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We conducted some tests to come up with our own best practices in creating PCB breakaway tabs. We also put out a white paper for the industry on the subject. Download our white paper by clicking here. Or follow the link to learn more about our testing techniques!
Rat bites, mouse bites breaks with holes Whatever you want to refer to them, they’re an essential to every board’s arsenal of tools. What’s that? You’ve not ever heard of them? That’s changed today!
“What’s a Mouse Bite?”
If you’re looking to construct some boards, you’ll likely purchase the PCBs in individual pieces and then assemble them one by one. If you’re creating an SparkFun number of boards, you’ll need the most efficiency you are able to get. When we construct large quantities of Qwiic Blower Fans, for example, we combine the boards into larger ones called panels. The panels go through the whole assembly process in a single piece, which allows us to reduce the time spent dealing with the boards as well as moving them around machines. After the process the panels are split to make individual boards! The boards that comprise an entire panel are connected using what’s known as the “v-groove” (sometimes called a “v-cut” or “v-score”) that’s the exact opposite of A v-shaped groove is cut through every side. This allows it to be snapped apart.
There are occasions when you shouldn’t be able to use v-groove however. Because of how they’re cut, v’grooves have to be straight and should extend across the entire length of an entire panel. This is a major problem for round boardssuch as those in the LilyPad ecosystem. It can also cause problems with boards that have straight edges, however they may also have connectors that extend over the edge of the board. How do we mix unusually-shaped boards or overhanging ones into panels that we still snap together after installation? Breakaway tabs!
Breakaway tabs are tiny sections of PCB specifically designed to separate from the final design. They break due to an array of small holes at each end that act as perforations along the edges of an envelope or postage stamp. These holes have earned them the name “mouse bites” because the marks left behind when they break appear to be the marks left by a tiny creature who was able to eat a piece from the paper. Sometimes, they’re used in conjunction with the design of a kit, such as those of the LilyPad ProtoSnap Plus, which places signal lines in between the mouse bites, allowing the user to design their application prior to taking the pieces apart and making them into a final project.
“Does anyone know how to do mouse bites correctly?”
The issue with mice’s bites is they’re very difficult to make right. Sometimes, they’re too fragile, but more often they’re not strong enough; at times they cause the floor in a mess, or the circuit maker refuses to produce them as they were originally designed. Therefore, when Nate was confronted with the idea of adding mouse bites into a design recently, he emailed an email to all engineers to ask for suggestions. Everybody had their own rules of thumb but there was no evidence. We didn’t have a set of “best practices” for mouse bites. Personally, I’ve got an enviable reputation for using mouse bites in my designs , outside of SparkFun, and Nate requested if I had any document detailing the ideal mice bite… Of course I did not. I used a rule of thumb like all others (20 millimeter holes and 40 millimeters from center to center) however, I chose to do it since the way it was done seemed appropriate and simple to remember. Then I was taken to search for any kind of authority regarding mouse bites.
A look at circuit fabricators’ websites indicates that the industry has settled on the following numbers that show holes without plating with dimensions that is 0.020″ which are spaced 0.030″ from each other. The forums suggest that this is the norm.
convention. There are those who advocate for greater or lesser spacing and others suggest bigger than smaller hole sizes, but they all must be within the range of a few thousandths of an inch. The trade organization IPC provides some advice in their IPC-7351 publication, also known as “Generic Requirements for Surface Mount Design and Land Pattern Standard,” which is a 100+ page document that only includes a few lines in chapter 188.8.131.52 to explain break-away tabs. The accompanying diagram suggests that the standard dimensions are 0.031″ in diameter and spaced 0.050″ from each other.
With all of these slightly different figures I did what any sensible individual would: I took the median, directed Nate to a blog post in one of the circuit fabs’ sites and declared it an evening.
No I purchased the PCBs worth $400 using the company card, and then snapped them all up one at each time!
The Scientific Method
Mouse bites are fairly easy to make, and there are only a few variables that could improve or decrease their effectiveness. This is a good thing for us, as it means we have a limited number of variables to be able to control. I came up with four parameters that include hole diameter the hole spacing edge clearance and corner radius of the tab. For each parameter I created a set of test coupons that are essentially small PCBs with one mouse’s bite at the center. The test coupons were stretched until it broke in a controlled fashion and the force needed to fracture it was documented in the spreadsheet. The force that broke was measured by using a fishing scale that is inexpensive. This scale was chosen due to the fact that it is sensitive to within 20 grams. Additionally, it features a peak hold function which can be used to determine the force that was applied prior to breaking. Test coupons were placed on a bench vise with silkscreen lines placed across the boards to ensure that they were consistently held during trials. Five trials were conducted for each test.
After all tests were completed after which they were put in a grid to be photographed. The strength of the break is just one of two main elements that determine the mouse’s effectiveness, the other (as frequently the situation) is the appearance. It does not matter how Goldilocks breaks strength when it leaves behind a sharp edge that is ugly and dangerous. This is why I ended up with an album of photos that looked something like:
“What did we learn, Palmer?”
The tests produced interesting data that eventually led us to a decision! Now we have an established set guidelines for the design of breakaway tabs that we’ll apply to all our designs to come. They’ll be the base to all of our designs moving forward, since the reality is that the most effective breakaway tabs are ones that ease the lives of the assemblers and your manufacturers and can alter slightly depending on the requirements of the particular design. These suggestions are a great beginning point I believe they’ll be able to work with most designs without modification.