Silicone Encapsulation for LED
Advantages and Handling Precautions
Application Note 5288
In recent years, LED brightness has improved tremendously due to advancement in LED die in terms of luminous efficiency and its ability to be driven under higher
powered conditions. Due to these developments, the
LED has been adopted in more applications, including
high brightness illumination, backlighting and ESS that
require high flux output and high driving current. Apart
from the improvement in die technology, the challenges
for the high brightness LED lies in the packaging technology, especially the encapsulation material, to ensure and
further enhance its reliability.
Conventionally, epoxy has been used as the encapsulation material. Optical grade epoxy has good light
transmission and high glass transition temperature (Tg)
that is suitable for LED application. With expectation of
higher luminous flux output, higher driving current and
better long term reliability, silicone materials has been
introduced to Avago’s LED product line-up as the new
generation of encapsulation material for LEDs.
Advantages of Silicone Encapsulant
Silicone can be formulated in different hardness properties, which are gel, elastic and hard types. Silicones used
in optoelectronic products are generally elastic type to
give the flexibility to encapsulated wire bond and die
attach. This characteristic provides the ability to absorb
thermal stress during the soldering process and during
operation as well.
The chemical structure of silicone provides several advantages over epoxy encapsulant used in optoelectronic
products. Its structure consists of bonding between Si
and O called siloxane bond which has higher energy
compare to the carbon (C) bond and oxygen (O) exist in
epoxy. This characteristic gives the thermal and UV stability of silicone, which are the main characteristics needed
for new generation LED products.
R = Organic groups
Figure 1. Example of Siloxane bond in silicone material
Prolonged exposure to high temperature, either from the
environment or the internal LED die junction temperature, will cause deterioration of the encapsulation material including reduction of transmittance and yellowing of
material. A similar deterioration issue applies to exposure
to UV emission from sunlight for outdoor products and
to the blue or white LEDs own near-UV emission. Silicone
material exhibits superb stability towards heat and UV
thanks to the strong siloxane bond, enabling long life
performance of high brightness LEDs with little intensity
Handling Precautions
Compared to epoxy encapsulant that is hard and brittle,
silicone is softer and flexible. Although its characteristic
significantly reduces thermal stress, it is more susceptible
to damage by external mechanical force. As a result,
special handling precautions need to be observed during assembly using silicone encapsulated LED products.
Failure to comply might lead to damage and premature
failure of the LED.
PCB might press or poke on the silicone. Use an appropriate rack to hold the PCBs.
1. Do not poke sharp objects into the silicone encapsulant. Sharp object like tweezers or syringes might
pierce through the silicone and induce failures to the
LED die or wire bond.
4. Use the recommended nozzle size. For automatic pick
and place, it is important to use nozzles that are within
recommended size. Generally, the outer diameter or
outer perimeter of the nozzle should be smaller than
the silicone. For exceptions and details, please refer to
the recommended nozzle size specified in the respective product datasheet.
2. Do not touch the silicone encapsulant. Uncontrolled
force acting on the silicone encapsulant might result
in excessive stress on the wire bond. The LED should
only be held by the body.
3. Do not stack assembled PCBs together. Abrasion
between two PCBs assembled with silicone encapsulated LEDs might cause delamination of the silicone.
Furthermore, components on the bottom side of the
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5. Clean the silicone surface gently using a cotton bud.
Silicone material attracts dusk and dirt easier than
epoxy due to its surface tackiness. To remove foreign
particles on the surface of silicone, a cotton bud can
be used with Isopropyl alcohol (IPA) or acetone. During cleaning, rub the surface gently without putting
much pressure on the silicone. Ultrasonic cleaning is
not recommended for a silicone encapsulated LED.
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Data subject to change. Copyright © 2005-2010 Avago Technologies. All rights reserved.
AV02-0681EN - July 21, 2010