8 Layers High Frequency PCB

A. Board Shape, Size, Layer Number Determination

Any piece of printed circuit board, there are problems with other structural components with the assembly, so the shape and size of the printed circuit board, must be based on the structure of the entire product . However, from the production process point of view, it should be as simple as possible, generally for the aspect ratio is not too disparate rectangular, in order to facilitate assembly, improve productivity and reduce labor costs.

In terms of the number of layers, must be based on the requirements of the circuit performance, board size and line density. For multilayer printed circuit boards, four-layer boards, six-layer boards are most widely used, four-layer boards, for example, is two wire layers (component surface and welding surface), a power supply layer and a ground layer.

The layers of a multilayer board should be kept symmetrical, and it is preferable to have an even number of copper layers, i.e., four, six, eight, etc. Because of the asymmetric lamination, the board surface is prone to warpage, especially for surface mount multilayer boards, more attention should be paid.

B. Component Location and Placement Direction

Component location, placement direction, first of all, should be considered from the circuit principle, catering to the direction of the circuit. Placement of reasonable or not, will directly affect the performance of the printed circuit board, especially high-frequency analog circuits, the location of the device and placement requirements, appear more stringent. Reasonable placement of components, in a sense, has heralded the success of the printed circuit board design. Therefore, when starting to arrange the layout of the printed circuit board and deciding the overall layout, the circuit principle should be analyzed in detail, and the location of special components (such as large-scale ICs, high-power tubes, signal sources, etc.) should be determined first before arranging the other components, so as to avoid factors that may cause interference as much as possible.On the other hand, the overall structure of the printed board should be considered from the perspective of the overall structure of the printed board to avoid uneven arrangement of components, clutter. This not only affects the aesthetics of the printed board, but also brings a lot of inconvenience to the assembly and maintenance work.

C. Wire Cloth Layer, Wiring Area Requirements

In general, multi-layer printed circuit board wiring is carried out in accordance with the circuit function, in the outer layer of wiring, the requirements of more wiring in the welding surface, the component surface less wiring, which is conducive to the maintenance of the printed circuit board and troubleshooting. Fine, dense wires and signal lines susceptible to interference are usually arranged in the inner layer. Large areas of copper foil should be more evenly distributed in the inner and outer layers, which will help reduce the warpage of the board, but also make plating on the surface to obtain a more uniform coating. In order to prevent the shape of the process to injure the printed wires and mechanical processing caused by interlayer short circuit, the inner and outer wiring area of the conductive pattern from the edge of the board should be greater than 50mil distance.

D. Wire Alignment and Line Width Requirements

Multilayer board alignment to separate the power layer, ground layer and signal layer to reduce the power, ground, signal interference. Adjacent two layers of printed circuit board lines should be as perpendicular to each other or go diagonal lines, curves, can not go parallel lines, in order to reduce interlayer coupling and interference of the substrate. And the wire should try to go as short as possible, especially for small signal circuits, the shorter the line, the smaller the resistance, the smaller the interference. Signal lines on the same layer, change direction should avoid sharp corners. The width of the wire should be based on the circuit requirements for current and impedance to determine the power input line should be larger, the signal line can be relatively small. For general digital board, power input line width can be used 50 ~ 80mil, signal line width can be used 6 ~ 10mil.

Layout should also pay attention to the width of the line should be as consistent as possible, to avoid sudden thickening and sudden thinning of the wire, in favor of impedance matching.

E. Drill Size and Pad Requirements

Components on the multilayer board drilling size and the selected components pin size, drilling too small, will affect the device mounted on the insertion and tin; drilling too large, welding, solder joints are not full. Generally speaking, the component hole hole diameter and pad size is calculated as follows:

Component hole hole diameter = component pin diameter (or diagonal) + (10 ~ 30mil)

Component pad diameter ≥ component hole diameter +18mil

As for the aperture hole diameter, mainly by the thickness of the finished board to determine, for high-density multilayer boards, generally should be controlled in the board thickness: aperture diameter ≤ 5:1 range. Calculation method for the through-hole pads are.

Through-hole pad (VIAPAD) diameter ≥ through-hole diameter +12mil.

F. Requirements for Power and Ground Layer Partitions and Vias

For multilayer printed circuit boards, at least one power layer and a ground layer. As all the voltages on the printed circuit board are connected to the same power supply layer, so the power supply layer must be partitioned isolation, the size of the partition line is generally 20 ~ 80 mil line width is appropriate, the voltage is super high, the thicker the partition line.

Solder holes and power supply layer, ground layer connection, in order to increase its reliability and reduce the welding process of large-area metal heat absorption and false welding, the general connection disk should be designed into the shape of a flower hole.

Isolation pads hole diameter ≥ drill hole diameter + 20mil

G. Safety spacing requirements

Safety spacing should be set to meet the requirements of electrical safety. Generally speaking, the minimum spacing of the outer conductor shall not be less than 4 mils, the minimum spacing of the inner conductor shall not be less than 4 mils, in the case of the wiring can be lined up, the spacing should be as large as possible, in order to improve the yield of the finished board and reduce the risk of failure of the finished board.

8 requirements to improve the board's anti-interference capability.

Multi-layer printed circuit board design, but also must pay attention to the board's anti-jamming ability, the general approach:

1. a. In the IC's power supply, ground near the addition of filter capacitors, the capacity is generally 473 or 104.
2. b. For the sensitive signals on the printed circuit board, should be added to the accompanying shielding line, and the signal source near as little wiring as possible.
3. c. Select a reasonable grounding point.