Electrostatic Discharge Control in PCB Production

Electrostatic discharge is a small event with a large consequence, and it is invisible in the two ways that matter. The discharge itself cannot be seen, and the damage it causes often cannot be seen either, because a component weakened by static may pass test and fail in the field months later. ESD control is therefore a programme built on prevention rather than on inspection, and its most important elements are the mundane ones: a grounded surface, a worn wrist strap and a rule about what may be handled where.

How Static Builds Up

Static charge builds whenever two materials separate, which in a board shop means whenever a board slides out of a bag, a cover is peeled from a panel or an operator walks across a floor. The charge accumulates on insulating surfaces and stays there until it finds a path to ground, and the path may be through a component that was never intended to carry it.

The voltage involved is far higher than most people expect. Walking across a synthetic floor can generate several thousand volts on a person, and a person can feel a discharge at around three thousand volts. The discharge that damages a modern component occurs well below the level that can be felt, which is exactly why damage prevention has to be systematic rather than based on whether anything was noticed.

Humidity strongly affects how much charge accumulates. Dry air allows charge to build and hold, while humid air provides a surface conduction path that dissipates it. This is why static problems appear in winter in heated buildings and in air conditioned areas, and why the humidity specification in sensitive areas is a process requirement rather than a comfort setting.

What Discharge Damages

The damage falls into two categories. Catastrophic failure destroys the component immediately and is found at test, while latent damage degrades a junction or a thin oxide without destroying it, leaving a component that passes its tests and fails earlier than it should in service. The second category is much more dangerous commercially, because it produces field failures whose cause cannot be established.

Boards are at risk at several stages. Handling bare boards, placing components, testing, packing and any manual rework all involve contact and movement. Finished assemblies are also at risk during shipping if the packaging does not provide a static shielding barrier, which is a common omission because the boards are assumed to be robust by then.

The risk varies with the component technology. Fine geometry devices, thin gate oxides and high speed interfaces are more sensitive than older parts, and the trend has been toward greater sensitivity rather than less. A process that managed static adequately ten years ago may not manage it now with the same components and the same procedures.

Grounding and Bonding

Grounding is the foundation of esd control. Every conductive surface that a person or a board might touch should be connected to a common ground point, and the connection should be verified rather than assumed. Work surfaces, floors, carts, racks and equipment frames all belong in the same bonded system, because a difference in potential between two surfaces is what drives a discharge through a board.

The connection has to be made through a resistance rather than directly. A hard ground on a work surface or a wrist strap presents a shock hazard to the operator, so the standard practice is a resistor in the path, typically around one megohm for a wrist strap and a similar magnitude for a work surface. This is one of the few cases in electronics where a resistance is deliberately inserted.

Verification is what keeps the system working. Wrist straps fail, cords break and mats lose their connection, and none of it is visible. A daily check of wrist straps and a periodic check of the work surfaces and floors, with the results recorded, is the only way to know that the protection is present. Testing equipment for wrist straps is inexpensive and takes seconds per operator.

Antistatic Materials and Packaging

Materials are classified as conductive, static dissipative or insulating, and their use depends on the application. Static dissipative materials allow charge to flow slowly to ground, which is what a work surface and a floor need. Conductive materials are used for shielding, such as the metallised bags that protect boards in transit, while ordinary insulating plastics should be kept away from sensitive work entirely.

Packaging is where many programmes are weaker than they appear. A pink antistatic bag prevents charge generation but does not shield against a discharge from outside, while a metallised shielding bag does both. Boards in transit need the shielding type, and the bags must be closed to work. A board in an open shielding bag on a plastic trolley is not protected, however good the bag is.

Trolleys, bins and containers used in the assembly area should be static dissipative or conductive, and the materials should be specified rather than chosen for convenience. Ordinary plastic bins are one of the most common sources of charge in an otherwise well controlled area, and replacing them is a small cost with an immediate effect on a measurable problem.

Operator Practices

Practices determine whether the equipment works. Wearing the wrist strap with the cord connected, handling boards by the edges, keeping components in their packaging until they are needed and never placing a board on an unprotected surface are all simple behaviours, and together they account for most of the difference between a good and a poor esd control programme.

Clothing matters more than people expect. Synthetic clothing generates charge when it moves, and a sleeve brushing a board can transfer it. Smocks made from a static dissipative material are the usual answer in assembly areas, and they should be worn closed rather than left open, which is a detail that requires a supervisory habit rather than a written rule.

Movement should be minimised in sensitive areas, because charge generation depends on friction and separation. Carrying a board across a room, sliding it across a bench or pulling it quickly out of a bag all generate charge. The practices that avoid this are unremarkable, and they are the ones most likely to be skipped when the area is busy.

Auditing and Measuring

An esd audit checks that the programme is actually in place: the grounding of the surfaces, the condition and use of the wrist straps, the footwear and floor system if it is used, the packaging in use for boards and components, and the awareness of the people in the area. The audit should be short and repeatable so that the same checks are made each time.

Measurement supports the audit. A surface resistance meter, a wrist strap tester and a static field meter are the usual instruments, and they provide numbers that can be trended. Instrument calibration belongs to the same programme as the other process measurements, because an uncalibrated resistance meter provides reassurance without evidence.

The findings should be closed like any other. A mat that has lost its connection, a trolley that is insulating and a bag type that is wrong for the product are all correctable at low cost, and the value of the audit is in correcting them before a field failure occurs rather than explaining it afterwards.

Practical Rules

Bond every conductive surface to a common ground through a resistance, verify the connection regularly and record the checks. Choose materials and packaging for the function rather than the appearance, and control the humidity where the process requires it.

Keep the practices simple and consistent, and audit them on a schedule with the results recorded alongside the production records. Electrostatic discharge control belongs in the quality system because its failures are invisible until they are far away, and prevention is the only control that a board shop can actually apply.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can you feel a damaging static discharge? No. Damage occurs well below the three thousand volts or so that a person can feel, which is why prevention must be systematic.

What is the most important control? Grounding, verified regularly. A work surface, a wrist strap and a bonded floor system cover most of the risk.

Why do static problems appear in winter? Because dry air allows charge to accumulate and hold. Humidity control is part of esd control, not a comfort issue.