The hazards of static electricity to semiconductor chips
In the microscopic world of modern electronic technology, static electricity is a potential threat that cannot be ignored. It has a wide range of sources. It may come from an inadvertent touch by a human body, or it may be hidden in the surrounding environment. Even the internal operation process of electronic devices can generate static electricity. For those extremely precise semiconductor chips, static electricity is like an invisible killer that can cause various serious damages.
Static electricity may penetrate the wafer-thin insulation layer inside the components. This is like breaking through a crucial line of defense, causing the current flow inside the chip to go out of control and triggering various malfunctions. The gates of MOSFET and CMOS components can also be damaged under the attack of static electricity, resulting in the failure of these important components to work properly and affecting the performance of the entire chip. The latching of flip - flops in CMOS devices is also one of the common "achievements" of static electricity. Once the flip - flop is latched, the logical operations of the chip will fall into chaos and cannot run according to the preset program.
In addition, static electricity can also short-circuit reverse-biased PN junctions and forward-biased PN junctions, damaging the current conduction paths inside the chip. Even more seriously, it can melt the bonding wires or aluminum wires inside the active devices, causing the internal connections of the chip to break. It's like a bridge being destroyed, and the signals cannot be transmitted normally, so the chip completely loses its function.
Necessity of eliminating electrostatic discharge interference and damage and preventive ideas
In order to enable electronic devices to operate stably and reliably, it is imperative to eliminate the interference and damage caused by electrostatic discharge (ESD). This requires us to adopt a variety of technical means and carry out prevention from multiple aspects. Among the numerous preventive measures, the design of the PCB is particularly crucial. Through reasonable layering, layout wiring, and installation, the anti - ESD design of the PCB can be achieved, greatly reducing the impact of static electricity on electronic devices.
Specific preventive measures in PCB board design
Use of multi-layer PCBs
In PCB board design, using multi - layer PCBs as much as possible is an important strategy. Compared with double - sided PCBs, multi - layer PCBs have unique advantages. They have ground planes and power planes, and the spacing between signal lines and ground lines is closely arranged. This structure can significantly reduce the common - mode impedance and inductive coupling, and the effect can reach 1/10 to 1/100 of that of double - sided PCBs. During the design process, we should try to make each signal layer closely adjacent to a power layer or a ground layer, so as to better shield electrostatic interference. For high - density PCBs with components on both the top and bottom surfaces, short connecting lines, and a large amount of filled ground, considering using inner - layer lines is also a good choice, which can further enhance the anti - ESD ability.
Power and ground grid design for double-sided PCBs
For double-sided PCBs, it is crucial to use a closely interwoven power and ground grid. The power lines should be placed close to the ground lines, and as many connections as possible should be made between the vertical and horizontal lines or filled areas. The grid size on one side is preferably less than or equal to 60mm. If conditions permit, controlling the grid size within 13mm can better suppress the influence of static electricity.
Circuit layout optimization
Ensure that each circuit is as compact as possible, which can reduce the propagation path of electrostatic interference. Place all connectors on one side as much as possible to facilitate management and shielding. If possible, introduce the power cord from the center of the card and keep it away from areas that are easily directly affected by ESD to reduce the interference of static electricity on the power cord.
Setting of chassis ground and filling ground
On all PCB layers below the connectors leading to the outside of the chassis, wide chassis ground or polygon-filled ground should be placed, and they should be connected together with vias at intervals of approximately 13 mm to form a good electrostatic discharge path. Install mounting holes on the edge of the card. Connect the top and bottom pads without solder mask around the mounting holes to the chassis ground to ensure that static electricity can be smoothly conducted to the chassis ground.
PCB assembly requirements
When assembling the PCB, do not apply any solder to the pads on the top or bottom layer to avoid forming unnecessary electrostatic accumulation points. Use screws with built - in washers to achieve close contact between the PCB and the brackets on the metal chassis/shielding layer or grounding plane, enhancing the reliability of electrostatic conduction.
Isolation area and ground wire connection settings
Between the chassis ground and the circuit ground on each layer, the same "isolation area" should be set. If possible, keep the spacing distance at 0.64 mm to prevent electrostatic interference between the two. At the top and bottom layers of the card near the mounting holes, connect the chassis ground and the circuit ground together with a 1.27 mm wide line along the chassis ground wire every 100 mm. Adjacent to these connection points, place pads or mounting holes for installation between the chassis ground and the circuit ground. These ground wire connections can be cut open with a blade as needed to keep them open, or jump-connected with magnetic beads/high-frequency capacitors to flexibly meet different electrostatic protection requirements.
Handling of unshielded circuit boards
If the circuit board will not be placed in a metal chassis or shielding device, no solder mask should be applied to the chassis ground lines on the top and bottom layers of the circuit board. In this way, they can serve as the discharge electrodes for ESD arcs to safely release static electricity.
Setting of the circular area
Setting a ring ground around the circuit is an effective anti - ESD measure. First, except for the edge connectors and the chassis ground, place a ring ground path around the entire periphery. Ensure that the width of the ring ground on all layers is greater than 2.5mm to guarantee sufficient conductivity. Connect the ring ground with vias every 13mm to enhance its integrity. Connect the ring ground to the common ground of the multilayer circuit. For double - sided boards installed in a metal chassis or shielding device, connect the ring ground to the circuit common ground; for unshielded double - sided circuits, connect the ring ground to the chassis ground, and no solder mask should be applied on the ring ground so that it can act as an ESD discharge rod. Meanwhile, place at least one 0.5mm - wide gap at a certain position on the ring ground (on all layers) to avoid forming a large loop. The distance between the signal wiring and the ring ground should not be less than 0.5mm to prevent the signal from being interfered with by the ring ground.
Layout of signal lines and ground lines
In the area that can be directly hit by ESD, a ground line should be laid near each signal line to provide electrostatic protection for the signal lines. The I/O circuits should be placed as close as possible to the corresponding connectors to reduce electrostatic interference during signal transmission. Circuits that are susceptible to ESD should be placed in the area near the center of the circuit, and other circuits should be used to provide shielding for them.
Use of resistors, magnetic beads and protectors
Normally, a series resistor and a ferrite bead are placed at the receiving end. For cable drivers that are easily hit by ESD, it is also possible to consider placing a series resistor or ferrite bead at the driving end to limit the current and filter. At the same time, a transient protector is placed at the receiving end and connected to the chassis ground with a short and thick wire (the length is less than 5 times the width, preferably less than 3 times the width) to quickly discharge the static electricity. The signal wires and ground wires coming out of the connector should be directly connected to the transient protector before being connected to other parts of the circuit to ensure that the signals undergo electrostatic protection treatment before entering the circuit.
Placement of filter capacitors
Filter capacitors should be placed at the connector or within a range of 25 mm from the receiving circuit. Connect them to the chassis ground or the ground of the receiving circuit with short and thick wires (the length should be less than 5 times the width, preferably less than 3 times the width). Connect the signal lines and ground lines to the capacitor first and then to the receiving circuit to further filter electrostatic interference through the filter capacitors.
Signal line and loop area control
Ensure that the signal line is as short as possible to reduce the influence range of electrostatic interference. When the length of the signal line is greater than 300mm, a ground line must be laid in parallel to enhance the shielding of the signal line. At the same time, ensure that the loop area between the signal line and the corresponding circuit is as small as possible. For long signal lines, the positions of the signal line and the ground line should be swapped every few centimeters to reduce the loop area and the influence of electrostatic induction. Drive the signal from the center of the network into multiple receiving circuits to make the signal transmission more stable.
Power supply and ground loop area control and capacitor placement
Ensure that the loop area between the power supply and the ground is as small as possible. Place a high-frequency capacitor near each power pin of the integrated circuit chip to provide high-frequency filtering for the power supply and reduce electrostatic interference on the power line. Place a high-frequency bypass capacitor within 80 mm of each connector to further enhance the protection against static electricity.
Filling ground and opening treatment
Whenever possible, fill the unused areas with the ground. Connect the filled ground of all layers every 60mm to form a complete ground plane and enhance the electrostatic discharge capacity. Ensure that any large ground - filled area (approximately larger than 25mm×6mm) is connected to the ground at two opposite endpoints to guarantee the conductivity of the ground - filled area. When the opening length on the power or ground plane exceeds 8mm, use a narrow line to connect the two sides of the opening to prevent electrostatic accumulation at the opening.
Precautions for wiring of special signal lines
Reset lines, interrupt signal lines, or edge-triggered signal lines should not be arranged near the edge of the PCB, because these areas are more susceptible to static electricity. Connect the mounting holes to the circuit common ground or isolate them. If metal brackets must be used together with metal shielding devices or chassis, a zero-ohm resistor should be used for connection. Determine the size of the mounting holes to ensure reliable installation of metal or plastic brackets. Large pads should be used on the top and bottom layers of the mounting holes. No solder mask should be used on the bottom pads, and it should be ensured that the bottom pads are not soldered using the wave soldering process to guarantee the connection stability and anti-static performance of the mounting holes.
Signal line arrangement and wiring details
Do not arrange protected signal lines and unprotected signal lines in parallel to prevent the unprotected signal lines from transmitting electrostatic interference to the protected signal lines. Special attention should be paid to the routing of reset, interrupt, and control signal lines. Use high-frequency filtering and keep them away from input and output circuits as well as the edges of the circuit board to reduce the impact of static electricity on these critical signal lines.
Notes on PCB installation position and bead wiring
The PCB should be inserted into the chassis and not installed at the opening or internal seams to avoid direct influence from external static electricity. At the same time, pay attention to the wiring under the magnetic beads, between the pads, and of the signal lines that may come into contact with the magnetic beads. Some magnetic beads have quite good electrical conductivity and may create unexpected conductive paths, leading to electrostatic interference problems.
Layout of multi-circuit board chassis
If a chassis or motherboard is to house several circuit boards, the circuit board most sensitive to static electricity should be placed in the middle, and the surrounding circuit boards should be used to provide shielding for it to reduce the impact of static electricity on it.
Through the above series of preventive measures in the PCB design and installation process, the interference and damage of electrostatic discharge to electronic equipment can be effectively eliminated, ensuring the stable operation of electronic equipment.