Introduction
7
An important role is played by the printed circuit board whereby LEDs are allocated so in
chapter two the main technologies for the realization of the substrates will be introduced.
After learning the main concepts on the thermal management, in chapter three some
electronic systems for the thermal protection and an overview on heat sink types will be
described.
Finally, in chapter four, an algorithm based on failure rate (related to junction temperature)
for the selection of the LEDs will be introduced with some considerations on the kind of vias
to employ in a substrate.
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Chapter 1: LED’s Go
A LED is a semiconductor device. When powered, electrons move through the
semiconductor material and some of them fall into a lower energy state. In the process, the
energy "saved" is emitted as light (photon). The wavelength (and color) of light can
be adjusted using semiconductor materials and processes manufacture different. Moreover,
the wavelength of the propagated light emitted is relatively narrow, providing more pure
colours.
1.1 Traditional LED and Power LED
In terms of thermal management there is an important difference between traditional LED
and power LED. A traditional LED produces a small amount of heat which can be dissipated
in part through the leads of the LED itself. Midpower and Power LED require a completely
different approach similar to that used to cool integrated circuits, then almost all surface
mount LED use the presence of ground pads to dissipate heat. To do this, each LED has a heat
sink (slug) that connects the LED pad as shown in Figure 1-1.
1.2 Importance of thermal management for LEDs
Unlike incandescent tungsten filament light bulbs, high-power LEDs do not radiate heat.
Instead, LEDs conduct heat from their PN junction to the thermal slug on the LED package.
Because the heat generated by LEDs is conducted, the heat has a longer, more expensive, path
to the atmosphere. The power dissipation on the junction of a chip is distributed in the
package and in the circuit board by means of heat conduction and it is transferred from the
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9
surfaces to the environment by means of radiation and convection
1
.
“Junction” refers to the p-n junction within the semiconductor die. This is the region of the
chip where the photons are generated (Figure 1-2).
Figure 1-3 shows the basic internal structure of a SMT (Surface Mount Technology) LED
Package and its method of mounting on a printed circuit board with the major routes of heat
flow.
Figure 1-1
Figure 1-2
1
Convection is the movement of molecules within fluids (i.e. liquids and gases). Convection is one of the
major modes of heat transfer and mass transfer. There are two major types of heat convention: 1) Heat is carried
passively by a fluid motion which would occur anyway without the heating process; 2) Heat itself causes the
fluid motion, while at the same time also causing heat transporting by this bulk motion of the fluid. This process
is called natural convection, or free convection. With natural convection, heat transport is generally more
complicated;
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Figure 1-3 – LED drawing with the major routes of heat flow
The LED consists of a chip mounted on a lead frame by solder or bonding adhesive. The
leads consist of a high conductivity material like copper.
The primary thermal path of the heat flow is from the junction through the lead frame to
the end of the leads by heat conduction. Another partial path travels from the surface of the
chip to the package surface. From the end of the leads simultaneous processes of heat
spreading by conduction and heat extraction over the surface of the board by convection and
radiation takes place. The heat transfer efficiency from the PCB to air has a significant effect
on the temperature difference between chip and air.
So in a LED, the heat path includes the thermal impedances from the junction to the slug,
the slug to the board, the board to the heat sink (if provided), and the heat sink to the
atmosphere (we will discuss later about thermal resistance in more accurately manner).
The heat path for a tungsten bulb is almost direct into the atmosphere, starting with the
thermal resistance from the filament to the glass and ending with the thermal resistance from
the glass to the atmosphere.
Longer lifetime, higher efficiency, and more flexible colour output make LEDs the
preeminent solution in architectural and entertainment lighting applications.
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The colour output of LEDs is programmable: a system of multiple LEDs combines, or
mixes, to create different colours. One application of colour mixing with LEDs is LCD
backlighting. In this application multicolour LEDs are used to create a white light. Backlight
LCDs require precise colour matching throughout the display. To achieve precise colour
matching the thermal and optical design must be optimized for each system. Optimal optical
design uses a colour sensor to sustain high colour accuracy because LEDs characteristics will
change when junction temperature change [1] [2].
1.3 Temperature influence on LEDs characteristics
1.3.1 Light Intensity
The junction temperature of the LED affects luminous flux, colour and forward voltage of
the device. Junction temperature can be affected by the ambient temperature and by self-
heating due to electrical power dissipation.
The equation for luminous flux as a function of temperature (°C) is given below [3]:
21
() ()
J
kT
VV
TTe
− Δ
Φ=Φ (1.1)
Where:
Φ
V
(T
1
) luminous flux at junction temperature T
1
Φ
V
(T
2
) luminous flux at junction temperature
T
2
k temperature coefficient
ΔT
J
change in junction temperature (T
2
-T
1
)
Typical temperature coefficients for various high-brightness LEDs are listed in the
following table and figure.
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Table 1-1
Figure 1-4 – Luminous flux versus ambient temperature for a typical red-orange AS/TS AlInGap LED
when operated at a constant current.
The degradation of flux as a function of increasing temperature for a typical red-orange,
absorbing-substrate (AS) or transparent-substrate (TS) AlInGaP LED is shown in figure.
Note, luminous flux has been normalized at 25°C.
As shown, an increase in the junction temperatures of 75°C can cause the level reduction
of luminous flux to one half of its ambient temperature value. From this, it is clear that
temperature effects on luminous flux must be accounted in the design of a LED assembly.
Dominant wavelength of LEDs, luminosity and forward voltage are all dependent on the
junction temperature of the LED. Because the colour and brightness properties are sensitive to
temperature, having control over the thermal performance of the LED lighting system is
essential. The plot in Figure 1-4 shows light intensity or brightness vs. junction temperature of
Lumileds Luxeon K2 LEDs
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1.3.2 Dominant wavelength shift
In addition to changes in light intensity, the dominant wavelength or radiated colour of the
LED will drift slightly with temperature. Even though the wavelength drift is slight, there will
be a noticeable change in the colour temperature of the backlight if the junction temperature
change is great. An area in which this is of particular importance is with white light. The
human eye can differentiate small colour changes in white light.
So the junction temperature of LEDs also affects their dominant wavelength, or perceived
colour. The equation for dominant wavelength λ
d
as a function of temperature is:
21
() ()
dd Jmateril
nm
TTTk
C
λλ
⎛⎞
=+Δ⋅
⎜⎟
°
⎝⎠
(1.2)
Where:
λ
d
(T
1
) dominant wavelength at junction temperature T
1
λ
d
(T
2
) dominant wavelength at junction temperature T
2
Figure 1-5
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1.3.3 Induced Failures for LEDs
LEDs are tipically encapsulated in an optically clear epoxy resin. At a certain elevated
temperature, know as the glass transition temperature T
g
, the epoxy resins transform from a
rigid, glass-like solid to a rubbery material. A drastic change in the coefficient of thermal
expansion (CTE) is generally associated with the T
g
, this temperature is calculated as the
midpoint of the temperature range at which this change in CTE occurs (see figure below).
To avoid catastrophic failure of LED packages, the junction temperature should be always
kept below the T
g
of the epoxy encapsulant. Manufacturers specify a maximum junction
temperature which is below the T
g
of the used epoxy encapsulant. If the junction temperature
is exceeded, the CTE of the epoxy encapsulant will permanently and dramatically change. An
higher CTE causes the epoxy encapsulant expansion, this causes more displacement of the
wire bond within the LED package, resulting in a premature breakage of the wire.
Figure 1-6 – Expansion-Temperature relationship for clear, epoxy, LED encapsulants.
Anyway, it should be clear that in order to have a long lifetime of LEDs it’s very important
to control the junction temperature which influences almost all characteristics of LED. In the
following figure is shown how high junction temperature can reduce the lifetime of LEDs.
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Figure 1-7
Generally, a 50 percent drop in light output for a constant-forward current indicates end-of-
life for HB-LEDs. With proper thermal management, HB-LED lifetimes can exceed 100,000
hours. As a simple rule of thumb, every 10°C drop in junction temperature will double the
lifetime of the LED.
1.3.4 Thermal Runaway
Related thermal runaway problems due the paralleling of LEDs are well known. In fact to
avoid thermal runaway in parallel LEDs it is recommended to use a constant current regulator
per string, but there is another thing that causes thermal runaway. There are few methods of
compensating for changes in colour output and brightness over temperature. Considering all
aspects of the design (including electronic, optical and mechanical), the simplest method of
colour compensation is to use junction temperature feedback, instead the most complex
method is to use colour sensor feedback.
The main issue that comes out of the colour sensor method is that as the LED temperature
increases, its luminous intensity degrades. As the colour sensor reports back a lower intensity
level, the processor will try to increase the intensity of each LED, which is accomplished by
driving the LEDs harder. By driving the LEDs harder, the power dissipated increases. Thus,
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