LED Package Design with Optimized Heat Transfer
Rafael C. Jordan, Ralph Schacht, Bernhard Wunderle, and Hermann Oppermann
Fraunhofer Institut für Zuverlässigkeit und Mikrointegration (IZM)
Gustav-Meyer-Allee 25, D-13355 Berlin, Germany
Email:
jordan@izm.fhg.de, Phone: +49(30)46403-219, Fax: +49(30)46403-271
ABSTRACT
1. Introduction
Ambient temperature, Joule heating by current through the LED and the current itself (electron scattering) are
the main reasons for degradation of an LED [1]. In the inner region the cause of degradation can be dislocation
or precipitation. Point, line and plane defects can be formed and typically result in a higher current leakage
without light emission. Although electrode degradation rate due to metal diffusion or alloy reaction depends on
temperature and current, but can be avoided with the right combination of ohmic, adhesion, barrier, and wetting
layers. More critical than the average junction temperature are hot spots, which are generated due to the
inhomogeneous band gap intra the junction layer [2]. Beside this permanent degradation of the LED also a
temporally decrease of the efficiency with increasing temperature is observed, especially for red and yellow
LEDs. Therefore enhanced heat dissipation is fundamental for an efficient UHB LED.
2. Chip selection
The design of the LED itself is important for the heat path through the assembly. In the following the focus will
be on blue LED chips. Three basic constructions are available: Epilayers grown on sapphire or silicon carbide or
epilayers grown on sapphire which are separated and typically rebonded on metal substrates. Whereas the first
two ones are available as p-side up and p-side down versions, the last one has always the optical layer on the
surface. As the p donated material is generally less conductive than the n donated material, the p-side is always
thinner and therefore closer to the optical active junction. This results in different advantages and disadvantages
for the sapphire and SiC based LEDs. The p-side up sapphire chips can be easily assembled by wire bond
technology, but as the sapphire has low heat conductivity this choice is critical for high current applications. The
p-side down design combines the advantage of good heat dissipation with the transparent substrate for light
transfer to the surface, but the LEDs need to be prepared for flip chip compatibility. Flip chip technology is
necessary because the chip can not be wire bonded due to the insulation character of the sapphire. P-side up SiC
have a better heat dissipation than sapphire chips but are also limited and p-side down SiC LEDs reabsorb some
of the emitted light within the SiC material. Special side wall shapes can improve this effect but result in a
complex light distribution with lower luminance.
Under these considerations surface emitting LED on metal substrate are first choice. p-side down sapphire chip
whit an appropriate flip chip technology would be a good choice as well. Tertiary a p-side down SiC should be
taken into account.
3. Thermal interface material and CTE mismatch compensation
Beside the heat conductivity the "Coefficient of Thermal Expansion" (CTE) from the chosen material is relevant
for a reliable assembly. Only a few board materials have a CTE in the range of the LEDs (about 5.5•10-6). These
materials are brittle as silicon or aluminum nitride or very expensive as molybdenum or copper/tungsten and
therefore not suitable for price-conscious production. Even copper with higher heat conductivity and smaller
CTE than aluminum is mostly rejected due to cost issues. Therefore aluminum is a common used core metal for
Insulated Metal Substrates (IMS), but soldering an LED directly on aluminum, with the necessary adhesion,
barrier and wetting layers, will destroy the semiconductor in short term, mostly already while cooling after the
soldering process. Therefore it is a common to use as thermal interface material (TIM) an adhesive to glue the
LED on the substrate to avoid the thermomechanical stress......
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