1. Sources and hazards of ESD
After friction between two different materials, one is positively charged and the other is negatively charged, resulting in a certain voltage between the two. The voltage depends on the nature of the material, the dryness of the air and other factors. If an object with static electricity is close to a grounded conductor, it will produce a strong instantaneous discharge, which is called electrostatic discharge. Generally speaking, an object with static electricity can be simply simulated as a small capacitor charged to a very high voltage in theory.
When an integrated circuit (IC) is subjected to ESD, the resistance of the discharge circuit is usually very small and cannot limit the discharge current. For example, when the cable with static electricity is inserted into the circuit interface, the resistance of the discharge circuit is almost zero, which will cause an instantaneous discharge peak current of up to tens of amps to flow into the corresponding IC pin. The instantaneous high current will seriously damage the IC, and the local heating heat will even melt the silicon die. ESD damage to IC generally also includes internal metal connection being burned out, passivation layer being damaged, transistor unit being burned out, etc.
ESD can also cause IC deadlock (latch). This effect is related to the activation of thyristor like structural units in CMOS devices. High voltage can activate these structures to form a large current channel, generally from VCC to ground. The locking current of serial interface devices is generally 1 ampere. The lockout current is maintained until the device is de energized. By then, however, the IC is usually burned out by overheating.
For serial interface devices, ESD will cause IC to work abnormally, communication error code, and serious damage. In order to analyze the fault phenomenon, Maxim company has conducted ESD test on RS-232 interface devices from different manufacturers. It is found that there are two common fault phenomena: one is crosstalk. The signal received by the signal receiver interferes with the transmitter, resulting in bit error (see Figure 1). Another fault is that a reverse current channel is formed inside the IC, so that the RS-232 signal level (± 10V) received by the receiver port is fed back to the power terminal (+ 5V). If the power supply does not have the voltage stabilizing function of absorbing current, excessive feedback voltage will damage other devices powered by single power supply (+ 5V)
For serial interface devices, the simplest protection measure is to add resistance capacitance elements on each signal line. The series resistor can limit the peak current, and the capacitor connected in parallel to the ground can limit the instantaneous peak voltage. This has the advantage of low cost, but limited protection capacity. Although the destructive power of ESD can be restrained to a certain extent, it still exists. Because the resistance capacitance element can not reduce the peak value of the peak voltage, it only reduces the slope of the voltage rise. Moreover, the resistance capacitance element will also cause signal distortion, which limits the length and communication rate of the communication cable. The external resistance / capacitance also increases the circuit board area. Another widely used technology is an applied voltage transient suppressor or or transzorbtm diode. This protection is very effective. However, additional devices will still increase the area of the circuit board, and the capacitance effect of protective devices will increase the equivalent capacitance of the signal line, and the cost is high, because the price of transzorbtm diodes is expensive (about 25 cents / each). A typical 3-generator / 5-receiver COM port requires 8 transzorbtm diodes, which costs up to $2.
An effective ESD test shall be conducted over the entire voltage range within the maximum test voltage. Because some ICs may pass the test at 10kV but be damaged by ESD at 4KV, such ICs actually have no antistatic ability. Manikin and iec1000-4-2 standard stipulate that the test must be conducted at an interval of 200V within the test voltage range, and the positive and negative voltages must be tested at the same time. That is, test from ± 200V, ± 400V, ± 600V to the maximum test voltage. Complete ESD test shall be conducted for all possible working modes of IC. Including power on working state and power-off shutdown state. If the serial interface device has automatic shutdown sleep mode, another ESD test shall be conducted for this state. All relevant test standards and procedures stipulate that the tested pin shall be discharged continuously for 10 times at each test voltage point. Considering that both positive and negative voltages shall be measured, the actual discharge shall be 20 times. After each round of discharge, the corresponding parameters of the tested device shall be measured to judge whether the device is damaged. For serial interface devices (RS-232, RS-485), the following criteria shall be followed:
● whether the power supply current is normal (the increase of power supply current generally means that the device deadlock occurs);
● whether the output level of signal transmission output terminal is still within the parameter specification range;
● whether the input resistance of the signal receiving input terminal is normal (generally between 3K Ω and 7K Ω).
Only when these indicators are qualified can we move to the next voltage test point. After all voltage points are tested, we should also conduct a comprehensive functional test on the IC to measure whether each parameter of the IC is still within the range defined by the parameter standard. Only the IC that can still meet the specified parameter standard after passing all these ESD tests is a real antistatic IC. It should be noted that according to the general standard ESD test can be completed, but it can not judge the quality of IC. Some ESD testers have some parameter measurement functions, but they can only be used as a reference because they are not parameter measurement for specific devices, but only general test means. Strict test should still be carried out according to the test procedures and test criteria described above.
