Although the current EDA tool is very strong, as the PCB size requires less and smaller, the density of the device is getting higher and higher, and the difficulty of PCB design is not small. How to implement high PCB high-end and shorten design time? This article describes the design techniques and points of PCB planning, layout and wiring.
Now the PCB design has time shorter, getting smaller circuit board space, increasingly high device density, extremely harsh layout rules and large-sized components make designers' work more difficult. In order to solve the difficulties in design, accelerate the listing of the product, now many manufacturers tend to adopt a dedicated EDA tool to implement the PCB design. However, dedicated EDA tools cannot produce ideal results, nor can they reach 100% of the openivity rate, and very chaotic, usually take a lot of time to complete the rest of the work.
There are many popular EDA tools in the market, but in addition to the use of the terms and function keys, it is different, how to use these tools to better implement PCB design? The designs carefully analyzed the design and carefully setting the tool software before starting the wiring. Here is a general design process and steps. Be
1, determine the number of layers of the PCB
The number of board sizes and wiring layers need to be determined in the early stage of the design. If the design requires the use of a high-density ball grid array (BGA) component, you must consider the minimum number of wiring layers required for these devices wiring. The number of wiring layers and the stack-up mode will directly affect the wiring and impedance of the printing line. The size of the board helps to determine the laminated mode and print wire width, and achieve the desired design effect.
Over the years, people always believe that the lower the number of circuit boards, the lower the cost, but there are many other factors that affect the manufacturing cost of the board. In recent years, the cost difference between multi-layer board has been greatly reduced. It is best to use more circuit layers when designing, making copper-wide distribution to avoid that there is a small number of signals that do not meet the defined rules and space requirements at the end of the design, thereby being forced to add new layers. The serious plan before the design will reduce a lot of trouble in the wiring. Be
2, design rules and restrictions
The automatic wiring tool itself does not know what should be done. To complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements, and to classify all special requirements, different design classifications are different. Each signal class should have priority, the higher the priority, the more stringent rules. Rules relate to printed wire width, maximum number, parallelism, and signal lines, and limitations of layers, which have great impact on the performance of wiring tools. Seriously considering design requirements is an important step in successful wiring. Be
3, the layout of components
To optimize the assembly process, the manufacturability design (DFM) rules will restrict the component layout. If the assembly department allows the component to move, it can be optimally optimized for the circuit, which is more convenient for automatic wiring. The defined rules and constraints will affect the layout design.
Considering the route path and the via region in the layout, as shown
Indicated. These paths and regions are obvious for designers, but the automatic wiring tool will only consider a signal at a time, by setting the wiring constraint condition and the layers set, the wiring tool can make the wiring tool can be envisioned by the designer. That is to complete the wiring. Be
4, fan out design
In the fan-out design phase, to connect the automatic wiring tool to the component pin, each pin of the surface mount should have at least one via, so that when more connection is required, the board can perform the inner layer. Connection, online test (ICT) and circuit processes. Be
In order to make the automatic wiring tool efficiency, it is necessary to use the maximum via size and print line as much as possible, and the interval is set to 50 mil is ideal. It is to be used to make a via type maximum number of wiring paths. When performing fan-out design, consider the circuit online test problem. Test clamps may be expensive, and usually order when they are about to be put into full production, if you consider adding nodes at this time, it is too late to implement 100% testability.
After careful consideration and prediction, the design of the circuit online test can be carried out in the initial design of the design, and it is achieved in the latter path and circuit online test, and the power supply and ground can also affect the wiring and fan out of the design according to the wiring path and circuit online test. . In order to reduce the induction generated by the filter capacitor cable, the via should be as close as possible to the pin of the surface mounting device, if necessary, can use the manual wiring, which may have an impact on the originally envisaged wiring path, and may even lead you to re- Which via is used to use, so you must consider the relationship between the via and the inductance, the priority of the via specifications must be considered. Be
5, manual wiring and key signal processing
Although this paper focuses on automatic wiring problems, manual wiring is an important process of designing printed circuit boards in the present and future. The manual wiring will help the automatic wiring tool complete the wiring work. As shown in FIGS. 2A and 2B, by performing a manual wiring and fixing the selected network (NET), it can be based on the path when the automatic wiring can be formed. Be
Regardless of the number of critical signals, it is first possible to wiring, manually wiring or binding automatic wiring tools. Key signals typically have to achieve desired performance through careful circuit design. After the wiring is completed, these signal wirings are checked by the relevant engineers, which is relatively easy. After checking, secure these lines and then start automatic wiring for the remaining signals. Be
6, automatic wiring
The wiring of the key signal needs to be considered to control some of the electrical parameters when the wiring, such as reducing distribution inductance and EMC, etc., the wiring of other signals is similar. All EDA vendors provide a way to control these parameters. After understanding the input parameters of the automatic wiring tool and the input parameters affect the wiring, the quality of the automatic wiring can be guaranteed to a certain extent.
The general rules should be used to automatically wiring the signal. By setting the restriction condition and the prohibition of the wiring zone to define the layer used by the given signal and the number of vias used, the wiring tool can be routed according to the engineer's design idea. If the number of layers and the number of the cloth used for the automatic wiring tool are not limited, each layer will be used when the automatic wiring is used, and there will be many via holes.
After setting the rules created by the constraints and applications, the automatic wiring will reach the results similar to expected, of course, some sorting work may also be required, and there is also a space for other signals and network wiring. After a portion of the design is completed, it is fixed to prevent the effects of the rear wiring process.
Wiring the remaining signals using the same step. The number of wirings depends on the complexity of the circuit and how much you define. After a class of signals are completed, the constraints of the remaining network wirings are reduced. However, it is that many signal wirings need to be manually intervened. The current automatic wiring tool is very powerful, usually 100% wiring. However, when the automatic wiring tool does not complete all the signal wirings, the remaining signal is required to make a manual wiring. Be
7, the design points of the automatic wiring include:
7.1 Slightly change the setting, try a variety of path wirings;
7.2 Maintaining the basic rules unchanged, trial different wiring layers, different printing lines and spacing widths, and different line width, different types of via holes such as blind holes, buried holes, etc., observing these factors What is the impact on design results
7.3 Letting Wiring Tools to process those default networks as needed;
7.4 The less important signal, the greater the freedom of the automatic wiring tool to the wiring.
8, the finishing of the wiring
If the EDA tool software you use can list the wiring length of the signal, check the data, you may find that some signal wirings of some constraints have a long length. This problem is relatively easy to handle, shortening the length of the signal wiring and reducing the number of overfores by manually editing. During the finishing process, you need to judge which wiring is reasonable, which wiring is unreasonable. Like the manual wiring design, automatic wiring design can also be organized and edited during the inspection process. Be
9, the appearance of the board
Previous designs often pay attention to the visual effect of the board, now different. Automatic design circuit boards are not better than manual design, but can meet the requirements of the requirements in electronic properties, and the complete performance of the design is guaranteed
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