Talk about what ESD is and how ESD enters electronic devices.

To prevent ESD, you must first know what ESD is and how ESD enters the electronic device. ESD can occur when one charged conductor approaches another conductor. First, a strong electric field is established between the two conductors, creating a breakdown caused by the electric field. When the voltage between the two conductors exceeds the breakdown voltage of the air and the insulating medium between them, an arc is generated. In the 0.7ns to 10ns period, the arc current can reach tens of amps, sometimes even more than 100 amps. The arc will remain until the two conductors are shorted or the current is low enough to sustain the arc. The generation of ESD depends on the starting voltage, resistance, inductance and parasitic capacitance of the object: # Examples of possible arcing are human bodies, live parts and machines. # Examples of possible spikes are hand or metal objects. # Examples of multiple arcs that may produce the same polarity or polarity change are furniture.

ESD can enter electronic devices through five coupling paths: # The initial electric field energy is capacitively coupled to a network with a large surface area and produces a high voltage of up to 4000 V/m at 100 mm from the ESD arc. # The electric charge/current injected by the arc can cause the following damage and failure:

a. Penetrate the thin insulating layer inside the component and damage the gate of the MOSFET and CMOS components (common).

b. Trigger lock in CMOS devices (common).

c. Short-circuit reversed-biased PN junction (common).

d. Short-circuit forward biased PN junction (rare).

e. Melt the weld line or aluminum wire inside the active device (rare). The # current causes a voltage pulse (V = L × dI / dt) on the conductor. These conductors may be power, ground or signal lines that will enter each component (common) connected to these networks.

#Arc generates a strong magnetic field with a frequency range of 1MHz to 500MHz and is inductively coupled to each adjacent routing loop, producing up to 15A/m of current 100mm away from the ESD arc.

# The electromagnetic field of the arc radiation is coupled to long signal lines that act as receiving antennas (rare). ESD finds the weak points of the device through a variety of coupling paths. The ESD frequency range is wide, not just discrete frequency points, it can even enter narrowband circuits. To prevent ESD interference and damage, these paths must be isolated or the device's ESD resistance must be enhanced. Table 1 describes the precautions for possible ESD and the circumstances in which it works.

Prevents the plastic chassis, air space and insulation from shielding the ESD arcs that are directed at the electronics. In addition to the use of distance protection, an anti-ESD environment with a breakdown voltage of 20kV is also required. A1. Ensure that the path length between the electronic device and the following items exceeds 20 mm. 1. Any point that the user can reach, including seams, vents, and mounting holes. At a given voltage, the arc travels farther through the surface of the medium than through the air. # Unconnected metal that any user can access, such as fasteners, switches, joysticks, and indicators. A2. Install the electronics in the groove or notch of the chassis to increase the path length at the seam. A3. Cover the seams and mounting holes with a mylar film in the chassis, which extends the seam/via edge and increases the path length. A4. Cover the unused or rarely used connectors with metal caps or shielded plastic dust caps. A5. Use a switch with a plastic shaft and a joystick, or place a plastic handle/sleeve on top to increase the path length. Avoid using handles with metal fixing screws. A6. Install the LEDs and other indicators in the holes in the device and cover them with a strap or cover to extend the edge of the hole or use a conduit to increase the path length. A7. Extend the membrane keyboard boundary to make it 12mm beyond the metal wire, or use a plastic tongue to increase the path length. A8. Place the heat sink close to the seam of the chassis, and the sides and corners on the metal parts of the vent or mounting hole should be rounded. A9. In plastic cases, metal fasteners that are close to the electronics or that are not grounded must not protrude into the chassis. A10. If the product cannot pass the indirect ESD test on the desktop/ground or horizontal coupling surface, a high support foot can be installed to keep it away from the table or the ground. A11. On a touch rubber keyboard, make sure the wiring is compact and extend the rubber sheet to increase the path length. A12. Apply an adhesive or sealant around the membrane layer of the membrane keyboard. A13. At the joint of the cabinet, use high-pressure silicone or gasket to achieve hermetic, ESD-proof, waterproof and dust-proof.

The chassis and shield utilize a metal chassis and shield to block ESD arcs and corresponding electromagnetic fields and protect the device from indirect ESD in order to block all ESD outside the chassis. For static-sensitive electronic devices, the ungrounded chassis should have a breakdown voltage of at least 20kV (rules A1 through A9); for grounded chassis, the electronic device must have at least 1,500V breakdown voltage to prevent secondary arcing, and requires The path length is greater than or equal to 2.2 mm. The following measures can make ESD shielding more effective. B1. If necessary, design a chassis made of the following shielding materials: 1. Metal plate; 2. Polyester film/copper or polyester film/aluminum platen; 3. Thermoformed metal mesh with welded joints. 4. Thermoformed metallized fiber mats (non-woven) or fabric (woven); 5. Silver, copper or nickel coating; 6. Zinc arc spray; 7. Vacuum metal treatment; 8. Electroless plating; The conductor filler material is added; 10. The handling of the joints and edges is critical. B2. Select a material with high conductivity (low resistivity), see Table 2. B3. Choose shielding materials, fastener materials, and gasket materials to minimize corrosion. Refer to Table 2. 1. The potential (EMF) of the components in contact with each other should be less than 0.75V. If in a salty, humid environment, the potential between each other must be less than 0.25V. 2. The anode (positive) component should be larger than the cathode (negative) component. B4. Use a shielding material with a gap width of more than 5 times to overlap the seam. B5. Electrical connection is made by welding, fasteners, etc. at a distance of 20 mm (0.8 inch) between the shield and the case. B6. Use a gasket to bridge the gap, eliminate grooving and provide a conductive path between the gaps. B7. Eliminate gaps, cracks and shields that are too thin. B8. Avoid straight corners and excessive corners in the shielding material. B9. Make sure that the hole diameter is 20mm or less and the length of the groove is 20mm or less. Under the same opening area, the hole is better than the groove. B10. If large openings and sensitive components are required, a second layer of shielding should be placed between the joystick and the indicator. B11. If possible, use a few small openings instead of a large opening. B12. If possible, the spacing between these openings should be as large as possible. B13. For grounded equipment, connect the shield to the chassis ground where the connector enters. B14. For ungrounded (double isolated) equipment, connect the shielding material to the circuit in common near the switch. B15. Place a ground plane or secondary shield (metal or copper/polyester film layering) in parallel near the electronics and bend the ground plane to connect to the chassis ground or the common ground of the circuit at the cable entry location. B16. Try to get the cable entry point near the center of the panel, not near the edge or corner. B17. The individual slots arranged in the shielding device shall be parallel to the direction in which the ESD current flows. B18. When considering indirect ESD problems, a partial shield should be installed under the horizontal board and backplane. 1. Connect the power connector and connector to the outside of the chassis or to the public ground of the circuit. # Use a metal plate with a metal bracket at the mounting hole to act as an additional grounding point, or use a plastic bracket for insulation and isolation. # Under the circuit board/backplane, it is cheap and easy to place a polyester film/copper or polyester film/aluminum platen and place a fastening sheet between the chassis and the connector metal body. # In the chassis, use a conductive coating or a conductive filler (see B1). B19. Install a partial shield on the control panel and keyboard location on the plastic chassis to block ESD: 1. The location of the power connector and the connector leading to the outside, to be connected to the chassis ground or circuit common ground. # Use a metal piece so that small high frequency capacitors can be soldered between the shield and the switch/joystick/indicator connection. # Use polyester film/copper or polyester film/aluminum platen in plastic, or use conductive coating or conductive filler. B20. Use a thin conductive chrome plating or chromate coating on the aluminum plate, but not anodized. B21. To achieve a shielding effect greater than 20 to 40 dB. B22. Remove the anodizing and coating at the joints, joints and connectors. B23. Good electrical continuity is achieved at the welded joint of stainless steel. B24. Use conductive filler material in plastics. Since the surface of the mold part usually has a resin material, it is difficult to achieve a low-resistance connection. B25. Use a thin conductive chromate coating on the steel material. B26. Directly contact the clean metal surface without relying on screws to connect the metal parts. B27. Add a ground plane immediately adjacent to the double panel and connect the ground plane to the ground point on the circuit at the shortest distance. B28. Connect the display to the chassis shield along the entire perimeter with a shield coating (indium tin oxide, indium oxide, tin oxide, etc.). B29. Provide an antistatic (weak conductive) path to the ground where the operator is frequently in contact, such as the space bar on the keyboard. B30. It is difficult for the operator to generate an arc discharge to the edge or corner of the metal sheet. Arcing to these points will cause more indirect ESD effects than arcing to the center of the metal plate. B31. Place a grounded conductive layer between the membrane keyboard circuit and its adjacent adjacent circuit. Grounding and bonding ESD arc current discharge first charges the parasitic capacitance of the metal object being hit and then flows through each of the possible conductive paths. The arc current is more likely to flow through the sheet, or a short, wide strip conductor rather than a narrow line. A low-impedance path is established between the metal parts by bonding, thereby minimizing the voltage difference between each other, and the grounding provides a path for finally discharging the accumulated charge. In order for grounding and bonding to effectively prevent ESD, it should be ensured that the ESD current density and current path impedance are as low as possible. C1. Multi-point grounding is used where the ESD current is expected to flow. C2. Single point grounding is used at locations where ESD current is not expected to flow. C3. Connect the metal part of the chassis to the chassis ground. C4. Make sure that each cable entry point is within 40mm (1.6 inches) of the chassis ground. C5. Connect the connector housing and metal switch housing to the chassis ground. C6. Place a wide conductive guard ring around the membrane keyboard, connect the outer ring of the ring to the metal chassis, or connect to the metal chassis at least at the four corners. Do not connect the guard ring to the PCB ground. C7. Near the connector, connect the signal on the connector to the chassis ground of the connector with an LC or bead-capacitor filter. C8. Ensure that the distance between the unisolated chassis ground and the electronic device is 2.2mm or more. C9. Add a magnetic bead between the chassis ground and the circuit common ground. C10. Make sure the bonding joint is short and thick. If possible, the aspect ratio should be as small as 5:1. C11. If multiple bonding joints are possible, avoid excessive ESD current concentration. C12. Ensure that the bonding joints and bonding wires are away from the susceptible electronic equipment or the cables of these electronic equipment. C13. When selecting the material of the bonding joint and the bonding wire and the fastener/fastening method, the erosion should be minimized as shown in Table 2. 1. The EMF between the parts close to each other must be less than 0.75V, if The EMF value must be less than 0.25V in a humid environment; 2. The anode (positive) component should be larger than the cathode (negative) component. C14. Ground the control metal handle to the shield with grounding fingers or conductive bushings. C15. Ensure that the bonding tape and bonding wires are away from PCBs susceptible to ESD. C16. Add a bonding tape or bonding wire in the hinge. C17. Weld metal sheets that cannot be separated by welding, brazing, lead welding, or bending of the iron. C18. From the point of view of operation/repair, the metal sheets that must be separated are bonded in the following manner: 1. Keep the metal surface clean and in direct contact. 2. Let the metal surface with a thin conductive coating be in direct contact. C19. The solid bond zone is superior to the braided bond zone. C20. Make sure the bonding is not wet. C21. Use multiple conductors to connect the ground plane or ground grid of all boards in the chassis together. C22. Ensure that the width of the bonding points and washers is greater than 5 mm. Protecting the power distribution system inside the power electronics is the main subject of ESD arc inductive coupling. The following steps will help the power distribution system protect against ESD. D1. Tighten the power cord and the corresponding return line tightly together. D2. Place a magnetic bead where each power cord enters the electronic device. D3. Place a transient suppressor, metal oxide varistor (MOV) or 1kV high frequency capacitor between each power supply pin and the ground of the electronics chassis. D4. It is best to place a dedicated power and ground plane on the PCB, or a tight power and ground grid, and use a large number of bypass and decoupling capacitors. The anti-ESD layout design allows PCB ESD design through layered design of the PCB, proper placement and routing, and ESD protection methods described above. To achieve the desired ESD resistance, it is common to retest such cycles through several test-solving problems, each of which may affect at least one PCB design. In the PCB design process, most of the design modifications can be limited to increasing or decreasing components through prediction. Adjust the PCB layout to make it the strongest ESD protection. E1. Use multi-layer PCBs as much as possible: 1. Compared to double-sided PCBs, the ground plane and power plane and the closely spaced signal line-ground spacing can reduce common impedance and inductive coupling. Reach 1/10 to 1/100 of the double-sided PCB. # Try to keep each signal layer close to a power or ground plane. # For high-density PCBs with components on the top and bottom surfaces, short traces, and many fill locations, consider using inner traces. Most of the signal lines, as well as the power and ground planes, are on the inner layer and are therefore similar to Faraday boxes with shielding. E2. For double-sided PCBs, a tightly interwoven power and ground grid is used. 1. The power cord is close to the ground. 2. Connect as much as possible between the vertical and horizontal lines or the filled area. 3. The grid size of one side is less than or equal to 60mm. 4. If possible, the grid size should be less than 13mm (0.5 inches). E3. Make sure that each circuit is as compact as possible. E4. Set all connectors aside as much as possible. E5. If possible, introduce the power cord from the center of the card and away from areas that are susceptible to direct ESD. E6. On all PCB layers under the connector (easy to be directly hit by ESD) on the outside of the chassis, place a wide chassis or polygon fill and connect them with vias every 13mm. together. E7. Place a mounting hole on the edge of the card. The top and bottom pads of the solderless solder are attached to the chassis ground. E8. When assembling the PCB, do not apply any solder to the top or bottom pads. Use a screw with an inset washer to make the PCB in close contact with the metal chassis/shield or ground plane bracket. E9. Set the same “Isolation Zone” between the chassis ground and circuit ground of each layer; if possible, keep the separation distance 0.64mm (0.025 inches). E10. At the top and bottom of the card near the mounting holes, the chassis ground and circuit ground are connected together by a 1.27 mm wide (0.050 inch) wire every 100 mm (4.0 in.) along the chassis ground. Adjacent to these connection points, pads or mounting holes for mounting are placed between the chassis ground and the circuit ground. These ground connections can be made with a blade to keep open; or magnetic bead/high frequency capacitors can be used to change the grounding mechanism during ESD testing. E11. If the board is not placed in a metal chassis or shield, solder resists should not be applied to the top and bottom chassis grounds of the board so they can act as discharge bars for ESD arcs. E12. Place a circular ground around the circuit in the following manner: 1. Place a circular path around the entire periphery except for the edge connector and the chassis ground. # Make sure that all layers have a ring-shaped width greater than 2.5mm (0.1 inches). # Every 15mm (0.5 inch) with a via to connect the ring. # Connect the ring ground to the common ground of the multilayer circuit. # For a double panel mounted in a metal chassis or shield, the ring ground should be connected to the circuit publicly. # Unshielded double-sided circuit should be connected to the chassis ground annularly, and no solder resist can be applied to the annular ground so that the annular discharge can act as a discharge rod for ESD, at least at a certain position on the annular ground (all layers) A 0.5 mm wide (0.020 inch) gap prevents the formation of a large loop. # The distance between the signal wiring and the ring ground should not be less than 0.5mm. E13. In the area that can be directly hit by ESD, a ground wire should be placed near each signal line. E14. The I/O circuit should be as close as possible to the corresponding connector. E15. Circuits susceptible to ESD should be placed close to the center of the circuit so that other circuits can provide some shielding. E16. The resistors and beads in series are usually placed at the receiving end, and for those cable drivers that are easily hit by ESD, it is also conceivable to place resistors or beads in series at the driving end. E17. A transient protector is usually placed at the receiving end. 1. Connect to the chassis ground with short, thick wires (less than 5 times the width, preferably less than 3 times the width). 2. The signal and ground wires coming out of the connector should be connected directly to the transient protector before they can be connected to other parts of the circuit. E18. Place a filter capacitor at the connector or within 25 mm (1.0 in.) of the receiving circuit. 1. Use a short, thick wire to connect to the chassis ground or receive circuit ground (less than 5 times the width, preferably less than 3 times the width). 2. The signal and ground lines are first connected to the capacitor and then to the receiving circuit. E19. Make sure the signal line is as short as possible. E20. When the length of the signal line is greater than 300mm (12 inches), be sure to lay a ground wire in parallel. E21. Ensure that the loop area between the signal line and the corresponding loop is as small as possible. For the long signal line, change the position of the signal line and the ground line every few centimeters or a few inches to reduce the loop area. E22. Drive signals from a central location of the network into multiple receiving circuits. E23. Make sure that the loop area between the power supply and ground is as small as possible, placing a high frequency capacitor close to each power supply pin of the integrated circuit chip. E24. Place a high frequency bypass capacitor within 80mm (3 inches) of each connector. E25. If possible, fill the unused area with land and connect the filling of all layers every 60mm. E26. Ensure that the ground is connected to the ground at two opposite end positions of any large ground fill zone (approximately greater than 25 x 6 mm (1 x 0.25 inches)). E27. When the length of the opening on the power supply or ground plane exceeds 8 mm (0.3 inch), connect the sides of the opening with a narrow wire. E28. The reset line, interrupt signal line, or edge trigger signal line cannot be placed near the edge of the PCB. E29. Connect the mounting holes to the circuit ground or isolate them. 1. When the metal bracket must be used with a metal shield or chassis, a zero ohm resistor is used for the connection. 2. Determine the mounting hole size to achieve reliable mounting of the metal or plastic bracket. Large pads should be used on the top and bottom layers of the mounting holes. Solder resists should not be used on the bottom pads, and the low-level pads are not soldered by wave soldering. E30. Protected signal lines and unprotected signal lines cannot be arranged in parallel. E31. Pay special attention to the wiring of reset, interrupt and control signal lines. 1. Use high frequency filtering. 2. Keep away from the input and output circuits. 3. Stay away from the edge of the board. E32. PCB should be inserted into the chassis, not installed in the opening position or the internal seam. E33. Pay attention to the wiring of the signal wires under the magnetic beads, between the pads, and possibly touching the magnetic beads. Some beads have very good electrical conductivity and may create unexpected conductive paths. E34. If a chassis or motherboard has several circuit cards installed, the most sensitive circuit card should be placed in the middle.