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Difference between ESD and EOS and their failures2020/04/06

ESD is a phenomenon of electricity discharge. The source of ESD mainly comes from the charges accumulated on the human body or cable. The events to damage electronic equipment will take place owing to the touch or plug. The novel electronic products adopt ICs fabricated by the advanced semiconductor processes to achieve the higher performance and cost benefit. Because the surge durability of ICs itself is pretty weak and it is easy to damage by the ESD event, it must be verified in the component-level ESD tests, including the HBM, MM and CDM. The HBM is discharged to a 1.5kΩ resistor after charged to a 100pF capacitor. The MM is discharged to a 500nH inductor after charged to a 200pF capacitor. The CDM is discharged by grounding after charged to ICs. The verification of the system-level ESD test must refer to IEC 61000-4-2, where is externally discharged to a 330Ω resistor after internally charged to a 150pF capacitor. The discharge resistance in the model is about one fifth of the HBM. Therefore, the discharge current can be roughly five times of the HBM. This is one of reasons why the system-level ESD test is stricter than the component-level ESD test.

Figure 1. Component-level and system-level ESD test models

The EOS is an electric overload event, mainly formed by low-frequency and high-energy surge greater than microsecond. Owing to the high-energy characteristics, the damage phenomenon can be obviously determined by the EOS, as shown in Figure 2. The damage resulting in the EOS could burn out the metal wiring layer of ICs, which can be found when ICs was analyzed by decapsulation process.

Figure 2. Damage resulting in the EOS

Figure 3. Surge spike on (1) lightning and (2) AC power of EOS

In addition, ICs and the bonding wires could be burnt by the EOS energy as well. This phenomenon can be found in X-ray analysis. What is more serious could directly burn out the package. It can directly find during the appearance inspection. Such large energy of EOS could come from surge spike on lighting or AC power, as shown in Figure 3.

The EOS test standards follow a voltage waveform of 1.2/50μs and a current waveform of 8/20μs in IEC 61000-4-5. Referring to the EOS in Figure 4, it can be found that it is charged to a 5μF capacitor and then discharged via an embedded resistor of 2Ω.

Figure 4. EOS test standard – IEC61000-4-5

The rest of the inductors and the resistors will modify the output waveforms to meet the standards. Because the EOS output current will be produced in low EOS voltage, it will seriously damage the electronic equipment. There are several EOS testers used in the industry. Figure 5 shows Thermo KeyTek EMCPro Plus is high-voltage EOS test equipment, featuring the maximum voltage up to 6kV and generally applying for connecting to electronic equipment for the outdoor network, such as STB or router. In addition, KAS KT200-SG in Figure 6 is low-voltage EOS test equipment, featuring an output voltage of 200V and applying for indoor electronic products, such as smart phones, TV sets and display panels.

Figure 5. Thermo KeyTek EMCPro Plus for high-voltage EOS

Figure 6. KAST KT200-SG for low-voltage EOS

Figure 7 shows Prima TVS 8/20 for low-voltage EOS equipment from China, where the maximum output voltage can reach 300V. It is assigned as EOS tester for electronic products in China. Some suppliers also use their EOS test machines. Figure 8 shows KeoSan Discharge Tester made by Korean supplier, where the internal structure is that a power supply in 1000V is charged and discharged to an external capacitor via an embedded relay. The capacitor is 2μF.

Figure 7. Prima TVS 8/20 for low-voltage EOS

Figure 8. EOS tester made by a supplier

The EOS energy is a microsecond level with IEC61000-4-5. Therefore, we can compare the differences of current waveform for the ESD and EOS. From the waveforms in Figure 9, and we can find that the discharge time is within the nanoseconds and belongs to a transient surge in high frequency although the waveforms of HBM, MM and IEC 61000-4-2 in the ESD range are different. The discharge time of the EOS current can be as long as microseconds.

Figure 9. Comparisons of EOS and ESD current waveforms

It can be realized that the surge energy of EOS can be 100 times greater than that for ESD at least. From the ESD damage in Figure 10, the ESD damage area from the human body and cable will be located on the component layer of ICs. It is damaged in the tiny range, and must be decapped to find the exact location, presenting the obvious EOS damage relative to Figure 2. Therefore, the TVS embedded with IEC 61000-4-5 8/20μs current waveform must be selected to meet the EOS standards for the systems.

Figure 10. ESD sources and damages

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