Printed Circuit Design & Fab - February 2009 - (Page 40)

THERMAL ManaGEMEnt designing with Conductive Materials, part 1 Complex electronic products present design engineers with an increased need for cost-effective thermal management. by HELENA LI Today, many designs continually push limits for heat transfer, resulting in high device operating temperatures. PCB materials that increase thermal conductivity to reduce peak operating temperatures can improve component life. For over 30 years, high temperature applications have used polyimide laminate and prepreg systems to provide device reliability in operating environments exceeding 200°C. Newer, lead-free compatible epoxy systems offer a cost-effective approach with requirements that are less critical. How materials react to cyclic thermal exposure remains a critical contributing factor in determining device reliability. Designing PCBs on materials that reduce or control thermal expansion will improve PTH reliability and reduce stress and fatigue on solder joints to SMT components. Low-loss materials for microwave and high frequency applications minimize heat generated by transmission line loss. Device reliability is a complex function of the heat generated by the operation of an electronic device, the tools used to dissipate or manage the heat, the thermal stability of the materials used and the environment in which the device is required to operate. Because of diversity of applications and the increasing demand for electronics, diverse thermal management tools have evolved to help mitigate reliability issues. These tools include gap fillers, active cooling systems, heat pipes and heat sinks. While many of these tools overlap in terms of potential benefits, the selection of which tools to apply depends on the ultimate constraints of the device in terms of cost, power requirements, weight, size and reliability. The breakdown of a typical electronic device demonstrates the various tools used to facilitate heat dissipation when designing for reliability. FiGurE 1 shows a generic device with an active component mounted on a typical circuit board. Complementing these active components may be heat 40 sinks, thermal interface materials, thermal vias and active cooling systems. Many of these approaches are implemented to compensate for the fact that most traditional electronic components and dielectric materials are thermal insulators that necessitate secondary cooling systems, such as heat sinks and cooling fans. Thermal interface materials are used to minimize gaps or variation between materials that can occur in assembly, which can retard heat transfer. Selection and optimization of thermal management tools are often based on a combination of experience, knowledge and device testing to understand failure mechanisms. Device failure is a function of the reliability of the components, materials, time and operating environment (humidity, temperature, thermal cycling, etc.). In most cases, failures can be grouped into one of several categories, such as component failure, connector/solder joint failure or board failure, to facilitate further root cause analysis. Ultimate causes of these FiGurE 1. Generic device with an active component mounted on a typical circuit board. FEBRUARY 2009 printEd CirCuit dESign & fAB

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - February 2009

Printed Circuit Design & Fab - February 2009 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar BGA Bulletin Interconnect Strategies Final Finsh Forum Defects Database Embedded Active Components In Multilayer LCP Packages Simulation: The Need for Speed Advanced Registration Systems The DC Design Squeeze Ad Index Do You Really Want a Better Autorouter? Designing With Conductive Materials, Part 1 Off th eShelf Marketplace On the Forefront

Printed Circuit Design & Fab - February 2009

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