RTC Magazine

There are many trends at the die level and circuit card level that can drive the decision of which cooling method to use for 3U COTS cards in rugged applications. One of these trends is the increased use of multicore processors on 3U cards. Placing more processor cores on a die increases power dissipation. However, as more cores are placed on a die, the size of the die increases, which actually decreases the power density in terms of W/cm2 (Figure 1). This is a good thing from a thermal standpoint because it reduces the spreading resistance down the heat removal path. Unfortunately, increasing device power dissipation, combined with the long-term trend of decreasing junction temperatures (i.e., from 125°C to 105°C, or 100° or less), tends to override the small benefit obtained from the decrease in power density.

One of the main causes of increased power dissipation for processors is the trend toward increasingly smaller transistor geometries, which results in large increases in static (or leakage) power. Some modern processors use new types of transistor materials that reduce the amount of static power. For example, Intel uses hafnium dioxide on its 45nm processors. These new transistor materials reduce the amount of gate oxide tunneling and sub-threshold leakage current, two of the dominant forms of leakage current.

At the board level, component miniaturization and the use of more highly integrated devices are increasing the functional density on 3U cards, which is directly related to heat density. As the amount of processing power per square inch per 3U card has increased, it has driven up heat density. Another factor driving up allowable power dissipations on 3U cards is the support for higher voltages in the new VPX (VITA 46) standards. VITA 46 defines support for 12V and even 48V, compared to the standard, traditional 3.3 and 5V supported by VME. VPX also supplements the traditional 0.8” pitch of VME with 0.85” and 1.0” pitches. This increase in pitch enables the use of more, hotter devices on the rear side of the circuit card, increasing the power dissipation per unit area and volume. The outcome is an almost exponential increase of power at the circuit card level. Susceptibility to this trend depends on the card’s functionality. The higher power cards are typically DSP cards that have multiple multicore processors on board for number crunching. In comparison to DSP cards, general-purpose processor and I/O cards typically follow more of a flat curve in terms of power.

While direct air and conduction cooling have been able to keep up with these power increases to date, it has been a challenge. The amount of thermal design, analysis and testing that is required on a rugged military COTS 3U card is many times what it was five years ago. This increased work is basically the result of the increase in power dissipation.

Direct Forced Air Cooling

Direct forced air cooling is typically the starting point in terms of cooling approaches for military COTS cards simply because most software and system development begins in a laboratory environment with air-cooled cards in a benchtop rack. Consequently, these cards are usually commercial temperature rated and not rugged.

Cooling begins at the device die and on most modern, high-power devices, the die is exposed. This is because most commercial cooling approaches employ an air-cooled heatsink on top of the die, which provides the shortest and lowest resistance heat removal path to the air. While forced air cooling takes advantage of this arrangement, it may not always be desirable to have a large piece of metal on the die. For this reason a heat spreader is sometimes placed between the heatsink and the die to spread the heat and to provide some protection for the die. Unfortunately, for today’s higher power devices, standard off-the-shelf aluminum heatsinks, with a few fins per inch, may be insufficient. To address these hotter devices the heatsink will likely need to be optimized. System designers can use computation fluid dynamics (CFD) tools to design the heatsink they require for air cooling. There are subroutines available within CFD tools to design the heatsink’s optimum number of fins per inch, thickness of fins, optimum gap and height, etc. Another trend in air cooling today is the availability of higher flow and pressure fans and blowers, which provide increased airflow for such high pressure drop heat sinks.