Thermal Management of the ACULED ® VHL Introduction Excelitas’ new ACULED ® VHL TM , with its superior four-chip design and smallest footprint, gives customers the most flexible multi-chip LED on the market. The product family contains various products from UV via VIS to IR with a variety of chip configurations, including sensors and thermistors. Excelitas’ ACULED ® DYO TM even enables customers to put together their own configuration. Please refer to the Custom Design Guide, “ACULED DYO - Design-Your-Own,” for more details on this product. Preventing the ACULED from overheating by taking away the heat from the package is a key point when designing the ACULED into your product. This application note describes the thermal management of the ACULED and further considerations in heat-sink design. www.excelitas.com Features and Benefits of the ACULED ® VHL and DYO High power light, UV and IR source Ultra compact footprint Excellent color mixing due to high packaging density Separate anode and cathode for each color and pad Various standard configurations available Combination of LED with sensors Design-Your-Own (DYO) Applications General illumination Entertainment and shop design Furniture lighting Architectural and landscape lighting Mood lighting Vision systems Backlighting Medical lighting Display and signs Customized chip configuration Author • Jörg Hannig Excelitas Technologies Luitpoldstrasse 6 85276 Pfaffenhofen Germany Phone: +49 8441 8917 0 Fax: +49 8441 71910 Email: [email protected]Technical Support • For additional technical support, please contact us at: [email protected]
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Thermal Management of the ACULED® VHL Introduction Excelitas’ new ACULED® VHLTM,
with its superior four-chip design and
smallest footprint, gives customers the
most flexible multi-chip LED on the
market. The product family contains
various products from UV via VIS to IR
with a variety of chip configurations,
including sensors and thermistors.
Excelitas’ ACULED® DYOTM even
enables customers to put together their
own configuration. Please refer to the
Custom Design Guide, “ACULED DYO -
Design-Your-Own,” for more details on
this product.
Preventing the ACULED from
overheating by taking away the heat from
the package is a key point when
designing the ACULED into your product.
This application note describes the
thermal management of the ACULED
and further considerations in heat-sink
design.
www.excelitas.com
Features and Benefits of the ACULED® VHL and DYO High power light, UV and IR source
Assuming a typical Rth JB of 5 K/W for the ACULED VHL, we get a safety power of approx.
200 mW. Of course the ACULED also transports some heat to the air on the front side, which
can be considered when used without heat-sink. Therefore, in practice, it can be operated at
50 mA and 600 mW without additional heat-sinks.
However, all these formulas are only very rough estimations since α is hard to determine in
reality. The most accurate approach is simulating the heat transfer processes with modern
thermal simulation software (see www.nusod.org) that can handle complex structures showing
the critical thermal paths. Figure 15 shows the ACULED VHL in this thermal simulation process done
by finite element simulation software. The results of this process, confirmed by
measurements of the real product, enabled us to improve the internal thermal management of the
ACULED, evidenced, for example, at the low thermal crosstalk.
Figure 15 Thermal simulation of the
ACULED (detail of the chip
area)
Upper left: finite element
mesh model
Upper right: temperature
simulation at all chip
operation
When calculating heat-sinks, we must consider the maximum current IF at which the ACULED
will be driven. This is included in Ptot in the previous formulas (see Equation 1). Figure 16 shows
that chips can be damaged by exceeding the maximum current of the heat-sink in operation. In
this figure, the curves of the pn-junction temperature TJ of a RGYB-ACULED are shown when
driven at IF = 350 mA, respectively, 700 mA on a small heat-sink of 25 x 25 x 18.5 mm³. Though
this heat-sink works properly at lower currents and power consumptions up to approx. Ptot =
6 W, it will fail with the ACULED VHL’s highest current because the junction temperature
exceeds the critical maximum after two minutes of operation. www.excelitas.com Thermal Management of the ACULED® VHL 21
TJ [
°C]
Figure 16
200
180 700 mA 350 mA
160
140
120
100
80
60
40
20
0
0 50 100
150 200 250 300
t [s]
TJ at different currents IF
with the same heat-sink
vs. time t of operation
Though the heat-sink
works properly at 350 mA
it is not suitable at higher
current.
Heat-Sink Types
The task of the heat-sink is to transport heat generated by the chips of the ACULED to the
environment or cooling medium. This is typically done by convection of air, but can also be
accomplished by a water-cooled system. Water cooling can be a good solution, particularly
when driving several ACULEDs at high current and, therefore, at high total power consumption (see
Equation 1). In some applications like machine vision, water is already used for other
cooling processes and can, therefore, be easily adapted. Also, the use of Peltier-coolers
(thermoelectric cooling) is an option for specific applications. Figure 19 shows examples of
different heat-sink types.
The main parameters that determine the type and size of heat-sink are the temperature
difference (e.g. ∆TBA), the surface area Ahs , and the flow rate v of the cooling medium (water or
fluid).
Heat-sinks against air
Standard heat-sinks made from a metal with good thermal conductivity, such as aluminium or
copper, are available to the market in a variety of dimensions and profiles. To enlarge the
surface area against air, these heat-sinks are equipped with fins or fingers. To support the
convection by heat radiation, most heat-sinks are black anodized. The principle of these heat-
sinks is quite easy - the heat-sink is heated by the ACULED and, therefore, is warming the
surrounding air. This warm air is rising, and cooler air follows. The cool air is again warmed by
the heat-sink, and so on. This is known as natural convection. As well as its area Ahs , the
properties of the surface (e.g. material, color) are also important for heat transfer to the air since
they have an influence on the surface emission factor and heat transfer coefficient α.
Heat-sinks and fans
If the natural convection of the air is not enough for cooling, a fan can force the air and strongly
decrease the thermal resistance. Heat-sinks with fans, such as those used with modern micro-
processors are also available. See Figure 7 for an example of additional cooling by a fan with
the ACULED VHL.
www.excelitas.com Thermal Management of the ACULED® VHL 22
Figure 17 shows how a fan can strongly decrease the thermal resistance Rth hs of a heat-sink by as
much as five times, depending on the air flow velocity vA. The curves indicate this for
different aluminium-made pin fin heat-sinks from the company PINBLOC, all on a 32 x 32 mm² base
with 81 pins at different heights (10, 15 and 20 mm). The power dissipation of the heatsinks
dramatically increases when the air flow is forced by a fan.
Figure 17 Power dissipation P (rising
curves) and heat-sink
resistance Rth hs (falling
curves) vs. air flow velocity
vA for different pin fin heat-
sinks (small picture).
picture and graphic
courtesy of PINBLOC
(Cologne, Germany)
Heat pipes
Heat pipes provide a very effective way of cooling by using evaporation and condensation of
fluids in a self-contained system. Figure 18 shows the principle of a heat pipe. The heat source
(e.g. the ACULED) heats up the fluid (e.g. water or alcohol) inside the heat pipe on its “warm”
side or end. The fluid, which is in a relative small volume inside the heat pipe, evaporates due to
the heat and is cooled down on the “cold” side through condensation. The cold side is
connected to another external heat-sink or designed for a good cooling by the surrounding
ambient. Since the temperature depends on the low pressure inside the heat pipe, the
temperature drop from the warm to the cold side can be defined by the vacuum.
Heat pipes do not require external water or electrical connections, do not have any moving
parts, and do not need maintenance. They are widely used for cooling high-end
microprocessors, for example, and have a thermal resistance or Rth hs < 1 K/W.
Heat-sinks with liquid cooling
Unlike heat pipes, which use the mechanism of evaporation and condensation of a liquid in a
contained system, a convection system uses the good thermal capacity of water or liquids. The
principle of these coolers is similar to air cooled systems, but now a liquid, rather than air,
transports the heat. Therefore, theses systems require water connection, pumps, and usually a
condenser to continuously cool down the liquid. They are used in applications where water
cooling already exists (for instance at machines) and high amounts of heat are generated -
such as heat generated by a large number of ACULEDs with a high packaging density.
Some companies provide micro-channel coolers that allow high heat flux capability of approx.
1000 W/cm² (remember, the ACULED VHL has up to approx. 10 W/cm² heat density). These
www.excelitas.com Thermal Management of the ACULED® VHL 23
liquid-cooled heat-sinks can take away a high amount of heat in a small footprint and are also used
for laser diode bars.
Figure 18 The principle mechanism of
heat pipes
Figure 19 Examples of different heat-
sink types for air cooling:
Top: standard heat-sinks
Bottom left: ceramic heat-
sink
Bottom right: heat-sink
with fan
Heat-Sink Mounting
When mounting the ACULED to a heat-sink, the most important issue is a very good thermal
contact between the ACULED’s substrate back side and the heat-sink itself. Figure 9 previously
pointed out the additional thermal resistance Rth TIM for the junction material between the
ACULED and the heat-sink, known as thermal interface material (TIM). This resistance should
be kept as small as possible. Depending on the kind of mechanical connection (clamping,
www.excelitas.com Thermal Management of the ACULED® VHL 24
screwing, gluing etc.), the TIM must also provide a good mechanical connection/adhesion. For
general mounting recommendations, please refer to the application note, “Mounting of the
ACULED® Product Family.”
Thermal grease and similar materials
Without any TIM, there can be a risk of having bad thermal conduction between the ACULED and
heat-sink due to air gaps resulting from micro-roughness of the materials. The use of thermal
grease helps avoid these gaps. However, since the thermal conductivity of thermal grease -
though much better than air - is in the range of 2 - 5 W / (mK), we must apply it as thinly as
possible. Its task is to displace only the air gaps and any additional (thick) layer
between the ACULED and the heat-sink will increase the thermal resistance. A few 10
micrometers of thermal grease is usually sufficient.
If good adhesion is necessary, a kind of thermal glue should be used. Generally speaking, you could
say that the better the adhesion of the glue, the lower the thermal conductivity. Therefore, the best
heat transfer is achieved by highly-conductive thermal grease and screwing or
clamping the ACULED to the heat-sink. Since the ACULED board is electrically isolated from
the solder pads and from the chip pads, there is no need to use insolating material, which also has
lower thermal conductivity.
Thermal conductivity tapes
Thermal tapes are typically double-sided adhesive films that are easy to handle, but bear the
risk of delamination. Due to their thickness of some 100 s micrometers and a relatively low
thermal conductivity of 1 - 2 W / (mK), they have a much higher thermal resistance than thermal
grease. Therefore, they should be used only at lower power or less critical applications.
Table 6 shows the properties of typical TIMs between the ACULED and heat-sink. The heat
transition resistance Rα (reciprocal value of the heat transfer coefficient α) can be used to
calculate the thermal resistance Rth TIM of the material when the area ATIM is known (usually the area
of the ACULED AACL):
Rth TIM
TIM
= Rα / ATIM
heat
transistion operating
resistance temperature
Rα [cm²K/W]
(15)
Table 6 Properties of different
remarks thermal interface materials
thermal high thermal conductivity; high pressure
grease 0.3 - 2 - 60 - + 200 °C helpful, no mechanical adhesion
elastomer lower thermal conductivity, high pressure
tapes 1 - 3 - 40 - + 200 °C required, suited for mass production
adhesive
tapes
phase change
materials
1 - 4 up to 250 °C
0.3 - 0.7 up to 200 °C
lower thermal conductivity, easy mechanical
mounting
Wax-like material with low glass transition
temperature, available as tape or paste
thermal glue 0.3 - 2 - 60 - + 250 °C thermal grease with glue; easy mechanical
mounting, curing necessary
During mounting, it is important to put some uniform pressure on the ACULED to achieve a
good mechanical and thermal contact and to help the interface material displace the air.
However, be careful not to put pressure on the silicone (refer to application note, “Handling of
LED and Sensor Products Encapsulated by Silicone Resin”). Also consider the “cool” side of the
heat-sink. For example, in an air-cooled system, ensure that the air can flow fast and easily
through the rims and fins of the heat-sink. When using natural convection, the rims of the heat- www.excelitas.com Thermal Management of the ACULED® VHL 25
sink should be vertical for optimized air flow, since warm air flows to the top (see Figure 20).
Also ensure that other heat sources, such as radiators, sunlight, microprocessors, and other
power consumers that can produce heat, are kept away from the LED or heat-sink - or that they
are considered in your thermal calculations. As described before, it is highly recommended to
control the heat by a thermistor on the ACULED, the board, or the heat-sink to avoid
overheating due to damaged fans, broken water tubes (with water cooled systems), or
additional heat sources.
Unlike other SMD-LEDs on the market, it’s not necessary to draw away the heat via the solder
pads. Therefore, the ACULED can be upside-down mounted on a simple FR4-PCB for electrical
connection and have a heat-sink attached to its back side. With this concept, the use of high-
priced IMS or ceramic PCBs is not necessary and multi-layer PCBs can even be used. This is
not possible with the previously-mentioned high thermal conductive materials (refer to Table 1
for λth of different PCB materials). However, if possible, the solder pads of the ACULED can be
used to assist the heat transportation (e.g. by designing big and thick copper pads and tracks or
even thermal vias on your PCB where the ACULED is mounted). Refer to Figure 21 for
examples on the recommended method for mounting the ACULED as a through-looking device with
the best thermal and electrical connectivity.
Figure 20 Orientation of the heat-
sink rims when used with
natural convection: best
(right) and less effective
orientations (left, middle)
www.excelitas.com Thermal Management of the ACULED® VHL 26
Figure 21 Clockwise from upper left:
(1) The ACULED mounted
as through looker on a
FR4 board, small heat-sink
(for 350 mA operation) and
PMMA standard optics.
(2) All parts mounted
together.
(3) 3 assemblies of (2)
connected together
(4) 3 blue ACULED VHL
under operation
www.excelitas.com Thermal Management of the ACULED® VHL 27
Symbols and Units
The following terms and their typical units are used in the application notes and datasheets of the
ACULED. Please note that not all of these are used in this particular note.
A [m²] area, surface
AACL [cm²] surface area of ACULED substrate back side
Ahs [cm²] surface area of heat-sink
Arad [mm²] radiating surface
ATIM [cm²] surface area of TIM with usually ATIM = AACL
α [W/(m²K)] heat transfer coefficient
Cnm [K/W] thermal crosstalk coefficient between ACULED pads n and m
Ee [W/m²] irradiance
EV [lx] illuminance [lux]
Φe [mW] radiant flux
ΦV [lm] luminous flux [lumen]
Ie [W/sr] radiant intensity
IF [mA] forward current
IFM [mA] surge current
IR [µA] reverse current
IV [cd] luminous intensity [candela]
Le [W/(m²sr)] radiance
LV [cd/m²] luminance
λdom [nm] dominant wavelength
λpeak [nm] peak wavelength
λth [W/(mK)] thermal conductivity
∆λ [nm] spectral half bandwidth
η [%] efficiency
ηopt [lm/W] optical (luminous) efficacy
PCn [W] power consumption of chip placed on pad n of the ACULED
Popt [mW] output power (optical)
Pth [W] thermal power (i.e. the amount of electrical power consumption that
is transformed into heat)
Ptot [W] power consumption (electrical) [Watt]
R [Ω] (electric) resistance
Rα [(Kcm²)/W] heat transition resistance; reciprocal value of the heat transfer
coefficient α
Rth [K/W] thermal resistance (general) [Kelvin per Watt] www.excelitas.com Thermal Management of the ACULED® VHL 28
Rth BA
Rth C
Rth Cn
Rth hs
Rth JA
Rth JB
Rth JBn
Rth P
Rth Pn
Rth TIM
RH
t
tLife
T
TA
TA max
TB
TB max
TC
TCn
TCT
TJ
TJn
TNTC
Top
Tsold
Tst
∆TB NTC
[K/W]
[K/W]
[K/W]
[K/W]
[K/W]
[K/W]
[K/W]
[K/W]
[K/W]
[K/W]
[%]
[s]
[h]
[°C] or [K]
[°C]
[°C]
[°C]
[°C]
[°C]
[°C]
[K]
[°C]
[°C]
[°C]
[°C]
[°C]
[°C]
[K]
thermal resistance from base (B) backside to ambient surrounding
(A)
thermal resistance of chip from junction (J) to chip substrate back
side
thermal resistance of chip from junction (J) to chip substrate back
side at chip placed on pad n of the ACULED
thermal resistance of heat-sink
thermal resistance from junction (J) to ambient air or surrounding (A)
thermal resistance from junction (J) to base (B) back side
thermal resistance from junction (J) to base (B) back side assigned to
pad n of the ACULED
thermal resistance of package from chip pad to base (B) back side,
indicates the mount without chip
thermal resistance of package from chip pad to base (B) back side
assigned to pad n of the ACULED
thermal resistance of TIM, usually between ACULED substrate (back
side) and heat-sink
relative humidity
time
life time of LED chip or module
temperature (general)
ambient temperature
maximum allowed ambient temperature
base temperature on back side of package (substrate)
maximum allowed base temperature on back side of package
(substrate)
temperature on back side of chip substrate
temperature on back side of chip substrate placed on pad n of the
ACULED
(correlated) color temperature
temperature at pn-junction of the LED chip; usually referred to the
maximum allowable junction temperature
temperature at pn-junction of the LED chip placed on pad n of the
ACULED. Usually the maximum allowable temperature is meant.
temperature inside NTC chip
operating temperature
soldering temperature (at backside of the ACULED VHL)
storage temperature
difference between base and temperature and NTC chip temperature
www.excelitas.com Thermal Management of the ACULED® VHL 29
∆TBA [K] difference between base and ambient temperature
∆TJA [K] difference between junction and ambient temperature
∆TJB [K] difference between junction and base temperature [Kelvin]
∆TJBn [K] difference between junction temperature of chip placed on pad n of
the ACULED and base temperature
∆TSafety [K] additional “safety” temperature to be subtracted from the TB max to be
on the safe side when calculating heat-sink dimensions
TCΦe [mW/K] temperature coefficient of radiant flux
TCΦV [mlm/K] temperature coefficient of luminous flux
TCλ dom [nm/K] temperature coefficient of dominant wavelength
TCλ peak [nm/K] temperature coefficient of peak wavelength
TCVF [mV/K] temperature coefficient of forward voltage
vA [m/s] (cooling) air flow velocity
VF or UF [V] forward voltage
VR or UR [V] reverse voltage
xn° [ - ] x coordinate in CIE color space for n-degree observer (usually n = 2
is used with light sources like LEDs: x2°)
yn° [ - ] y coordinate in CIE color space for n-degree observer (usually n = 2
is used with light sources like LEDs: y2°)
2ψ [°] viewing angle (usually at half of maximum intensity)
Abbreviations
The following abbreviations are used in the application notes. Please note that not all of these
abbreviations are used in this particular note.
ACULED® The trademarked name for Excelitas’ range of All Color Ultrabright LEDs.
BOM Bill of material
ccw Counter clockwise
CCT Correlated color temperature
CIE Commission Internationale de l'Eclairage = International Commission on
Illumination
COB Chip-on-board
CRI Color rendering index, value to measure the quality of light used for illumination
purposes. 100% means best natural appearance of illuminated colors by the light
source.
DYOTM Design-Your-Own, indicates an ACULED with customized chip configuration
DUT Device under test
ESD Electro-static discharge
www.excelitas.com Thermal Management of the ACULED® VHL 30
FR4 Flame resistant 4, low cost PCB material made from epoxy resin and fiberglass
mat
FWHM Full width at half maximum
IMS Insulated metal substrate, PCB substrate made from aluminum or copper to
provide excellent heat management
IR Infra-red, radiation above 700 nm within the scope of this application note
LED Light-emitting diode
NTC Negative temperature coefficient, used as acronym for an NTC resistant.
Thermistor to control (LED-) temperature
PCB Printed circuit board
PD Photo-diode
PMMA Polymethyl methacrylate, transparent thermoplastic; in optical grade used for
lenses
pn junction Layer in the LED chip, where positive (p) and negative (n) charged carriers
recombine to light respectively radiation.
PPA Polyphtalamide (plastic)
PT100 Thermistor made from platin with 100 Ω at 0 °C. Has a positive temperature
coefficient (PTC).
SMD Surface mount device
TIM Thermal interface material
UV Ultra-violet, with LEDs radiation below 405 nm within the scope of this application
note
VHLTM Very high lumen. This is the name for the newest generation of standard
monochromatic and multi-colored four-chip ACULEDs.
VIS Visible light, radiation between 405 and 700 nm within the scope of this application
note
www.excelitas.com Thermal Management of the ACULED® VHL 31
Notes
1. Excelitas maintains a tolerance of ± 5% on flux and power measurements.
2. Excelitas maintains a tolerance of ± 2 nm for dominant wavelength measurements.
3. Excelitas maintains a tolerance of ± 1 nm for peak wavelength measurements.
4. Excelitas maintains a tolerance of ± 2 K/W for thermal resistance measurements depending on chip
properties.
5. Due to the special conditions of the manufacturing processes of LEDs, the typical data or calculated
correlations of technical parameters can only reflect statistical figures. These do not necessarily
correspond to the actual parameters of each single product, which could differ from the typical data and
calculated correlations or the typical characteristic line. If requested, e.g. because of technical
improvements, these typ. data will be changed without any further notice.
6. Proper current derating must be observed to maintain junction temperature below the maximum.
7. LEDs are not designed to be driven in reverse bias.
8. All drawings are not to scale.
9. All dimensions are specified in [mm] if not otherwise noticed.
Technologies Corp. or its subsidiaries, in the United States and other countries. All other trademarks not owned by Excelitas Technologies Corp. or its subsidiaries that are depicted herein are the property of their
respective owners. Excelitas reserves the right to change this document at any time without notice and disclaims liability for editorial, pictorial or typographical errors.
600195_01 APP0707
www.excelitas.com Thermal Management of the ACULED® VHL 32