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Keeping Your Cool with Thermal Analysis
December 31, 1969 |Estimated reading time: 5 minutes
By Patrick Carrier, Mentor Graphics Corp.
Thermal issues increase due to the competing requirements of higher complexity, lower cost, and faster time to market. Most thermal issues can be solved early in the design cycle at the board and chip level through thorough analysis. Possible solutions often prove more effective than elaborate, costly modifications made later on.
Modern electronic systems are increasingly plagued with thermal issues due to the competing requirements of higher complexity, lower cost, and faster time to market. Appropriate thermal management is vital to meeting these needs while still designing products that behave reliably. Trying to solve these thermal issues can be frustrating; often complicated mechanical solutions are added at the end of the design cycle and therefore have a substantial effect on time to market. Much like unwanted radiated electrical emissions, most thermal issues can be solved early in the design cycle at the chip and board level through thorough analysis. Less effective, more costly mechanical modifications can be avoided. The result is a more robust design that also is cost effective and meets tough thermal requirements.
Thermal analysis and problem solving require an inclusion and understanding of the three heat transfer mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through a thermally conductive material, such as metal. Convection is heat transfer through liquids, such as water or air. Radiation is the heat transfer from one hotter surface to a cooler one.
Figure 1. A PCB before and after implementation of conductive cooling methods including addition of ground planes, appropriately placed mounting screws, and metal slugs at the base of several hot components. Red areas are 100°C, cooling to purple at 0°C.
One of the most effective means of heat transfer in integrated circuits (ICs) and PCBs is conduction (Figure 1). Conduction starts at the IC level. Smaller packages dissipate less heat than larger ones, and it is the package-to-PCB contact area that determines how much heat can be dissipated through conduction. The greater the number of pins on the package, the more thermally conductive connections there are between IC and PCB. Power and ground pins are especially effective since they often are connected to solid metal planes in the PCB, which conduct the heat away efficiently. The effect can be enhanced further by adding other thermal connections between the IC and the PCB, such as thermal glue or a metal slug. Many small, high-power packages have a metal slug at their base to enhance thermal conductivity into the PCB. The heat conducted away from the ICs to the PCB may be transferred further to the significantly cooler chassis by using board-edge connections such as wedge locks, or connections on the board such as mechanical standoffs or mounting screws.
Figure 2. A PCB before and after addition of a heatsink to the hottest components, and a change in airflow direction. Red areas are 100°C, cooling to purple at 0°C.
Another effective means of controlling thermal issues is by deploying cooling methods that take advantage of convection (Figure 2). Natural convection is the transfer of heat upward from ICs and PCBs; it can be summarized as “heat rises.” Natural convection is quite helpful for PCBs that are oriented horizontally, as heat rises from the components through the air. It is something to be wary of, however, for PCBs oriented vertically, as heat may rise from hot components and further exacerbate thermal stress on the components that sit above them. The same is true in systems that use forced convection – hotter ICs may heat up other ICs that are downstream of the directed airflow. Proper airflow orientation can aid in making sure such ICs do not get victimized by the so-called thermal wake. Convection can further be taken advantage of, and often is, by the addition of heatsinks to hot ICs. Heatsinks conduct heat from the IC packages and provide a greater surface area through which convection can occur.
A somewhat less-effective means of controlling thermal issues is through radiation. Radiative heat transfer occurs from hotter parts to cooler ones. Radiation increases with surface area, and is highly dependent on the emissivity of the radiating component. Black surfaces typically will exhibit an emissivity close to 1; organics and oxide metals will have lower emissivities ranging from 0.5 to 0.9. Black IC packages are the best radiators on a PCB. Radiation also can be increased by moving cooler chassis metal closer to highly emissive components – a metal guard or a chassis wall for example. This is one reason why PCBs mounted near the end of rack-mounted electronic systems typically are cooler than those near the center.
As with any system of competing requirements, there are always tradeoffs to be made. Placing a PCB close to a chassis wall will increased radiative heat transfer, but heat transfer through forced convection may be reduced seriously due to the reduction in volume of air directly adjacent to the PCB. Similarly, changing airflow direction may cool off hotter components but also can heat up components due to the effect of the thermal wake. Deliberate component placement can result in a lower overall system temperature, but can be detrimental as well if, for instance, too many hot ICs are placed in close proximity. Managing these tradeoffs requires thermal analysis early in the design cycle, considering each heat transfer mechanism.
Design disasters can be avoided by having an understanding of the thermal profile of the electronic system. Doing analysis early in the design phase lets designers stay abreast of potential thermal issues and maximizes their ability to implement various cooling methods at the IC and PCB levels. Mechanical fixes – chassis modification, fans, liquid cooling – often need not be added. Avoiding these extraneous features speeds time to market and cuts costs. Reliability issues also become less likely, with a lower risk of product failure in the field and the subsequent need to perform emergency design changes after the product has shipped. Thermal analysis provides a means to solve what is becoming an exceedingly difficult design challenge, and allows designers to keep their cool throughout the product lifecycle. SMT
Patrick Carrier, technical marketing engineer, Mentor Graphics Corp., may be contacted at patrick_carrier@mentor.com.