The author is Standard Products Marketing Manager, 32-Bit Embedded Controller Division, Transportation & Standard Products Group, Semiconductor Products Sector, Motorola, Inc.
Introduction
Huge increases in computing processing performance accompanied by equally dramatic decreases in microprocessor and microcontroller costs have been one of the most important technological, as well as societal, drivers in the history of electronics. One aspect of these trends that is not often highlighted but is important is the migration of computing technology from the carefully controlled confines of the lab and office to the many rugged and demanding environments where manmade equipment operates.
Early computers were housed in special rooms kept within strict temperature ranges and protected from environmental shocks. Today''s embedded microcontrollers provide orders of magnitude higher computing performance, and must also operate in harsher environments, such as extreme cold of the upper atmosphere or in the heat found under the hood of an automobile or inside factory equipment. In these environments -- unlike in a piece of desktop equipment -- implementing bulky and expensive cooling fans or heat sinks is often not an option. And while operating in these harsh environments, today''s controllers must do so at ever increasing quality and reliability levels.
Applications In Harsh Environments
Many examples of the types of applications that use embedded controllers in harsh environments are found in the avionics and aerospace industry, where safety and reliability are critical factors. If one thing is common across the systems in these industries, it is this: system failure is not an option. Companies in these industries that are designing semiconductors into their products and systems must be assured that the devices will operate reliably over wide temperature ranges. Manufacturers of aerospace and avionics systems, such as helicopter controls, fuel and altimeter controls, and global positioning systems, strive to find the most cost-effective system components and methods for testing. Concurrently, they must ensure the highest reliability of the end product/system without sacrificing the safety of employees or the end user. So, not only are the environmental conditions especially demanding, but the costs of failure are unacceptable.
The individual electronic components, including microcontrollers (MCUs), a type of semiconductor, must be rigorously tested prior to production and release to customers. As MCUs frequently act as the "brain" of many embedded systems, they must direct certain actions according to the temperature of the environment. Ensuring that the MCU and other semiconductor devices can withstand extreme temperatures is crucial to maintaining system reliability and a safe environment.
Extreme Temperatures Affect Semiconductors
One of the key properties of the physics behind semiconductor devices such as MCUs is that their performance can vary widely over temperature. A device that may function fine at room temperature may no longer work fast enough at temperatures above and below normal ranges. Or, the complex timing of the device may shift such that it may no longer work properly at temperatures outside normal ranges. Also, at elevated temperatures, many of the failure mechanisms of semiconductor devices are accelerated, potentially reducing the operating life of the device if the physics of reliability are not thoroughly understood and accounted for in the device''s design.
In order to extend the temperature range within which MCUs can properly function, great care must be taken in the design process, often building extra margin and performance into the operation of the circuits. Once the circuits are designed, the resulting devices have to be rigorously characterized to ensure that there is sufficient operating margin over the extended temperature range, and then carefully qualified and tested in production to guarantee proper functionality of outgoing product.
Temperature extremes are not the only environmental shock that these devices must endure. Many applications are demanding in terms of the physical vibration or movement that the systems components must tolerate. Also, thermal cycling in some aviation applications -- where devices are continuously moving between high and low temperature extremes -- can put semiconductor devices under tremendous thermal stress.
In the atmosphere, the temperature typically decreases 6.5°C for every 1000m of altitude, so an aircraft taking off and landing between the ground and 20,000m results in a temperature cycle of 130°C. The stresses caused by this type of temperature cycling can result in mechanical failure of the MCU packages due to mismatches in the thermal coefficients of expansion between the various materials in the controllers.
System designers who create applications in which computational power must be applied in these extreme environments may have to go to great lengths to make it possible. Complex and bulky heating and cooling systems may be created to keep the MCUs and other semiconductor devices within the narrow temperature ranges within which they are specified to operate. However, these systems can take up valuable space in avionics applications, which ultimately adds great costs to the end system. In any aviation application, space and weight are key considerations due to fuel costs in launching these machines in the air and maintaining them in flight.
Semiconductor devices that are designed to function in harsh environments may enable system designers to reduce or eliminate the need for temperature control systems and avoid purchasing additional, costly equipment to test vibration and movement conditions. Implementing such a semiconductor device can greatly improve reliability and reduce system costs.
As computing power is applied to more and more diverse applications, such as instrumentation equipment, fuel control devices, GPS-based navigation products, and systems control in aerospace applications, the need for MCUs that can handle the rigors of extreme environments increases. Semiconductor manufacturers are responding by providing devices like those in Motorola''sMPC500 family. Designed and tested to withstand the stresses of extreme heat and cold (minus 55 to 125 deg Celsius), vibration, and other hazards, these devices enable designers to bring new levels of sophistication to many types of electronic systems cost effectively and reliably.