SEMICOSIL®

Silicones are characterized by a property profile that makes them interesting for many applications. Silicone elastomers together with silicone gels, which are less densely crosslinked, are permanently stable up to at least 180 degrees Celsius. They have a strongly water-repellent (hydrophobic) surface, are electrical insulators and attenuate mechanical vibrations.

In many applications, it has proved particularly useful for the physical and technical properties of silicone elastomers and gels to remain unchanged over a wide temperature range (between -45 and +180 degrees Celsius); not even continued thermal and mechanical loads (even when exposed to oxygen and ozone) will cause them to age.
In this respect, they differ from all organic elastomers. The latter show distinctly less thermal stability, and, with time, they become brittle and crack.

To cure UV-active silicone encapsulants, electronics manufacturers need a UV irradiation system. The UV radiator must have a radiation spectrum that can stimulate the platinum catalyst to convert to its active form.

In order to determine ideal curing conditions, WACKER SILICONES is cooperating for instance with Fusion UV Systems, a manufacturer of UV irradiation systems.
“The aim of our joint project is to define a process window for UV activation of the silicones – so that silicone processors can integrate the curing process smoothly and to the best advantage in their production process,” explains Petra Burger, a technical service manager at Fusion UV Systems’ German subsidiary.

All commercial UV radiators generate UV radiation by excitation of metal atoms. This involves supplying external energy to raise the outer electrons of the atoms to a higher energy level. When these electrons subsequently fall back to their “normal” energy level, they emit energy as UV radiation.

Which metal atoms are used depends on the desired radiation spectrum. To activate the platinum catalyst contained in the new silicone system, UV light in the 250 – 400-nm wavelength range is required (1 nm = 1 nanometer = 1 millionth of a millimeter).

A special feature of UV radiators supplied by Fusion UV Systems is that the metal atoms are energized by microwaves. Radiators based on this technology have an exceptionally long service life during which neither the radiant power nor the emitted UV spectrum varies. A UV irradiation setup of this kind comprises the irradiation unit, i.e. the actual UV radiator plus the microwave generators, the necessary control technology and a cooling-air system. The irradiation unit is enclosed in an opaque housing to ensure safe operation of the system and to

protect the operator from UV radiation.

Coating

To coat a PCB populated with components, electronics manufacturers often spray the liquid silicone onto the components or spread it over them with a slot die. In the latter case, the slot die moves over the board and the silicone flows down like a liquid curtain onto the components.

Potting

In many applications, a component or circuitry assembled in a housing has to be potted. To this end, a low-viscosity silicone is poured into the space between the electronic component (or the circuit) and the walls of the housing until all the components are covered. The silicone then cures.

Glob-Top Process

A small amount of silicone encapsulant is dripped from above onto the component to be embedded. After curing, and provided it has the “right” flow properties (not too thick and not too thin), the silicone forms a dome-shaped covering, or glob-top, over the substrate.

Dam-and-Fill Process

Sometimes it is expedient to first frame the component to be embedded with a dam of shear-thinning encapsulant. Next, a low-viscosity silicone is dispensed over the component and fills the area inside the dam. A shear-thinning compound is non-sag as long as it is not subjected to shear forces, but liquefies if it is. The stronger the shear forces, the less viscous the material becomes.

Once the shear forces are removed, the viscosity immediately increases again to its original value, and the compound regains its non-sag property. When the shear-thinning silicone cures upon UV activation at room temperature, the geometrical shape applied – in this case by the pre-formed dam – becomes permanent.