Component & Board Level Cooling.

Transcription

1 << >> Component & Board Level Cooling (C) Flomerics Ltd 2002

2 Section Title Components PCBs Packages Interface Materials Heat Sinks Thermoelectric Coolers Heat Pipes

3 Printed Circuit Boards Structure consists of multiple layers of a conductor material embedded in a dielectric Typically Fr4 and Cu Layers measured in oz-copper, 1oz-cu = 35 micron Serves as a heat sink for for the components Highly Orthotropic Conductivity Typically In plane ~ 100x Through Plane Thermal Vias improve through-plane conductivity

4 Conductivity of a PCB The orthotropic conductivity may be calculated from the volume-averaged conductivities of the two materials Effective Conductivities K in-plane = CC * k c + (1-CC) * k d K thru-plane = [ CC/k c + (1-CC)/ k d ] -1 CC = Conductor Volume /Total Volume k c = Conductivity of conductor material k d = Conductivity of dielectric material

5 Conductivity of a PCB Small components are more sensitive to changes in PCB conductivity: Large Chip Small Chip Normalised Temperature % Copper in board

6 Components What is an electronics package? (Buzzword definition) The combination of engineering and manufacturing technologies required to convert an electronic circuit into a manufactured assembly Martin Miller, Electronic Packaging, Microelectronics and Interconnection Dictionary What is an electronics package? (Intelligible definition) An intermediary between the IC chip (die) and the PCB

7 Basic Packaging Encapsulant Die Level 1 interconnect Substrate Level 2 interconnect PCB Heat generated on the active side of the Die Complex heat paths dependent upon package construction, environment, and board composition.

8 Components Heat transfer from a PQFP die Encapsulant Die Die Flag Lead PCB

9 Components Peripherally leaded Pin Grid Arrays Ball Grid Arrays Chip-Scale

10 Components Thermal Resistance of a Package: Defined as: R Tj = temperature at active surface of die ( junction ) Tx = temperature at some reference point P = package power Junction Board Junction Ambient Junction Case jx = T j T P x

11 Package Characterization θja (Natural Convection) Most common concept Defined by JEDEC 51_2 : θ ja = T j T Ta = ambient temperature, taken inside a specific enclosure defined by JEDEC (Still-Air Test) Measurements taken either with a High-k (2S2P) and Low-k (1S0P) board P a

12 Package Characterization θjma (Forced Convection) Speeds from LFM Defined by JEDEC 51_6 : θ jma = T j T P Ta = ambient temperature, taken upstream in the wind tunnel Board orientation is an important factor a

13 Package Characterization θjc (junction to case resistance) Ambient The thermal resistance from the junction to the outside surface of the package (case) closest to the chip mounting area when that same surface is properly heat sunk so as to minimize temperature variation across that surface. θ jc = T j T P c Case Junction Die Substrate PCB

14 Package Characterization θjb (junction to board resistance) The thermal resistance from the junction to the board. θ jb = T j T P b

15 Components Thermal Resistance of a Package Try to represent the many complex heat flow paths with a single thermal resistance Useful for comparison of packages Be cautious with use in actual design environments θ ja highly environmentally dependent θ jb and θ jb can be used in conjunction to provide reasonable accuracy

16 Component 2-Resistor Models 2-Resistor Compact Models: A significant improvement over singleresistor metrics Simple topology Can be used in thermal analysis tools Can be derived experimentally or computationally Typical accuracy for most cases is < 20% θ jc θ jb T J B

17 Interface Materials Interface materials reduce the interface resistance Q Low conductivity (stagnant) air is replaced by material of higher conductivity ΔT i R ct = ΔT i /Q contact resistance

18 Interface Materials Thermal paste (ceramic mixed with silicon grease or hydrocarbons) - Fluids, naturally fill the gap - Thermal resistance is very low - allows for thinnest possible interface Thermally conductive compounds - Initially flow as freely as grease to fill gaps - Cure with heat to a rubbery state - Approximately same performance as grease

19 Interface Materials (cont d ) Conductive elastomers - Deform with pressure to fill irregular gaps - Need pressure to be applied (~1 MPa) - Provide electrical insulation Adhesive tapes - Double sided adhesive to stick to adjacent surfaces - Resistances relatively high - Provide thermal and mechanical benefit

20 Interface Materials (cont d ) Phase Change Materials - Behave like thermal greases after they reach their melt temperature - Interface becomes thinner until surfaces contact or material viscosity prevents further flow

21 Heat Sinks Increase the area for heat transfer Ideally, R = 1 / ha R = thermal resistance A = total surface area h = heat transfer coeff.

22 Heat Sinks Single Fin Efficiency η = tanh (ml) / (ml) l δ m = [2 h / k δ] 0.5

23 Heat Sinks Thermal Resistance R = 1 / η ha Also need to 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 for a given flow rate

24 Heat Sink Quick Calcs. Provided here are some quick calculations to be used for first order approximations of heat sink performance Estimate of HTC h = (V/H) 0.5 W/m 2 K The minimum recommended fin spacing S min = (H/V) 0.5 m H = heat sink length in the flow direction m V = approach velocity m/s

25 Heat Sinks Heat sink design and efficiency vary greatly depending on the construction and application. Factors in selecting heat sink Available volume Rsa Interface Resistance Spreading Resistance Pressure Drop Flow by pass

26 Rsa ( C/W) ΔP (Pa) Heat Sinks Typical Data Provided Approach Velocity (m/s)

27 Heat Sinks Simple volume averaged performance comparison of various heat sink types 300 Volumetric Resitance ( C-cm^3/W) 250 Bonded Fin 200 Parallel Fin Vapor Base Impingement Pin Fin 50 Folded Fin Rsa ( C/W) Data collected from various vendors and based on 200 LFM

28 Cold Plate Liquid Cooled Heat Exchanger

29 Thermoelectric Coolers AKA Peltier Device Reverse effect of a thermocouple Advantage: Precisely controlled cooling Disadvantage: Increases the amount of heat Typically COP = q Q + q

30 Heat Pipes High effective conductivity 1,000-50,000 W/m-K Used primarily to transport heat in constrained areas - Laptops - Densely packed boards - High heat flux components Limits based on maximum operating temperature and and heat flow

31 The Thermal Budget A Useful Tool in Helping With Heat Sink Selection Defined as: T budget = Q * Rja [K] Breaks the Problem Into Clearly Defined Heat Paths For a Clear Design Understanding Assists to Focus on Problem Areas Eg. The Component Interface Material is Taking up 35% of my Thermal Budget Due to That High Heat Flux! Lets Put a Lid on First.