Beating the Heat Dealing with the Thermal Challenge: Past, Present and Future

Transcription

1 Beating the Heat Dealing with the Thermal Challenge: Past, Present and Future MEPTEC The 4th Annual The Heat is On: Thermal Solutions for Advancing Technology February 28 th 2008 Joseph Fjelstad Verdant Electronics

2 Historical Perspective Thermal management has largely been a challenge shared by circuit design, electronic manufacturing technologies and heat removal technologies Transistors replaced vacuum tubes Integrated circuits replaced discrete transistors CMOS replaced BiPolar/MOS Next shift? will photons displace electrons?

3 Electronic Vacuum Tube Assembly Lee de Forest s Audion tube use as an amplifier. Sold it to the telephone company as an amplifier of transcontinental wired phone calls. Tubes generate heat and convection is primary path but metal chassis also helps to spread heat

4 ENIAC an early thermal challenge 30 separate computing units plus power supply and forced-air cooling system. Weight: 60,000 pounds. Dimensions: 100' x 10' x 3' 19,000 vacuum tubes, 1,500 relays, hundreds of thousands of resistors, capacitors, and inductors. 500,000 soldered joints Power consumption ~ 200 kilowatts

5 Tsubame Super Computer Tokyo Institute of Technology 5000 processors. 76 Racks 20 Cooling Units 80 Teraflops. Power consumption ~ 1,200 kilowatts

6 CMOS Chilled Thermal Developments Thermal solutions that were in development during the bipolar era are being resurrected today The introduction of CMOS temporarily obviated the need for extreme thermal management. Today the thermal solutions are back on the front burner Image source: Electronics Cooling

7 Range of Chip Wattages is Rising

8 Thermal Management: Electronics' Rodney Dangerfield? Thermal problems have historically been addressed at the end of the design process. The tendency too often is assume that thermal management is a pedestrian activity. It is not! Thermal problems are often very complex It is, in fact, becoming increasingly important to have thermal concerns moved up earlier in the design process. Key concern is at the thermal interfaces and getting effective transference of heat without also transferring mechanical stress

9 Electro-Mechanical Co-design 3D PKG Design PKG Mechanical Analysis Thermal Analysis Wiring DRC DRC Board Source: Ichiro Anjo, Elpida Physical Design DRC Die PKG Wire bond pad V [mv] time [psec] Eye pattern Electro-Magnetic Analysis SI PI Electrical Analysis Device simulation S-parameter, RLC Extract Current density PKG model System Simulation(SPICE) SI Device model

10 Overheat Protection is a Must Stepped Phase System Protection First level passive thermal protection (e.g. heat spreader or heat pipe) to remove heat Fan assist at first thermal threshold Next CPU/system clock speed reduced Nest overheat condition warning to user Finally automatic system shutdown

11 Thermal Management Benefits Proactive thermal management helps preempt potential electrical and mechanical problems and boost system performance Inverse relationship between long term reliability and thermal excursions and thermal extremes endured CTE mismatches creates stress and strain on physical elements of construction and must be addressed Cooler systems can operate faster

12 Addressing the Thermal Challenge System energy in Is it greater than energy out? Cooling is not necessarily free Operating temperatures continue to rise as operating frequencies increase Prospective solutions? 1) Slow down (not likely in this lifetime ) 2) Use multiple core technology 3) Shorten electrical paths 4) Eliminate parasitic effects 5) Clean up critical electrical channels

13 Thermal Transfer Modes Primary Thermal Transfer Modes Conduction - Thermal transfer normally through solids. Conductivity of solids vary widely.

14 Electronic Material Properties Material Copper Epoxy/Glass Silicon Aluminum Thermal conductivity 400 W/mK 0.3 W/mK 1.0 W/mK 250 W/mK CTE 18 ppm/ C ppm/ C 3 ppm/ C 23 ppm/ C

15 Factoring Thermal Conductivity with CTE Source: Carl Zweben - Advanced Packaging Feb 2006

16 Thermal Transfer Modes Primary Thermal Transfer Modes Conduction - Thermal transfer normally through solids. Conductivity of solids vary widely. Convection - Typically works in concert with conduction and involving a liquid or gas of different temperature passing over a surface to provide heat transfer with faster flow rates providing greater transfer. Secondary Thermal Transfer Modes Radiation - Thermal transfer from one body to another through space without a transfer medium Phase Change - Includes evaporation and condensation as well as freezing, melting and sublimation all occur through either a loss or gain of thermal energy.

17 Heat Paths In Electronic Assemblies Printed circuit board a natural first path Component leads and solder connections provide path to circuit board Metal power and ground planes serve as heat spreaders Heavy metal planes improve thermal transfer but make soldering more difficult Thermal vias improve heat removal Exposed edge of metal core can be clamped to rack to extend conductive path

18 Heat Paths In Electronic Assemblies

19 Thermal Management Solutions

20 Liquid and Gas Cooling Immersion liquid cooling Jet vapor or gas impingement cooling 1) Free path 2) Submerged and 3) Confined Liquid spray cooling Pulsed air cooling

21 Spray Cooling Technology To condenser Coolant Semiconductor chip Package or Interconnection Substrate

22 Heat Pipe Cooling Predicated on closed loop evaporative cooling cycle sealed system with microchannels and fluid evaporation at heated interface and cooling at distal end better thermal performance than solid metal can be very low profile (~0.5mm) Image sources: Acrolab (top) and Wikipedia

23 Thermoelectric Cooling Based on the Peltier effect Peltier observed that when electric current passed across the junction of two dissimilar conductors (i.e. a thermocouple) there was a heating effect not accountable by Joule heating alone. Solid state cooling system that actively cools by drawing heat away from the device of interest passing it to heat sink or head spreader Requires additional energy budget to operate

24 Thermal Modeling Benefits Multiphysics analysis of a Peltier cooler showing temperature contours Transient thermal analysis of a BGA package Source: Ansys Source: Gradient Design Automation

25 Measures for Thermal Modeling Thermal Conductivity (W/mK) Thermal Resistivity ( C m/w) Thermal Resistance ( C/W) Thermal Diffusivity (cm 2 /s) Specific Heat Capacity (kj/kg C) Also Tg, Td, CTE, Young s modulus, flow rates, etc. And for performance related metrics? Watts per Gigahertz?... Baud-Watts?

26 Embedded Device Issues and Benefits Advantage of greater integration in less space More compactness higher energy density Encapsulants should be appropriately filled Internal metal must serve a dual role Integral heat sinks and heat spreaders Offers shorter signal paths Allows for improved signal integrity Shock and vibration immunity Improved design security

27 Embedded Component Technologies Help Address Thermal Problems First Patents pending

28 Summary Thermal management concerns ebb and flow over time but will never go away and they are currently on the rise Energy management is a multi-dimensional and multi-factorial problem. Combining good design practices with intelligent thermal planning is required Addressing thermal issues early is going to be increasingly important over time.