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In our last few posts, we have explored different aspects of PCB design. Starting with preparing to place components, then placing the components, followed by adding electrical constraints, and recently defining and assigning physical and spacing constraints. Today, we will explore another important aspect, routing. We will look at the challenges you might face while routing a PCB and what Allegro PCB Editor provides to overcome those challenges.
Let's get started right away.
You have a dense ball-grid array (BGA) on your PCB to connect to a high pin-count IC package. So, how do you connect the pins? The outer rows can be managed, depending on what pads you are using. For example, you know the diameter of the balls and the pitches (the distance between the centers of two adjacent balls), so you can easily calculate the space between two pads - subtract the pad diameter from the pitch (pitch includes half of each pad being measured from the middle). And, you figure out the type of pads you are using have a smaller pad diameter. Well and good. You have something like the following. But what about the inner pads?
Even with your smaller pad types, you can only do that much. So, fanout comes to your rescue. You add vias and connect the traces to the vias - something like in the following figure. Problem solved!
So much about the challenge and its solution. How does PCB Editor help you? It has a fully automated method of creating fanouts, or pin escapes, for SMD pads. Simply choose Route – Create Fanout and use options to control fanout direction, pin selection, and fanout via. To top it, you can choose Via Structures, which combine patterns of vias and connect lines (clines) into a single via structure symbol.
A hex pattern package is highly staggered and drawing straight segments is near impossible through the maze of pads. Now, if you have to route a differential pair, that will be a nightmare. Imagine two traces come out of the blue circles in the image and you have to take the traces through the maze of hex pattern. You cannot have a straight segment at all - what with all the spacing constraints.
The solution, of course, is the ability to zig-zag through the maze - a snake breakout. PCB Editor lets you do exactly that.
So, you are in Add Connect and you can't see a straight path, simply right-click and choose Snake Mode. Move the cursor through the channel and the traces will display as arcs.
You have other conveniences and choices in Snake Mode. For example, for a single trace, you can choose a lane - meaning, you can choose to trace the top or bottom of a channel (a channel here is, of course, the space between two rows of pads through which you are tracing). Or, you can center the trace - take the trace midway between two rows. All this with a simple right-click.
This is a big one - keeping track of any constraints being violated while tracing. Say, you are tracing a net and there is a Total Etch Length constraint but you have already violated it. Don't you wish for a heads up? Real-time feedback immediately provided rather than waiting to the end.
PCB Editor does exactly that. You get a heads-up display showing you any violations and displaying a comparison of the constraint value and the real value. You can off course decide to take immediate action or complete the task and take remedial action. Don't worry, you will not miss it because a DRC marker will appear for the violation. For example, if the actual etch length violates the constraint, just add Delay Tune right away or after connecting the nets, while reviewing and noticing the DRC marker.
There are many more challenges in routing but then there are elegant and intuitive solutions to most of them too. If the challenges listed in this post make sense to you and you want to know more about the solutions mentioned, try out this Rapid Action Kit (RAK) on Routing and HDI. You can use the sample boards accompanying the RAK to try out the steps too. The RAK also gives steps to perform various useful tasks, such as routing a group of nets at once or using the scribble mode to free-form scribble a path through tight areas.
Note: The above link can only be accessed by Cadence customers who have a valid login ID for https://support.cadence.com