PCB Layout: Placement and Routing Practice

PCB layout is where a schematic becomes a piece of hardware, and most of the problems that appear during bring up were decided on the screen rather than in the fabrication shop. Placement sets the shape of every current path on the board, the routing strategy decides how much of that path behaves like an antenna, and the design review is the last point at which either can be changed cheaply.

The work divides into three questions that are answered in order. First, where does each block sit relative to the connectors, the power entry and the heat sinks. Second, which nets carry fast edges or large currents and therefore need a defined return path. Third, which of the remaining decisions can be left to the router and which have to be constrained by hand.

Placement Decides the Rest of the Layout

Component placement is the highest leverage step in the whole exercise. A part that sits between a driver and its load can be routed almost any way and still work, while the same part placed on the far side of the board turns a short net into a loop that radiates. The effort spent moving blocks on the screen is repaid many times over during routing.

Start with the fixed items: connectors, mounting holes, indicators and anything that has to align with a mechanical part. Then place the power conversion stage close to the input, keep the clock and its load together, and keep analog front ends away from switching nodes. Functional blocks placed as blocks make the routing obvious instead of forcing it.

Current Paths Before Signal Paths

The tracks that carry real current define the copper that everything else has to live with. A power stage that shares a thin ground return with a sensitive analog block will inject switching noise into it through the impedance of that shared copper, no matter how careful the analog routing is afterwards.

The practical rule is to draw the high current loops first, on the layer and width they actually need, and then check what is left for everything else. Where a ground plane is split between a noisy and a quiet region, the two should be joined at a single defined point rather than connected by the general pour.

Define the Signal Return Path

Every fast signal travels as a loop, and the return current takes the path of least impedance, which on a plane is directly under the trace. A routing strategy that crosses a gap in that plane, or changes reference layers without an adjacent ground via, forces the return current to detour and creates an impedance discontinuity that shows up as ringing and radiation.

This is why the layer arrangement matters as much as the routing itself. Assign a continuous reference plane to the fast nets, bring the reference with the signal when a layer change is unavoidable, and keep the stitching vias dense around any region where a plane is interrupted by a connector or a cut out.

Routing Strategy for Dense Areas

The area around a large device is where the routing strategy is decided. Escaping the pins in an order that keeps related signals together, and reserving channels for the nets that will need them later, prevents the situation where the last few connections have to be made by a long detour across the board.

Differential pairs, impedance controlled nets and length matched groups belong on the layers where their reference is cleanest. Everything else can be routed for convenience. Separating the board into a controlled region and an unrestricted one keeps the constraints manageable and stops the design rules from being applied where they do not matter.

Thermal and Mechanical Reality

Copper that carries current also carries heat, and the thermal path from a device to the ambient is a layout decision. Thermal vias under a power pad, a copper area on the opposite side and clearance from temperature sensitive parts all belong in the placement plan, not in a later revision after a prototype runs hot.

Mechanically, the board has to survive assembly and use. Components near a score line or a mounting screw will see stress, tall parts near the edge can be damaged in handling, and a heavy component on a thin board needs support. These constraints are easy to satisfy at the layout stage and expensive to discover at the assembly house.

Design Review and Documentation

A structured design review catches the errors that the tools cannot: a connector mirrored, a net tied to the wrong rail, a via that lands on a pad that will be soldered. Walking the schematic block by block against the layout finds these far more reliably than a visual scan of the finished artwork.

The review should end with a documented set of checks, which is the same discipline described in the note on layout review mistakes. Rules that were applied by hand rather than by the constraint manager should be listed, because the next revision will otherwise start without them.

Handover to Fabrication and Assembly

The layout is finished when another party can build from it. That means fabrication data with a stackup, impedance requirements and drill information, and assembly data with orientation, polarity and a bill of materials that matches the reference designators. A package that needs a phone call to interpret is not finished.

Where the board has been designed with the design guidelines for manufacture in mind, the handover is mostly a formality. Where it has not, the supplier will either ask questions that delay the order or quietly adjust the design, and the second outcome is the one that costs money.

Process Control and Verification

On a design of this kind, design review is the item that decides how the rest of the board is arranged. 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.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

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

FAQ

Should placement be finished before routing starts? In practice the two iterate, but the major blocks should be fixed first. Starting to route before the placement is stable guarantees that the routing has to be redone.

How wide should a power trace be? Width follows the current and the permitted temperature rise, and the calculation is the same one used for any conductor. The width of the return path matters just as much as the width of the forward path.

Is a ground pour always a good idea? Only where it is continuous and connected. An isolated island of copper connected by a single via can be worse than no pour at all, because it couples noise rather than screening it.