To prevent ESD, it is first necessary to know what ESD is and the process of ESD entering electronic devices. When a charged conductor approaches another conductor, ESD may occur. Firstly, a strong electric field is established between two conductors, resulting in breakdown caused by the electric field. When the voltage between two conductors exceeds the breakdown voltage of the air and insulating medium between them, an arc will be generated. Within a time range of 0.7ns to 10ns, the arc current can reach tens of amperes, sometimes even exceeding 100 amperes. The arc will continue until two conductors come into contact with a short circuit or the current is too low to sustain the arc.

The generation of ESD depends on the initial voltage, resistance, inductance, and parasitic capacitance of the object:

Examples of electric arcs that may occur include human bodies, charged devices, and machines.

Examples that may generate sharp arcs include hands or metal objects.

Examples of multiple arcs that may produce homopolarity or polarity changes include furniture.

ESD can enter electronic devices through five coupling pathways:

The initial electric field is capacitive coupled to a network with a larger surface area and generates a high voltage of up to 4000V/m at a distance of 100mm from the ESD arc.

The charge/current injected by the arc can cause the following damages and faults:

a. Penetrating the thin insulation layer inside the components and damaging the gates of MOSFET and CMOS components (common).

b. Trigger lock in CMOS devices (common).

c. Short circuit reverse biased PN junction (common).

d. PN junction with short circuit forward bias (rare).

e. Melt the welding or aluminum wires inside the active device (rare).

Current can cause voltage pulses (V=L x dI/dt) to be generated on conductors, which may be power, ground, or signal lines. These voltage pulses will enter every component connected to these networks (common).

The arc will generate a strong magnetic field in the frequency range of 1MHz to 500MHz, which is inductive coupled to each adjacent wiring loop, generating a current of up to 15A/m at a distance of 100mm from the ESD arc.

The electromagnetic field radiated by an arc is coupled to long signal lines, which act as receiving antennas (rare).

ESD will identify the weak points of the device through various coupling pathways. ESD has a wide frequency range, not just some discrete frequency points, it can even enter narrowband circuits. In order to prevent ESD interference and damage, it is necessary to isolate these paths or enhance the device's ability to resist ESD. Table 1 describes the preventive measures for possible ESD and the scenarios in which they can be effective.

Prevent problems before they occur

Plastic cases, air spaces, and insulators can shield ESD arcs directed towards electronic devices. In addition to utilizing distance protection, it is also necessary to establish an ESD resistant environment with a breakdown voltage of 20kV.

A1. Ensure that the path length between the electronic device and the following items exceeds 20mm.

Any point that users can access, including seams, vents, and mounting holes. Under a constant voltage, the arc propagates further through the surface of the medium than through air.

Any ungrounded metal that users can come into contact with, such as fasteners, switches, joysticks, and indicators.

A2. Install electronic devices in the grooves or notches of the chassis to increase the path length at the seams.

A3. Cover the seams and mounting holes inside the chassis with polyester film, which extends the edges of the seams/through holes and increases the path length.

A4. Cover unused or rarely used connectors with metal caps or shielded plastic dust covers.

A5. Use switches and joysticks with plastic shafts, or place plastic handles/sleeves on top to increase path length. Avoid using handles with metal fixing screws.

A6. Install LEDs and other indicators into the internal holes of the device and cover them with straps or covers to extend the edges of the holes or use conduits to increase the path length.

A7. Extend the boundary of the thin film keyboard beyond the metal wire by 12mm, or use plastic grooves to increase the path length.

A8. The edges and corners of the metal components of the heat sink near the chassis seam, ventilation port, or installation hole should be made into a circular arc shape.

A9. In plastic chassis, metal fasteners close to electronic devices or not grounded should not protrude from the chassis.

A10. If the product cannot pass indirect ESD testing on the desktop/ground or horizontal coupling surface, a high support foot can be installed to keep it away from the desktop or ground.

A11. On the touch rubber keyboard, make sure the wiring is compact and extend the rubber pads to increase the path length.

A12. Apply adhesive or sealant around the circuit layer of the thin film keyboard.

A13. At the junction of the chassis, high-pressure resistant silicone resin or gaskets should be used to achieve sealing, ESD prevention, waterproofing, and dust prevention.

Chassis and shielding

The use of metal chassis and shielding covers can prevent ESD arcs and corresponding electromagnetic fields, and protect equipment from indirect ESD effects, with the aim of blocking all ESD outside the chassis. For electrostatic sensitive electronic devices, the ungrounded chassis should have a breakdown voltage of at least 20kV (rules A1 to A9); For the grounding chassis, electronic devices must have a minimum breakdown voltage of 1500V to prevent secondary arc, and the path length must be greater than or equal to 2.2mm.

The following measures can make ESD shielding more effective.

B1. If necessary, a chassis made of the following shielding materials should be designed:

Metal plate;

Polyester film/copper or polyester film/aluminum pressure plate;

Hot formed metal mesh with welded joints.

Thermoformed metalized fiber mats (non woven) or fabrics (woven);

Silver, copper or nickel coatings;

Zinc arc spraying;

Vacuum metal treatment;

No electroplating;

Adding conductive filling materials to plastic;

The handling of junction points and edges is crucial.

B2. Choose a material with high conductivity (low resistance coefficient), as shown in Table 2.

B3. Choose shielding materials, fastener materials, and gasket materials to minimize corrosion as much as possible. Refer to Table 2.

1. The potential (EMF) between components in contact with each other should be less than 0.75V. If in a saline and humid environment, the potential between each other must be less than 0.25V.

The size of the anode (positive electrode) component should be larger than that of the cathode (negative electrode) component.

B4. Overlay shielding materials with a gap width of more than 5 times at the joint.

B5. Electrical connections are achieved every 20mm (0.8 inches) between the shielding layer and the box through welding, fasteners, and other means.

B6. Use washers to bridge gaps, eliminate slotting, and provide conductive pathways between gaps.

B7. Eliminate gaps, cracks, and situations where the shielding is too thin.

B8. Avoid straight corners and excessive bends in shielding materials.

B9. Ensure that the aperture is less than or equal to 20mm and the length of the groove is less than or equal to 20mm. Under the same opening area conditions, using a hole is better than a groove.

B10. If large openings and sensitive devices are required, a second layer of shielding should be installed between the control lever and indicator.

B11. If possible, use a few small openings instead of a large one.

B12. If possible, the spacing between these openings should be as large as possible.

B13. Connect the shielding layer to the chassis ground at the point where the connector enters the grounding device.

B14. For ungrounded (double isolated) equipment, connect the shielding material to the common ground of the circuit near the switch.

B15. Place a ground plane or secondary shield (metal or copper/polyester film layer) in parallel near the electronic device, and bend the ground plane so that it can be connected to the chassis ground or the common ground of the circuit at the cable entry position.

B16. Try to place the cable entry point as close to the center of the panel as possible, rather than near the edges or corners.

B17. The slots arranged in the shielding device should be parallel to the direction of ESD current flow.

B18. When considering indirect ESD issues, a local shielding device should be installed below the horizontal circuit board and backplane.

At the power connector and where the connector leads to the outside, it should be connected to the chassis ground or the common ground of the circuit.

Use metal sheets with metal brackets at the installation holes to serve as additional grounding points, or use plastic brackets to achieve insulation and isolation.

Under the circuit board/backplane, polyester film/copper or polyester film/aluminum pressure plates should be placed, and a fastening sheet should be placed between the chassis and the connector metal body, which is both cheap and easy to implement.

In the chassis, conductive coatings or conductive fillers (see B1) should be used.

B19. Install local shielding devices at the control panel and keyboard positions on the plastic chassis to prevent ESD:

The position of the power connector and the connector leading to the outside should be connected to the chassis ground or circuit common ground.

Use metal sheets so that small high-frequency capacitors can be soldered between the shielding device and the connection of the switch/lever/indicator.

Use polyester film/copper or polyester film/aluminum pressure plates in plastics, or use conductive coatings or fillers.

B20. Use thin conductive chromium plating or chromate coating on aluminum plates, but cannot use anodic electroplating.

B21. To achieve a shielding effect greater than 20 to 40dB.

B22. Remove anodizing and coatings at seams, joints, and connectors.

B23. Achieve good electrical continuity at the welded joints of stainless steel.

B24. Conductive filling materials should be used in plastics. Due to the resin material typically present on the surface of the molded parts, it is difficult to achieve low resistance connections.

B25. Use a thin conductive chromate coating on steel materials.

B26. Allow clean and tidy metal surfaces to come into direct contact without relying on screws to connect metal components.

B27. Add a ground plane next to the double-sided board and connect it to the grounding point on the circuit at the shortest distance.

B28. Connect the display to the chassis shielding device using shielding coatings (indium tin oxide, indium oxide, tin oxide, etc.) along the entire periphery.

B29. At locations where operators frequently come into contact, an anti-static (weakly conductive) path to the ground should be provided, such as the space bar on the keyboard.

B30. It is difficult for operators to generate arc discharge to the edges or corners of the metal plate. Arc discharge to these points will cause more indirect ESD effects than arc discharge to the center of the metal plate.

B31. Place a grounded conductive layer between the thin film keyboard circuit and its adjacent circuit.

Grounding and Bonding

When the ESD arc current is discharged, the parasitic capacitance of the hit metal object is first charged, and then it flows through every possible conductive path. Arc current is more likely to flow through sheet-like or short and wide strip conductors rather than narrow lines. A low impedance path is established between metal components through bonding, thereby minimizing the voltage difference between them, while grounding provides the final path for releasing accumulated charges. In order to effectively prevent ESD through grounding and bonding, it is necessary to ensure that the ESD current density and current path impedance are as low as possible.

C1. Use multi-point grounding at the location where ESD current is expected to flow.

C2. Use single point grounding at locations where ESD current is not expected to flow.

C3. Connect the metal part of the chassis to the ground of the chassis.

C4. Ensure that the distance between each cable entry point and the chassis ground is within 40mm (1.6 inches).

C5. Connect both the connector housing and the metal switch housing to the chassis ground.

C6. Place a wide conductive protective ring around the thin film keyboard and connect the periphery of the ring to the metal case, or at least at four corners to the metal case. Do not connect the protective ring to the PCB ground.

C7. Near the connector, connect the signal on the connector to the chassis ground of the connector using an L-C or magnetic bead capacitor filter.

C8. Ensure that the distance between the uninsulated chassis and electronic devices is greater than or equal to 2.2mm. C9. Add a magnetic bead between the chassis ground and the circuit common ground.

C10. Ensure that the bonding joint is short and thick. If possible, try to achieve a aspect ratio of 5:1 or less.

C11. If possible, use multiple bonding connectors to avoid excessive concentration of ESD current.

C12. Ensure that bonding joints and bonding wires are kept away from easily affected electronic devices or cables of these electronic devices.

C13. When selecting materials for bonding joints and bonding wires, as well as fasteners/fastening methods, erosion should be minimized as much as possible, as shown in Table 2.

1. The EMF between components that are close to each other must be less than 0.75V. If in a humid environment, the EMF value must be less than 0.25V;

The size of the anode (positive electrode) component should be larger than that of the cathode (negative electrode) component.

C14. Ground the control metal handle to a shielding device with a grounding fork or conductive liner. C15. Ensure that the bonding tape and bonding wire are kept away from PCBs that are susceptible to ESD.

C16. Bonding straps or wires should be added to the hinge.

C17. Weld metal sheets that cannot be separated by welding, brazing, lead welding, or bending of shaped iron.

C18. From the perspective of operation/maintenance, the metal sheets that must be separated should be bonded in the following way: 1. Keep the metal surface clean and in direct contact. 2. Let the metal surface with a thin conductive coating come into direct and close contact.

C19. Solid bonding tape is superior to woven bonding tape.

C20. Ensure that the bonding area is not damp.

C21. Use multiple conductors to connect the ground plane or grid of all circuit boards in the chassis together.

C22. Ensure that the width of the bonding point and washer is greater than 5mm.

Protecting power supply

The power distribution system inside electronic devices is the main object subject to inductive coupling of ESD arcs. The following steps will help the power distribution system prevent ESD.

D1. Twist the power cord and corresponding circuit cord tightly together.

D2. Place a magnetic bead at each point where the power cord enters the electronic device.

D3. Place a transient suppressor, metal oxide varistor (MOV), or 1kV high-frequency capacitor between each power pin and the ground adjacent to the electronic device chassis.

D4. It is best to arrange a dedicated power and ground plane or a tight power and ground grid on the PCB, and use a large number of bypass and decoupling capacitors.

ESD resistant layout and wiring design

The anti ESD design of PCBs can be achieved through layered design, appropriate layout, wiring, and installation, as well as the aforementioned ESD prevention methods. To achieve the expected ESD resistance, it is usually necessary to go through several cycles of testing, problem-solving, and retesting, each of which may affect at least one PCB design. In the PCB design process, most design modifications can be limited to adding or removing components through prediction.

To adjust the PCB layout and wiring to have the strongest ESD prevention performance.

E1. Use multi-layer PCBs as much as possible:

Compared to double-sided PCBs, the ground plane and power plane, as well as the tightly arranged signal line ground distance, can reduce common impedance and inductive coupling, achieving 1/10 to 1/100 of that of double-sided PCBs.

Try to place each signal layer as close as possible to a power or ground layer.

For high-density PCBs with components on both the top and bottom surfaces, short connection lines, and many filling grounds, inner layer wires can be considered. Most signal lines, as well as power and ground planes, are located on the inner layer, making them similar to Faraday boxes with shielding capabilities.

For double-sided PCBs, a tightly interwoven power and ground grid should be used.

The power cord is tightly attached to the ground wire.

Connect as much as possible between vertical and horizontal lines or fill areas.

The grid size on one side is less than or equal to 60mm.

If possible, the grid size should be less than 13mm (0.5 inches).

E3. Ensure that each circuit is as compact as possible.

E4. Try to put all connectors aside as much as possible.

E5. If possible, introduce the power cord from the center of the card and keep it away from areas that are easily affected by ESD.

E6. On all PCB layers below the connectors that lead to the outside of the chassis (easily hit by ESD), place a wide chassis ground or polygon filled ground, and connect them together with through holes at a distance of approximately 13mm.

E7. Place installation holes on the edge of the card, and connect the top and bottom solder pads with unobstructed flux around the installation holes to the chassis ground.

During PCB assembly, do not apply any solder to the top or bottom solder pads. Use screws with embedded washers to achieve close contact between the PCB and the bracket on the metal chassis/shielding layer or ground plane.

E9. The same "isolation zone" should be set between the chassis ground and circuit ground on each layer; If possible, maintain a spacing of 0.64mm (0.025 inches).

E10. Connect the chassis ground and circuit ground with 1.27mm wide (0.050 inches) wires every 100mm (4.0 inches) along the chassis ground wire near the installation holes on the top and bottom layers of the card. Place solder pads or mounting holes for installation between the chassis ground and circuit ground adjacent to these connection points. These ground wire connections can be cut open with a blade to maintain an open circuit; Alternatively, a magnetic bead/high-frequency capacitor can be used as a jumper to change the grounding mechanism during ESD testing.

E11. If the circuit board will not be placed in a metal chassis or shielding device, solder mask should not be applied to the ground wires of the top and bottom chassis of the circuit board, so they can serve as discharge rods for ESD arcs.

E12. A circular ground should be set around the circuit in the following ways:

Place a circular ground path around the entire periphery, except for the edge connectors and chassis ground.

Ensure that the circular width of all layers is greater than 2.5mm (0.1 inches).

Connect the rings with through holes every 13mm (0.5 inches).

Connect the ring ground to the common ground of the multi-layer circuit.

For double-sided boards installed in metal cases or shielding devices, the circular ground should be connected to the circuit common ground.

Unshielded double-sided circuits should be connected to the chassis ground in a circular manner, and solder mask should not be applied to the circular ground so that it can serve as the discharge rod for ESD. At least a 0.5mm wide (0.020 inches) gap should be placed at a certain position on the circular ground (all layers) to avoid forming a large loop.

The distance between the signal wiring and the circular ground should not be less than 0.5mm.

E13. In areas 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. For circuits that are susceptible to ESD, they should be placed in an area close to the center of the circuit, so that other circuits can provide them with a certain shielding effect.

E16. Usually, series resistors and magnetic beads are placed at the receiving end, and for cable drivers that are easily hit by ESD, series resistors or magnetic beads can also be considered at the driving end.

E17. Transient protectors are usually placed at the receiving end. 1. Use short and thick wires (less than 5 times the width, preferably less than 3 times the width) to connect to the chassis ground. The signal wire and ground wire coming out of the connector should be directly connected to the transient protector before connecting to other parts of the circuit.

E18. Filter capacitors should be placed at the connector or within a range of 25mm (1.0 inch) from the receiving circuit. 1. Use short and thick wires to connect to the chassis ground or the receiving circuit ground (with a length less than 5 times the width, preferably less than 3 times the width). 2. The signal line and ground wire are first connected to the capacitor and then to the receiving circuit.

E19. Ensure that the signal line is as short as possible.

E20. When the length of the signal line is greater than 300mm (12 inches), a parallel ground wire must be laid.

E21. Ensure that the loop area between the signal line and the corresponding circuit is as small as possible. For long signal lines, swap the position of the signal line and ground wire every few centimeters or inches to reduce the loop area.

E22. Drive signals from the center of the network into multiple receiving circuits.

E23. Ensure that the loop area between the power supply and ground is as small as possible, and place a high-frequency capacitor near each power pin of the integrated circuit chip.

E24. Place a high-frequency bypass capacitor within a range of 80mm (3 inches) from each connector.

E25. If possible, fill unused areas with land and connect all layers of fill ground every 60mm distance.

E26. Ensure that the two opposite endpoints of any large ground fill area (approximately greater than 25 x 6mm (1 x 0.25 inches)) are connected to the ground.

E27. When the length of the opening on the power or ground level exceeds 8mm (0.3 inches), narrow wires should be used to connect both sides of the opening.

E28. The reset line, interrupt signal line, or edge trigger signal line cannot be arranged near the edge of the PCB.

E29. Connect the mounting holes to the circuit common or isolate them. When the metal bracket must be used together with the metal shielding device or chassis, a zero ohm resistor should be used for connection. 2. Determine the size of the installation hole to achieve reliable installation of metal or plastic brackets. Large solder pads should be used on the top and bottom layers of the installation hole, and solder resist should not be used on the bottom layer solder pads. Ensure that the low layer solder pads are not welded using wave soldering technology. E30. Protected signal lines and unprotected signal lines cannot be arranged in parallel.

E31. Special attention should be paid to the wiring of reset, interrupt, and control signal lines. 1. High frequency filtering should be used. 2. Stay away from input and output circuits. 3. Stay away from the edges of the circuit board.

E32. The PCB should be inserted into the chassis and not installed in open positions or internal seams.

E33. Pay attention to the wiring of signal wires under the magnetic beads, between solder pads, and those that may come into contact with the magnetic beads. Some magnetic beads have excellent conductivity and may create unexpected conductive paths.

E34. If a chassis or motherboard needs to have several circuit cards installed inside, the circuit card that is most sensitive to static electricity should be placed in the middle.

Author: John R. Barnes

Consulting Engineer

Lexmark International