oshparkoshpark wrote 01/27/2017 at 19:48 • 4 min read • Like

We’re huge fans of Sparkfun’s old school ProtoSnap kits. These include a small support tab with traces running between different sections of the board. They’re great for kit designs, allowing for both immediate out-of-box functionality, and eventual customization.

Due to our shipping and panelization process, this type of board is not fully supported, but can be fabricated with careful consideration and experimentation.

Design Considerations

These designs are almost always unique, since the number of shapes, sizes, and orientation varies wildly. The design also may need adjusting based on the intended user.

in orientation, intended usage, and shape. As a result, a bit of experimentation is always going to be required. However, here’s a few design tips to get things going.

Designing for Failures

Since these typically are for kits, the most likely failure that will occur is a pulled up trace. This is when the trace on the two boards doesn’t break, and starts pulling up traces inside your layout. Not good. Fortunately, the design can account for this in several ways.

Any of these techniques will generate a “stress concentration point” for a pulled up trace. This will cause a the trace to tear in a well-defined place, ensuring that it won’t affect your board.

The simplest one simply having vias near the tab, as seen on the sparkfun board above. A ripped up trace will usually break at the edge of the via pad.

Another option is to actually narrow the trace near the board edge. This takes up less space than a via, but still provides a good weak point in a desirable location. However, make sure you don’t go too far below the design rules!

The last option is less ideal, but may suit certain layouts. Simply jog the trace, creating a 90* bend or two.

Indicating on the design

Here’s the measurements on our typical support tabs. Your design may require different configurations, so these are a rough guideline.

Typical tabs have the following features

Note, the milling tool will add a 34 mil corner radius on the tabs. As a result, you can place the outermost holes tangent to the tab edge.

Failure modes (and how to cope)

See our Tips and Tricks page for more