Keeping it cool

As the design requirements for the enclosures become smaller, clock speeds increase, and more functionality is packed within an ever-decreasing space, the need for thermal management in today’s embedded system designs becomes critical.

Thermal management awareness for an embedded system is nothing new to electronic system design engineers. It is a battle between the design engineer and the electronic system that has been underway since the first circuits were designed. As the design requirements for the enclosures become smaller, clock speeds increase, and more functionality is packed within an ever-decreasing space, the need for thermal management in today’s embedded system designs becomes critical.

The embedded engineers enemy: Heat
The life expectancy and reliability of electronic systems is inversely proportional to the temperature of the components within the system itself. Decreasing the heat that is generated from the circuit board or efficiently removing the heat from the individual components will have a direct impact on the life and reliability of the overall embedded system.

A general rule of thumb that design engineers have used is based on the Arrhenius equation: k=A*exp(-Ea/R*T); which predicts that for each 10°C (18°F) rise in the operating temperature reduces the electronic component life by about 50 percent.  Conversely, each 10°C (18°F) decrease in the operation temperature increases the electronic component life by about 100 percent.

The strength and versatility of the PC/104 form factor allows a system design engineer to individually select the PC/104 boards that precisely meet the overall design requirements of the entire system. This advantage of utilizing off-the-shelf circuit boards reduces the design time of the entire system. The PC/104 board manufacturers will define the ambient temperature under which their boards are guaranteed to function. Unfortunately, many data sheets and specifications for the individual PC/104 circuit boards do not provide a thermal profile of the board itself or details on how hot the individual components on the board will reach.

It is wrong and dangerous for the system design engineer to assume that because each of the individual PC/104 circuit boards meet the ambient temperature range defined by the system design, the embedded system will not experience failures or system performance anomalies due to temperature. PC/104 boards that tend to “run hot” and are placed within a small hermitically sealed enclosure with no fans or cool airflow is a recipe for intermittent system level failures or, worse, a complete system level failure after it is deployed to the field.

What to do? What to do?
Thermal design analysis and management needs to be a part of the overall system level design from the very beginning stages. All of the PC/104 circuit boards that are going to be utilized in the system need to be procured in the initial stages of the system design and functionally tested, including a thermal analysis evaluation in the lab.

Each board, and preferably all the boards together, needs to be powered up and functionally running. At the beginning stages of a system level design the software and firmware code are not ready to be loaded onto a board. Almost all PC/104 board vendors supply some type of example code or demo software that demonstrates the features of their boards. This code can be utilized to test the thermal properties of the circuit board. Download freeware from the Internet that targets the CPU, DSP, or FPGA located in the system, or internally develop some very simple software or firmware routines that can be loaded into the PC/104 board. The main objective is to get the PC/104 boards running with some type of software or firmware running at the intended final clock speed and pulling a representative current from the power supply. A thermal analysis of the system in relation to the enclosure and the end application can then be obtained and evaluated.

Were HOT! Now what?
There are many options available for addressing heat issues in relation with the system design. Heat sinks are the most readily available solution. Heat sinks come in a vast assortment and range of sizes. There are several special epoxies that are available that allow a heat sink to be “glued” to a component to provide an increased surface area to dissipate heat away from the IC. Incorporation of internal component mounted fans and enclosure mounted fans are available and provide a constant airflow directly to the components and the internal system circuit board.

There is also the option of mechanically modifying the embedded system enclosure and mounting custom designed heat sinks to its side, therefore enabling heat removal from individual or multiple components. These types of heat sinks effectively turn the entire enclosure itself into a large heat sink.

System design restrictions that prevent the utilization of fans or advanced cooling systems and that require hermetically sealed enclosures must resort to slowing down the system clock or periodically placing the CPU into a low-power or sleep mode to decrease the amount of power required by the system.

The option of utilizing a different PC/104 board featuring decreased features, reduced clock speeds, or alternate CPU, FPGA, or DSP architecture is sometimes the only solution available to eliminate thermal problems with a system level design.

Early identification is key
Today’s PC/104 system design engineers need to address thermal management of the overall embedded system at the beginning stages of the design. Early identification of potential PC/104 board ICs that may contribute to heat related failures, or overall reliability of the entire system in the system enclosure, will allow the system-level design engineer to make the necessary changes to correct any thermal problems.