicals. Since most of the restricted chemicals are unavailable and have not been used in conformal coating formulation for years, ozone depletion isn’t the current concern. If there are regional agencies (see next paragraph) that have stricter requirements than the EPA, those generally will need to be followed.

CARB (California Air Review Board) and other regional regulations – Local agencies continue to play  a larger-and-larger role in environmental restrictions. CARB was one of the early regulatory bodies, laying down VOC (volatile organic compounds – smog-producing chemicals) restrictions by product category. Other regional agencies followed their lead. Global warming potential (GWP) is the latest environmental topic of discussion.

This concludes our guide on conformal coating. We hope that it answered your questions and provided proper guidance in selecting the best products and methods for your needs. Like any challenge, selecting the best coating and coating process can be broken apart, analyzed, and solved.

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