Thermal Design And Analysis of High Power Electronic Boards

1
Thermal Design And Analysis of High Power
Electronic Boards

• J. Collado
• CHT, April 13, 2000

2
Thermal Requirements

• Maximum junction temperatures and dereating
  criteria
• Thermal environments in which the equipment is
  required to operate
• Ambient temperature
• Altitude/Pressure
• Thermal radiation environment
• Solar heat loads
• Air velocity
• Government specifications
• Mission profile and duration
• Design constraints
• Noise and vibration limits
• Maximum allowable air exhaust temperature
• Size limit
• Weight limits

3
Temperature Effect on Electronic Boards
4
Governing Equations

• Fouriers Law of Heat Conduction

• q -kA dT/dx
• Integrated Equation Forms
• q (kA/L)(T1-T2)
• q G(T1-T2) (T1-T2)/ q1-2
• where
• q1-2 1/G R L/kA
• or
• q1-2 1/G R LN(d2/d1)/2pLk (Radial)
• k material thermal conductivity (W/in-C)
• G thermal conductance (W/C)
• d equivalent diameter

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Thermal Resistance to Conduction

• Thermal Resistance, R (C/Watt)
• R1/G (the inverse of thermal conductance, G)
• R1-2 is the resistance from point 1 to point 2
• For electronic components R1-2 q1-2
• Rj-c qj-c Junction to Case Thermal Resistance
• Rc-b qc-b Case to Board Thermal Resistance
• Rj-b qj-b Junction to Board Thermal Resistance
• Rj-b Rj-c Rc-b qj-b qj-c qc-b

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Thermal Resistance Equations (Cont.)

• Junction Temperature Calculation
• Thermal Via Holes Thermal Resistance

r2
-
r1 rOUT
r
r
)
ln(
r1
q

1
2
p
1
L1K
2
rin r Cu after plating
rout
L2

q
p
2
2
2
-
)
(
r
r
K
PWB
in
out
L2
4
rin
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Typical Mounting Configuration For High Power
Transistors
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Typical Assumptions

• Component case is isothermal
• Board bottom is isothermal under the part
• Heat sink is isothermal under the part
• No convection or radiation heat transfer
• Extra footprint area for PWB calculations
• lead vias are defaulted to be under the part

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Type of Electronic Packages

• Through Hole
• Part leads pass through a circuit board
• e.g., DIPs, Axial Diodes, Transistors, PGAs
• Surface Mount Devices (SMD)
• Parts leads stop at the circuit board surface
• e.g., SOICs, PLCCs, QFP, SOTs
• Parts leads stop at the circuit board surface
• e.g., SOICs, PLCCs, QFP, SOTs

10
Effects of Thermal Vias and Cu Pad Sizes on a
Typical Transistor
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View of High Power Board Used For Case Study
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Effects of Thermal Vias on PWB Operating
Temperature
Board With No Thermal Vias
Board With Thermal Vias