PCB Tech - Understand several key points of RF PCB design

In electronic products and equipment, PCB is an indispensable component, which plays a role in the electrical and mechanical connection of the circuit system. How to arrange and combine the components in the circuit on the PCB according to certain requirements is one of the main tasks of PCB design. Layout design is not simply arranging the components on the PCB or connecting the circuits. Practice has proved that a good circuit design must have a reasonable layout of components, so that the circuit system can achieve stable and reliable work after physical combination.

Conversely, if the layout of the components is unreasonable, it will affect the working performance of the PCB, or even fail to work. Especially today, when integrated devices are widely used, if integrated circuits are still mounted in the form of wiring boards, not only the circuit is bulky, but also it cannot work stably. Therefore, in the product design process, layout design and circuit design have the same important position.

Here is a brief introduction to the precautions for RF PCB design.

1. Layout considerations

(1) Structural design requirements

The structure of the product needs to be clarified before the PCB layout. The structure needs to be reflected on the PCB (the contact part of the structure and the PCB, that is, the position and shape of the cavity shell). For example, the thickness of the outer side of the cavity shell, the thickness of the intermediate cavity, the size of the chamfering radius and the size of the screw on the cavity, etc. (in other words, the structural design is based on the outline (structure part) drawn on the completed PCB Specific design is carried out (if the structure has been moulded in batches, it is another matter) (the screw types are M2\M2.5\M3\M4, etc.).

In general, the thickness of the outer cavity is 4mm; the width of the inner cavity is 3mm (2mm for dispensing process); and the chamfer radius is 2.5mm. Taking the lower left corner of the PCB as the origin, the position of the compartment on the PCB must be an integer multiple of 0.5 grid point, and at least the grid point must be an integer multiple of 0.1. This is conducive to structural processing, and the error control is more accurate. Of course, this needs to be designed according to the type of specific product.

(2) Layout requirements

Prioritize the layout of the RF link, and then layout other circuits.

1. The precautions for the RF link layout are based on the sequence of the schematic diagram (input to output, including the sequence of each component and the spacing between components. The distance between some components should not be too large, Such as π network.) Carry out the layout, layout into a "one" shape or "L" shape.

In the actual RF link layout, due to the space constraints of the product, it is impossible to completely realize the "one"-shaped layout, which forces us to make the layout into a "U" shape. It is not impossible to arrange it in a U shape, but it is necessary to add a compartment in the middle to isolate it from the left and right, and to shield it. As for why we need to block this article, I won’t talk more about it.

There is also a need to add compartments in the lateral direction. That is, the in-line shape is isolated from the left and right by the compartment. This is mainly because the part that needs to be isolated is very sensitive or easily interferes with other circuits; in addition, there is another possibility that the gain of the line from the input end to the output end of the circuit is too large, and it needs to be separated by a cavity.

2. Chip peripheral circuit layout

The layout of the peripheral circuit of the radio frequency device strictly refers to the requirements on the datasheet, and can be adjusted due to space constraints (place as close as possible to the chip when the process requirements are guaranteed); the peripheral circuit layout of the digital chip will not be discussed.

If the structure has a metal bottom plate, try not to put any components on the contact surface of the PCB and the bottom plate, and avoid slotting on the metal bottom plate.

2. Wiring precautions

Layout according to the 50 ohm impedance line width (generally need to be used as a reference for the interlayer), try to lead out from the center of the pad, route the line in a straight line, and try to walk on the surface. Make a 45-degree angle or arc trace where you need to turn. It is recommended that the pads on both sides of the capacitor or resistor be used as the inflection point. If you meet the requirements of device wiring matching, please strictly follow the reference length and shape of the datasheet. For example, the length of the trace between an amplifier tube and the capacitor (or the length of the trace between the inductors) requirements and so on.

In the PCB design, in order to make the design of the high-frequency circuit board more reasonable and have better anti-interference performance, the following aspects should be considered (general practice):

(1) Reasonably choose the number of layers

When wiring high-frequency circuit boards in PCB design, using the middle inner plane as the power and ground layer can play a shielding role, effectively reducing parasitic inductance, shortening the length of signal lines, and reducing cross-interference between signals.

(2) Wiring method

The wiring must be turned at an angle of 45° or a circular arc, which can reduce the emission and mutual coupling of high-frequency signals, and reduce signal reflection.

(3) Cable length

The shorter the trace length, the better, and the shorter the parallel distance between the two lines, the better.

(4) Number of vias

The more the number of vias, the better.

(5) Wiring direction between layers

The wiring direction between layers should be vertical, that is, the top layer is the horizontal direction, and the bottom layer is the vertical direction, so that interference between signals can be reduced.

(6) Copper coating

Increasing the grounding copper can reduce the interference between signals.

(7) Land package

Packing the important signal lines can significantly improve the anti-interference ability of the signal. Of course, it is also possible to pack the interference source so that it cannot interfere with other signals.

(8) Signal line

The signal wiring cannot be looped and needs to be wired in a daisy chain manner.

Three, grounding treatment

(1) RF link grounding

The radio frequency part adopts multi-point grounding method for grounding treatment. The copper gap of the radio frequency link is generally used for 20mil to 40mil. Ground holes need to be drilled on both sides, and the spacing should be as consistent as possible. For the grounding pad of the grounding capacitance and resistance on the radio frequency path, make a grounding hole as close as possible. The grounding pads on the device need to be grounded via holes.

In order to make better contact between the cavity shell and the PCB board. Generally, two rows of grounding holes are punched and placed in a staggered manner.

The contact position between the PCB and the compartment needs to open a window,

The place where the ground copper on the bottom of the PCB is in contact with the bottom plate needs to be opened (windows are not allowed on this layer of signal lines) to make better contact.

In order to have a closer contact between the PCB and the base and the cavity shell (better shielding and heat dissipation), screw holes need to be placed on the PCB board.

Screw placement method between PCB cavity shells: Place a screw at each intersection of the compartment. In the actual design, it is difficult to realize, and it can be adjusted appropriately according to the function of the module circuit. However, there must be screws on the four corners of the cavity shell.

Screw placement method between PCB bases: each small cavity in the cavity shell needs screws, and the number of screws depends on the size of the cavity (the larger the cavity, the more screws are placed). The general principle is to place screws on the opposite corners of the cavity. Screws must be placed next to SMA heads or other connectors. The SMA head or connector will not deform the PCB board during the plugging process.