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Heat Sink 101: Everything You Ever Wanted to Know about Heat Sinks Alexandra Francois-Saint-Cyr Application Engineering Manager MECHANICAL ANALYSIS DIVISION 热仿真 http://www.refangzhen.com Agenda What is a Heat sink? Heat Sink Design Criteria Application Examples Questions and Answers 2 热设计 http://www.resheji.com 1

http://www.refangzhen.com 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 Component dissipating heat PCB Heat Transfer by Conduction Heat Transfer by Convection and Radiation 3 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 R SA Pressure drop Flow by pass Natural or forced convection Manufacturability Cost 4 2

Thermal Resistance Definition Conductive Heat Transfer R k = T/Q = L/kA (K/W) Similar to R Ω = V/I (Ohms) k: Material thermal conductivity Face A at fixed T 1 > T 2 L Q x T 1 T 2 L/kA Face A at fixed T 2 < T 1 Convective Heat Transfer R h = T/Q h = 1/hA (K/W) T = T s - T V,T Surface A Q h T S h: Heat transfer coefficient T S 1/hA T 5 The Thermal Budget A useful tool in helping with Heat Sink selection Defined as: T budget = Q * R JA [K] Breaks the problem into clearly defined heat paths for a clear design understanding T A RSA Sink to Ambient Resistance T s RCS Case to Sink Resistance T C RJC Junction to Case Resistance T J http://www.resheji.com 6 3

Case to Sink Resistance: Spreading Resistance Component encapsulant to heat sink Component substrate to PCB via solder balls 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 http://www.mhtl.uwaterloo.ca/old/onlinetools/strip_source/intro.html 7 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 flow 8 4

Heat Sink Fin Efficiency 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 η = (tanh ml)/(ml) m = (2h/kδ) 0.5 h = Convection Coefficient k = Conductivity of Fin Material δ = Fin Thickness L = Fin Height δ An ideal fin (Tbase = Ttop) would have an efficiency of 1 L 9 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) Flat Plate Geometry Circular Tube Characteristic Length L (m) Plate length Tube Diameter Flow Regime Laminar Turbulent Laminar Turbulent <10 5 >10 5 <2300 >2300 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 Re Nu 0.664Re 0.5 Pr 0.3 0.037Re 0.8 Pr 0.33 4.36 for constant Q 3.66 for constant T 0.023Re 0.8 Pr 0.4 Fluid properties defined at T = (T s +T )/2 Tube correlations assumes fully developed temperature and velocity profiles; smooth surface http://www.resheji.com 10 5

Heat Sink Quick Calculations Here are some quick calculations to be used for first order approximations of heat sink performance Based on laminar flow over a flat plate Air at 300K Merged boundary layer Estimate of Heat Transfer Coefficient h = 1.26e-3 (V/H) 0.5 (W/in 2 C) The minimum recommended fin spacing S min = 1.3 (H/V) 0.5 (inches) H = heat sink length in the flow direction (inches) V = approach velocity (ft/min) T w Fin T w Fin V,T 11 http://www.refangzhen.com Heat Sink Flow Resistance vs. Thermal Resistance Simple volume averaged performance comparison of various heat sink types 300 Data collected from various vendors and based on 200 LFM Volumetric Resitance ( C-cm^3/W) 250 200 150 100 50 Impingement Bonded Fin Vapor Base Folded Fin 0 0.01 0.1 1 10 100 Rsa ( C/W) Parallel Fin Pin Fin 12 6

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 13 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 14 http://www.resheji.com 7

Application Example 1: CARMA Board Thermal Design CARMA board was designed by the California Institute of Technology for use in the Owen Valley Radio Observatory Heat sink design needed for 4 components in a row U136, 137, 138 and 139 FBGAs dissipating 13W each U136 U137 U138 U139 400 ft/min; 25 C incoming air Gravity direction 15 Application Example 1: CARMA Board Thermal Design Baseline plate fin heat sink Aluminum 13 fins, 0.035 in thick, 0.34 in tall Base: 1.45 in 1.45 in, 0.115 in thick Heat sink TIM: 0.9 C.in 2 /W 16 8

Application Example 1: CARMA Board Thermal Design FloTHERM Model 17 Application Example 2: Integra-Luxtec Light Model Fan 40 C Air Inlet 300W Bulb Heat Sink Aluminum Heat Sink Air Exhaust (50% open) http://www.resheji.com 18 9

Flow Distribution http://www.refangzhen.com Application Example 2: Integra-Luxtec Light Model Airflow rotating due to fan blade rotation 19 Application Example 2: Integra-Luxtec Light Model Heat Sink Temperature Distribution 22 fins 27 fins Bulb temperature decreased to 148ºC compared to 174ºC in original 22 fin design Cut Plane Location 20 10

Application Example 2: Integra-Luxtec Light Model Additional fin designs were analyzed Increased number of fins Fin with limbs (F shape fin) Elongated F shape fin Fan pulling instead of pushing air through the heat sink F shape fin yielded the lowest bulb temperature Temperature [Deg C] 220 210 200 190 180 170 160 150 140 130 120 Bulb Temperature vs Fin Count & Geometry 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Fin Count single fin ave temp 2 xtra fins ave temp 2 xtra fins, long, ave temp single fin ave temp (pull fan) 21 Application Example 2: Integra-Luxtec Light Model Optimized Heat Sink 27 fins with limbs (F shape) Bulb temperature decreased to 123ºC compared to 174ºC in original 22 fin design Cut Plane Location 22 http://www.resheji.com 11

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 http://www.ovro.caltech.edu/~dwh/carma_board/ Web References - www.cfd-online.com - www.coolingzone.com - www.electronics-cooling.com 23 http://www.refangzhen.com 12