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Sensible Design: Resins–Are They All About Chemistry?

In my previous columns, I have looked at many different aspects of resin systems by going right back to basics, questioning the key reasons for potting and encapsulation, examining how different resins systems vary from one another, and exploring how their individual properties can be exploited to maximise performance under a wide range of environmental conditions. I hope this has proven useful.  

When it comes to resin selection and application, there are a plethora of factors to consider. Resins come in many forms and have lists of properties that would challenge even a graduate chemist. For this month’s column, I’m going to take a closer look at thermally conductive resins, flexible resins, elevated cure temperatures, resin types for different applications, and resin systems that enable wider operating temperatures. Without further ado, let’s look at these topics in our signature five-point format. 

1. What are the benefits of a thermally conductive encapsulation resin? 

As electronics have become smaller and more powerful over the years, the amount of heat generated per unit area on a PCB has increased as well; however, it is well known that electronics will perform much better at low temperatures. Thermally conductive resins are designed to allow heat to be dissipated away from sensitive components. The typical thermal conductivity of an unfilled resin is 0.20-0.35 W/m.K. For a resin to be classified as being thermally conductive, it must have a thermal conductivity of >0.8 W/m.K. This is usually accomplished by using selected ceramic fillers, which offer a combination of thermal conductivity and chemical stability. 

2. Which resin chemistry types typically allow the widest operating temperature ranges, and why? 

Providing exceptional performance at high temperatures, silicone resins have the broadest temperature range (-50 to +250°C), but these are generally soft resins and are not as chemically resistant as some of the other resins. Some polyurethanes can go down to temperatures lower than silicones (-60°C) but have a maximum operating temperature of +150°C, while epoxies are designed more for higher temperature applications (-40 to +200°C) but have excellent adhesion to a wide range of substrates and excellent chemical resistance. 

3. Why are some resins suited to different applications?  

It’s all in the chemical bonds! Epoxies are tough yet can be brittle due to the high crosslink density that is possible. But this high crosslink density also means that the resins are very resistant to chemicals and have excellent adhesion to a wide range of substrates. Polyurethanes are generally made up of long flexible polyols linked together by reactive isocyanates, giving rise to the classic hard and soft segment polymer, which means the resins are normally tough yet flexible. The reactivity of the isocyanates also means the resins have good adhesion. Silicones are soft yet very flexible resins due to the presence of silicon in their chemical structure. This makes them very temperature stable and capable of withstanding a wide temperature range, particularly high temperatures. 

4. Is it a good idea to use elevated temperatures to accelerate the curing process? 

Elevated cure temperatures are used to speed up the production process and reduce the cycle time. However, there are a few points that need to be considered. It is best to wait until the material has reached its gel time before subjecting the resin to a high temperature; if this is not possible or desirable, then the use of a temperature ramp is advised.  

In the case of epoxy resins, care must be taken due to the exothermic nature of the epoxy curing reaction, particularly with fast-curing unfilled resin systems. Also, the amount of resin being cast at one time in a unit is critical. A large amount of resin has the potential to generate a lot of heat, which speeds up the curing reaction.  

For silicones, care must be taken when curing as the catalysts used are very susceptible to being poisoned. It is recommended that silicone resins be cured in a separate oven from other resin types. If they are to be cured in the same oven as other resins, then the oven should be well ventilated before putting the silicone resin inside—no other resin types should be present. 

5. Why would I potentially require a flexible encapsulation resin for my application, and what types of applications would typically suit this type of encapsulation resin? 

Flexible resins find a wide range of applications as they can accept and absorb physical and thermal stresses well. If a unit will be subjected to thermal cycling, either continuously or infrequently, then a flexible resin is designed to withstand the stresses induced under such conditions. Similarly, in the case of physical shock, where the electronics need to be protected against vibrations, then a flexible resin will absorb the stresses far more effectively than, perhaps, a more rigid resin. 

Conclusion 

Every customer and customer project is different; while 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, dispensing method/equipment to be used, curing times, and temperature limitations that may be imposed during the production process. The more information that the customer can provide regarding the resin’s ultimate operating conditions—temperature range, likely chemical exposures, and so on—the better.  

Certainly, technical datasheets can be a great help when you embark on a new production schedule with new components and resins, but if you foresee any problems with matching resin types to your production procedures that are not easily resolved by studying the literature, be sure to contact your supplier’s technical support team for further advice.  

Next time, I will take an in-depth look at some of the most frequently asked questions we get as resin experts and explore various options in response to these enquiries 

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