Vibration and Shock Design for PCB Assemblies

A product that is transported, mounted in a vehicle or used on machinery will see vibration and shock, and the assembly is usually the part that is designed last for it. The failures that follow are not random: a heavy part on a small footprint, a board supported at two points, a connector at the centre of a panel.

What the Environment Supplies

The requirement should be a spectrum rather than a single figure. Sinusoidal vibration at a resonant frequency, random vibration from a vehicle, and a shock pulse from a drop or an impact each load the assembly differently.

The mounting also matters. A board bolted at four corners behaves differently from one held in a card guide, and the same board in a different enclosure has a different response. Our design release notes describe where the environment should be recorded.

Resonance

Every board has natural frequencies. When the excitation matches one, the deflection at that mode is amplified, and the amplification at the edges of the board can be many times the input.

The response should be measured with an accelerometer or modelled before the design is frozen, because a board that is stiff enough at one frequency can be resonant at another.

Component Mass and Footprint

The load a component applies to its joints rises with its mass and falls with the number of joints that share it. A large electrolytic capacitor on two small pads is a fatigue test on those two pads.

Where the mass cannot be reduced, the options are more joints, a larger footprint, or a mechanical anchor. Adhesive under a large part is a common answer, and it should be specified rather than added in production. Our reliability notes describe how the joint margin is assessed.

Board Support and Stiffness

Support points divide the board into spans, and the shorter span has the higher natural frequency. Moving a support or adding one changes the response more than any other single change.

Stiffness can also be increased by the stackup, by thicker laminate or by adding material, but the effect on the rest of the design should be considered before the thickness is changed.

Connectors and Cable Loads

A cable attached to a board connector transfers its own vibration into the connector. The retention must therefore anchor the cable to the structure rather than only to the board.

Where the connector is at the edge of the board, the edge deflection is at its maximum, which combines the worst two effects. Our cable assembly notes describe the strain relief that removes the load.

Board Level Test

The test is a swept sine to find the modes, a random vibration at the specified level, and a shock pulse. The assembly should be monitored electrically during the test so that an intermittent fault is recorded rather than missed.

Where the product will be shipped, a packaged drop and transport test should be added, because the loads through the packaging are different from the loads through the mounting. Our quality notes describe how the results are recorded.

Design Measures

Reduce mass, increase the number of joints, add support, and keep the heavy parts away from the board edges and from the centre of a span where the deflection is greatest.

Where a part cannot be moved, an adhesive or a bracket converts the load into a distributed one. The measure should be specified on the assembly drawing, with the material and the coverage, so that it is applied consistently. Our fabrication notes notes list the attributes that should be stated.

Process Control and Verification

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Is a stiffer board always better? It raises the natural frequency, which may move the board out of the excitation range, but it does not reduce the stress in a component that is heavy relative to its joints.

Can a conformal coating help? It adds a small amount of damping and it holds small parts, but it is not a structural measure and it should not be used as one.

What does gopcb provide for vibration and shock? We provide spectrum based requirement definition, resonance measurement and modelling, component mass and footprint review, support and stiffness design, cable and connector load path review, and board level and packaged testing with electrical monitoring.