2/28/05ME 2591 Electronics Cooling Reference: Cengel, Heat Transfer, 2 nd Edition, Chapter 15

2/28/05ME 2592 Introduction u All electronic components generate heat due to I 2 R (joule heating) u Modern integrated circuits (ICs) –approaching components/chip –heat fluxes up to 100 W/cm 2 u Failure rate of electronic devices increases exponentially with operating temperature u Silicon p-n junctions –Absolute limit of  125  C for “safe” operation –  85  C desirable for extended life

2/28/05ME 2593 Supporting Electronic Equipment u Chip Carrier –heat flow paths –junction-to-case thermal resistance ( R jc ) u Printed Circuit Board (PCB) –Single-sided –Double-sided –Multilayer u Enclosure –sealed –vented

2/28/05ME 2594 Common Cooling Methods u Conduction cooling –copper “heat frames“ attached to PCB –copper layers and “vias” within multilayer PCBs u Air Cooling with or w/o heat sinks –natural convection & radiation –forced convection with fans u Liquid cooling –direct immersion –indirect (hxer, pump)

2/28/05ME 2595 Electronics Cooling Models u Component Model –Semiconductor device –Heat sink, mini-fan, heat pipe, hxer –PCB u Enclosure Model –Chassis assembly –Environment u Air Flow Model –Chassis-mounted fans –Air intakes, exhausts, –Shrouds, ducted flow u System Model –Combines component, enclosure, and air flow models

2/28/05ME 2596 Component Model u Physical system u Thermal circuit u Typical values u Analysis

2/28/05ME 2597 Enclosure Model u Physical system u Thermal Circuit u Analysis

2/28/05ME 2598 Air Flow Model u Physical system: u Thermal Circuit u Analysis

2/28/05ME 2599 System Model u Thermal circuit u Analysis

2/28/05ME Example u Given: Enclosure containing 16 TO-3 style power transistors, mounted in sets of four, on four EG&G 435 series heat sinks. Transistor specsEnclosure specs R jc = 7.0  C/WL = 60 cm R cs = 0.09  C/WH, W = 20 cm q c = 10 Wt = 3 mm T j,max = 125  Ck = 0.4 W/m-K h o = 10 W/m2-K T o = 25  C  o = 0.8 Heat Sink specs Find: Fan flow rate needed to keep T j below 125  C