Modeling of Heat Dissipation in SMD Chip
Components
Radu Bunea, Paul Svasta and Norocel-Dragos Codreanu
Center for Technological Electronics and Interconnection Techniques
“Politehnica” University of Bucharest, UPB-CETTI
Bucharest, Romania
[email protected]
Abstract— The ever-rising pressure to reduce circuit board size
to fit the ever-shrinking chassis challenges design engineers not
just with size constraints, but power and thermal constraints
concurrently.
The power dissipation of a circuit is not
necessarily reduced as technologies advance and circuits become
smaller. It may not be an acceptable option to reduce the power
dissipation and there may even be a drive to increase it.
Reliability is a major concern of all electronic products
manufacturers, and it is given by the reliabilities of the electronic
components. One factor that determines the operation time of
electronic components is temperature, and to be more precisely
heat dissipation. There appears the problem of how heat is
dissipated from SMD components, especially when no forced
cooling is available.
Keywords-SMD, heat dissipation, thermovision, simulation.
I.
THEORETICAL ASPECTS OF HEAT TRANSFER FROM SMD
COMPONENTS
The heat transfer from SMD components has always been a
subject of discussion because of the high number of variables
that are involved in the process. We have to take into account
the size of the component, the possibility of forced convection,
the pad size, the track size and many other aspects. We tried to
investigate the factors that influence the peak temperature of
the component bodies and provide some guidelines for the
industry when designing high-density boards.
Most of the electronic modules which imply very small size
SMD components do not provide convection cooling of the
electronic components. In our model we did not take into
consideration convection, but only conduction. In figure 1 and
figure 2 we present the model for the heat conduction from the
SMD resistor. The heat is generated in the resistive layer of the
component by Joule heating, and is then conducted through the
component body towards the solder joint. The solder joint
absorbs part of the heat and generates internally an amount of
heat (also by Joule effect) and transfers the rest to the pad. Here
takes place an interesting phenomenon. The heat flux will be
divided and will be conducted both through the PCB track and
through the substrate. The most heat is conducted by the PCB
track because of the high thermal conductivity (380W/m·K for
copper), but is also transferred to the substrate on all track
length.
Fig. 1 Heat conduction through the SMD system
To compute the temperature at any distance from the
components, we will start with the computation of the
temperature of the component body. The heat generated in the
component is [1]:
Q=I2Rt
(1)
where Q is the generated heat by Joule-Lenz effect, I is the
electric current, R is the component resistivity, and t is the
time.
The thermal balance equation states that the heat generated
by the component is partly stored inside the component body
and partly dissipated to the environment [1],[2]:
Q = Wa+Wev
(2)
Or
t
t
0
0
 u  i  dt  Cth  T  Ta   
1
 T  Ta   dt
Rth
(3)
where Cth is the thermal capacity of the body and Rth is the
thermal resistance of the body. It results:
TCM  Rth  P  Ta
(4)
where TCM is the component body temperature, Rth is the
thermal resistance, P is the electric power dissipated in the
component and Ta is the environment temperature.
The conduction thermal resistance is:
A
Rthc  λth 
l
(5)
where λth is the thermal conductivity of the material, A is the
cross-section area of the body and l is the distance from the
initial and final point of interest.
By using recurrently the formula of the thermal conduction
power, we can obtain the temperature at any point on the
board [2]:
dQ
dT
 th A
dt
dx
(6)
where λ is the conductivity of the material, A is the
conduction area, dT is the temperature difference and dx is the
distance between the initial point and the point of interest.
Note that the formula can be used only for homogeneous and
isotropic materials. It can be observed that the area of
conduction has a major influence on the conducted heat, and
subsequently to the maximum temperature of the component.
Taking into account the previous theory aspects, we propose
a heat conduction model as equivalent schematic of electric
circuits:
Fig. 2 Model of heat transfer through SMD components at stationary
functioning regime
II.
THERMOGRAPHICAL ANALYSIS OF SMD COMPONENTS
In order to validate the model proposed, we developed a
series of test boards on which we took into account the
component sizes, distance between components, pad size and
track dimensions (Figure 3).
sprayed from a tube. This has given a picture with uniform
temperatures at room temperature which means that the effects
of ambient reflections were eliminated. The very thin painting
is expected to not produce any change in the heat transfer
distribution. We have used a FLIR SC640 thermovision camera
with Germanium macro lens, in order to obtain a veru precise
temperature distribution on a small area.
We used components with common sizes: 0402, 0603,
0805, 1206 and 2510. The 10Ω resistors were daisy-chained
and charged in steps of 10mA till the rated power. It was
observed, as expected, that the temperature of smaller size
components was higher.
In order to investigate the influence that the distance
between components has on the maximum temperature of the
component bodies (thermal coupling), the resistors were
charged two by two. The components were placed at different
distances from each other: 0.25mm, 0.5mm, 1mm, 2mm,
5mm. For all component sizes, it was noted that the
component body temperature increased as the distance
between them decreased. At 5mm gap between components,
the thermal coupling is negligible, as the component
temperature is the same as if the component was alone on the
board. The maximum heat difference occurred for large
components, and it was 4°C between the smallest and largest
gap.
(a)
Figure 3. Test board for investigating the influence of the distance between
components and track width on the maximum temperature of the component
body.
Because the boards were not provided with coating material
there were problems detected in measurement of bare copper
tracks and solder joints. It is a well known issue of the infrared
measurement that the shiny surfaces are difficult to measure.
The observed phenomenon was that the temperature of the
tracks seams to be higher than the rest of the board, although
their emissivity is much lower. This is due to reflected heat
from ambient. A solution possible to be tried in latter
experiments will be to do the measurements in a closed (dark)
box. We have decided to coat the boards with a mate dye,
(b)
Figure 4. Temperature distribution on a board with 0402 size resistors with (a)
0.25mm, and (b) 5mm gap between them. The temperature is higher than the
case of single components for (a) and equal for (b).
The pad size influence the amount of solder paste and
subsequently the size of the solder joint. More solder paste
would result in a solder joint with a higher cross-section area.
Even though the solder joint thermal resistance can be up to 10
times lower than the thermal resistance of the copper track, the
significantly larger cross-section area leads to a higher heat
flux that can be conducted through it [3], [4].
As a result, the dimension of the PCB track has a much
more significant influence on the heat conduction, as its very
small cross-section area will highly restrict the heat conducted
from the solder joint. We developed boards with different
PCB tracks widths leaving from the pads: 0.25mm, 0.5mm,
1mm and 2mm. In all cases, it was proven that higher track
widths lead to a better heat conduction and smaller component
temperature. This could be also found through calculus, as
stated in the first paragraph.
Fig. 6. First setup of the study: the boards contain daisy-chained test
points (resistors and copper straps) and the boards are daisy-chained together.
Figure 5. Temperature distribution on a board with 0603 size resistors with
different tracks widths. The temperature is higher in the case of smaller width
tracks.
III.
Contrary to our expectations, the maximum current that
could be applied to the circuit was of 0.3A.
From this setup, we were able to draw two conclusions.
First, the thermal behavior and the current capabilities of the
circuits were different, according to the soldering technology
used: the worst was the IR convection with cooling, as it can
be concluded from figure 7. Second, the solder joint did not
heat very much. The heated elements were the tracks and,
even more, the component bodies. As it can be observed in
figure 8, the temperature of the 0 Ohm resistor is 172.6°C. The
solder joint temperature is 64.1°C, but this temperature is
mostly obtained by conduction from the resistor.
STUDY ON THE INFLUENCE OF SOLDER JOINTS ON THE
MAXIMUM TEMPERATURE OF THE COMPONENT
The results until this point lead to the necessity of further
investigation of the solder joint behavior when charged at
different currents.
For the first tests, we used FR4 substrate boards with
1.5mm width tracks and 1206 0 Ohm resistors. We used the 0
Ohm in order to reduce to a minimum the power dissipation
on the component bodies. The resistors were daisy-chained to
have multiple points of observation. More points of
observation that would show the same behavior would suggest
a correct test and correct results [6].
The team developed identical boards for the test, the only
difference being the soldering process: IR convection with and
without cooling and vapor phase soldering. All the boards
were daisy-chained together, to obtain the same observation
parameters (voltage and current) and compare them at the
same time.
Fig. 7. Different thermal behaviors of the circuits, given by the different
soldering technologies.
Figure 9. Simulation of a 1206 SMD resistor powered at 0.5W. The results
are very close to a real component in similar conditions (substrate, track width
and thickness, pad area etc.).
V.
Fig. 8. The body of the 0Ώ resistor heats very much, and the temperature
of the solder joint increases by conductivity.
In order to reduce the thermal influence of the component
bodies, we replaced the resistors with copper straps (2.5mm
diameter), which have very high current capability. The result
was that the circuits could withstand higher currents (up to
3A), but the elements that heated very much were the tracks,
and again by conduction the solder joint.
IV.
HEAT TRANSFER SIMULATION
The designers of electronic modules have a very useful tool at
hand to optimize their projects from thermal point of view:
simulation. They can evaluate the thermal response of the
system even before the fabrication of the first prototype, and
find solution for optimization. This leads to dramatic decrease
of the costs of development of the electronic product, as fewer
prototypes are necessary and the time to market is decreased.
Nowadays there is available a wide range of thermal
simulation software, each with advantages and disadvantages.
One software that is widely used in thermal analysis in the
electronic design field is Flotherm, a powerful 3D
computational fluid dynamics software that predicts airflow
and heat transfer in and around electronic equipment,
including the coupled effects of conduction, convection and
radiation. Its drawbacks are the limited grid dimension and the
fact that it can only deal with heat and not be able to simulate
electric current input[5].
There are software programs that overcome the drawbacks of
Flotherm, but are not so oriented on electronics. One such
example is Ansys, a multi-field solver. The designer can
analyze the thermal response as response of an electric current
input, and go even further and take into account mechanical
movement (for example pressure applied to a membrane and
electric output – pressure sensor).
CONCLUSIONS
The component size affects the maximum temperature of
the component body for the same electrical resistivity and the
same current: a smaller dimension component will have a
higher temperature.
The distance between components affect the maximum
temperature of the component body: the closer the components
are, the higher the temperature. At a distance of 5mm between
the components, the thermal coupling can be considered null
for all component sizes.
Even though the thermal conductivity of the solder joint
can be up to ten times lower than the thermal conductivity of
copper, its high cross-section area compensate and the heat flux
is conducted through the solder joint without any significant
restriction.
The tracks dimensions have a major effect on the maximum
temperature of the component. This is where the heat flux is
restricted by the small cross-section area, and designers can use
this fact in order to optimize the thermal regime of the
electronic component (increase the track with, therefore the
cross-section area).
ACKNOWLEDGMENT
The work has been funded by the Sectoral Operational
Programme Human Resources Development 2007-2013 of
the Romanian Ministry of Labour, Family and Social
Protection
through
the
Financial
Agreement
POSDRU/6/1.5/S/19 and by ANPCDEFP/Romania under the
project
E-learning
microsystems
technologies"
(MSYSTECH), contract no. LLP-LdV/ToI/2008/RO/003.
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[6]
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