What are PCB Mouse Bites and V-Grooves?
PCB Mouse Bites and V-Grooves are two different design/fabrication techniques in PCB manufacturing. You may have seen circuit boards with little notches on the side or seen others that are flat. Today we will discuss what these two are and which may be the right use for your project. Printed circuit boards (PCBs) are normally printed from large panels of materials which are standard sizes for the manufacturing and printing process. Before bare boards are shipped out and populated, the fabrication shop will usually depanelize the boards into individual units. Depanelization is a process where the larger manufactured panel is snapped into individual boards and any leftover material is removed. It is worth noting that pick and place equipment can also run panelized boards for efficiency but this planning must be done ahead of time and should be communicated to your fabrication shop.
The panelized layout of printed circuit boards, or its array, is usually done by your supplier. They know what size materials they stock and how many of your boards can be created with each size. This allows them to mitigate wasted materials and keep your costs down. This is also why many shops have a MOQ for boards.
PCBs are typically made from performant composite materials in order to stop thermomechanical warping, insulate where needed, and enable channels either integrated into the layers or on the exterior to conduct as required by the engineer’s design. The reinforcement material used is typically a glass fiber that helps dimensional accuracies over a range of usage temperatures with epoxy or phenolic-based matrix to aid this necessary attribute.
PCB assemblies are typically mounted on standoffs or bosses and will need to resist fatigue and cracks forming during the mounting and use of the circuit board. This makes PCB design a challenge for long-lasting products with even the best manufacturers failing at times to ensure that their designs meet the specification of the final application.
In addition to thermal-mechanical challenges, the board also needs to resist moisture ingress and layer delamination through hydrolysis. This is a problem for PCBs especially in instances where exposed surfaces of reinforcement material are present. A clean surface is preferred to limit this ingress along interface cracks between the glass fiber and matrix material.
PCB mouse bites resemble the holes of a postage stamp and are drilled into a PCB. This can allow boards to snap under pressure with some cards using a cutting process to reduce the perforation size needed. After snapping the board there will be a rough surface left which resembles a mouse’s bite. This can be used to create better grip during mounting. The holes used for the breakout tabs may vary, but in most cases, manufacturers will use five holes in a breakout tab with the following dimensions:
Hole size: 0.020 inch (0.5mm) diameter
Spacing: 0.030 inches (0.76mm) apart from each other
When designing the placement of the breakout tabs around the board, there are a few things to keep in mind:
- Avoid placing these tabs close to sensitive components or traces since the stress of depanelization may damage them
- Have enough tabs that the board can be handled properly both by a person and equipment.
- Don’t put too many tabs, this will just slow down the manufacturing process and increase wear on parts.
- Locate tabs where they have at least 0.125 inches of clearance to the nearest components.
Alternatively, a v-groove can be cut into the board on one or both sides allowing the snap edge to be smooth and perfect for mounts that first require a slide fit into a groove.
V-Grooves vs Mouse Bites
PCB mouse bites can be used along with routing to reduce wasted material on a PCB panel through better tessellation of shapes for more complex geometry. Conversely, v-grooves are normally added in a linear fashion to a board but are difficult to add to non-rectangular patterns.
In many cases, you may use both along with routing to get the greatest number of boards from a PCB panel to reduce unit costs. Mouse bites can be more economical over v-grooves however v-grooves are used in particular for larger boards that do not require non-rectangular shapes to be routed.
The challenge with PCB mouse bites is that usually unless using bespoke manufacturing tools each hole used will need to be drilled, which can take longer per PCB mouse bite to complete. Routing can help reduce the number of perforations needed and ensure that the majority of the edges are smoother and easier to handle.
When designing a PCB mouse bite, you can either choose to offset the perforation away from the leading edge to create a mounting grip or recess it to reduce snagging on slot mounting PCBs.
The challenge with v-grooves is that they need to be added ideally in straight lines not only due to the tooling used and improved feed rate but also because the geometry needs to be repeated on the other side of the board. V-grooves are normally added at a 30-degree or 45-degree angle normal to the surface of the PCB.
When adding them you will need to ensure the remaining material is approximately half of the board’s thickness.
What Occurs During PCB Depanelization?
A board is typically snapped into individual units by an assembly person or technician. They may use rotary cutting tools or manual hand tools. There are other methods but those require expensive and specialized tooling.
Effective v-groove and mouse bite routing is essential in high quality and safe circuit boards. Since the PCB is created of a glass fiber reinforced composite, glass particulates could enter the lungs of a person has to sand the edges of a board due to irregular or poor routing. Another item to look out for is when snapping the PCBs out of the panel, it may cause stresses within the board that needs to be minimized as best as possible to ensure that conductive channels are not cracked or broken. While a cracked connection may work it will significantly reduce the life expectancy of the board especially during thermal-mechanical loading and power surges during on-off events.
During depanelization, any part of the composite that is not properly bonded during the panel’s manufacturing process may delaminate further causing failure of the board. As this delamination occurs inside the board between its layers, it typically forms something similar to a bubble-like cavity that grows with stress. These could range in sizes and may be difficult to find. This could occur both during and after manufacturing.
As such any processes on a PCB board should be conducted carefully to reduce the risk of failure before the designed service life of the product.
In this article, we haven’t yet mentioned specialized PCB boards used in applications such as submarines where hygrothermal resistance is critical to the longevity of the PCB board. While extensive testing occurs in environmental chambers to try and mimic such conditions repeatedly over the service life of the component, in general, it is best to keep any snap locations clean without any torsional loading required to remove the board from the panel.
If your PCB board is going into an environment where shock or fatigue is a challenge such as in a spaceship, then reducing the risk of delamination will be critical. Forces can cause resonance and quickly make a good-looking board unusable; removing rough surfaces will help along with conducting a modal analysis of how the board is to be mounted.
One interesting technique when using v-grooves is if a conductive connection is needed at the interface a conductive through-hole can be created and the v-groove can run through the center during the grooving process. The section will snap and need yet leave enough metal for a conductive interface. Alternatively, these can be used for snap insert connections where a smoother action is desired than just a routed profile.
When routing a mouse bite consider rounding the ends of the bite location during the process; this can stop stresses propagating deeper into the board during the snapping process. Remember that a PCB board is designed to be a resilient composite and glass fiber delamination can propagate cracks deep into the board.
PCB Mouse Bite and V-Groove Summary
You can use the various techniques described here to create cost-effective PCB patterns that tessellate well on a panel. Not only that but also longer-lasting parts through understanding where and when to use a mouse bite compared to the v-groove.
Remember that whenever designing a snap that stress is retained to only that location through rounding stress propagation points. If you are after a smoother surface for a slot mount, use a v-groove on those surfaces.
If you need a grip point for mounting try using a mouse bite but be prepared to sacrifice time for hole drilling operations and try to reduce this time with routing processes. When creating your board and considering which snaps and operations to use, consider where it will be used.
Vinatronic has been helping company’s build circuit boards for over 25 years. Our knowledge of PCB and PCB assembly has helped hundreds of companies be successful. If you have further questions about mouse bites and v-grooves, feel free to reach out to our team. We are always available for consultation.
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