Dealing With the Shock and Vibration of Your System

January 9, 2020 John Burkhert

Have you ever dropped your phone? Of course, that is a rhetorical question. How many screens have I cracked (?) would be a better one. It is pretty amazing that the devices continued to work, especially the time I was going full tilt on my bike when the phone became the victim of unrelenting gravity. There’s a reason these computing machines are nearly bullet-proof. The chassis of a smart-phone is one of the under-appreciated Engineering Marvels™. Light and stiff rules the day.

I wish I had taken a picture of the shaker table at the Velodyne Lidar factory. It was more bolted together than a nuclear vessel. (or is it ‘wessel’?) The giant castings could have belonged on an oil refinery if they were approximately 100% more dainty. The space between the massive bolts was just enough to get the comically oversized wrench around them and yet, it wasn’t over-built. A contraption that shakes other contraptions to their breaking point is going to be a brute. There is nothing quiet about stress testing in this way either. Knowing how and where those pricey Lidar sensors are deployed makes this type of stress testing mandatory.

shaker table

Image Credit: Bruel and Kajaer - Typical shaker table

It doesn’t have to be installed on the roof of a self-driving car to benefit from abuse in the name of reliability. A prototype mounted on the shaker table will reveal any resonant frequencies where it will over-react to the stimulation. A strobe camera will create a freeze-frame view of the unit showing the sympathetic convulsions that it is going through. From there, the Product Engineers will find a way to dampen the micro-movements.

At the office, we have a robotic coffee maker. The first thing it does is grind up the beans. Then it waits for a spell before dumping out the whole cup of liquid in a one-second splash. The grinding of the beans vibrates the whole machine making an empty paper cup dance around like it’s going to jump off of the little pedestal. It would be a shame if that happened right before the motivation streams out of the nozzle. The grinder needs to be mounted with some rubber bushings to isolate it from the rest of the ‘bot before it shakes itself apart.

cup

 

Sustaining Engineering is about making good things better. One of the ways we make things better is to drop them. We expect phones and watches to handle it. What about a laptop? At Google, we would take a pre-production unit and drop it onto a concrete surface from three feet above. The drop-test revealed a flaw that we had to fix. We had a number of flex circuits that tied different functions to the main logic board. Most of them were pretty simple for things like the camera or various wireless functions.

The flex for the USB-3 wasn’t so simple. Combining super speed lanes and the battery charging power had us using a number of layers. The stiffness of the flex was more than enough for the PCB connector to pop off when it hit the concrete floor. We had to add a bracket to positively anchor the connector to the board. That iteration lived through the three-foot ordeal. I still wouldn’t recommend drop-testing your own personal gear.

Underfill for the larger components and even glue in some cases are used to hold components down. We derate electrical components for higher reliability and the same notion can be applied to hardware. The mounting hole spacing can be decreased or the hardware size can be increased to keep pieces torqued down through hard use. If two is enough, use three.

Military programs will normally want to be twice as strong and reliable as what would be considered good enough for commercial practices. Ensuring traceability for every component such that defects can be rooted out whenever they turn up is another requirement. The heavy-duty form factors require continuous improvement. One way that manifests on the PCB is with a wider toe fillet on components that would have them.

PCB

Image Credit: Author

Stress risers on a PCB collect around the corners. Rounding them off helps. Circular boards are awesome. Copper holds its shape better than the dielectric material so use thicker copper and thinner core materials. Taken to an extreme, there are metal-core PCBs.  A slug of copper acts as the backbone. Stiffness and dimensional stability come with the package. This isn’t four-ounce copper, more like 45-oz copper (0.062”) which is drilled but not etched. It looks like edge-plating but the weight of the board leaves little doubt that this is burly.

PCB

Image Credit: Author - Full Metal Jacket

All in, a product may be exposed to shock and vibe in the normal course of operation. We only hurt the tools we love. Simulating the way we do so helps uncover the weak points so they can be upgraded when it comes time for the new and improved version. Lessons learned in the reliability lab can be put to use in the field but only if we’re paying attention. Just like we protect our high-speed signals, the mechanical challenges require some iteration and collaboration to get to the sweet spot.

About the Author

John Burkhert

John Burkhert Jr is a career PCB Designer experienced in Military, Telecom, Consumer Hardware and lately, the Automotive industry. Originally, an RF specialist -- compelled to flip the bit now and then to fill the need for high-speed digital design. John enjoys playing bass and racing bikes when he's not writing about or performing PCB layout. You can find John on LinkedIn.

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