ESD Control on the Assembly Floor: Wrist Straps and Beyond

ESD control on an assembly floor is a programme rather than a piece of equipment, and the parts of it that fail are almost never the parts that cost the most money. A wrist strap that is worn but not tested, a bench that is grounded through a painted leg, or a plastic bag used for a bare board will each defeat an otherwise well equipped line without producing any visible evidence at the time.

What Static Damages and When

Electrostatic discharge damages a device by forcing a current through a structure that was never designed to carry it, and the damage ranges from an immediate open circuit to a small shift in a parameter that fails much later. The second kind is the expensive one, because it leaves the factory and fails at the customer.

Charge arrives at a device in three ways: directly from a charged operator or tool, by induction from a charged surface nearby, and from the device itself carrying a charge that it picked up while sliding across a surface. Only the first is obvious, and the other two are the reason a full programme is needed. A device can also sit on a bench, acquire a charge from the surface beneath it, and discharge the moment somebody picks it up.

The Three Pillars of ESD Control

A workable programme rests on three things: all conductors at the same potential, insulators kept away from the product or neutralised, and everything that touches a board handled in a defined way. The idea is to remove the difference in potential that drives the current, rather than to shield the device.

That is why the grounded bench, the grounded operator and the grounded tool all connect to one common point. Where two of them are grounded separately, a potential difference can exist between them, and a board moved from one to the other carries a discharge with it.

Wrist Straps, Footwear and Clothing

A wrist strap is the most direct connection an operator has to ground, and it works only if the band is snug, the cord is continuous and the resistor inside it is intact. Straps should be tested at the start of every shift and logged, because a broken cord is invisible to the wearer.

Where the operator has to move around, conductive footwear and a dissipative floor replace the cord but depend on a clean, wax free surface. A smock made of a dissipative fabric stops clothing charge from reaching the board, and it only works when it is buttoned and the cuffs are not rolled up. Footwear and floor are specified together, because a conductive shoe on an ordinary floor is no better than a normal shoe.

Workstation Grounding and Common Point

Every conductive item at the bench, including the mat, the iron, the tool bodies and the storage racks, should be bonded to a single common point that is connected to earth through a resistor. The resistor limits the current if an operator touches a live conductor, which is a safety requirement rather than an ESD one.

The connection has to be by design rather than by contact. A mat laid on a metal frame with a painted finish is not grounded, and an iron whose tip is isolated from its body may still carry a potential from its own supply. Both need an explicit bonding strap that someone can see and test.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/Through-Hole-PCB-Assembly2.png" alt="Ioniser neutralising charge above an assembly bench” />

Ionisers and Insulators

Insulators cannot be grounded, so the charge they hold has to be neutralised with an ioniser. Air blown across a bench carries positive and negative ions that cancel the charge on a plastic housing, a tape reel or a connector body without touching the product.

Ionisers need maintenance to keep working. The emitter points become coated and stop producing ions, and the balance drifts so that the air stream itself charges the part it was meant to protect. Cleaning the points and checking the balance should be part of the routine maintenance schedule. A failed ioniser is worse than no ioniser, because it creates the impression of protection where none exists.

Packaging, Handling and Transport

A board that leaves the ESD protected area has to be packaged in a material that does not generate or hold charge. Static dissipative bags, conductive tote boxes and dissipative foam all serve this purpose, and ordinary plastic bags and bubble wrap do not, however well they protect the board mechanically.

Handling is the other half. Bare boards and assemblies should be picked up by the edges, gloves should be chosen for the material they touch, and a board should never be slid across a bench top even if the bench is grounded. Our component reliability notes describe how that damage shows up later.

Humidity, Temperature and the Environment

Low humidity is the enemy. Below about thirty percent relative humidity, insulating materials generate and hold charge far more readily, which is why an ESD problem often appears in winter or in an air conditioned room with no humidity control.

Keeping the assembly area above forty percent relative humidity removes a large part of the risk without any other change. A hygrometer at the bench, with a defined action when the reading falls, is a cheap control that many lines overlook until the failure rate rises. Humidity control also helps the printing process, so the investment tends to pay for itself twice.

Auditing and Measuring the Programme

An ESD programme is audited by measurement rather than by inspection. The resistance from the bench surface to the common point, the resistance of each wrist strap, the surface resistance of the packaging and the decay time of the ionisers are all values that can be recorded and trended.

Electrical overstress failures found at test should also feed back into the audit. Custom components that keep failing at a particular station are telling you something about that station rather than about the supplier, and our judging PCB quality notes describe how that evidence is used.

Training and Behaviour

Everything above depends on people doing the same thing every time, which is a training problem rather than a technology problem. An operator who understands that a charged tape reel can damage a device is far more reliable than a rule pinned to the wall.

Refresher training and a visible audit routine keep the standard from drifting. Our short circuit inspection notes and the fabrication notes checklist show where the handling records sit alongside the rest of the job documentation. A short monthly walk around the floor with a checklist and a meter finds most faults before they reach a product.

FAQ

What is a triboelectric charge? It is the charge that appears when two materials rub or separate, which is how a board sliding out of a bag or a tape peeling off a reel becomes charged. It is the most common source of charge in an assembly area.

Is a grounded bench enough on its own? No. The operator, the tools, the packaging and the environment all take part, and a single ungrounded item can transfer a charge to a board that is sitting on an otherwise perfect bench.

How does gopcb control ESD in production? We keep the assembly area inside a defined humidity band, bond all conductive items to one common point, test wrist straps and footwear each shift, maintain the ionisers on a schedule, and audit the results against the product requirements.