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.
Additional Considerations for This Build
Practical attention to failure criterion pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating failure criterion explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, drop test is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.
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.
Process Control and Verification
On a design of this kind, drop test is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.
Process Control and Verification
On a design of this kind, drop test is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. 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.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
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.
