Environmental protection for electronic assemblies

5 min read

Paul Whitehead, strategic accounts manager at adhesives specialist Intertronics, explains how to approach the environmental protection of an electronic product.

A product development team’s worst nightmare — designing a brilliant product only for it to fail at the environmental testing or qualification phase. Protecting an electronic assembly from its environment throughout the product’s lifetime is not always an easy task, so what can design engineers do for the best chance of success?

The best time to consider environmental protection is as early as possible during the design stage. By looking at the product’s working environment, establishing what you are protecting against and what level of protection you need, you can build a product that is much more likely to pass testing first time.

Manufacturers typically protect electronic assemblies from dust, debris, moisture, pressure, impact, vibration and chemicals. Defining the environmental conditions will help determine which tests you will need to conduct on the product.  For example, do you need ingress protection (IP) rating against dust and/or water, and if so, what level? There is a big difference between IP63 (protected from direct sprays up to 60⁰ from the vertical) and IP68 (protected against long periods of immersion under pressure).

Many devices will need protection from extreme temperatures, which means conducting thermal cycling testing to ensure the product won’t fail in hot or cold environments. Relative humidity is another important factor, with different testing conditions based on the final environment the product will be used in. You may also need to explore a flammability rating, like UL 94, which has six classifications from HB to 5VA.  If the product will be used indoors or inside a passenger vehicle, meeting UL 94 flammability standards will be more pertinent than for an outdoor product.

As well as defining the environment, consider the sector that the product will operate in. Companies that supply into military applications will also need to consider Mil-Spec testing, which assesses physical characteristics and operational durability. If you are manufacturing a wearable device, compliance with ISO 10993 or meeting skin sensitivity requirements may be needed. If you operate in an industry where intellectual property is important, you may wish to consider protecting your design by encapsulating or potting in such a way that it helps prevent your competitors from reverse engineering your product. The protective material's characteristics will be important — for example, cured hardness will be a factor when looking at vibration protection or where there are significant differences in the coefficients of thermal expansion in the assembly.

Once you have established what you want to protect, what level of protection is needed and what testing can be done to assess for it, you can start looking at how to do it. Design engineers can consider two approaches to environmental protection, primary and secondary. While primary protection can be an enclosure (sealed or gasketed), overmoulding or complete potting, secondary protection involves adding layers of polymeric materials, either by encapsulation or conformal coating. The level of protection depends on the design; for example, if the electronics will be housed in a sealed box which offers primary protection, then no additional protection or a conformal coating might be additionally used. However, if the electronics will be in an open top enclosure, potting/total encapsulation might be a more suitable option.

One recent example of a potting application for secondary protection comes from a project with sustainable fishing start-up SNTech. The company’s flagship product is a sophisticated kit of ten LED lights that fits onto fishing gear to enable more precise fishing. When the team were developing a second version of the device for deep sea fishing (suitable for being submerged up to 1,000 metres), SNTech got in touch with Intertronics for help building a pressure-resistant device.

Because light colour and intensity were important to the device’s functionality, Intertronics recommended Opti-tec 4210, an optically-clear potting compound for the top of the device, and IRS 3071, a more cost-effective but non-optically clear potting compound for the bottom of the device. Intertronics also supported SNTech with building a process, to ensure the materials could be used effectively and bubbles weren’t introduced during the potting process. The device has since passed pressure tests at 100 bar and is commercially available.

Materials for environmental protection

The most common chemistries for the environmental protection of electronic assemblies are polyurethanes (PUs), epoxies, silicones, acrylates and modified acrylates, acrylated urethanes, and fluorochemicals. An experienced adhesives partner can support you through the specification process, advising you on which chemistry will be best to meet your technical, process, and aesthetic requirements, to help you narrow down a small group of materials to test. For example, if you needed a material with high thermal conductivity (for performance), an extended pot life at room temperature and a suitable rheology to fill gaps (for process), Intertronics may recommend trialling Polytec TC 437, which is a two-part thermally conductive epoxy. Material selection is always a complex balancing act.

Materials for environmental protection cure via different mechanisms, including chemical (mixing two parts together), or the addition of energy (heat or light), or a combination of these. If there are a number of chemistries that would meet functional requirements, it is worth thinking about process and cure in making a selection. Consider both the costs and process resources needed for application and curing for any candidates. Two-part materials may need mixing equipment. Thermal or light curing products will need ovens and lamps. Think about capital costs, and running, maintenance and training overheads for additional equipment; this may be offsetable against the benefits of production speed and productivity.

Manufacturers who want speed of cure may wish to consider a light curable material, as these cure “on demand” in seconds. One good example comes from Intertronics’ work with Nottingham Trent University’s Advanced Textile Research Group on the encapsulation of electronic yarn, where the team selected a UV curing resin because it is solvent-free, gives process time improvements, and avoids potential damage to components from process heat.

Another interesting example comes from a customer who manufactures electronic tracking devices that need to survive extremely aggressive testing, exemplified by a ten-minute battering with a sledgehammer. The customer’s original polyester potting compound was evaluated to protect the electronics, but its uneven cure led to exothermic hot spots and shrinkage, causing component damage. While inexpensive, the polyester was also not sufficiently shockproof, shattering under impact testing.

Intertronics recommended the IRS 3071 flame-retardant polyurethane potting compound, which was found to be waterproof, impact and thermally resistant, as well as easy to process. Importantly, its electrical transmission characteristics allowed the transmission of tracking signals, and the fire-retardant specification helped with fire proofing.

Once a material has been narrowed down, deciding on the application method is typically dependent on the production volume. If you have high volumes, try to choose a material that is readily automatable, and if you have low volumes, try to choose a material that lends itself to manual production. For conformal coatings, encapsulants and potting compounds, application methods vary from spraying, dispensing, or metering, mixing and dispensing — any of these can be manual processes, or can be semi- or fully automated with robotics. There is a broad range of equipment to choose from depending on your accuracy and repeatability requirements, and whether the materials are single part, or multi-parts requiring mixing.

Materials can be supplied in various types of packaging: in bulk containers, aerosol cans, pre-filled dispensing cartridges or syringes, twin-pack sachets or bags, and so on. For a low-volume application of a two-part potting compound, purchasing the material in a twin-pack bag will offer processing and handling advantages. If the application grows to a higher volume, a metering, mixing and dispensing machine may make financial sense; its cost can be offset against a faster automatable process and material savings by purchasing in bulk.