Manufacturers use light-curable electronics encapsulants to benefit from their fast on-demand curing, which reduces operating costs and increases productivity. New light curing technology has incorporated secondary moisture cure to assist with the curing of shadowed areas, but how well does it compare in terms of performance?
Encapsulants are used to protect PCB components against moisture, chemicals, and rapid and extreme temperature changes while providing mechanical support and electrical insulation. Liquid glob top encapsulants provide a protective layer over electronic components, wire bonds and other connections, and are usually based on epoxies, silicones, or light-curable materials.
Light-curable materials and silicones have a lower modulus than epoxies and are typically more flexible, reducing stress on the components, wire bonds, and closer connections. While silicones are often two-part materials, light-curable encapsulants are single part and do not require mixing. A major benefit of light-curable encapsulants compared with conventional encapsulants is their very fast curing, which enables faster processing, greater output and lower operating costs.
A limitation of light curing is the curing of shadowed areas where light cannot penetrate. Recent developments have resulted in the release of dual-curable (light/moisture or light/heat) encapsulants. Secondary moisture mechanisms allow the curing of material over time in shadowed areas not available to UV light, eliminating the need for a second heat cure process or other complicated arrangements.
However, one of the challenges of these dual-cure encapsulants is their relative instability, and the requirement for cold/frozen storage and shipping. For light/moisture cure encapsulants, this is due to the inherent moisture adsorbed in the fillers used to provide thixotropy for the encapsulant formulation. In contrast, Dymax has recently developed a technology that enables the formulation of a high performance light/moisture cure encapsulant with room temperature storage and shipping stability.
But do the process advantages and the friendly shipping/storage benefits result in performance compromises?
Testing the performance of new materials
Dymax tested heat-humidity and thermal shock resistance of a number of commercially available dual-cure encapsulants on solder-masked populated test printed circuit boards, including light/moisture and light/heat varieties. Different locations on each board were encapsulated with a dome shape coverage at 2-2.5 mm thickness.
To evaluate heat and humidity resistance, a humidity chamber was set to 85˚C, 85% RH for 500 hours. Thermal shock resistance was tested by exposing test boards to -55˚C and 125˚C with 30 minutes dwell time at each temperature and 15 second transition time between highest and lowest temperatures. Boards were tested under these conditions for 500 cycles. Any cracks or delamination of encapsulants on and around the components were inspected with magnification.
Custom-designed, multi-pattern FR4 boards were used to test salt spray corrosion resistance according to ASTM B117. Encapsulants were applied at 2 mm thickness to the entire board with a drawdown bar and UV cured. Encapsulated boards were exposed to 5% sodium chloride solution at 35˚c for 500 hours in a salt spray chamber. Upon completion of the test, samples were maintained at 25˚C, 50% RH for a 24-hour stabilization period and visually inspected for the appearance, crack or delamination, and corrosion on the copper by a microscope camera. Encapsulated boards were subjected to a modified voltage transient test before and after reliability tests according to UL-746E.8.
Results
Dymax’s results show that its light and moisture dual-curable, 100% solids encapsulant exhibits an excellent balance of properties, when benchmarked against other commercially available light/moisture and light/heat dual cure encapsulants. After 85°C/85% RH damp heat testing, and also after 500 cycles of thermal shock, it was the only one which showed no delamination, cracking, or yellowing. The Dymax material performed best in a salt spray corrosion test, with no signs of the severe circuit corrosion seen with the other materials.
The Dymax encapsulant cures tack free in seconds with UV light which allows faster processing of parts. Additionally, it cures in shadowed areas relatively fast and it does not require cold or frozen storage and shipping. This material does not require heat to fully cure, making it an ideal choice for heat-sensitive substrates.
For manufacturers involved with chip-on-board, chip-on-flex, wire bonded assembly, or any vulnerable electronics, this material features excellent flexibility and increased durability. It offers manufacturers a compelling option for encapsulating electronics, even where shadow areas may be a concern.
Full details of the testing regimes and results can be found in Dymax’s white paper, available via https://www.intertronics.co.uk/wp-content/uploads/2020/04/wp016eu_high_performance_light_and_moisture_dual_curable_encapsulant.pdf
