Layout Review Checklist: Placement, Ground and Decoupling

Most layout mistakes are not exotic. They are the same ones that appear in every design review: a part that cannot be replaced, a decoupling capacitor too far from its device, a ground that was never thought about, a connector that collides with the enclosure. A checklist exists because these errors are cheap to find before the board is built and expensive after.

This article works through the review points that a placement and a routing should be checked against, in the order they naturally arise.

Placement Review

The review starts with fit. The outline has to match the mechanical drawing, the tooling marks have to be present, and every part has to be checked in three dimensions, not two, so that a tall capacitor beside a connector cannot clash with the enclosure. The placement also has to be balanced, with no area crowded and none empty by accident.

Serviceability comes next. Components that will be replaced during the product life, adjustable parts and anything that has to be reached with a tool need clear access, and a connector has to be reachable for the cable that will mate with it. The order used to place the parts is what makes this review short, because the constraints were respected as the work was done.

Thermal and Sensitive Components

Heat is a placement problem before it is a routing problem. Parts that dissipate have to be separated from parts that measure: a temperature sensor beside a regulator will report the regulator, and a crystal beside a hot inductor will drift. The distance is a design decision, and it should be recorded rather than left to the smallest gap that the placement allows.

Where a heatsink is required, the space it occupies belongs in the layout as a keep-out, and the air path it depends on has to be clear of tall components. Mechanical parts such as buzzers and inductors often interact as well, and separating the ones that generate from the ones that are sensitive is cheaper than filtering the result.

Placement of Fast Devices and Shared Supplies

Speed is a function of distance. A fast memory placed close to its processor can be routed with short traces and a small loop area; the same device placed across the board has to be routed to reach it. Devices that share a supply belong together for the same reason, because the current loop they form is then short and the decoupling is more effective.

The rule is not absolute, but the exceptions should be deliberate. Where a fast device must be distant, the reason is usually mechanical, and the consequence, which is a longer routing delay and a larger loop, should be understood before the decision is made.

Routing Direction and Separation

Signals are grouped by what they are: inputs and outputs, high and low voltage, fast and slow, analogue and digital. Each group is routed with its own direction and its own area, so that groups run alongside each other only where it is unavoidable. Parallel runs between an input and an output are a coupling path, and separating them costs nothing at the layout stage.

Where two layers have to run in the same area, the traces on one should cross the other at right angles rather than run parallel, because parallel traces on adjacent layers couple both through the dielectric and along their length. Where a group has to cross another, a grounded trace between them is a barrier, and it should be routed as a deliberate feature rather than discovered as an option.

Choosing and Handling Ground

The ground connection is the single most important decision in the layout, because every signal current returns through it. A design that treats ground as a convenience rather than a network will produce a board whose noise performance is unpredictable, and the symptom will appear somewhere far from the cause.

The usual requirement is that the return currents of sensitive circuits are kept out of the returns of noisy ones, and that the two meet at a defined place, typically the point where the supply enters the board or where the conversion stage sits. A continuous plane with a defined division achieves that; several disconnected islands connected by long traces do not.

Decoupling Capacitor Placement

A decoupling capacitor works through the loop it forms with the device it serves, and the loop includes the vias and the traces between them. Placing the capacitor far from the pin leaves a longer path, and at the frequencies the capacitor is meant to address, that path is the dominant impedance.

The most effective position is on the opposite side of the board directly beneath the device, with short vias connecting capacitor, plane and pin, so that the supply reaches the capacitor before it reaches the chip. Wide traces rather than thin ones, and vias placed immediately at the pad rather than at the end of a stub, are what make the difference. The planning of power and ground conductors covers the same territory at the level of the whole board.

Trace Geometry: Width, Corners and Layer Changes

Width follows current first and impedance second: a trace that is wide enough is never a mistake, and where the process allows a wider line, the narrower one buys nothing. Angles are the other easy decision. A right angle is a point where the trace width changes and where the impedance is discontinuous, so corners are drawn at forty-five degrees or as arcs, particularly on high-voltage and high-frequency nets.

Ground should be the widest conductor on the board, followed by power, with signals narrowest, because the impedance of the return path matters as much as the impedance of the signal. Reducing the number of vias on a signal path, and avoiding parallel runs of the same net on adjacent layers, both reduce the opportunities for coupling. The geometry of a via and the pad around it is covered by the rules for via design.

Mixed Analogue and Digital Grounding

On a board that contains both domains, the routing problem is the same as on any other: the digital section draws current in fast bursts, and the analogue section measures small signals. If the two share a return path, the measurement contains the burst.

The practical approach is to keep the two areas separate in placement, to keep the analogue signals away from the fast digital nets, and to join the grounds at one point rather than in several places. The reason the domains are kept apart is described in the <a href="https://www.gopcba.com/mixed-signal-pcb-design-guidelines/” title=”mixed-signal layout guidelines”>mixed-signal layout guidelines, and the single joining point is what makes the separation meaningful rather than decorative.

FAQ

Is a single ground plane always better than a split one? A single unbroken plane is usually the better default, because it avoids the question of where the two halves are joined. A split is justified when currents genuinely must be kept apart.

How many decoupling capacitors does a device need? Enough to cover the frequency range it generates, which usually means several values rather than one, placed where their loops are short.

Do these checks replace a design review? They organise it. A checklist makes the review repeatable, and repeatability is what stops a known mistake from returning in the next revision.