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