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Electronics Cooling Reference: Cengel, Heat Transfer, 2nd Edition, Chapter 15 2/28/05 ME 259 1 Introduction All electronic components generate heat due to I2R (joule heating) Modern integrated circuits (ICs) – approaching 1010 components/chip – heat fluxes up to 100 W/cm2 Failure rate of electronic devices increases exponentially with operating temperature Silicon p-n junctions – Absolute limit of 125C for “safe” operation – 85C desirable for extended life 2/28/05 ME 259 2 Supporting Electronic Equipment Chip Carrier – heat flow paths – junction-to-case thermal resistance (Rjc) Printed Circuit Board (PCB) – Single-sided – Double-sided – Multilayer Enclosure – sealed – vented 2/28/05 ME 259 3 Common Cooling Methods 2/28/05 Conduction cooling – copper “heat frames“ attached to PCB – copper layers and “vias” within multilayer PCBs Air Cooling with or w/o heat sinks – natural convection & radiation – forced convection with fans Liquid cooling – direct immersion – indirect (hxer, pump) ME 259 4 Electronics Cooling Models 2/28/05 Component Model – Semiconductor device – Heat sink, mini-fan, heat pipe, hxer – PCB Enclosure Model – Chassis assembly – Environment Air Flow Model – Chassis-mounted fans – Air intakes, exhausts, – Shrouds, ducted flow System Model – Combines component, enclosure, and air flow models ME 259 5 Component Model Physical system Thermal circuit Typical values Analysis 2/28/05 ME 259 6 Enclosure Model Physical system Thermal Circuit Analysis 2/28/05 ME 259 7 Air Flow Model Physical system: Thermal Circuit Analysis 2/28/05 ME 259 8 System Model Thermal circuit Analysis 2/28/05 ME 259 9 Example Given: Enclosure containing 16 TO-3 style power transistors, mounted in sets of four, on four EG&G 435 series heat sinks. Transistor specs Rjc = 7.0 C/W Rcs = 0.09 C/W qc = 10 W Tj,max = 125 C Enclosure specs L = 60 cm H, W = 20 cm t = 3 mm k = 0.4 W/m-K ho = 10 W/m2-K To = 25 C o = 0.8 hi 3.9 V / L W/m2 - K Heat Sink specs Rsa 0.9 1V C/W (curvefit to data, V in m/s) Find: Fan flow rate needed to keep Tj below 125C 2/28/05 ME 259 10