1.1 General Principles for Component Wiring on Printed Circuit Boards
1. Power cord design
According to the current of the printed circuit board, try to increase the width of the power line, reduce the loop resistance, and make the direction of the power line and ground line and the direction of data transmission consistent, which helps to enhance the anti-noise ability.
2. Ground design
(1) The common ground wire should be placed at the edge of the board to facilitate the mounting of the printed board on the rack.
(2) Digital ground and analog ground should be separated as far as possible
(3) The ground wire of each stage of the printed circuit board should be self-contained closed loop to ensure that the ground current of each stage of the circuit mainly flows in the loop of this stage, reducing the current coupling between the stages.
3. Signal line design
(1) The low-frequency wires are placed close to the printed board and the low-frequency and DC wires such as power supply, filtering, and control are placed on the edge of the printed board. The high-frequency line is placed in the middle of the board surface, which can reduce the split capacitance of the high-frequency wire to the ground wire and the casing, and also facilitate the connection of the ground wire of the board to the frame.
(2) The high-potential conductor and the low-potential conductor should be as far as possible from the best wiring to minimize the potential difference between adjacent conductors.
(3) Avoid long-distance parallel traces. The wiring on the printed circuit board should be short and straight. A jumper wire can be used if necessary.
(4) Simultaneous installation of analog circuits and digital circuits on the printed circuit board At this time, the grounding systems of the two circuits should be completely separated, and their power supply systems should be completely separated.
(5) Use appropriate plug-in forms such as connectors, plug-in ends, and lead-outs.
1.2 Printed wire size and graphics
When the component structure layout and wiring scheme are determined, it is necessary to specifically design the drawing of the printed conductor.
1. Width of printed conductor
The thickness of the copper clad copper foil is generally 0.02 mm to 0.05 mm. The minimum width of the printed conductor depends on the current carrying capacity of the conductor and the allowable temperature rise. The working temperature of the printed board should not exceed 85 °C. After the wire is heated for a long time, the copper foil will fall off due to poor bonding strength.
2. Printed conductor spacing
The minimum spacing of the wires is primarily determined by the insulation resistance and breakdown voltage between the wires in the worst case. Generally, the wire spacing is equal to the wire width, but not less than 1 mm. For micro devices, not less than 0.4 mm. The surface mount board has a pitch of 0.12 to 0.2 mm, or even 0.08 mm. The following three factors should be considered in the specific design:
(1) The wire pitch of the low-frequency low-voltage circuit depends on the welding process. The time required for automated welding is larger, and it should be smaller when manually operated.
(2) The wire pitch of the high voltage circuit depends on the operating voltage and the electrical strength of the substrate.
(3) The high frequency circuit mainly considers the influence of distributed capacitance on the signal.
3. Printed wire pattern
There are two ways to arrange components on a printed board: irregular arrangement, regular arrangement. The irregular arrangement is suitable for high frequency circuits, which can reduce the length and distribution parameters of printed conductors, but is not conducive to automatic insertion. The rules (coordinate grids) are arranged in a neat arrangement, and the automatic insertion efficiency is high, but the lead wires may be long. The width of the wires on the same printed board should be the same, and the ground wire can be appropriately widened.
When a large area of copper foil is used, the center hole of the pad is slightly larger than the diameter of the device lead, but the pad is too large to form a solder joint. Generally, the pad outer diameter D>(d+1.3) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum pad diameter can be Dmin = (d + 1.0) mm.
(1) Printed board design steps and methods Know the circuit that the printed circuit board board needs to accommodate,
(2) The conditions for designing the printed board and the types, specifications and main dimensions of the various components in the circuit.
(3) Determining the position and special requirements of the printed circuit board in the whole machine (or extension) during the layout and wiring of each component and wire. Its connection form.