ESD Safe Handling of Boards and Assemblies
Static damage is the defect that leaves no evidence behind. A device that has been exposed to a discharge may fail immediately, may drift slowly, or may pass every test and fail in the field after a month of use, and nothing about the board will show which of those happened.
Protecting an assembly is therefore a discipline rather than a device. Wrist straps, smocks and mats are the visible part, while the parts that decide the outcome are the layout decisions that make a board less sensitive in the first place and the handling rules that keep a charge from reaching it.
How a Discharge Reaches a Device
There are three routes and each needs a different defence. A charge can travel through the body of a person who touches the board, it can be induced by a nearby charged object without any contact, and it can arrive through the board itself when the assembly is slid across a surface.
The first route is blocked by grounding the operator, the second by keeping charged materials away from the work and by using dissipative packaging, and the third by controlling the surfaces the board touches and by keeping the humidity in a range where charge does not build.
Layout Measures That Reduce Sensitivity
A board with a large exposed ground area near the connector edge gives a discharge somewhere to go other than a signal pin. Guard traces, series resistors on connector pins and a short path to a protective device all reduce the energy that reaches the sensitive node.
Keeping signals away from the board edge, bringing ground and supply pins to the outside of a package and providing a low impedance return are layout habits that cost nothing and remove a class of field failure. They are part of the same set of decisions described for mixed signal design where noise and protection interact.
Workstations and Flooring
A workstation is only protected when every element is at the same potential. The mat, the bench, the floor and the operator all have to be connected through a resistance that limits the current while still draining the charge, and the resistance is specified rather than left to chance.
The floor covering matters as much as the bench, because an operator who walks across a synthetic floor while wearing a strap that has come loose is ungrounded. Testing the strap at the start of each shift is the control that keeps the system honest, and it takes seconds.
Packaging and Transport
Finished boards should travel in a bag or a tray that is dissipative rather than in one that is insulative. A common misunderstanding is that a conductive bag is the best choice, when in fact a fully conductive package can discharge through a device when it is opened at an unprotected bench.
Inside a cabinet or on a trolley, boards should be separated so that they cannot rub against one another, since friction between two boards generates charge on both. Foam, dividers and edge protectors are simple measures that prevent the problem in transport.
Humidity and Environment
Charge builds up most readily in dry air, which is why an assembly area with low relative humidity reports more static damage than the same area in a humid season. Holding the room above a specified relative humidity is a cheap control, and it should be monitored rather than assumed.
Air conditioning that dehumidifies aggressively can push a room below the safe range without anyone noticing, and ionisers are then used to compensate. Where ionisers are fitted, they need their own maintenance schedule, since a dirty emitter stops working long before it fails outright.
Handling Practice
Boards should be held by the edges, and never by the connector or the component area. Gloves that are insulative are worse than bare hands in some situations because they prevent the operator from grounding the board on contact, so the glove specification should follow the station requirement.
Where a board must be placed on a surface, the surface should be the mat rather than a notebook, a plastic bag or the top of a machine. This is the kind of rule that is broken under time pressure, and it is the reason audits matter.
Component Sensitivity and the Labels
Components carry a sensitivity class that describes the voltage they can survive, and the class determines how carefully they are handled. A part in a low class needs the full routine at every step, while one in a robust class is tolerant of ordinary handling.
The class should be visible on the packaging and understood at the station where the part is opened, not only in the purchasing record. A reel that arrives in a protective bag and is then poured onto an unprotected tray has lost the protection that was paid for.
Assembly Process Considerations
Processes that generate charge need attention in their own right. Tape peeling during feeder operation, board separation from a vacuum nozzle and depanelling all create discharges that can reach a device, and the equipment manufacturers provide dissipative versions for exactly that reason.
Conveyors, nozzles and handling fixtures should be checked as part of the same programme as the workstations. The failure mode is subtle: a nozzle that lifts a charged board does not fail visibly, it simply transfers the charge to the next component it touches.
Verification and Records
The programme is verified with periodic audits that measure the resistance of the mats, the straps and the floor, and by testing the ionisers. The records show whether the programme exists at all, which is what a customer audit asks for.
Damage that does occur should be recorded with the handling step it is attributed to, so that patterns emerge. Without that record, static damage is always attributed to a supplier, and the actual cause stays in the factory.
Additional Considerations for This Build
Practical attention to ESD 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 ESD 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, relative humidity is the item that decides how the rest of the board is arranged. 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.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
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.
<img src="https://www.gopcba.com/wp-content/uploads/2025/08/7502-1-scaled.png" alt="Operator wearing a wrist strap at an ESD bench” />
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.
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
Is a conductive bag always better than a dissipative one? No. A dissipative bag discharges slowly and protects the device, while a fully conductive one can deliver a fast discharge when it is opened.
Do wrist straps need testing? Yes, at the start of every shift, because a strap that has lost continuity looks exactly like one that works.
Can a coated board be handled without protection? A coating reduces the exposure of the conductors but does not remove it, and the handling rules still apply.
Is humidity control really necessary? In a dry climate it is one of the cheapest and most effective measures available, and in a humid one it is usually satisfied by the ambient air.
