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Test Method of System-Level ESD2020/05/08

Introduction


The material surface in a certain process such as friction and induction to make charges static is called ad static charge. The electromagnetic discharge (ESD) is a transfer of static charge between the objects in different potentials. The damage produced by ESD is formed by the artificial factor and very hard to avoid. Static charge will accumulate on human body, instrument or product itself to easily form a discharge path when the electronic products are manufactured, assembled, tested, stored and transported, so that they are damaged by the ESD.

The ESD is an issue that damages electronic products and cannot be solved easily. Compared with other overstress, static charge is a very quick transient pulse. Once the electronic products are struck by the ESD, they often present the unstable effect, such as malfunction. The function suddenly fails and can be restarted by rebooting if the condition isn’t serious. The electronic products are permanently damaged due to the ESD stress or current if the condition is quite seriously.

Component-level ESD

According to the discharge models, the component-level ESD can be classified as three models: human body model (HBM), machine model (MM) and charged device model (CDM).

The three ESD models above are very important to the semiconductor processes and electronic product assemblies, where the ESD stress produced by the HBM is the widest damage to the electronic products (semiconductor devices). The HBM produces static charge owing to the action of human body, such as the carpet carries positive charges and the rubber shoes carry negative charge owing to the friction when we wear a pair of rubber shoes to walk on the carpet. At this time, the sole will be induced to carry positive charge. And the upper part of the human body carries negative charge. If a hand contacts the semiconductor device at this time, a discharge path will be produced to damage the device by the ESD current.

The regulations governing the protection of electronic products on HBM are internationally and increasingly conscientious. The electronic devices have passed the component-level ESD tests (standard ±2000V) when shipped. The equivalent circuit model in the HBM is as shown in Figure 1. The rise time of the discharge waveform is about 5 ~ 10ns, as shown in Figure 2. The discharge voltage in 8000V will produce a peak current about 5.5A. However, electronic equipment will suffer from the more serious ESD conditions in the system level. Even though semiconductor devices have passes the ESD tests before shipment, the installed products can’t pass the system-level requirements.

System-level ESD

Figure 1. Equivalent circuit model of HBM Figure 2. Equivalent discharge waveform of HBM
 
 
Today, the system-level ESD standard internationally accepted is IEC 61000-4-2. The equivalent circuit module and the current waveform are as shown in Figure 3 and Figure 4. IEC 61000-4-2 defined the rise time from 700ps to 1ns, and the pulse duration was 60ns. Compared with the HBM, the system-level and component-level capacitances and discharge resistances are 150pF and 100pF and 330Ω and 1500Ω. In the circumstance with the larger energy-storing capacitance and smaller discharge resistance, the energy produced by the ESD will be more in the system-level ESD test. In the same 8000V stress, the peak current produced by the system level will reach 30A, five times more than the component level. It can be seen the system-level test presents more damages.

Figure 3. Equivalent circuit model of IEC 61000-4-2 Figure 4. Equivalent discharge waveform of IEC 61000-4-2
 
 
This is the main reason why ICs products sometimes can’t pass the system-level ESD test after passing the component-level ESD test.

In order to ensure the functions of the electronic products, the international companies request the OEM products must meet IEC 61000-4-2. In addition to passing the component-level tests, the final electronic products will be continuously tested by the system-level ESD specifications. It is very important to guarantee the product reliability, and the products must reach the standard when sold on the international market.

Test
 
Figure 5 shows the ESD gun for the system-level ESD test according to IEC 61000-4-2.
 
 
 
Figure 5. ESD gun (ESS-B3011, NoiseKen)
 
This is a basic ESD gun (ESS-B3011) produced by NoiseKen, where the equivalent circuit is as shown in Figure 6.

 
 
Figure 6. Equivalent circuit of an ESD gun

For the environment setting, the environmental temperature is 15oC to 35oC, the moisture is kept at 30% to 60% and the pressure is optimized at 68kPa to 106kPa. The tests are contact and air discharges.

Contact Discharge Test

The test method is to simulate the ESD phenomenon when a metal tool contacts the electronic product. In this test method, a head of an ESD gun uses pointed-end metal. During the test, the metal head of an ESD gun holds the point under test. The ESD performs the device under test by contact test via the pointed-end metal, usually tested to +/-4kV. This test regulation reserves the testing conditions allowed to be higher than +/-4kV.

The contact discharge can be detailed as direct discharge and indirect discharge. Figure 7 shows the system-level ESD device in the indirect discharge mode. The device consists of a wooden table and a ground reference plane, where there is an insulating plane used to insulate the device under test and a horizontal coupling plane on the table. The horizontal coupling plane is connected to two resistors in 470kΩ, and then connected to a ground reference plane.
 
 
Figure 7. Diagram of ESD device in indirect discharge mode
 
When the ESD gun strikes the horizontal coupling plane, it will produce electromagnetic interference coupled to the device under test.

Air Discharge Test

The test method is to simulate the ESD phenomenon when a finger contacts the electronic product. In this test method, a head of an ESD gun uses an 8mm discharge head (round). During the test, an ESD gun is activated in a close distance without contacting the test point. The head of the ESD gun perform the non-contact discharge test to the electronic products by air.

 
Figure 8. Contact discharge head and air discharge head of an ESD gun

Figure 8 shows the contact discharge head and the air discharge head of ESD gun. The voltage is tested from low to high, usually tested to +/-8kV. This test regulation reserves the testing conditions allowed to be higher than +/-15kV.

Table 1 shows the system-level ESD tests for the contact discharge and air discharge.

The positive and negative polarities are tested each 10 times at least with an interval of one second. The testing results of ESD must be determined according to the extent that the functions of an electronic product are influenced.

 
 
Table 1. System-level ESD tests for the contact discharge and air discharge

The influenced extents are divided as four classes, including Class A, Class B, Class C and Class D, as shown in Table 2. Class A (Criterion A): indicates a product can normally operate before/after and during the test without degrading any functions and failures, calling this product to meet Class A. Class B (Criterion B): indicates any functions of a product are influenced during the test and temporarily degraded in the moment of discharge, calling this product to meet Class B. Class C (Criterion C): indicates any functions of a product can be operated before the test, but they are degraded or failed by the ESD during the test and must be manually reset or rebooted to recover. This condition meets the results in Class C.
Table 2. System-level ESD test classes

Class D (Criterion D): indicates any functions of a product can be operated before test, but fail during the test and can’t be restored although manually reset or rebooted. A product in this condition has been seriously damaged to only meet the results in Class D (NG). According to IEC 61000-4-2, the product verifications meet Class A or Class B to be accepted, not accepted for Class C and Class D.

Conclusions

Following the international standards to perform the ESD tests, the product reliability can be ensured. With the more advanced chip processes, IEC 61000-4-2 can be divided as four ESD classes, 2kV, 4kV, 6kV and 8kV via a resistor in 330Ω. The present electronic systems are required to resist Level 3 or Level 4 discharge voltage at least. Taking Level 4 for example, the maximum ESD current can reach 30A, 20 times more than the chip-level ESD current. If calculated according to the ESD standards above-mentioned, it is obvious the next generation of ICs will suffer from the catastrophic damage even though there is only one discharge. It can be predicted that ICs itself will be difficult to pass the system-level ESD test.

In consideration of the cost, it will be extremely difficult to protect the electronic equipment without being damaged.

In the past, the manufacturers mostly made the ESD tests on the housing according to the international standards. Although they can pass the system standards, it was found recently that many products returned by the customers were damaged. After the housing was removed, there was burnt at the input/output end. This indicated the housing test was not enough to ensure the product reliability. The signal ports on the product should be further tested, as shown in Figure 9.

To summarize the contents above, it will be a best solution for the input/output port on the system product with an ESD protection device recently. The external ESD protection device will play the first line of defense. Because the input/output port of the electronic product provides a discharge path to ICs, it will be the most effective way to suppress the ESD before coupled into the PCB. The selection of the ESD device is prioritized for the low clamping voltage and short response time, such as TVS, so that it can provide the protection in the moment of static charge pulse and provide the system from the ESD damage.

 
 
Figure 9. ESD test at signal port

 
 
References

[1] EMC - Part 4-2: Testing and Measurement Techniques - Electrostatic Discharge Immunity Test, IEC 61000-4-2 International Standard, 2008.

[2] Consortium of ESD Protection Technology for Circuits and System
 
 
 

 

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