ESD Control on the Electronic Assembly Line

Static electricity is generated by movement and it is invisible. A person walking across a floor can accumulate a charge of several thousand volts, and the discharge that follows when a hand approaches a board is fast enough to damage a semiconductor junction without producing any other evidence.

ESD control is the set of measures that keep those charges from reaching the device. It is a discipline of paths and materials rather than a single product, and it works only when the whole workstation is designed around it and audited after it is installed.

How Damage Happens

Charge is generated by friction, by separation and by induction. A charged object that comes close to a conductive body causes a discharge through whatever path exists, and the energy is delivered in a few nanoseconds into a junction that was never designed to absorb it.

The damage can be immediate, in which case the device fails at test, or latent, in which case the junction is weakened and fails later in the field. The second category is the reason the control measures apply to the whole process rather than only to the parts that fail.

Grounding and the Common Point

Grounding means giving every conductive item at the workstation a defined path to a common point, so that no potential difference can build up between them. The wrist strap, the worksurface, the floor mat and the tooling all connect to that point, usually through a resistor that limits the current should a person touch a live conductor.

The common point itself then connects to the building ground. Measuring the resistance from each item to that point is the verification, and it is a measurement that has to be repeated, because a strap that is worn or a lead that is broken still looks correct.

Worksurfaces, Flooring and Footwear

The worksurface provides a controlled path to ground for anything placed on it, and its resistance is specified within a band: conductive enough to drain a charge, resistive enough to prevent a rapid discharge. A surface that is too conductive can damage a device as effectively as no protection at all.

Flooring and footwear form the second path, for people who are moving rather than seated. Wrist straps are only effective while they are worn and connected, and in a workshop where people move between benches the combination of a dissipative floor and heel straps is what provides continuous protection.

Ionisation Where Grounding Cannot Reach

Insulators cannot be grounded, and charge on an insulator will not drain away. Plastic housings, tape, foam and the surfaces of some components all fall into that category, and the charge on them creates a field that can induce a discharge in a nearby conductor.

Ionisers neutralise that charge by supplying ions of both polarities to the air around the work area. They are used where insulators must be handled, and their performance has to be maintained: the emitter points become contaminated and lose efficiency, and the balance between positive and negative ions drifts.

Packaging and Transport

A device is at risk whenever it is moved, and the packaging is what protects it between operations. Bags, tubes and trays are specified as conductive, dissipative or shielding, and the categories are not interchangeable: a shielding bag protects the part from a discharge outside it, while a dissipative bag prevents the part from becoming charged.

The packaging has to be closed, and it has to be opened at a protected workstation, with the bag held away from the body while the device is removed so that no charge is induced as the two separate. A part that is taken out of a bag at an unprotected bench has lost the protection the bag provided, and the operation that follows is where the damage happens.

Humidity and Its Limits

Dry air allows charge to accumulate, while humid air provides a natural leakage path along surfaces. That is why the problem becomes worse in winter and in air conditioned rooms, and why relative humidity is part of the specification for an ESD protected area.

Humidity is a supplement and not a substitute. Raising it reduces the charge that builds up but does not provide a defined path to ground, so the protection measures still have to be in place and the humidity reading is used as an indicator rather than as a control. Where a process is stable in summer and fails in winter, the humidity record is usually the first place the cause is found.

Auditing the Workstation

A workstation audit checks the installation rather than the intention. The resistance of each path to the common point, the continuity of the connections, the condition of the straps and leads, and the presence of unauthorised insulators in the work area are the items that matter.

The audit should also check the tools and the equipment. A soldering iron tip should be grounded, a dispenser should not be made of an insulating plastic that rubs against a part, and the documentation that travels with the board should be held in a way that does not produce a charge when it is turned over. The findings and the corrections belong in a record, in the same way that the quality control routine is recorded for the product itself.

Training and Records

Most ESD failures in production are caused by a small number of habits: a strap left unfastened, a board carried in a bare hand, a part placed on a plastic tray. Training that explains why the habits matter is more effective than a set of rules that are followed because they are rules.

Records complete the loop. The audit results, the resistance measurements and the training records are what allow a problem to be investigated later, and they demonstrate that the control measures were in place during the period when a suspect product was built. The same documentation habit runs through the production requirements recorded in the fabrication notes.

Sensitive Devices and Their Ratings

Not every part has the same tolerance. Devices are classified by the voltage they can survive in a defined test model, and a part with a low rating needs protection that a robust one does not.

The rating should be known before the product is planned, because it decides how strict the area has to be, which packaging is acceptable and whether ionisation is needed at the bench. The information is in the datasheet, and it is the input to the classification of sensitive devices through the assembly process.

FAQ

Does a wrist strap alone provide protection? Only while it is worn and connected, and only for a seated operator. Moving between benches requires a floor and footwear path as well.

Can ESD damage be repaired? The damage is inside the device, so it cannot be repaired. The part has to be replaced, which is why prevention is the only economical approach.

Is a conductive surface better than a dissipative one? No. A controlled resistance limits the discharge current, while a fully conductive surface allows a rapid transfer of energy.