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Several important elements in the later check of PCB design
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Several important elements in the later check of PCB design

December 9, 2019

PCB Image:Several important elements in the later check of PCB design

When the routing in finished on one PCB Prototyping board and there is no error in both the connecting and the distance, does it mean that this piece of PCB board is finished. Of course, it is not true. Many beginners, including some experienced engineers, tend to be rash in the process and ignore the follow-up checking due to urgency in time, impatience or overconfidence. As a result, some basic bugs occur, for example, wires are not wide enough; silk screen of the component marks is printed on the holes; the socket is too near; loop occurs in the signal, and so on., which may lead to electrical or technique problems. In very serious cases, new boards need to be remade and wastes occur. Therefore, after the routing on the PCB board is finished, the another important step is the follow-up checking.

1. Components enclosing

1).The distance between the soldering pads. If it is a new component whose drawing needs to be worked out by oneself, a proper distance should be kept because the soldering distance can directly affect soldering between the components.

2).The size of the pore (if there are pores). For plug-in components, enough space should be kept for the size of the pores, which is generally no less than 0.2mm.

3).Outline silk screen. Silk screen of the outline should be larger than the practical one to ensure that components can be installed smoothly.

2. Layout

1).It is not appropriate to place IC near the edge of the board.

2).Components on the same circuits should be placed next to each other, for example, decoupling capacitance should be placed next to the power pin of IC; components from the same functions of the circuits should be placed in the same area with clear arrangements to ensure the functions are performed.

3).The position of the socket should be placed based on practical situations. Sockets are usually led to other parts through leads and assembled to the edge of the board on the basis of proximity and for ease of assembling.

4).The direction of socket. All the sockets have their own directions. Once assembled wrongly, all the wires need to be replaced and remade. For sockets which are plugged in the flat direction, the direction of the socket should face to the outside of the board.

5).No components should exist in the KeepOut area.

6).Interference source should be far away from sensitive circuits. High-speed signal, high-speed clock or high current switching signal all belongs to interference source, which should be far away from sensitive circuits, like reset circuits, simulation circuits. However, they can be separated by pavement.

3. Wiring

1).The size of wiring. The width of wires should be based on the specific technique and carrying capacity. The width should not be less than the minimum width required by PCB manufacturers. Meanwhile, in order to guarantee the current-carrying capacity, a length of 1mm/A tends to be suitable for the width.

2).Differentiating signal line. For differentiating line, such as, USB, Ethernet, routing lines should has about the same length, be parallel and on the same level, whose gap is decided by the resistance.

3).Pay attention to returning path of the high-speed wires. It is easy for high-speed wires to generate electromagnetic radiation. If the area formed by routing paths and returning paths is too large, a single-turn coil will be formed to radiate electromagnetic interference to the outside, as is shown in Image 1. Therefore, you should pay attention to the returning path in wiring. Power layers and ground board are set in the multi-layer board to effectively solve the problem.

4).Pay attention to the simulating signal. Simulating signal should be separated from digital signal, whose wiring should bypass the interference source(such as clock, DC-DC power), and the shorter the wires are, the better.

4. EMV and integrity of signal

1).Terminating resistor. High-speed wires or high-frequency and long digital signal lines should be matched with a capacitance at the ends.

2).Signal lines should be input and small capacitance should be connected. Signal lines should be input from the connectors which had better be near the connectors and be connected with small capacitance. The degree of capacitance should be determined by the signal strength and frequency, which should be too high, or it will affect the integrity of signal. For low-speed signal, such as the “enter” key, small capacitance of 6+ with 330pF can be chosen.

3).Drive capability. For switching signal of high drive current, triode drive can be installed. For the bus whose fan out capability is high, bumper drive can be added.

5. Silk screen

1).The name of the board, time and PN code.

2).Remark. For the pins of some connectors (in rows) or key signals, remarks should be made.

3).Components number. Components should be marked in the right place. For intensive components, marks can be placed in groups. Please be noted that components number shouldn’t be placed in the holes.

6. Others

Mark dot. For PCB which needs machine soldering, two to three Mark dots can be added.