The new LED encapsulants LUMISIL® 740 and LUMISIL® 770 are two-component systems. They cure at room temperature via a platinum-catalyzed addition reaction. The cured rubber grades have a refractive index of 1.41, which is typical of polydimethylsiloxanes. The two products thus belong to the group of normal refractive index (NRI) encapsulants. They effectively protect the sensitive LED semicon-ductor chip against environmental influences. They can additionally serve as carriers for luminescent dyes, which can selectively influence the color of the light emitted by the LED.
A special feature of LUMISIL® 740 and LUMISIL® 770 is their extremely high heat, light and thermal-shock resistance. Corresponding aging tests confirm this. After 500 hours of storage at 245 degrees Celsius, LUMISIL® 740 test specimens display neither yellowing nor embrittlement. Even after 1,000 hours, the change in the cured rubber grades is insignificant. LUMISIL® 740 has proven particularly resistant to combined heat and light exposure. In a thermal shock test going from +125 to -45 degrees Celsius, both materials withstand more than 1,000 test cycles. The encapsulation compounds are thus able to compensate thermomechanical stresses that can arise as a result of the different thermal expansions of the LED materials.
Due to their stability, the new silicones are particularly suitable for encapsulating LED chips with strong heat generation and intense light emission. The two grades differ mainly in the hardness of the cured products. LUMISIL® 740 cures to form a material with a Shore A hardness of 50, while LUMISIL® 770 is formulated to be slightly harder at Shore A 70. Its property profile makes
LUMISIL® 740 ideal for the encapsulation of multi-chip LEDs that are applied using the chip-on-board technology – mounted directly onto the printed circuit board, tightly packed, without casing. LUMISIL® 770, on the other hand, is the material of choice for encapsulating single-chip LEDs.
The viscosity of the two encapsulation materials has been adjusted so that they can easily be applied by dispensing. LUMISIL® 740 and LUMISIL® 770 are self-adhesive. They adhere to the semiconductor chip and to conventional reflector and casing substrates without pretreatment.