The easy handling and fast, “on demand” curing of light-curable encapsulants helps electronics manufacturers improve productivity and reduce operating costs. These benefits, along with their technical properties, make them a suitable candidate to test for PCB encapsulation projects. However, they do have some limitations, which new materials have been designed to overcome. Here Kevin Brownsill, Learning and Development Manager at adhesives specialist Intertronics, discusses light-curable materials for PCB encapsulation, and explains how dual-curing materials can overcome their limitations.
Encapsulation is widely used to protect components and their interconnects from their environment, provide electrical insulation, and offer mechanical support. Typically, a manufacturer will start by establishing the level of environmental protection required, including thermal shock, cycling, and vibration, and understanding any regulatory requirements, such as ISO 10993 for some medical electronics, Mil-Spec for military assemblies, NASA outgassing, etc.
There are numerous materials available for manufacturers to trial, typically based on epoxies, silicones, or light-curable materials. Depending on the application, there are formulations available with properties including: high flexibility, optically clear, or thermally conductive. Choosing a formulation involves considering many factors including adhesion, viscosity, hardness, modulus, environmental resistance, thermal conductivity, cost, and processing methods.
Process benefits of light-curable materials
Light-curable materials offer manufacturers significant productivity benefits over other encapsulant formulations, due to their easy handling and quick “on demand” curing (5 to 15 seconds). This can save hours, or even days, of cure time compared with alternative encapsulants.
As well as their fast curing, light-curable encapsulants are single part and do not require mixing, unlike most silicone alternatives. There may be some technical benefit over other materials — light-curable materials have a lower modulus than epoxies and are typically more flexible, which can reduce stress on components/wire bonds.
Light-curable encapsulants are generally considered good candidates for glob top and chip on board (COB) applications, protecting wire bonds, encapsulating components on flexible substrates, and safeguarding other areas of vulnerable circuitry. Overall, these materials cut costs and processing times associated with electronic assembly, improving productivity.
Curing shadowed areas
To cure fully, light-curable encapsulants must be accessible to UV light, which means that curing is more difficult in shadow areas where light cannot penetrate. While these materials are typically thixotropic and form dome shaped protection over components — limiting the amount of material that flows under components — shadowing can still occur under larger chips and surface mount components.
To aid with the curing of shadow areas, manufacturers have introduced materials with secondary cure mechanisms (heat or moisture). The bulk of the curing is done using UV light, while the secondary moisture cure eliminates the need for a secondary heat cure or other complex processes. The moisture cure happens over some hours/days after the light cure, and requires no active processing. Ambient atmospheric moisture is enough to effect the cure.
Dual cure encapsulants
One example product designed for applications with shadow areas is the Dymax 9000 Series, designed with UV/visible light curing and a secondary ambient moisture-cure system. Moisture cure is achieved in two days, compared with the typical period of seven days with other systems. The materials in this series are available in multiple viscosities to allow the optimisation of performance and dispensing. Because it can be shipped and stored at room temperature, the product offers easier handling than many other dual cure products, which require frozen/cold storage and shipping, due to the inherent moisture adsorbed in the fillers used to provide thixotropy.
Manufacturers may be concerned that the formulation changes required to achieve dual-curing could result in performance compromises. To investigate, Dymax tested the heat-humidity and thermal shock resistance of a number of dual-cure encapsulants on solder masked populated PCBs. The latest formulations have been assessed to show superior performance in reliability tests such as heat and humidity resistance (85°C / 85% RH), thermal shock resistance (-55 °C to +125 °C), and corrosion resistance against salt spray and chemicals
Choosing a material
For manufacturers involved with COB, chip-on-flex, wire bonded assembly, or any vulnerable electronics, selecting the right encapsulant is a complex balance of technical and process considerations. New dual curable materials are a compelling option for encapsulating electronics, offering room temperature storage and shipping, easier curing of shadow areas, and excellent performance characteristics.
An experienced adhesives provider can guide you through the materials that are available on the market, and make recommendations on the best candidates to test. They can support you with building a process that is repeatable, accurate, and productive.
For more information on UV curing adhesives for the electronics industry, visit https://www.intertronics.co.uk/product/uv-light-curing-adhesives-for-electronics-assembly/.
