Tom’s Circuits – Robotics Design Tips, Part 3: How to Avoid Vibration Failures

If your product moves, think about protecting printed circuit boards (PCBs) from vibration failures!

When a design does not take this into account, PCBs can often crack, fasteners will loosen, and your product will fail by falling apart. The key to avoiding these types of vibration failures is to understand board vibration and choose good fasteners.

Typical failures due to shock, vibration and thermal fatigue are cracked capacitors, broken component leads, broken PC board traces, cracked solder joints, PC board delamination, electrical shorts, and plated via barrel-to-pad disconnection.

Most vibration failures are due to mechanical resonances. Energy coupled into the structure at the resonant frequency will build up and cause large displacements, leading to failure. In general, the idea is to improve the design by finding the lowest resonant frequencies, and then increase them by adding stiffeners.

The source of vibration can be a shock due to dropping. The impulsive deceleration at impact contains all the vibration frequencies. This impulse can be damped with rubber bumpers. Rubber with the correct durometer provides smoother deceleration. There are opportunities for clever designs, such as varying the cross section of the rubber foot. This changes the spring constant to provide smoother deacceleration for small drops while still giving some protection for higher drops. Care must be taken with this sort of design to avoid making the problem worse by lowering the resonant frequency and causing larger displacements.

Shake tables use a sine wave vibration pattern to explore the resonances. By watching the product on the shake table and varying the vibration frequency, the resonances can be seen directly. Unfortunately, some of the resonances come into play only in a narrow range of frequencies. These are called high-Q mechanical resonances. Set the shake table to random vibration mode to find these hidden resonances. This test is typically more difficult to pass than swept-sine vibration.

Diagnosing problems on a shake table is scary. You can’t see inside the package, and there is also additional hardware on the shake table to hold the product in place. Don’t lean in too closely to see the product on the shake table while it is running, for fear that chunks will fly off and hit your face. A safer, more informative approach is to create a computer model and use modal analysis to understand the vibration frequencies. This can anticipate problems that would otherwise be found in drop testing or seeing what breaks on the shake table. Modal analysis is a feature of some mechanical CAD packages.

This simple example uses Fusion 360 to simulate a metal plate and a circuit board.

FR4 PC board64 mm by 65 mm, 1.55 mm thick
ScrewsM2.5 x 12mm, Stainless Steel
NutsM2.5 Stainless Steel
Aluminum plate64 mm by 65 mm, 1.55 mm thick
Standoffs5 mm height, 5 mm diameter, Al
Power dissipation12.9 W (2 W/in²)

In this example, the top plate represents a piece of FR4 PC board material, and the bottom plate is made of Aluminum. The goal of modal analysis is to find the resonant frequencies. Increasing the frequency of the first mode is usually desirable.

I had to choose a way to couple the vibrations into the design. This simulation holds the assembly by the screws and shakes the board up and down. If the vibration is instead coupled in through an edge of the plate, the plate and circuit board flex like a diving board. Vibrations normal to the face of the circuit board, up and down in this case, are the ones most likely to damage the PC board. Side to side vibrations can also cause damage, but they are more likely to cause the fasteners to loosen and fall out.

Aluminum material properties are already built into Fusion 360. Here is what I used for FR-4 material properties:

FR-4 is a difficult material to model accurately. It is anisotropic, which means that its properties change depending on the direction that you measure them. The fiberglass weave in the FR-4 plastic adds strength, but only in the direction of the fibers.

The deflections in the simulator output are amplified to show the motion. They can be adjusted to show the true range of motion. Fusion 360 can also animate the motion to show the board flopping around.

The frequency of the first resonance of the FR4 is 1317 Hz. In a real circuit board, the frequency will be lower due to the extra mass of the copper traces, solder, and the components soldered to the board. The frequency of the first resonance of the aluminum base plate is 2273 Hz. To keep a stiff structure, the resonant frequency of the mounting plate should be at least twice as high as the frequency of the first resonance.

The simulator shows that the first two resonance hot spots are on the left-hand side of the circuit board. By adding another standoff near this point, this resonance would be eliminated, and the frequency of the first resonance would increase. Mode 4 is the first mode with a resonance in the aluminum plate. In this design, it is nearly at the design goal of being at twice the resonant frequency of the PC board. Adding a standoff would change this resonance, also. Adding and removing screws to increase resonant frequency is a good way to improve a design without breaking a lot of prototypes.

A good design goal is to increase the resonant frequency by a factor of two for each layer of packaging. For example, in a mobile robotics application, the wheels, suspension, or rubber feet have a resonance of their own. At each level of hardware, the lowest resonant frequency should be a factor of two more more higher, with the highest frequencies in the innermost core of the product. Otherwise the resonances can align in frequency, and mechanical vibration will build up in the core of the product.

Design in enough margin to avoid problems caused by unit-to-unit variation in fastener tightness and material properties.

To prevent fasteners from loosening, use a locking mechanism. Good locking mechanisms are friction locking with Nyloc nuts, or Loc-Tite. Belleville (conical spring) washers have the advantage that they maintain spring compression. This does not provide locking unless the washers are serated. For PC board applications, the serations can damage to the PC board surface. By cutting into the board, serations create a low-resistance ground connection. This is tempting to overdo, leading to board damage. It is considered poor form to pass your FCC EMI certification by overtorquing the ground connections!

Avoid helical spring washers (the classic ‘lock-washer’) because they are not effective. To watch a test of locking, see this YouTube video about the Junker Test. The output of the test is a graph that shows how the bolt loosens in vibration, which causes a loss of torque in the screw. A loose screw weakens a product, or worse, the screw falls out and creates an electrical short-circuit in the product.

For more about locking mechanisms, see page 6 of NASA Reference Publication 1228 starting on page 6.

Learn more about the benefits of Belleville washers in Thermal Stresses in Printed Circuit Boards.

 

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