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PCB wiring can be successfully completed depends mainly on the layout, and the higher the wiring density, the more important the layout. Almost every designer has encountered such a situation. When there are only a few wirings left, they find that they cannot be routed anyway. They have to delete a large number or all of the wiring, and then re-adjust the layout! Reasonable layout is to ensure smooth The premise of wiring.
There is no absolute criterion for determining whether a PCB layout is reasonable, and some relatively simple criteria can be used to judge the pros and cons of the PCB layout.
The most commonly used criterion is to keep the total length of the flying leads as short as possible.
How to make the layout to make the total length of the flying line the shortest and ensure that the layout density is not too high to be realized is a very complicated problem. Because adjusting the layout means adjusting the placement position of the package. The pads of a package are often associated with several or even dozens of networks at the same time. Reducing the length of the flying leads of one network may increase the length of the flying leads of another network. How to adjust the position of the package to the optimal point is really not a practical standard. In actual operation, the designer mainly depends on the experience of the designer to observe whether the flying line displayed on the screen is simple and orderly and whether the calculated total length is the shortest.
Flying leads are the main reference standard for manual layout and wiring. When manually adjusting the layout, try to make the flying leads take the shortest path. When manually routing, they are often connected to each pad according to the route indicated by the flying leads. Protel's flying line optimization algorithm can effectively solve the shortest path problem of flying line connection.
The connection strategy of the flying line Protel provides two flying line connection methods for users to choose: sequential flying line and shortest tree flying line.
The flying line connection strategy can be set on the flying line mode page in the wiring parameter setting, and the shortest tree strategy should be selected.
The dynamic flying line adopts a flying line strategy of finding a nearby point to connect to the network and ensuring the shortest length of the entire network connection. Therefore, the dynamic flying line and the total length of the shortest tree flying line provide us with a relatively best judgment standard when laying out.
Specifically: When PCB layout, we use the following methods to ensure the effectiveness of the layout in the dynamic flying line state.
(1) Quickly move a package within the entire board. If the flying lead connected to this package does not change much, it means that the number of nodes in the electrical network connected to the package pin is small, which is close to a one-to-one connection. The location of this package cannot be placed arbitrarily and has a higher positioning priority. The best placement location of the package can be found by referring to the flying lead length displayed in the lower right corner of the screen.
(2) Quickly move a package within the entire board. If the flying lead connected to this package changes greatly, it means that there are many nodes in the electrical network connected to the pins of this package. This package may not be placed in a fixed location. The location has a lower priority for positioning. You can find the relatively best placement location for the package according to other criteria (such as whether the layout is beautiful, etc.) and refer to the flying lead length displayed in the lower right corner of the screen.
(3) Move the package, the position where the flying lead is the smallest shown in the lower right corner is relatively best.
(4) If the flying lead connection relationship between the two packages does not change no matter how they are moved, it means that the two packages have a strong constraint relationship and should be placed together first; if a package is moved no matter how the position is The connection relationship between the flying leads remains unchanged, indicating that this package has a strong constraint relationship with these packages, and should be placed at the center of gravity of these packages or relatively close to the center of gravity; if a package moves, the flying leads can be Constant change, that is, the connection node can always be found nearby, indicating that this package has a weak constraint relationship with all other packages. The location of this package can be finalized and the determined location can be more flexible.
Specifically: When PCB layout, we use the following methods to ensure the effectiveness of the layout in the dynamic flying line state.
(1) Quickly move a package within the entire board. If the flying lead connected to this package does not change much, it means that the number of nodes in the electrical network connected to the package pin is small, which is close to a one-to-one connection. The location of this package cannot be placed arbitrarily and has a higher positioning priority. The best placement location of the package can be found by referring to the flying lead length displayed in the lower right corner of the screen.
(2) Quickly move a package within the entire board. If the flying lead connected to this package changes greatly, it means that there are many nodes in the electrical network connected to the pins of this package. This package may not be placed in a fixed location. The location has a lower priority for positioning. You can find the relatively best placement location for the package according to other criteria (such as whether the layout is beautiful, etc.) and refer to the flying lead length displayed in the lower right corner of the screen.
(3) Move the package, the position where the flying lead is the smallest shown in the lower right corner is relatively best.
(4) If the flying lead connection relationship between the two packages does not change no matter how they are moved, it means that the two packages have a strong constraint relationship and should be placed together first; if a package is moved no matter how the position is The connection relationship between the flying leads remains unchanged, indicating that this package has a strong constraint relationship with these packages, and should be placed at the center of gravity of these packages or relatively close to the center of gravity; if a package moves, the flying leads can be Constant change, that is, the connection node can always be found nearby, indicating that this package has a weak constraint relationship with all other packages. The location of this package can be finalized and the determined location can be more flexible.
The dynamic flying line is undoubtedly a powerful layout tool, but because every time the package is moved, the shortest tree of the relevant network must be recalculated, which takes a certain amount of time. Therefore, when using dynamic flying leads on low-end PCs or large designs, you will feel that the mobile packaging is not very flexible. At this time, you can solve this problem by setting part of the flying line mode and controlling the contacts of the display flying line network.
When the PCB factorymoves the package in the dynamic flying lead state, press the R key to adjust the redisplay frequency of the flying lead. The redisplay frequency is divided into 5 levels. When it is 1, the redisplay frequency of the flying line is the highest, which is suitable for faster machines; when it is 5, the redisplay frequency of the flying line is the lowest, which is suitable for slower machines.
