PCB Tech - How to Design High Frequency PCB Board

How to Design High Frequency PCB Board

SMT circuit board is one of the indispensable components in surface mounting design. SMT circuit board is the support of circuit components and devices in electronic products. It implements the electrical connection between circuit components and devices. With the development of electronic technology, PCB boards are becoming smaller and denser, and PCB boards are increasing. Therefore, PCB boards are required to have higher and higher overall layout, anti-jamming ability, process and manufacturability.

The main steps of high frequency PCB board design

1: Draw a schematic diagram.

2: Component library creation.

3: Establish the network connection relationship between the schematic diagram and the components on the printed circuit board.

4: Wiring and layout.

5: Create PCB production usage data and label production usage data.

The following issues should be considered in the design of high frequency PCB board:

1. Make sure that the circuit schematic element graphics are consistent with the actual objects and that the network connections in the circuit schematic are correct.

2. The design of high frequency PCB board not only considers the network connection relationship of the schematic diagram, but also some requirements of the circuit engineering. The requirements of the circuit engineering mainly include the width of power cord, ground wire and some other wires, the connection of lines, the high frequency characteristics of some components, the impedance and anti-interference of components, etc.

3. Installation requirements of the whole high frequency PCB board system, mainly considering installation holes, plugs, positioning holes, reference points, etc. to meet the requirements, the placement and accurate installation of various components in the prescribed location, at the same time, to facilitate installation, system debugging, ventilation and cooling.

4. For the manufacturability of high frequency PCB board and its technological requirements, it is necessary to be familiar with the design specifications and meet the production process requirements so that the designed high frequency PCB board can be produced smoothly.

5. Consider that the components are easy to install, debug and repair in production, and at the same time, the graphics, bonding pads, through holes on high frequency PCB board should be standardized to ensure that the components do not collide with each other and are easy to install.

6. The purpose of designing high-frequency PCB board is mainly to apply, so we should consider its practicability and reliability, and reduce the layer and area of high-frequency PCB board at the same time, so as to reduce costs. Appropriately larger pads, passes and passes are conducive to improving reliability, reducing passes, optimizing routes, making them compact, uniform and consistent. Make the overall layout of the panel more beautiful.

In order to achieve the desired purpose of the printed circuit board, the overall layout of high frequency PCB board and the placement of components play a key role, which directly affects the installation, reliability, ventilation and cooling, and the straightness of wiring of the entire printed circuit board.

Outer dimensions of printed circuit boards take precedence. When PCB size is too large, printed lines are long, impedance increases, noise resistance decreases, cost increases, heat dissipation is poor, and adjacent lines are susceptible to interference. Therefore, a reasonable positioning of PCB size and shape is given first. To determine the position of the special elements and the unit circuit, the whole circuit should be divided into several unit circuits or modules according to the flow of the circuit, and centered on the core components of each unit circuit (such as integrated circuits). The other components should be arranged on the PCB board in a certain order uniformly, neatly and compactly, but not too close to these large elements, and at a certain distance. Especially for larger, higher components, keep a certain distance around them to help with welding and repairing. For high power integrated circuits, the color heat sink should be considered, and enough space should be left for it and placed in the well ventilated and cool position of the printed board. Also, do not over-concentrate. Several large components should be placed on the same board at a certain distance, and in the direction of 45 angles, smaller integrated circuits such as SOP should be arranged along the axis, and resistive capacitive components should be arranged vertically and axially, all of which are relative to the transmission direction of the PCB production process. This enables the components to be arranged regularly, thereby reducing the defects that occur in the welding. Light-emitting diodes for display should be placed on the edge of the printed circuit board because they are used for observation during application.

Some switches, fine-tuning elements, etc. should be placed in an easy-to-operate place. Distribution parameters between components should be considered in the same frequency circuit. Distribution parameters between components should be considered in the general high frequency circuit. The general circuit should arrange the components as parallel as possible, which is not only beautiful, but also easy to install and weld, and also easy to batch produce. Components located on the edge of the circuit board must be 3-5cm away from the edge. The thermal expansion coefficient, thermal conductivity, heat resistance and bending strength of PCB plates should be considered in order to avoid adverse effects on components or PCB in production.

After determining the position and shape of the components on the PCB, consider the wiring of the PCB.

With the position of the element, it is a principle to route the printed circuit board as short as possible according to the position of the element. Short route, small occupancy of channels and area, so that the straight-through rate will be higher. The wires at the input and output ends of the PCB board should try to avoid adjacent parallel lines, preferably with ground lines between the two lines. In order to avoid circuit feedback coupling. If the printed circuit board is a multilayer board, the direction of the signal line of each layer is different from that of the adjacent layers. For some important signal lines, it should be agreed with the line designer that the specially differential signal lines should run in pairs, try to keep them parallel, close to each other, and have little difference in length. All components on the PCB board minimize and shorten the wires and connections between components. The minimum width of the wires in the PCB board is mainly determined by the adhesive strength between the wires and the insulation layer substrate and the current value through them. When the copper foil thickness is 0.05mm and the width is 1-1.5mm, the temperature will not be higher than 3 degrees through 2A current. Width of 1.5mm lead can meet the requirements, for integrated circuits, especially digital circuits, usually 0.02-0.03mm is selected. Of course, as long as allowed, we use as wide as possible wires, especially power and ground wires on PCB boards, and the minimum spacing of wires is mainly determined by the worst-case interline insulation resistance and breakdown voltage. For some integrated circuits (IC), the spacing can be less than 5-8mm from a process perspective. Print guides generally have the smallest arcs at bends, avoiding routes that bend less than 90 degrees. In general, the wiring of the printed circuit board should be uniform, compact and consistent. Avoid using large area of copper foil in the circuit as far as possible, otherwise, when heat is generated for too long time in the process of use, copper foil expansion and shedding will occur easily. If large area of copper foil must be used, grid wire can be used. The port of the wire is the pad. The center hole of the pad is larger than the lead diameter of the device. When the pad is too large, it is easy to form virtual welding. The outer diameter of the pad D is usually not less than (d+1.2) mm, where D is the aperture. For some components with higher density, the minimum diameter of the pad is preferable (d+1.0) mm. After the design of the pad is completed, the shape frame of the device should be drawn around the pad of the printed board, and the words and characters should be marked at the same time. Normal text or frame height should be about 0.9 mm, line width should be about 0.2 mm. And do not press the text and character contours on the pad. If it's a double-deck, the bottom characters should mirror the label.

In order to make the designed product work better and effectively, the anti-jamming capability of PCB has to be considered in the design, and it is closely related to the specific circuit.

The design of power cord and ground wire in the circuit board is especially important. Depending on the size of the current flowing through the circuit board, increase the width of the power cable as much as possible to reduce the resistance of the loop, and keep the power cable in line with the line direction and the direction of data transmission. It helps to enhance the anti-noise ability of the circuit. There are both logical and linear circuits on the PCB to separate them as far as possible. Low frequency circuits can be grounded in single-point parallel connection. Actual wiring can connect parts in series and then in parallel. High frequency circuits can be grounded in multiple-point series connection. The ground wire should be short and thick. For high frequency components, a large area of ground foil with raster can be used. The ground wire should be as thick as possible. If the ground wire is very thin, the ground potential changes with the current, which reduces the noise resistance. Therefore, the grounding wire should be enlarged so that it can reach the allowable current three times higher than that on the printed circuit board. If the grounding wire is designed to be 2-3mm or more in diameter, most of the grounding wires in digital circuits can be looped to improve noise resistance. In the design of PCB, it is common to configure appropriate decoupling capacitance at key parts of the printed circuit board. The electrolytic capacitance of 10-100uF connected across the line at the input end of the power supply, generally near the 20-30 pins, should be equipped with a 0.01PF ceramic capacitor. Generally near the pins of integrated circuit chips with 20-30 pins, a 0.01PF magnetic capacitor should be installed. For larger chips, there will be several pins, it is best to add a decoupling capacitor near them. A chip with more than 200 feet will have at least two uncoupling capacitors on each side. If the gap is insufficient, a 1-10PF tantalum capacitor can also be arranged on 4-8 chips. For the components with weak anti-jamming ability and great power change, the decoupling capacitor should be directly connected between the power cord and ground wire of the component, no matter which kind of lead to the capacitor is not too long.

After the component and circuit design of the circuit board is completed, the process design of the circuit board should be considered. The purpose is to eliminate all kinds of unfavorable factors before the start of production, and at the same time to consider the manufacturability of the circuit board, so as to produce high-quality products and batch production.

Previously, when we talked about the positioning and wiring of components, we already involved some aspects of the process of the circuit board. The process design of the circuit board is to organically assemble the circuit board and components designed by us through the SMT production line, so as to achieve good electrical connection to achieve the location layout of the products we designed. Welding pad design, wiring and anti-interference also need to consider whether the board we designed is easy to produce, whether it can be assembled with the modern assembly technology-SMT technology, and at the same time, the design height should be achieved in the production so as not to cause undesirable products. There are the following specific aspects:

1). Different SMT production lines have different production conditions, but in terms of PCB size, PCB veneer size is not less than 200*150 mm. If the long edge is too small, you can use a jigsaw, and the ratio of length to width is 3:2 or 4:3 PCB surface size is larger than 200 * When 150 mm, the mechanical strength of the printed circuit board should be taken into account.

2. When the size of the printed circuit board is too small, it is difficult for SMT whole line production process and not easy for batch production. The best way to use the form of collage is to combine two, four and six single boards according to the size of the single board to form a whole board suitable for batch production, and the size of the whole board should be suitable for the size of the clippable range.

3). In order to fit the production line, the veneer should have a 3-5mm range without any components, and the veneer should have 3-8mm process edge. There are three forms of connection between process edge and PC B: A without edge, B with edge, B with separation slot, C with edge and no separation slot. There is a blanking process to build up the country. Depending on the shape of the PCB board, different forms of puzzle can be applied for the purpose. For PCB process edges, according to the positioning method of different models, some need to have positioning holes on the process edge, the diameter of holes is 4-5cm, which is higher than edge positioning accuracy in comparison. Therefore, for PCB processing models with positioning holes, positioning holes should be set, and the hole design should be standard, so as to avoid inconvenience to production.

4). In order to better position and achieve higher mounting accuracy, setting a benchmark for PCB directly affects the batch production of SMT production line. The shape of the datum point can be square, circular, triangle, etc. And the diameter should be in the range of 1-2mm, 3-5mm around the datum point, without any components or wires. At the same time, the datum point should be smooth and flat, without any pollution. The design of the datum should not be too close to the edge of the board, but should be 3-5mm apart.

5). From the overall production process, the shape of the board is best pitch, especially for wave soldering. Rectangles are easy to transfer. If there are slots in the PCB board, slots are allowed in the form of process edges for a single SMT board. However, the slot is not too large and should be less than 1/3 of the length of the edge.

In the high frequency PCB board design, the power supply is designed as one layer. In most cases, it is much better than the bus design, so the circuit can always follow the path with the smallest impedance. In addition, the power board must provide a signal loop for all signals generated and received by the PCB, which can minimize the signal loop, thereby reducing noise. Low-frequency circuit designers often ignore these noises.
In high-frequency PCB design, we should follow the following principles:
The unity and stability of power and the ground.
Careful consideration of wiring and proper termination can eliminate reflections.
Careful consideration of wiring and proper termination can reduce capacitance and perceived crosstalk.
Noise suppression is required to meet EMC requirements.

High-frequency circuit board manufacturing material requirements:

1. The dielectric loss (Df) must be small, which mainly affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss.
2. If the water absorption rate is low, the high water absorption rate will affect the dielectric constant and dielectric loss.
3. The dielectric constant (DK) must be small and stable. Generally, the smaller the signal, the better the signal transmission rate, which is inversely proportional to the square root of the material's dielectric constant. High dielectric constant can easily cause signal transmission delay.
The thermal expansion coefficient of copper foil is consistent with the thermal expansion coefficient of copper foil, because the inconsistency in the process of cold and heat changes will cause the copper foil to separate.
Generally, a high-frequency board can be defined as a frequency above 1 GHz. Currently, the high frequency is a medium matrix of fluorine, such as polytetrafluoroethylene (PTFE) commonly known as Tefluron.
Matters needing attention in high frequency circuit board processing:
1. The impedance control requirements are strict, the relative line width control is very strict, and the general tolerance is about 2%.
2. Due to the special plate, the adhesion of PTH is not high, so it is usually necessary to roughen the holes and the surface with plasma treatment equipment to increase the adhesion of PTH. Hole copper and welding resistance ink.
3. Do not grind the plate before welding, otherwise the adhesion will be poor, only micro-corrosion solution and other roughening can be used.
4. The plates are mostly made of PTFE, and ordinary milling cutters will have a lot of edges when they are formed. Spe cial milling cutters.
High frequency circuit board is a special circuit board with high electromagnetic frequency. Generally, high frequency can be defined as a frequency higher than 1 GHz. Its physical properties, precision and technical parameters are very high. It is often used in automobile collision avoidance systems, satellite systems, radio systems and other fields.

In a word, the generation of bad products is possible in every link, but in this link of PCB design, we should consider from all aspects, so that we can achieve the purpose of designing this product very well, and try our best to design high-quality high-frequency PCB board to minimize the possibility of bad products in mass production which is suitable for SMT production line.