PCB Technical - Pay attention to the essentials of detail in PCB drawing

Wiring is detailed and limited in the PCB design process. Even engineers who have laid wiring for more than ten years often feel that they are not good at wiring, because they have seen all kinds of problems and know what bad consequences this wire will lead to when it is laid out. Therefore, they do not know how to lay. But ace still has, they are having very rational knowledge, taking the emotion that a few ego create to go wiring again at the same time, the line that cloth comes out is very beautiful have artistic feeling.

Here are some good wiring tips and essentials:

First of all, a basic introduction is made. The number of PCB layers can be divided into single layer, double layer and multi-layer. Single layer is basically eliminated now. Double deck board what sound system uses now is quite much, it is to regard the result as commonly put rough model board child, multi-layer board points to 4 reach the board of 4 above namely, to the density requirement of component not tall will tell 4 layers is enough commonly. From the Angle of through hole can be divided into through hole, blind hole, and buried hole. A through-hole is a hole that goes directly from the top to the bottom; The blind hole is worn from the top or bottom hole to the middle layer, and then it does not continue to be worn. This advantage is that the position of the hole is not blocked from the beginning to the end, and other layers can still walk on the position of the hole. The buried hole is this hole that goes through the mesosphere to the mesosphere, is buried, the surface is completely invisible.

Before automatic wiring, wiring with high requirements of the interactive line in advance, input and output side line should not be adjacent parallel, to avoid reflection interference. If necessary, ground cables can be used for isolation, and the wiring of two adjacent layers should be perpendicular to each other, because parallel layers tend to produce parasitic coupling. The distribution rate of automatic wiring depends on good layout, wiring rules can be set in advance, such as the number of bending lines, the number of through-holes, the number of steps, etc. It is to undertake exploration type wiring first commonly, connect short line quickly, pass maze type wiring again, the connection that wants cloth undertakes global wiring route optimization, it can disconnect the line that already cloth according to need and try to re - route again, improve overall wiring effect thereby.

For layout, one rule is to keep digital and analog as separate as possible, and one rule is to keep low speed away from high speed. The basic principle is to separate digital grounding and analog grounding. Digital grounding is a switching device, and the current is very large at the moment of the switch, and very small when it does not move. Therefore, digital grounding can not be mixed with analog grounding. A recommended layout could look like the one below.

1. Precautions for wiring between power supply and ground wire

(1) To add decoupling capacitance between the power supply and ground wire. Be sure to connect the power supply to the pin of the chip after the decoupling capacitor, the following figure lists several wrong connection method and a correct connection method, we refer to the next, is there such a mistake? Decoupling capacitor generally has two functions: one is to provide the chip with instantaneous large current, and the other is to remove the power supply noise. On the one hand, the noise of the power supply should be minimized to affect the chip, and on the other hand, the noise generated by the chip should not affect the power supply.

(2) try to widen the power supply and ground wire, ground wire is wider than the power line, the relationship is: ground wire & GT; The power cord & gt; Signal lines.

(3) can use a large area of copper layer as ground, in the printed board is not used in the place are connected with the ground, for ground use, or made of multi-layer, power supply, ground each occupy a layer.

Second, digital circuit and analog circuit mixing processing

Nowadays, many PCBS are no longer single-function circuits, but are made up of a mixture of digital and analog circuits, so the interference between them needs to be considered when routing, especially the noise interference on the ground.

Due to the high frequency digital circuits, analog circuit sensitivity is strong, for signal lines, high frequency signal as far as possible away from the sensitive analog device, but for the whole PCB, PCB ground wire to the outside world nodes can have only one, so must be within the PCB processing, digital circuit and analog circuit problems, and within the circuit board, The ground of the digital circuit and the ground of the analog circuit are actually separate, only at the interface (plug, etc.) where the PCB is connected to the outside world. The ground of the digital circuit is a little short of the ground of the analog circuit, please note that there is only one connection point, there are also incommon ground on the PCB, this depends on the system design.

Three, the processing of the line corner

Normally there will be a change in thickness at the corner of the line, but when the line diameter changes, there will be some reflection phenomenon. Corner mode for line thickness variation, right Angle is bad, 45 degrees is better, rounded corner is. However, rounded corners are troublesome to PCB design, so it is generally determined by the sensitivity of the signal. Generally, a 45 degree Angle is enough for the signal, and only those very sensitive lines need rounded corners.

Four. Check the design rules after laying the line

No matter what we do, we should check it after we finish it, just like we should check our answers if we have time left in the exam, which is an important way for us to get high marks, and it is the same for us to draw PCB boards. In this way, we can be more sure that the circuit boards we draw are qualified products. Our general inspection has the following aspects:

(1) whether the distance between line and line, line and component pad, line and through-hole, component pad and through-hole, through-hole and through-hole is reasonable, whether to meet the production requirements.

(2) Whether the width of the power cord and ground cable is appropriate, whether the power supply and ground cable are tightly coupled (low wave impedance), and whether there is room in the PCB for the ground cable to be widened.

(3) Whether measures are taken for key signal lines, such as short length, protective lines, input lines and output lines are clearly separated.

(4) Analog circuit and digital circuit part, whether there are independent ground wire.

(5) Whether the graphics (such as ICONS and notations) added to PCB will cause signal short circuit.

(6) Modify some unsatisfactory lines.

(7) Whether the process line is added on the PCB, whether the resistance welding meets the requirements of the production process, whether the resistance welding size is appropriate, and whether the character mark is pressed on the welding pad of the device, so as not to affect the quality of electric equipment.

(8) Whether the outer frame edge of the power supply layer in the multi-layer board is reduced, such as the copper foil exposed outside the board of the power supply layer is easy to cause short circuit.

All in all, the above skills and methods are experiences, which are worth learning when we draw PCB board. In the process of drawing PCB, in addition to the skilled use of drawing tools, we should also have solid theoretical knowledge and rich practical experience, which can help you to complete your PCB map quickly and effectively. But there is also a very important point, that is, we must be careful, no matter wiring or overall layout every step should be very careful and serious, because a small mistake you may lead to your final products become waste, and then can not find where the wrong, So we would rather spend more time on the drawing process to carefully check the details than to go back and check if something goes wrong, which might take more time. In short, the PCB process pays attention to the details.