What is PCBA Board Components are Susceptible to Electrostatic Breakdown of the Reason?
A. Electrostatic Breakdown Analysis
MOS tube an ESD sensitive equipment, its own input resistance is very high, and gate - source inter-pole capacitance is very small, so it is very susceptible to external electromagnetic fields or electrostatic inductance and charged (a small number of charges may be in the inter-pole capacitance on the formation of a suitably high voltage (think about the U = Q / C) will be damaged), but also due to the electrostatic occasions in the more difficult to discharge the charge, simply caused by electrostatic breakdown. Electrostatic breakdown has two methods: one is the voltage type, that is, the gate of the thin oxide layer breakdown, so that the gate and the source of the short circuit between the gate and perhaps the gate and the drain short circuit; the second is the power type, that is, the metallization film aluminum strip is fused, forming an open gate or perhaps the source of an open circuit. JFET tube and MOS tube, the same, there is a very high input resistance, only the MOS tube's input resistance is higher.
Reverse-biased pn junctions are more prone to thermally induced 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 required under forward-biased conditions. This is due to the fact that most of the power is consumed in the center of the junction region when reverse-biased, while more is consumed in the body resistance outside the junction region when forward-biased. For bipolar devices, the area of the emitter junction is usually smaller than that of any other junction and the junction surface is closer to the surface than other junctions, so degradation of the emitter junction is often observed. In addition, pn junctions with breakdown voltages higher than 100V or leakage currents less than 1nA (e.g., the gate junction of a JFET) are more sensitive to electrostatic discharges than conventional pn junctions of similar scales.
All things are relative, not certain, MOS tube is only relative to other equipment to be more sensitive, ESD has a great characteristic is random, and not without touching the MOS tube can be able to break it down. In addition, even if ESD occurs, it is not necessarily the tube breakdown. The fundamental physical characteristics of electrostatic:
a, Attractive or repulsive force;
b, The presence of an electric field, and the earth has a potential difference;
c, The discharge current will occur.
These three scenes that ESD will generally form the following three scenes of electronic components:
a, Components adsorbed dust, change the impedance between the lines, affecting the function and life of the components;
b, Due to the electric field or current damage to the components of the insulating layer and conductors, so that the components can not work (complete damage);
c, Due to instantaneous breakdown of the electric field soft or the current occurs overheating, so that the components are injured, although they can still work, but the life of the Damaged.
So ESD damage to the MOS tube may be one, three two cases, not necessarily every time is the second case. In these three cases, if the component is completely damaged, it can be detected and excluded in production and quality testing, and the impact is less. If the component is slightly damaged, in the normal test is not easy to be found, in this scenario, often due to repeated processing, and even have been in use, only to be found damaged, not only to view is not easy, and the loss is also difficult to predict. Static electricity on the electronic components of the damage is no less than the loss of serious fire and blasting accidents.
B. The Prevention of Static Electricity
PCBA board in what case will suffer electrostatic damage? Can be said that: electronic products from production to use the whole process are subject to electrostatic damage threat. From the production of equipment to plug-in welding, machine assembly, packaging and transportation until the application of the product, are under the threat of static electricity. In the entire electronic product production process, each stage of each small step, electrostatic sensitive components may be affected by static electricity or damaged, but in fact most of the main and simple to ignore the point is in the transmission and transportation of components in the process. In this process, transportation due to the movement of simple exposure to the external electric field (such as through the high-voltage equipment near the workers move frequently, rapid movement of vehicles, etc.) static electricity and damage, so the transmission and delivery process requires special attention to minimize the loss, to avoid frivolous disputes. Protection words plus Zener voltage regulator tube maintenance.
Nowadays, the mos tube is not so simple to be broken down, especially the high-power vmos, mainly many have diode maintenance. vmos gate capacitance is large, inductance is not high voltage. Unlike the dry north, the south is not easy to generate static electricity in the humidity. There is also the majority of CMOS equipment has now increased the internal IO port maintenance. But touching the CMOS device pins directly with your hands is not a good habit. At least make the pin weldability deterioration.
First, the input resistance of the MOS tube itself is very high, and the gate source inter-polar capacitance is very small, so it is very susceptible to external electromagnetic fields or electrostatic inductance and charged, and a small number of charges can be formed in the inter-polar capacitance of the appropriate high voltage (U = Q / C), the tube will be damaged. Although the MOS input has anti-static maintenance measures, but still need to be treated with care, in the storage and delivery most good with metal containers or perhaps conductive material packaging, do not put in easy to electrostatic high voltage chemical materials or chemical fiber fabrics. Assembly, commissioning, things, instruments, workstations, etc. should be well grounded. To avoid damage caused by electrostatic interference formed by the operator, such as should not wear nylon, chemical fiber clothing, hand or something in touch with the integrated block before most good first ground. To equipment leads straighten zigzag or manual welding, the use of equipment is necessary to good grounding.
Second, the maintenance diode at the input of the MOS circuit, its conduction current tolerance is generally 1mA, in perhaps too large transient input current (more than 10mA), should be connected to the input maintenance resistor. Therefore, the application can choose a MOS tube with internal maintenance resistance should be. Also due to the maintenance circuit to absorb the instantaneous energy is limited, too large instantaneous signal and too high electrostatic voltage will make the maintenance circuit useless. So when welding soldering iron is necessary to reliable grounding, to prevent leakage breakdown equipment input, general use, can be powered off after using the residual heat of the soldering iron for welding, and first weld its grounded pins.
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