Advanced Packaging - Proactive Thermal Planning

In recent years, long-ignored thermal issues have been wreaking havoc on electronic systems' reliability. Anyone who has opened a computer case and seen the massive, often elaborate heatsinks attached to microprocessors knows how severe thermal issues can be. And while applying a heatsink is an effective way to cool a microprocessor, a proactive approach, applying various methodologies, is the only solution for successfully designing complex electronic systems to meet thermal requirements.

ICs account for the greatest amount of power dissipated on a PCB electronic design. Board designers are plagued with trying to distribute power throughout the PCB for a steadily increasing number of voltages, and concerns over heat generated by wide traces have also increased. However, such issues are a distant concern compared to the power dissipated by the ICs on the board. For that reason, proactive thermal planning must begin with the package.

Microprocessors pose a large thermal concern because they dissipate more power than any other IC on the PCB. However, it is not the power itself which causes the problem, but the power-per-unit-area of the package. A small IC that dissipates a moderate amount power creates more of a thermal concern than a large IC that dissipates a large amount of power (Figure 1). The small ICs on the left hand side of the board dissipate only one fourth, or less, of the power of the larger components on the right side, yet they get much hotter, as seen by the large yellow and amber segment surrounding that segment of the board. Their package temperatures approach 100°C and, as a consequence, make the left side of the board quite hot. On the other hand, while the IC in the bottom right has a die temperature approaching 100°C, its large package allows it to stay cool.

Techniques to Control Thermal Issues
The most obvious method to control thermal issues in ICs is to use the largest package possible. Of course, there are other factors such as cost and system integration that need to be considered when choosing a package design, and such factors also play a part in the overall system design. To properly control thermal issues, use all heat transfer mechanisms. That includes conduction, convection, and radiation.

Conduction is an effective method for dissipating heat in an IC. The greater number of pins, the less thermal resistance there is between the package and the board, and the easier heat can be conducted away by the board. This can be taken a step further by applying thermal glue between the IC and the board, or even by attaching a metal slug to the base of the IC package to increase its conductivity.

Changing package material to one with greater emissivity maximizes heat transfer due to radiation: black surfaces will typically exhibit an emissivity close to 1; organics and oxide metals will have lower emissivities ranging from 0.5 to 0.9.

And of course convection, more specifically forced convection, can be used to dissipate power by placing a heatsink on the package. A heat sink is typically the most common way to control heat on a package, but incorporating conduction and radiation can also be very effective (Figure 2). Small components on the left side now have a metal slug included with their package, which has been modified to use a material with an emissivity of 0.9. The result is that the small components dissipate their heat more effectively, and drop in temperature from 100°C to about 65°C. Consequently, the board temperature in the area of those components also drops.

Board-Level Solutions
That is not to say thermal issues cannot also be controlled at the board level. By simply adding more ground planes to the board, thermal conductivity can be dramatically increased to help conduct heat way from hotter components. Adding screws or standoffs to the board also conduct heat into the chassis. Similarly, the high thermal conductivity of existing screws and other board mounting mechanisms can be exploited by moving components closer to these heat conductors. These are very cost-effective methods for controlling thermal issues. Such board changes will result in reduction of component temperatures similar in scale to the effects of packaging changes.

Thermal Planning is Critical
By addressing thermal issues at both the package and board level, the arsenal of possible solutions is greatly expanded beyond mechanical solutions, such as adding more fans to the chassis.
No longer does the burden of cooling the system fall solely on the mechanical engineer at the end of the design cycle, where solutions are limited and design changes can be costly and risk time to market. Worse yet, such design changes may not even be implemented and thermal issues ignored, reducing component lifespan and creating quality and reliability issues in the product. Such design disasters can be avoided with proactive thermal planning.

Patrick Carrier, Technical Marketing Engineer, may be contacted at Mentor Graphics, Patrick_Carrier@Mentor.com.

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