The "mystery" of failure of chip tantalum capacitors during power-on testing

Electronic engineers who have used a large number of chip tantalum capacitors may have encountered this phenomenon: the chip tantalum capacitors used have qualified performance during testing and screening, but when they are installed on the board and powered on for testing, they often break down and short-circuit.

There are two simple reasons for this phenomenon:

1. The chip tantalum capacitors used have serious quality defects;

Since the chip tantalum capacitor is a universal component, it can be used in various electronic circuits as a filter or instantaneous discharge power supply. For circuits with different uses, the power supply of the whole machine is very different, and the difference in the strength of the electrical signal in the circuit can even be more than a hundred times, such as the chip tantalum capacitors used in mobile phones and the filter capacitors and discharge capacitors used in high-power power supplies. Due to their different use conditions, the national standard must cover all use conditions. Therefore, the leakage current index that plays a decisive role in the reliability of the capacitor is very wide; as long as the leakage current meets K≤0.01CR×UR [CR is the rated capacity, UR is the rated voltage], under normal circumstances, there will be no problem in low-power personal electronic products powered by batteries, but when used on high-power electronic machines, meeting the above indicators cannot guarantee reliability at all.

Therefore, depending on the circuit power supply and reliability conditions, chip tantalum capacitors with different reliability must be selected. Especially for military electronic circuits, even the robustness of the capacitor must be considered.

The premise of this article is that you must choose the right chip tantalum capacitor with no quality problems. Failures caused by choosing chip tantalum capacitors of low quality will not be analyzed.

2. Why is it possible that a tantalum capacitor with no quality problems may still have a breakdown short circuit problem during power-on testing?

The reasons for this problem are as follows;

2.1. The loop resistance of the external power supply during the power-on test is too low, resulting in excessive surge voltage and surge current during the test. The capacitor actually withstands overvoltage and overcurrent shocks far exceeding the allowable values.

It is important to note that during the power-on test, the high surge due to low loop resistance is completely different from the working conditions of the capacitor when the circuit is actually working alone.

This type of circuit is basically a switching power supply circuit [also called a DC-DC circuit]. Many of our users do not have a good understanding of the electrical signal characteristics of this type of low-impedance circuit; therefore, when selecting capacitor specifications, they do not take into account that at the moment of switching, a voltage and current pulse with a very short duration (less than 1 microsecond) and extremely high energy density will appear in the circuit. The instantaneous voltage of this pulse can reach 2-10 times the steady-state voltage, and the current can instantly reach more than ten times the steady-state current.

Therefore, the rated voltage of the capacitor is too low, and sometimes the capacity is not enough. This causes the capacitor to fail due to instantaneous overvoltage and overcurrent during power-on testing.

When the capacitor is working normally, the power supply is often composed of a separate battery. Therefore, the voltage applied to the capacitor at the moment of switching is very stable, and the current is much smaller than that of the industrial power grid. In addition, a surge protection circuit is generally installed in the circuit. Therefore, in actual use, there will be no overvoltage and overcurrent during power-on testing.

Some engineers believe that only a high-power startup test on an unlimited current industrial grid can ensure that the circuit meets the reliability requirements in actual operation; in my opinion, this understanding is very wrong and even extremely harmful. The reasons are as follows;

1. The high-power power supply test without protective resistor will generate too high surge voltage, which will instantly far exceed the actual withstand voltage of the capacitor. Circuit designers must be fully aware of this point and avoid causing the circuit to be subjected to overvoltage and overcurrent shocks due to their own misunderstandings, resulting in a decrease in overall reliability.

There are many ways to conduct robustness testing. Reasonable experimental methods must be carefully selected. Unreasonable experiments and inspections may actually damage the reliability of components. The maximum DC surge current that any chip tantalum capacitor can safely withstand is shown in the following formula;

  I=UR/1+ESR

In the above formula, I is the maximum DC current that a certain chip tantalum capacitor can withstand. UR is the rated voltage of the product. 1 refers to the loop resistance of the circuit. ESR is the actual equivalent series resistance of the product.

From the above formula we can get the following conclusions:

1. The DC surge current that any chip tantalum capacitor can safely withstand is different; products with higher rated voltage can withstand greater DC surge current.

2. The lower the ESR, the higher the DC surge current that the product can safely withstand. The larger the capacity, the lower the ESR. Therefore, when considering reliability, products with large capacity can not only have greater output power, but also higher reliability.

3. The larger the loop resistance, the higher the surge current impact that the capacitor can withstand. Conversely, the higher the loop resistance, the higher the surge current passing through the capacitor will be suppressed. It is very important to understand this point.

According to the above analysis, in order to avoid the sudden failure of chip tantalum capacitors during power-on testing, the following points must be noted;

1. When using an industrial external power supply for power-on testing, the maximum DC surge withstand capacity of the powered circuit must be considered. The safe maximum surge current that a circuit can withstand must be limited to the maximum allowable value of a component in the circuit. The maximum peak discharge current in the charge and discharge circuit cannot exceed 50% of the maximum allowable value of a component. The excessive surge peak voltage and current of the industrial external power supply must be limited by a resistor with appropriate resistance.

In a word, when using an external industrial power supply to test the whole machine, a resistor with a suitable resistance value must be installed at the input end to limit the surge. Do not use a low-resistance external power supply to directly test the electronic machine suddenly.

2. A reasonable time-delay protection circuit must be installed in front of the capacitor used in the high-power filter circuit and the discharge circuit. The design of the protection circuit can refer to my blog article [Reliability protection design of switching power supply circuit].

3. When using an external power supply to perform performance and reliability tests on an electronic device, it is necessary to ensure that the characteristics of the input circuit electrical signals are close to the actual application conditions. Destructive performance tests on electronic devices or sub-circuits cannot be performed, which may result in a decrease in the reliability of the device.

Unfortunately, my above analysis is basically based on many circuit designs and applications in mainland China, some of which are even in the military industry. Many circuit designers do not have enough understanding or are wrong about this due to lack of experience.