Vibration Resistance in PCB Design

Vibration damage in electronics is rarely caused by the vibration itself and usually caused by resonance. A board has natural frequencies, and when the excitation in the environment matches one of them the deflection is amplified many times over. Vibration resistance is therefore mostly about knowing where the board’s resonances are, moving them away from the excitation, and making sure that the components most at risk are attached well enough to survive the deflection that remains.

How Vibration Damages an Assembly

A board that is excited at its natural frequency bends, and the bending produces strain at the points where the board is fixed and where components are attached. The largest strain occurs at the edges of the board and at the leads of heavy components, which is why a large electrolytic capacitor or a transformer is a common failure point in a vibrating product.

The damage accumulates as fatigue in the solder joints and in the leads, in the same way that thermal cycling causes fatigue. The failure appears after a period in service, and it is often first seen as an intermittent connection rather than as a clean break, which makes it difficult to diagnose without knowing the mechanical environment.

Mechanical shock is a different case. A single event with a high acceleration, such as a drop or a handling impact, produces a large deflection in one direction and can fracture a joint or a component body outright. Shock and vibration require related but distinct measures, and a design that survives one may not survive the other.

Resonance and Board Stiffness

The natural frequency of a board depends on its stiffness, its mass and the way it is supported. A thin board supported only at the corners has a low frequency and a large deflection, while a thicker board with more mounting points has a higher frequency and smaller deflections. The first step in vibration resistance is therefore mechanical: stiffen the board or support it more often.

The excitation frequency is set by the environment, and the design aim is to place the natural frequency above it or, where that is not possible, to damp the response. Raising the frequency is usually achieved by adding mounting points, by increasing the board thickness or by adding stiffening features such as ribs or a bonded metal plate.

The relationship is not linear, and a small change in support can move the frequency substantially. This is why the mounting arrangement is a design decision with an electrical as well as a mechanical consequence, and why it should be settled early rather than adjusted after the first test.

Layout Measures

Heavy components should be placed near mounting points or near the board edges where the deflection is smallest, rather than in the middle of a span. Components with a high centre of mass are the most vulnerable, and their position relative to the supports has more effect than any other layout decision.

Large components should be oriented so that their long axis is aligned with the direction of least bending where the vibration direction is known. Where the direction is not known, a compromise is required, and the components should be placed where the deflection is lowest. The same reasoning applies to connectors, which are heavy and which carry the additional load of a cable.

Small chip components are less vulnerable in themselves, but the joints that attach them are subject to the strain produced by the board’s bending. The strain is highest at the edges of the board and near the fixings, so the density of small components in those regions carries a slightly higher risk than in the centre.

Component Attachment

The attachment method determines how much of the board’s deflection reaches the component. A leaded component with compliant leads tolerates more movement than a leadless package, and a ball grid array with large balls tolerates more than one with small ones. Where the package is rigid, the joint absorbs the whole strain.

Underfill and adhesives are the standard measures for large area packages in high vibration environments. Underfill mechanically couples the package to the board, distributing the strain over the whole area rather than concentrating it in the joints. Adhesive applied at the corners of a large component achieves something similar with less process complexity.

For heavy leaded components, additional mechanical support in the form of a clamp, a bracket or a tie is often more effective than any change to the solder joint. A capacitor held by a clamp cannot move relative to the board, and the joint is protected from the load that its mass would otherwise impose.

Mechanical Shock

Shock loads are transient and can be much larger than the continuous vibration loads. The design response is to reduce the mass that has to be accelerated, to increase the stiffness of the path to the fixings and to avoid arrangements where a heavy component can lever against the board.

Component placement relative to the board edge matters for shock as much as for vibration. A heavy component near a corner of the board can produce a local flexure during an impact that is much larger than the board’s overall deflection, and the resulting joint damage appears without any obvious cause.

Where the product is expected to be handled roughly, the enclosure and the board mounting contribute as much as the board design. Standoffs, edge supports and the stiffness of the chassis determine how much of the shock reaches the assembly, and they are frequently cheaper to improve than the board itself.

Reliability Testing

Vibration testing is done with a defined profile of frequency, amplitude and duration, applied in one or more axes, and the assembly is monitored during the test for intermittent behaviour. Monitoring during the test is important because a failure that appears as an intermittent contact may not be visible afterwards.

Resonance search is a useful preliminary. Sweeping the frequency at a low level and measuring the response locates the natural frequencies, and the test profile can then be compared with them. If the dominant excitation coincides with a natural frequency, the design change is obvious even without a full qualification.

Shock testing applies a defined pulse in each axis, and the assembly is examined afterwards for joint and component damage. The examination should include a microsection of the joints at the highest stress locations, because a joint that has cracked internally may still pass a continuity check in the laboratory.

Practical Rules

Locate the board’s natural frequencies, move them away from the excitation by stiffening or by adding supports, and place heavy components where the deflection is lowest. Use underfill, adhesive or mechanical support for large or heavy parts.

Test with a profile that represents the environment, monitor during the test and microsection afterwards. Record the mounting arrangement and the test profile with the build records, because vibration performance depends as much on the mounting design as on the board, and the field data will show whether the assumption was correct.

FAQ

What causes most vibration failures? Resonance. When the excitation matches a natural frequency of the board, deflection is amplified many times over.

Where should heavy components be placed? Near mounting points or board edges, where deflection is smallest, rather than in the middle of a span.

What is underfill for? It mechanically couples a large package to the board, spreading the strain instead of concentrating it in the solder joints.