Thermal Management Guide

Thermal Management Guide
2005.8.
SEOUL SEMICONDUCTOR CO., LTD.
148-29, Kasan-Dong, Keumchun-Gu, Seoul, Korea
TEL : 82-2-3281-6269 FAX : 82-2-857-5430
Rev 3
1. Introduction
LED have the special character that LED break out radiant power and heat
when It is operating. Recently photo efficiency of LED is just 20% and almost
residual power converts heat. But heat cause bad reliability and changes of
electrical and optical character negatively. So power LEDs must dissipate heat
from chip in that package.
SSC Power Package is the latest product in SMT Package. Z-Power
LED(includes white, red, green, blue, amber, etc.) is composed of lead frame,
inner heat sink(slug), and thermoplastic body(housing). The chip is mounted on
reflector made of metal. To dissipate heat from a package, it uses a metal PCB.
The bottom of Z-power LED is soldered on thermally improved metal PCB.
Therefore, Z-Power package is proper one for a large output in the range of
more than 1W. To get reliability and optimized performance, appropriate
thermal management design is absolutely needed.
As all of the other electric materials, Z-power LED has thermal limits as well.
Operating temperature is limited by junction Temperature(Tj) inside chip and
operating power. So operating temperature shouldn’t be over maximum Tj.
These all brief explanation will be an introduction of thermal management to a
design engineer. The concept to improve thermal design will be as follow below.
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
2.Explanation of Basic Relationships
Power dissipation (Pd) in P-N junction of a chip is distributed by transferring
heat through package. And it is transmitted by radiation and convection from
free surface on package to the outside, by radiation or/and convection. But it is
possible to neglect heat radiation transfer. Figure 1 is showing inside structure
for discussion of static properties of Z-power LED.
MOLDING COMPOUND
BOND WIRE
DIE
LEAD
DIE ATTACH
HEAT
SINK-SLUG
HEAT SINK
SOLDER
SOLDER PAD
DIELECTRIC
ALUMINIUM PLATE
ALUMINIUM PLATE
Figure 1.
Z-power LED is composed by mounted chip on bottom of heat sink slug and
solder pad of AI-PCB. Heat sink slug is composed the materials as copper that
has high thermal conductivity.
SEOUL SEMICONDUCTOR CO., LTD.
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
3. Explanation of Thermal Analysis
Thermal resistance in Z-power LED package exists between P-N junction and
heat spreader (such as resin, slug, housing etc).
This value of thermal
resistance can be determined by structure of package, for example, geometry ,
materials and size of LED bare chip, properties of materials used in LED
package. In case of Z-power LED, The value of thermal resistance is RΘJB
which is from junction to metal PCB bottom.
Thermal resistance value is depending on the application that heat flows from
junction in the chip to environment. In case of thermal resistance, RΘJA can be
affected by many factors, such as solder pad design, a position of component,
material of PCB, and structure of PCB. RΘBA decides its particular character
by transmitting heat to undefined part. (for example, external heat sink)
RΘJS is the thermal resistance from junction in chip to slug, RΘSB is the thermal
resistance from junction in chip to slug.
In SSC, The standard thermal
resistance is RΘJB from junction in chip to bottom of the metal PCB
TA
TJ
TS
TB
TA
R
TJ
R
θ JS
TS
R
θ SB
TB
θ BA
TA
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RΘ of Z-Power LED is calculated from junction to metal PCB bottom.
The Equation to get the value of thermal resistance of Z Power LED will be as
follow
RθJA = RθJS + RθSB + RθBA
-----------------
(1)
RθJB = RθJS + RθSB
-----------------
(2)
TJ = RθJB·PD + TB
-----------------
(3)
Where: PD – Power dissipation
TB – Temperature of metal PCB bottom
*Equation 1,2,3 can be calculated from the “Thermal Ohm’s law”
Conditions:
- Not considering thermal resistance of plastic housing body connected by the
method in a row to approach “resistance network”(Figure 2.)
Negligible
Figure 2
(little heat
transfer)
Negligible
(radiation)
RΘJS
RΘJE
RΘJ
(Junction
to Epoxy)
S
RΘSB
RΘEA
(Epoxy to
ambient)
RΘBA
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- Not considering the value of thermal resistance, Rθaj while it’s transmitted
from fluid of circumstance to Junction, Rθj while it’s transmitted by radiation.
The value of RθJB of Z-Power LED can get in Equation3 by measuring Tj, and
also the value of resistance can be differed by Package design, or Chip
Cf. After finishing to design a LED package, the value ,RΘJS+RΘSB, cannot be
changed. (Namely, it is constant)
4. Calculation of Junction Temperature
The method to get the value of Tj can be measured by VF(forward voltage) at
low current experimentally . Normally VF is the value changed by temperature.
When applying voltage, the temperature of chip goes up, and VF goes down. In
case of measuring Tj , VF should be measured with applying pulse low current,
when it is Tj = Ta. The reason why it applies pulse current is to minimize the
effect of heat that can be generated from chip. If we figure out the relation of Ta
and VF at every temperature, we can measure Tj indirectly, and get thermal
resistance RΘJB in Equation 3 by measuring input power
Some supposition need to explain the measurement of the Tj.
i) The input power proportionally converts heat emerge in LED
ii) At low currents, there is no heat in junction of LED
iii) VF is in inverse ratio to Tj in LED
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- Example
-VF = 2.03V, TJ = 105℃
2.25
At Z-power LED White(1W), TA = 25 ℃
2.20
VF
2.15
2.10
2.05
2.00
1.95
20
40
60
80
100
120
140
TJ
5. Calculate Thermal Resistance and Heat sink Sourcing
RΘJB = (TJ – TB)/ Pd = (125℃-111℃)/1.4W=10℃/W
where: Z-power LED White(1W)
TB = 110℃(Temp of PCB bottom in 1W LED)
Tj = 125℃(Max. Junction Temperature)
Vf = 4.0V, If = 350mA (Max VF at 350mA)
Pd = Vf X If = 4.0V X 0.35A = 1.4W
In this case, RθJB =10℃/W
But Z-power LED have low RθJB is 8℃/W at 350mA in P1 package
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6. Summary
Normally thermal management is divided by the inside, and outside thermal
management. In case of the inside thermal management, it manages from
junction to outer surface of a package, and in case of the outside thermal
management, it manages from a package to undefined part. In this case,
controlling ambient temperature will be very important.
The outside thermal management includes a selection of cooling mode, heat sink
design, material and adhesion(combination) process.
After selection of cooling mode, cooling system can be designed. Thermal
resistance RΘJB and RΘBA have to be optimized for applications. But according to
previous comments RΘJB can not be changed. Namely just only RΘBA must be
optimized for using.
Generally Junction temperature of Z-power LED has to be maintained under
the permitted temperature(125 Celsius)mentioned on the datasheet of Z-power
LED
Life time is bound up with Junction temperature. At high junction temperature,
Life time is reduce and at low junction temperature Life time is increasing.
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
[ Supplement to a Heat management for Heat Sink]
7.Heat Sink
Heat sink is a protective device that absorbs and dissipates the excess heat
generated by a system. It is very important heat sink of shape and surface
Area, because it is main factor for heat generation. Usually If heat sink would get
Wider surface area, thicker plate and much more fin, heat dissipation is getting
better.
8 Heat sink categories
Cf. area :25mm × 25mm
Normal load limit
Typical height
Mechanism
Passive heat sink
5~50 watts
~10mm
Natural convection
Active heatsink
(ex. Fan)
10~160 watts
35~80mm
Forced convection
Liquid cooled cold plates
-
10~20mm
Fluid flow
Phase change recirculating
system (ex. Heat pipe)
100~150 watts
5~10mm
Phase transition
9. Heat Sink Classification by type
< T ype 1>
< T ype 2>
< T ype 3>
< T ype 4>
H e a t S in k D e p th
F o o t p rin t A re a
B a s e T h ic k n e s s
F in
< T ype 5>
SEOUL SEMICONDUCTOR CO., LTD.
148-29, Kasan-Dong, Keumchun-Gu, Seoul, Korea
TEL : 82-2-3281-6269 FAX : 82-2-857-5430
-General heat sink type using natural convection is extrusion(Type 1,2,3,5)or
plate(Type 4). According to location, array direction of LED packages, and
quantities of used LED packages, extrusion heat sink can be Type 1,2,3,5
and Type 4 is normally used as plate type which has no typical.
-In case of Type 2,3, fin’s pitch and length are different for each other and they
can be used for other purpose
-Single side cutting for Type 1,2,3,4,and Cross(Both side) cutting for Type 5 can
be divided by Fin cutting Type.
-In previous page there are 5 different heat sinks listed. But they are not all kind
of heat sink. Therefore you should sort out it according to your purpose.
10.The method to optimize Heat Sink
It is possible to optimize heat sink geometry by use of computer simulation,
however, heat sink should be sorted under thermal experiment in
practical environment, because it can not be applied to all environmental factors.
Therefore, the factors of heat sink are Interval of Fin(s), Fin ‘s Thickness(Tf),
Base Thickness(Tb), Heat Sink’s Depth(Dh), Fin’s Height(FS), and the number of
Fin(N), except experimental condition used. In case of Fin Thickness (Tf), it
should be within maximum 1.0mm because it has to be less than extrusion
dimension possible. So, the other 5 things which are factors to design could be
considered as factors for sorting, barring Fin Thickness (Tf)
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In case of designing each variable without optimizing total Heat Sink, create
figures which users want to set as designing variables. In this way, it becomes
the matter of one dimension so that optimizing time is able to be easier and faster.
In order to sort out, Constraint function is necessary.This function includes
temperature condition and measurement condition.
Temperature condition means that calculated degree on the surface of heat sink
(TH) is the same or smaller than target-designed temperature (TG) in the standard
of junction temperature.
Measurement condition is constraint condition decided by means of geometrical
figure or in the range that designer sets outline dimension which is maximally
permitted.
Constraint function:
G1(x) = TH - TG ≤ 0
G2(x) = DH – (LFaw +TB ) ≤ 0
G3(x) = - DH + DHLL ≤ 0
G4(x) = - S + SLL ≤ 0
G5(x) = FH - Whaw ≤ 0
G6(x) = - FH + FHLL ≤ 0
G7(x) = - TB + TBLL ≤ 0
G8(x) = - N + NLL ≤ 0
G9(x) = DH - TB - 5S ≤ 0
Where,
Whaw : Heat Sink width of maximum allowance
LFaw :Heat Sink Fin length of maximum allowance
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
DHLL, SLL , FHLL , TBLL , NLL :The value of Lower Limit for each designing
variable.
G9(x) :Constraint function whose interval of fins is less than 5.
This function is applied because the product is able to be transformed or
damaged when it comes to general extrusion process.
*Notes
The above formula is not able to be absolute solutions and it has some
exceptions because this is one of the examples for the way of selection of Heat
sink.
11. Heat Sink Test Example
- Test Purpose
The Heat Sink Test Example helps end-users select the best heat sink by
measuring temperature difference at equilibrium or steady state after the
selection of the one of Heat Sinks which is fit to the constraint condition
mentioned in the clause function G(3).
- Test conditions
LED – Z-power LED White(5W)
Ta = 25ºC
-Heat Sink and Z-power LED are assembled by means of Thermal grease
-Using test box to confirm reappearance and to control natural convection.
-Heat Sink is Horizontal on insulating sheet.
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
- Test Results
1
TB(ºC)
RθBA(ºC /W)
Size : 99.85 x 70.08 mm
S : Irregular (Random) TB : 3.18mm,
DH : 23.90mm
FH : 20.5mm
N : 8ea
Power Dissipation:5W
Footprint: 625mm2
37.9
2.58
Size : 59.60 x 53.08 mm
S : Irregular (Random) TB : 3.70mm,
DH : 25.95mm
FH : 22.10mm
N : 8ea
Footprint: 625mm2
Power Dissipation:5W
51.5
5.3
Size : 49.90 x 44.85 mm
S : Irregular (Random) TB : 8.90mm,
DH :27.82mm
FH : 19.00mm
N : 11ea
2
Foot print: 625mm
Power Dissipation :5W
56.1
6.22
Size : 50.14 x 49.80 mm
S : Irregular (Random) TB : 2.42mm,
DH :29.84mm
FH : 26.00mm
N : 48ea
Power Dissipation :5W
Foot print: 625mm2
44.7
3.94
Size : 61.00 x 58.00 mm
S : Irregular (Random) TB : 3.90mm,
DH :20.50mm
FH : 17.00mm
N : 121ea
2
Foot print: 625mm
Power Dissipation :5W
51.9
5.38
Specification & Size
2
3
4
5
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12. Tj Vs forward current
100
Blue
Red
Green
90
o
Junction Temp( C)
80
SSC Heat Sink, FH:25mm
size(45mm*45mm)
N:7 footprint:625mm2
1W R G B
70
60
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
450
500
Forward Current(mA)
100
Red
Green
Blue
o
Junction Temp( C)
90
80
70
60
50
SSC Heat Sink Type 4
Size : 50.14 x 49.80 mm
S : Irregular (Random)
TB : 2.42mm, DH :29.84mm
FH : 26.00mm N : 48ea
Foot print: 625mm2
Power Dissipation :5W
2.5W R G B
40
30
20
10
0
0
100
200
300
400
500
600
700
800
900
1000
Forward Current(mA)
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13. Relative light Output Vs TJ
RED
GREEN
BLUE
WHITE
AMBER
CYAN
ROYALBLUE
160
Relative Light Output [%]
140
120
100
80
60
40
20
0
0
20
40
60
80
100
120
o
Junction Temperature, TJ [ C]
14. Tj Vs Tc (Tc is temperature of package case)
100
Max
Min
90
80
o
∆Tj ( C )
70
60
50
40
30
SSC Heat Sink Type
Size : 49.90 x 44.85 mm (DH :27.82mm))
o
R?BA( C /W) = 6.22
Measurement on Lead
20
10
0
0
10
20
30
40
50
o
60
70
80
90
100
∆Tc ( C )
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Tc Point
ΔTc in graph means difference Temperature between On and Off,
Also ΔTj is same meaning with ΔTc.
For example, When Tc temperature is 55℃ on LED in 25℃ ambient temperature,
ΔTc is 30℃, and ΔTj is 40 ~ 45℃. So real Tj is about 65~70 ℃ for Tj= Ta + ΔTj.
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
15. Life time Vs TJ
50% Degradation graph of Luminous output
275,000
250,000
P3 White, Blue, Green & Cyan
P3 Red & Amber
P1
225,000
Time (Hr)
200,000
175,000
150,000
125,000
100,000
75,000
50,000
25,000
0
20
30
40
50
60
70
80
90
100
110
o
Junction Temperature ( C)
*This calculation can be done using the
Arrhenius Model as shown below
R(t)=exp(-‫ג‬t)
where
R(t)= Probability that unit will operate at time t
λ = failure rate
t= Time component is on
λ1 = failure rate at junction temperature T1
λ2 = failure rate at junction temperature T2
EA = activation energy, in units eV
k = Boltzmann's constant (8.617×10-5eV/°K)
T = junction temperature in °K(°K = ℃ + 273)
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16. simulation
1W blue
At IF=350 mA
In air
Resin surface Temp
113.2℃
Reflector surface Temp
17. Actual measured data
Resin surface Temp
102.6℃
1W blue
At IF=350 mA
In air
71.1℃
Reflector surface Temp
79.4℃
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RΘ(oC/W)
18. Rθ Vs Heat sink area
Heat sink area (cm2)
19. Rθ Vs Fan on/off
Fan O n/O ff
Heat sink
60.00
25*25*5mm,
fin 20,
50.00
surface area
6800mm2
T j / RΘja
40.00
Aluminum
Tj
RΘja
30.00
20.00
LED 1W red
350mA
Fan 1.2W
10.00
0.00
fan on
fan off
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
18. Thermal Adhesive
When the power product of emitter type is attached on the metal PCB, we
recommend a reflow process. If customer wants to attach a metal PCB on big
heat sink or when It is not possible to attach the emitter type on heat sink in
reflow process, we recommend to use thermal adhesive.
There are two kind of thermal adhesive grease and tape. It ordinary use
thermal adhesive tape to attach on the wide face and thermal adhesive grease to
attach on the narrow face. The flat face is better. When customer uses thermal
adhesives, avoid to bring out air void between thermal adhesive and the
attached face. The void block the thermal transfer in package.
Customer can consult following the data sheet about some kind of thermal
adhesive.
Type
Product
Name
Thermal
Adhesive
Tape
Bond play
100
Thermal
Adhesive
Grease
Thermal
Conductivity
Company
11 K/W thickness :0.127mm
area: 50mm2 (on slug)
0.8W/mK
Burgquist
company
9882
5K/W thickness: 0.05mm
Area:625mm2(on metal pcb)
0.6W/mK
3M
Bond play
100
2K/W thickness: 0.127mm
Area:625mm2(on metal pcb)
0.8W/mK
Burgquist
company
384
1.2K/W thickness :about0.01mm
area: 50mm2 (on slug)
0.757W/mK
Henkel
TCR
1.4 K/W thickness:about0.01mm
area: 50mm2 (on slug)
2.0W/mK
Electrotu
be
0.815W/mK
Holdtite
Thermalink
@38
Thermal Resistant
Experiment Result in SSC
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20. TJ Vs heat sink area
1000
o
Junction temperature( C)
1 Chip
4 Chip
100
10
100
1000
2
Heat sink area(mm ) (1.5t Al Heat sink)
20. TJ Vs MCPCB Thickness
2.5*2.5*T square Al MCPCB
on 1W P3 white.
45.00
40.00
35.00
Temp(℃)/RΘ
30.00
25.00
Tj
RΘ(J-B)
20.00
15.00
10.00
5.00
0.00
1.2
1.6
MCPCB thickness(mm)
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22. Simulation results for passive heat sink(type 1)
Before
simulation
After
simulation
Note : Above these two figures shows just temperature distribution
qualitatively not quantitatively
23. Trend of temperature change vs. base thickness change
(input load limit : 5~50 watts)
1W Z-power LED (1EA)
40
38
9
Rtheta
8
34
7
32
6
30
5
28
4
26
3
24
2
22
1
20
2
4
6
RΘ is from junction
to heat sink
Rtheta[K/W]
temperature['C]
36
10
chip
inner heat sink
MCPCB
heat sink
0
base thickness[mm]
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
2.5W Z-power LED (1EA)
50
10
8
to heat sink
40
6
35
4
Rtheta[K/W]
temperature['C]
45
RΘ is from junction
30
25
20
chip
inner heat sink
MCPCB
heat sink
2
2
Rtheta
4
6
0
base thickness[mm]
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24. Trend of temperature change vs. quantities of LEDs
Chip
Inner Heatsink
MCPCB
Heat Sink
o
Temperature( C)
40
30
20
10
0
1
2
3
Chip Quantity(EA)
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TEL : 82-2-3281-6269 FAX : 82-2-857-5430
24. Simulation for array LED on Metal PCB
When 1W blue LEDs array among 0.5~5mm each Led on 200*25*2 mm
aluminum metal PCB , The following data is about Temperature of Tj,
MCPCB and RΘ from junction to MCPCB.
The number of LED is 10.5, 10, 9.5, 8.5, 8 and 7 ea according to distance
among LEDs.
℃/RΘ
Array LED on MCPCB
Distance among LEDs