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Home > News > These experiences teach you how to design a double-layer pcb board
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These experiences teach you how to design a double-layer pcb board

Double-layer PCB board - word solution

PCB (Printed Circuit Board), the Chinese name for printed circuit boards, also known as printed circuit boards, is an important electronic component, a support for electronic components, and a carrier for electrical connection of electronic components. Because it is made by electronic printing, it is called a "printing" circuit board.

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The double-layer pcb board is a double-layer circuit board, and the double-layer circuit board has wiring on both sides of the circuit board, but to use the two-sided wire, it is necessary to have an appropriate circuit connection between the two sides. The "bridge" between such circuits is called a via. The via 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 a wire connection component on the TopLayer (top layer), which is the top layer drawing board; select BottomLayer (bottom layer), draw the wire connection component on the bottom layer, and draw the board on the bottom layer. The above is to draw a double-layer pcb, which means drawing wires on the top and bottom of a pcb board. The double panel solves the difficulty of wiring interleaving in a single panel (it can be conducted to the other side through the hole), that is, the wiring is provided on both sides, and the components can be soldered on the front side or welded on the reverse side, which is more suitable for use in the ratio. Single panel is more complex on the circuit.

Double-layer pcb board - design and wiring principles

The double-layer grounding wire is designed to be formed by a grid-like frame, that is, a plurality of parallel ground lines are arranged on one side of the printed board, and the other side is a vertical ground line of the board, and then metallized vias are connected at the intersections thereof ( The via resistance is small).

In order to take into account that each IC chip should be provided with a ground wire, a ground wire is often laid every 1 to 115 cm. Such a dense ground wire makes the signal loop area smaller, which is beneficial to reduce radiation. The design method of the ground network should be before the signal line, otherwise it is difficult to implement.

Double-layer pcb board signal line wiring principle

After the two-layer board is determined by the reasonable layout of the components, the power line of the ground-screen copy board is designed first, and then the important lines---sensitive lines, high-frequency lines, and general lines---low-frequency lines. The key leads preferably have separate power supplies, ground loops, and very short leads, so sometimes a ground wire is placed on the critical line next to the signal line to allow it to form a minimum working loop.

The wiring principle of the top and bottom of the four-layer board is the same as that of the double-layer board. It is also the key crystal, crystal circuit, clock circuit, CPU and other signal lines. It must follow the principle that the circulation area is as small as possible.

When the printed circuit board IC circuit works, the circulation area is mentioned many times in the past, and its actual source is in the concept of differential mode radiation. For example, the definition of differential mode radiation: the circuit operating current flows in the signal loop. This signal loop generates electromagnetic radiation. Since this current is differential mode, the radiation generated by the signal loop is called differential mode radiation, and its radiation. Field strength calculation formula: E1=K1·f2·I·A/γ

In the formula: E1---Differential mode copy board printed board circuit space γ radiation field strength is seen by the differential mode radiation formula, its radiation field strength is proportional to the working frequency f2, the circulation area A, the working current I, such as After the working frequency f is determined, the size of the circulation area is a key factor that can be directly controlled in our design. At the same time, as long as the working speed and current of the circulating current satisfy the reliability, the larger the better, the narrower the jump edge on the signal, the more harmonic The larger the wave component, the wider the electromagnetic radiation, and the higher the power, the greater the current (which has been pointed out above), which we do not expect.

The following gives several logic circuits that meet the loop current reference values allowed by the Radiation Class B standard. It can be seen that the faster the circuit is switched, the smaller the allowable area.

The key connections, if possible, can be surrounded by ground lines. After the PCB copying board is completed, all the gaps can be covered by the ground wire, but it must be noted that these grounding wires should be short-circuited with the low-impedance joint of the large ground layer, so that good results can be obtained (note: there is a gap requirement) Conditions should be met, such as creepage distance, etc.).

Double-layer pcb board - wiring skills

Using an autorouter to design a pcb is appealing. In most cases, automatic routing does not cause problems with purely digital circuits (especially low frequency signals and low density circuits). However, when attempting to use the automatic routing tool provided by the wiring software for wiring analog, mixed signal or high-speed circuits, some problems may occur and 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. If the ground path begins with the upper layer, the ground of each device is connected to the ground via a pull wire on the layer. For each device of the lower layer, it is connected to the upper layer by a through hole on the right side of the circuit board to form a ground loop. The immediate red flag that the user sees when checking the wiring pattern indicates that there are multiple ground loops. In addition, the ground loop of the lower layer is at a level. It can reduce the effect of digital switching δi/δt on analog circuits. However, it should be noted that the two double-layer boards have a ground plane on the lower layer of the board. This is designed so that engineers can quickly see the wiring when troubleshooting, which is often seen on the device manufacturer's demonstration and evaluation board. But a more typical approach is to lay a ground plane on the upper layer of the board to reduce the electromagnetic dry (emi).

Double-layer pcb board - design steps

1, prepare the circuit schematic

2, create a new pcb file and load the component package library

3, planning the circuit board

4, load the network table and components

5, automatic placement of components

6, layout adjustment

7, network density analysis

8, wiring rules set

9, automatic wiring

10, manually adjust the wiring

Double-layer pcb board - PCB design experience (embedded hardware experience)

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1. Clearance spacing is generally 10mil minimum, and high-density wiring is at least 5mil.

2. From the wire coming out of the soldering station, it is necessary to make at least 10 mils of the outgoing line and then change direction. Do not slant the line, which will produce an acute angle.

3. The main power line (larger current) vias are paralleled by two holes to prevent one via failure circuit from working.

4. The power inlet capacitance is 100uf and 104 ceramic. The output capacitor capacity should be large enough to meet the circuit requirements (the voltage will not be pulled low when the current is high). The closer the turn-off diode is to the output pin of the power chip, the better.

5. The power supply part of the resistor and capacitor should be calculated for power, and the package should meet the power requirements.

6. Multiple RF circuits, which can be placed on different layers to reduce interference

7. Pay attention to the position of the lead, to meet the schematic diagram, not the same signal can be drawn at any position

8. When the signal lines of the same characteristics are routed, the signal characteristics are the same, the trace distance is as long as possible, and the number of vias is the same.

9. Put some power supply decoupling capacitor filter capacitors on the opposite side of the pin to save space and shorten the wiring distance.

10. The wiring adopts the warp and weft wiring, and the upper and lower layers are clearly wired, which can also reduce the via holes and reduce the interference.

11. When drawing the schematic diagram, it is necessary to strictly calculate the rated current rating of the power chip to meet the actual load requirements.

12. When wiring, put the in-line components around, do not put them in the core wiring area, which will cause interspersing and affect the warp and weft. Prevent interspersing, because the welding layer of the line may be scratched during welding, so that adhesion may occur when the tube is welded

13. Banned copper under the network chip

14. The crystal oscillator is rigorously dropped when soldering, because excessive oscillation will affect its performance.

15. The four corners of the board are best rounded to prevent scratches.

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