PCB Tech - Safety distance and related safety requirements of PCB specification system

1. Common errors in schematic diagrams:

(1) There is no signal connected to the ERC report pin:

a. I/O attributes are defined for the pins when the package is created;

b. The inconsistent grid attributes were modified when components were created or placed, and the pins and wires were not connected;

c. When creating a component, the pin direction is reversed, and the non-pin name end must be connected.

(2) The component went out of the drawing boundary: no component was created in the center of the component library chart paper;

(3) The network table of the created project file can only be partially imported into the PCB: when generating the netlist, it is not selected as global;

(4) When using multi-part components created by yourself, never use annotate.

2.Common errors in PCB:

(1) When the network is loaded, it is reported that NODE is not found:

a. The components in the schematic diagram use packages that are not in the PCB library;

b. The components in the schematic diagram use packages with inconsistent names in the PCB library;

c. The components in the schematic diagram use packages with inconsistent pin numbers in the PCB library, such as triodes: the pin numbers in sch are e, b, and c, while the pin numbers in PCB are 1,2,3.

(2) It can't always be printed on one page when printing:

a. It is not at the origin when creating the PCB library;

b. The component has been moved and rotated many times, and there are hidden characters outside the boundary of the PCB board. Select to show all the hidden characters, reduce the PCB, and then move the characters to the boundary.

(3) The DRC reporting network is divided into several parts:

It means that the network is not connected. Look at the report file and use CONNECTED COPPER to find it.

In addition, remind friends to use WIN2000 as much as possible to reduce the chance of blue screen; export the file several times and make a new DDB file to reduce the file size and the chance of PROTEL freezing. If the design is more complicated, try not to use automatic wiring.

In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are done for it. In the entire PCB, the wiring design process is the most limited, the skills are the smallest, and the workload is the largest. PCB Wiring includes single-sided wiring, double-sided wiring and multilayer wiring.

There are also two ways of wiring: automatic wiring and interactive wiring.Before automatic wiring, you can use interactive to pre-wire the more demanding lines.The edges of the input and output terminals should be avoided adjacent and parallel to avoid reflection interference. Ground wire isolation should be added when necessary, and the wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is likely to occur in parallel.

The current high-density PCB design has felt that through holes are not suitable. It wastes a lot of valuable wiring channels. In order to solve this contradiction, blind hole and buried hole technologies have appeared, which not only completes the role of through holes., It also saves a lot of wiring channels to make the wiring process more convenient, smoother, and more complete. The design process of the PCB board is a complex and simple process. To master it well, a vast electronic engineering design is required. Only when personnel experience it by themselves can they get the true meaning of it.

1. 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. The wiring of the ground wire should be taken seriously, and the noise interference generated by the electricity 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 described:

It is well known to add decoupling capacitors between the power supply and ground.

Try to widen the width of the power and ground lines, preferably the ground line is wider than the power line, their relationship is: ground line>power line>signal line, usually the signal line width is: 0.2~0.3mm, the smallest width can reach 0.05~0.07mm, 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, to form a ground net to use (the ground of the analog circuit cannot be used in this way) Use a large area of copper layer as a ground wire, which is not used on the printed board All places are connected to the ground as a ground wire. Or it can be made into a multi-layer board, and the power supply and the ground wire occupy one layer each.

2. Common ground processing of digital circuit and analog circuit

Nowadays, 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 on the ground wire. interference.

The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines are as far away as possible from sensitive analog circuit devices. For ground lines, the entire 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. 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.

3. 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 workload of production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. First, consider using the power layer, and then the ground layer. Because it is best to preserve the integrity of the ground layer.

4. Treatment of connecting legs in large area conductors

In large-area grounding (electricity), the legs of commonly used components are connected to them, and 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. 2. It 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 the heat can be over-dispersed due to the cross-section during soldering The possibility of creating virtual solder joints is greatly reduced. The processing of the power (ground) layer legs of the multilayer board is the same.

5. The role of the network system in cabling

In many CAD systems, the wiring is determined based on the network system. The grid is too dense, although the path has increased, but the step is too small, and the amount of data in the field is too large, which will inevitably have a higher storage space for the device At the same time, it also has a great impact on the computing speed of computer electronic products. Some paths are invalid, such as those occupied by the pads of the component legs or occupied by mounting holes, fixed holes, etc. Too sparse and too few paths have a great impact on the distribution rate. Therefore, there must be a dense and reasonable grid system to support the routing.

The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or less than an integral multiple of 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.

6. Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules set by the designer, and at the same time, it is also necessary to confirm whether the rules set meet the requirements of the printed board production process.General inspections include the following aspects:

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, 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 supply and the ground line (low wave impedance)? Is there any place in the PCB that allows the ground line to 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 the analog circuit and digital circuit part have their own independent ground wires.

Whether the graphics (such as icons and annotations) added to the PCB will cause signal short circuit.