PCB Design Basics: Libraries, Placement, Routing and Checks

PCB design is easy to start and difficult to finish well. The tool will accept any arrangement of parts and any route that does not violate the rules it has been given, and the judgement about whether the result is good belongs to the designer rather than to the software.

This article sets out the stages a board goes through from library preparation to the checks that release it, and the standard that separates a board that merely works from one that can be built, tested and maintained.

Before Layout: Libraries and Schematic

Nothing is drawn until the libraries exist. The footprint library is the more demanding of the two, because it decides whether the component fits the pads and whether the board can be assembled at all. Each footprint is created from the manufacturer drawing, and it is worth building the footprint library before the schematic library so that the pin definitions have something to correspond to.

The schematic library is more forgiving, but the attributes it carries are not. Pin numbering, the mapping between a symbol and its footprint, and the assignment of parts to packages all have to be correct, because these are the links that the netlist will follow. The checks that the netlist enables are the same ones that the workflow from rules to DRC depends on.

The Board Frame and Mechanical Features

The outline is drawn first, at the size and shape the product requires, and the fixed features are placed before anything else: connectors, switches, mounting holes and any part whose position is determined by the enclosure. The areas around the mounting holes where no copper may be placed are marked at the same time, because they constrain the routing later.

The division between the routing area and the rest is a decision, not a default. Where the enclosure has a rib, where a battery sits, or where a cable passes, the board may be mechanically present but electrically unavailable. Marking those regions in the layout database prevents a route from being drawn through them and discovered later.

Placement

Placement decides how difficult the rest of the work will be. The parts that cannot move are locked first, the core devices are placed with their supporting components beside them, and the interfaces are positioned against the connectors they serve. Everything else is arranged around those decisions.

Good placement is visible before a single trace is drawn: the functions sit in blocks, the power stages are separated from the sensitive circuits, and the connections between blocks follow the signal flow rather than crossing the board. The order used to achieve that is described in the notes on placing components in a defined sequence.

Routing: Three Levels of Quality

The first level is connection. Every net is routed and the board has no opens; that is the entry requirement rather than the goal. The second level is electrical performance: the critical nets meet their length, impedance and reference requirements, and the returns are continuous. The third level is legibility, where the routing is orderly enough that the next engineer can follow it.

All three levels are achievable, and the cost of reaching the third is small compared with the cost of not reaching the second. A board that is electrically correct but visually chaotic is difficult to review, difficult to modify and, in practice, more likely to contain a mistake that nobody noticed.

Optimisation and Silkscreen

Optimisation takes longer than the original routing, and that is not a sign that the work was done badly. Moving a trace to open a channel, rerouting a group so that the layers balance, or adjusting a via to reduce a stub is the work that turns a connected board into a good one, and it has to be done with the constraints in view rather than by eye.

The silkscreen belongs to the same stage. Reference designators are moved until they are legible and clear of pads, polarity marks are checked, and the bottom-side legend is mirrored so that it reads correctly from the side it is viewed from. This is also the point at which the legend is checked against the assembly drawing, because a mark that is present but unreadable is the same as no mark. Reading the legend on a printed assembly is the test, and it is worth doing on paper rather than on a screen.

Netlist Checks and Physical Checks

When the layout is complete, two inputs are compared against the result. The first is the netlist: a design rule check confirms that the copper matches the schematic, and comparison against the netlist catches the connections that were broken or added during routing. The second is the mechanical drawing, which the outline, the hole positions and the placement of the connectors have to match.

Both checks exist because a layout can be internally consistent and still wrong. An error in the schematic produces a netlist that the board faithfully reproduces, so the check is against the source document rather than against the drawing, and the source document is what has to be reviewed first.

DFM and Manufacturability Checks

The final stage is a check against the fabricator and the assembler. DFM analysis compares the design with the process capabilities of the shop that will build it: the minimum trace and space, the smallest drill, the annular ring, the clearance to the edge and the solder mask sliver between adjacent pads.

The rules that come back from this stage are the ones that a general rule set cannot supply, because they depend on the specific process. A design that was drawn to nominal rules and then adjusted to the fabricator’s capability will be built without a query, and the guidelines that keep a board manufacturable are what that adjustment is measured against.

When Requirements Conflict

PCB design is a negotiation between requirements that cannot all be satisfied. A board that needs six layers for signal integrity and a shield layer between routing layers may be constrained to four by cost, and the answer is a compromise: the cross-talk margin falls, and it has to be accepted deliberately rather than discovered.

Recording the compromise is what keeps it honest. Where a layer was dropped for cost, or a spacing reduced to fit a connector, the decision belongs in the design notes so that the next revision knows what was traded away. The rules that keep a layout efficient describe the choices, but the reason a particular one was made is knowledge that only exists in the review.

FAQ

How long should the checks take? Long enough to compare the board against the schematic and the mechanical drawing rather than to press a button and read a report. The automated checks find rule violations, not mistakes in intent.

Is a design complete when the checks pass? It is complete when the checks pass and someone has read the design. The two are different, and the second is the one that catches a wrong net that is correctly routed.

Why maintain a personal checklist? Because a checklist accumulates the mistakes that have already occurred. It is the cheapest form of quality control available, and every entry in it was paid for once.