ESD Packaging And Handling For PCB Assembly

An electrostatic discharge is a short, high current event that begins when two objects at different potentials touch. The energies involved are small, which is why a discharge is often invisible, but the voltages are not: a person walking across a synthetic carpet can reach fifteen kilovolts, and a discharge of a few hundred volts is enough to break the gate oxide of a small transistor. The person who caused the damage feels nothing, and the board may still pass its electrical test.

This article explains where the charge comes from, why an assembled board is more sensitive than a bare one, what the packaging has to provide, and how the handling practices on a line are verified.

Where The Charge Comes From

The mechanism is triboelectric charging, which transfers charge when two materials separate. Common plastics, adhesive tape, foam, and even a dry airflow generate charge in ordinary use, and the amount is higher when the humidity is low. The human body is a capacitor of a few hundred picofarads, and when it discharges into a device the current rises in a few nanoseconds, which is why the effect is described by a model rather than by a voltage rule.

Two models are used to qualify parts. The human body model represents a charged person discharging through a finger, and the charged device model represents a device that has itself become charged and then touches a grounded surface, which produces a faster and often more damaging event. The first failure mode is a ruptured gate oxide, which appears as a short or a leak, while the second is a junction or metal damage that can appear as a parameter shift rather than as a hard failure. A latent defect of the second kind passes the factory test and fails later, which is what makes control worth the effort.

The cost of the damage is what justifies the discipline. A hard failure is found at test and the board is scrapped, which is a visible and bounded loss. A latent failure is shipped and then appears in the field, where the cost of diagnosis, of a replacement, and of the reputation of the product is far larger than the value of the board. Because the two cannot be distinguished at test, the only practical control is to prevent the discharge rather than to detect its effect.

The damage threshold also varies with the device rather than with the board. A small geometry logic device may be damaged by a few tens of volts, an analogue part may tolerate several hundred, and a passive component is largely immune. The sensitivity of an assembly is therefore set by its most sensitive part, and the handling requirement is written for that part rather than for the average of the bill of materials.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/1.webp" alt="Board being packed into a metalised shielding bag” />

Why An Assembled Board Is More Sensitive

A bare board with only copper features is not especially sensitive, because copper conducts the charge away. Once components are mounted, however, the assembly carries semiconductor devices with thin oxides, and the connector contacts and the exposed test pads become the entry points for a discharge. A board that is being handled during assembly is therefore exposed at exactly the moment when it is most sensitive, and a board that has been coated or enclosed in a shield is less exposed afterwards.

The exposure points are worth identifying. An edge connector with gold fingers is a direct path to the circuit, a board with a large area of exposed copper can couple a discharge capacitively into nearby traces, and a card that is inserted into a system while the system is powered can receive a discharge through its own connector. The assembly and test sequence should be reviewed with those points in mind, because the same board can be safe in one step and vulnerable in the next.

What The Packaging Has To Provide

Packaging falls into two families. A dissipative or anti static material, which is usually pink or black, prevents the generation of charge and slowly bleeds away what is present, but it does not shield the contents from a discharge that arrives from outside. A shielding material, which has a metalised layer or a metal foil, forms a Faraday cage around the board so that an external field cannot reach it, and that is what a moisture barrier bag does when it is sealed.

The choice follows the sensitivity of the contents. A board with exposed connectors and fine geometry is normally packed in a sealed shielding bag, with the board inside a dissipative inner bag or wrapped in dissipative foam, and the bag is closed so that it is a continuous conductor. The bag also has a moisture function, and the desiccant and humidity indicator inside it are what make it possible to know whether the contents have been exposed. Packaging requirements of this kind are usually referenced from a standard rather than invented on a drawing.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/5lRxYcI_XX3h4-BipULu61gw3L-RLFsml7hKUIgIa_VH27O-NxAY4kwXW-anx9eybam9Oi6IQU1Z6Ikty88PAv96DDaMknpa0qy0QqunYNmrcMEdlDPmwA9BhROsA1D9O822fX5pNp5xOnz-tn-6Lh4qo_E3y_iCy67kzO0tfMdAZvnqq2fkM1twbpuC3hx-1.jpg" alt="Operator wearing a wrist strap at a dissipative work bench” />

Handling Practices On The Line

A protective area is defined by its surfaces rather than by its walls. Work benches are covered with a dissipative mat that is connected to ground through a resistor, operators wear a wrist strap that is tested at the start of each shift, heel grounders or dissipative shoes are used where a strap is impractical, and insulators such as tape, foam, and plastic bags are kept out of the area unless an ioniser is used to neutralise them. The tools matter as well: a soldering iron with an ungrounded tip injects charge at the exact point of contact, so the iron tip, the rework station, and the test fixtures are all grounded.

Behaviour is the part that training can change. A board should never be carried loose or in an ordinary plastic bag, a stack of boards should not be slid across a bench, and components should not be removed from their packaging before they are needed. Where a board must be moved between areas, it goes in a closed dissipative tote or on a grounded cart rather than in a hand.

Storage, Environment And Verification

Storage areas raise the same questions as the line. Shelves and bins are dissipative, carts are grounded through their casters, and the humidity is controlled, because low humidity increases charging and high humidity, which reduces it, is also a moisture risk for the board. The compromise is usually a controlled relative humidity in the region of forty to sixty percent, which is high enough to limit static generation and low enough to keep the laminate dry.

Verification is an audit with instruments. The resistance of a work surface and of a wrist strap is measured with a test set against the limits in the standard, a shielding bag is checked for continuity, and the grounding of the tools and the benches is confirmed. The training record is the other half of the evidence, because a measurement that shows a bench is correct says nothing about whether the operator uses the strap. The overall quality framework that these records support is described under PCB design quality characteristics, the protection after assembly under board level protection, and the fabrication sequence under PCB design and fabrication.

FAQ

Does a pink bag protect a board from static discharge? It prevents charge generation and is dissipative, but it does not shield the contents. A sealed metalised shielding bag is what stops an external discharge from reaching the board.

Can a damaged board still pass electrical test? Yes. A latent ESD defect often shifts a parameter slightly without breaking it, so the board passes test and fails in service. That is the main reason the controls are applied rather than relying on test.

Is a wrist strap enough on its own? No. The strap protects against the operator, but insulating materials on the bench, an ungrounded soldering iron, or a discharge from a fixture can still reach the board.