I’ll never forget the day that my wife and I were cleaning out the garage, and she stumbled across some evidence of mice activity. Let’s just say that it wasn’t a pleasant discovery, and she took a break while I finished in the garage. Mice often have that effect, so let’s use the adjoining cartoon as a mental picture to wash away any skittishness that might be associated with the little critters.
Mice are typically pretty small, and when they nibble on something, they leave tiny bite marks as evidence. You may also find “mouse bites” on your circuit board, but these shouldn’t be taken as an indicator of a rodent infestation. Instead, those tiny holes help facilitate breaking a circuit board out of its manufacturing panel. These holes have to be precisely located and dimensioned, and yes, they resemble the nibbling that you might find left by a mouse.
So put away your cheese and traps because the mouse bites we’re going to talk about don’t have anything to do with real mice. Instead, we will be looking at PCB mouse bites dimensions to give you a better understanding of how circuit boards are manufactured.
What Exactly is a Mouse Bite in a Circuit Board?
A mouse bite in a circuit board is part of the manufacturing panel used to fabricate and assemble the circuit board. Some people use the term “mouse bite” to describe over-etching in copper, but that isn’t what we are talking about here.
Circuit boards are typically laid out in a manufacturing panel by your PCB contract manufacturer using their Computer-Aided Manufacturing (CAM) systems. The panel gives both the fabricator and the assembler more board material for handling during the manufacturing processes, which is critical for smaller boards. Panels are usually a uniform size, and multiple instances of the PCB design can be laid out within the outline of the panel. Not only does a panel make it easier to manufacture a board, but multiple boards in the panel can be processed for almost the same cost as a single board.
Once the circuit boards have completed their fabrication and assembly processes, they need to be separated from their panels in a process called “depanelization.” Depanelization is accomplished by either cutting the boards out along pre-scored lines known as “V-grooves,” or breaking them out. Boards to be broken out must already be routed around their outlines to separate them from the panel and are only held in place by small tabs of material. These “breakout tabs” are removed, freeing the board from its manufacturing panel.
The tabs have small holes drilled into them that make breaking out the PCBs easier and reduce the boards’ stress. With each tab broken along the line of the holes, the remaining material is left with a “mouse bite” appearance until it is smoothed out. Next, we’ll look at these mouse bite holes in more detail to find out what their recommended sizes and dimensions should be.
Locations, Sizes, and Mouse Bite Dimensions
The holes used for the breakout tabs can vary, but most manufacturers will use five holes in a breakout tab with the following dimensions:
The mouse bite hole sizes and spacings appear random to minimize the cleanup required after the board is broken out of the panel. Although a smaller hole would take less effort to smooth out, it would also require drilling more holes and leave more bulk material. However, If the holes are too big, the board is left with larger “mouse bites” that have to be smoothed out. The placement of the holes within the breakout tab is also essential. The goal is to drill them closer to the edge of the PCB to reduce the amount of leftover tab material that has to be removed.
The placement of the breakout tabs around the board outline also has some constraints to keep in mind:
- There need to be enough tabs to fully support the board in the panel during PCB assembly operations.
- Avoid using too many tabs to save time, effort, and wear on the router bits.
- Avoid placing the tabs near areas of sensitive components or circuitry that would be more susceptible to breakout stresses put on the board.
- Locate tabs where they have at least 0.125 inches of clearance to the nearest components.
Sometimes stress damage to components or circuitry from breaking the tabs may not be discovered until later or result in intermittent problems that are difficult to debug. For this reason, locating breakout tabs away from these sensitive areas is critical to the success of the circuit board. On the other hand, if not enough tabs are used or are not strategically placed to support the board during manufacturing, the board may flex too much for reliable assembly. Here is where the knowledge and experience of your local PCB contract manufacturer are essential.
Your PCB Contract Manufacturer Has the Experience in Panels You Need
There are many variables to work with when setting up a manufacturing panel for a printed circuit board, and fortunately, your PCB CM has a lot of experience doing this. The CM will confirm whether a V-groove or a breakout panel best suits your design based on different design criteria and the board’s specific needs. They also understand your business needs and will tailor the panel’s creation according to your expected production volumes and the board’s specific requirements. At VSE, we design panels for our customer’s PCBs as a regular part of our manufacturing workflow. Our engineering and manufacturing teams have years of experience working with different panel requirements. You can be assured that we will create a perfect panel for your application.