Detailed explanation of multilayer PCB laminate structure (5)

5 Summary of multilayer board design principles


In the introduction of this chapter and the previous chapters, we have emphasized some principles that need to be followed in PCB design. Here we summarize these principles for readers' reference during design and can also be used as a check after design completion. Reference basis.


(1) Requirements for the PCB component library


① The packaging of the components used on the PCB must be correct, including the size of the component pins, the spacing between the pins, the number of the pins, the size of the frame, and the direction of the frame.


② Polar components (electrolytic capacitors, diodes, triodes, etc.) positive and negative or pin numbers should be marked in the PCB component library and PCB board.


③ The pin numbers of the components in the PCB library and the pin numbers of the schematic components should be the same. For example, the problem of inconsistent pin numbers in the diode PCB library components and the pin numbers in the schematic library was introduced in the previous chapter.


④ The components that need to use heat sinks should consider the size of the heat sink when drawing the component package. The components and the heat sink can be drawn together to form the overall package.


⑤ The inner diameter of the pin of the component and the pad must match, and the inner diameter of the pad must be slightly larger than the pin size of the component for installation.


(2) Requirements for PCB component layout


① The components are arranged uniformly, the components of the same functional module should be arranged as close as possible.

②The components using the same type of power supply and ground network are arranged as much as possible, which is beneficial to complete the electrical connection between each other through the internal electrical layer.

③The interface components should be placed on the side, and the type of interface should be indicated with a character string. The direction of the wiring lead should usually leave the circuit board.

④Power conversion components (such as transformers, DC / DC converters, three-terminal voltage regulators, etc.) should have sufficient heat dissipation space.

⑤The pins or reference points of the components should be placed on the grid points, which is conducive to wiring and aesthetics.

⑥The filter capacitor can be placed on the back of the chip, close to the power and ground pins of the chip.

The first pin of the component or the marking in the direction of identification shall be marked on the PCB and shall not be covered by the component.

The label of the component should be close to the component frame, the size is uniform, the direction is neat, does not overlap with the pads and vias, and it cannot be placed in the area covered by the component after installation.


(3) PCB wiring requirements


① Power supplies of different voltage levels should be isolated, and power supply traces should not cross.

② Traces use 45 ° or rounded corners. No sharp corners are allowed.

③ The PCB trace is directly connected to the center of the pad. The width of the wire connected to the pad must not exceed the outer diameter of the pad.

④ The line width of the high-frequency signal line is not less than 20mil, and it is surrounded by a ground wire to isolate it from other ground wires.

⑤ Do not wiring the interference source (DC / DC converter, crystal oscillator, transformer, etc.) to avoid interference.

⑥ Make the power cord and ground wire as thick as possible. Where space permits, the width of the power cord is not less than 50mil.

⑦ Low voltage and low current signal line width is 9 ~ 30mil, and it should be as thick as possible when space permits.

的 The spacing between signal lines should be greater than 10mil, and the spacing between power lines should be greater than 20mil.

⑨ The line width of high-current signal lines should be greater than 40mil, and the pitch should be greater than 30mil.

The minimum size of the via hole is preferably 40mil outer diameter and 28mil inner diameter. When a wire is connected between the top layer and the bottom layer, a pad is preferable.

It is not allowed to route signal lines on the internal electrical layer.

The interval width between different areas of the internal electrical layer is not less than 40mil.

When drawing the boundary, try not to let the boundary line pass through the pads of the area you want to connect to.

Laying copper on the top and bottom layers, it is recommended to set the line width value to be larger than the grid width, completely covering the free space, and leaving no dead copper, while maintaining a distance of 30mil (0.762mm) or more from other lines (can be set before copper Safety clearance, change back to the original safety clearance value after copper plating).

Teardrop the pads after wiring.

External grounding of metal case components and modules.

⑰ Place mounting and soldering pads.

⑱ DRC check is correct.


(4) Requirements for PCB layering


① The power plane should be close to the ground plane, tightly coupled to the ground plane, and arranged below the ground plane.

② The signal layer should be adjacent to the internal electrical layer and should not be directly adjacent to other signal layers.

③ Isolate digital circuits from analog circuits. If conditions permit, layer analog signal lines and digital signal lines and use shielding measures; if you need to arrange them on the same signal layer, you need to use isolation and ground lines to reduce interference. Ground and ground should be isolated from each other and cannot be mixed.

④ The high-frequency circuit has a large external interference. It is best to arrange it separately. Use an intermediate signal layer directly adjacent to the internal electrical layer for transmission. In order to reduce the external interference, use the copper film of the internal electrical layer.