6 PCB Design Skills

- Aug 04, 2018-

1. Design process from schematic diagram to PCB

Set up component parameters -> input principle network table -> design parameters setting -> manual layout -> manual wiring -> verification design -> review ->CAM output.

2. Parameter setting

Adjacent conductor spacing must meet electrical safety requirements, and should be as wide as possible for ease of operation and production.The minimum spacing should be at least suitable for the withstand voltage. When the wiring density is low, the signal line spacing can be appropriately increased. For the signal line with a large difference between high and low level, it should be as short as possible and the spacing should be increased.

The distance between the edge of the inner hole of the welding plate and the edge of the printing plate is more than 1mm, which can avoid the defect of the welding plate during processing.When the wire connecting to the welding plate is thin, the connection between the welding plate and the wire line should be designed as water drop shape. This advantage is that the welding plate is not easy to peel, but the wire running and the welding plate are not easy to break.

3. Component layout

The practice shows that even if the circuit schematic diagram is designed correctly and the printed circuit board is not designed properly, the reliability of electronic equipment will be adversely affected.

For example, if the two thin parallel lines of the printing plate are very close together, the delay of the signal waveform will be formed and reflected noise will be formed at the end of the transmission line.Due to the inconsiderate interference caused by the power supply and ground wire, the performance of the product will be decreased. Therefore, when designing the printed circuit board, the correct method should be adopted.

Each switching power supply has four current loops: the charge switch ac loop the overall output ac loop the input loop of the input signal source current loop the output load current loop the input loop

Through an approximate dc current to charge the input capacitance, the filter capacitance mainly plays a broadband energy storage role.Similarly, the output filter capacitor is used to store high frequency energy from the output rectifier while eliminating the dc energy from the output load circuit.

Therefore, the connection end of the input and output filter capacitors is very important. The input and output current circuits should be connected to the power only from the connection end of the filter capacitor.If the connection between the input/output circuit and the power switch/rectifier circuit cannot be directly connected to the terminal of the capacitor, the ac energy will be radiated to the environment by the input or output of the filter capacitor.

The ac circuit of the power switch and the rectifier consists of high amplitude trapezoidal current, which has a high harmonic component. The frequency is much higher than that of the switching base frequency, and the peak amplitude can be up to 5 times of the continuous input/output dc current, and the transition time is usually about 50ns.

The two circuits are most likely to produce electromagnetic interference, so must the other printed wiring in power source to cloth before these ac circuits, each loop three main components of the filter capacitor, the power switch or rectifier, inductor or transformer shall be placed adjacent to each other, adjust the current path between the element position make them as short as possible.

The best way to establish a switch power distribution is similar to its electrical design. The optimal design process is as follows:

2. Design the power switch current circuit; 3. Design the output rectifier current circuit; 4. Control circuit connected to the ac power circuit

When designing the input current source circuit and input filter and designing the output load circuit and output filter, the following principles should be followed when designing the layout of all components of the circuit according to the functional units of the circuit:

The dimensions of the PCB should be considered first.When the PCB size is too large, the printing line is long, the impedance increases, the anti-noise ability decreases, and the cost increases.Too small is not good heat dissipation, and adjacent lines are susceptible to interference.The optimal shape of the circuit board is rectangular, with the aspect ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm from the edge of the circuit board

The latter welding should be considered when placing the device, not too dense

The configuration revolves around the core elements of each functional circuit.Components should be uniformly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between components, and the decoupling capacitance should be as close as possible to the VCC of the device

The distribution parameters between components of a circuit that operates at high frequencies should be considered.In general, the components should be arranged as parallel as possible.In this way, not only beautiful, but also easy to install and weld, easy to batch production

The position of each functional circuit unit is arranged in accordance with the flow of the circuit to facilitate the flow of signals and to keep the signals in the same direction as possible

The primary principle of the configuration is to ensure the wiring distribution rate. When moving the device, pay attention to the connection of the flying wire and put the connected devices together

The loop area is minimized to suppress the radiation interference of the switching power supply

4. The wiring

The switching power supply contains high frequency signals, and any printed line on the PCB can play the role of antenna. The length and width of the printed line will affect its impedance and reactance, thus affecting the frequency response.Even the printed lines that pass through the dc signal are coupled from the adjacent printed lines to the rf signal and cause circuit problems (and even radiate interference signals again).

All printing lines that pass through the ac current should therefore be designed as short and wide as possible, which means that all components connected to the printing line and to other power lines must be placed very close together.

The length of the printed line is proportional to its inductance and impedance, while the width is inversely proportional to the inductance and impedance of the printed line.The length reflects the wavelength of the printed line's response. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, the more radiofrequency energy it can radiate.

According to the current of printed circuit board, try to add the width of leased power line to reduce the circuit resistance.At the same time, make the power line, ground line and the direction of current consistent, so as to enhance the anti-noise ability.

Grounding is the bottom branch of the four current circuits of the switching power supply. As a common reference point of the circuit, it plays an important role and is an important method to control the interference.Therefore, the grounding wires should be carefully considered in the layout. Mixing various grounding wires will result in unstable power supply.

5. Check the

Wiring design is completed, it is necessary to carefully check the wiring design by the designers is in line with the rules, rules at the same time also need to confirm whether accord with the demand of the PCB production process, general inspection line to line, line and element bonding pad, the line and communicating pores, element bonding pad and communicating pores, through hole and the distance between the through hole is reasonable, whether to meet the production requirements.

Whether the width of the power cord and ground wire is appropriate, and whether there is still room for the ground wire to be widened in the PCB.Note: some errors can be ignored. For example, some of the Outline of the plug-in are placed outside the box.In addition, after each modification of the wire and hole, copper should be re-coated.

According to the "PCB inspection table", the review includes design rules, layer definition, line width, spacing, welding disc and hole setting, as well as the key review of the rationality of device layout, power supply and ground wire network routing, high-speed clock network routing and shielding, decoupling capacitor placement and connection, etc.

6. Design output

Notes for the output of optical drawing files:

Output layers include the wiring layer (bottom layer), the screen printing layer (including top layer screen printing, bottom layer screen printing), the welding resistance layer (bottom layer welding resistance), the drilling layer (bottom layer), and the NC Drill file (NC Drill).

When setting up the Layer of screen printing Layer, do not select Part Type. Select Outline, Text and Line of top Layer (bottom Layer) and screen printing Layer

When setting each Layer, select the Board Outline. When setting the Layer of screen printing Layer, do not select Part Type. Select the Outline and Text of top Layer (bottom Layer) and screen printing Layer