ESD Control in Electronics Assembly: Grounding and Audits
Static discharge damages electronic assemblies in ways that are often invisible, and it does so at voltages far below the level a person can feel. An ESD control program exists to keep those voltages from reaching the parts.
What Damage Looks Like
A discharge can destroy a component outright, which is the easy case because it is found at test. More often it degrades a junction or a thin oxide layer, leaving a part that works at the time and fails later.
The latent failure is the reason the program is worth the effort. A part damaged during handling but passing test will fail in the field, and the field failure will be attributed to the design or to the supplier.
Damage also requires less voltage than most people assume. A device with a sensitivity of a few tens of volts can be damaged by a discharge that the operator never notices.
The Three Elements of Control
Every control program has the same three parts: keep the charge from building, drain the charge that does build, and protect the parts from the charge that cannot be drained.
Keeping the charge from building means controlling the materials in the area, including the clothing, the packaging and the furniture. Draining means providing a path to ground through a controlled resistance. Protecting means using packaging and handling methods that shield the part.
A program that covers one element and not the others fails, which is why an audit that checks wrist straps alone is not an audit of the process.
Grounding and the Person
A wrist strap grounds the operator through a resistor that limits the current in case of contact with a live circuit. The strap must fit the wrist, must contact the skin, and must be tested daily, since a strap that has become disconnected is indistinguishable from a working one.
Where the operator must move around, a heel or toe ground combined with a conductive floor serves the same purpose. The floor’s resistance must be within a range that drains charge without creating a shock hazard.
Smocks and gloves are part of the same system. A synthetic garment that is not treated generates charge as the wearer moves, and it is a source rather than a protection. Our design release checklist notes where the handling requirements are recorded for the product.
Worksurfaces and Tools
A worksurface is dissipative rather than conductive, and it is connected to ground through a resistor. The resistance is high enough to be safe and low enough to drain a charge in a fraction of a second.
Soldering irons must have their tips grounded, which is a property of the station rather than of the iron alone. An ungrounded iron can inject a discharge into the joint it touches.
Tools, fixtures and the surfaces of automated equipment should be dissipative as well. Placing a board on an insulating plastic tray after removing it from a dissipative mat defeats everything that came before.
Ionizers and Insulating Materials
Some materials cannot be grounded, including most plastics and ceramics. An ionizer neutralises the charge on such a surface by supplying both positive and negative ions, which drift to whatever surface carries a charge.
Ionizers are used where a part cannot be grounded, and they require maintenance because their emitters become contaminated and their output falls. The output should be measured on a schedule rather than assumed.
Where an insulating material can be removed from the area entirely, that is more reliable than neutralising it. The best control is often a decision about what is allowed into the room.
Packaging and Transport
Boards and components move between departments and between companies, and the packaging must protect them throughout. A shielding bag provides a Faraday cage around the part; a pink antistatic bag does not.
The distinction matters because pink polyethylene prevents charge generation but does not shield against a discharge from outside. Using the wrong bag is a common and invisible error.
Within the factory, a tote must be dissipative and must not allow parts to slide against an insulating surface. Our quality notes describe the handling rules that accompany inspection.
The Layout Contribution
The design determines how sensitive the assembly is. Adding series resistance at an exposed connector, choosing a protection device with a low trigger voltage and placing it close to the connector all reduce the energy that reaches the sensitive circuit.
A protected net that is routed long distances from the connector to the protection device gains inductance and loses effectiveness. The protection must be at the point of entry.
Our solder defects notes cover the assembly faults that can look like ESD damage and are sometimes misdiagnosed as such.
Auditing and Records
The audit should check the items that fail silently: strap test records, floor and worksurface resistance measurements, ionizer output, and the training record of the people in the area.
Measurements should be recorded with their date so that a trend is visible. A floor resistance that has risen steadily over a year is a maintenance issue that can still be corrected before a part is damaged.
An ESD audit that produces a number rather than an opinion is the version that changes behaviour, because it can be compared over time and against the specification.
Process Control and Verification
On a design of this kind, ionizer is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Process Control and Verification
On a design of this kind, ionizer is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
FAQ
Can ESD damage be detected after the fact? Sometimes, by comparing a damaged part’s electrical signature against a reference. Usually the damage is found only when the part fails.
Is a humidity controlled room enough? Low humidity increases charge generation, and controlling it helps. It does not replace grounding.
What does gopcb provide for ESD control? We provide a documented control program covering grounding, dissipative surfaces, ionizers and packaging, with measurement records for each, training records for the operators, and handling rules carried through to packing and shipping.
