Meeting the military challenge: Designing for wide temperature range systems

Story

December 16, 2011

Dr. Qi Chen

Arbor Technology

Designing embedded systems for wide temperature range rugged applications requires careful attention to detail and consideration of many aspects. Only by utilizing a comprehensive design methodology will successful implementations be achieved.

For many embedded applications, particularly those in military transportation, or where systems must be operated in harsh outdoor environments, a wide operating temperature range of -40 °C to + 85 °C is typically expected, and this can be a challenging requirement to meet.

To achieve such a rugged system specification, the design needs to take account of many special details, starting at the circuit level and also encompassing component selection, thermal design and PCB layout, and design verification testing and life-cycle management, to name just a few. It will be seen that many factors need to be considered before a successful system can be realized.

Components matter

Selection of major components is an important factor, and all the active components, such as the processor chips, memories, and regulators, need to use appropriate industrial temperature grade parts to ensure operation within specification at the temperature extremes found in wide temperature range applications. Less widely considered is that all passive components too need appropriate specifications, and resistors, capacitors, and crystals all need to be carefully selected.

Resistors, for example, need to be chosen so that they meet the tolerance requirements for resistance value across the full temperature range, and so that their power dissipations are not exceeded. This is a particularly important consideration for resistors that require accurate values but must be placed near other hot components.

Capacitors are used in large numbers in embedded designs, mostly for power supply smoothing and decoupling active components. High value bulk capacitors are needed to smooth supply rails, and electrolytics are typically used in general-purpose designs because they provide a high capacitance value in a small physical volume. However, wide temperature range system designs generally need to avoid use of electrolytics because these types of devices are not solid-state and contain gel-like materials internally. These materials can potentially change properties over time when operated at temperature extremes, partially drying out or even failing with extended use at high temperatures.

Solid-state ceramic capacitors are therefore the best choice for capacitors used in wide temperature range rugged applications. There are a number of ceramic materials for capacitors, and for these applications it is preferable to only use materials such as NPO or X7R, which have a low temperature coefficient, meaning that the change in capacitor value with temperature over the full operating range is relatively small. Standard materials such as Y5V can vary by as much as -80 percent/+20 percent, whereas an X7R variation is much lower at only ±15 percent, and NP0 has effectively no significant variation, being a very small ±30 ppm/C.

Stable capacitance values are particularly important in any circuits where the capacitor value is a carefully selected design parameter. Examples of this include filter circuits in regulator control loops and clock and timing circuits. Although these low-temperature-coefficient ceramic devices are more expensive than general-purpose parts, the improvements in consistent system operation and reliability outweigh the added cost.

Choosing low-temperature-coefficient components also applies to clock circuit crystals, which should, for example, be specified to be only 20 ppm variation. This ensures that clocks do not change frequency with temperature in any significant way that would affect system operation. This could otherwise lead to problems at the system level, which may only occur at temperature extremes under certain conditions where clocks unexpectedly drift too far away from their nominal frequency.

Thermal design is key

To reduce internal sources of heat in the rugged embedded system, it is much preferred to use switching regulators in the power section instead of linear or LDO-type regulators. Switching regulators are more efficient, 90 percent being typical, whereas LDO efficiency depends on the voltage drop and current flowing and can be very low in comparison. Depending on the voltage drop across the regulator, 50 percent efficiency would not be unusual. The higher efficiency of the switching regulator compared to an LDO means that the switching regulator therefore produces much less excess heat. Even though a switching regulator is used, it should also still be implemented so that the design has been tuned to be as efficient as possible to further minimize any excess heat. This is particularly so at high power load conditions of 10s of Amps when the CPU is drawing maximum power.

A wide temperature range embedded rugged system needs to be designed so that all aspects of heat removal from all the heat generating components are considered. Usually the processor is the main heat producing part, but chipset and regulator parts also can generate significant amounts of heat. The processor and chipset should therefore be chosen to be as low power as possible, while still providing for the application processing requirements, and be verified to operate over the temperature range required. The heat removal from the back surface of the die at the top of the processor package is a key part of the design; a CNC-machined aluminum heat spreader is a good solution to take heat off the board and transfer it to the case, which is itself designed to be a heat sink. The heat spreader can also be used to touch most of the other main heat generating components on the PCB, such as parts of the power section, conducting their heat away from the board to the exterior heat sink surfaces of the case as well. Fans contain moving parts and are therefore vulnerable to wear and eventual failure. Although airflow from fans is very effective to remove heat from components, fanless designs are preferred because they have much higher reliability. Fanless systems are possible to achieve by factoring in careful thermal design considerations.

PCB layout and design

Effective PCB layout is also a key aspect for wide temperature range systems. It is necessary to ensure that hot components are both distributed over the PCB to avoid creating hotspots and also that these hot components are not too near to other heat sensitive components such as clock crystals. Also the PCB’s metal layers can be designed where necessary so that they conduct heat (or not) in a controlled way from one part of the PCB to another. In short, the PCB, heat spreader, and case need to be designed as one overall thermal system.

PCB materials are chosen so that they have a high glass transition temperature (Tg). This is the temperature at which the board material starts to physically deform, and at the maximum ambient operating temperature of + 85 °C, all parts of the PCB are designed to be well below the maximum temperature that the PCB itself can handle before deforming. Careful control of heat generation and heat flow are therefore all part of the design process, and they are not afterthoughts added later. By careful design of the PCB, heat spreader/heat sinks, and case, it is possible to achieve an optimal thermal solution that is very reliable in operation at all temperatures within the operating range.

Design verification is vital

Once a system design has been generated using suitably chosen components and the thermal design has been completed, the next step is design verification. This is a key step in proving that the design as a whole meets the specification requirements. A number of demanding tests are used, which are designed to stress the system in different ways to expose any weaknesses. For wide temperature range systems, long-term testing involving extended operation at -40 °C and +85 °C is performed to ensure that the basic specification is met. For embedded systems, a boot-up test is particularly important and is performed a large number of times at all temperatures including the extremes. Thermal shock testing is also used, where the temperature is rapidly ramped from -40 °C directly to +85 °C, generally including both normal operation and multiple boot-up tests during the temperature ramp. Any failures in these tests are fully investigated and the design corrected where needed.

Once design verification is completed, there are further tests that can then be performed during production on boards and systems before delivery to customers. At Arbor Technology, for example, we perform vibration stress screening according to MIL-STD-781D task 401 on production boards to eliminate any that have assembly defects that may show up later during operational life. Figure 1 is an example of such a wide temperature range board: The Arbor EasyBoard-841E has specific design features included for applications such as external environments on vehicles, and is specified to operate over a -40 °C to +85 °C ambient temperature.

Figure 1: The Arber EasyBoard-841E wide temperature range rugged board

Product life-cycle management

For wide temperature range and high-reliability applications, a long product life cycle is a key element. To control and maintain quality and meet specifications at all times during the life cycle of the product, the manufacturer needs to pay particular attention to products that go End-Of-Life (EOL) during that time. Components that go EOL must only be replaced by compatible ones that meet the same specification, and the design verification step needs to then be repeated. Also a replacement component generally needs to be a true drop-in, even with the same footprint; otherwise the thermal environment may change because of needing to make changes to the PCB layout. This level of detailed Bill-Of-Materials (BOM) control therefore needs to extend to cover all components in the design, including capacitors and resistors, as well as the more obvious large chips.

Consistent application of design methodology

Achieving a wide temperature range embedded system design requires careful attention to detail at all stages of product design, from component selection to thermal design, through verification testing, and even into maintaining BOMs over the product life cycle. Design success is therefore a result of methodically applying a consistent integrated design methodology that attends to all these factors.

Dr. Qi Chen has more than 23 years of experience in technical and management roles for industry-leading companies in the U.S. and U.K. She obtained her Ph.D. in the U.K. and previously worked as Sr. Director of Engineering for both Ampro and Ampro Adlink before joining Arbor Technology as Vice President of Engineering and General Manager of their U.S. office in San Jose. She can be contacted at [email protected].

Arbor Technology 408-452-8900 www.arbor.com.tw