PCB (Printed Circuit Board) is an important electronic components, it’s base board of electronic components and carrier of electrical connections. it is called “printed” circuit board because it is made using printing technology.

Double-layer circuit board is layout wiring on both sides, but to use the two sides of the wire, must have appropriate circuit connection between the two sides. The “bridge” between such circuits is called via. The vias filled with or coated with metal holes in PCB, it can be connected with the two sides of the wire. Use PROTEL to draw double-sided pcb board, in TopLayer (top layer) to draw wire connection components, is in the top drawing board; select BottomLayer (bottom), in the bottom of the wire to connect components, is the bottom of the drawing board. Above is to draw double-layer pcb, meaning a pcb board in the top and bottom are painted wire. The double-sided PCB solves the difficulty of wiring in a single layer (which can pass through the hole to the other side), that is, both sides of the front and back wiring, the components can be welded on the front, can also be welded on the back, it is more suitable for use Single-side  complex on the circuit.

The two-layer board ground wire is designed as a grid-shaped frame, that is, the parallel lines are printed on the side of the printed circuit board, the other side is the vertical ground of the copy board, and then connected with the metal vias( resistance to be small).

In order to take into account each IC chip should be installed near the ground, often every 1 ~ 115cm place a ground line, so dense ground to make the signal loop area smaller, is conducive to reducing radiation. The ground network design method should be in the layout signal line before, otherwise it is more difficult to achieve.

Double-sided PCB board signal line wiring principle:
After Double-sided board rational distribution of components, followed by the first design of the network copy board power cord, and then layout important line — sensitive line, high frequency line, then layout the standard line — low line. The key lead line is best have a separate power supply, ground circuit, lead line must be very short, so sometimes design a ground line close to the signal line near the key lead line, let it form a smallest working loop.

Four-layer board top and bottom wiring principle is as same as double-sided PCB, also the first layout key crystal, crystal circuit, clock circuit, CPU and other signal lines, must follow principle the circulation area as small as possible.

PCB IC circuit working, above mentioned many times of circulation area, the actual its difference concept of differential mode radiation. Such as differential mode radiation definition: the circuit current flowing in the signal loop, the signal loop will produce electromagnetic radiation, because this current is differential mode, so the signal loop radiation is called differential mode radiation, the radiation Field calculation formula: E1=K1·f2·I·A/γ

In the formula: E1 — differential mold copy board circuit space γ radiation field intensity from the differential mode radiation formula can be seen, the radiation field strength and the operating frequency f2, circulation area A, operating current is proportional to, such as when the operating frequency f is determined, the size of the circulation area is the key factor that can be directly controlled in our design. At the same time, the working speed and current of the circulation are better meet reliability,not the bigger the better, the narrower the jump, the larger the wave component, the wider, the higher the electromagnetic radiation, the greater the power of its current must be large (the above has been pointed out), which we do not expect.

Several logic circuits are given below to meet the allowable loop area reference values for the radiation Class B standard. It can be seen that the faster the circuit switch, the smaller the area allowed.

The key to the line, if possible around it can be surrounded by the ground. After the PCB copy board is finished, the available ground will cover all the voids, but it must be noted that these covered ground lines should be shorted with the low impedance of the large strata, which can achieve good results (Note: Should meet the conditions, such as creepage distance, etc).

Some tips of double pcb board:

The use of automatic router to design pcb is attractive. In most cases, automatic routing of pure digital circuits (especially low-frequency signals and low-density circuits) will not be a problem. However, when attempting to use the automatic routing tool provided by the cabling software to do the wiring of analog, mixed-signal or high-speed circuits, there may be some problems and may cause extremely serious circuit performance problems. There are many things to consider about wiring, but the more troublesome problem is the grounding,If the ground path is started by the upper layer, the grounding of each device is connected to the ground via the cable on the layer. For each device in the lower layer, a ground circuit is formed by connecting the upper hole on the right side of the circuit board to the upper layer. The immediate red flag that the user sees when checking the routing mode indicates that there are multiple ground loops. In addition, the lower ground circuit is at a level. Can reduce the digital switching δi/δt the impact of analog circuits.It should be noted, however, that the two sheets have a ground plane in the lower layer of the circuit board. This is designed so that engineers can quickly see wiring when doing troubleshooting, often in the device manufacturer’s demonstration and evaluation board. But the more typical approach is to lay the ground plane on the top of the board to reduce electromagnetic interference.

    Steps of double side PCB:
    prepare the circuit schematic
    create a new pcb file and load the component package library
    planning circuit board into the network table and components
    automatic component layout
    layout adjustment
    network density analysis
    wiring rules set
    automatic routing
    manually adjust the wiring

PCB design experience of double side pcb board(embedded hardware experience):

    clearance spacing is better more than 10mil, high-density wiring is at least 5mil.
    The minimum line is 10mil out from welding line,no oblique line otherwise produce acute angle and unaesthetic.
    The main power line(current is relatively large)through the hole with two vias in parallel way to prevent a via failure lead to circuit board don’work.
    Power supply inlet capacitor with 100uf and 104 ceramic way the export capacitor capacity should be large enough to meet the circuit requirements (high current will not pull the voltage instantly). Turn off the diode from the power chip output pin closer the better.
    Power part of the resistance and capacitance to calculate the power, the package to meet the power requirements.
    A number of RF circuits, the  RF can be crossed on different layers, reducing interference.
    Pay attention to the lead position, to meet the schematic, not the same signal can be any position can lead.
    The same characteristics of the signal line wiring signal characteristics to the same, as long as the distance from the same length, the same number of vias.
    Will be some of the power supply of the decoupling capacitor filter capacitor can be placed on the back of the pin, save space and shorten the wiring distance.
    Wiring with latitude and longitude wiring, the upper and lower wiring is clear, but also to reduce the hole, reduce interference.
    When drawing the schematic, it is necessary to strictly calculate the rated current of the power chip rated current to meet the actual load requirements.
    When wiring, place the in – line elements around the core wiring area, which will cause interspersed and affect the latitude and longitude. To prevent interspersed, because the welding may scratch the line of the welding layer, so that when the welding pin may produce adhesion.
    Network chip under the ban copper
    Welding crystal crystal strict fall, because excessive shock will affect its performance.
    The four corners of the board are best rounded to prevent scratches.