Printed Circuit Design & Fab - May 2008 - (Page 29)

LAMINATES THERMALLY CONDUCTIVE Microwave Materials Increasing the thermal conductivity of the base materials improves thermal management in RF designs. by RUSS HORNUNG, MIKE SMITH and CHET GUILES Looking at the thermal issues which are impacting microwave design in today’s marketplace, we see several drivers that will continue to require better and more effective thermal management in the near future. Some of these factors are relevant to all electronics packaging, such as increased watt density on smaller package footprints, but others are of particular importance to the microwave and RF industries. Among the more important considerations for the microwave engineer are: ■ Power density (Watts/square inch of PCB area) continues to increase ■ Packaging is getting smaller & hotter (inherently increasing watt density still more) ■ Tower mounted & outdoor electronics increase environmental exposure FIGURE 1. Junction temperature and mean time-to-failure analysis courtesy of TriQuint. MAY 2008 while requiring higher reliability Complex waveforms decrease amplifier efficiency, resulting in more energy lost to heat ■ Higher temperatures reduce component reliability ■ Dielectric constant of many materials varies significantly with temperature When the heat generated by an active device mounted on a PCB cannot be dissipated, then the temperature of the device rises. With all the drivers pointing toward more power and smaller board surface area, the watt density becomes such that temperatures can easily exceed the qualificated temperature for many of the individual components. Since reliability of components follows a first order Arrhenius equation, a 10˚C increases in temperature doubles failure rate, so even 1˚C temperature increase matters. FIGURE 1 shows data presented by Eli Reese of TriQuint, and presented at IWPC in November 2005, relating junction temperature to mean time-tofailure. The key to reducing junction temperature is to increase the rate at which heat can be removed from the device and from the working area of the PCB immediately adjacent to the device. The cost of device failure can be ■ significant since device failure accounts for as much as 70 to 90% of equipment downtime, and in the case of towermounted electronics, the cost can run several thousand dollars for a simple repair that results in a short downtime. In bad weather this can extend to a full day and cost as much as $25,000. Between actual repair costs and potential lost revenue the implications of this trend towards increased operating temperatures of PCB’s is substantial. The industry is looking seriously at solutions that can provide a cost-effective way to mitigate the risk of failure. Heat is generated every time an active device is in operation. Heat itself does not become a problem until there is an increase in temperature above a critical point (typically 105 to 120˚F). In simple terms, the whole business of managing heat in a PCB assembly is about preventing the junction temperature from getting high enough to “fry” the active devices. Heat is moved by one of three basic modes: conduction, convection or radiation. In a PCB assembly all three are in effect to one degree or another. The most effective is direct conduction, where a warmer body is in direct PRINTED CIRCUIT DESIGN & FAB 29

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - May 2008

Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

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