Transcript ME 259 Heat (and Mass) Transfer

Electronics Cooling
Reference: Cengel, Heat Transfer,
2nd Edition, Chapter 15
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Introduction

All electronic components generate heat
due to I2R (joule heating)

Modern integrated circuits (ICs)
– approaching 1010 components/chip
– heat fluxes up to 100 W/cm2

Failure rate of electronic devices increases
exponentially with operating temperature

Silicon p-n junctions
– Absolute limit of 125C for “safe”
operation
– 85C desirable for extended life
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Supporting Electronic Equipment

Chip Carrier
– heat flow paths
– junction-to-case thermal resistance (Rjc)

Printed Circuit Board (PCB)
– Single-sided
– Double-sided
– Multilayer

Enclosure
– sealed
– vented
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Common Cooling Methods



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Conduction cooling
– copper “heat frames“ attached to PCB
– copper layers and “vias” within multilayer
PCBs
Air Cooling with or w/o heat sinks
– natural convection & radiation
– forced convection with fans
Liquid cooling
– direct immersion
– indirect (hxer, pump)
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Electronics Cooling Models



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Component Model
– Semiconductor device
– Heat sink, mini-fan, heat pipe, hxer
– PCB
Enclosure Model
– Chassis assembly
– Environment
Air Flow Model
– Chassis-mounted fans
– Air intakes, exhausts,
– Shrouds, ducted flow
System Model
– Combines component, enclosure, and air
flow models
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Component Model

Physical system

Thermal circuit

Typical values

Analysis
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Enclosure Model

Physical system

Thermal Circuit

Analysis
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Air Flow Model

Physical system:

Thermal Circuit

Analysis
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System Model

Thermal circuit

Analysis
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Example

Given: Enclosure containing 16 TO-3 style
power transistors, mounted in sets of four, on
four EG&G 435 series heat sinks.
Transistor specs
Rjc = 7.0 C/W
Rcs = 0.09 C/W
qc = 10 W
Tj,max = 125 C
Enclosure specs
L = 60 cm
H, W = 20 cm
t = 3 mm
k = 0.4 W/m-K
ho = 10 W/m2-K
To = 25 C
o = 0.8
hi  3.9 V / L W/m2 - K
Heat Sink specs
Rsa 
0.9
1V

C/W
(curvefit to data, V in m/s)
Find: Fan flow rate needed to keep Tj below
125C
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