THERMAL DESING AND ANALYSIS OF HIGH ELECTRONIC BOARDS
J. Collado
Spring, 2000
CHT
Page 2 of 18
TABLE OF CONTENT
1.
INTRODUCTION .....................................................................................................................................3
2.
THERMAL REQUIREMENTS .................................................................................................................3
3.
TEMPERATURE IMPACT ON ELECTRONIC BOARDS .......................................................................4
3.1
4.
TEMPERATURE EFFECT ON COMPONENT FAILURE ............................................................................................. 4
THERMAL DESIGN CONSIDERATIONS ...............................................................................................4
4.1 FUNDAMENTAL CONCEPTS ................................................................................................................................. 4
4.2 THERMAL RESISTANCE TO CONDUCTION ........................................................................................................... 5
4.2.1
Component Internal Thermal Resistance ................................................................................................. 5
4.2.2
Component to Mounting Surface Heat Transfer ...................................................................................... 5
4.2.3
PWB to Heat Sink Heat Transfer ............................................................................................................. 6
5.
CASE STUDY “THERMAL DESIGN AND ANALYSIS OF A TYPICAL HIGH POWER ELECTRONIC
BOARD” ..........................................................................................................................................................8
5.1
5.2
ANALYSIS INPUTS............................................................................................................................................... 8
ANALYSIS METHOD ................................................................................................................................... 10
6.
RESULTS ..............................................................................................................................................11
7.
CONCLUSIONS ....................................................................................................................................11
8.
REFERENCE.........................................................................................................................................11
APPENDIX .....................................................................................................................................................17
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1. Introduction
The reliability of an electronic device depends primarily on its operating temperature. For maximum
reliability, devices must operate at temperature below their maximum rated value. Heat affects the
electrical characteristics of semiconductors, the chemical properties of metals used in semiconductors
devices and integrated circuits production and the mechanical properties of printed wiring board (PWB)
materials. Heat affects these materials in a form detrimental to circuit operation. For example, gold and
aluminum can form inter-metallic compounds at high temperatures, thus weakening lead bonds within the
component. High temperatures also increase the activity rate of corrosive elements in solder pastes.
The majority of electronic components, passive and active, as well as the traces on the printed wiring
boards generate heat when they are operated. This heat raises the temperature of the component in which
it originates, the surface upon which the component is mounted, and other devices close to it. The thermal
management of high power PWBs is of increasing importance as the power density of components and
circuits continues to rise. The situation is complicated by the severe environmental requirements, limited
weight tolerance, and the use of boards with multiple sheet of copper embedded in the electrically insulating
boards. Given the high thermal conductivity of copper, the thermal properties are drastically altered by the
embedded layers, possibly introducing overall anisotropy. As a result of these conditions, thermal analysis
must be an integrated step with component placement, routing, mechanical analysis, and reliability
prediction. The use of thermal analysis software helps to identify thermal problems during the early stages
of design. It also provides various options to resolve possible thermal problems during the design process.
This process reduces costly corrections and reduces design cycle time by predicting the thermal
performance before the first prototype is developed.
A study case selected to illustrate the modeling techniques in the thermal design of a high power PWBs is a
power supply board of an electronic engine control. The engine controller is subjected to severe
environmental constraints. External unit cooling is provided mainly by natural convection and, to a smaller
extent, to thermal radiation to the engine bay. The ambient air temperature can range from -55C to 75C
(118C during unit qualification testing). Inside the engine controller are 6 boards, 2 power supply boards, 2
Processor boards, and 2 Input boards. Each board is bonded to an aluminum heat sink, and the heat sink
are screwed to the chassis to maximize conduction cooling of the boards. The total power dissipation of a
single power supply board is 25.8 Watts. Thermal math modeling of the design was accomplished using
SINDA. SINDA (Systems Improved Numerical Differencing Analyzer) is a finite difference thermal analysis
program.
2. Thermal Requirements
Thermal management of electronic boards, especially high power electronic boards, begins at the procuring
activity with the derivation of the thermal environments and performance criteria. This effort culminates in
the baseline requirements and other essential thermal design information to be included with the equipment
specifications that will be used in defining the thermal management needs of the unit and boards. The
requirements should be based on availability and life cycle cost objectives. The following items should
typically be included in the baseline thermal requirements of each equipment specification:
1. Maximum junction temperatures and dereating criteria
2. Thermal environments in which the equipment is required to operate:
a. Ambient temperature
b. Thermal radiation environment
c. Solar heat loads
3. Design constraints:
a. Noise and vibration limits
b. Size limit
c. Weight limits
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3. Temperature Impact on Electronic Boards
The temperature at which an electronic device operates is one of the primary determinants of its reliability.
Heat dissipated in the device during operation escapes through a path consisting of one or more series of
thermal impedances terminating in the surrounding air. The presence of these nonzero thermal
impedances cause the temperature of the device to rise above that of the heat sink.
3.1 Temperature Effect on Component Failure
High temperature is an enemy of most electronic PWBs and components. It causes slow progressive
deterioration that eventually results in catastrophic failure. The time to failure of each part is a statistical
function of its stress level and the complex interrelationship of thermal history and chemical structure. The
failure of individual parts leads to unit failure. Many failure modes involve changes in the chemical
composition or physical structure of the board and the electronic devices. The rate of these chemical
reactions or physical changes increases with increasing temperature. Thus, operation at high temperature
increases the failure rate, or decreases the mean time between failures, by accelerating the rate at which
these undesired changes occur. Conversely, failure is also accelerated at very low temperatures. Thermal
stresses resulting from differential thermal contraction of dissimilar material, embrittlement of materials, and
shorting and corrosion caused by condensed water trapped within the encapsulation are typical failures
induced by very low temperatures. The table below summarizes some of the effects and the possible
failure mechanism induced by very high and very low operating temperatures.
Condition
Hot Temperature
Cold Temperature
Thermal Cycling
Thermal Shock
Effects
Insulation Deterioration
Oxidation
Over Expansion
Softening/Hardening
Chemical Changes
Contraction
Viscosity Increase
Embrittlement
Ice Formation
Temperature Gradients
Expansion/Contraction
High Temperature Gradient
Possible Failure Modes
Shorting
Rust
Physical Damage
Physical Breakdown
Dielectric Loss
Wear, Structural Failure
Loss of Lubricity
Structural Failure, Cracked Parts
Structural Failure
Repeated stress variation causes
mechanical failure of part, solder
joints, and board delamination
Mechanical failure, Cracks, Rupture
4. THERMAL DESIGN CONSIDERATIONS
4.1 Fundamental Concepts
Thermal Resistance
Energy, in the form of heat, flows naturally from a hotter region to a cooler one. The greater the
temperature difference (T), the greater the heat-flow rate (Q) will be. This relation can be written as:
T = Q
Where the proportionality factor () is called the thermal resistance. If T is expressed in C and Q is in
Watts (W), then the units of  are C/W.
Thermal Dissipation
Electronic equipment requires electrical power to operate. Because the parts are not perfectly efficient,
much of the power is converted to heat, which causes the temperature of the equipment to rise. The
temperature will continue to rise unless the heat can find a path by which to leave the equipment. The
purpose of the cooling system is to provide an effective heat path. Therefore, the most important heat-flow
rate is usually the thermal dissipation.
Sinks
In the cooling of equipment, heat must be transferred from the part in which the heat is generated to an
ultimate sink. This ultimate sink is a heat reservoir, such as a body of land, ambient air, the sky, or the
ocean. It is the final destination of the heat. Such a heat reservoir can be considered to be infinite. That is,
it is so large that the heat transferred from the equipment does not produce a significant increase in the
reservoir’s temperature.
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The Cooling Equation
The essence of the problem of cooling electronic equipment can now be described. For a component
having a dissipation (Q), a sink temperature (T sink) (which could be the ultimate sink, local sink, or heat sink
temperature), and package such that the thermal resistance between the part and the sink is , the
operating temperature (Tcomp) is given by:
Tcomp = Tsink + (Q)
4.2 Thermal Resistance to Conduction
4.2.1 Component Internal Thermal Resistance
The temperature at which a semiconductor device operates is often referred to as the “Junction
Temperature”. Heat dissipated in the device during operation escapes through a path consisting of one or
more series thermal resistances terminating in the surrounding ambient, refer to Figure below.
Junction-Ambient
Case-Ambient
DIE
CASE
Junction
Junction-Case
The presence of these nonzero thermal resistances causes the temperature of the device to rise above that
of the ambient. Each of the components of the overall thermal resistance causes a rise in temperature,
which is linearly dependent on the power dissipated in the device. The  value for each thermal resistance
represents the amount of temperature rise across the resistance as a function of the power dissipation.
Usually,  is given a subscript indicating the two point between which the resistance is measured. Thus the
junction temperature of an operating device is given by:
TJunction
T
 T Amb

 ConductionSink  Pd 

 case Sink

  junctioncase  Pd    case Amb
1
1





 case Amb  case Sink


Where:
Tjunction
= Component Junction Temperature
Tamb
= Ambient Temperature
Tconduction Sink = Temperature of the Mounting Surface
Pd
= Power Dissipation
junction-case = Junction to Case Thermal Resistance
case-Amb
= Case to Ambient Thermal Resistance
case-sink
= Case to Conduction Sink Thermal Resistance
The internal thermal resistance of a given device (junction-case and case-Amb) are dependent on several factors,
the package type, the package material, the way the die is attached to the package, etc. Normally these
thermal resistances are provided by the component manufacturer in the component data sheets.
The case-sink is dependent on the way the component is mounted to its mounting surface. This thermal
resistance is normally calculated by the thermal design engineer based on the actual mounting
configuration used.
4.2.2 Component to Mounting Surface Heat Transfer
Heat is generated within the active components in the unit. This power must somehow be transferred to the
component mounting surface with minimal temperature rise. The problem can be compounded by
extremely high power densities, 0.4 W on a part with a 0.05” X 0.2” case footprint for 40 W/in 2 (typical of
diodes). This is an order of magnitude higher than a 208 Pins Quad Flat Pack with a power dissipation
equal to 4 W and a case footprint equal to 1” X 1” for 4 W/in2 power density.
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The heat path from the component to its mounting surface is commonly accounted for by hand calculations
for each active component. Many components have similar mounting techniques, so the same hand
calculation may apply to numerous parts thus does not need to be redone very often. Due in part to the
extremely tight reliability and weight requirements of aerospace electronic equipment, there are three basic
design features available to minimize the thermal resistance from the component to its mounting surface.
All three involve mainly conduction as the surface area available for radiation directly off the component is
negligible, and natural convection cooling is only a secondary cooling path.
4.2.2.1 Component Leads
Conduction out of the component leads is always the first path considered. Typical lead resistances vary
from 1000C/W per lead for Kovar leads on microcircuits to 12C/W per lead for copper leads on diodes.
The lead thermal resistance is given by:
Lead = L / (KA)
Where,
L = Lead length (measured from the edge of the component case to the soldered pad on the PWB)
K = Thermal conductivity of lead material
A = Cross sectional area of the lead
4.2.2.2 Bonded Joints
For any component with significant thermal dissipation, a conductive bonding material could be used to fillet
between the component case and its mounting surface. Either electrically conductive (soft solder or metal
filled cements) or non-conductive (epoxy or ceramic filled cements) bonding materials may be used,
depending on whether electrical isolation is required. The selection of the bonding agent is normally based
on component shape, bonding technique, structural (bond strength) and removal requirements, as well as
thermal conductivity. The mounting surface is generally a conductive copper plane or aluminum heat sink
bonded to the PWB to then spread and carry the heat away from the component.
The thermal resistance across rectangular or circular bond interfaces is given by:
Lead = L / (KA)
Where,
L = Bond pad on the PWB)
K = Thermal conductivity of bonding material
A = Cross sectional area of the bonded interface
The thermal resistance for heat flow by conduction between isothermal cylindrical component bonded to the
mounting surface is given by:
1

4 KW
( a 2  1)1 / 2
2 
 

 2
 2
1/ 2
( a  1)1 / 2 tan( 1 ) 
 1  ( a  1) tan( 2 ) 

1
2
 tan 
  tan 
  2 KW (2  1 )
a

1
a

1










Where,
K = Thermal conductivity of bonding material
W = Length of the bonded section (or lead)
 = Fillet angle
a = (2x/D + 1) = (x/r + 1)
x = Gap between bonded body and mounting surface
D = Diameter of cylindrical body
r = Radius of cylindrical body
4.2.3 PWB to Heat Sink Heat Transfer
Once in the component mounting surface, the heat must somehow be transferred to the heat sink or metal
chassis. The PWB is a very poor lateral conductor, thus extreme care must be used in insuring proper
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paths are available. This heat transfer is generally accounted for by developing a detailed thermal math
model of all possible paths within the PWB. The thermal math model must correctly account for the
component power density by outlining all significant dissipaters. The most common error resulting in
optimistic temperature predictions results from nodes that are too large. This result in much lower power
densities being applied to the PWB surface thus missing potential local hot spots. Nodes that are too large
also miss the constriction resistance that is present at local hot spots. As a general rule, all significantly
dissipating components (over 0.5 Watts) must also be outlined with node boundaries to insure proper power
densities, and all mounting points must also be outlined with node boundaries. Models of this type require
complex model generating software, which are readily available from commercial software vendors. There
are three basic paths to transfer heat from the PWB to the heat sink or chassis.
4.2.3.1 Lateral Conduction to Mounting Points (X-Y Direction)
The PWB is a very poor lateral conductor being mostly epoxy (K = 0.01 W/inC) and typically 0.06” to 0.09”
thick. Therefore continuos copper planes embedded in the PWB or aluminum heat sinks bonded to the
PWB surface are used to enhance this lateral conduction. Copper planes within the PWB are generally
0.0014” to 0.0028” thick per layer and can be used to conduct small quantities of heat to nearby mounting
points only if they are continuous. Any discontinuity in the copper adds an extremely large series resistance
to the heat flow path. Even a 0.05” gap in the copper plane (sometimes required for electrical isolation)
between the component and the PWB mounting point will result in a significant temperature increase. If the
discontinuity occurs around the component case, it is doubly bad as it not only prevents conduction to the
mounting point, but also prevents any spreading of the heat for radiation and convection heat transfer.
These discontinuities must be properly accounted for in the thermal model. Assuming full copper planes
around all components and mounting points can have a catastrophic effect as significant series thermal
resistances will be missed resulting in optimistic temperature predictions.
4.2.3.2 Conduction Through the PWB (Z-Direction)
Even if the heat gets to a local mounting point, the PWB can act like an insulator in preventing heat from
being conducted from a top copper layer through the board into the chassis boss. Adding thermal vias
around the screw hole is commonly used to reduce that Z-direction resistance at the mounting boss.
Thermal via holes are drilled holes, usually 0.016” in diameter of bigger, with 0.001” thick copper plating on
the walls. These thermal vias greatly reduce the Z-direction conduction resistance in the PWB epoxy by
adding approximately 150C/W per via parallel resistance.
For very high power boards or for large, boards in general an aluminum heat sink is usually bonded to the
back plane of the board (side opposite to the components) in order to improve the board structural stiffness
or to greatly improve the lateral heat transfer capacity of the board. In this case, for components with
extremely high thermal dissipations (over 1 Watt), or component with very high power densities, the power
can not be efficiently conducted laterally in the copper planes even if they are continuous and must
somehow be given a Z-direction path directly to the metal heat sink. The Figure below shows a typical via
hole pattern for a high power semiconductor.
Copper Pad
Component Could be Soldered or
Bonded to this Pad
Via Hole
0.016” Drilled Hole, With 0.001” Copper (Cu)
Plating, Connecting Cu Pad on Top of The
PWB With Similar Cu Pad on the Back of The
PWB (Closest to Heat Sink)
Section A-A
Component Footprint
Typically no Thermal Vias Are
Placed Directly Under The Component
Section A-A
The approximate thermal resistance across a single thermal via is given by:
First we need to consider the lateral thermal resistance within the Copper pad from the heat source to the
thermal via for the top copper pad as well as the lateral thermal resistance to heat spreading from the
thermal via for the bottom copper pad. This is given by:
1 
Where:
r2 = Via thermal radius (*)
ln( r2  r1 )
2LK
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r1 = Thermal Via Outer Radius
L = Thickness of Copper Layer
K = Thermal Conductivity of Copper
(*) The value of r2 can be approximated by considering the total copper pad area (A) and dividing
this area by the total number of thermal vias within the copper pad, then if we consider this area
equal to the area of a circle we can determine the radius. This approximation assumed that
heat will travel an equal distance to each thermal vias.

4
(r2 ) 2 
A
Total # ofthermalvias
Now we need to consider the thermal resistance across the PWB. This is given by:
L
2 
K

4
2
(rout
 rin2 )
Where:
L = Thickness of the PWB
K = Thermal Conductivity of Copper
Rout = Outer Radius of the Thermal Via
Rin = Inner Radius of the Thermal Via
Finally the total thermal resistance across a single thermal via is given by:
Total = (21) + 2
But normally a number of thermal vias are used to greatly reduce the thermal resistance across the board,
so the effective thermal resistance for a given number of thermal vias is given by:
 Effective

 1 

 # vias  

Total



1
The final path that need to be identify when considering the thermal design of the PWB are the thermal path
between the heat sink and the chassis and the thermal path between the chassis and the ultimate heat sink
(ambient air, cooling liquid, heat exchanger, etc.). For the purpose of this paper these two elements will not
be discussed.
5. CASE STUDY “Thermal Design and Analysis of a Typical High Power Electronic Board”
5.1
Analysis Inputs
Physical Design
The EEC150 (Electronic Engine Control) consists of the chassis, 2 Power Supply boards, 2 Input boards, 2
Processor boards, 7 interconnect boards, 6 pressure sensors, and the PB sensor heater (high pressure).
Internally the unit is broken down into two channels, A and B, with each channel consisting of a power
supply board, an input board, and a processor board.
The two rectangular sections of the chassis are geometrically identical to each other. Each section is
machined from a large block of alloy A356 (AMS4218) aluminum. The base and side walls are 0.110” thick
with overall unit dimensions being 18.38” X 14.35” X 5.05” for the fully assembled. Each section of the
housing also provides an internal network of ribs 0.1” thick and 0.2” high. This rib network provides
additional support for the board mounting posts. There are two different types of post, those only
supporting the Power Supply board and the power supply heat sink, 0.317” high, and those supporting the
Power Supply board, power supply heat sink and the Processor/Input module (Processor board, Input
board, and common heat sink), 0.417” high with 0.1” high aluminum spacers (.517” total height). The outer
diameter for all the posts is 0.5” and the inner diameter is 0.17”. The two rectangular sections of the
housing are held together by means of 18 #10-32 screws. All external and internal surfaces are hard
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anodized. The unit mounts to the outer section of the engine case across four vibration isolators. These
vibration isolators also conductively isolate the unit from the engine case.
Each of the 6 PWB’s integrating the unit, 2 Power Supply boards, 2 Input boards, and the 2 Processor
boards, consist of multilayers of copper and Type GFG epoxy sheets held together with layers of Pre-Preg
material (adhesive). There are 12 copper planes integrating each of the Power Supply boards for a nominal
board thickness of 0.074”, and 10 copper planes for each of the Input and Processor boards with a nominal
board thickness of 0.064”. In all cases, the copper layers in the PWB’s consist of 1 oz/ft 2 of copper per
plane, this yields an effective thickness of 0.0014” per layer. Figures 2 shows the component layout for the
Power Supply, which is the only board that we will consider for detailed analysis on this report.
Each Power supply PWB is bonded to an alloy AMS4027 aluminum heat sink 0.08” thick. The bonding
material used in the bonding process is 3M-Y9469 with a nominal bond thickness of 0.005”. The Power
Supply heat sinks are attached one to each half of the housing by mean of 26 #6-32 screws.
Component Mounting
The method used to mount the different electronic components on the boards is of paramount importance
in determining their operating temperatures. In this thermal design all the electronic components are
mounted directly to the PWB’s. As previously mentioned, the critical factors to determine the mounting
method of an electronic component are its physical size, power dissipation, and lead material (where
applicable). The following is a description of the mounting methods used for all the components in the
Power Supply boards. Appendix shows a component specific description of the mounting methods.
1. All surface mounted components are soldered directly to copper pads on the top surface of the PWB’s.
2. High power surface mounted transitors, diodes, and resistors (in excess of .5 Watts) have been
soldered to enlarged copper pads with a large number of thermal vias connecting top copper pads to
identical copper pads on the opposite side of the PWB near the heat sink. Thermal vias reduce the
thermal resistance across the PWB while the enlarged copper pads help to minimize the thermal
resistance across the PWB by increasing the effective heat transfer area. Refer to Figure 3 for a typical
mounting configuration of a high power surface mounted component.
3. Medium power surface mounted resistors (less than 0.4 Watts) are soldered to enlarge copper pads on
the top surface of the PWB’s.
4. None of the Flat Pack, PQFP, CQFP, SOIC, and DIP type components in the unit will be bonded. The
thermal dissipation within most of these types of components is low to moderate. The components in
this group with moderate power dissipation levels are large in size and their lead material is copper.
These two factors combine provide adequate heat transfer capability for these types of components.
Thermal Configuration
During the steady state operation of the unit, the heat dissipated in operating electronic components is
conducted out through the case and leads (where applicable), across any solder or bonding (where
applicable) material present to the mounting surface. Once the heat enters the PWB’s, it is spread laterally
and conducted downward to the aluminum heat sink. From the heat sink, heat is conducted laterally,
across the screw interfaces, and to the mounting points. In this design the mounting points are the metal
posts coming off the housing. The contact resistance value used for the #6-32 screws connecting the
Power Supply heat sinks to the housing is 1.73 C/W. These values include the resistance across the dry
bolted interface as well as the resistance through the post. The later case also includes the thermal
resistance across the double interface of the spacers. The determination of this contact resistance is based
on empirical charts and it will not be discussed in this paper. During steady state operation there is thermal
radiation interchange between the PWB’s that are facing each other and between the PWB’s facing the
housing and the housing, as well as natural convection within the unit.
Once the heat is in the chassis shelf, it conducts outwardly to the unit walls. Heat is also transferred
between the two halves of the housing across the bolted interface. The contact resistance term used for
#10-32 screws was 1.6 C/W, again how this value was obtained from empirical charts and it will not be
discussed in this paper. The heat is dissipated to the aircraft environment in two ways. First, it is
transferred by means of natural convection to the Nacelle environment. The ambient air heat transfer
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coefficient for the operating unit is 4 BTUHrFT2F. The value of the external air heat transfer coefficient
was provided by the engine manufacturer. Secondly, heat is also radiated to the Engine case as well as to
the Nacelle wall. A vast majority of the heat within this unit is transferred to the environment by convection
rather than radiation. Conduction to the engine case is negligible when compared to convection and
radiation off the unit, therefore it was ignored for the purpose of analysis.
Operating Conditions
The method of analysis considered steady state operation for typical power dissipation (75.4 Watts). The
analysis also assumed that both channels, A and B, were operating simultaneously during unit operation.
These power levels were used to determine case, and junction (where applicable) temperatures for each
individual component within the power supply boards.
Environments
The only thermal environment analyzed corresponds to the EEC150 qualification test environment. Under
this operating condition the Nacelle wall, Engine Wall, and Nacelle air temperature were taken to be 118C.
The Nacelle air temperature was used as a convection heat sink. The Nacelle wall temperature and the
Engine wall temperature were used as radiation sinks.
Material Properties
Air
Aluminum AMS4218
Aluminum AMS4027
Bond, type 3MY9469
Copper C11000
Copper C19400
GFG
Solder Mask
Hard Anodized
Thermal
Conductivity
WinC
7.774E-4
39.20E-1
41.20E-1
4.400E-3
97.00E-1
65.94E-1
1.311E-2
4.400E-3
N/A
Optical Property
Emissivity
N/A
0.10
0.10
N/A
0.10
0.10
0.68
N/A
0.60
Assumptions
1)
The Nacelle air temperature, the Nacelle wall temperature, and the Engine case temperature are
isothermal and infinite heat sinks.
2)
The heat transfer coefficient for the internal air (inside the unit) is 0.0037 Win2C.
3)
A view factor of 1.0 between the PWBs facing each other and the PWBs facing the housing.
4)
Conduction across electrical connectors is negligible.
5)
The Nacelle wall and the Engine case surface emissivities are 0.5.
5.2
ANALYSIS METHOD
A detailed unit level mathematical thermal model was developed for the EEC, PWBs, and heat sinks to
represent all significant thermal exchange within the elements integrating the unit as well as the thermal
exchange to the aircraft engine environment. Although individual models were created to represent the
different PWBs and heat sinks integrating the unit, all of them were connected to form a single unit thermal
model. The model accounts for conduction from the PWBs to the heat sinks, and from the heat sinks to the
chassis. The model also accounts for PWB to PWB, PWB to chassis, and chassis to environment thermal
radiation. Convection from the chassis to the environment as well as internally was also accounted for in
the model. The power distribution within the PWBs was such as to represent the actual power density
within the unit. A detailed thermal model of a Power Supply board was also created. The model account
for all significant thermal path within the board, including the interlayer connections, all significant copper
pads and the thermal vias connecting top and bottom copper pads. The mounting post, internal air, and
housing and board average temperatures predicted by the unit level thermal model were used as
conduction, convection, and radiation heat sinks respectively.
The nodes and connectors for the unit level thermal model as well as the boundaries were created
numerically using SINDA/G. This software is a finite difference thermal analysis program. SINDA/G utilized
Page 11 of 18
a steady state routine STDSTL to arrive at a solution. The node and connectors for the detailed Power
Supply thermal model was created geometrically using SINDA/3D. SINDA/3D is a pre and post processor
for SINDA/G. The output from the pre-processor is a SINDA/G input file. SINDA/G is used to arrive at a
solution, and the output from SINDA/G is used by the post-processor to create color contour plots of the
predicted temperatures among other things.
6. RESULTS
The results from the thermal analysis are various and can be presented in various forms. Figure 4 is one
example of how the results could be presented. Figure 1 shows the temperature delta increase across a
PWB 0.08” thick for a particular type of component (TO-220) as a function of power dissipation and for
several mounting configurations. The main purpose of the Figure is to show that similar temperature delta
increase can be achieved by different mounting configuration. For example when there is no sufficient
board area to have large copper pads on the board so that the heat transfer area is large we can
compensate by increasing the number of thermal via holes. In the event that large number of thermal via
holes could not be used because of electrical routing problems we can compensate by increasing the
copper pad size. Results are also shown in the form of temperature color contour plots. Figures 5 and 6
show the predicted temperature for the same board for two different type of component mounting
configurations. Both Figures are based on the same board level power dissipation and environment, but
Figure 5 shows the predicted board temperature when no means of thermal management is used and
Figure 6 shows the predicted board temperature when enlarged copper pads and thermal via holes are
used to mount all the high power components. We can see that there is an 8C temperature delta decrease
on the predicted surface temperature for the case where the thermal via holes are used (Figure 6). This
shows the effectiveness of the thermal via holes in reducing the thermal resistance across the PWB.
Results of the thermal design are also presented in the Appendix where the actual thermal design for each
of the high power components is shown. This is very important because the mechanical designer would
require this information in order to incorporate the changes into the design.
7. CONCLUSIONS
The subject of thermal design of any electronic printing wiring board (PWB) is a very complex process not
only from the thermal stand point but from all aspects. In this paper, it was the author intention to present
the very basic concepts behind proper thermal design and thermal management of high power PWBs.
There are many areas of the thermal design of PWB that were only mentioned or not mentioned at all in this
paper. Passive thermal systems that rely basically on improving thermal conduction across a board were
discussed in this paper. Active thermal management systems, thermo-electric coolers, phase change
materials, force convection, and liquid cooling, were not discussed. These types of systems are often used
when the power dissipation is of the order of hundreds of Watts. On the other hand, it was the intention of
the author to show that the thermal design of a PWB is a team effort between the electrical, mechanical,
structural designers. Proper thermal management often time involves making compromises between what
is the optimum design and what is economically feasible.
8. REFERENCE
1)
Thomas, L. C., “Heat Transfer, Professional Version,” Prentice-Hall, 1993.
2)
Mullen, J., “How to Use Surface Mount Technology,” The Staff of the Texas Instruments
Information Publishing Center, 1984.
3)
Nakayama, W. and Bergles, A. E., “Cooling Electronic Equipment: Past, Present, and Future,”
1988.
4)
Lazzaro, G. and Andrikowich, T., “Thermal Management of Aero and Space Electronic Boards,”
1998.
5)
Graebner, J. E., “Thermal Conductivity of Printed Wiring Boards,” AT&T Bell Laboratories, 1995.
6)
EPP Staff Report, “Considerations in Device Cooling,” Electronic Packaging and Production, July
1981.
7)
Azar, K., “The History of Power Dissipation,” Electronic Cooling, January 2000.
8)
Swager, A. W., “Circuit Desing Requires Thermal Expertise,” Electronic Design News, June 1989.
Page 12 of 18
Figure 1. EEC150. Isometric view of typical unit
Figure 2. EEC150. Power Supply Board Component Layout
Page 13 of 18
TJUNCTION
THERMAL VIAS
0.019” O. D./ 10 PLCS
THETAJ=C= 6.3 C/W
0.165”
TCASE
COMPONENT
FOOTPRINT
A
0.380”
0.190”
THETACFG= 3.7 C/W
A
THETACFG=
37.4 C/W
THETA10 VIAS=7.7C/W
COPPER PAD
TOP OF PWB
0.330”
SOLDER
0.003”TK.
THETAC-A=
65 C/W
COMPONENT
COPPER PAD 0.0014”TK.
0.33” X 0.38”
CFG
0.006”TK
THETASM= 4.1 C/W
COPPER GROUND PLANE
0.0014” TK. / 2 PLCS
CFG
0.052” TK.
COPPER PAD
0.38” X 0.48” X 0.0014”
3M Y9469 BOND
0.005” TK.
SOLDER MASK 0.003” TK.
SECTION A-A
THETABOND= 6.2 C/W
TSINK
TAMB.
AL HEAT SINK (AMS4027)
0.07” TK.
Figure 3. EEC150. Mounting Configuration for a Typical High Power Surface Mounted
Transistor
TO-220
TEMPERATURE DELTA ACROSS A BOARD 0.08" THICK
ACTUAL PAD SIZE USED IN CALCULATION IS X TIMES THE FOOTPRINT OF THE COMPONENT (O.183in 2)
70
65
60
TEMPERATURE DELTA ACROSS THE BOARD (C)
55
60 V IA S/1.5 PA D
60 V IA S/2.0 PA D
60 V IA S/2.5 PA D
60 V IA S/3.0 PA D
60 V IA S/3.5 PA D
50 V IA S/1.5 PA D
50 V IA S/2.0 PA D
50 V IA S/2.5 PA D
50 V IA S/3.0 PA D
50 V IA S/3.5 PA D
50 V IA S/4.0 PA D
40 V IA S/1.5 PA D
40 V IA S/2.0 PA D
40 V IA S/2.5 PA D
50
45
40
40 V IA S/3.0 PA D
40 V IA S/3.5 PA D
40 V IA S/4.0 PA D
30 V IA S/1.5 PA D
30 V IA S/2.0 PA D
30 V IA S/2.5 PA D
30 V IA S/3.0 PA D
30 V IA S/3.5 PA D
30 V IA S/4.0 PA D
20 V IA S/1.5 PA D
20 V IA S/2.0 PA D
20 V IA S/2.5 PA D
20 V IA S/3.0 PA D
20 V IA S/3.5 PA D
20 V IA S/4.0 PA D
10 V IA S/1.5 PA D
10 V IA S/2.0 PA D
35
30
25
20
15
10
10
10
10
V IA S/2.5 PA D
V IA S/3.0 PA D
V IA S/3.5 PA D
V IA S/4.0 PA D
10
5
0
0
0.25
0.5
SLOPE=THETA C-S
0.75
1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
3.25
3.5
3.75
4
POWER DISSIPATION (W)
Figure 3. TO-220. Temperature Delta Across A 0.08” Thick PWB for Various Mounting Configurations
Page 15 of 18
Figure 4. EEC150. Power Supply Board Predicted Temperature (Using Thermal Management Techniques
Described in the Appendix)
Page 16 of 18
Figure 5. EEC150. Power Supply Board Predicted Temperature (Without Using Any Thermal Management)
Page 17 of 18
APPENDIX
MOUNTING METHODS USED FOR THE DIFFERENT HIGH POWER COMPONENTS ON THE EEC150
1. The components mounted using this configuration have their respective bases soldered to enlarged copper
pads on the top of the board. These copper pads are connected to similar copper pads on the opposite side of
the board, closed to the heat sink. The connection between the two copper pads is made via a number of
thermal vias. The actual size of the copper pads and the total number of thermal vias connecting the two pads
vary with component and power dissipation. The table above shows the actual copper pads sizes and total
number of thermal vias used for the different components. Figure 1 and 2 shows tipical configurations for this
method.
2. The metal end caps on this type of components are each to be soldered to enlarged copper pads on the top of
the board. The enlarged copper pads increase the effective heat transfer area for the given parts. By
increasing the heat transfer area, the temperature delta increase across the board is reduced. The actual size
of the copper pads varies with component and power dissipation. The table above shows the actual copper
pad sizes used for the different components.
POWER SUPPLY BOARD
SCHEMATIC
IDENT.
CR201
CR202, 203, 207
CR206, 215
CR208
CR211
CR212
Q201, Q213
Q202, 214
Q203, 215
Q208, 212
Q702
Q704, 705, 718-720, 732-735
Q712, 713
Q722, 724
Q726, 727
Q729, 730
Q738
Q740-742, 746
Q744
R201
R219
R221
R244
R248
R702
R710, 711, 767-769, 834-837
R740
R752, 753
R765-767
R803, 810
R838
R870-872, 894
R915, 924
U204
U205
U206
U207
PART
NUMBER
MUR3020PT
MURB1620
MURB1620
MURB1620
MURS360T3_4A
MURS360T3_4A
IRF540S
IRF540S
IRF540S
IRF640S
IRF9530
MTD6N10
IRF9530
IRF9530
MTD6N10
IRF9530
IRF9530
MTD6N10
IRF9530
M55342K09B1E
93077-R402F
88034-K27R4
93077-R200F
88034-K3091
93076-R133F
93076-301F
93076-2R37F
93076-R162F
93076-2R37F
93076-R301F
93076-2R37F
93076-301F
88034-K2001
MC7815BD2T
MC7915BD2T
MC7815BD2T
MC7915BD2T
CASE
TYPICAL
MAXIMUM MOUNTING TOP COPPER
2
TYPE
POWER (W) POWER (W) METHOD PAD SIZE (IN )
TO-252
0.000
3.225
1
0.890 X 0.790
D2PAK
1.003
1.604
1
0.594 X 0.693
D2PAK
1.029
2.069
1
0.594 X 0.693
D2PAK
0.415
1.245
1
0.594 X 0.594
DO-214
0.228
0.416
2
0.198 X 0.298
DO-214
0.204
0.445
2
0.198 X 0.396
D2PAK
0.451
1.016
1
0.594 X 0.693
D2PAK
1.967
1.991
1
0.594 X 0.693
D2PAK
1.967
1.991
1
0.594 X 0.693
TO-236AB
0.599
1.288
1
0.594 X 0.644
SMD-220
0.337
0.862
1
0.594 X 0.693
TO-252
0.108
0.270
1
0.297 X 0.397
SMD-220
0.267
0.742
1
0.594 X 0.693
SMD-220
0.087
0.237
1
0.594 X 0.693
TO-252
0.125
0.388
1
0.297 X 0.397
SMD-220
0.347
0.939
1
0.594 X 0.693
SMD-220
0.469
1.705
1
0.594 X 0.693
TO-252
0.108
0.270
1
0.297 X 0.397
SMD-220
0.076
0.176
1
0.594 X 0.693
CHIP
0.085
0.117
2
0.074 X 0.148
CHIP
0.147
0.283
2
0.149 X 0.228
CHIP
0.326
0.326
2
0.198 X 0.198
CHIP
0.073
0.640
2
0.149 X 0.228
CHIP
0.160
0.170
2
0.074 X 0.148
CHIP
0.128
0.298
2
0.089 X 0.198
CHIP
0.065
0.163
2
0.100 X 0.100
CHIP
0.028
0.371
2
0.148 X 0.297
CHIP
0.124
0.333
1
0.148 X 0.198
CHIP
0.023
0.225
2
0.100 X 0.100
CHIP
0.075
0.204
2
0.100 X 0.100
CHIP
0.024
0.292
2
0.198 X 0.198
CHIP
0.065
0.163
2
0.100 X 0.100
CHIP
0.288
0.410
2
0.148 X 0.198
TO-220
0.490
0.588
1
0.594 X 0.693
TO-220
0.546
1.509
1
0.594 X 0.693
TO-220
0.648
1.635
1
0.594 X 0.693
TO-220
0.476
0.571
1
0.594 X 0.693
TOTAL NUMBER OF THERMAL VIAS REQUIRED
BOTTOM COPPER
PAD SIZE (IN2)
0.890 X 0.790
0.594 X 0.693
0.594 X 0.693
0.594 X 0.594
N/ A
N/ A
0.594 X 0.693
0.594 X 0.693
0.594 X 0.693
0.594 X 0.644
0.594 X 0.693
0.297 X 0.397
0.594 X 0.693
0.594 X 0.693
0.297 X 0.397
0.594 X 0.693
0.594 X 0.693
0.297 X 0.397
0.594 X 0.693
N/ A
N/ A
N/ A
N/ A
N/ A
N/ A
N/ A
N/ A
0.148 X 0.198
N/ A
N/ A
N/ A
N/ A
N/ A
0.594 X 0.693
0.594 X 0.693
0.594 X 0.693
0.594 X 0.693
THERMAL
VIAS / PART
40
21
21
37
0
0
21
21
21
29
33
6
36
28
6
21
23
6
14
0
0
0
0
0
0
0
0
4
0
0
0
0
0
28
28
28
28
TOTAL VIAS
PER GROUP
40
63
42
37
0
0
42
42
42
58
33
48
74
56
12
42
23
24
14
0
0
0
0
0
0
0
0
8
0
0
0
0
0
17
17
17
17
768
Page 18 of 18
0.693”
COMPONENT
FOOTPRINT
A
THERMAL VIAS
0.016” O. D./ 21 PLCS
A
0.594”
0.380”
COPPER PAD
TOP OF PWB
0.380”
SOLDER
0.003”TK.
COMPONENT
COPPER PAD 0.0021”TK.
0.594” X 0.693”
CFG
0.009”TK
COPPER GROUND PLANE
0.0014” TK. / 2 PLCS
CFG
0.052” TK.
COPPER PAD
0.594” X 0.693” X 0.0021”
3M Y9469 BOND
0.005” TK.
SOLDER MASK 0.003” TK.
SECTION A-A
AL HEAT SINK (AMS4027)
0.08” TK.
FIGURE 1. EEC150. THERMAL CONFIGURATION FOR IRF540S TRANSISTORS ON THE
POWER SUPPLY BOARD, D2PAK TYPE CASE / MAXIMUM POWER DISSIPATION 1.991W
A
A
THERMAL
VIAS / 4 PLCS
SOLDER PAD
2 PLCS
COPPER PAD
2 PLCS
COMPONENT
METAL END CAP / 2PLCS
SOLDER
0.003” THICK
COPPER PAD
0.0021”TK./ 2 PLCS
THERMAL VIA
4PLCS
COPPER GROUND PLANE
0.0014” TK. / 2 PLCS
CFG
0.052” TK.
SOLDER MASK
0.003” THICK
COPPER PAD
0.148” X 0.198” X 0.0021”
3M Y9469 BOND
0.005” TK.
SECTION A-A
AL HEAT SINK (AMS4027)
0.08” TK.
FIGURE 2. EEC150. THERMAL CONFIGURATION FOR R752 AND R753 ON THE POWER SUPPLY
BOARD, CHIP TYPE RESISTORS / MAXIMUM POWER DISSIPATION 0.333 W