ESD Protection Devices: Selection and Placement

A protection device that is not on the board, or that is on the board but a centimetre from the connector, provides much less protection than its data sheet implies. The selection and the placement are one decision.

What the Device Must Do

The device must remain invisible during normal operation and become a low impedance path during a transient. Its specifications therefore include the standoff voltage, the capacitance, the leakage and the clamping voltage.

The standoff voltage must be above the maximum normal signal level, including any overshoot, so that the device does not conduct in normal operation.

The capacitance must be low enough not to affect the signal. On a fast data line, a few picofarads at the wrong place will round the edges and reduce the margin. Our EMI immunity notes describe the related filtering measures.

Types of Device

A transient voltage suppressor diode clamps at a defined voltage and recovers. It has a higher capacitance and is suited to power rails and to slow signals.

A varistor clamps by changing resistance and degrades with each event. It handles more energy and is used where the transient is large, such as on a mains input.

A polymer device changes resistance on heating and returns. It handles low energy events and is used on interfaces where the capacitance must be very low. Each type has a place, and the choice follows from the line’s speed and the transient’s energy.

Placement and the Return Path

The device must be between the connector and the circuit it protects, so that the transient current is diverted before it reaches the sensitive part. A device placed after a series element protects only what is behind it.

The return path from the device to the reference must be short and wide. A protection device with a long thin ground return has an inductance that raises the voltage seen by the circuit when the device conducts.

The reference should be the same one the circuit uses, which for an interface is usually the connector ground. A device referenced to a different ground plane routes the transient current through the board. Our industrial assembly notes describe the grounding used for interfaces.

Series Elements

A resistor or a ferrite bead between the connector and the circuit limits the current that the protection device must divert and slows the transient. It is often the element that makes a small device sufficient.

The series element also adds impedance to the signal and, in the case of a ferrite bead, may add loss at high frequency. The value must be chosen against the signal requirement.

Where a series resistor is used, its power rating must accommodate the transient current. A small resistor that fails open during an event leaves the circuit unprotected and is a common weakness in a design that looks correct on paper.

Clamping and Coordination

Where several devices are used at different points, their clamping voltages must be coordinated so that the first device conducts before the second. Otherwise the second device absorbs the event and the first does nothing.

The coordination is achieved by choosing a higher clamping voltage at the input and a lower one at the circuit, which is the opposite of what intuition suggests.

The transient voltage rating of the protected circuit must be above the clamping voltage of the device, with margin for the inductance of the current path. Our design release checklist notes where the requirement is recorded.

Verification

The verification is an injection test at the connector with the transient specified by the applicable standard, with the circuit powered and monitored.

The measurement should be made at the protected device’s pins, not at the connector, because the protection is what happens at the device. A measurement at the connector shows the transient and not the residual.

Where the test fails, the usual cause is the return path inductance or a device with too high a clamping voltage, and both are visible in the measurement.

Layout Checklist

Place every protection device within a few millimetres of the connector and on the same side as the interface signals. Provide a short, wide return to the connector ground.

Route the protected signal away from the unprotected side of the device and do not allow a stub that bypasses the protection. A trace that branches before the device sends part of the transient straight through.

Keep the transient current path away from the sensitive circuit, since the magnetic field of the transient is itself a disturbance. Our solder defects notes describe the assembly conditions that can make a joint in the protection path open.

Process Control and Verification

On a design of this kind, clamping voltage is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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.

Process Control and Verification

On a design of this kind, clamping voltage is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Process Control and Verification

On a design of this kind, clamping voltage is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

Does a protection device eliminate ESD damage? It limits the voltage and the current that reach the circuit. Where the residual is still above the circuit’s rating, a second device or a series element is needed.

Can the same device protect several lines? An array device can, and its placement must be such that each line’s return path is short. A shared return that is long for one line reduces the protection on that line.

What does gopcb provide for ESD protection? We provide device selection matched to the line speed and the transient energy, placement and return path design, coordination where several devices are used, and injection testing to confirm the residual at the protected device.