Sensible Design: Top Design Tips for a Successful Coating Process

phil_kinner-100x100.jpgBoard design, cycle time, and coating flow can all impact a designer’s path to a successful coating process, so what are the key factors to bear in mind in achieving the best outcomes for your coating processes? In this month’s column, I’ll be highlighting some of the key challenges and commonly asked questions about conformal coatings that every design engineer should take into consideration when specifying the coating process. As assemblies become ever more densely populated, and housing/casing designs become more permeable to save weight, the use of conformal coatings is becoming essential to protect the assembly from its operating environment and ensure acceptable reliability for the application intended, especially when operating in hostile environments. Let’s explore some of the key factors that will help following our five-point Q&A format.  

1. How does board design impact the coating process?
The board’s design can have a significant impact on the success of a coating operation by influencing which methods can be used and determining what degree of coverage can be achieved within a required cycle time. In general, the less areas there are on a board that must be free from coating, the faster the coating cycle time will be. Should the coating cycle time be too long, then a decision can be made to either determine the most vulnerable parts of the assembly and focus on that area in the available cycle time, or to split the coating across several workstations.

Designers should be aware that coating the edges of an assembly is not considered as value-added for most applications. Most design standards do not allow innerlayer circuit traces to be closer than 0.63 mm from the edge of the board. The act of routing the board edges results in smearing the epoxy resin over the cut glass fibers or the reinforcement material, sealing off the innerlayers. A designer might specify coating board edges where such sealing does not occur, such as with V-scored, punched, or sheared edges.

One area where designers can really help production is to specify where coatings are optional or “don’t care” in the engineering drawing. It is best practice to specify the areas that need to be coated and the areas that don’t as well as the “don’t care” areas to help the coating process run as smoothly as possible.

Another helpful tactic is placing connectors and components that must not be coated along one edge of the assembly to simplify the conformal coating application process. This may allow dip coating to be explored as a potential alternative methodology, speeding up application times and reducing costs. Also, avoid large arrays of discrete components, which can pose a huge coating challenge due to the high levels of capillary forces present. The net result is often areas of no coverage/protection on the board as well as areas of excessive thickness that are prone to stress-cracking, de-lamination, and other coating defects.

Similarly, tall components present challenges of their own by the creation of shadowed or hard to reach areas. Splashing is another associated problem. The trick is to avoid placing tall components next to “must-coat” components to avoid this eventuality.

2. How is coating thickness specified?
When specifying the thickness of coating on an engineering drawing, the industry practice is to measure coating on a flat, unencumbered area of the assembly, and not on items like component leads. As many modern assemblies are very component-dense, it is often difficult to find a flat, unencumbered area of the assembly. Consequently, it is a common and accepted practice to use witness or process control coupons for such measurement.

Designers will need to know that the “nominal thickness” measured on flat, unencumbered areas or witness coupons will have no relation to the thickness of coating achieved on the corner of a discrete, or the leads of ICs, where thickness might be on the order of 1 micron or less, with a nominal thickness of 25 or 50 microns. In a recent study performed by the IPC’s “Conformal Coating—State of the Industry,” there were some very eye-opening results for many folks who assumed they were getting 25 microns everywhere. 

Again, the key takeaway is that nominal coating thickness is a process indicator only; the actual coverage on leads and components is far more important to the reliability of the assembly and this should be understood by all parties. The goal is the greatest degree of coverage possible, and cross-sections should be performed to understand how the application process is delivering this critical-to-success parameter. The witness coupon will only tell you if your process has changed significantly.

3. Why is coating cycle time important?
The coating cycle time is a critical requirement in Lean manufacturing to ensure the production takt time can be met to maintain a balanced production line. It is advantageous for the coating cycle time to be as fast as possible to ensure any accumulated production can be cleared as rapidly as possible, should any previous operations suffer a stoppage.

If we look at a selective coating machine as an example, depositing stripes of coating material that can be programmed to stop and start on demand, the coating stripes can be deposited to create a pattern, avoiding areas such as switches, connectors and test points which must not be coated to prevent interference with form, fit or function. The coating stripes deposited are usually in the 8–15mm range, for optimum accuracy and minimizing overspray and splashing. When the area to be coated is less than 8 mm in width, then it is necessary to utilise a dispensing step, which is a cycle time killer. Due to the combination of machine X/Y positional accuracy, material fluid dynamics and component topography, 2-3 mm is usually as close to keep-out areas as anyone would be comfortable coating for a repeatable process. So, must-coat and keep-out areas within 2–3 mm of each other present a problem and, again, dispensing will be required, another process step which kills cycle time.

4. How to improve coating flow
For many coatings, it is difficult to limit the flow of coating from the application site to adjacent sites, as the coating will have some level of capillary flow (often referred to as wicking), taking the coating to unanticipated areas. This brings us back to the importance of avoiding putting components which must not be coated close to components which must be coated. Placing a keep-out zone at the base of a tall component, such as an electrolytic capacitor, will create a lot of problems in manufacturing as the coating flows down the tall part into the keep-out zone.

Coating flow is also important on low standoff devices, such as BGAs or CSPs, which may have microvias under them. Coating can wick under the parts and down the vias to the opposite side of the assembly. To avoid this problem, the vias under low standoff components can be tented with solder mask or have the vias filled with solder or a suitable via-fill material. In addition, the presence of conformal coating beneath BGAs, for example, can significantly reduce the reliability of the solder joints. The coating process should be designed to aim to minimise coating flow beneath BGAs or use an underfill material to improve the mechanical reliability of these devices.

5. What other processes can impact the coating?
Flux residues from “no-clean” solder pastes and selective soldering or wave soldering can significantly impact the ability of the coating to adhere and perform its function of protecting the assembly. Compatibility studies should always be performed prior to the implementation of coating over a “no-clean” process and care must be taken to ensure the soldering processes remain under tight controls.

The designer should ideally be aware of what kinds of manufacturing practices may occur following the application and cure of the coating, as other materials such as thermal greases/putties and rework/repair chemicals can all have an impact on the integrity and overall performance of a coating. Also, when selecting adhesives for assemblies, care should always be taken to ensure that they are compatible with the selected coating materials and processes. Adhesives that are not compatible can have a detrimental effect on the overall performance of the coating.   

Having started the conversation about the importance of making sound early-stage design decisions, it is important to understand "what affects what" on the surface of the board to ensure successful conformal coating is achieved. Implementing these lessons will prevent potential production disasters not only with conformal coatings, but also in other areas of production as well. In my next column, I will be covering more coating tips and tricks.

This column originally appeared in the January 2022 issue of Design007 Magazine.

Phil Kinner is the global business and technical director of conformal coatings at Electrolube.  Download your free copy of Electrolube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, and watch the micro webinar series “Coatings Uncoated!

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2022

Sensible Design: Top Design Tips for a Successful Coating Process

01-26-2022

Board design, cycle time and coating flow can all impact on a designer’s path to a successful coating process, so what are the key factors to bear in mind to achieve the best outcomes for your coating processes? In this months’ column, I’ll be highlighting some of the key challenges and commonly asked questions about conformal coatings that every Design Engineer should take into consideration when specifying the coating process. As assemblies become ever more densely populated, and housing /casing designs become more permeable to save weight, the use of conformal coatings is becoming essential to protect the assembly from its operating environment and ensure acceptable reliability for the application intended, especially when operating in hostile environments. Let’s explore some of the key factors that will help following our five-point Q&A format.

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Sensible Design: The Rise of Resins in IoT Applications

01-05-2022

The Internet of Things (IoT) is a platform enabling embedded devices connected to the internet, to collect and exchange data with each other. Devices can begin to interact and work with each other, even learning from each other’s experience as humans do. The potential for the IoT and connectivity is endless, as everyday objects can connect and share intelligence and knowledge. What role do resins play?

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2021

Sensible Design: Are Encapsulation Resins Suitable for EV Applications?

10-27-2021

In this month’s column, I will be exploring how resin chemistries can be incorporated by design engineers facing ever increasing new challenges from E-mobility applications.

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Sensible Design: How to Increase the Reliability of Conformal Coatings

09-23-2021

For this month’s column, Phil Kinner examines the main methods for achieving the best outcome for your conformal coating process, ultimately increasing the protection afforded by your coating and lifetime of your product.

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Sensible Design: Selecting Resins for Effective Resistance in End-Use Environments

08-24-2021

In this series on resins, I started by going back to basics, questioning the core rationale for resin choice and application. This month, let’s take a closer look at the fundamental chemistries of potting and encapsulation resins and examine how each resin type offers individual properties that can be exploited to maximise performance under a wide range of environmental conditions.

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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

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Sensible Design: Conformal Coating Vs. Encapsulation Resin

05-24-2021

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Sensible Design: Growth and Trends in the Thermal Management Market

04-20-2021

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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

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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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