Effective Resin Chemistries for the Protection of Electronic Components
Alistair Little, Technical Director of Electrolube’s global resins business, describes the different types of circuit and component encapsulating resins, providing some background on their properties and applications, with examples from Electrolube’s own product portfolio.
Resins are widely used for potting and encapsulation in the electronics and electrical industries, and are generally found in three major categories, depending on their chemistry; epoxy, urethane and silicone. Electrolube develops, manufactures and supports a wide variety of resin products within these categories.
Epoxy Resin Chemistry
Epoxy resins enjoy wide popularity among electronics industries due to their excellent electrical and mechanical properties, as well as their ability to offer protection against chemicals and high temperatures. Their use enables protection for electrical components against dust, moisture and short circuits, and as standard epoxy resins have better thermal conductivity than air, they provide more efficient dissipation of heat from components, thus prolonging service life.
Today there are a wide range of different epoxy based resin systems available, offering a wide range of different properties. The curing process for epoxy resins is generally slow, but a range of different cure speeds and cured properties can be obtained using amines, amine modified polymers or polyamides as hardeners. However, while the reaction can be very fast, it can also be very exothermic, leading to the possibility of a runaway reaction. This can be mitigated by modifying the hardener chemistry or by using fillers, which will help absorb the heat of reaction and which can be used as a flame retardant. Note here that the temperature at which a resin is cured will affect not only its cure speed, but also the quality of the final cured resin. It is therefore good practice to carry out some trials before committing to a specific cure rate.
The increasing use of electric vehicles has led to a change in the market requirements for many resins, but particularly epoxies. Since e-mobility is a fast changing and evolving market, the various performance criteria required for the components of the vehicles is constantly evolving and improving. Since resins are used in many areas on a vehicle, from sensors and displays to the motors themselves Electrolube is at the forefront of providing solutions to meet the requirements of the automotive electronics industry.
It is almost a given that with each new design generation of components, they reduce in size but will become more powerful. Generally speaking, increased power will equate to more heat being released, hence the drive to develop new resins with improved upper temperature operating ranges, this is relevant for both for continuous and short term operating temperatures. There is also a requirement to increase the thermal conductivity of resins enabling improved heat dissipation from electrical components.
Electrolube’s epoxy resin portfolio includes a variety of clear, white and black, single- and two-part products with a host of useful properties to meet most requirements from potting and sealing to dipping, including exceptional electrical and thermal characteristics, flame retardancy and resistance to chemicals and fuels.
Polyurethane Resin Chemistry
Generally epoxy resins provide a hard and rigid encapsulation when cured, whereas polyurethane resins are elastomeric or rubbery in their cured state, which is particularly useful if the circuit to be potted contains delicate components. Like epoxy resins, polyurethane resins provide chemical, dust and moisture resistance, as well as excellent electrical insulation and good adhesion to most substrates, both metal and plastic. Unlike their epoxy counterparts, polyurethanes generally have a lower exotherm during cure, even for the faster curing systems. However, it is important to note that in general cured polyurethane resins should not be allowed to rise above 130°C continuous service temperature.
Polyurethane resins are widely used in a wide range of applications, where their inherent flexibility reduces the amount of stress applied on components compared to epoxies. This means that components with thin or fragile legs can be potted with ease. Also a number of polyurethane resins are thixotropic, meaning that the viscosity is low during mixing, but once poured, the resin viscosity rapidly increases, allowing for the accurate placement of a resin and will not flow through any small holes on the PCB.
The exponential increase over the past few years in the use of LED’s has led them to become the default lighting choice for the majority of applications. However LED’s do require some level of protection, depending upon the environment they are exposed to. Optically transparent polyurethane resins have become a very popular choice to protect LED’s in the more challenging environments. These resins have been specifically developed with a fully aliphatic polymer backbone to offer the best yellowing resistance due to UV degradation. In many cases designers now look at the use of resins not just to provide a level of protection for the lighting unit, but also to improve the aesthetics. Depending upon the resin choice, the LED’s can appear as discrete light sources, to a solid bar of light.
As previously, mentioned most polyurethane resins have an upper operating temperature of 130°C, but by careful formulation it is possible to increase this maximum temperature limit to 150°C, yet still retain the same flexibility and chemical resistance that would be expected for a polyurethane.
Electrolube’s polyurethane resins come in white, black, blue, clear straw, hazy/cloudy and optically clear formulations and, in common with the epoxy range, offer a host of properties to meet the needs of high-temperature environments and those exposed to chemical contamination, mechanical stress or shock and moisture ingress.
Silicone Resin Chemistry
As with some polyurethane resins, optically clear silicone resins have superior resistance to UV light, and have been shown to maintain their clarity throughout rigorous laboratory exposure testing regimes. This well-documented UV resistance makes them ideal for LED applications where resin colour stability is important to achieving minimal colour temperature shift of the LED lighting unit itself.
Though not as popular as epoxy and polyurethane resins, silicone resins do offer some distinct advantages when used as an encapsulating resin because the cured products have a high degree of flexibility, excellent chemical, dust and moisture resistance, and good electrical insulating properties. Silicone resins tend to be more expensive than epoxies or polyurethanes, but are ideally suited where high continuous operating temperatures (200°C) are required. Moreover, the exothermic temperature when working with silicone systems is very low indeed, ensuring compatibility with heat-sensitive components.
In contrast to the epoxy and polyurethane curing mechanisms, silicone curing can take between 24 and 48 hours to fully cure, dependent upon the potting depth or sample thickness and the geometry of the unit to be potted. A limitation is that silicones often require moisture to be present to drive the reaction forward, so attention needs to be taken concerning the curing conditions, particularly with respect to humidity. Silicone resins also demonstrate excellent adhesion to most substrates, including both metals and plastics.
In addition to optically clear resins, Electrolube also supply other pigmented two-part silicone resins for a range of applications. These can have diverse requirements, from extreme temperature resistance, to excellent flexibility and high thermal conductivity. For certain industry segments silicones offer many advantages.
The majority of Electrolube resins are two component systems that, when mixed together in the correct ratio, react to form polymeric materials. By careful formulation, the properties of the cured resin can be tailored to meet individual customer requirements
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