Sensible Design: To Coat or Encapsulate—Making An Informed Choice for Electronics Protection

phil_kinner-100x100.jpgOne of the most frequently asked questions we receive from customers is, “Which is better to protect my PCB: a coating or a resin?” PCBs are found in many domestic, industrial, automotive, and military devices and require protection from their environment. Lack of protection can lead to reduced performance or, in the worst case, a complete failure. Protection can be offered in the form of conformal coatings or potting and encapsulation resins.

In this month’s column, I will demystify why one may be more suitable for your application than the other. Like any good engineering answer, it depends on the degree of environmental protection required. The first point for consideration is often the design of any housing within which the PCB will be enclosed. If an assembly is enclosed in a housing that is designed to be the primary environmental protection, then a conformal coating is often used to provide a back-up to the primary protection provided by the housing. Where the housing is not suitable or capable of providing primary protection of the assembly from its operating environment, then most times, a resin might be a better choice.

If we look at both coatings and resins, then we can consider what is common between the two. Both are generally organic polymers, which can cure to form an electrically insulating layer that provides some degree of chemical and thermal resistance. There is a degree in commonality in the chemistry of the polymers used, with acrylic, epoxy, polyurethane, and silicone being the most often encountered.

Without further ado, let’s explore coatings and resins in more detail in our five-point format.

1. What is an encapsulation resin?
Potting and encapsulation resins offer the highest level of protection for PCBs. Resins can be applied from 0.5 millimetres upwards and are generally applied much thicker than this. The increased thickness leads to a significant increase in weight and often results in a greater per-unit price than a coating. However, the increased thickness does mean that the PCB is far better protected against chemical attack, particularly in the cases of prolonged immersion.

Also, a resin can provide superior protection against physical shock (depending upon the formulation) since the bulk of the resin will help to dissipate the forces. A layer of dark-coloured resin can also completely hide the PCB, which allows for some security of the design. Depending on the choice of the resin, removal of the resin can also result in the destruction of the PCB. With so many varying options to protect electronic circuitry, there’s a lot of ground to cover, and depending on the application, sometimes a conformal coating may be more suitable—particularly with the two-part coating series that performs like a resin.

2. What is a conformal coating?
Conformal coatings can be used to protect the PCBs in a variety of applications, ensuring optimum performance in the harshest of conditions. They are generally thin films applied in the 25–250-micron dry film thickness range, leading to a minimal weight increase of the assembly. Conformal coatings conform to the contours of the board, providing maximum protection with minimal weight or dimensional change to the PCB. This is possibly the primary advantage of conformal coatings over potting and encapsulation resins.

Often, coatings are clear, so the coated components are easy to identify, and the coating can be easily reworked and components replaced as required. The chemical and thermal resistance of coatings is generally good for short exposures. A coating applies relatively little stress upon the components. This is a particular advantage where a component has thin leads or legs. The majority of coatings are 1K (single-component) systems, which have a long useable life, a low curing or drying temperature, and short drying time.

Being a single-part solution, they are clearly easier to process and apply; however, the majority of 1K coatings are solvent based in order to modify their viscosity for application purposes. Conventionally, coatings can be applied manually by use of a paintbrush, spray gun, or even manually dipped. Increasingly, however, coatings are applied by robotic selective coating systems to provide a more controlled and more consistent process.

3. Is it possible that a conformal coating can perform like a resin?
To confuse matters further, the simple answer to this is yes! We’ve developed a wide range of two-part (2K) conformal coatings, which combine the protection and properties of a resin with the ease of application of a conformal coating but without the use of solvents, giving them an environmental advantage. The 2K coatings provide excellent coverage, and their superior flexibility offers protection of delicate components.

2K coatings also deliver excellent mechanical properties and abrasion resistance, but, being two-part, they require more sophisticated application equipment than 1K coatings, and they are more difficult to remove, making board repair more difficult. Based on similar two-part chemistry to resins, 2K coatings are designed to be applied by selective coating equipment in the range of 200–400 microns (0.2–0.4 millimetres), combining many of the advantages of both technologies and minimising many of the drawbacks of each.

4. What are some key differences between resins and coatings?
The most noticeable differences are the methods of application (aerosol, conformal coating spray equipment, manual spray gun, and brushing for coatings, compared to mixing and dispensing equipment and resin packs for resins). The materials are applied at different thicknesses (500 microns for resins). Coatings are generally approved to international standards (IEC-1086), military standards (MIL-I-46058C), industry standards (IPC-CC-830), or national safety standards (UL746), while for resins, it is very much dependent on the application, and there are very few standards.

Due to the coating thickness, coatings occupy less volume and have a lower increase in overall weight compared to resins. There are both coatings and resins that are based on epoxy, polyurethane, and silicone chemistries, but there are also acrylate, acrylic, and parylene coatings that do not have a direct resin equivalent. Ninety-nine percent of resins are 100% solid systems, so they have low or no VOCs released during curing, while many coatings are high in solvent content, although there are two-component (2K) and UV-curable acrylate systems that are also available as 100% solids.

5. Why would I choose a resin instead of a coating?
Fundamentally, the choice will often depend on the design of the housing. If the housing is designed to be the primary protection, then a coating will often be used to provide secondary protection or additional insulation. If the housing is not the primary barrier to the environment, then an encapsulation resin will often be used to augment or replace the housing.

The choice between a resin or a coating normally comes down to application specifics. If the unit involved is to be subject to long-term immersion in various chemicals or subject to long-term thermal and/or physical shock cycling, then a resin is generally preferred. Also, if there are a large number of large components on a PCB, it is sometimes better to use a resin to encapsulate these than to coat them. Another scenario where a resin would be preferable is when the unit will be used in a situation where it is not easily accessible, or a long continuous service life is required. In this case, a resin would be recommended to provide the extra protection and durability needed.

However, the 2K conformal coatings have demonstrated excellent performance in condensing environments and immersion tests. In recent trials simulating highly condensing and immersion conditions, a urethane coating, 2K301, gave the highest overall values in terms of circuit protection and showed the least change during condensing/immersion events. The very large difference in thickness between the 2K conformal coating and a urethane resin potted assembly didn’t show a large increase in performance.

Indeed, the 2K coating achieved much the same results as the resin applied at one-tenth of the thickness. 2K conformal coatings can be applied more thickly than previous coating technologies without the risk of cracking. They can also be readily applied by selective coating techniques to provide a coating with better thickness and sharp edge coverage that has performance somewhere between where a conventional conformal coating fails and potting is required.

Conclusion
Every customer and their project is different. Whilst we can advise a customer as to which products are best suited to their needs, based on our years of experience, it all boils down to the unit, the environment it will be subjected to, the dispensing method/equipment to be used, the curing times, and the temperature limitations that may be imposed during the production process. The more information that a customer can provide regarding the ultimate operating conditions—temperature range, likely chemical exposures, and so on—then all the better.

Look out for my next column, where I will look at more considerations to improve the lifetime and reliability of electronic assemblies.

This column originally appeared in the December 2020 issue of Design007 Magazine.

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2020

Sensible Design: To Coat or Encapsulate—Making An Informed Choice for Electronics Protection

12-28-2020

One of the most frequently asked questions we receive from customers is, “Which is better to protect my PCB: a coating or a resin?” In this month’s column, Phil Kinner demystifies why one may be more suitable for your application than the other and explores coatings and resins in more detail.

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Sensible Design: Top Tips for a Successful Thermal Management Process

11-10-2020

Jade Bridges concentrates on some of the complications you are likely to encounter when selecting and applying a thermal interface material and looks a little more closely at thermal resistance, viscosity, and vibration, as well as their effects on performance. Read on to learn how to fully optimize your thermal management process.

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Sensible Design: Pushing the Boundaries of Thermal Management

10-23-2020

Thermal management plays a central role in circuit and electronic assembly design, ensuring improved reliability and increased performance of devices. But what if you could push the boundaries even further and extend the long-term stability? To explore how this can be achieved, Jade Bridges touches on some of the latest advances in thermal management technology.

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Sensible Design: Key Benefits of Resins and Differences From Coatings

09-17-2020

Alistair Little focuses on the benefits of using a thermally conductive encapsulation resin and compare the difference between using a resin and a conformal coating. He also looks more closely at the best way to mix a resin pack and what to be wary of if air bubbles get trapped in the cured resin.

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Sensible Design: How to Overcome Conformal Coating Challenges

08-19-2020

Phil Kinner takes a look at some issues you are sure to face, from areas of the coated board that simply refuse to cure fully to masking components to the orange peel effect and examining the difference between a critical and a non-critical area of a board.

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Sensible Design: Resins–Are They All About Chemistry?

06-15-2020

When it comes to resin selection and application, there are a plethora of factors to consider. Alistair Little looks at thermally conductive resins, flexible resins, elevated cure temperatures, resin types for different applications, and resin systems that enable wider operating temperatures.

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Sensible Design: Thermal Management—Keeping Cool Starts From Within

06-09-2020

Thermal management plays a significant role in protecting electronic circuitry. Jade Bridges takes a fresh look at popular subjects within the field of thermal management and explores what occurs when devices overheat, as well as the benefits of thermal gap fillers and how to best avoid pump-out.

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Sensible Design: Thermal Management—Five Tips for Application Success

05-25-2020

With so much to consider when choosing a thermal management material, it’s important to do your calculations, consider the equipment’s operational, and environmental conditions and experiments. Underestimating these could compromise the reliability of an electronic assembly and shorten its life expectancy. Jade Bridges shares five tips to improve your thermal management process.

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Sensible Design: Conformal Coating Enemies—Challenges Sabotaging Your Process

05-15-2020

Phil Kinner examines the arch enemies to conformal coatings.

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Sensible Designs: Resins—Five Tips for Potting and Performance

05-05-2020

Readers continue to ask, “Does thicker coverage achieve better performance? What is the best advice for manual potting? We chose an inappropriate resin, so how will this affect our application?” In this column, Alistair Little explores these and other issues based upon frequently asked questions from Electrolube’s customers.

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2019

Sensible Design: Top Tips for Conformal Coating Selection

12-31-2019

Over the past few months, I have covered the topic of conformal coatings in as much depth as possible. In this column, I’m going to explore some of the essential factors for designers in coating selection. As we have all experienced, sometimes, things are not always as simple or straightforward as we would like them to be, and in any engineering discipline, there is always the slightest chance that something might go wrong. Thankfully, the key to kicking that possibility is to be as fully prepared as possible. Thus, I’m going to concentrate on helping you avoid coating pitfalls in my five-point guide.

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Sensible Design: Design Challenges and the Impact on Coating Success

11-15-2019

Phil Kinner concentrates on essential factors regarding the challenges board designs can pose on designers to help you implement a more successful coating operation, as well as issues that may arise with coating coverage, cycle time, and coating flow.

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Sensible Design: Five Key Factors for Flexible Resins and Potting Sensitive Components

10-18-2019

In this month’s column, I am going to concentrate on protecting sensitive components and take a more in-depth look at flexible resins, their reworkability, and some of the common problematic consequences that you may encounter. Potting compounds play an important role in the electronics industry where they serve to protect sensitive components from chemicals, moisture, dust, and damage, but their selection can baffle many. Let’s explore some frequently asked questions in more detail.

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Conformal Coatings: How to Design Out Production Problems

08-29-2019

In my last few columns, I’ve covered quite a bit of ground regarding the important considerations for conformal coating selection and performance, and the suitability of conformal coatings for LEDs and protecting circuitry from the harshest environments. I hope these columns have provided plenty of food for thought as well as given you a basic understanding of coatings and their benefits and limitations. In this column, I’m going to look at the different angles that design engineers and purchasing professionals come from and explore how these can sometimes conflict when selecting conformal coatings.

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Sensible Design: Resins Fit for a Purpose—Failure Mitigation and Environmental Concerns

07-18-2019

So far, in my columns on resin chemistries and encapsulation/potting techniques, I have tended to concentrate on the properties of these materials. My insights have included how they are best applied in the factory, and the steps that must be taken to get the best performance from them once they are in the field to protect an electronic assembly or lighting fixture against the elements. In this column, I am going to address the question of resin failures—in particular, how to avoid them—and to get a better idea of where and how appropriately selected and applied modern resins are making a big difference in the world of extreme electronics installation and implementation.

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Thermal Management: Why It Should Be High on Your Circuit Protection Agenda

06-06-2019

In my previous column, Jade Bridges highlighted a few cautionary notes on the pain points associated with thermal management products, particularly the choices that you will be confronted with, such as which material or product type (i.e., pad or paste) is best suited to your application. In this column, she will underline the importance of getting it right, and touch on the consequences if you don’t.

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Sensible Design: Five Tips to Further Improve Resin Encapsulant Performance

05-16-2019

There are a number of different factors that influence the protection afforded by potting compounds. The act of encapsulating a component or PCB means that it is surrounded by a layer of resin, which completely seals a component or an entire PCB from the environment in which it operates. When mixed, a two-part resin starts a chemical reaction, which results in the resin becoming fully polymerised to provide a homogenous layer.

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Sensible Design: Important Considerations for Conformal Coating Selection and Performance

04-25-2019

Having covered the subject of conformal coatings in depth over the past few months, now is an appropriate time to review some of the key pointers I have tried to share in my various columns. I present some of my thoughts on the essentials in this five-point guide.

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Sensible Design: Thermal Management Materials—Golden Rules for Product Selection

03-28-2019

Selecting the right type of thermal management method that will suit a particular electronic assembly and its predicted operating conditions is far from easy. There are a number of stages in the selection process that you should consider taking before you decide upon a particular material or material format, whether paste or pad. In this column on achieving effective thermal management of electronic assemblies, I will revisit our trusted question-and-answer format to bring you some essential pointers, beginning with a few cautionary notes on pain points—the occasional pangs of agony you will have to face during the decision-making process.

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Sensible Design: Getting the Best Performance from Encapsulation Resins

03-07-2019

When I last broached the subject of potting and encapsulation resins, I went into some depth on the subject, explaining their chemistries and physical properties, how they behave when being mixed, applied and cured. For this column, I’m going to return to our tried-and-trusted Q&A format to offer four commonly asked questions about resins and their application, together with my responses, which I hope will help you achieve the best outcomes for all your potting and encapsulation jobs. So, setting material choice aside for the moment, let’s start with a key aspect of potting: getting the resin in place.

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2018

Sensible Design: Top Tips for Successful Potting

12-19-2018

For effective potting, ideally, the layout of the circuit components should be such that the material can flow smoothly around them without too much turbulence. When possible, it is always good practice to space components in a regular pattern. Irregular spacing—particularly bunching of components in discrete areas of the PCB—causes the formation of eddies in the resin as it is poured, which can lead to voids and air entrapment, which compromise the thermal performance of the resin.

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Do's and Don'ts of Thermal Management Materials

10-18-2018

Selecting a thermal management material that is broadly applicable to a particular electronic assembly and its predicted operating conditions is a good starting point; however, as with many of these things, the devil is very much in the details! Find out the key considerations in choosing your materials.

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Conformal Coatings: An Evolving Science

09-26-2018

One of the trends impacting the electronics assembly industry is the continuing miniaturization of electronics products. This article sheds more light on coating problems posed by this trend, as well as provide key considerations when it comes to coating properties, selections, and applications. Read on to find answers to five of the best coating-related questions that frequently arise during preliminary consultations.

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Thermal Management Materials: Easing the Decision-Making Process

08-02-2018

There are many different types of thermally conductive materials, and choosing between them will be dictated by production requirements and application design, as well as critical performance factors that must be achieved.

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Protecting PCBs from Harsh, Challenging Environments

07-03-2018

Think very carefully about the sort of environment your PCB is likely to encounter. It is easy to over-engineer a product so that it will survive the very worst of conditions, but worst conditions may only be fleeting or transient. Therefore, a resin solution with a lower temperature performance specification will often cope. Take temperature extremes, for example. Your application may experience occasional temperature spikes of up to 180°C, which you might feel deserves treatment with a special resin.

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My Top Coating Queries

04-04-2018

This is my first of many columns for 2018, and I have decided to share some top trending queries that concern many different applications and areas. LEDs are always a hot topic, as are volatile organic compounds (VOCs) and harsh environment concerns.

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2017

Heat Transfer and Thermal Conductivity: The Facts

12-26-2017

In my first two columns, I presented a broad introduction to the subject of thermal management of electronic circuits. This month I’m taking a closer look at thermal interface materials—how they can be applied to achieve efficient heat transfer, and the significance of bulk thermal conductivity in relation to heat transfer and thermal resistance.

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Thermal Management—The Heat is On

09-25-2017

Thermal management materials are designed to prolong equipment life and reduce incidences of failure. They also maintain equipment performance parameters and reduce energy consumption by reducing operating temperatures, and minimising the risk of damage to surrounding components. Indirectly, they maintain brand reputation, as the reliability of the equipment will be very dependent upon the effectiveness of the thermal management technique used.

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Resins: Cutting Through the Technical Jargon

08-21-2017

This month, I’m going to cut through some of the more heavy-going tech-speak, taking a few of my customers’ more frequently asked questions about resins to try to help you refine your selection process. There’s a lot of ground to cover, but for the purposes of this column, let’s concentrate on the PCB’s operating environment, caring for the components that are to be encapsulated, and the special needs of applications like LED lighting and RF systems.

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Casting a Spotlight on Resin Applications

05-03-2017

Over the last few columns, I’ve given readers pointers on virtually every aspect of potting and encapsulation resins, ranging from their formulations and special properties to their applications, benefits and limitations. It’s probably high time, therefore, to take a step back from the do's and don’ts and focus instead on how these resins are bringing very real benefits to practical electronic and electrical engineering applications. A good starting point is to look at the special requirements of an industry that is enjoying explosive growth: LED lighting.

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2016

Resins: Five Essentials to Achieve the Right Cure

12-19-2016

In my previous column, I looked at some of the critical things you need to consider before selecting your resin. Of course, when it comes to the choice and application of resins, there’s a lot of information to take in, and over the following months I hope to distill this and provide some useful tips and design advice that will help you in your quest for reliable circuit protection.

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Why are Resin Properties So Important?

11-21-2016

I started this series of columns on resins by going back to basics, questioning the core rationale for potting and encapsulation with resins, their fundamental chemistries and how each resin type differs one from the other—indeed, how their individual properties can be exploited to maximise performance under a wide range of environmental conditions. I hope readers found this useful. Of course, when it comes to the choice and applications of resins, there’s a great deal more to discuss.

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The Little Guide to Resins

10-17-2016

I would like to start this series of columns by going back to basics, questioning the core rationale for potting and encapsulation with resins, their fundamental chemistries and how each resin type differs one from the other—indeed, how their individual properties can be exploited to maximise performance under a wide range of environmental conditions.

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Conformal Coatings - Beware the Boards that ‘Bare’ All!

09-21-2016

This month, Phil Kinner departs from his usual format of providing five essential facts about conformal coatings. Instead, he provides an account of a customer’s problem—no company names mentioned, of course—that brought into question the adhesion performance of a coating that they had been using successfully for some time.

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When Coatings Go Wrong

08-23-2016

This month, I consider some of the more common, and often very frustrating, problems that may be encountered when coating electronic circuit boards and components. I also discuss some practical solutions. As we all know, nothing in life is straightforward.

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Coatings—Five Essentials for Designers

06-28-2016

In an ideal world, PCB designs would not have an inherent weak point for corrosion; unfortunately, in the real world, they do. When a weak point is revealed, you are better equipped to deal with it. Often the spacing of components, board finish and distance to ground planes can be optimised for corrosion resistance.

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