PCB Tech - Overview of the four-layer PCB value preservation method

The quality of the four-layer PCB circuit board is defined in IPC. The surface process is anti-oxidation. If the vacuum package is not opened, it will be used within half a year, and the vacuum package will be removed within 24 hours, and the temperature and humidity are controlled. In an environment where the board is not unpacked, it should be used within one year. After it is opened, it should be pasted within a week and hours. The temperature and humidity must also be controlled. The gold board is equivalent to the tin board, but the control process is stricter than that of the tin board.

Generally speaking, a four-layer circuit board can be divided into a top layer, a bottom layer and two middle layers. The top and bottom layers are routed with signal lines. The middle layer first uses the command DESIGN/LAYER STACK MANAGER to add INTERNAL PLANE1 and INTERNAL PLANE2 with ADD PLANE as the most used PCB power layers such as VCC and ground layers such as GND (that is, connect the corresponding network labels. Be careful not to use ADD LAYER, this will increase MIDPLAYER, which is mainly used for multi-layer PCB signal line placement), so PLNNE1 and PLANE2 are two layers of copper connecting the power supply VCC and the ground GND.

Four-layer PCB refers to the PCB Printed Circuit Board made of 4 layers of glass fiber. Usually, SDRAM uses a 4 layer board. Although it will increase the cost of the PCB, it can avoid noise interference.

The general principles of multi-layer PCB circuit board layout and routing The general principles that PCB designers need to follow in the circuit board routing process are as follows:

(1) The principle of setting the spacing of printed traces of components. The spacing constraint between different networks is based on the principle of setting the spacing of the printed traces of the components by electrical insulation, manufacturing process, and components. Determined by factors such as size. For example, if the pin pitch of a chip component is 8mil, the [ClearanceConstraint] of the chip cannot be set to 10mil. PCB designers need to set a 6mil PCB design rule for the chip separately. At the same time, the spacing setting should also consider the production capacity of the manufacturer.

In addition, an important factor affecting components is electrical insulation. If the potential difference between two components or networks is large, electrical insulation needs to be considered. The gap safety voltage in a general environment is 200V/mm, which is 5.08V/mil. Therefore, when there are both high-voltage and low-voltage circuits on the same circuit board, it is necessary to pay special attention to sufficient safety clearance. When there are high-voltage circuits and low-voltage circuits, it is necessary to pay special attention to sufficient safety distance.

(2) The choice of the wiring form at the corner of the line. In order to make the circuit board easy to manufacture and beautiful, it is necessary to set the corner mode of the circuit and the selection of the wiring form of the corner of the circuit when designing the PCB. Can choose 45°, 90° and arc. Generally, sharp corners are not used, and it is better to use arc transition or 45° transition, and avoid 90° or sharper corner transitions.

The connection between the wire and the pad should also be as smooth as possible to avoid small sharp feet, which can be solved by teardropping. When the center distance between the pads is less than the outer diameter D of a pad, the width of the wire can be the same as the diameter of the pad; if the center distance between the pads is greater than D, the width of the wire should not be greater than that of the pad diameter. When a wire passes between two pads without connecting with them, it should keep the largest and equal distance from them. Similarly, when a wire and a wire of a wire pass between two pads without connecting with them, it should be kept to the maximum And equal spacing, the spacing between them should also be uniform and equal and keep the maximum. The spacing between them should also be uniform and equal and kept to the maximum.

(3) How to determine the width of printed traces. The width of the trace is determined by factors such as the level of current flowing through the wire and anti-interference. The larger the overcurrent flowing through the current, the wider the trace should be. The power line should be wider than the signal line. In order to ensure the stability of the ground potential (the larger the change of the ground current, the wider the trace should be. Generally, the power line should be wider than the signal line, and the power line should have less impact than the signal line width), the ground line should also be longer. The wide ground wire should also be wider. Experiments have proved that when the copper film thickness of the printed wire is 0.05mm, the current carrying ground wire of the printed wire should also be wider and can be calculated according to 20A/mm2, that is, 0.05mm thick, 1mm wide wire can flow through 1A Current. Therefore, for the general, the general width can meet the requirements; for high voltage and high voltage, the width of 10-30 mils for signal lines can meet the requirements of high voltage and high current signal lines with a line width greater than or equal to 40 mils, line ~ The spacing between lines is greater than 30mil. In order to ensure the anti-stripping strength and working reliability of the wire, the widest possible wire should be used to reduce the line impedance and improve the anti-interference performance within the allowable range of the board area and density.

For the width of the power line and ground line, in order to ensure the stability of the waveform, if the wiring space of the circuit board allows, try to thicken it as much as possible. Generally, it needs at least 50mil.

(4) Anti-interference and electromagnetic shielding of PCB printed wires. The interference on the wires mainly includes the interference introduced between the wires, the interference introduced by the power line) the anti-interference and electromagnetic shielding of the printed wires. The interference on the wires mainly includes the interference introduced between the wires, the crosstalk between the PCB signal lines, and the crosstalk between the signal lines. Reasonable arrangement and layout of the wiring and grounding methods can effectively reduce the sources of interference and make the PCB design The printed circuit board has better electromagnetic compatibility performance.