Optimum Encapsulation - Wacker Chemie AG


Optimum Encapsulation

WACKER has responded to growing demand for LED encapsulants by developing a portfolio of four new specialty silicones. These encapsulation compounds are tailored to a variety of LED grades and their associated manufacturing processes. As a result, it is possible to increase the luminous efficacy of LEDs and make them more cost-efficient to produce.

A sample LED encapsulated in silicone is taken for quality-control purposes and placed in a test dish.

Light-emitting diodes – LEDs – are becoming ever more powerful and efficient. Nowadays, white LEDs can generate well over 100 lumens from one watt of electrical power. Such luminous efficacy values far exceed those of any traditional lamps. White light-emitting diodes have been increasingly replacing conventional luminaires for several years now – and the market is growing at a correspondingly fast pace. According to a Boston Consulting Group study, six out of every ten luminaires retailed in 2020 will be LEDs. And for professional luminaires, the share will be as high as 80 percent.

“Our materials are intended to increase the lifetime of LEDs, as well as maximizing their luminous efficacy.”

Dr. Jeong-Han Kim Head of the Center of Electronics Excellence

Yet LED-based lighting is still relatively expensive, with the industry making every effort to further improve cost efficiency – and thereby offer its customers more lumens per dollar. “Strategies include optimizing the LED technology, faster production techniques and using materials that increase the lifetime of LEDs, as well as maximizing their luminous efficacy,” says Dr. Jeong-Han Kim, head of the Center of Electronics Excellence (COEE) in Seoul. COEE is a corporate R&D facility where the silicones made by WACKER are customized to meet the demands of the East Asian electronics and lighting sectors.

At the heart of an LED is the light emitting semiconductor chip, which is encapsulated to protect it and to optimize outcoupling. The encapsulant used for this purpose often also serves as a carrier for the luminescent pigments that are used to tune the color of the emitted light. The encapsulant influences both the production cycle time and the quality of the LED.

Developed in South Korea

Display screens at the Hong Kong stock exchange: the selective use of semiconductor materials and doping enables LEDs to glow in a great many different colors.

WACKER has responded to growing demand for suitable encapsulants – especially for bright and highly efficient white LEDs – by developing a portfolio of high-quality silicone encapsulants at the Center of Electronics Excellence (COEE) in Seoul. This portfolio comprises four new products: LUMISIL® 590, LUMISIL® 591, LUMISIL® 740 and LUMISIL® 770, which are tailored to the different designs, applications and operating conditions of modern LEDs.

Molded Packaging

An LED – by this is meant the electronic component suitable for installation in a circuit, the so-called LED package – consists of the semiconductor chip along with all the electrical, mechanical, thermal and optical interfaces to the surroundings. There are currently several different processes used for mounting chips in packages. but the key step in all of them is the encapsulation of the sensitive chip.

R&D experts at WACKER’s Center of Electronics Excellence in Seoul develop encapsulation materials tailored to LEDs.

LEDs become hot during operation and develop more and more heat the higher their electrical power consumption. The heat released can raise the temperature of the encapsulant significantly. At the same time, the encapsulant is also exposed to the light emitted by the chip. Heat and light age the encapsulating material, with the rate and extent of the damage largely determined by the type of material. The encapsulant can gradually become brittle. It then loses its protective function, becomes increasingly less transparent and discolors. All this degrades the luminous efficiency. If the LED is to achieve a long service life, the encapsulant must withstand these stresses in the long term.

Silicones are known for their resistance to heat, changes in temperature, light, UV radiation and many chemical effects. “In this respect, they are far superior to the epoxy resins that are widely used for encapsulating LED chips,” says Dr. Hyun-Kwan Yang, who is a developer at COEE. Because of their compliance, silicone elastomers are also capable of damping vibrations and absorbing thermomechanical stresses. They can also be made highly transparent and their flow properties adjusted as needed. “For all these reasons,” adds Dr. Yang, “silicones’ share of the LED-encapsulant market has grown significantly in recent years.”

Adhesion without Pretreatment

Structure of a Silicone Lens

All four products are two-component formulations that can be easily applied by automated dispensing, and therefore permit inexpensive mass production. They cure at room temperature by a platinum-catalyzed addition reaction to form highly transparent silicone elastomers. They have also been modified to adhere to the semiconductor chip and to conventional reflector and casing substrates without the need for pretreatment.

The individual grades differ mainly in their refractive index and in the hardness of their vulcanizates. LUMISIL® 590 and LUMISIL® 591 feature a refractive index of 1.53 and are therefore termed “high refractive index encapsulants.” They are based on phenyl-substituted polysiloxanes. LUMISIL® 740 and LUMISIL® 770, on the other hand, have a refractive index of 1.41, which is typical of conventional silicones, and are therefore “normal refractive index encapsulants.”

High Crosslink Density

WACKER’s R&D center in South Korea also simulates industrial production processes, such as applying the silicone by dropping it on to the LED chip, to find the right formulation for specific applications.

LUMISIL® 591 is characterized by the particularly high crosslink density of the vulcanized product. With a hardness of 40 Shore D, it is the hardest of the four new silicone products. The three other encapsulants have hardnesses in the medium to high Shore A range.

The refractive index determines how much of the light can enter the encapsulant from the LED chip. The interface between the chip and encapsulant represents a hurdle for the passage of the emitted light due to the large difference between the refractive indexes of the two materials – the semiconductor materials used have a refractive index greater than 2. Most of the light therefore remains trapped in the interior of the semiconductor crystal as a result of total internal reflection. The smaller the refractive index difference, i.e. the greater the refraction of the encapsulant, the more light can pass through. LUMISIL® 590 and LUMISIL® 591 are therefore ideal for manufacturing high-efficiency LEDs.

Silicone encapsulants with a normal refractive index resist thermal loads and light exposure better than phenyl-substituted silicones – they only embrittle and yellow at a very slow rate. Vulcanized LUMISIL® 740 can therefore withstand 1,000 hours’ storage at 245 °C without noticeable yellowing and with only relatively little mechanical damage. It is therefore ideal for encapsulation of multi-chip-on-board LEDs, in which multiple high-performance LEDs are tightly packed and mounted on the circuit board without an enclosure. Such arrays generate so much heat, that other encapsulants would quickly become cracked.

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.