Feature: Enclosures
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, causing bubbling in the material. To avoid this, the material must be kept dry during storage and application.
Epoxy potting compounds Epoxies are a cost-effective option that offer high temperature and chemical resistance and perform well in salt water and mineral/oil-based environments. Tese materials are hard-wearing and environmentally resistant, suitable for automotive and marine applications. One key drawback is that epoxies are prone to yellowing due to the bisphenol in the resin, therefore they are selected in applications where yellowing is not an important factor. Epoxies are oſten used for smaller
applications, as their exotherm during cure in larger volumes can lead to loss of clarity. Teir 2:1 mix ratio makes them easy to handle, and they are available both in twin-packs and bulk. A typical starting point is 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 for front and rear coverage.
Silicone potting compounds Silicones offer good clarity, are suitable for potting large areas and are relatively soſt when cured. Tey 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. Tis soſtness can be a disadvantage, too, requiring an additional lens, since the product may not be hard enough to form a lens itself, and is susceptible to abrasion. Silicones are UV resistant and tend not
to discolour, whilst offering temperature resistance to 200˚C. We typically recommend to with Wacker RT 601 or 604. RT 601 is a highly transparent, low- viscosity material that offers medium-cured hardness, with 604 being a soſter material.
36 May 2024
www.electronicsworld.co.uk
Figure 2: Dymax LED UV light curing adhesives, coatings and temporary masking products are optimised for use with LED UV curing lamps
Silicone potting materials are easy to
handle and apply. Cure times are usually long – a day or two. Whilst the curing process can be accelerated with heat, too high a temperature can impact the material’s clarity – so it’s a careful balance. One example comes from our work
with a manufacturer of outdoor signage and lighting products. Te customer wanted to encapsulate its LED strips to protect them from weather and other external factors, but was experiencing quality issues with the materials it was testing, such as delamination. Tis was aesthetically unacceptable as it altered the colour temperature of the light output and had the potential for functional failure. We recommended that the customer
evaluate Wacker Silgel 612, which provides good adhesion, optical clarity and is waterproof. Its gel-like texture means it can absorb shock or vibration, protecting the electronic circuitry from damage and thermal cycling.
Processing requirements When specifying a potting compound for an LED assembly, it’s worth considering the material’s packaging. For low-volume applications, a cartridge or twin-pack is most suitable, whereas for high volumes, bulk containers are more efficient.
PUs are the most flexible chemistry in
terms of packaging, and they are static mix, which makes them a good option for trials, smaller runs or R&D projects. The materials used for potting LEDs
are typically two-part, meaning they must be both mixed correctly and dispensed accurately. Available process options range from manually mixing and dispensing from a twin-pack; dispensing from a cartridge through a static mixing nozzle; right through to metering, mixing and dispensing equipment, fed from bulk containers. Epoxies and silicones typically only come in twin- packs and bulk, and many manufacturers find success with a 2-K-DOS metering, mixing and dispensing system. For applications requiring the highest accuracy, a Preeflow eco-DUO precision metering, mixing and dispensing system offers accuracy of ±1%. This system can be combined with a robot that delivers positional accuracy. Considering the processing requirements
early on can deliver better returns on investment. For cost-effectiveness, it is worth investing in a preliminary technology and adding to it later. An experienced materials and equipment provider can recommend suitable materials and processing equipment.
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