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We don't have any high refractive index materials that cure with UV. The reason is because the UV activators needed to cure the material limit the transmission of the material. Furthermore, the UV exposure to the silicone damages the silicone itself, making it go yellow which further reduces the transmission.UV-cured silicones are usually not suitable for high transmission, high refractive applications.
PCB protection encapsulants are used to protect passive components such as capacitors and resistors from dust, moisture and mechanical impact. LOCTITE ECCOBOND 3838T, a black color epoxy with low modulus to minimize stress on chip cap and UV9060F, a light blue UV acrylate with moisture cure that cures in shadowed areas are the first PCB protection encapsulants that come to mind.
Alternative, more or less colorless, options can be the tried and proven ABP 6892, the clear silicones 8035 and STYCAST SC 3613 and finally the UV/Heat Dual cure LOCTITE 3217. Quite a lot of options depending on your package.
It is also likely that you would need a DAM if you want the encapsulant to stay on the PCB. In this case we can recommend the black ABP 8420 epoxy or the black ABP 8142B silicone. Other potential candidates can be the more thixotropic LOCTITE 3621 red epoxy and LOCTITE 3217 that can be uv cured before the fill operation.
There are two main ways to encapsulate a semiconductor device: transfer molding a thermosetting Epoxy Mold Compound (EMC) or using liquid encapsulation directly on the Printed Circuit Board (PCB). Encapsulation in EMC is better than liquid encapsulation for applications that require higher filler content, and for the same reason, volume for volume, EMC is usually cheaper than liquid.
Liquid casting is however much more flexible. EMC requires are large significant investment in molding machines and mold plates for specific devices which must produce large quantities before there is a return on that investment. Liquid encapsulation on the other hand has a high flexibility and low through put: meaning that it is best suited for a multiple diverse low quantity orders.
Another advantage of potting and encapsulating devices in liquids is that in general, the result is flatter and thinner than devices encapsulated by a transfer molding process around a lead-frame with EMC. The space requirements for devices will continue getting more and more stringent as the component density of devices increases. This trend makes encapsulation and potting in liquids a sure bet for the future.
Transfer molding EMC may also cause wire sweep: where the pressure of the flowing molten compound shifts the fine-pitched bonding wires connecting a die. Wire sweep can cause shorts and mutual inductance. Liquid encapsulation is applied from above and does not disturb bonding wires. That’s why it is the industry standard for larger or extra sensitive dies and chips.
Bare die may be mounted to the PCB with die attach but that’s not enough to protect them or their connecting wires from moisture, mechanical stress, temperature, and chemical corrosion. That’s why it makes sense to insulate and encapsulate them with a protective potting layer of liquid encapsulant.
In non-Newtonian thixotropic encapsulants thixotropy refers to the temporarily lower viscosity of an epoxy gel when shear, pressure, or temperature is applied. This means that they can be gel-like when at rest, but flow like a liquid when agitated.
Being able to control the viscosity of liquid encapsulants means you can dial in how far they will spread over a surface area, and also how high they will build up to. This is important in Glob Top encapsulation where devices are literally encapsulated by a “Glob” from above.
Glob Top is a process of encapsulation by using a fluid encapsulant gel with higher viscosity so that it fully encapsulates a device without flowing beyond the necessary area. As mentioned, by controlling the temperature, shear, and pressure, the viscosity and flow of liquid can be adjusted so that the final surface area and height of the “Glob” can be controlled quite effectively.
Dam & Fill on the other hand refers to a two-step process of encapsulation, where a ring or frame of higher viscosity liquid encapsulant is applied as a “Dam” around a device, and then a lower viscosity liquid “fill” is used to then fully encapsulate a device. The liquid used to build the frame or dam usually has more abrasive filler and a higher viscosity at rest than the liquid used for the fill.
Dam & Fill is a more selective protective coating and can be framed to cover areas in specific shapes, and is for example also used for optical bonding. In optical bonding the frame or dam is used to create a space between the glass and the display or touch screen. Then an optically clear fill is used as an adhesive and fixative.
Optically clear liquid encapsulation is also used for LED devices, and encapsulate optical or optoelectronic devices that require both a high level of light transmittance as well as a good level of mechanical protection. In a similar process to Glob Top encapsulation, a literal glob of clear encapsulant is dropped directly into place on top of the LED. In another production process, optically clear liquid encapsulant is used to cast LED’s final body. Phosphor, colored dyes, and diffusant concentrates can be added to the liquid encapsulant to alter its optical properties.