Optimum Encapsulation - Wacker Chemie AG


Optimum Encapsulation

Stress Caused by Blue Light

The liquid silicone is applied by dropping it on to the LED in a single operation. It encapsulates the chip, at the same time forming its optical lens. LUMISIL® therefore also helps to simplify production processes.

In practice, the encapsulation materials are simultaneously exposed to heat and light – particularly blue light in the case of white LEDs. The aging process that this causes can be simulated in the laboratory by the blue-flux test, in which disk-shaped test specimens of the cured encapsulants are irradiated at a temperature of 85 °C with blue LEDs operated at a power of 5 watt. At certain intervals, the light transmittance and yellowing of the test disks are measured. Tests show that LUMISIL® 770 retains its high light transmittance even after 1,000 hours. As with every polymer, slight yellowing is detectable in silicone, too. However, it is not apparent to the naked eye and can be considered negligible.

Corrosive gases, too, cause LEDs to age. This is particularly noticeable with the effect of hydrogen sulfide, which is mainly emitted in exhaust fumes in large towns and can pose a problem for exterior lighting applications. In the interior of the LED packages, many surfaces are silvered so that they reflect the incident light toward the encapsulant. However, silver tarnishes immediately in the presence of hydrogen sulfide – even trace amounts – due to the formation of black silver sulfide. This sulfur corrosion may be severe enough even to show through the encapsulation. The encapsulation should therefore also act as a barrier to hydrogen sulfide and other gases that can form hydrogen sulfide.

The sulfur corrosion test shows that the two high-refractive silicones act as a particularly effective gas barrier – due to the presence of phenyl groups in the molecular structure. “The best performance is exhibited by LUMISIL® 591,” says Dr. Yang, who subjected the silicone to extensive applications testing at COEE in Seoul. “The light output of LEDs encapsulated with this particularly hard material decreased least in the test as a function of the exposure time to sulfurous gases.” He added that, with this product, the extremely high crosslink density had an additional inhibiting effect on gas diffusion.

Bond Wires at Risk

APPLICATION AREAS

  • For LEDs intended to operate at high powers and with maximum luminous flux, encapsulation with LUMISIL® 740 or LUMISIL® 770 is recommended because of the high heat and light exposure that they are exposed to during service.
  • LUMISIL® 740 is particularly suitable for chip-on-board technology. An extra benefit is its extremely low VOC content.
  • If, on the other hand, the refractive index should be as high as possible to optimize the luminous efficacy, the preferred materials are LUMISIL® 590 and LUMISIL® 591.
  • LUMISIL® 591 is tailor-made for encapsulating single-chip LEDs, as are used for backlighting displays of smart phones, laptops and TVs. Besides their high refractive index, their excellent barrier effect with respect to sulfurous gases is another benefit in these applications.

LEDs must also be resistant to temperature changes. In aircraft, cars or street lighting, they are continually subjected to severe temperature fluctuations, which lead to mechanical stresses due to the different thermal expansions of the installed materials. The thin bond wires are particularly susceptible. If they break off, the LED fails immediately. The LED industry therefore places great importance on thermal shock resistance.

To investigate how much the new silicone encapsulants contribute to thermal shock resistance, LED chips were encapsulated with the new silicone products, and the packages that were produced in this way were investigated in industry-standard temperature cycling tests. In this test, the LED to be analyzed is repeatedly rapidly cooled to –45 °C and heated to +125 °C in half-hour cycles in the test rig. After several thermal cycles, it is tested whether the LED still lights up.

In these tests, LUMISIL® 740, LUMISIL® 770 and LUMISIL® 590 – whose vulcanizates reached a medium hardness, and which are therefore relatively flexible – performed best: the LEDs encapsulated with these products withstood over 1,000 cycles. “With the very hard LUMISIL® 591 (40 Shore D), the LEDs still withstood 950 cycles,” explains Dr. Yang. “Silicone is thus superior to rival high-refractive-index (HRI) materials of comparable hardness.”

Another special feature of LUMISIL® 740 and LUMISIL® 770 is that they contain extremely few volatile substances, which WACKER achieves by pretreatment of the feedstocks used. The LED industry makes every effort to avoid VOCs, which can interfere with the production processes. The two new normal-refractive-index (NRI) silicones provide maximum safety in this respect.