Board Level Drop Test and Its Limitations

A drop test is used to qualify a product for the handling it will see, and it is often required before a design is released. It also produces results that are easy to misread.

What the Test Applies

The test applies a large acceleration over a short time. The board flexes, the joints are strained, and the failure is usually at the joint or at the component body rather than in the board.

The input depends on the drop height, the orientation and the mass, and it is measured with an accelerometer rather than assumed from the height. Our vibration and shock notes describe the same measurement discipline.

Board Level and Product Level

A board level drop test uses a fixture that constrains the board and a known input. It is repeatable and it does not represent the product.

A product level drop test uses the real enclosure and the real mass distribution. It represents the product and it is less repeatable, because the orientation at impact varies.

What Determines the Result

The board thickness, the support positions, the component mass and the joint geometry all contribute. The bending of the board is usually the dominant factor, not the acceleration itself.

A stiffer board bends less and it also transmits more of the input to the components. The two effects are opposite and the optimum depends on the design. Our warpage notes describe the stiffness that the stackup provides.

Failure Criteria

The criterion should be defined before the test: a functional failure, a resistance change above a limit, or a visible crack. The three give very different results.

A resistance criterion catches the damage before it becomes a failure, which is usually what the qualification is for. Our reliability notes describe how the limit is set.

Monitoring During the Test

The assembly should be monitored electrically during the drop so that an intermittent open is recorded at the moment it occurs. A measurement taken after the drop sees a joint that has closed again.

The monitoring has to have the bandwidth to see the event, which is short. A slow logger records that nothing happened. Our probe notes describe the measurement arrangement.

Limitations

The test is not a fatigue test. A product that passes one drop has survived one event, and the requirement for repeated handling is a different question.

It also does not reproduce the environment, because the impact in service is usually onto an unknown surface at an unknown angle. The result should be treated as a comparison between designs rather than as a prediction.

Interpreting the Result

Where two designs are compared, the difference is meaningful if the input and the fixture are the same. Where a design passes, the margin should be established by increasing the input until it fails.

That step test gives a margin figure, which is more useful than a pass. Our quality notes describe how the result is recorded.

Process Control and Verification

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.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

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.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. 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.

FAQ

Does a passing drop test prove the design is rugged? It proves it survived the input applied with the fixture used. The margin has to be established separately.

Should the test be run with the enclosure? Both are useful. The board level test compares designs and the product level test confirms the whole assembly.

What does gopcb provide for drop testing? We provide board level and product level tests with the input measured rather than assumed, criteria defined before the test, electrical monitoring with the bandwidth to capture the event, step testing to establish the margin, and results recorded so that two designs can be compared.