Detailed manual layout and routing using flying leads in PCB design
Whether the wiring of a printed board can be completed smoothly depends mainly on the layout, and the higher the density of the wiring, the more important the layout. Almost every designer has encountered such a situation. When there are only a few wires left, they find that they can't be laid out anyway. They have to delete a lot or all of the wiring and re-adjust the layout! A reasonable layout is guaranteed. The premise of wiring.
Whether a layout is reasonable or not has no absolute criteria. Some relatively simple criteria can be used to judge the pros and cons of the layout.
The most common standard is to keep the total length of the flying line as short as possible.
In general, the shorter the total length of the flying line, the shorter the total length of the wiring (note: this is only correct relative to most cases, not absolutely correct); the shorter the line, the footprint occupied by the line The smaller the board area, the higher the routing rate. While the traces are as short as possible, the issue of wiring density must also be considered.
How to lay out to make the total length of the flying line the shortest and to ensure that the layout density is not too high to be realized is a very complicated problem. Because adjusting the layout is to adjust the placement of the package, a packaged pad is often associated with several or even dozens of networks. Reducing the length of one network fly line may increase the length of the flying line of another network. How to adjust the position of the package to the best point can not give a practical standard. In actual operation, it depends on the experience of the designer to check whether the flying line displayed on the screen is simple, orderly and the total length calculated is the shortest.
Flying lines are the main reference standard for manual layout and routing. When manually adjusting the layout, try to make the flying line take the shortest path. When wiring manually, connect the pads according to the path indicated by the flying line. Protel's flying line optimization algorithm can effectively solve the shortest path problem of flying line connection.
Fly-line connection strategy Protel offers two fly-wire connections for users to choose from: sequential flying lines and shortest tree flying lines.
The flying line mode page can be set in the flying line mode page in the wiring parameter setting, and the shortest tree strategy should be selected.
The dynamic flying line has been mentioned in the section about flying line display and control: After executing the display network flying line, displaying the package flying line and displaying one of the flying line commands, the flying line display switch is turned on, and after executing all implicit flying line commands The flying lead display switch is turned off.
When the flying line display switch is turned on, not only the specified network flying line is automatically displayed on the screen, but also when you manually adjust the layout to move the package position, the flying line connected to the package is automatically displayed. In addition, when the connection package flying leads are automatically displayed, all the flying leads are automatically turned off except for the flying line display connected to the package.
Execute the "Edit / Move / Move Package" command. If the current flying line display switch is turned on, all the flying leads are automatically turned off except for the flying line connected to the package.
When the flying line strategy is "the shortest tree", the starting end point of the flying line is changed. We know that the shortest tree flying line does not display the flying line according to the connection order of the pins in the network table, but determines the connection order of the package pins in a network according to the actual position of the package pins through the shortest tree calculation; When the position of a package changes, the connection order calculated according to the shortest tree theory also changes, that is, the start and end points of the flying line change. Therefore, when the package is moved under the "shortest tree" strategy, The flying leads connected to the package pins change with the package position. This is called a dynamic flying line.
The dynamic flying line adopts the flying line strategy of connecting the network to the nearest point and ensuring the shortest connection length of the whole network. Therefore, the dynamic flying line together with the total length of the shortest tree flying line provides a relatively best judgment standard for our layout.
Specifically: When laying out, we ensure the effectiveness of the layout in the dynamic flying state by the following methods.
(1) Quickly move a package within the entire board. If the flying line connected to this package does not change greatly, it means that the number of nodes in the electrical network connected to this package pin is small, and the connection is close to one-to-one. The location of this package cannot be placed arbitrarily and has a higher positioning priority. The optimal placement of the package can be found by referring to the length of the flying line displayed in the lower right corner of the screen.
(2) Move a package quickly within the entire board. If the flying line connected to this package changes greatly, it indicates that there are many nodes in the electrical network connected to this package pin. This package is not necessarily fixed in some way. The location has a lower positioning priority, and the relative optimal placement of the package can be found according to other criteria (such as whether the layout is beautiful) and referring to the length of the flying line displayed in the lower right corner of the screen.
(3) In the mobile package, the position where the length of the flying line displayed in the lower right corner is the smallest is relatively optimal.
(4) If the two packages have no change in the flying line connection relationship between the two positions, it means that the two packages have strong constraint relationship and should be placed first together; if a package moves position and some packages anyway The connection relationship between the flying lines is unchanged, indicating that this package has a strong constraint relationship with these packages, and should be placed preferentially at the center of gravity of these packages or relatively close to the center of gravity; if a package moves position, the flying line can Constantly changing, the connection node can always be found nearby, indicating that the package has a weak constraint relationship with all other packages. The position of this package can be finalized and the position can be flexible.
Dynamic flying lines are undoubtedly a powerful layout tool, but it takes time to recalculate the shortest tree of the relevant network every time you move the package. Therefore, when using a dynamic flying line on a low-end PC or a large design, the mobile package will feel less flexible. At this time, this problem can be solved by setting part of the flying line mode and controlling the display of the contacts of the flying line network.
When moving the package in the dynamic flying state, press the R key to adjust the re-display frequency of the flying line. The re-display frequency is divided into 5 levels. When it is 1, the fly-line re-display frequency is the highest, which is suitable for the faster machine. For 5 o'clock, the fly-line re-display frequency is the lowest, suitable for slower machines.