Printed Circuit Design & Fab - February 2009 - (Page 41)

THERMAL ManaGEMEnt failures may include chemical or electrical degradation of base materials, connection failures caused by thermal expansion mismatches, air gaps causing a reduction in heat transfer and oxidation caused by high temperature or mechanical failures. One cause of temperature related failures of boards or components relates to change or degradation at the molecular level. This type of failure is best modeled as a first order kinetic reaction, typically described as an Arrhenius Equation, which is proportional to the inverse log of the temperature. A simplified Arrhenius equation and a resulting reliability plot against operating temperature are displayed in FiGurE 2. Since failure rates, often described as a mean-timeto-failure (MTTF), increase exponentially with temperature, a 10° C-increase in temperature can double the failure rate. In an operating device where reliability is critical to success, even 1° C can matter. The key to improving reliability is to reduce device temperature by increasing the rate at which heat is removed from the device and from the working area of the PCB immediately adjacent to the device. Understanding heat transfer then becomes the next step. Heat is generated every time an active device is in operation. Device operating temperature is a result of the balance between heat generation and heat dissipation. Heat itself does not become a problem until there is enough heat to result in an increase in temperature above a critical point, in many cases about 105° F to 120° F. Since many designs are set based on function, the heat generation side of the equation is already determined by the time it comes to managing heat dissipation. As such, it is important to understand the basics of heat transfer to determine possible strategies for reducing device temperature. 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 from a “hot body” to a “cooler body” by one of three basic modes: conduction, convection or radiation. In a PCB assembly, all three are in play to one degree or another. Conduction can be the most effective for heat transfer, where the cooler body is in direct (and preferably intimate) contact with the warmer one, and the heat moves from hot to cold materials in an attempt to reach equilibrium. The rate at which heat is carried from one to the other depends on the thermal (temperature) gradient, the coefficient of heat transfer of materials involved (thermal conductivity), the amount of material involved in the thermal path (thickness), the quality of the interface and to a lesser extent, the heat capacity of the cool body that is absorbing the heat. The combined effects of thermal conductivity, the material thickness in the thermal path and the interface effects on heat flow are often characterized in terms of the thermal impedance. Convection is the transfer of heat from a hot body to a cooler fluid that carries it away through molecular motion. This can happen naturally in a fluid based on resulting density gradients caused by temperature variation. Convection may be aided by forcing the cooling fluid to flow past the warm body, thus carrying away the heat faster. Conversely, convection heat transfer can be significantly impeded by device enclosures that restrict air flow, resulting in higher device temperatures. Radiation is the removal of heat from a body by the emission of energy in the form of electromagnetic radiation, which may be in the infrared (heat) or even visible (light) parts of the spectrum depending on the temperature of the radiating body. RF signals, such as those generated by an antenna, are also a type of radiation that dissipates energy. Thermal Management Design Options The number of approaches and possible solutions to reduce temperature through heat removal from active devices is almost endless. This remains an active area for development across all application areas, and the tools available to designers will continue to evolve. Tools for increasing convection include: active cooling such as forced air or conditioned air; water; and vapor cooled. While active systems, such as cooling fans, may be extremely effective in heat removal, they add additional design and power requirements, increase costs and add to device size and weight. They also add to potential reliability concerns, as failures of these systems typically result in device failures. Tools for increasing conduction include heat sinks, heat spreaders, heat risers and heavy metal backplates. Additional conduction methods involve thermal vias (a PTH used to leverage thermal conductivity of copper), thermal coins (inserts of metal conductors in PCB cut-outs under active components to improve heat transfer), thermal interface materials, thermally conductive adhesives, gap fillers and other ways to create a thermally conductive printed circuit board using conductive materials. Each of these tools brings their own advantages and disadvantages. Heat sinks and backplates add cost and weight to a system. Design and optimization of these systems requires a consideration of the metallurgy (usually copper, brass or aluminum) to balance heat transfer requirements with material thermal conductivity, heat capacity, density, machinability, processing requirements and costs. Thermal vias can be extremely cost effective, but there are practical limits in the area covered. Heat transfer in thermal vias is limited to thru-plane heat transfer and can cause reliability concerns if the PTH fails due to stresses from thermal expanprintEd CirCuit dESign & fAB 41 FiGurE 2. Simply Arrhenius equation and a resulting reliability plot against operating temperature. FEBRUARY 2009

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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