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Paths of the Heat Flow
from Semiconductor
Devices to the Surrounding
Krzysztof Górecki, Janusz Zarębski
Department of Marine Electronics
Gdynia Maritime University, POLAND
S
Outline
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Introduction
Elements of heat flow path
Thermal models of semiconductor devices
Results
Conclusions
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Introduction (1)
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One of the main problems restricting the development of the microelectronics
is efficient abstraction of the heat generated in the semiconductor structure to
the surrounding.
The limited efficiency of practical cooling systems causes that the internal
temperature of semiconductor devices increases, over the ambient
temperature.
The device internal temperature rise is a basic factor worsening the reliability
of electronic elements and circuits comprising these elements.
It is so important to develop efficient methods of cooling devices and
electronic circuits.
Producers of semiconductor devices aim at reducing the thermal resistance
between the semiconductor chip and the device case.
The main task of the classical device package is to protect it from corrosion
and mechanical hazards and it has to guarantee the possible low value of the
thermal resistance between the semiconductor chip and the case surface of
the device.
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Introduction (2)
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Cases of devices have different constructions depending, among
others, on the semiconductor chip size, the manner of the device
setting-up and the power dissipated into the device.
The construction of the case of semiconductor devices is very
important, but it has not a decisive meaning in the global thermal
resistance between the chip structure and the surrounding.
Depending on the applied system of the device cooling it is
necessary to take into account thermal properties of the other
elements of the heat abstraction path.
In the paper we present the initial effects of our research devoted
to the influence of the properties of the device case, the manner of
the montage, the size of paths on the printed circuit board, the size
of the heat-sink, systems of the affected cooling and the thermal
property of the case of the whole electronic equipment on the
thermal parameters of semiconductor devices.
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Elements of heat flow path (1)
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The heat generated in the semiconductor chip is dissipated to the device
case due to the thermal conduction phenomenon.
The construction of the device case is determined by the device producer,
whereas further elements of the heat flow path depend on the
constructor-engineer of the electronic equipment.
The heat can be transported from the case to the surrounding by
conduction, convection and radiation.
Conduction is realized by the device metal terminals, next by the solder
areas and conductive paths on PCB. The second possibility of the heat
transport by conduction is realized from the device case surface through
the insulating washer to the heat-sink.
Convection exists on each surface of the contact between the solid-state
situated on the heat flow path and the surrounding fluid.
The simplest case of convection is the natural one occurring on the device
case surface devoid of any contact with the heat-sink, on the surface of
the PCB or on the heat-sink.
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Elements of heat flow path (2)
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Radiation comes from each surface of the elements existing in the heat flow path,
beginning from the device case, through the PCB and the heat-sink to the case of
whole the electronic equipment.
Usually, the device is the component of the electric equipment situated inside its case
(EC). This EC protect the considered equipment from mechanical shocks, but on the
other hand, it makes the generated heat abstraction difficult.
Depending on the size, construction and the kind of material from which the device
case is made, convection and radiation have the dominant role in the heat transport
from the device case to the surrounding.
Such parameters as the heat conductivity, thermal emissivity, the heat transfer
coefficient characterizing the heat flow path have various values depending on the
kind of material and the geometrical sizes of these elements.
Additionally, the temperature of the cooling surface or the difference between the
temperatures of the cooled surface and the cooling fluid influence the values of the
parameters mentioned above.
The description of the device heat properties are a very important and complex task.
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Thermal models of
semiconductor devices (1)
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To estimate the values of the devices internal temperatures the device
thermal models, describing the heat transport from the chip to the
surrounding are used.
The effectiveness of the heat transport from the semiconductor chip to the
surrounding can be described using the heat conduction equation with
appropriate boundary and initial conditions, or by lumped thermal models,
based on the concept of transient thermal impedance Zth(t).
The phenomena that determine the effectiveness of this process are not
easy to model. Due to the dependence of the efficiency of heat dissipation
mechanisms on the device temperature, the thermal model of such a
semiconductor device is nonlinear.
In the thermal analysis of the semiconductor device a form of the electrical
analog of the thermal model is commonly used. This analog is of the form
of the RC network.
Often, semiconductor device thermal models characterizing the heat flow
path from the semiconductor structure to the device case are provided by
the manufacturers of these devices.These models do not allow including in
the design the impact of the PCB, heat sink, pins, the case of electronic
equipment comprising the considered device, on the course of the transient
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thermal impedance of the device under consideration.
Thermal models of
semiconductor devices (2)
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From the perspective of a designer of electronic equipment is important to
determine the total thermal transient impedance from the structure of the
semiconductor device to the surrounding, taking into account all the
mechanisms of heat dissipation and all the ways of its movement.
The effectiveness of heat transfer from the semiconductor device case to
the surrounding is affected by many factors, the inclusion of which is not
trivial. Such factor may be:
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temperature,
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power dissipated in the investigation device,
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thermal coupling between devices,
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the size of the heat sink and other elements making up the path of the
heat flow and its spatial orientation,
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the coolant flow rate in the cases of liquid cooling components,
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the length of leads,
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solder surface fields,
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properties of the case of electronic equipment containing the
investigated semiconductor device.
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Thermal models of
semiconductor devices (3)
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The general form of the device thermal model
Tj
The junctioncase thermal
model A
B The thermal
interface model
The case –box
pth
D thermal model
C The heat-sink
thermal model
The case –PCB
E thermal model
The heat-sink-box
F thermal model
The PCB-box
G thermal model
Ta
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The box-ambient
H thermal model
The structure of each component of the considered model (blocks A – H ) is
of the form of the nonlinear RC networks.
The influence of the external equipment case on the device thermal
properties are modeled by changing the temperature of the air existing inside
the equipment case depending on the air temperature outside the whole
equipment and the power dissipated in the device.
To formulate the proper dependencies describing the model, some
measurements of the thermal resistances and the transient thermal
impedances of devices operating at various cooling conditions are
indispensable.
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Results (1)
Rth [K/W]
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The dependence of the thermal resistance of the diode ZPY56 with the glass case
DO-41 on the diode current.
•the nominal length (l = 25 mm) of the
220
B
ZPY 56
metal leads (dashed lines)
200
•the shortened ones when l = 5 mm (solid
180
lines),
160
•A – the copper leaf of the area S = 38x15
140
A
mm,
120
C
•B – the cooper leaf of the area S = 15x3
100
80
mm
0
100
200
300
400
500
600
•C – the monolithic cooper of the
i [mA]
dimensions 3x27x75 mm.
As seen, increasing the solder areas causes a decrease of the device
thermal resistance, similarly to an increase of the diode current or
shortening of its metal leads.
after increasing the diode metal leads 5 times for the device having long leads, the
thermal resistance value was reduced over a dozen or so percent, whereas
shortening these metal leads from 30 to 5 mm at the same value of the solder
areas, caused a decrease of the considered thermal parameter to about 30%.
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Results (2)
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The device thermal transients are well described by the course of its transient
thermal impedance Zth(t), which allows estimating the time indispensible to get the
thermal steady-state in the device.
Fig. shows the courses of the transient thermal impedance normalized with respect
to the thermal resistance for the transistor BC109 at three kinds of the device
cooling. 1
0,9
BC 109
0,8
Zth(t)/Rth
0,7
large heat-sink
0,6
0,5
small heat-sink
0,4
0,3
0,2
no heat-sink
0,1
0
0,0001
0,001
0,01
0,1
1
10
100
1000
t [s]
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The set-up time of the thermal steady-state (when Zth(t)/Rth ≈ 1) depends on the
device thermal conditions. For the transistor BC109 without any heat-sink, with the
small and large heat-sinks the set-up time is equal to 200 s, 600 s and 80 s,
respectively.
It is worth mentioning that the large heat-sink assures practically ideal cooling of the
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device case, therefore in such conditions the set-up time has the lowest value.
Results (3)
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Fig illustrates the influence of the space orientation of the heat-sink on the power
MOS transistor situated on the heat-sink
In this figure the courses a, b and c represent the heat-sink placed horizontally with
the upwards and downwards cooling fins as well as the heat-sink situated vertically.
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IRF840 on the large heat-sink
5,8
5,6
b
a
Rth [K/W]
5,4
20%
5,2
5
c
4,8
4,6
4,4
4,2
4
0
5
10
15
20
25
30
p [W]
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The most efficient heat abstraction from the device on the heat-sink assures its
vertical position.
The worse case of the heat abstraction is when the heat-sink is situated horizontally
with downwards cooling fins.
For these two cases the differences of the device thermal resistance are equal to
about 20%.
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Results (4)
A great influence on the device thermal parameter value has its case.
In Fig. the results of measurements of the dependence of the thermal
resistance of the monolithic voltage regulator LT1073 on its input voltage
VSUP at different cooling conditions are presented.
The investigated IC was situated on the PCB of the dimensions 110x105 mm.
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80
LT1073
Rth [K/W]
75
c
70
a
b
65
d
60
55
50
0
2
4
6
8
VSUP [V]
10
12
14
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• For the PCB situated horizontally (curve a)
the thermal resistance of the device is by
about 5% higher than in the vertical position
of the PCB (curve b).
• Situating the PCB inside the perpendicular
metal box of the dimensions 83x148x150 mm
(curve c) causes an increase of the device
thermal resistance value by another 5%.
• Using the external heat-sink (curve d)
causes a decrease of the thermal resistance
value of the LT1073 by even more than 20%.
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Conclusions
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From the presented results it is seen that the multipath flow of the heat
dissipated in the device causes essential changes of its thermal parameters
values.
The complexity of the description of transportation of the heat dissipated in
the device and removed to the surrounding often causes that the projects of
the cooling systems are made by the method of “trial and error”.
Therefore, the sense of purpose of the investigations leading to formulate the
device thermal model including the heat flow multipath is fully fulfilled.
Formulating the multipath thermal model of the device is the main aim of the
research project realized currently by the authors.
This task demands among others performs a lot of measurements of the
device thermal parameters with the use of various device cooling systems and
formulating the analytical dependencies describing the influence of the cooling
system technical parameters on the device thermal parameters.
The results of the preliminary investigations show that the thermal model
under test should be the nonlinear one, taking into account a lot of factors, as:
the ambient temperature, the kind of the device case, the solder areas, the
heat-sink dimensions and its space orientation and the device dissipated
power.
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