Sensible Design: Resins—Fundamental Considerations for Circuit Designers

Beth_Turner_300.jpgWelcome to the first of my new series on encapsulation resins. I am going to focus on the all-important subject of resin selection, types of chemistries available and their properties, application, mix and cure. First, though, let me introduce myself. I'm Beth Turner and have worked for Electrolube for five years, starting as a member of the Global Technical Support Team. I’ve spent my career as a research and development chemist working on adhesives, resins, and conformal coatings at Apollo Chemicals before moving to Electrolube, and have previously worked on specialist projects, including bio-functional nano-coatings and coatings affording protection to electronics in harsh environments.

When it comes to the choice and applications of resins, there’s a great deal to talk about, and over the forthcoming months, I hope to provide some useful tips and design advice that will help in your quest for reliable circuit protection. For this resins series, I will start by reviewing some fundamentals that will help circuit designers understand some of the essential properties of resins before deciding which is most suitable for the task at hand.

Selecting the Correct Resin for Your Application
Choosing the correct encapsulation resin is possibly the most critical aspect of the design process. It is imperative to understand where and how the finished unit is going to be used and what performance criteria are expected of it. It is best practice to draw up a list of the ambient or standard operating conditions that the unit will be exposed to, and then list what the extremes of those operating conditions are likely to be.

Another important factor to consider is the duration of time that the unit will be exposed to in the most extreme of operating conditions. For example, there is a vast difference between specifying a chemically resistant resin that can withstand fully permanent immersion in antifreeze, and one that only requires resistance to occasional splashes of antifreeze that are wiped clear after short periods of exposure.

Similarly, if an application reaches a maximum temperature of 150°C, but this only occurs once a day for a couple of minutes and the rest of the time the normal operating maximum is 90°C, then it is sufficient to specify a resin with a maximum operating temperature of, say, 100°C. Most polymeric materials will tolerate wider temperature excursions than originally quoted for continuous operation over short periods, similar to those seen in typical reflow profiles.

Other application requirements for consideration might include flame retardancy. Is UL certification essential as part of material qualification or is it customary to accept a manufacturer’s internal test report? For LED lighting assemblies, optical clarity and UV resistance is important. Information on changes to correlated colour temperature (CCT), colour space (L*a*b*) and total colour difference (delta E*) can be useful when qualifying a resin to pot directly over a PCB with LEDs and lenses. Or perhaps a design engineer wants to protect their circuit design from potential IP theft, in which case it is better to select an opaque resin. RF communication is used in many industries; these communication devices can benefit from encapsulation resins with low loss tangent and low dielectric constant properties to provide electrical insulation.

Electrification in the transport industry requires high power density electrical components to sustain long ranges and high horsepower. To manage the heat generated by small, high power devices, thermally conductive encapsulation resins can effectively dissipate heat away from hot components to a heat sink to sustain long-term performance. There are several different thermal characterisation techniques including steady state and transient methods; each method gives some variety in results so it's worth bearing that in mind when comparing literature from different manufacturers.

What Types of Resin Chemistries are Available?
Resin chemistries fall within three major classes: epoxy, polyurethane, and silicone. Epoxy is the strongest and most chemically resistant of the three, but it is brittle, challenging if not impossible to remove for rework, and is typically limited to operating temperatures between -40°C and +150°C. Epoxy resins offer excellent adhesion to a wide range of substrates.

The tough and flexible polyurethanes are suitable for applications operating at lower temperatures. Typically, polyurethane resins are only suitable for applications reaching maximum temperatures of 110°C for long periods (though some can go to 130°C). The chemical resistance of a polyurethane resin is generally lower than that of an epoxy, but polyurethanes outperform epoxies in water and high humidity environments. Polyurethane resins are typically used in marine applications, where water penetration resistance is critical, and for applications subject to a high level of physical stress, such as the potting of accelerometers, or sensors embedded in road surfaces. If large temperature swings are expected over a short timeframe, then the flexible nature of the resin is advantageous as there is a low probability of stress cracking occurring.

Silicone resins have the widest operating temperature range (-50°C to +220°C) and when cured are the most flexible of the three resin chemistries. Their adhesion to certain substrates is poor, as is their chemical resistance, particularly to chemicals more commonly encountered in everyday use, such as alcohols, aromatics, and ketones.

While they do tend to differ widely in terms of performance characteristics, all resin types have excellent electrical insulation properties across their respective temperature ranges. Resin chemistries have advanced considerably in recent years and there are now resins available that exhibit properties and performance criteria which is often well beyond the normal boundaries expected for that resin type.

How Much Resin Should You Apply?
As a rule of thumb, the amount of resin that needs to be applied must be sufficient to cover the top of the highest component of the board, and the thickness of the resin layer must provide the desired level of protection. While most customers will determine the minimum thickness of resin layers for their particular applications by trial and error, the relevant technical datasheet will provide good guidance and consulting your supplier will often help resolve a problem.

What Are the Key Factors for a Successful Mix Ratio?
The mix ratio is quite possibly the most critical aspect of resin mixing. Brace yourselves to face long-term adverse repercussions if you get it wrong. There are two methods of mixing a resin with its associated hardener, either by hand or by using specialist dispensing equipment. If mixing by hand, then the ratio of the weight of the two components is the more useful method to employ. If mixing via dispensing equipment, then the volume ratio is used.

If the job is reasonably small, then you are likely to use a resin pack, which provides the resin and hardener in precise quantities, in separate compartments of the pack. When you are ready to use the product, you simply remove the clip or other separating device between the compartments and “massage” the resulting pouch, ensuring that both components are completely mixed. For further support on mixing techniques, there is a how-to video in the Knowledge Base section of the Electrolube website. For larger production jobs, the resin and hardener are supplied in separate bulk containers, suitable for use with two-component metering and mixing systems. It is important to check the shot sizes for each component regularly to ensure that the correct amounts of resin and hardener are being dispensed.

When mixing bulk resin and hardener, it is important to avoid introducing excessive amounts of air, which will form micro-bubbles within the cured resin. The liquid resin and liquid hardener used for polyurethanes are particularly sensitive to moisture, so it is important to avoid humid conditions when potting to avoid micro-bubble formation. These micro-bubbles may expand when hot and create voids. If you are not happy with bulk materials mixing and incurring the potential problems of introducing too much air (and moisture) into the mix, then it might be more appropriate to use automated metering and mixing equipment, which will accurately mix resin and hardener in the correct proportions, and usually in an inert atmosphere.

Remember, incorrect ratios will lead to a poor cure and the physical properties, tensile strength, elongation, and modulus will differ from those specified in the manufacturer’s data sheet.

Important Considerations for Cure Temperature
Once a resin and its associated hardener are mixed, the reaction can be very fast but also very exothermic, which can lead to the possibility of a runaway reaction; remember that the exotherm will also increase as potting volume increases. Not all reactions have a high exotherm; the exotherm temperature can be controlled by adjusting the chemistry or by using a filled system, as the fillers absorb some of the heat as well as reduce the concentration of the active component within the hardener that promotes this rapid cure. Controlling the temperature during the cure profile is important because the components requiring potting or encapsulation might be adversely affected by raised temperatures.

It is important to follow the recommended cure profile to ensure resins are fully cured and the physical properties achieved match those specified on the manufacturer’s datasheet.

I hope these points have been useful and informative. Please contact our technical support team if you need any advice with your resin application requirements. In the meantime, please watch for my next column, where I will be examining resins in more depth.

This column originally appeared in the July 2021 issue of Design007 Magazine. 

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2021

Sensible Design: Resins—Fundamental Considerations for Circuit Designers

07-27-2021

New Electrolube columnist Beth Turner starts a series on the all-important subject of resin selection, types of chemistries available and their properties, application, mix and cure.

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Sensible Design: Thermal Management—Good Design Practice for Heat Dissipation

07-06-2021

Trial and error is an essential process in the development of new and innovative products however, excessive testing can be unnecessary and costly. Incorporating thermal management at the preliminary stage of your design process will ultimately lead to more reliable and cost-effective end products.

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Sensible Design: Conformal Coating Vs. Encapsulation Resin

05-24-2021

Without a doubt, this is the biggest question we get asked regularly at Electrolube is “When is it suitable to use a conformal coating or an encapsulated resin?”. There are a number of considerations that will determine the answer, however, it depends largely on your how the circuit will be housed within the assembly as well as the type of environment in which it will operate.

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Sensible Design: Growth and Trends in the Thermal Management Market

04-20-2021

Electrolube’s Jade Bridges reminds us why thermal management plays such a vital function in circuit and electronic assembly design, and explores the factors contributing to the current growth curve in the market.

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Sensible Design: The Role of Resins and Conformal Coatings in Your Applications

03-10-2021

This month, Phil Kinner examines some of the key differences between conformal coatings, encapsulation resins and potting compounds to help designers make more informed decisions, and ultimately help to increase the performance, reliability and lifetime of your electronic circuitry. He also looks at issues that some of you may have with mixing resin packs and air bubbles, what can go wrong and why.

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Sensible Design: How to Avoid Problematic ‘Blushing’ and ‘Bubbling’ in Conformal Coatings

02-15-2021

In this month’s column, Phil Kinner revisits a couple of general enquiries Electrolube has received about "bubbling' and also, the less common appearance of cloudy, white patches following the application of a conformal coating.

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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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