Antistatic method of PCB design
时间：【2018-08-21】 共阅【1043】次 【打印】【返回】
In the design of PCB board, the anti-ESD design of PCB can be realized by layering, proper layout and installation. By adjusting PCB layout and wiring, ESD can be well prevented. Multilayer PCB can be used as much as possible. Comparing with double-sided PCB, ground plane and power plane, as well as closely spaced signal Line-ground line can reduce common mode impedance and inductive coupling to 1/10 to 1/100 of double-sided PCB. It has a very short connecting line.
Static electricity from the human body, environment and even the inside of electronic equipment can cause various kinds of damage to precision semiconductor chips, such as penetrating the thin insulating layer inside the components; destroying the gate of MOSFET and CMOS components; locking triggers in CMOS devices; short-circuit reverse-biased PN junctions; short-circuit forward-biased PN junctions; melting active devices. Internal welding line or aluminum wire. In order to eliminate the interference and damage of electrostatic discharge (ESD) to electronic equipment, a variety of technical measures are needed to prevent it.
In the design of PCB board, the anti-ESD design of PCB can be realized by layering, proper layout and installation. In the design process, most of the design modifications can be limited to components or components. By adjusting the layout and routing of PCB, ESD can be well protected. Here are some common precautions.
Using multi-layer PCBs as much as possible, the ground plane and power plane, as well as the tightly arranged signal Line-ground spacing, can reduce the common mode impedance and inductive coupling to 1/10 to 1/100 of the double-sided PCB. Try to keep each signal layer close to a power layer or ground layer. For high-density PCBs with components on top and bottom surfaces, short connections, and lots of padding, you might consider using inner lines.
For double-sided PCB, closely interconnected power and ground grids should be used.The power cord is close to the ground wire and should be connected as much as possible between the vertical and horizontal lines or the filled area. One side of the grid size is less than or equal to 60 mm, if possible, the grid size should be less than 13 mm. Make sure that each circuit is as compact as possible.
Put all connectors together as far as possible.
If possible, the power cord will be introduced from the center of the card and away from the area that is easy to be directly affected by ESD.
On all PCB layers beneath the connectors leading out of the chassis (easy to hit directly by ESD), place wide chassis or polygonal padding and connect them through holes at about 13 mm intervals.
Installation holes are placed on the edge of the clamp and are connected to the ground of the chassis by the top and bottom pads of the flux-free solder around the installation holes.
When PCB is assembled, do not apply any solder on the top or bottom pad. A screw with a built-in washer is used to achieve close contact between the PCB and the metal chassis/shield or the bracket on the ground surface.
The same "isolation zone" should be set between the cabinet floor and the circuit floor on each floor; if possible, keep the distance between the cabinet floor and the circuit floor at 0.64 mm. Connect the cabinet floor and the circuit floor with 1.27 mm wide wires at 100 mm intervals between the top and bottom layers of the card near the mounting holes. Neighboring these joints, a pad or mounting hole for mounting is placed between the chassis floor and the circuit floor. These ground connections can be cut off by blades to keep them open, or to connect with magnetic beads / high frequency capacitors.
If the circuit boards are not placed in metal chassis or shielding devices, no resistance flux can be applied on the top and bottom chassis ground wires of the circuit boards, so they can be used as discharge electrodes for ESD arcs.
The following way is to set up an annular area around the circuit:（1）Except for the edge connector and chassis, the ring road is placed around the entire periphery.
（2）Ensure that the width of all circles is greater than 2.5mm.
（3）The rings are connected to each other through 13mm holes.
（4）The ring is connected to the common ground of multilayer circuits.
（5）For double panels mounted in metal chassis or shielding devices, the loop should be connected to the circuit in a common way. Unshielded double-sided circuits should be connected to the chassis ring, so that the ring can serve as the ESD discharge rod, at least a 0.5mm wide gap should be placed at a certain position on the ring (all layers), so that a large loop can be avoided. The distance between the signal wiring and the annular ground should not be less than 0.5mm. In the area that can be hit directly by ESD, a ground wire should be laid near each signal line.
The I/O circuit should be as close as possible to the corresponding connector.
Circuits that are susceptible to ESD should be placed near the center of the circuit so that other circuits can provide a shielding effect.
Series resistors and magnetic beads are usually placed at the receiving end. For cable drivers that are vulnerable to ESD hits, it is also possible to consider placing series resistors or magnetic beads at the driving end.
A transient protector is usually placed at the receiving end. Connect to the chassis with short and thick lines (less than 5 times the width, preferably less than 3 times the width). The signal and ground wires from the connector are connected directly to the transient protector, and then to the rest of the circuit.
In the range of the connector or from the receiving circuit 25mm, the filter capacitor should be placed.
（1）Connect to the chassis or receiving circuit with short, thick wires (less than 5 times the width, preferably less than 3 times the width).
（2）The signal line and ground wire are connected to the capacitor first and then connected to the receiving circuit.
Make sure the signal line is as short as possible.
When the length of the signal line is greater than 300mm, a ground wire must be laid in parallel.
Ensure that the loop area between the signal line and the corresponding loop is as small as possible. For long signal lines, the location of signal lines and ground wires should be changed every few centimeters to reduce the area of the loop.
Driving signals from the center of the network to multiple receiving circuits
Make sure the loop area between the power supply and the ground is as small as possible, and place a high-frequency capacitor near each power pin of the IC chip.
Driving signals from the center of the network to multiple receiving circuits
Where possible, fill unused areas with land and connect all layers of filling at 60 mm intervals.
Make sure you connect to the ground at two opposite endpoints in any large geofill area (about 25 mm by 6 mm).
When the opening length of the power supply or ground plane exceeds 8mm, the two sides of the opening should be connected with a narrow line.
The reset line, the interrupt signal line or the edge trigger signal line can not be arranged near the edge of the PCB.
Connect the mounting holes to the circuit, or isolate them.
（1）Metal brackets must be used with metal shielding devices or chassis, to use a zero ohm resistance to achieve the connection.
（2）Determine the size of the mounting hole to achieve reliable installation of metal or plastic support, in the top and bottom of the mounting hole to use a large pad, the bottom pad can not use flux, and ensure that the bottom pad does not use wave soldering process for welding.
The protected signal line and the unprotected signal line can not be arranged in parallel.
Special attention should be paid to reset, interrupt and control the wiring of signal wires.
（1）High frequency filtering is necessary.
（2）Keep away from input and output circuits.
（3）Away from the edge of the circuit board.