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PCB Layout
gesptechnologypcbMay 31, 2017 Blog 0 Comment327views

In the PCB design, the wiring is a crucial step to complete the product design, and it can be said that the preparatory work is done for it, in the PCB, the wiring design process to limit the highest, the most excellent techniques, the largest workload. PCB wiring with single-sided wiring, double-sided wiring, and multi-layer wiring. There are two ways of wiring: automatic routing and interactive wiring, in the automatic routing, you can use the interactive pre-string on the more stringent requirements, the input and output side of the line should be avoided parallel to avoid reflection interference. If necessary, should be added to ground isolation, two adjacent layers of wiring to each other perpendicular to the parallel prone to parasitic coupling.

The wiring rate of the automatic wiring depends on the good layout, and the wiring rules can be set in advance, including the number of times of alignment, the number of vias, the number of steps, and the like. Generally first to explore the type of cloth warp, quickly connected to the short-term, and then the maze-type wiring, the first cloth to the wiring of the overall routing optimization, it can be broken according to the need to cloth line. And try to re-wiring to improve the overall effect. The current high-density PCB design has felt that the through-hole is not well adapted, it wastes a lot of valuable wiring channels, in order to solve this contradiction, there blind hole, and buried hole technology, it not only completed the role of vias But also save a lot of wiring channels to make the wiring process is more convenient, more fluid, more perfect, PCB board design process is a complex and simple process, in order to master it, need the majority of electronic engineering design Personnel to their own experience, in order to get the true meaning of which.

1 power, ground handling

Even if the wiring in the entire PCB board is done very well, but due to power, ground considerations are not thoughtful interference caused by the performance of the product will decline, and sometimes even affect the success rate of the product. So the electricity, ground wiring to be taken seriously, the electricity, ground noise generated by the interference to a minimum, to ensure product quality.

For each engineer engaged in the design of electronic products are aware of the noise between the line and the power supply of the reasons, only to reduce the type of suppression noise to express:

  1. It is well known to add a decoupling capacitor between the power supply and the ground.
  2. As far as possible to widen the power, ground width, preferably ground than the power line width, their relationship is ground> power line> signal line, usually, signal line width: 0.2 ~ 0.3mm, Fine width of up to 0.05 ~ 0.07mm, the power line is 1.2 ~ 2.5mm
  3. The PCB of the digital circuit can use a wide wire to form a loop, that is, to form a ground to use (analogue circuit can not be used) with a large area of copper for the ground line, in the printed circuit board has not been used on the ground are connected with the ground as a ground. Or made of multi-layer board, power, ground each occupied a layer.

2 digital circuit and analogue circuit common ground processing

Now there are many PCB is no longer a single functional circuit (digital or analogue circuit) but by the digital circuit and analogue circuit mix. So when wiring needs to consider the interference between them, especially on the ground noise interference.

Digital circuit frequency is high, the sensitivity of the analog circuit is strong, the signal line, the high-frequency signal line as far as possible away from the sensitive analog circuit devices, the ground, the whole PCB on the outside world only one node, so Must be handled within the PCB number, the problem of the common ground, and in the board of digital and analog ground is actually separated from each other between them are not connected, but in the PCB and the external interface (such as plugs, etc.). Digital and analogue to a little short, please note that there is only one connection point. There are also on the PCB on the ground, which is determined by the system design. 3 signal lines on the electric (ground) layer

In the multi-layer printed circuit board wiring, because the signal line layer is not finished cloth line left is not much, and then add more layers will cause waste will also increase the production of a certain amount of work, the cost has increased accordingly To solve this contradiction, you can consider wiring in the electrical (ground) layer. First consider the power layer, followed by the formation. Because it is best to preserve the integrity of the formation.

4 large area conductor in the treatment of connecting legs

In a large area of the ground (electricity), the commonly used components of the legs connected to the connection legs to deal with the need for comprehensive consideration, the electrical performance, the component legs of the pad and copper surface is better, but the Components of the welding assembly there are some bad risks such as ① welding need high-power heater. ② easy to cause virtual solder joints. So taking into account the electrical performance and process needs, made of cross-pad, known as the heat shield (heat shield) commonly known as the thermal pad (Thermal), so that the welding can be too much heat due to cross-section of the potential Greatly reduced. Multilayer board of the power (ground) layer of the same treatment.

5 cabling in the role of the network system

In many CAD systems, cabling is determined by the network system. Grid is too dense, although the channel has increased, the pace is too small, the figure of the amount of data is too large, which must have a higher storage requirement of the equipment, but also the object of computer electronic products operating speed Great influence. And some passages are ineffective, such as those occupied by the pads of the component legs or by the holes, which are occupied by the holes. The grid is too sparse, the passage of too little impact on the cloth rate. So there must be a dense and reasonable grid system to support the wiring.

The distance between the legs of the standard component is 0.1 inch (2.54 mm), so the foundation of the grid system is generally set to an integer multiple of 0.1 inches (2.54 mm) or less than 0.1 inches, such as 0.05 inch, 0.025 inches, 0.02 Inch and so on.

6 Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer. It also needs to confirm whether the rules are in line with the requirements of the PCB production process. The general inspection has the following aspects:

  1. Line and line, line and component pad, wire and through-hole, component pad and through-hole, through-hole, and through the hole between the reasonable, whether to meet the production requirements.
  2. Is the width of the power and ground lines appropriate and whether the power supply is tightly coupled to the ground (low impedance)? Whether there is a place where the ground can be widened.
  3. Whether the key signal line to take the best measures, such as the shortest length, plus protection lines, input lines, and output lines is clearly separated.
  4. Analog circuits, and digital circuit parts, whether there is a separate ground.
  5. After the graphics in the PCB (such as the icon, marked) will cause the signal short circuit.
  6. To modify some of the unsatisfactory lines.
  7. Whether the process line on the PCB? Whether the resistance to meet the requirements of the production process, resistance to welding size is appropriate, whether the character mark pressure device pads, so as not to affect the quality of electronic equipment.
  8. Whether the edge of the outer edge of the power plant in the multilayer board is reduced, such as the copper foil exposed to the power source is liable to cause a short circuit.

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