Go PCB Layout and cabling experience

In the design, the layout is an important link. Layout results will directly affect the effect of the wiring, so you can think that a reasonable layout is the first step in PCB design success.

Layout of two kinds of ways, one is interactive layout, the other is automatic layout, generally in the automatic layout based on the interactive layout of the adjustment, in the layout can also be based on the situation of the door to the circuit redistribution, the two gate circuit to exchange, making it the best layout for easy wiring. After the layout is completed, the design files and related information can also be returned to the schematic diagram, so that the relevant information in the PCB and the schematic diagram, so that in the future of the document, change the design can be synchronized, while the simulation of the relevant information to update, so that the electrical performance and function of the circuit board-level verification.

Consider the overall aesthetics

A product's success or not, one is to pay attention to the intrinsic quality, the second is to take into account the overall aesthetic, both are more perfect to think that the product is successful.

On a PCB board, the layout of the components required to be balanced, dense and orderly, not top-heavy or Yitou.

Inspection of layouts

• Is the size of the printed board consistent with the size of the processing drawing? Can meet PCB manufacturing process requirements? Are there any positioning marks?
• Is there any conflict between the components in two-dimensional and three-dimensional space?
• is the layout of the components orderly and neatly arranged? Did you finish all the cloth?
• Can I replace the components that need to be replaced frequently? is the plug-in board convenient for inserting the device?
• Is there a proper distance between the thermal element and the heating element?
• Is it convenient to adjust the adjustable components?
• Do you have a radiator in place where heat is needed? Is air flow unobstructed?
• is the signal flow smooth and connected to the shortest possible?
• are plugs and sockets incompatible with mechanical design?

PCB cabling Experience (I.)

This is a matter of great concern. Aside from other factors, only in the PCB design process, I have the following points to understand, for your reference:

2. to have a reasonable direction : such as input/output, AC/DC, strong/weak signal, high/low frequency, high voltage/low pressure, etc..., their direction should be linear (or separate), not to mingle with each other. The aim is to prevent mutual interference. The best direction is the straight line, but generally not easy to achieve, the most unfavorable trend is the ring. For DC, small signal, low voltage PCB design requirements can be lower. So "reasonable" is relative. The direction of the line between the upper and lower layers is basically perpendicular. The whole board does not want uniform, can not squeeze in together.
4. Choose a good pick-up location : Small pick-up location I do not know how many engineers have done how many of the discussion, it is important. In general, the requirements of a common point, such as: The forward amplifier of a number of ground should be joined to the main line and so on .... In reality, it is difficult to fully do so by various restrictions, but we should try to follow it. The problem is quite flexible in practice. Everyone has their own set of solutions. It is easy to understand if the specific circuit board can be explained.
6. reasonable arrangement of power filter and decoupling capacitors : In general, only a few power filter and decoupling capacitors are drawn in the schematic, but they do not indicate where they are bell. In fact, these capacitors are for switching devices (gate circuit) or other needs to filter and decoupling parts, set up, the arrangement of these capacitors should be as close as possible to these components, too far away will not work. Interestingly, when the power filter and decoupling capacitance is properly arranged, the problem of the pick-up point appears less obvious. The decoupling capacitance of the SMD device is best in the device belly position on the other side of the board, Power and ground to first over capacitance, and then into the chip .
8. The line is fastidious : have the condition to do wide line never do fine, high-voltage and high-frequency line should park slippery, must not have sharp chamfer, turn also cannot adopt right angle. Ground should be as wide as possible, preferably using a large area of copper, this docking location problem has a considerable improvement.
10. Some problems occur in the post-production, but it is the PCB design brought, they are: too much wire hole, sinking copper process slightly inadvertently will bury the hidden trouble. Therefore, the design should be as far as possible to reduce the line hole. The parallel line density is too large to be easily connected to one piece during soldering. Therefore, the line density should be determined according to the level of welding process. The distance of the solder joint is too small, it is not conducive to manual welding, only to reduce the work efficiency to solve the welding quality. Otherwise it will leave a hidden danger. Therefore, the determination of the minimum distance of the solder joint should consider the quality and ergonomics of the welding personnel comprehensively. The size of the pad or over-wire hole is too small, or the size of the pad fits properly with the drilling size. The former is unfavorable to the artificial drilling, the latter is unfavorable to NC drilling. It is easy to drill the pad into a "C" shape and then drill off the pad. The wire is too thin, and the large area of the non-wiring area is not set to apply copper, easy to cause corrosion uneven. That is, when the non-wiring area after the corrosion, thin wire is likely to corrode the head, or may be broken, or completely broken. Therefore, the role of applying copper is not only to increase the ground area and anti-jamming. Many of these factors will greatly compromise the quality of the board and the reliability of future products.

PCB Cabling Experience (ii)

PCB design, cabling is the completion of the product design an important step, you can say that the preparation of the previous work is for it, in the entire PCB, the design process to the highest limit, the most fine skills, the largest workload. PCB wiring has single-sided cabling, double-sided cabling and multilayer cabling. There are two ways of wiring: automatic wiring and interactive cabling, before the automatic wiring, you can use the interactive pre-strict line routing, input and output side should avoid adjacent parallel, so as not to produce reflection interference. If necessary, ground insulation should be added, two adjacent layers of the wiring to each other perpendicular, parallel prone to parasitic coupling.

The pacchiosi rate of automatic wiring, depends on the good layout, the wiring rules can be predetermined, including the number of bends, the number of vias, the number of steps and so on. Generally first to explore the warp cloth, quickly connect the short line, and then to the maze-type wiring, the wiring to be cloth to the global routing path optimization, it can be broken according to the needs of the cloth lines. and try to re-route again to improve the overall effect.

The current high-density PCB design has felt that through the hole is not very suitable, it wasted a lot of valuable wiring channel, in order to solve this contradiction, there have been blind hole and buried hole technology, it not only completed the role of the through hole, but also save a lot of wiring channels to make the wiring process more convenient, more smooth, more complete, PCB Board design process is a complex and simple process, want to master it well, but also need the vast number of electronic engineering designers to experience, in order to get the true meaning.

1. Treatment of power and ground

Even in the entire PCB board wiring completed is very good, but due to the power, grounding the consideration of the interference caused by bad, will make the performance of the product decline, and sometimes even affect the success rate of the product. Therefore, the wiring of electricity, grounding wire to be taken seriously, the electricity, ground noise caused by the interference to a minimum to ensure product quality. For each engineer engaged in the design of electronic products understand the cause of noise between the ground and the power line, now only to reduce the noise suppression to express:

It is well known that a decoupling capacitor is added between the power supply and the ground.

As far as possible to widen the power, the ground width, preferably the ground than the power cord width, their relationship is: Ground > power line > signal line , usually the signal line width is: 0.2~0.3mm, the most fine width up to 0.05~0.07mm, the power cord is 1.2~2.5 MM on the digital circuit of the PCB can be a wide range of ground wire to form a circuit, that is, a ground network to use (analog circuit can not be used in this way)

Use a large area of copper layer for ground use, on the printed circuit board to the unused places are connected with the ground as ground. Or made of multilayer board, power, ground each occupy a layer.

1. Co-processing of digital circuit and analog circuit

There are now many PCBs that are no longer a single functional circuit (digital or analog), but are composed of digital and analog circuits. As a result, it is necessary to consider the interference between them in the wiring, especially the noise disturbance on the ground.

The high frequency of the digital circuit, the sensitivity of the analog circuit is strong, to the signal line, the high-frequency signal line as far away from the sensitive analog circuit device, to the ground, the entire PCB to the outside world only one node, so must be in the PCB internal processing number, mold common problem, And in the board of the digital and analog is actually separate between them are not connected, just in the PCB interface with the outside world (such as plug, etc.). Digitally with analog ground a little short, note that there is only one connection point. There is also a non-common on the PCB, which is determined by the system design.

1. The signal line is on the Electric (ground) layer

In the multi-layer PCB wiring, because in the signal line layer is not the end of the line is not much, then more layers will cause waste will also increase the production of a certain amount of work, the cost is correspondingly increased, in order to resolve this contradiction, you can consider the electrical (ground) layer on the wiring. first, the power layer should be considered, followed by the formation . Because it is best to preserve the integrity of the strata.

1. Treatment of connecting legs in large area conductors

In a large area of grounding (electricity), the legs of common components and their connection, the treatment of the connecting leg needs to be considered comprehensively, in terms of electrical performance, the component leg pads and copper surface full connection, but the components of the welding assembly there are some bad hidden dangers such as:

① welding requires a high-power heater. ② easily cause false solder joints. So take into account the electrical performance and process needs, made of cross-flower pad, known as thermal isolation (heat shield) commonly known as hot pad (Thermal), so that in the welding due to excessive heat dissipation of the cross-section of the possibility of a significant reduction in the welding point. The multi-layer board is treated the same as the ground leg.

1. The role of network system in cabling

In many CAD systems, cabling is determined by the network system. The grid is too dense, although the path has increased, but the step is too small, the data volume of the graph field is too large, this will inevitably have higher requirements for the storage space of the equipment, but also the object of computer-type electronic products computing speed has a great impact. Some channels are not valid, such as the pad occupied by the component leg or by the mounting hole, positioning hole occupied. The mesh is too sparse and the channel is too small to affect the pacchiosi rate greatly. Therefore, there should be a reasonable grid system to support the wiring. Standard components The distance between the legs is 0.1 inches (2.54mm), so the base of the grid system is generally set to 0.1 inches (2.54 mm) or less than 0.1 inches in full multiples, such as: 0.05 inches, 0.025 inches, 0.02 inches, etc.

1. Design rule check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the cabling design conforms to the rules set by the designer, and also to confirm whether the rules are in accordance with the requirements of the production process of the printed circuit board, and the general inspection has the following aspects:

• Line and line, wire and component pads, wire and through hole, component pad and through hole, through hole and through the distance between the hole is reasonable, whether meet production requirements.
• Is the width of the power and ground wires appropriate and is the coupling between the power supply and the ground (Low wave impedance)? Is there any place in the PCB where the ground can be widened?
• For the key signal line whether to take the best measures, such as the shortest length, add protection line, the input line and the output line is clearly separated.
• Analog circuit and Digital circuit section, whether there are separate ground.
• If the graphics (superscript, mark) added to the PCB will cause short-circuit signals.
• Modify some of the undesirable alignments.
• Is there a process line on the PCB? Resistance welding is in line with the requirements of the production process, the size of the solder is suitable, the character mark is pressed on the device pad, so as not to affect the quality of the Denso.
• The outer frame edge of the power stratum in the multilayer board is reduced, such as the copper foil outside the power formation is easy to cause short circuit.

Go PCB Layout and cabling experience