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CHE/ME 109 Heat Transfer in
Electronics
LECTURE 20 – ELECTRONICS COOLING
FUNDAMENTALS
BACKGROUND - VACUUM TUBES
• FUNCTION LIKE DIODES• TEMPERATURES FOR THE TUNGSTEN
(WITH A DASH OF THORIUM) FILAMENT (CATHODE) CAN RUN UP TO 2400 C
• HEAT LOADS ARE THE RANGE OF 100's TO 1000's W WERE TYPICAL
• TUBES ARE STILL FABRICATED AND STILL IN USE
• AMPLIFIERS FOR INSTRUMENTS• HIGH POWER ( >10000 W) HIGH
FREQUENCY (> 50 MHz) USE TUBE BASED UNITS
http://www.electrontubes.net/industrial.htm
BACKGROUND - TRANSISTORS
• LOWER POWER OPERATION FOR THE SAME FUNCTION AS TUBES
• MATERIALS OF CONSTRUCTION ALLOW OPERATION AT RELATIVELY HIGH TEMPERATURES
• LED TO DEVELOPMENT OF INTEGRATED CIRCUITS AND MICROPROCESSORS
http://media.digikey.com/photos/Intersil%20Photos/20-16-SOIC.jpg
INCREASING HEAT LOAD
• POWER ELECTRONICS AND TELECOMMUNICATION DEVICES
• HIGHER DEVICE DENSITIES
• PROBABILITY IS FOR CONTINUED INCREASES IN HEAT FLUXES
https://vicariousconversations.com/~vicario1/MWT/img/wiki_up/heat.JPG
THERMAL MANAGEMENT H
• ATTEMPTS TO BALANCE HEAT LOADS AND COOLING CAPABILITIES
• GENERAL FORMS OF COOLING FOR DEVICES ON BOARDS
• CONDUCTION - THROUGH CONNECTION TO BOARDS AND THEN TO HEAT SINKS
• CONVECTION - BOTH NATURAL AND FORCED USING HEAT SINK TECHNOLOGY
• RADIATION – INTERNAL AND EXTERNAL SOURCES
THERMAL MANAGEMENT IN SATELLITES
• Young-Keun, C., et. al., A Study on Thermal Modeling and Heat Load Mitigation for Satellite Electronic Components, 15th Annual AIAA/USU Conference on Small Satellites, Utah State University Research Foundation, 2001 (http://www.smallsat.org/proceedings)
• PRIMARY HEAT DISSIPATION VIA CONDUCTION AND RADIATION
• NEED TO CONSIDER LOCAL HEAT LOAD FOR EACH COMPONENT
• ONE METHOD MAY BE TO INCREASE HOUSING THICKNESSES TO ENHANCE CONDUCTION TRANSFER
• ALSO ANALYZE THE EFFECT OF COMPONENT LAYOUT
THERMAL MANAGEMENT IN SATELLITES
FABRICATION OF ELECTRONIC DEVICES
• DEVICES ARE MOUNTED ABOVE THE PCB DUE TO DIFFERENT COEFFICIENTS OF THERMAL EXPANSION
• JUNCTION TEMPERATURES ARE THE MAXIMUM IN THE DEVICE
• CALCULATED FROM
CHIP CARRIER
• CAN BE USED FOR THERMAL MANAGEMENT
• TYPICAL DEVICE SCHEMATIC FOR FLIP-CHIP PLASTIC BALL GRID ARRAY PACKAGE (SOLDER TECHNIQUE)
http://www.electronics-cooling.com/assets/images/2003_Feb_A1_Figure3.jpg
MECHANISMS FOR HEAT DISSIPATION
• INVOLVE ALL THREE MODES OF HEAT TRANSFER
• CONDUCTION THROUGH THE PINS TO THE BOARD (THE MORE PINS, THE HIGHER THE RATE OF CONDUCTION)
• CONVECTION TO THE SURROUNDING AIR
• RADIATION TO THE SURROUNDINGS http://www.njr.co.jp/pdf/ee/ee
05007.pdf
JUNCTION TO CASE RESISTANCE
• CONTROLLED BY THE BONDING AGENT PROPERTIES
• ALSO AFFECTED BY THE GEOMETRY OF THE SYSTEM
• COOLING THROUGH THE PCB
• PCB’s CAN BE FABRICATED WITH HEAT FRAMES TO CONDUCT HEAT AWAY FROM THE DEVICES
• THE HEAT FRAME IS CONNECTED TO A COLD PLATE, WHICH SERVES AS A HEAT SINK.
THERMAL STUDY FOR A BOARD
• HEAT LOADS ON THE BOARDS ARE ADDITIVE, SO THE MAXIMUM TEMPERATURE IS TYPICALLY IN THE CENTER OF THE BOARDhttp://
www.thermoanalytics.com/applications/electronics.html
THERMAL STUDY FOR A BOARD
http://www.thermoanalytics.com/applications/electronics.html
CASE AND OTHER COMPONENTS
• BOARD CONFIGURATION IN THE CASE IS A FACTOR IN THERMAL MANAGEMENT
• AIR FLOW PATTERNS ARE AFFECTED BY RELATIVE LOCATION OF BOARDS AND OTHER EQUIPMENT
• LOCATION OF VENTS CAN IMPACT THE FLOW IN THE UNIT
• VERY HIGH HEAT DENSITY UNITS, CPU’s AND POWER SUPPLIES, MAY HAVE LOCAL FAN COOLING
• LAYOUT SHOULD ALLOW FOR MAXIMUM COOLING BY THE METHODS THAT ARE NOT DEPENDENT UPON A MOTOR– RADIATION– CONDUCTION– NATURAL CONVECTION
COOLING LOADS
• HEAT BALANCE REQUIRES THAT HEAT IS DISSIPATED AT THE RATE IT IS GENERATED AT STEADY STATE
• MOST DESIGNS INCLUDE SOME INSURANCE TO ALLOW FOR COOLING RATES ABOVE THAT ANTICIPATED IN DESIGN SERVICE
THERMAL ENVIRONMENT
• THE RANGE OF ENVIRONMENTS TO WHICH THE SYSTEM WILL BE EXPOSED MUST BE CONSIDERED
• NATURAL CONVECTION WILL NOT WORK WELL IN SPACE DUE TO LOW GRAVITY
• FORCED CONVECTION WILL NOT WORK IN A VACUUM
• AIRCRAFT TYPICALLY EXPERIENCE A RANGE OF PRESSURE AND TEMPERATURE CONDITIONS ON EACH FLIGHT