Electrostatic damage magnetostriction, the cause of device failure, finally understood

What is the culprit of device failure? Sometimes the device is "end to sleep", sometimes there is pressure but not obvious.

The "end of life" of a device is a cumulative decay effect that results from physical or chemical changes.

As we all know, electrolytic capacitors and some types of thin film capacitors "have a dead end" because of the chemical reaction of the dielectric under the combined action of trace impurities (oxygen, etc.) and voltage. The structure of integrated circuits follows Moore's Law and becomes smaller and smaller, and the risk of dopant migration at normal operating temperatures causes the device to fail over decades (rather than the original hundreds of years).

In addition, the fatigue caused by magnetostriction causes mechanical fatigue of the inductor, which is a well-known effect. Some types of resistive materials oxidize slowly in the air, and as the air becomes more humid, the rate of oxidation increases. Again, no one would expect the battery to be effective forever.

Therefore, when selecting a device, it is necessary to understand its structure and possible aging-related failure mechanisms; these mechanisms may be affected even when the device is used under ideal conditions. This column does not discuss failure mechanisms in detail, but most reputable manufacturers will focus on the aging of their products and are generally familiar with working life and potential failure mechanisms. Many system manufacturers provide information on the safe working life of their products and their limiting mechanisms.

However, under the right working conditions, the life expectancy of most electronic devices can reach decades or even longer, but some still fail prematurely. The reason is often the pressure of not being noticed.

A useful statement that quotes Murphy's Law is that "the laws of physics do not work because you didn't pay attention to it." Many stress mechanisms are easily overlooked.

Anyone who designs electronic products for use in the marine environment will consider salt spray and humidity – this is justified because they are terrible!

In fact, many electronic devices may encounter chemical challenges that are less horrible, but can still cause harm. Human (and animal) breaths contain moisture and are slightly acidic. The kitchen and other home environments contain a variety of mildly corrosive fumes such as bleach, disinfectants, cooking fumes, oils and alcohol, all of which are not very harmful, but we should not take it for granted that we The circuit will be "safe for life" under the condition of being well protected. Designers must consider the environmental challenges of the circuit and, where economically feasible, should be designed to minimize any potential hazards.

Electrostatic damage (ESD) is a stress mechanism, and warnings related to this are the most common, but we often turn a blind eye.

When the PCB is in production, the factory takes adequate measures to eliminate ESD during the manufacturing process, but after delivery, many PCBs are used in systems that do not have adequate protection against ESD caused by general operation. It is not difficult to do adequate protection, but it will increase the cost and is often overlooked. (Probably because of the economic downturn). Evaluating what ESD protection is required for system electronics in the most extreme cases of normal use and considering how to implement it should be part of any design.

Another factor is overpressure. Few people require semiconductors or capacitors to suffer from significant overvoltages, but it is common for large value resistors to encounter voltages that are much larger than the absolute maximum values ​​listed in the data sheet. The problem is that although the resistance is high enough to not heat up, there may be a slight arc inside, causing it to drift slowly and deviate from the specification, eventually shorting. Large wirewound resistors typically have a breakdown voltage of hundreds of volts, so this problem was not common in the past, but today's widely used small surface mount resistors, which have breakdown voltages of less than 30 V, are quite susceptible to overvoltage.

High currents can also cause problems. Everyone is familiar with ordinary fuses - it is a piece of wire that, if excessive current flows through it, heats up and blows, preventing shorts in the power supply and other similar problems. However, if there is a very high current density in a very small conductor, the conductor may not become very hot, but may eventually fail. The reason is the so-called electromigration (sometimes called ion migration). That is, the momentum transfer between the conductive electrons and the diffused metal atoms causes the ions in the conductor to gradually move, causing a substance transport effect. This makes thin conductors carrying large DC currents become thinner and thinner over time and eventually fail.

But some parts will fail like a fuse, that is, a fuse, such as a conductive trace on a wire or semiconductor chip. A common cause of this phenomenon with high currents is that the capacitor charging current is too large. Consider a 1 μF capacitor with an ESR of 1 Ω. If it is connected to a 110 V, 60 Hz AC source, approximately 41 mA of AC current flows through it. However, if connected to AC power when the voltage is at its maximum (110√2 = 155.6 V), only the ESR will limit current and the peak current will reach 155.6 A. Although its duration is less than 1 μs, it is enough to damage many small signals. Semiconductor device. Repeated surges can damage the capacitor itself, especially electrolytic capacitors. This is a particularly common failure mechanism in inexpensive low voltage switching power supplies ("wall power adapters") for charging small electronic devices.

If plugged in at the wrong time of an AC cycle, the rectifier and capacitor will carry a very large inrush current. If this happens multiple times, the device may eventually be damaged. Using a small resistor in series with the rectifier limits this inrush current and minimizes the problem.

If we are fortunate, ESD or overvoltage/overcurrent events can immediately damage the device, so it's easy to know the problem. But more often, damage caused by stress causes the device to fail, and the pressure that initially caused the failure has long since disappeared. It is very difficult, if not impossible, to diagnose the cause of such failure.

Regardless of the circuit design, it is necessary to consider the operating life and failure mechanism of the device used, and whether there are any potential problems or pressure sources that can cause damage to the device under the most extreme conditions of use allowed. Any such issues should be considered and minimized as much as possible in the final design.