How to do a PCB routing like a pro?
DesignSpark PCB Pro software has been recently launched for our users in the US. For just under $450 USD, you can now purchase our schematics and PCB design tool with new enhanced features and expanded configurability. This article highlights some of the features of DS PCB Pro that might be very handy when you are routing your next board.
Throughout the article, I am using example designs that are available in the software. Those include RIAA amplifier (riaa amp.pcb) and chipKIT Max32 board (chipKit Max32.pcb) files.
Automatic Routers in DS PCB Pro
We are proud to announce that the automatic router in our software has been improved. Our Pro version of DS PCB now offers a new router called “Trace Router” for automatic routing. It is a shape-based router that is optimised to achieve low track lengths and fewer number of vias. The trace router is based on a multi-pass “iterative – improvement” algorithm in which tracks are routed iteratively by minimizing tracking errors, e.g. design rule violations, shorts, obstacles, etc.
The dialog window for Trace Router is demonstrated below. It can be accessed from Tools - Trace Router. Let us go through each area separately.
Routing passes signify the number of iterative passes you would like to attempt on the whole design or on the selected nets. The number of routing passes will increase with the size of the board. If the routing is 100% complete, no more passes will be required.
In the next are, Tracks, you can select the following options:
- Use minimum width (the minimum width specified in the Net Class will be used instead of Nominal width)
- Keep Preroutes (keep existing routes during autorouting)
- Keep Fixed (keep fixed routes during autorouting)
- Fix New (tracks routed during autorouting are saved in case another session of autorouting is done later)
- Routing Style (you can choose the track style, 90 or 45 degrees)
By checking Optimizing Passes, you can provoke the router to optimize the routing by trying to reduce track length and via count. This will be done after Routing Passes are complete. You can specify any number of Optimizing Passes, but it can adversely affect the routing time.
Fanout Passes can be defined to route short escape track to vias from SMD pads. This is carried out before Routing passes. The number of Fanout Passes can be set to any number as well, but it is recommended not to exceed 20.
Routing By Net Class option allows selecting the net classes to be used for routing. A new dialog window pops up, and then the required classes can be selected from the list by toggling/untoggling the To Route column.
The last section of Trace Router dialog is a Cost option, which can be used to change the Cost Factors of the autorouter. It is recommended that the settings in this section do not change unless you are comfortable with editing Cost Factor parameters.
This feature allows you to define areas that you might want to exclude when using an autorouter. The routing area can be selected only on electrical layers, and the shape of the area must be enclosed. Tracks will not be able to cross the area that is constrained in the defined shape. If you do not want tracks on either side of the board, for example, routing areas must be added on each layer separately. The design below shows an example of a rectangular routing area defined around connector CONN1. Autorouting function did not include tracks crossing that area. To define the routing area, go to Add - Routing Area and select the shape from several options available: rectangular, shape, circle and square.
Pull tight track
Pull tight track is very similar to manual routing except the tracks are automatically pulled around obstacles creating the shortest possible path. The track between source and target pads is “pulled tight” close to obstacles. In the image below, the tracks between R3 and C3 as well as R2 and C2 were drawn using Pull tight mode. The first track (R3-C3) is wrapped around the pad of R1, whereas the track between R2 and C2 is close to the pad of C3.
To select this mode of routing, go to Add - Pull Tight Track. Once in Pull tight mode, there is an option of auto-finishing the track if you are close enough to the target pad. To do that, select Auto-finish in the context menu.
Trunk routing allows you to add multiple tracks in parallel for common functional signals such as differential pair, buses or memory signals. In DS PCB Pro, this option can be found in Add - Trunk Route. There are three steps to perform trunk routing. Firstly, the items that need to be routed together, which may include pads, vias or track ends, must be selected. Then, the starting point of the trunk router from which every track will start must be chosen. Lastly, move the mouse to add the segment of the trunk route as you would do if you are adding a single track. The trunk route option can be used to add a new set of tracks to the design or modify the tracks that already exist.
The image above shows a simplified version of trunk routing with two traces only. The starting point of seven tracks is demonstrated for chipKit Max32 board in the image below.
Auto Mitre Track
This feature can be used to automatically adjust the sharp corners of the track by adding a short 45-degree segment. Both straight and curved options are available. You can select a track and choose Auto Mitre from the context menu to adjust that particular track. To operate on all tracks, go to Tools - Auto Mitre - Mitre All Tracks. For example, the track from CONN1 to U1 has been edited.
Auto Neck Track
Sometimes the track can be too wide for certain areas of the board. The auto neck feature of DS PCB Pro allows you to change the width of the track dynamically so that it can pass by the obstacles without causing a design rule error. To perform neckling, the minimum width of the track must be specified in the track style, which is available in Design Technology - Net Classes. Auto Neck can be enabled in the context menu when adding or editing the track.
This option is available to improve the presentation of tracks by reducing their length and via count. Smoothing results in producing tracks at a 90-degree angle. The use of this Smooth Track does not produce an error during DRC check. To use this feature, select a track and go to Adjust Track - Smooth Track. The example below demonstrates before and after stages of the track between R3 and C3. Smoothing of this track (shown in the grey area) decreased the number of corners as well as the length.
This article has summarized some of the features available for routing PCBs in DS PCB Pro. More details can be found in the Help section of the software. Context-sensitive help provides an intuitive explanation of all the options in DS PCB Pro. You can search by topics in the Index tab of the dialog window. This will be the first place to go in case you are unsure about any feature, but do not forget that you can ask us questions on our discussion forums.