PCB Tech - Analysis of components on PCBA board susceptible to electrostatic breakdown

1. Analysis of electrostatic breakdown
The MOS tube is an ESD sensitive device, its input resistance is very high, and the capacitance between the gate and the source is very small, so it is very easy to be charged by the external electromagnetic field or static electricity (a small amount of charge may form a considerable amount on the capacitance between the electrodes. High voltage (think U=Q/C) will damage the tube), and because it is difficult to discharge the charge in the case of strong static electricity, it is easy to cause electrostatic breakdown. There are two ways of electrostatic breakdown: one is the voltage type, that is, the thin oxide layer of the gate breaks down, causing a short circuit between the gate and the source, or a short circuit between the gate and the drain; the other is the power type, that is, metal The aluminum strip of the chemical film is blown, resulting in an open gate or an open source. The JFET tube, like the MOS tube, has a very high input resistance, but the input resistance of the MOS tube is higher.

Reverse-biased pn junctions are more prone to thermal failure than forward-biased pn junctions, and the energy required to damage the junction under reverse-biased conditions is only about one-tenth of that under forward-biased conditions. This is because in reverse bias, most of the power is consumed in the center of the junction area, while in forward bias, it is mostly consumed on the bulk resistance outside the junction area. For bipolar devices, the area of the emitter junction is usually smaller than that of other junctions, and the junction surface is closer to the surface than other junctions, so the degradation of the emitter junction is often observed. In addition, a pn junction with a breakdown voltage higher than 100V or a leakage current less than 1nA (such as the gate junction of a JFET) is more sensitive to electrostatic discharge than a conventional pn junction of similar size.

Everything is relative, not absolute. MOS transistors are only more sensitive to other devices. ESD has a great feature of randomness. It is not possible to break down MOS transistors without encountering them. In addition, even if ESD is generated, the tube may not be broken down. The basic physical characteristics of static electricity are: (1) There is attraction or repulsion; (2) There is an electric field, and there is a potential difference with the earth; (3) A discharge current is generated. These three situations, namely ESD, will generally affect electronic components in the following three situations: (1) The component absorbs dust, changes the impedance between the lines, and affects the function and life of the component; (2) The insulation layer of the component is destroyed by the electric field or current. And the conductor, make the component unable to work (completely destroyed); (3) due to the instantaneous electric field soft breakdown or current overheating, the component is injured, although it can still work, but the service life is damaged. Therefore, the damage of ESD to the MOS tube may be one or three cases, and it is not necessarily the second case every time. In the above three cases, if the component is completely destroyed, it must be detected and eliminated during production and quality testing, with less impact. If the component is slightly damaged, it is not easy to be found in the normal test. In this case, the damage is often found only when it has been processed many times or even when it is in use. Not only is it difficult to check, but the loss is also difficult to predict. The damage caused by static electricity to electronic components is no less than the damage caused by serious fire and explosion accidents.

Second, the prevention of static electricity

Under what circumstances will PCBA boards suffer electrostatic damage? It can be said that the entire process of electronic products from production to use is threatened by static electricity. From device manufacturing to plug-in assembly and welding, complete machine assembly, packaging and transportation to product application, all are under the threat of static electricity. In the entire production process of electronic products, at every small step in every stage, electrostatic sensitive components may be affected or damaged by static electricity. In fact, the most important and easily neglected point is the transmission and transportation of the components. the process of. In this process, transportation is easily damaged due to static electricity generated by the external electric field (such as passing near high-voltage equipment, frequent movement of workers, rapid movement of vehicles, etc.). Therefore, special attention must be paid to the transportation and transportation process to reduce losses and avoid indifference. Disputes. If you protect it, add Zener voltage regulator to protect it.

The current MOS tube is not so easy to be broken down, especially the high-power vmos, which are mainly protected by diodes. The vmos gate capacitance is large, and no high voltage can be induced. Unlike the dry north, the humid south is not prone to static electricity. In addition, IO port protection has been added to most CMOS devices. But it is not a good habit to directly touch the pins of CMOS devices with your hands. At least make the pin solderability worse.

First, the input resistance of the MOS tube itself is very high, and the capacitance between the gate and the source is very small, so it is very easy to be charged by the external electromagnetic field or electrostatic induction, and a small amount of charge can form a very high voltage on the capacitance between the electrodes. (U=Q/C), damage the tube. Although the MOS input terminal has anti-static protection measures, it still needs to be treated with care. It is best to use metal containers or conductive materials for storage and transportation, and do not place it in chemical materials or chemical fiber fabrics that are prone to static high voltage. When assembling and debugging, tools, instruments, workbenches, etc. should be well grounded. To prevent the damage caused by the electrostatic interference of the operator, if it is not suitable to wear nylon or chemical fiber clothes, it is best to touch the ground before touching the integrated block with hands or tools. When the device leads are straightened and bent or manually welded, the equipment used must be well grounded.

Second, the current tolerance of the protection diode at the input end of the MOS circuit is generally 1mA when it is turned on. When there may be excessive transient input current (over 10mA), the input protection resistor should be connected in series. Therefore, a MOS tube with internal protection resistors can be selected for application. In addition, because the instantaneous energy absorbed by the protection circuit is limited, too large instantaneous signals and excessively high electrostatic voltage will make the protection circuit useless. Therefore, the electric soldering iron must be reliably grounded during soldering to prevent leakage of electricity from penetrating the input terminal of the device. In general use, the residual heat of the electric soldering iron can be used for soldering after the power is turned off, and the grounding pin should be soldered first.