Circuit boards do not exist in a vacuum. They're used in products and those products have other requirements that the board needs to accommodate. A lot of times these circuit boards have a specific shape that allows them to be fixtured either against another circuit board, a bracket, the edge of an enclosure, or something else, and the edge of this circuit board needs to be exactly correct for the final assembly to work. For most circuit boards a small amount of tolerance for the edge of the PCB is acceptable and can be accounted for. But sometimes it needs to be exactly right.

For those situations many customers prefer to have their PCB's "tab-routed" instead of "v-scored". A quick lesson in what "tab-routed" and "v-score" means is probably necessary.

V-Scored

A v-scored PCB means that when the individual PCB’s are put into a panel for automated assembly, a small v-shaped groove is cut into the top and bottom of the PCB along its edge (See Figure A). This makes it possible to break these PCB’s apart (depanelize) after the automated assembly process is finished. This v-scoring method has been used for many years and is the most inexpensive option for production work. V-Scores can only be placed along straight edges of PCB’s, so any curve to the PCB will have to be routed.

Figure B - Close up view of the v-groove

V-Scored PCB’s tend to leave a bit of a rough edge after they are separated from each other. Of course, roughness is relative because some may find it perfectly acceptable for their application whereas others might find it entirely unusable. Unfortunately there’s only one way to know for sure whether or not it will work for your application, and that’s to order some PCB’s that are v-scored and see if they’ll work for you.

There’s a number of ways to separate v-scored PCB’s. The easiest and simplest way is to simply snap them apart using your bare hands. The trouble with this method is that oftentimes small bits of the fiberglass will hang onto each edge of the adjacent PCB’s and create long string-like fibers. Usually just taking a piece of scrap PCB and rubbing it along the edge will clean up these fibers and smooth the edge down a bit. But a better way to separate these v-scored PCB’s is to use a tool specifically designed for such a purpose. Worthington Assembly utilizes a number of cab’s Maestro 3E tools for just this purpose.

Figure C - A popular v-score depanelizer

The Maestro 3E has a sharp long knife on the bottom where the bottom v-groove rests, while a round blade is drawn across the top v-groove. The force of the two blades cutting through this v-groove separates the two adjacent PCB’s and results in a clean if somewhat rough edge. How rough? Well, that’s going to be different for each person. Some might find it perfectly smooth whereas others might find it completely unacceptable. The only way to know for sure is to order some for yourself and see how they turn out.

Tab-Routed

Tab-routing a PCB means that when the individual PCB’s are put into a panel for automated assembly, a router bit cuts the edge of the PCB up until it reaches a tab, where it will pull out of the PCB, skip the tab, and then carry on with the rest of the routing process. The tabs that it leaves behind are then drilled along the PCB edge in order to give it a perforated edge that can easily be snapped off.

As the high speed router creates the edge of the PCB it leaves a perfectly smooth edge. The dimensional accuracy of a routed PCB edge is going to be far greater than that of a v-scored PCB edge. If your application requires a very accurate PCB edge, or if there is a curve associated with your PCB edge, then you are going to want to specify the need to use tab-routing when ordering your circuit board assemblies.

Some designers may decide that they would prefer the smoother and more accurate edge that you get with a tab-routed panel. However, there are a number of drawbacks to this method. The primary drawback being the fact that wherever these tabs are broken off, they will leave a very rough section. This rough section is almost always proud of the edge of the PCB, making it difficult to fit into tight enclosures.

A popular method to deal with this is to simply file down this rough section with a literal file or even a benchtop belt sander. The drawbacks to doing this is obvious. There’s obviously the risk of damaging your expensive assembly, but also the time involved in performing this work. For a couple dozen pieces it’s probably not much of a big deal, but if you had to do this to hundreds or even thousands of assemblies, the costs of performing this operation would add up quick.

This obviously raises some questions. What if you needed the edge of your PCB to be very accurate, very smooth, and curved, but you did not want to pay for the added costs of sanding down the breakaway tabs on each assembly? Is it possible to get the best of both worlds? I’m happy to report that it is almost certainly possible using the “recessed tab method”, but it takes some effort and collaboration between your PCB fab, your assembler, and your designer to make it all work.

Recessed Perforated Tabs

Figure E - Close up view of the routed edge and perforated tabs

The size, shape, and placement of the perforated tabs can have a significant impact on the manufacturability of the panel. If there are too few tabs or they are placed in the wrong area, then it might result in a very flimsy panel which is difficult to fabricate and difficult to assemble. If the tabs are too small, then they might be easily broken during production and the perforated edge might be difficult to create. So usually the first step in this process is to send your design to the assembly house to get their recommended panel layout. They will work with the fab and use their CAM software to determine the appropriate size and placement of their perforated tabs to make sure that it’s easy to manufacture and remain rigid enough for assembly. They will then send a drawing of their recommended panel to you for review. From here, you can decide to make small adjustments to the location and size of the tabs if needed, but more importantly you can identify areas of your PCB where you can recess the tabs into the edge of the PCB such that when the tabs are broken off, the left over material is not proud of your edge and results in a continuous line which can easily and comfortable fit into your enclosure (Figure F).

Figure F - Notice the two recessed tabs on the curved edge of the PCB

The location chosen for these recessions is important because you need to make sure there are no components, holes, traces, etc, that would be affected by recessing these tabs. This can be tricky to find but if done right it can result in significant cost savings over the life of the product.

A good rule of thumb would be to keep everything at least 1.1mm from the PCB edge. This will leave enough room for the drills of the mouse bites to be recessed sufficiently behind the PCB edge, and enough room so that when stress is put on the perforations, it does not damage any nearby traces or components.

dimensions for mouse bite clearance — Dimensions for mouse bite clearance

If you have any questions about how to make this work, please don’t be afraid to get in touch with us. You can email us at info@worthingtonassembly.com or you can email the author of this article at cdenney@worthingtonassembly.com