PCB wiring is very important in the entire PCB design. How to achieve fast and efficient wiring and make your PCB wiring look tall is worth studying. Sorted out the 7 aspects that need to be paid attention to in PCB wiring, and come to check the omissions and fill the vacancies!

1. Common ground processing of digital circuit and analog circuit

Many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground wire. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated and they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

2. The signal line is laid on the electric (ground) layer

In the multi-layer printed board wiring, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.

3. Treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of common components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding and assembly of components, such as: ① Welding requires high-power heaters. ②It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multilayer board is the same.

4. The role of the network system in cabling

In many CAD systems, wiring is determined based on the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. So there must be a reasonable grid system to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integral multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.

5. Treatment of power supply and ground wire

Even if the wiring in the entire PCB board is completed very well, the interference caused by the improper consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the power supply and ground wire should be taken seriously, and the noise interference generated by the power supply and ground wire should be minimized to ensure the quality of the product. Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is expressed: it is well-known to add the noise between the power supply and the ground wire. Lotus capacitor. Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire “power wire” signal wire, usually the signal wire width is: 0.2 ~ 0.3mm, the finest width can reach 0.05 ～0.07mm, the power cord is 1.2～2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, a ground net can be used (the ground of the analog circuit cannot be used in this way). A large area of ​​copper layer is used as a ground wire, which is not used on the printed board. Connected to the ground as a ground wire in all places. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.

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 formulated by the designer, and at the same time, it is necessary to confirm whether the established rules meet the requirements of the printed board production process. The general inspection has the following aspects: line and line, line Whether the distance between the component pad, line and through hole, component pad and through hole, and through hole and through hole is reasonable and whether it meets the production requirements. Is the width of the power line and the ground line appropriate, and is there a tight coupling between the power line and the ground line (low wave impedance)? Is there any place in the PCB where the ground wire can be widened? Whether the best measures have been taken for the key signal lines, such as the shortest length, the protection line is added, and the input line and output line are clearly separated. Whether there are separate ground wires for analog circuit and digital circuit. Whether the graphics (such as icons and annotations) added to the PCB will cause signal short circuit. Modify some undesirable line shapes. Is there a process line on the PCB? Whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character logo is pressed on the device pad, so as not to affect the quality of the electrical equipment. Whether the outer frame edge of the power ground layer in the multi-layer board is reduced, if the copper foil of the power ground layer is exposed outside the board, it is easy to cause a short circuit.

7. Via design

Via is one of the important components of multi-layer PCB, and the cost of drilling usually accounts for 30% to 40% of PCB manufacturing cost. Simply put, every hole on the PCB can be called a via. From the point of view of function, vias can be divided into two categories: one is used for electrical connections between layers; the other is used for fixing or positioning devices. In terms of process, vias are generally divided into three categories, namely blind vias, buried vias and through vias.

Blind holes are located on the top and bottom surfaces of the printed circuit board and have a certain depth. They are used to connect the surface line and the underlying inner line. The depth of the hole usually does not exceed a certain ratio (aperture). Buried hole refers to the connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the circuit board. The above-mentioned two types of holes are located in the inner layer of the circuit board, and are completed by a through-hole forming process before lamination, and several inner layers may be overlapped during the formation of the via. The third type is called a through hole, which penetrates the entire circuit board and can be used for internal interconnection or as a component mounting positioning hole. Because the through hole is easier to realize in the process and the cost is lower, it is used in most printed circuit boards instead of the other two kinds of through holes. The following via holes, unless otherwise specified, are considered as via holes.

1. From a design point of view, a via is mainly composed of two parts, one is the drill hole in the middle, and the other is the pad area around the drill hole. The size of these two parts determines the size of the via. Obviously, in high-speed, high-density PCB design, designers always hope that the smaller the via hole is, the better, so that more wiring space can be left on the board. In addition, the smaller the via hole, the parasitic capacitance of its own. The smaller it is, the more suitable it is for high-speed circuits. However, the reduction in hole size also brings about an increase in cost, and the size of vias cannot be reduced indefinitely. It is restricted by process technologies such as drilling and plating: the smaller the hole, the more drilling The longer the hole takes, the easier it is to deviate from the center position; and when the depth of the hole exceeds 6 times the diameter of the drilled hole, it cannot be guaranteed that the hole wall can be uniformly plated with copper. For example, the thickness (through hole depth) of a normal 6-layer PCB board is about 50Mil, so the minimum drilling diameter that PCB manufacturers can provide can only reach 8Mil.

Second, the parasitic capacitance of the via hole itself has a parasitic capacitance to the ground. If it is known that the diameter of the isolation hole on the ground layer of the via is D2, the diameter of the via pad is D1, and the thickness of the PCB board is T, The dielectric constant of the board substrate is ε, and the parasitic capacitance of the via is approximately: C=1.41εTD1/(D2-D1) The main effect of the parasitic capacitance of the via on the circuit is to extend the rise time of the signal and reduce The speed of the circuit.

3. Parasitic inductance of vias Similarly, there are parasitic inductances along with parasitic capacitances in vias. In the design of high-speed digital circuits, the damage caused by parasitic inductances of vias is often greater than the impact of parasitic capacitance. Its parasitic series inductance will weaken the contribution of the bypass capacitor and weaken the filtering effect of the entire power system. We can simply calculate the approximate parasitic inductance of a via with the following formula: L=5.08h[ln(4h/d)+1] where L refers to the inductance of the via, h is the length of the via, and d is the center The diameter of the hole. It can be seen from the formula that the diameter of the via has a small influence on the inductance, and the length of the via has the greatest influence on the inductance.

4. Via design in high-speed PCB. Through the above analysis of the parasitic characteristics of vias, we can see that in high-speed PCB design, seemingly simple vias often bring great negatives to circuit design. effect.