PCB News - Manual layout and routing using flying leads

Whether the wiring of a PCB printed board 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 a prerequisite to ensure smooth wiring.

There is no absolute criterion for determining whether a layout is reasonable. Some relatively simple criteria can be used to judge the pros and cons of the layout.

The most commonly used criterion is to keep the total length of the flying leads as short as possible.

Generally speaking, the shorter the total length of the flying line, it means that the total length of the wiring is also shorter (note: this is only correct for most situations, not absolutely correct); the shorter the trace, the smaller the trace occupies. The smaller the board area, the higher the distribution rate. While the traces are as short as possible, the issue of wiring density must also be considered.

How to lay out the shortest total length of the flying line 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.

Fly line connection strategy

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.

Dynamic flying line
It has been mentioned in the section about flying line display and control: After executing one of the commands of display network flying line, display package flying line and display all flying lines, the flying line display switch will be turned on, and the flying line will be displayed after executing the implied all flying lines command. The switch is off.

After the flying line display switch is turned on, not only the specified network flying line is automatically displayed on the screen, but also whenever you manually adjust the layout to move the package position, the flying line connected to the package is also automatically displayed. In addition, when automatically displaying the flying leads connected to the package, all the flying leads are automatically closed except for the flying leads connected to the package.

Execute the "Edit/Move/Move Package" command, if the current flying lead display switch is on, all flying leads will be automatically closed except for the automatic display of the flying lead connected to the package.

When the flying line strategy is "shortest tree", the starting and ending points of the flying line are 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 after calculating the shortest tree according to the actual location of the package pins; When the location of a package changes, the connection sequence calculated according to the shortest tree theory will also change, that is, the start and end points of the flying line will change. Therefore, when the package is moved under the "shortest tree" strategy, The flying lead connected to the package pin will change with the change of the package position, which is the so-called dynamic flying lead.

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 laying out, 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 of the package according to some other criteria (such as whether the layout is beautiful, etc.) and refer to the length of the flying line 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 in the center of gravity of these packages or relatively close to the center of gravity; if a package moves the position of 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 package 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 moving 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 grades. 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.

The above is an introduction to manual layout and wiring using flying leads. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.