Reliability is critical to electronic design, particularly in applications where safety and functionality are paramount.
By Jon Gabay for Mouser Electronics.
Reliability is critical to electronic design, particularly in applications where safety and functionality are paramount. From automotive systems to outdoor infrastructure, electronics must perform reliably under demanding conditions to ensure users’ safety and the application’s success.
In today’s world, we depend on electronic devices and components to function as expected when needed, but the reliability level depends on the specific application. For instance, the reliability requirements for a simple smartwatch to track fitness data are significantly different from those of a medical device used for patient monitoring, where a malfunction could have severe consequences.
While there are many safety and reliability standards and organizations, electrical engineers must understand the different screening levels for high-reliability electronic components to ensure their designs meet the required standards.
The levels of screening and reliability of the selected components in a design can vary depending on the application’s specific needs, and choosing the right parts is essential.
The components must withstand the environmental conditions of their intended use, including temperature extremes, vibration, and shock. In addition, they must be tested to ensure that they meet the required safety and reliability standards.
Various reliability standards exist for mechanical components, such as sealed enclosures and shock/ vibration survivability. Still, the standards you choose (or need) to adhere to vary, depending on your specific application. In this article, we will focus on the different screening levels for electronic systems and components and provide guidance on selecting which is appropriate for particular applications.
Reliability through Standards Design engineers must be familiar with the dominant general reliability and safety standards agencies for electronic components.
Two of the most significant agencies in this field are the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). The ISO was founded in 1947 and has over 800 committees with more than 24,500 published international standards. Similarly, the IEC was formed in 1906 and initially dealt with the standardization of terminology, symbols, ratings, and preliminary standards of electric and electronic components. While a detailed discussion of either or both agencies’ numerous accomplishments is beyond the scope of this article, it’s important to recognize the significance of these organizations in ensuring the safety and reliability of electronic components and systems.
As technology evolved, many other standards for the reliability of PCBs and systems emerged, such as the Institute for Printed Circuits (IPC) standards
for printed circuit boards (PCBs). Since reliability is inversely proportional to the number of interconnections, the medium for those interconnections (i.e., PCBs) dramatically affects the overall system reliability. IPC standards cover repair, solderability, stencil design, and much more. The technology accumulated in making PCBs, and even the specific application constraints like high- frequency PCB designs, are captured in the IPC standards from start to finish (Figure 1). Being familiar with these standards assists in designing electronic systems and components that meet the required safety and reliability standards.
High-Reliability Standards and Screenings When designing life-critical medical, automotive, and aerospace equipment, ensuring the highest levels of reliability and safety is paramount. Orbital and aerospace designs must meet tough requirements for radiation-hardened (rad-hard) electronics, which offer radiation tolerance from high-energy cosmic particles. The atmosphere mostly protects terrestrial equipment, so space-bound satellites, probes, and crafts need the rad-hard designation for the toughest designs.
Each of these standards has different levels of screening and testing to ensure high reliability in various applications. For instance, the Automotive Electronics Council Qualification (AEC-Q) prescribes screening levels depending on the component type and intended use. Components used in safety-critical systems, such as airbags or anti-lock brakes, require more stringent screening than those used in non- safety-critical systems.
Semiconductor Reliability
Semiconductors have specific reliability concerns that require attention from design engineers. While the basic principles (i.e., prevent overvoltage and overcurrent, operate within the specified temperature and vibration ranges) still apply, handling and soldering techniques are more critical for high- density semiconductors.
Semiconductor makers have long warned of static sensitivity, but early semiconductors were relatively robust and could withstand repeated stress. For instance, transistor- transistor logic (TTL) chips used robust bipolar transistor technology that could withstand static discharge. However, the situation changed with the introduction of complementary metal-oxide semiconductor (CMOS) technology and its higher- density capability. Especially with the lower-voltage- process technologies in use, modern CMOS 1.8V microprocessor cores cannot withstand the stresses. The 12V 4000 series CMOS parts released in the late 1960s were more resilient, but modern devices with smaller feature sizes have become more sensitive to damage.
Design practices like power rail protections, buffering, and controlling waveforms during startup and shutdown are essential to ensure the reliability of high-density CMOS parts. Careful consideration should also be given to board-to-board interfaces to prevent stress on the components. Shielding is usually unnecessary unless the design is subject to high electromagnetic interference/radio frequency interference (EMI/RFI) fields or emissions are a concern for FCC/TÜV testing. However, chokes on data and clock lines can help lower emitted noise.
Proper handling and soldering techniques are also critical for high-density semiconductors. The manufacturing world has adopted antistatic measures for workers and machines to protect sensitive parts. For instance, technicians and assemblers are required to wear antistatic wrist straps, and antistatic mats are used to prevent charges from accumulating on the work surface. These measures help minimize the risk of electrostatic discharge (ESD), which can damage sensitive semiconductor components.
In addition, with the shrinking geometries of modern semiconductor devices, the package’s thermal properties and management have become more critical. High-density semiconductors generate more heat in smaller packages, and the heat must be dissipated to prevent thermal runaway, which can lead to premature failure of the device. Proper heat sinking and cooling techniques, including thermal interface materials, can help ensure the reliability of high- density semiconductors.
Overall, with proper handling, soldering, and design practices, design engineers can maximize the reliability of modern semiconductors. By incorporating best practices and considering the specific requirements of each application, engineers can ensure that their designs meet the required safety and reliability standards.
What’s Best for Your Design?
So, how do you choose a suitable reliability rating for your application? The first step is to determine how critical the system or application is. Is it a safety- critical system like an airbag or anti-lock brakes? Or is it a non-critical system like a radio or entertainment display? This will help you determine which screening level is appropriate. The second step is to consider the environmental factors the component will endure. Will it be exposed to extreme temperatures, shock, vibration, or other harsh conditions? Parts subjected to harsh environments must be screened for a higher reliability level. It’s also important to consider the cost of components. Higher levels of reliability typically come with higher prices, so it’s important to balance the necessary level of reliability with the budget for the project.
Understanding the right reliability rating for high- reliability electronic components can be daunting, but understanding the different screening levels and considering the criticality of the system, environmental factors, and cost can help make the decision easier.
The market segments you serve will often dictate the necessary levels of reliability. Starting with your chosen components, resistors are one of the most common components in your design. There are many types of resistors to choose from, and they are all reliable as long as they are handled and appropriately soldered and not overstressed in their operating circuits. Two main concerns for resistor reliability are precision and temperature rating. Selector guides on websites of distributors like Mouser Electronics make choosing the precision rating you need easy. While low-precision pull-up resistors can be used, higher precision is needed for sensitive circuits like analog and audio. Carbon composite and wire-wound resistors are the toughest and can withstand the most punishment, while variable resistor reliability is maximized when sealed trim pots are used.
Capacitors are another major factor in circuit reliability. Like resistors, there are many capacitor technologies to choose from, and the designer must pick the best one for the design. Capacitors have an extensive range of values and must be handled and soldered properly to be reliable. They also have tolerances for accuracy; but some capacitors, like electrolytic capacitors, have hour-based lifetime ratings. While low-tolerance applications can use lower- precision components, RF and analog circuits require higher precision to operate effectively. Designers must employ good design practices and circuit protection devices to ensure that capacitors are not exposed to overvoltage conditions, which can cause them to explode.
Other passive components, like inductors, coils, and transformers, are typically tough and resilient, but proper soldering is crucial. Overheating a winding can break down an insulating coating and cause shorts and value changes. Inductors require the same respect as resistors and capacitors in a circuit and should not be overdriven with overvoltage or overcurrent. It is important to ensure they always operate within their rated temperature ranges.
Conclusion
Selecting the correct reliability rating for various applications can be demanding. Choosing the right components and adhering to specific reliability standards are essential to ensure that electronics can perform reliably in demanding conditions. Engineers must consider factors such as the criticality of the system, environment, and cost when selecting the appropriate screening levels for components. The reliability ratings of components vary depending on the specific needs of each application, and it is crucial to work with a trusted supplier to ensure that the correct parts are chosen. By incorporating best practices and considering the specific requirements of each application, engineers can ensure that their designs meet the required safety and reliability standards.