LED Technology Choosing an LED potting compound
Potting and encapsulation are commonly used to protect LED assemblies from shock, vibration, the environment, and for thermal management. Depending on the application’s needs, there are numerous potting compounds available, with formulations offering properties including being low density/ lightweight, flame retardant, flexible, optically clear, and thermally conductive. It can be challenging to know which material to specify, given the different chemistries, properties, and processing requirements. Here Kevin Brownsill, head of technical: learning and development at adhesives specialist Intertronics, shares insight on potting compound selection for an LED assembly
L
ED applications exist in automotive engineering, indoor and outdoor lighting, marine lighting, signage, infrastructure, architecture and interior design, and more. Manufacturers may need to protect these electronic assemblies from dust, debris, moisture, pressure, impact, vibration, and chemicals. With environmental protection, there are two main options — seal them into an enclosure with a lens, or pot them with resin. LEDs generate heat as well as light, and so there are often requirements for thermal management of LED PCB assemblies. Potting the non-optical side of the assembly with a thermally conductive potting compound can help with this.
The level of protection required from potting depends on the final environment the product will function in. For example, underwater swimming pool lights must be fully waterproof to IP68, as well as have good resistance to chlorine and other chemicals. Companies that supply into military assemblies, such as lighting for war ships, should consider Mil-SPEC testing of physical characteristics relating to operational durability, ruggedness, and reliability. If the potting compound is being designed into a medical device, the designer will need to consider meeting ISO 10993.
Choosing a material
After establishing environmental protection level required and determining the industry’s testing regime, the design engineer can begin to narrow down an appropriate chemistry. This involves considering many factors, including the desired harness, resistance to yellowing, clarity, thermal conductivity, cost, and processing methods.
For products where the LED must be seen, designers typically specify an optically clear coating on the front of the product. On the rear of the device, this is less important and
36 November 2023
developing a pressure-resistant version of the device for deep sea fishing (suitable for being submerged up to 1,000 metres) and required a potting compound. Because light colour and intensity were important to the device’s functionality, we 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.
Pisces Deep - courtesy of SNTech
designers may consider a different material that offers the desired thermal management characteristics. For LED assemblies that require a clear finish, there are three main chemistries, polyurethanes (PUs), epoxies, and silicones, which each bring advantages and disadvantages.
Polyurethane potting compounds We typically recommend that customers test polyurethane materials first, due to their versatility, hardness, reasonable price, and UV resistance. These two-part materials cure at room temperature and their 1:1 mix ratio means they are easy to handle and use. PUs are available in various packaging styles, from small to large two-part cartridges, twin-packs, and bulk containers. They have good long-term UV resistance, and can be formulated with anti-UV additives that avoid yellowing, making them popular for outdoor applications. Many of our customers start by trialling
Components in Electronics
Opti-tec 4200, a clear polyurethane resin. Available in double syringe or twin-pack sachets, the material is easy to mix and apply and cures to a clear, scratch resistant finish. It is also available as Opti-tec 4220, a semi- opaque, milky-white version that provides an aesthetically pleasing finish by allowing a high transmission of light, while hiding underlying electronics.
One drawback of PUs is that they are hygroscopic and are therefore sensitive to moisture before cure — PUs readily absorb moisture from the air, which reacts to form a gas and can cause bubbling in the material. To avoid this, the material must be kept dry during storage and application. One recent example of a polyurethane potting application is our 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. SNTech was
Epoxy potting compounds Epoxies are a cost-effective option that offer high temperature and chemical resistance and perform well in salt water or mineral/ oil-based environments. These materials are hard-wearing and environmentally resistant, and are used in automotive and marine applications. One key drawback is that epoxies are prone to yellowing due to the bisphenol in the resin — they are therefore selected where yellowing is not an important factor.
Epoxies are often used for smaller applications, as their exotherm during cure in larger volumes can lead to a loss in clarity. Their 2:1 mix ratio makes them easy to handle, and they are available both in twin- packs and bulk. A typical starting point to trial would be Opti-tec 5020, a material designed for applications where optical clarity, good wetting, and low viscosity are important. It is available in both black and clear versions, which could be considered for both the front and back of a device.
Silicone potting compounds Silicones offer good clarity, are suitable for potting large areas, and are relatively soft when cured. They are available in gel formulations, where they retain a jelly-like consistency even post cure, which can be advantageous for products in environments that are subject to extreme shock, temperature or thermal cycling. This softness
www.cieonline.co.uk.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70
