ESD Control In The Assembly Area

An assembly area is a controlled environment in which every surface, every tool and every person is held at the same potential, so that a static discharge cannot pass through a component. Control is not a single product but a system of grounding, materials and procedures, and it fails at the weakest link rather than at the average of its parts.

This article describes how the ground reference is built, how materials are selected, how a wrist strap and a workstation are verified, and what a surface resistance measurement actually tells you.

The Ground Reference

An ESD control system needs one reference point, and that point is the equipment ground of the building or the common point of the workstation itself. The common point is bonded to the equipment ground with a conductor thick enough to carry a fault current, and everything else, including bench mats, floor, shelving, tools and people, is connected to that point through a resistance. The resistance is deliberate. A direct bond would let a fault current pass through a person, so the connection is made through a resistor that limits current while still draining charge.

That resistance also sets how fast a charged object equalises with the reference. A mat with a very high resistance drains slowly, so a board placed on it stays charged for a long time. A mat with too low a resistance drains quickly but offers less protection if a person touches live equipment. Most specifications place the mat inside a band that drains a board in a fraction of a second while keeping the current through a person below a few milliamps. Boards that carry exposed conductors near the edge need particular attention, as described under ESD and PCB edge traces.

Materials And Their Surface Resistance

Materials in an assembly area are classified by surface resistance, and the classification drives where each one may be used. A conductive material drains charge almost instantly and is used for tote boxes, magazines and tool handles. A dissipative material drains charge in a controlled way and is used for bench mats, floor finish, garments and gloves. An insulative material holds charge and cannot be drained, so it is banned wherever it can touch a board or come close to one.

The practical problem is that the classification describes a surface, and the surface changes. A dissipative mat cleaned with the wrong solvent, worn smooth by traffic or contaminated with flux can become insulative in patches. A conductive tote sprayed with mould release agent behaves as an insulator. This is why the resistance of a surface must be measured in service and not merely accepted from the certificate that arrived with it. Where a coating is used to cover conductors, the guidance on conformal coating as board protection is a useful companion.

Personal Grounding

A person is a charge generator, and walking across a floor can build several kilovolts on a body within a few steps. The wrist strap is the usual answer: a conductive band against the skin, connected to the common point through a resistor and a coiled cord. The band must contact skin rather than a sleeve, and must not be worn over a watch or bracelet that lifts it clear of the arm. The cord has to reach a proven ground point, and that connection is the part that fails most often, because the cord is unplugged for convenience and not plugged back.

Where a cord is impractical, a heel strap or conductive footwear on a dissipative floor provides the path instead, and the two methods are not equivalent. A corded strap drains through the arm and works even when the person is seated, while footwear relies on both feet contacting a floor that has itself been verified. Sitting still with the feet on a footrest breaks the path. For work on energised equipment the strap is deliberately omitted, and the component is handled only with a dissipative tool while the bench provides the reference.

Workstation Layout And Field Sources

The layout of the workstation decides whether charge is generated at all. Sources of static are moved away from the bench: adhesive tape dispensers, plastic bags, foam packaging, shrink film and anything else that is peeled or rubbed. Where such a material must be used it is used away from the board, and the board is covered while it is handled. Air from a plain compressed air nozzle generates charge by friction, so blow-off is done with ionised air instead.

An ioniser is not a substitute for grounding. It neutralises charge on insulators that cannot be drained, which is useful for a plastic housing or a bare laminate surface, but it does nothing for a conductor that is already bonded or for a person with no path. Ionisers also need to be balanced and cleaned, because contaminated emitters drift and can charge a part rather than neutralise it. The most reliable layout removes the insulator, and ionisation is the fallback for the materials that cannot be removed.

Verification And Records

The system is verified on two time scales. Daily checks cover the wrist strap and its cord, the bench mat and the ground connection at the point where the operator works. Periodic checks cover the floor, the shelving, the chairs and the ionisers, and they are made with a calibrated instrument and a recorded result. The measurement is normally a resistance between two points, or a resistance to the common point, taken with an electrode of a defined weight so that contact pressure is repeatable.

Records matter because the failure is rarely dramatic. A mat that has drifted out of the dissipative band still looks and feels the same, and it will still pass a visual audit. The only way to know is the number on the meter compared with the number from last month. A programme that verifies but does not record cannot separate drift from a bad day, and cannot show a customer that the area was in control when a batch was built. The same evidence discipline appears in every definition of board quality.

Where Control Usually Fails

The most common failure is a break in the path rather than a wrong material. A bench mat bonded at one corner with a loose snap at the operator end is grounded only on paper. A floor that is dissipative in the aisles but finished with insulative tile under the bench is not a path. A ground wire shared with noisy equipment puts a voltage on the mat instead of removing one. Each of these passes a visual inspection and fails a resistance measurement.

The second most common failure is the material that arrives between the operator and the board: packaging, a plastic bag used to carry a part, a foam insert, an adhesive label peeled over an open assembly. Control of what enters the area is as important as control of the area itself, and it depends on receiving inspection and on operators knowing which packaging has been approved for use inside the boundary.

FAQ

What resistance should a bench mat have? It is set by the specification the area is built to rather than by one universal number, but a surface resistance in the dissipative band, on the order of a million to a hundred million ohms measured point to point, is typical of a mat at an operator position.

Can a wrist strap be worn on either arm? It can, provided the band touches skin and the cord has a clear path to the common point. The hand that handles the board is the usual choice, because the band then moves with the hand instead of dragging across the bench.

Is a conductive floor enough on its own? Only with footwear that has itself been verified and with both feet on the floor. A conductive floor without a path from the person does not ground the person, and it does not ground the bench.