Safety distance and related safety requirements of PCB specification system
1. Common mistakes in schematic diagram:
(1) The ERC report pin has no access signal:
a. I/O attributes are defined for the pins when the package is created;
b. The inconsistent grid attribute is modified when the component is created or placed, and the pin is not connected to the line;
c. When the component is created, the pin direction is reversed and must be connected to the non-pin name end.
(2) The component runs outside the drawing boundary: no component is created in the center of the component library chart paper;
(3) The created project file network table can only be partially transferred to pcb: when the netlist is generated, it is not selected as global;
(4) Never use annotate when using a multi-part component created by yourself.
2. Common errors in PCB:
(1) The report NODE was not found when the network was loaded:
a. The components in the schematic use packages that are not in the pcb library;
b. The components in the schematic use packages with inconsistent names in the pcb library;
c. The components in the schematic use a package with inconsistent pin numbers in the pcb library, such as a triode: the pin number in sch is e, b, c, and the pcb is 1, 2, 3.
(2) It is always impossible to print to one page when printing:
a. The pcb library is not created at the origin;
b. Move and rotate the component multiple times, there are hidden characters outside the pcb board. Select to display all hidden characters, reduce the pcb, and then move the characters into the boundary.
(3) The DRC reporting network is divided into several parts:
Indicates that the network is not connected. Look at the report file and use the option CONNECTED COPPER to find it.
Try to use WIN2000, reduce the chance of blue screen; export files several times, make new DDB files, reduce file size and PROTEL zombie chance. If you are more complicated, try not to use automatic routing.
In the PCB design, wiring is an important step to complete the product design. It can be said that the previous preparation work is done for it. In the whole PCB, the wiring design process is the highest, the trick is the finest, and the workload is the largest. PCB The wiring has single-sided wiring, double-sided wiring, and multilayer wiring.
There are also two ways of layout: automatic layout and interactive layout. Before automatic routing, you can use interactive pre-wired lines that require more stringent requirements. The edges of the input and output ends should avoid adjacent parallel to avoid reflection interference. If necessary, ground wire should be isolated. The wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.
The routing rate of automatic routing depends on a good layout. The wiring rules can be preset, including the number of bending of the traces, the number of vias, the number of steps, etc. Generally, the exploration warp is first performed, and the wiring is quickly Short-term communication, then labyrinth wiring, first optimize the routing path of the wiring to be clothed, it can disconnect the laid line as needed. Try to re-route again to improve the overall effect.
For the current high-density PCB design, it has been felt that the through-hole is not well adapted. It wastes a lot of valuable wiring channels. To solve this contradiction, blind hole and buried hole technology have emerged, which not only completes the function of the via hole. It also saves a lot of wiring channels to make the wiring process more convenient, smoother and more perfect. The PCB board design process is a complicated and simple process. To master it well, it also needs extensive electronic engineering design. When people go to their own experience, they can get the truth.
3. Power, ground processing
Even if the wiring in the entire PCB board is completed very well, the interference caused by the inconsistency of the power supply and the ground line will degrade 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 wires should be minimized to ensure the quality of the products.
For every engineering engineer engaged in the design of electronic products, the reason for the noise between the ground and the power line is understood. Only the reduced noise suppression is described:
It is well known to add a decoupling capacitor between the power supply and the ground.
Try to widen the power supply and ground line width. It is better to ground the ground line than the power line. Their relationship is: ground line > power line > signal line. Usually the signal line width is 0.2~0.3mm, and the finest width is up to 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 is used for use (the ground of the analog circuit cannot be used in this way), and a large-area copper layer is used for the ground line, and the printed circuit board is not used. The place is connected to the ground as a ground wire. Or it is made into a multi-layer board, and the power supply and the ground line each occupy one layer.
4. Co-processing of digital circuits and analog circuits
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 interference between them when wiring, especially on the ground. interference.
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 is as far as possible away from the sensitive analog circuit device. For the ground line, the whole human PCB has only one node to the outside, so The problem of processing the number and the common mode must be carried out inside the PCB, and the digital ground and the analog ground inside the board are actually separated from each other, but only at the interface where the PCB is connected to the outside (such as a plug, etc.). The digital ground is slightly shorted to the analog ground. Please note that there is only one connection point. There is also no common ground on the PCB, which is determined by the system design.
5. The signal line is laid on the electrical (ground) layer
In the wiring of multi-layer printed boards, since there are not many lines left in the signal line layer, the addition of layers will cause waste and increase the workload for production, and the cost will increase accordingly. To resolve this contradiction, consider wiring on the electrical (ground) layer. First consider the power plane, followed by the ground plane. Because it is best to preserve the integrity of the formation.
6. Treatment of connecting legs in large-area conductors
In a large area of grounding (electricity), the legs of common components are connected to them, and the treatment of the connecting legs needs to be comprehensively considered. In terms of electrical performance, the pads of the component legs are perfectly connected with the copper surface, but There are some hidden dangers in the soldering assembly of the components. For example, 1 welding requires a high-power heater. 2 It is easy to cause a virtual solder joint. Therefore, taking into account the electrical performance and process needs, a cross-shaped solder pad is called a heat shield, which is commonly called a thermal pad, so that the cross-section is dissipated heat during soldering. The possibility of creating a virtual solder joint is greatly reduced. The processing of the ground (ground) leg of the multilayer board is the same.
7. The role of the network system in the wiring
In many CAD systems, the wiring is determined according to the network system. The mesh is too dense, although the path is increased, but the step is too small, the data volume of the field is too large, which inevitably has higher storage space for the device. The requirements, but also the computing speed of the computer-like electronic products have a great impact. And some of the paths are invalid, such as occupied by the pads of the component legs or by the mounting holes, the holes occupied by the holes, etc. Too thin, too few paths have a great impact on the routing rate. Therefore, there must be a sparse and reasonable grid system to support the wiring.
The distance between the legs of the 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 0.1 inches of multiples, such as: 0.05 inches, 0.025 inches, 0.02 Inches and so on.
8. 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 also 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 and component pad, wire and through hole, component pad and through hole, whether the distance between the through hole and the through hole is reasonable, and whether the production requirements are met.
Is the width of the power and ground wires appropriate? Is there a tight coupling between the power supply and the ground (low wave impedance)? Is there a place in the PCB where the ground wire can be widened?
Whether the best measures are taken for the critical signal lines, such as the shortest length, the added protection line, the input line and the output line are clearly separated.
Whether the analog circuit and the digital circuit part have separate ground lines.
Whether the graphics (such as icons, and markers) added to the PCB will cause a signal short circuit.
Modify some undesired line shapes.
Is there a process line on the PCB? Does the solder mask meet the requirements of the production process, whether the solder mask size is appropriate, and whether the character mark is pressed on the device pad to avoid affecting the quality of the electrical equipment.
Whether the edge of the outer layer of the power supply layer in the multi-layer board is reduced, such as the copper foil of the power supply layer is exposed outside the board, which is likely to cause a short circuit. The purpose of this document is to explain the process of designing the printed board using the printed circuit board design software PowerPCB of PADS. And some considerations, to provide a working group of designers with design specifications to facilitate communication and mutual inspection between designers.