Printed circuit board, also known as printed circuit board, is an important electronic component, a support for electronic components, and a carrier for the electrical connection of electronic components. It is made using electronic printing and is therefore called a “printed” circuit board.
The double-layer PCB is a double-layer circuit board. The double-layer circuit board has both sides of the circuit board. However, to use the wires on both sides. It is necessary to have a proper circuit connection between the two sides. This “bridge” between circuits is called a via.
The pilot hole is a small hole filled or coated with metal on the PCB, which can be connected to the wires on both sides.
When using PROTEL to draw a double-sided pcb board, draw the wire connection components on the TopLayer (top layer).
Which is the top board; select BottomLayer (bottom layer), draw the wire connection components on the bottom layer, which is the board on the bottom layer.
The above is to draw a double-layer PCB, which means draw wires on the top and bottom layers of a PCB board.
The double-sided board solves the difficulty of staggering in the single-sided board (it can be connected to the other side through the hole), that is, both the front and back sides
The components can be welded on the front side, or can be welded on the reverse side, it is more suitable for use than single-sided On more complicated circuits.
Double-layer PCB board-design and wiring principles
First prepare the circuit schematic diagram, and then create a new pcb file and load it into the component package library and planning circuit board. Load the netlist and components into the EDA software and use the automatic component placement tool. After completion, perform manual placement to adjust the network density analysis.
Next, you can use the automatic routing tool, and you need to manually adjust the routing for more special lines. The double-layer board ground wire is designed as a grid-like frame, that is, there are more parallel ground wires on one side of the printed circuit board, and the vertical ground wire on the other side, and then they are connected with metalized vias where they cross ( The via resistance should be small).
In order to consider that there should be a ground wire near each IC chip, a ground wire is often placed every 1 to 115 cm. Such a dense ground wire makes the area of the signal loop smaller, which is helpful to reduce radiation. The design method of the ground network should be before the signal line, otherwise it is more difficult to achieve.
The wiring principle of double-layer PCB board signal line
After the reasonable layout of the components of the double-layer board is determined, first design the power line of the grounding grid copy board, and then design the important lines-sensitive lines, high frequency lines and ordinary lines-low frequency lines.
The key lead is the best. The ground wire loop provides a separate power supply, and the lead is very short, so sometimes a ground wire is placed next to the signal wire on the edge of the key wire to form a minimum duty cycle.
The wiring principle of the top and bottom of the four-layer board is the same as the signal line of the double-layer board, and the key crystal, crystal oscillator circuit, clock circuit, CPU and other signal lines are first laid out, and the principle of the smallest circulating area must be observed.
Differential Mode Radiation and Controlling Circulating Area in PCB Design
When the printed circuit board IC circuit works, the circulation area is mentioned many times before, and its origin is actually the concept of differential mode radiation. For example, the definition of differential mode radiation: the working current of the circuit flows in the signal loop.
This signal loop will generate electromagnetic radiation. Because this current is differential mode, the radiation generated by the signal loop is called differential mode radiation. Field strength calculation formula: E1 = K1 · f2 · I · A / γ.
In the formula: E1 — the radiation field strength at the circuit space γ of the differential mode copy board is visible from the differential mode radiation formula, and its radiation field strength is proportional to the operating frequency f2, the circulating area A, and the operating current I, such as when After the working frequency f is determined, the size of the circulating area is a key factor that can be directly controlled in our design.
Reducing Electromagnetic Radiation in PCB Design through Grounding Techniques
At the same time, as long as the circulating speed and current meet the reliability, the larger the better, the narrower the upper and lower edges of the signal, its harmonic The larger and wider the wave component, the higher the electromagnetic radiation, and the higher the power, the higher the current (which has been pointed out above), which is undesirable.
The following gives several kinds of logic circuits that can meet the reference value of circulating area allowed by the radiation class B standard. It can be seen that the faster the circuit switching speed, the smaller the allowable area.
If possible, the key cable can be surrounded by a ground wire. In addition, after the wiring of the copy board is completed, all gaps can be covered with ground wires, but it must be noted that these covered ground wires must be short-circuited with the low-impedance connection point of the large ground layer, so that good results can be obtained (note: Clearance requirements must be met, such as creepage distance, etc.).
Double-layer PCB-wiring skills
The use of automatic wiring devices to design PCBs is attractive. In most cases, the operation of automatic wiring on purely digital circuits (especially low-frequency signals and low-density circuits) will not cause problems.
However, when trying to use the automatic routing tools provided by the routing software to do analog, mixed signal or high-speed circuit wiring, some problems may occur, and it may cause extremely serious circuit performance problems. [1] There are many things to consider about wiring, but the more troublesome problem is the grounding method.
– Connect the ground of each device through the pull wire on the upper layer, if the ground path starts from the upper layer.
– For each device in the lower layer, connect the through hole on the right side of the circuit board to the upper layer to form a ground loop.
If there are multiple ground loops, the user will immediately notice it when checking the wiring.In addition, the ground loop of the lower layer is horizontal. Can reduce the impact of digital switching δi / δt on analog circuits. Note that the lower layer of the printed circuit board for these two double-layer boards must have a ground plane.
This design is to allow engineers to quickly see the wiring when doing troubleshooting. This method often appears on the demonstration and evaluation boards of device manufacturers. But a more typical approach is to lay a ground plane on the upper layer of the printing circuit board to reduce electromagnetic interference (emi).
Double-layer PCB board-design operation steps
1. Prepare the circuit schematic.
2. Create a new PCB file and load it into the component package library.
3. Planning the circuit board.
4. Load the netlist and components.
5. Automatic layout of components.
6. Layout adjustment.
7. Network density analysis.
8. Wiring rule setting.
9. Automatic wiring.
10. Manually adjust the wiring.
Double-layer PCB board-design experience (embedded hardware experience)
1. Clearance spacing is generally at least 10mil, and at least 5mil for high-density wiring.
2. The wire coming out of the welding base should be at least 10mil and then change direction. Do not slant the wire, it will produce an acute angle and is not beautiful.
3. Arrange the vias (for relatively large current) of the main power line in parallel with double holes to prevent the circuit from malfunctioning due to the via.
4. The power supply inlet capacitor adopts 100uf and 104 ceramics. The closer the shutdown diode is to the output pin of the power chip, the better.
5. You must calculate the power of resistors and capacitors in the power supply and ensure that the package meets the power requirements.
6. Multiple RF circuits can cross RF on different layers to reduce interference.
Common considerations in PCB design
7. Pay attention to the position of the lead, to meet the schematic diagram, it can be led out at any position if the signal is the same.
8. The signal characteristics of the signal lines with the same characteristics should be the same, the distance of the traces should be as long as possible, and the number of vias should be the same.
9. Decouple and filter capacitors on pins and opposite side save space and shorten wiring distance for some power supplies.
10.Warp and weft wiring is used, with clear upper and lower wiring. This reduces vias and interference.
11. When drawing the schematic diagram, it is necessary to strictly calculate. The rated current and rated power of the power chip to make it meet the actual load requirements.
12.Put in-line components around when wiring.Avoid placing them in the core wiring area.
Prevent interleaving and routing issues by avoiding penetration. Scratching the solder layer can cause adhesion problems when soldering pins.
13. It is forbidden to catch copper under the network chip.
14. The crystal oscillator falls rigorously during welding, because excessive shock will affect its performance.
15. Make the four corners of the board rounded to prevent scratches.
