ESD Programme Audit for an Assembly Floor

An ESD programme is a set of arrangements that keep static charge away from sensitive parts, and an audit is the check that the arrangements are still in place. The two are often confused: a shop can have wrist straps, mats and ionizers and still have no programme, because the items are not tested, not grounded to a common point, and not replaced when they fail. What follows is what an audit should actually measure, and the failure modes it usually finds.

What an ESD Programme Actually Consists Of

The programme has four parts: a defined protected area with a boundary and a common ground, personal grounding for everyone who enters it, control of insulators and of charge generation inside it, and protection of parts in transit between areas. Each part has to be documented, and each has to have an owner. Where any of the four is missing, the protection is incomplete regardless of how much equipment has been purchased.

The programme also depends on the sensitivity of the parts being handled. A shop building boards with human body model ratings of a few hundred volts needs a different level of control from one handling only robust parts. The sensitivity should come from the component data, and the protection should be chosen to match it rather than to a general standard that may be more or less strict than the product needs.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/Rigid-Flex-PCB-1.jpg" alt="Technician testing a wrist strap at an ESD test station on the assembly floor” />

Wrist Straps, Foot Grounders and Their Testing

A wrist strap works only if the band is in contact with skin, the cord is intact and the resistor inside it is within its tolerance. All three fail regularly: a band that is worn loose, a cord that has been stretched until the conductor is broken inside the insulation, and a resistor that has gone open. Testing is therefore a daily check for the person and a periodic check for the assembly, with the result recorded.

Foot grounders are harder to verify because they depend on the floor and on the shoe. A foot grounder tested on a conductive floor can read correctly and fail on a different floor, and a heel strap that has picked up an insulating layer of dust will read open. The test should be made in the position the person actually works in, which means on the floor they stand on, not on a test bench. Where the floor is waxed or has been sealed with an insulating finish, no foot grounder will work, and the floor itself becomes part of the audit.

Workstation Grounding and the Common Point

Every element at a workstation should be connected to a single common point, and that point should be connected to ground through a resistor or a defined impedance. The purpose of the single point is to prevent a difference in potential between two surfaces that a part might touch, which is a discharge path through the part. Where mats, benches and tools are grounded separately to different points, the potential difference between them is uncontrolled.

The connections themselves fail quietly. A ground cord that has been unplugged to move a bench, a mat that has been cut and has lost continuity across the cut, and a snap that has corroded are all common. Continuity should be measured from the surface of the mat to the common point, not from the cord to the common point, because the mat itself is part of the path. The measurement should be made at the far corner of the mat as well as near the connection, since a damaged mat can read correctly at one end.

<img src="https://www.gopcba.com/wp-content/uploads/2026/04/power-electronics.png" alt="Field meter measuring ionizer balance at a workstation position” />

Ionizers and Their Maintenance

An ionizer neutralises charge on insulators that cannot be grounded, such as plastic housings and tapes. It only works while its emitters are clean and its balance is within specification, and both degrade. A dirty emitter produces fewer ions, and an unbalanced ionizer deposits charge of one polarity on the parts it is meant to protect, which is worse than no ionizer at all in some cases.

Maintenance is therefore part of the audit. The emitters should be cleaned on a defined interval, and the balance and the decay time measured with an electrostatic field meter and a charge plate. The measurement should be made at the position where the parts actually sit, not directly under the ionizer, because the coverage falls off with distance and with obstructions. Where the ionizer is used only intermittently, it should still be maintained, because the failure is invisible until a part is damaged.

Packaging and Transport Within the Shop

Parts are most vulnerable between operations, when they are in a tote, on a cart or on a conveyor. A conductive or dissipative tote keeps the charge on its contents equalised; an insulating tote or a plastic bag can hold charge generated by handling or by separation. The distinction between a dissipative and an insulating material is not visible, so the packaging should be identified by marking and checked periodically.

Conveyors and carts are part of the path. A metal conveyor is a grounding path only if the boards are in contact with it and the conveyor is grounded; a board riding on plastic guides is isolated. Where a cart moves between areas, its wheels and its shelves determine whether it carries charge. The transport arrangement should be reviewed as a route rather than as individual items, because a single insulating step breaks the chain.

The Audit: What to Measure

An audit should produce numbers rather than impressions. The useful measurements are the resistance from the working surface to the common point, the resistance from a wrist strap through the person to the common point, the surface resistance of the floor and of the totes, the balance and decay time of the ionizers, and the resistance of the packaging. Each has a limit, and each should be recorded with the location and the date.

The audit should also check the physical state of the items, because a resistance measurement can pass while an arrangement is unusable. A wrist strap that is coiled on a bench passes its own test and protects nobody; a mat that is covered with an insulating sheet of paper is electrically fine and functionally useless. The physical check catches these, and it is the reason a visual audit and a measurement audit are both needed.

Failure Modes the Audit Usually Finds

The most common finding is the unplugged ground. The second is the missing wrist strap, worn on the bench or left in a drawer. The third is the damaged mat, with a cut or a worn spot that has broken the conductive layer. After those come the unmaintained ionizer, the non-conductive tote used because the right one was not available, and the sealed floor that no longer dissipates.

All of these are organisational failures rather than technical ones, and they respond to ownership rather than to purchase. A named person responsible for testing each day and for keeping the records turns most of them into non-issues. Where the same finding appears in two consecutive audits, the arrangement should be changed rather than the reminder repeated: a strap that keeps being left off may mean the workstation layout makes it inconvenient to wear. The ESD protection arrangements should be reviewed with the line layout as a whole.

Records, Frequency and Corrective Action

The records should show the daily checks, the periodic measurements, the ionizer maintenance and the corrective actions. They should be kept where the audit can find them and where an operator can see them, because a record that lives in an office does not influence behaviour on the line. The frequency of the periodic measurement follows from the environment and the sensitivity of the parts, with a monthly interval being typical for a stable area.

Corrective action should follow the same pattern as any other process nonconformance: a defined action, a named person and a date. Where a measurement fails, the item should be removed from service until it is repaired or replaced, not marked for attention. Where the same item fails repeatedly, it should be replaced rather than repaired, and the reason for the repeated failure should be examined. The preventive maintenance schedule is the natural place for the checks whose frequency is longer than daily, and the training standards should cover what the operator has to do when a strap fails.

FAQ

Is a wrist strap always better than a foot grounder? A wrist strap is more reliable because it does not depend on the floor, but it depends on the person wearing it properly. A foot grounder allows free movement, which suits a workstation where the operator has to step to a machine. Where both are available, the choice should follow the task rather than a preference.

Can an ESD event be detected after the fact? A latent failure may show up as a leakage change or as a lower breakdown voltage, and both are measurable only on a sample. In production the event is invisible, which is why the programme relies on prevention and measurement of the arrangements rather than on detection of events.

Does humidity matter? It does. Low humidity allows charge to build and to persist, so a shop at 20 % relative humidity needs more control than the same shop at 50 %. Where the humidity cannot be controlled, the ionizers and the grounding become more important rather than less.