ESD Damage Investigation and Control Verification
Electrostatic damage is difficult to prove because the damage is often invisible and the event leaves no record. A device that fails after handling may have been damaged weeks earlier, or it may have been damaged by the test fixture that is now measuring it.
What the Discharge Does
A discharge delivers a small amount of energy in a very short time. If the energy reaches a thin oxide it can puncture it, and if it reaches a junction it can melt a filament a few micrometres across.
The result can be a hard failure, a degraded parameter, or a latent defect that passes every test at the time and fails later. The latent form is what makes the investigation difficult. Our reliability notes describe how the latent damage appears over time.
Human Body and Machine Models
The human body model describes a person charging and discharging into a device through a finger. It is a fast event with a modest energy and it is the model that most handling damage follows.
The machine model is a lower impedance event with more energy, and it is the model for a device touching a metal object. The charged device model describes the device itself being charged and then discharged, which is fast enough to damage an internal node that the other two models do not reach.
Where the Damage Happens
The dangerous moments are not only at the bench. A device can be damaged when it is removed from a tube, when it is placed on a conveyor, when a board is separated from a panel, or when a connector is mated.
Each of those conditions should be assessed against the model that fits it, and the control should be applied at the point of the risk rather than to the whole facility. Our fabrication notes notes list the process attributes that should be stated.
<img src="https://www.gopcba.com/wp-content/uploads/2025/08/21.png" alt="Worksurface resistance measured at a bench” />
Investigating a Suspected ESD Failure
The first step is to establish that the failure mode could have been caused by a discharge. A punctured oxide and a melted filament leave characteristic damage that a cross section and a decapsulation can show.
Where the failure is a parameter drift rather than a break, the evidence is a comparison with a known good device from the same batch, measured under the same conditions. Our failure analysis notes describe how the comparison is arranged.
Verifying the Controls
An ESD control programme is verified by measuring: the resistance of the wrist strap and the worksurface, the field on the bench, and the charge on the materials that touch the board.
Measuring the control rather than inspecting it is the only way to find a worn strap or a worksurface that has become insulated by a coating. Our cleanliness notes describe the measurement discipline.
Materials and Packaging
Every material that touches a board should be defined by its surface resistance and by whether it is dissipative or conductive. A label on the bag is not a measurement.
Packaging that has been reused should be checked, because a bag that has been creased and folded can lose its dissipative layer. The same applies to foam and to the inserts in a tote.
Training and Behaviour
The controls only work when they are used. A wrist strap that is worn but not connected, or a worksurface that is used through a plastic sheet, is a control that exists in the documentation.
The check should therefore be part of the standard work and the result should be recorded, so that a lapse is visible before it becomes a failure. Our workmanship notes describe how the behaviour is confirmed.
Process Control and Verification
On a design of this kind, worksurface is the item that decides how the rest of the board is arranged. 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. 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.
Process Control and Verification
On a design of this kind, worksurface is the item that decides how the rest of the board is arranged. 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. 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.
Process Control and Verification
On a design of this kind, worksurface is the item that decides how the rest of the board is arranged. 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.
Process Control and Verification
On a design of this kind, worksurface is the item that decides how the rest of the board is arranged. 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 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
Can ESD damage be detected by a functional test? A hard failure is detected. A latent defect and a parameter shift are often not, which is why the controls are measured rather than the units tested.
Is a wrist strap enough? It protects against the human body model. The charged device model and the machine model need other controls, including the materials and the handling.
What does gopcb provide for ESD control? We provide process risk assessment by discharge model, measured verification of straps, worksurfaces and ionisers, defined materials for every item that touches the board, checking of reused packaging, records of the routine measurements, and failure analysis that distinguishes a discharge from other damage.
