# Speed and improve PCB routing

& -December 05, 2013

PCB routing methods continue to evolve, and flexible routing techniques can reduce wire length and free up space on a PCB.

Traditional PCB routing is limited by fixation of coordinates of wires and by the absence of any-angle traces. Removing these restrictions, we can significantly improve quality of routing. Including real-world examples, this article covers the advantages of any-angle routing, the advantages of flexible routing, and a new algorithm for constructing Steiner trees.

Background
First, we would like to introduce some terms [1 – 3]. Let's define any-angle routing as wire routing that uses both segments and arcs at any angle. It is wire routing, but is not limited to using only line segments with 90 and 45-degree angles. Topological routing is wire routing without snapping to a grid and coordinates. No regular or irregular grids like shape-based routers are used. And let's use the term flexible routing for wire routing without shape fixing and with the following possibility of their shift with real-time wire shape recalculation. Only the arcs of circles from obstacles and common tangents to them are used to form the shape of wires [4, 5]. (Obstacles include contacts, coppers, keepouts, vias and other objects.)

Figure 1 shows parts of two PCB models. There are green wires and red wires on different layers of the PCB model. Blue circles are vias. A red component is highlighted. In addition, there are some red circle contacts. There is a model using only line segments and 90 degree angles between them in the Figure 1a. And there is PCB model using arcs and any-angles in Figure 1b. Maybe any-angle routing looks unusual, but it has a lot of advantages. It is routed similarly to how engineers did it manually half a century ago.

Figure 1. Parts of two PCB models. top: a version of traditional design; bottom: the same design but at any-angle

Figure 2 shows a real manually routed PCB which was developed by an American company Digibarn in 1972. This is a PCB of a computer based on the Intel 8008. Virtually the same any-angle routing is shown in Figure 2 as in the Figure 1b. Why did people use any-angle routing? Because it has many advantages.

Figure 2. A printed circuit board of a computer based on the Intel 8008 developed in 1972  (The DigiBarn computer museum)

Advantages of Any-Angle Routing
There are several advantages to any-angle routing. First, space can be saved on a PCB by not using the angles between the segments. (A polygon always takes up more space than the inscribed circle.)
Conventional auto-routers can route only three wires between closely spaced components (on the left and in the center in the Figure 3). The space we have during any-angle routing is enough to route four wires on the same path without violating design rules checking (DRC), on the right in the Figure 3.

Figure 3. The left and center figures: conventional auto-routers can route only three wires between closely spaced components. The right figure: the space we have during any-angle routing is enough to route four wires at the same path without violating DRC.

Suppose we have a square chip and want to connect the chip contacts with two other lines of contacts (see Figure 4). It takes a large area using only 90 degree angles (see Figure 4 top).

Figure 4. Square chip routing: (top) in orthogonal layout wire routing requires a large area; (middle) any-angle routing not only helps decrease wire length, but also fits them in a smaller area while ensuring all requirements are met; (bottom) turning the chip (any-angle) delivers even greater effect where area can be further reduced by more than two times.

Using any-angle routing we can shorten the distances between the chip and other contacts (Figure 4 middle). And, it reduces the area. In this example, area shrinks from 30 cm2 to 23 cm2.

Any-angle turning of the chip delivers even greater effect. In this example, the area was reduced from 23 cm2 to 10 cm2 (Figure 4 bottom). Figure 5 shows a real PCB. The any-angle routing with chip turning was the only way to route this board. This is not just a theory but it is actually applied solution (sometimes the only possible).

Figure 5. The any-angle routing with chip turning was the only way to route this board.

Figure 6 shows an example of a simple PCB. The result of a topological router is shown in Figure 6a while the result of the best shape-based auto-router is in Figure 6b. There is a picture of a real PCB in the figure 6c. The best auto-routers cannot route this board because the components are turned at an arbitrary angle. You need more area, and the device will be larger without turned components.

Figure 6. Examples of PCB routing: (a) a topological auto-router (100% of wires were routed); (b) the best shape-based auto-router (56.3% of wires were routed); (c) a real PCB.