Digital Isolator Creepage and Clearance

A digital isolator carries logic signals across a barrier that must survive thousands of volts, so the package rating is only half of the story. The creepage across the board surface, the clearance through the air, and the way the two grounds are separated all decide whether the measured withstand voltage matches the datasheet. Layout mistakes here are not performance problems; they are safety problems.

What a Digital Isolator Replaces

A digital isolator transfers a logic level across a galvanic isolation barrier using capacitive or magnetic coupling rather than light. The advantage over an optocoupler is speed and life, since there is no emitter to age, and the advantage over a transformer coupled interface is that the device handles DC levels and provides a defined logic output without external signal conditioning.

Applications range from isolated gate drive and fieldbus interfaces to medical equipment and mains referenced measurement. In every case the purpose is the same: to allow data to pass while preventing any metallic connection that could carry fault current, expose a user to line voltage, or create a ground loop that corrupts the measurement.

The device datasheet will quote an isolation voltage, a working voltage and a set of creepage and clearance figures. Those figures describe the package. Whether the board achieves them depends on pad geometry, solder mask, copper pours and the mechanical enclosure, so the layout has to be designed against the same standard that the part was qualified to.

Creepage and Clearance Defined

Clearance is the shortest distance through air between two conductive parts, while creepage distance is the shortest path along the surface of the insulating material between the same two parts. They are different numbers because air breaks down at a different rate from a contaminated surface, and moisture, dust and condensation all reduce the surface performance far more than they affect the air gap.

Pollution degree and material group are the two parameters that link the physical layout to the required numbers. A board in a clean, dry enclosure is treated as pollution degree 1, while an industrial cabinet with condensation is pollution degree 2 or 3 and demands noticeably larger creepage for the same working voltage. Material group describes how the laminate behaves when tracking occurs on the surface.

A slot or a routed cut under the package increases creepage without affecting clearance, because the surface path has to go around the slot while the air path stays the same. That is why isolated packages often sit above a milled slot in industrial designs, and it is also why conformal coating, which protects the surface from contamination, can reduce the creepage requirement in some standards.

Isolation Ratings and Working Voltage

Functional isolation is the minimum level and only prevents the circuit from malfunctioning. Basic insulation protects against electric shock, and reinforced insulation provides the same protection as two layers of basic insulation, which is what most mains referenced interfaces require. The distinction decides whether the working voltage can be continuous or only transient.

Reinforced insulation typically doubles the required creepage and clearance for a given working voltage compared with basic insulation, and the standard usually imposes a minimum distance regardless of how low the working voltage is. For a 250 volt rms mains referenced circuit, a common requirement is 8 millimetres of creepage and 5 millimetres of clearance, but the applicable standard and pollution degree must be checked before the layout is fixed.

The isolator package may offer more than the board needs or less. A narrow body part with 5 millimetres of creepage cannot satisfy a reinforced requirement of 8 millimetres no matter how it is soldered, so the package has to be selected after the requirement is known. Choosing the part first and the standard second is the most common way to end up with a board that cannot be certified.

Layout Across the Barrier

The barrier has to be a genuine break in the copper. Split the ground plane into two regions, keep every trace on each side within its own region, and place the isolator so that the package body straddles the gap with the pads on either side. No signal, no power and no test point should cross the line except through the isolator itself.

Keep the region under and around the package free of copper on all layers where the standard requires it, because an inner layer plane that runs beneath the package reduces the effective clearance to the plane rather than to the surface. This detail is easy to miss on a four layer board and it is exactly what a compliance test will find.

Route the two sides with independent reference planes and separate stitching vias. If the primary side has a ground pour, the secondary side needs its own, and the two should not be connected by a row of vias that happens to follow the board outline. Anywhere the grounds meet, the isolation is gone, so the mechanical drawing should show the barrier as a controlled feature, the same way via to trace clearance is treated on high density boards.

Power, Ground and Data Across the Barrier

An isolator needs supplies on both sides, and each side should be decoupled with its own capacitor placed close to the pin. Where the secondary side is powered from an isolated converter, that converter is part of the same barrier and its transformer has to meet the same creepage requirement as the signal isolator, including the pad geometry under its own package.

Keep the data path short on both sides so that the isolator is not driven by long stubs. The primary side typically connects to a controller pin that can drive a few milliamperes, and the secondary side drives whatever the field interface needs. Series resistors on the output are useful for limiting current into a long cable, but they also slow the edges and should be chosen with the data rate in mind.

If the two sides need to exchange more than a few signals, a single multi channel isolator is usually better than several single channel parts. One package means one barrier to control, one set of creepage dimensions to verify and one place for the compliance engineer to inspect, which shortens both the layout work and the certification review.

Our guide to manufacturable PCB design covers the general rules that keep barrier features producible. The barrier must survive assembly, so keep it clear of the board edge, of mounting holes and of any mechanical feature that could bridge it with a fastener or a metal standoff.

Common Layout Mistakes

The most frequent mistake is a ground pour that runs from one side of the board to the other because the outline was drawn before the isolation requirement was known. The second is a via that connects the two ground regions under the package, usually added to fix a thermal or stitching complaint without anyone noticing what it does to the barrier.

A third mistake is placing a connector that carries both isolated and non isolated signals on the same side of the barrier. Even if the schematic is correct, the mechanical arrangement invites a field wiring error that shorts the two grounds together through the cable shield. Physical separation of the two connector groups is a better defence than a warning label.

The last common error is relying on an unpopulated component footprint to provide the barrier. A pad pattern in the middle of the isolation gap reduces creepage even when no part is fitted, and the reduction is invisible on the schematic. If a placeholder is needed for a future option, place it on one side of the barrier rather than across it.

Testing and Verification

The type test applies the rated isolation voltage between the two sides for one minute and measures the leakage current. Before that, the production test usually applies a lower voltage, often 1.5 to 2 times the working voltage plus a margin, on every unit. The board has to be laid out so a probe can reach both ground regions without touching anything else, which is a small design decision that saves time on every unit built.

Insulation resistance measured with a high voltage megohmmeter detects surface contamination that a hipot test may pass. Both tests are normally applied at the connector level, with all pins on one side tied together, so the connector pinout should group the isolated signals physically to make that fixturing simple and repeatable.

Do not forget the mechanical assembly. A metal standoff placed between the two regions, a screw that touches a plated hole on the far side, or a thermal pad that spans the barrier will all defeat the isolation in a way that no electrical test of the bare board can detect. Review the assembly drawing for anything conductive that could bridge the line, and specify a minimum distance from the barrier to every mounting feature.

Additional Considerations for This Build

Practical attention to clearance distance pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating clearance distance explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

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

FAQ

Can I use a single ground plane with a slot cut in it? Yes, provided the slot spans the full barrier, including the area under the isolator package, and the creepage path measured around the slot meets the requirement. The slot must reach far enough that no copper bridges it anywhere on the board.

Does the isolator rating alone satisfy the safety standard? No. The package is qualified for a given creepage and clearance, but the board, the enclosure and the pollution degree decide what the finished product can be certified to. Both have to be considered together.

Is a common mode transient immunity figure important? Yes in motor drive and power conversion applications. A fast common mode transient across the barrier can corrupt data or latch the output, so the dynamic immunity specification matters as much as the static isolation voltage.