Printed Circuit Design & Fab - February 2009 - (Page 42)

THERMAL ManaGEMEnt sion. Thermal coins are potentially more cost-effective than a large heat sink and potentially more reliable than thermal vias, but they add to assembly complexity and costs. Thermal interface materials can be either electrically conductive or electrically insulating and cover a wide variety of materials, such as thermal pastes, greases, phase change materials, tapes, bonding plies and prepregs. Their primary advantage is that they can displace air gaps and reduce impedance to heat flow at interfaces. Some types of materials can also offer thermal-mechanical decoupling to minimize stresses from thermal expansion and to reduce vibration related failures or joint failures and improve reliability through thermal cycling. Circuit board thermal conductivity is often neglected as a potential area to improve heat transfer since this has historically not been a controllable design option. The primary purpose of these materials is to provide electrical isolation of the various components and traces. This has yielded materials that are also good thermal insulators that trap heat within the active components of the board. By modifying the base properties of these materials, it is possible to create materials that provide not only the electrical insulation required for typical circuit boards, but also improve the heat transfer rates relative to traditional materials. This benefit is demonstrated by comparing a traditional FR-4 material with laminates of increasing thermal conductivity in the same circuit design with a 0.5-Watt heat source. Design optimization usually begins with thermal modeling tools to identify hot spots or other issues. There are several good software packages on the market today that can help designers understand the affects and trade-offs of the various thermal management tools. This can be time consuming and requires a good understanding of the model limitations to fully translate the results into practice. Unfortunately most modeling software assumes isotropy which may result in over or underestimation of the heat removal. In most PCB materials, the heat transfer coefficient in the perpendicular direction (down through the PCB) is different from that in the plane of the board. The models also rarely account for potential issues, complications or variation in materials, fabrication or assembly. Interfaces interfere with the heat transfer mechanism, through imperfections in conduction. This often results in reduced efficiency in heat transfer which results in excessive temperatures. Addressing these potential issues is generally where thermal testing of subassemblies and devices can help. Such testing can vary from power cycling or single point thermocouple measurements to thermal imaging equipment. This level of testing can help designers further optimize their designs by identifying potential problems or lower cost substitutions, such as trade-offs in materials choice (aluminum vs. copper, cast vs. machined heat sinks, etc.). The first step towards ensuring long-term survivability of a device in a thermally stressing environment is to select materials that can meet the thermal excursions in the PCB manufacturing and assembly process. IPC specifications for Lead-Free and High Reliability materials include such factors as Tg (glass transition temperature), Td (thermal decomposi42 tion temperature), total thermal expansion perpendicular to the plane of the board (Z-axis) and short term stress tests such as T260, T288 and T300. The key to success is to avoid latent defects such as hidden cracks in PTH copper that can later propagate and cause resistance change or can open when the board is at operating temperature. Arguably, Tg is usually the first consideration in material selection as it relates to the survivability of PTH’s during thermal cycling. A high Tg material will exhibit lower overall Z-expansion from room temperature to the operating temperature and will normally retain its adhesion to copper up to and even somewhat beyond the Tg. Z-Axis thermal expansion from 60° C to 260° C is also a key metric in comparing materials for potential PTH reliability. Decomposition Temperature (Td) is a determinant in assessing long term survivability of materials at elevated temperatures, and while the temperature of a PCB rarely reaches anything approaching its Td, it is reasonable to say that a material with a high Td is likely more survivable at any long-term temperature exposure than a material with a lower Td. T260, T280 and T300 tests are good measures of the likelihood of a PCB substrate surviving short-term exposure at the extreme thermal limits. These tests are measured by ramping samples to the specified temperature and holding them isothermally until irreversible change occurs, usually in the form of delamination. Beyond the temperatures observed during PCB fabrication and assembly, the working environment of the finished PCB, the thermal environment in which it will actually live and work, will throw new challenges at the devices and substrates employed in making the board. Any active device on a PCB will generate its own heat as the junctions on its silicon chip flip on and off billions of times per second. When a piece of electronic gear is turned on and off over time, the temperature inside the enclosure will cycle up and down. Beyond these temperatures, electronics today often encounter elevated service times and temperatures in certain difficult environments such as down-hole drilling, underhood automotive electronics and some space applications. The combination of the environment with the heat generated from the device results in a widely varying thermal operating zone that requires special design consideration to insure device reliability. High temperature electronics are devices that are designed to operate in severe conditions, such as engine mounted sensors, jet-engine control systems and chemical process or oildrilling instrumentation. Operating temperatures for these devices can exceed 250° C. These temperatures can start to chemically degrade polymers and lead to device failure. Selecting a thermally robust system that is designed to handle high temperatures for long periods is critical for success. pCd&f Ed. Note. The complete article including a Chinese translation will be included in the March 2009 edition. hElEna li is with Arlon-Med and can be reached at helenalihai@vip.sina.com. 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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