散熱器設計準則--Mentor Graphics公司heatsink資料HeatSink_101

Heat Sink 101:Everything You Ever Wanted to Know about Heat Sinks

Agenda
 What is a Heat sink?
 Heat Sink Design Criteria
 Application Examples
 Questions and Answers
 
 

What is a Heat Sink?
 Heat sinks enable a more efficient heat transfer from a heat source to the adjacent fluid by using an extended surface area
 Heat gets moved from heat source to heat sink by conduction
 Heat sink transfers heat to ambient air by convection
 Heat can also be radiated to surrounding environment

Heat Sink Design Criteria
 Heat sink design and efficiency vary greatly depending on the construction and application
 Factors in selecting heat sink
 Component power dissipation and maximum junction temperature
 Available volume/space
 Interface material/Mounting system
 Spreading resistance
 Thermal resistance RSA
 Pressure drop
 Flow by pass
 Natural or forced convection
 Manufacturability
 Cost

Thermal Resistance Definition
 Conductive Heat Transfer
? Rk = DT/Q = L/kA (K/W)
? Similar to RW = DV/I (Ohms)
 Convective Heat Transfer
 
The Thermal Budget
 A useful tool in helping with Heat Sink selection
 Defined as: DTbudget = Q * RJA [K]
 Breaks the problem into clearly defined heat paths for a clear design understanding

Case to Sink Resistance:Spreading Resistance
 Kennedy Charts can be used to estimate the spreading resistance
 See the Reference section for more details
 On line calculator from the University of Waterloo
Component encapsulant

Case to Sink Resistance:Interface Materials
 Thermal paste (ceramic mixed with silicon grease or hydrocarbons)
 Fluids, naturally fill the gap
 Thermal resistance is very low
 Thermally conductive compounds
 Initially flow as freely as grease to fill gaps and then cures with heat to a rubbery state
 Approximately same performance as grease
 Conductive elastomers
 Deform with pressure to fill irregular gaps
 Provide electrical insulation
 Adhesive tapes
 Double sided adhesive to stick to adjacent surfaces
 Resistances relatively high
 Phase Change Materials
 Behave like thermal greases after they reach their melting temperature
 Interface becomes thinner until surfaces contact or material viscosity prevents further

 A temperature gradient exits between the top and the base of the fin
 Due to conduction resistance within the fin
 This can be quantified using the fin efficiency formula
 h = (tanh mL)/(mL)
m = (2h/kd)0.5
h = Convection Coefficient
k = Conductivity of Fin Material
d = Fin Thickness
L = Fin Height
 An ideal fin (Tbase = Ttop) would have an efficiency of 1
Heat Sink Fin Efficiency

Heat Sink Calculations
 To find a suitable heat sink for your application, you can use correlations to obtain h values and the Fin Efficiency formula
 Flat plate and ducted flow correlations available from most Flow and Heat Transfer books (please see References section)
 Also, consider the effects on flow impedance
 Few fins - low surface area, low pressure drop
 Many fins - high surface area, high pressure drop
 There is an optimum number of fins for a given flow rate

Heat Sinks in Natural Convection
Applications
 When designing a heat sink for a natural convection application, consider
 Heat sink orientation (compared to gravity)
 Pin fin heat sink maybe be more appropriate than a plate fin heat sink
 Surface finish
 Heat transfer by radiation is more predominant
 High emissivity surface will help dissipate more heat away from the heat sink

Thermal Design Tools
 Hand calculations/Spreadsheet
 Excellent tool for early heat sink design exploration
 Finite Element Analysis (FEA)
 3D numerical analysis
 Typically doesn’t calculate convective heat transfer and radiation explicitly
 Computational Fluid Dynamics (CFD)
 3D Conjugate fluid flow and heat transfer numerical analysis  Lab tests
 Most value when used as a model validation - rather than for parametric investigation
 
....
References
 Frank White, Fluid Mechanics
 Frank P. Incropera, David P. Dewitt, Fundamentals of Heat and Mass
Transfer
 Idelchik, I.E., Flow Resistance: A Design Guide for Engineers
 Steinberg, Dave S., Cooling Techniques for Electronic Equipment
 Kennedy, D.P., “Heat Conduction in a Homogeneous Solid Circular Cylinder
of Isotropic Media”, IBM TR 00.699, 1959
 Tony Kordiban, Hot Air Rises and Heat Sinks
 CARMA Board Project, California Institute of Technology

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