Strain Gauge: Panel Strain Measurement During Depaneling
Depaneling bends the board, and the bend is what breaks ceramic components. The damage is usually a microcrack that passes the functional test and fails after a number of thermal cycles, which is why the bending is measured rather than judged.
Where the Strain Comes From
Breaking a tab applies a moment to the board, and the moment produces a strain that falls off with distance from the tab. A router that is pushed too fast does the same through the cutting force.
The strain is highest at the board edge and it decays inward. Our tab design notes describe the geometry that reduces it.
How It Is Measured
A strain gauge is bonded to the board surface at the location of interest and its output is recorded during the operation. The gauge axis has to align with the direction of the bend.
The reading is a microstrain and it depends on the gauge length and the adhesive. Our measurement notes describe the discipline that also applies here.
What Limit Protects a Ceramic Component
The limit is set by the component type and its orientation relative to the bend. A component that is oriented across the bending axis sees more strain than one aligned with it.
The orientation is a layout decision and it costs nothing to change. Our component reliability notes describe the principle.
Support and Its Effect
Edge support under the board limits how far it can deflect, and it is the most effective single control. A board that is supported close to the cut sees far less strain than one held only at the ends.
Support has to be part of the fixture rather than improvised. Our fabrication notes list the points to confirm.
Depaneling Method and Strain
Hand breaking produces the largest strain because the force is applied without control. Routing produces less and laser produces the least, though the heat affected edge is a separate consideration.
The method should be chosen against the components near the edge. Our board failure notes describe the damage that follows.
Detecting the Damage Afterward
A microcrack is not visible from above. It is found by a bend test on a sample, by acoustic microscopy, or by a functional test that stresses the component electrically.
The sample test is the practical production check. Our inspection notes describe what each technique can see.
Verification
The verification is a strain reading taken at the location of the most sensitive component during the actual depaneling operation, with the fixture and the method recorded alongside it.
Our quality notes describe how the records are kept.
Process Control and Verification
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. 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.
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.
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.
FAQ
Can the strain be predicted from the tab width? Only approximately. The measured value depends on the fixture and the operator as much as on the tab, which is why it is measured.
Does a lower strain guarantee no damage? It reduces the probability and it raises the number of cycles to failure. It does not make the board immune.
What does gopcb provide during depaneling? We provide edge support built into the fixture, a depaneling method chosen against the components near the edge, strain measured at the most sensitive component during production rather than in a trial, and a bend test on a sample taken from the run.
