FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect. Thermal Management

FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect Thermal Management

Figure information preceeding page Free convection thermoelectric cooler (Peltier cooler) with heat sink surface temperature contours, and rising warmer air and falling cooler air flow trajectories, predicted using a CFD analysis package, courtesy of NCI "CFD Free Convection Peltier Cooler" by Heatlord - I created this model and CFD analysis for WikipediaPreviously published: http://www.novelconceptsinc.com. Licensed under CC BY-SA 3.0 via Wikipedia - http://en.wikipedia.org/wiki/file:cfd_free_convection_peltier_cooler.gif#/media/file:cfd _Free_Convection_Peltier_Cooler.gif FYS4260/FYS9260 Frode Strisland 2

Learning objectives Types of heat transfer mechanisms Approaches to cool electronics One dimensional thermal resistance calculations to estimate temperature and heat flow Background literature: Halbo & Ohlckers Chapter 6 and 7 Harper: Thermal Management Electronic Packaging and Interconnection Handbook 4 th ed.

Thermal management Heat is an unavoidable by-product of every electronic device and circuit. Heat leads to problems in electronics, and needs to be controlled thermal management FYS4260/FYS9260 Frode Strisland 4

What causes heat generation in circuits? Semiconductor switching Resistive heating Energy conversion (there may be several other effects) FYS4260/FYS9260 Frode Strisland 5

Heat generation: Semiconductor switching The power P dissipated from a semiconductor during switching is where P = CV2 2 f C = input capacitance in farads V = peak-to-peak voltage swing of signals in volts f = switiching frequenzy in Hertz. FYS4260/FYS9260 Frode Strisland 6

Heat generation: Semiconductor switching Major challenge to keep power dissipation in modern IC's low enough when the number of switcing devices and f (the clock frequency) increases. Strategies to make heat generation managable Lower input capacitance (smaller dimensions) Lower switching voltages (5.0 V 3V 1 V) Efficient cooling strategies P = CV2 2 f FYS4260/FYS9260 Frode Strisland 7

Heat Generation Energy conversion All energy conversion is connected with a conversion efficiency (and the rest is loss (which is usually heat)) Examples Power converters (AC-DC, DC-DC converters) Light diodes (converting electrical power into light (and heat)) Motors (convertion electrical power into mechanical energy) FYS4260/FYS9260 Frode Strisland 9

Why Thermal Management? Temperature increases lead to several unattractive effects, such as Temperature changes circuit operation Temperature affects the physical construction Failure rates increases FYS4260/FYS9260 Frode Strisland 10

Temperature effect on circuit operation Increasing temperature of an active electronic component changes its electrical parameters, such as gain, leakage and offset Passive components also typically change values with a temperature dependency, e.g. resistors Temperature Coefficient of Resistance may vary from several ppm/k to several hundred ppm/k Exceeding manufacturerer specifications often cause failures FYS4260/FYS9260 Frode Strisland 11

Temperature effect on circuit operation Gain tracking in an operational amplifier If R 2 physically is placed close to a heat dissipating component, the R 2 /R 1 ratio will change, and thus the amplifier gain. V in R 1 R 3 + - R 2 Vout Gain tracking of R 1 and R 2 FYS4260/FYS9260 Frode Strisland 12

Temperature effects on Physical Construction Different materials have different coefficient of tempermal expansion (CTE). Thermal stresses occur when materials are constrained during expansion or contraction. Extended stress cycles may need to fatigue and failure. Thermal stresses might be permanent (for example due to soldering) or occur due to component (self-) heating FYS4260/FYS9260 Frode Strisland 13

Failure rates increases with increasing temperatures The failure rate F of an electronic component increases with temperature, and can often be described by the Arrhenius equation derived in statistical mechanics where A = constant F = failure rate E A = activation energy (Joule) k B = Boltzmann's constant T = local temperature in Kelvin F = Ae E A k B T FYS4260/FYS9260 Frode Strisland 14

Activation Energies for common Failure Mechanisms in Electronic Circuits Table 3.2 in Electronics Packaging and Interconnection Handbook 4th ed FYS4260/FYS9260 Frode Strisland 15

Temperature dependence on Arrhenius type failure rates The relationship between failure rates at two different temperatures can be expressed like: F 1 = Ae F 2 Ae E A k B T 1 E A k B T 2 where F 1 and F 2 are the failure rates at temperatures T 1 and T 2, respectively. FYS4260/FYS9260 Frode Strisland 16

Temperature dependence on Arrhenius type failure rates Let us consider a device with an activation energy of 1.0 ev operating at 50 C when temperature increases to 60 C. Failure rate increase factor: 2.9 F 1 = Ae F 2 Ae With an activation energy of 0.65 ev (by many used as a rule of thumb), the failure rate increase factor becomes 2: E A k B T 1 E A k B T 2 The failure rate doubles for every 10 K increase in operation temperature FYS4260/FYS9260 Frode Strisland 17

Heat Flow Theory (The extremely condensed need-to-know version 1 ) Second law of Thermodynamics: Heat flows spontaneously from a hotter region to a cooler region. 1) FYS4260 will only introduce simple 1-dimensional approximations. This will help you understand which effects are important and which can be neglected. FYS4260/FYS9260 Frode Strisland 18

Three heat transfer mechanisms Convection Radiation Conduction FYS4260/FYS9260 Frode Strisland 19

Heat transfer mechanisms: Conduction Thermal conduction: Heat flows through a solid, liquid or gas, or between two media that are in intimate contact. FYS4260/FYS9260 Frode Strisland 20

Microsopic view on heat conduction Thermal conduction involves the transfer of kinetic thermal energy from one electron to another with no visible global effects Dielectrics: Heat transfer dominated by lattice vibrations Metals: Added energy transport through free electrons Electrical and thermal conductivity closely linked Liquids: Lower conduction than solids due to weaker inter-molecular bonds Gases: Even weaker (and random) inter-molecular bonds causing gases to have poor thermal conductivity FYS4260/FYS9260 Frode Strisland 21

Heat conduction: Fourier's law The heat flow rate dq equals the product of dt the cross-sectional area A of the area normal to the heat flow path multiplied with the thermal conductivity of the medium and the temperature gradient: dq dt = KA dt dx where K = thermal conductivity of medium [W/m K] A = cross-sectional area normal to heat flow path [m 2 ] T = temperature of medium [K] X = position [m] t = time [s] q = heat generated per unit volume [Joule/m 3 ] FYS4260/FYS9260 Frode Strisland 22

Introducing Thermal resistance Based on Fouriers law, we define Q as the heat flow normal to the cross-sectional area, and the corresponding power P: P = dq = power in W dt Inserting this into Fourier's law and rearranging: P = dq dt = KA dt dx Pdx = KAdT x T P 0 dx = KA 2 T1 dt Px = KA T T = xx KKKK PP = RRRR where the thermal resistance RR = xx KKKK FYS4260/FYS9260 Frode Strisland 23

Example: Heat conduction in a copper rod 100 C Cu rod 60 C 15 cm long, 1 cm wide, 1 cm high Thermal conductivity K= 385 W/mK Thermal resistance of the Cu rod: R = L KA = 0.15m 385 W = 3.9 K/W m K (0.01 m)2 Power transferred: P = T R = 40K 3.9 K/W = 10.2 W FYS4260/FYS9260 Frode Strisland 24

Electrical Analogy to Thermal Networks Thermal characteristics can be translated into the language of electrical circuits! Ohm's law U=RI T = RP Electrical conductivity σ thermal conductivity K When several materials are stacked in series, the equivalent or total thermal resistance becomes the sum of the individual resistances FYS4260/FYS9260 Frode Strisland 25

Example: Thermal resistances in series R 1 R 2 R 3 R 4 R 5 R 1 : thermal resistance of silicon die R 2 : thermal resistance of conductive epoixy die attach R 3 : thermal resistance of alumina substrate R 4 : thermal resistance of substrate attach (solder) R 5 : thermal resistance of copper case T junction : junction temperature T case : case temperature FYS4260/FYS9260 Frode Strisland 26

Example: Thermal resistances in series R 1 R 2 R 3 R 4 R 5 T = T junction T case = R 1 + R 2 + R 3 + R 4 + R 5 P = R Tot P FYS4260/FYS9260 Frode Strisland 27

Heat spreding Heat dissipating silicon chip Copper heat spreader Alumina substrate Heat spreading is generally a 3D phenomenon By placing a high thermal conductivity material next to the heat source, we can diffuse heat more efficiently away Flow in x and y directions might be larger than in the z direction FYS4260/FYS9260 Frode Strisland 28

Heat spreading angle Heat spreading angle α is always >0 If the lower material has poor thermal conductivity, α approaches, but never reaches 90 One approximation for heat spreading angle is as follows: α = arctan K 1 K 2 Here, K 1 and K 2 are the thermal conductivities of the current and the underlaying layer, respectively. FYS4260/FYS9260 Frode Strisland 29

Heat dissipating silicon chip Copper heat spreader Alumina substrate Geometry dependence on heat flow LL We recall that RR = KKKK Due to geometry, the resistance may differ in different directions For example in the figure, R x R y (which one is largest?) What is the equation for the resulting R tot? y x FYS4260/FYS9260 Frode Strisland 31

Thermal networks with more than on paths If there is more than one heat path, the equivalent thermal resistances can be considered to the the parallel equivalent to electrical resistors in parallel. Confer previous example: 1 R tot = 1 R x + 1 R y FYS4260/FYS9260 Frode Strisland 32

Thermal conductivity in PCBs Type Effective thermal conductivity (W/m C) FR-4 without Cu 0.2 1 Cu conductor layer, 35 µm 1.7 2 layers, 35 µm 3.1 4 layers, 2 x 35 µm, 2 x 70 µm 15-25 Metal base board, 0.5 mm core 50-100 Table 6.9: Typical values for the effective thermal conductivity of different types of PCBs. Slide 33

Thermal Design, continued Fig. 6.26: LLCC package with thermal solder lands and thermal vias connected to a metal core in the PCB. Slide 34

Convection CUP heat sink and fan cooling system Convection is the transfer of thermal energy between two surfaces as a consequence of a relative velocity between them. Convection only occurs in fluids, and the transfer mechanism is the mixing of fluids Most practical example: Interface between a solid surface and a fluid (usually air) FYS4260/FYS9260 Frode Strisland 35

Convection cooling due to Newtonian cooling Convection cooling is also known as Newtonian cooling, and is based on the assumption that the heat transfer factors are relatively temperature independent. In mathematical terms: QQ cc = h cc AA ss TT ss TT AA = h cc AA ss TT where Q c = heat transferred from a surface to ambient by convection [W] A s = surface area (m 2 ) T s = suface temperature [K] T A = ambient temperature [K] h c = convection heat transfer coefficient [W/K m 2 ] FYS4260/FYS9260 Frode Strisland 36

Convection cooling (cont'd) The heat flow equation can be rearranged: TT = 1 h cc AA ss QQ cc From this, we can define the convective surface resistance RR ss = 1 h cc AA ss Observe that this is just a definition, not a law; the remperature coefficient actually depends on temperatures, fluid velocity, viscosity, density and surface geometry FYS4260/FYS9260 Frode Strisland 37

Natural (free) and forced convection Natural convection: energy exchange caused entirely by differences in temperatures and densities causing fluidic movement Forced convection: The heat transfer is propelled by artificial means, e.g. fans. FYS4260/FYS9260 Frode Strisland 38

Detour: Keep in mind the wind chill index when considering the effect of forced convection! FYS4260/FYS9260 Frode Strisland 39

Estimating forced convection heat transfer coefficients Engineering estimates are used, for example: where h = BB vv0.75 LL 0.25 B = constant of air properties and surface configuration v = linear velocity of air L = characteristic lenght of surface in direction of flow FYS4260/FYS9260 Frode Strisland 40

Geometry is important in natural convection Flat plate heat flow Relative heat transfer coefficients: 0.56 Top surface hot 0.52 Bottom surface hot 0.26 FYS4260/FYS9260 Frode Strisland 41

Radiation All objects with temperature above 0 K emit thermal radiation. Radiation cooling is the transfer of heat by electromagnetic emission, mainly in infrared wavelengths Thermal radiation is a purely surface related phenomenon, and we distinguish between Black bodies A thermal camera (IR camera) measures the levels of thermal radiation Non-black bodies FYS4260/FYS9260 Frode Strisland 42

Black body A surface that absorbs the entire thermal radiation incident upon it, neither reflecting nor transmitting any of the incident radiation. The black body, at any given temperature, radiates more energy, both in the total spectrum and for each wavelenght interval than any other temperature radiator FYS4260/FYS9260 Frode Strisland 43

Emissivity The emissivity ε is the ratio of radiated flux E emitted by a body to that emitted by a black body E b at the same temperature εε = EE EE bb Black body: ε = 1 FYS4260/FYS9260 Frode Strisland 44

Emissivities at different surfaces Surface treatment can change radiation properties completely! Polished Aluminum ε = 0.03 Black anodisesd aluminum ε = 0.86 Aluminum with 10 nm gold: ε = 0.04-0.23 FYS4260/FYS9260 Frode Strisland 45

Stefan-Boltzman law The rate of emission of radiant energy from the surface of a body, R, can be expressed by the Stefan- Boltzmann law: R = εσt 4 R = Q A [W] Q = εσat 4 where ε = surface emissivity [J/ s m 2 ] σ = Stefan-Boltzmann constant Q = heat transferred [W] A = radiating surface area [m2] T = surface temperature [K] FYS4260/FYS9260 Frode Strisland 46

Summary of heat flow calculations Heat flows from hot to cooler regions Electrical analogy using thermal resistance networks makes calculations easy! The total thermal resistance R tot of multiple parallel paths are 1 1 1 1 = + + R tot R conduction R convection R radiation You should be able to estimate which effects are important in your design! FYS4260/FYS9260 Frode Strisland 47

Thermal Design, continued Thermal Resistance R jc : Thermal resistance junction - case R jl : Thermal resistance junction - lead R ja : Thermal resistance junction - ambient T a : Ambient temperature T j : Junction temperature Fig. 6.23: Thermal model of an IC and package. Slide 48

Strategies for improved cooling Thermal vias Cooling fins Fan Thermally conducting gas: helium, fluorocarbon Liquid: water, fluorocarbon, oil Boiling liquid Heat pipe Impingement cooling Microbellows Microgrooves Slide 49

Examples of Cooling strategies Fig. 6.27: a): Forced air convection in a channel between two PCBs (Texas Instruments) b): water-cooled heat exchanger for edge cooling of PCBs and temperature distribution (qualitative). Slide 50

Cooling liquid heat convection coefficients Fig. 6.28: Heat convection coefficient in different cooling media for natural convection, forced convection and boiling Electronic Pack.. Chapter 6: Printed Circuit Board Design 08.10.99 Slide 51

Heat Pipes cooling A heat sink (aluminium) with heat pipe (copper) and fan "Heat Pipe Mechanism" by Zootalures. Licensed under CC BY- SA 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/file:heat_pipe_mechanism.p ng#/media/file:heat_pipe_mechanism.png FYS4260/FYS9260 Frode Strisland 52

Microbellows cooling Fig. 6.29: "Microbellows cooling": A jet of water or other cooling liquid impinges on the backside of the chip. The bellow structure is necessary to accommodate thermal expansion Slide 53

Forced Liquid Cooling Fig. 6.30: Cooling by forcing liquid through microscopic, etched channels in the semiconductor chip [6.32]. The channels are approximately 400 µm deep and 100 µm wide. Slide 54

End of lecture: Thermal Management Important issues: Thermal flow is complex and calculations or even advanced three dimensional calculations can only give estimates of the physical reality. Using thermal resistance calculations, you should be able to assess which effects are significant and which ones can be neglected.