In PCB design, the layout is an important step in the completion of product design, it can be said in front of all the preparatory work for it to do, the entire PCB in order to limit the maximum wiring design process, the finest technique, the maximum workload. There are single-sided PCB layout wiring, double-sided and multi-layer wiring wiring.
Wiring there are two ways: automatic routing and interactive routing, automatic routing before you can use the interactive advance of more stringent requirements for wiring, the input and output terminal adjacent parallel edges should be avoided to avoid reflection interference. If necessary add ground isolation, two wiring layers adjacent to each other vertically, parallel prone to parasitic coupling.
Routability automatic routing, depends on good placement, routing rules can be set in advance, including bending frequency alignment, the number of vias, step number and the like. Usually the first to explore the warp cloths quickly to short-term communication, then the labyrinth wiring, put the cloth to make global connection wiring path optimization, it can disconnect the wire cloth have been needed. And try to re-wiring, to improve the overall effect.
The current high-density PCB design has been felt through holes do not adapt, and it wasted a lot of valuable routing channels, in order to solve this problem, there has been blind and buried vias technology, which not only completed the vias role many also save a routing channel the wiring process to complete a more convenient, more smooth, more perfect, PCB board design process is a complex and simple process, to a good grasp of it, the majority of electronic engineering design needs personnel to their own experience, to get the true meaning.
1 Power, ground handling
Even throughout the PCB wiring board finish was very good, but due to the interference power, ground caused by thoughtless, product performance will decline, and sometimes even affect the success rate. So the power ground wiring to be taken seriously, the noise interference, ground produced a minimum, in order to ensure product quality.
For each product in the electronic design engineering staff understand the reason between the ground and the noise generated by power lines, now only reduces noise suppression formula to make statements:
1. It is well known among plus power and ground decoupling capacitors.
2. Try to widen the power, ground width, it is better than the power ground line width, their relationship is: Ground> power cables> signal lines, generally signal line width: 0.2 ~ 0.3mm, the smallest width of up to 0.05 ~ 0.07mm, the power cord is 1.2 ~ 2.5 mm
3. Available for PCB width digital circuit ground wire to form a loop, constitutes a ground network to use (analog circuit can not be used as such)
4. Copper layer with a large area for ground use on the PCB is not a place to spend connected to ground with a ground. Or made of plywood, power, ground floor of the occupation.
2 Total handle digital circuits and analog circuits
There are many PCB is no longer a single-function circuit (digital or analog), but by the digital and analog circuit mixed composition. Therefore, when the wiring need to consider issues of mutual interference between them, especially to line noise.
High-frequency digital circuits, analog circuits strong sensitivity of the signal line, the high-frequency signal lines as far away from the sensitive analog circuit devices on the ground, the entire PCB to the outside world is only one node, it must be in the PCB internally processed, analog common ground issues, and in the board internal digital and analog ground are actually disconnected between separate them, but in the PCB and interface connected to the outside world (such as plugs, etc.). Digital and analog ground a little short, please note that only one connection point. There are no common ground on the PCB, which is determined by the system design.
3 Signal wire in the power (ground) layer
When multilayer printed wiring, since no signal line layer fabric finish line is not much left over, then more layers will result in increased production of waste will give a certain amount of work, but also a corresponding increase in cost for the to counteract this, you can consider the electric wiring (ground) layer. We should first consider the use of a power supply layer, followed by the formation. Because the best is to retain the integrity of the formation.
4 LARGE AREA conductor connection leg
In large areas of ground (electric), a commonly used components connected thereto leg, the leg is connected to the processing needs to be a comprehensive consideration, the electrical performance, and the copper pad surface element leg full access as well, but welding assembly member there is some bad risks such as:
① welding requires high-power heater.
② likely to cause Weld point.
So both the electrical performance and technology necessary to make crosswise pad, called the thermal isolation (heat shield) commonly known as thermal pad (Thermal), this enables the welding due to excessive heat generated sectional Weld points may is greatly reduced. Multilayer same electrical connection (ground) layer leg treatment.
5 The role of network cabling system
In many CAD systems, network cabling system is the basis for the decision. Grid too dense, although access has increased, but the step is too small, too large data map field, which is bound to the storage space of the device have higher requirements, but also the object computer electronics computing speed great impact. And some passages are ineffective, such as by holes or mounting leg member occupies pad, fixed holes are occupied and the like. Grid too sparse, too little impact on the path routability great. So there must be a reasonable mesh density wiring system to support the conduct.
The distance between the legs of standard components is 0.1 inches (2.54mm), so that on the basis of the grid system is typically set to 0.1 inches (2.54 mm) or less than 0.1 inches of integer multiples, such as: 0.05 inches, 0.025 inches, 0.02 inch.
6 Design Rule Check (DRC)
After wiring design is complete, the need to carefully check the wiring design complies with the rules established by the designer, but also need to confirm the rules developed meets the needs of the PCB production process, generally check the following aspects:
1. Line to line, from the line and the component pad, line and through hole component pads and the through-hole, through-hole and through-hole between reasonable and whether to meet production requirements.
2. Power line and ground width is appropriate and whether the tight coupling (low impedance) between the power supply and ground? Is there a place to make ground in widening the PCB.
3. For critical signal lines whether to take the best practices, such as minimum length, unprotected line, line input and output lines are clearly separated.
Analog and digital circuitry portion, is there separate ground wire.
If we were added to the PCB graphics (such as icons, graphing) may cause short-circuit signal.
Some undesirable linear modified.
Whether on the PCB plus a processing line? Solder meets the requirements of the production process, solder size is appropriate, the character flag is pressed on the device pads, so as not to affect the quality of electrical equipment.
Edge of the frame in multilayer formation whether to reduce power, such as power strata exposed copper-board likely to cause a short circuit.