Silicones / AL32 / AL52
The ÅngstromLink AL-32xx Series silicone gels are chemically inert, optical grade encapsulation gels, designed especially for sensitive optical assemblies. They offer a service temperature range of -40ºC (-40ºF) to +150ºC (+302ºF).
The ÅngstromLink AL-52xx Series offers a range of chemically inert optical grade fluids designed especially for sensitive optical assemblies. These fluids have a service temperature range of -40ºC (-40ºF) to +150ºC (+302ºF).”
Silicones are remarkably versatile materials that have found their way into many of the most challenging encapsulation applications. Optically clear silicones have enabled the development of challenging applications such as HBLED lighting. These optically clear silicones have high purity, are transparent in the near UV and visible wavelengths (see typical absorption spectrum shown I the left hand graph below), and the refractive index and viscosity can be controlled to a certain degree. Fluid silicones can be used for temporary refractive index matching, prototyping for proof of concept and easily removed/cleaned off. Other applications include cores for liquid light guides. Curable silicone gels do the same refractive index matching with the added capability of staying put and not needing containment. Both fluid and cured forms offer service temperature ranges of -40°C to +200°C (+260°C for 1 to 2 minutes for soldering operations).
The curable silicones have work times and modulus that is controllable to a certain degree. Their elastomeric properties provide protection of sensitive photonics assemblies from mechanical shock, thermal shock, dust, and ambient atmosphere, and soft stress-relief for stress sensitive components and delicate wire bonds. Silicone cure chemistry is a thermal process, controllable by heat, and produces no cure by-products. Adhesion to substrates has everything to do with preparation of the substrates for bonding.
The curable silicones are initially two parts. When mixed together they remain fluid for a period of hours (pot life), and ultimately cure to a soft gel consistency – usually within twenty-four hours at room temperature. No UV exposure or temperature bake is required to initiate the curing process. Elevated temperature will accelerate the curing process and can reduce cure times to minutes (i.e., at a temp of 150°C). The long pot life of these silicones allows the material to fill in voids in a complex assembly and to permit time for any trapped air bubbles to float to the surface and escape. The two parts are supplied in a variety of packages, ranging from separate bottles to ready-to-use cartridges with static mixers. The technical people at FOC are well versed in proper mixing methods, de-airing and dispensing, and can provide valuable support with these issues.
FOC offers curable gels (AL-3200 Series) and fluids (AL-5200 Series) in a range of refractive indices. The refractive index of each product is embedded in the product name by the last two digits – i.e., AL-3252 has a refractive index of 1.52 (@ 589nm). One notable exception in the 5200 Series is AL-5241F, which is a non-silicone fluid that does not have the characteristic absorption bands of silicone around 1200 and 1400nm. The AL-5241F absorption spectrum is shown below next to the AL-3252 for comparison purposes.