Modern society is saturated with printed circuit boards. Despite this, it can be difficult to explain such a technical field to those looking to gain entry or even just knowledge.
Regarding layout design, I like to start with a classic topology logic puzzle: on a piece of paper, draw three evenly spaced houses on a line, and somewhere above or below them, three evenly spaced utilities (gas, electricity, and water). The goal now is to connect all three houses to all three utilities in the plane without crossing any lines or passing lines through houses or utilities. No matter how the lines are drawn or in which order, fitting the last connection in will prove impossible. This problem, known informally as the “three utility problem,” gives good motivation for using vias in layout.
Most boards, instead of three “components” and nine connections, feature hundreds and thousands; modern PCB design is several times more constrained and flexible than the puzzle. To achieve a design that maximizes space and performance, layout work begins before a single trace is run between parts. Mastering PCB layout basics will help designers increase efficiency during board design and improve manufacturability.
A Five-Point Plan for PCB Layout Basics
The basics of PCB layout can be outlined as a five-step process:
- Part creation: This may or may not fall to the layout designer, depending if there is a dedicated librarian or not. Part creation involves designing the land pattern for either SMT or through-hole components, including the pads, drilled holes, placement information (polarity, pin 1, etc.), and the general space requirements to avoid arranging components too tightly. These land patterns are associated with reference designators in the schematic and are transferred to the board via the netlist.
- Stackup: The stackup is the physical attributes of the board as defined by the materials used, their thickness, and the distribution of signal/plane layers throughout the board. These variables are immediately responsible for the impedance profile for single-ended and double-ended (e.g., differential pairs) traces. Field solver software, whether standalone or as part of a layout software package, is used to accurately model trace width and spacing and segue into the greater board design rules.
- Design rules: The combination of constraints due to manufacturing or devised by the designer for some particular characteristic that acts as both guiding principles and safeguards against time spent laying out unproducible features. Most design rules can be sorted between those unlikely to change from board to board and those defined by the board or component characteristics, such as the drilled hole diameter for vias.
- Placement: The layout designer places components in such a way that groups circuits to promote short, direct traces. Ostensibly, a schematic with even a moderate amount of components cannot be fully detangled, but this is alleviated with additional routing planes. Design documents may require certain key components, like connectors, to be placed in a designated position from a particular board feature. Placement can occur on the top and bottom sides in exceptionally dense assemblies, but there are advantages to limiting parts to one side of the board. As a prelude to routing, vias fan out should be utilized to ensure the placement is not so tight as to disallow breakout from fine-pitch components and as a guide for power plane design.
- Routing: The culmination of the layout process has the designer connect all the components together as specified in the netlist. Depending on the particular function of a circuit, nonstandard copper features can be utilized to improve parameters like electrical/thermal conductivity or for entirely new purposes, such as waveguides in RF. Before any signal routing, designers should connect any power nets to establish a stable distribution network that avoids winnowing that can starve load delivery and lead to thermal hotspots. This also has the advantage that designers will know where to avoid routing over split planes to prevent signal integrity issues. After the power nets are connected, designers can begin by routing the most important signals (differential pairs, clocks, etc.) before finally finishing with the less critical signals.
Good Artwork Powers Current and Future Revisions
At this point, the significant layout tasks are complete, yet the precision work of manufacturing remains distant. The best-laid board can rapidly become scrap with poor instructions provided to the shop. To combat this, designers need to furnish manufacturing artwork that is unambiguous and goes beyond the minimum callouts to anticipate potential deviations:
- Enclosed cutouts: A polygon sitting within the board’s perimeter can be easily misconstrued in several ways. Clearly label cutouts with bold text and large font on the appropriate layers.
- Drill drawing: Artwork is often thought of as existing only digitally to the designer, but it is often the case that it may be printed out for reference on the floor. Unlike a computer screen, a printed image has a fixed resolution. It can be difficult to read areas of high drill traffic if symbols are overlapping and easily mistakable. Layout designers should accommodate operators on the floor by choosing distinguishable drill symbols and ensuring artwork is properly scaled when printing to manufacturing files.
- Reference designators: In a dense assembly, designers will have trouble fitting in all reference designators without shrinking to nigh illegible font sizes. Not only does this pose issues for the manufacturer, but it can also make diagnosing second-hand rework nearly impossible. Instead, designers should focus their efforts on prioritizing select components such as:
- Polarized components (incl. pin 1/a or pin n text for connector orientation)
- Test points to assist in clarifying board testing
- ICs
Mistakes and refinement are a natural part of designing printed circuit boards. As the board develops from proof-of-concept to prototype to mass production, certain design strategies may fall into disfavor that scales poorly in cost or ease of manufacturing. This may include small to moderate changes like a reduction in via count or alterations to circuitry (new packages, new components, etc.), or on the extreme end, a reduction in layer count. While it’s infeasible for a designer to be able to predict alterations based on market whims or an expansion/contraction of circuit functionality, a well-designed board during the initial stages of product development will ease the challenge of future revisions.
Your Contract Manufacturer Is Anything But Basic
PCB layout basics are easy in theory to disseminate, but every board will have its quirks. The best remedy is a design team acquainted with various board styles. However, no one has seen everything in PCB, it is much easier for those with considerable experience to adapt to the evolving needs of a board or the industry in general. For that reason, we here at VSE see ourselves as particularly well-situated for the challenges of modern PCB design. As a team of engineers, we build electronics for our customers – it’s that straightforward. Alongside our valued manufacturing partners, we provide exceptional PCBs in several life-saving fields.
