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Sensible Design: All Resins Are Not Created Equal

With so many different encapsulation resin options on the market, selecting a resin that is best suited for your application can present a real challenge. Today, there is a wide range of different resin systems available that offer a comprehensive range of different properties. It is often assumed that properties can vary moderately, however, resins can offer significantly different benefits. This month, I will explore the different factors you should consider when comparing resins for your application. Every project will differ in terms of the unit, the dispensing method and equipment that will be used, the cure time, and temperature limitations during the production process.

First, I establish the the resin’s ultimate operating conditions, temperature range, and likely chemical exposures. I will look at resins that have the biggest impact on performance and reliability. Let’s look at where to start and what you need to factor in using our signature five-point format.

Resin Chemistry
It’s easy to assume that all polyurethane resins or epoxy resins would have broadly similar strengths and weaknesses as they belong to the same chemical family, but this isn’t the case. Different materials used in the part A and part B can have a big impact on the performance of the end material. Different fillers and loading can affect thermal conductivity, and various additives can influence processability, cure time, and operating temperature range. An example is whether a polyurethane part A is polybutadiene (polyBD) or polyether/polyester based.

PolyBD resins generally have exceptional resistance to water, even holding up well against saltwater immersion, but are susceptible to attack by organic solvents such as oils and fuels. Resins with a polyether/polyester part A base, however, generally have a better resistance to organic solvents. But while they offer good water resistance at lower temperatures, they typically are not as strong as the polyBD based materials when the conditions are both hot and wet. Even within those broad categories there are differences in performance, which is why it’s best to bring your application and conditions to specialists in the first instance so a resin can be matched with the best combination of properties for your needs.

Thermal Conductivity
The trend for miniaturisation means high power density devices are increasingly common. Thermal dissipation away from hot areas of the device to heat sinks and metal casings is more important than ever.

The thermal conductivity of a resin is influenced by the type of conductive filler used and even particle size distribution and particle shape, as this will affect the packing of the particles, and thus heat transmission, through the material. Thermal conductivities quoted on datasheets are of limited value as different test methods can give different values for the same material. This means that two resins with the same stated TDS value can perform very differently in-application.

Some methods are most accurate when measuring lower thermal conductivity materials, such as the guarded hot plate method, while others such as the laser flash method are particularly strong for testing at high temperatures. Several methods (including both already mentioned) rely on precise test sample dimensions, which means the testing is time-consuming and can be influenced by operator technique and precision. We use a modified transient plane source (MTPS) instrument which is a non-destructive technique suitable for liquids, solids, pastes, and powders, and which gives accurate and repeatable measurements, even for smaller sample sizes.

To achieve the most accurate comparison it is important to test side by side in-application, if possible, to see the real-world heat reduction achieved. This is not just relevant for resins; it also applies when considering thermal interface materials.

Bio-based Raw Materials
Sustainability is becoming ever more of a priority globally, for manufacturers and for us here. We take our ESG targets very seriously and have been devoting a lot of R&D time to the use of bio-based raw materials in our resins, which led to the introduction of our UR5645 resin, a chemically resistant, high temperature bio-based PU. There is an element of futureproofing in using bio-based resins, as synthetic raw materials derived from crude oil will not be available forever. Results demonstrated by our resin development chemist, Beth Turner, have highlighted that there can be significant technical benefits from transitioning over to bio-based raw materials. The results also show that not all bio-based raw materials are created equally. A lot depends on their origin as some cheaper bio-based fillers did not offer the same performance benefits as other options, and users cannot assume that all resins advertised as using bio-based materials will offer a technical advantage. There are numerous opportunities for bio-based encapsulation resins for use in the electronics industry, given the distinct performance advantages they provide in harsh environments, underwater applications, and hot and humid operating environments.

Resin Cost
Comparing the cost of resins can potentially cause confusion because resins are often priced in kilograms but used in litres as the unit to be potted is of a fixed volume—which means density can have a big effect on the actual cost per potted unit.

If we consider two resins which have the same apparent cost per kg, but resin A is 1 kg/L (typical for an unfilled resin) and resin B is 2 kg/L (a highly filled resin), it will cost twice as much to pot a unit of fixed volume X using the denser resin vs. the lighter one. The difference in densities is not usually this extreme as other requirements which are influenced by filler content, such as thermal conductivity and flame retardance, mean it’s unlikely for both a completely unfilled and a heavily filled resin to be in the running for the same application. However it can still be an important consideration when choosing a resin for a cost-critical end-use.

Mixing
Not all mixers are created equally, both in terms of manual operators when using the resin bi-packs for prototyping, and also when using a mix-and-dispense machine for larger production runs. An improperly mixed resin will not cure properly, leaving voids and a sticky or uneven finish. Even if there are no visible defects, it is highly likely the material will not have the final cured properties expected, which could lead to unit failures later on and will certainly not allow a fair comparison between materials at the approval stage.

It’s important to have a good resin pack mixing technique. There are several videos available on our website and our YouTube channel that give a good grounding in the best way to use our resin packs.

If using a machine, it’s important to work with your resin provider and your machine manufacturer to identify a static mixer with the correct attachment for your machine, the correct shape and mixing style, and with sufficient elements, diameter, and length to give the resin a good mix. The resin chemistry, densities, and viscosity difference of parts A and B, as well as the mix ratio, desired flow rate, and shot size, will all influence the amount of mixing required and consequently the selection of the mixer. Depending on whether the resin is filled, heating, stirring, and recirculation may also need to be considered on the machine’s storage tanks as well.

Not all resin manufacturers offer the same level of customer service or technical support, so do ask them what level of support you can expect to receive before embarking on your project with them. I hope you have enjoyed this month’s column and picked up some top tips on comparing resin systems. I cannot stress enough how looking at bio-based resin systems could be of significant benefit when considering a resin system for your next application.

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