PCB Design Guide for First-Time Designers

Components in preferred packages may be unavailable, and alternate replacements may be necessary. There isn’t much designers can do to circumvent this process, but luckily, updating the BOM assembly information via netlist associations at the schematic level is possible. Designers can even be proactive and create alternate part land patterns for components, such that implementation only requires the association mentioned above and any minor routine cleanup.

While many variables are in play, including the choice of substrate material, the trace factors most directly influencing the impedance will be the trace width and the dielectric height. The trace width can be misleading, as the acid etching will form a trapezoidal shape due to the solution having more contact with the upper sides of the trace as it works around the etch protectant. The base of the trace is always wider than the top, and manufacturers’ datasheets often report this value as the “singular’ width of the trace (in reality, the top width will be about a half-mil shorter).

Building the impedance profiles is only half of the stack-up usage. The designer will want to begin planning how they will distribute power planes, ground planes, and routing planes throughout the stackup to maximize signal and power layers while keeping reference plane distance to a minimum. Distribution should be balanced — think of the innermost core as a double-sided mirror. This method will not work for every design — sometimes, it may be necessary to borrow a plane layer for signal or vice versa, but it will often provide an excellent starting point.

Routing planes should always have an adjacent ground plane on one side for reference. Most designers will insert ground planes on layers two and the second-to-last layer to reference the outer-layer signals, an internal power plane or signal layer on layers three and the third-to-last layer. This stack-up monomer will continuously repeat on larger boards, though power or routing design needs may cause slight deviations.

1. Focus first on the most “central” signals or circuitry that represent the core functionality of the board and will have cascading effects if designed poorly. These will usually be things like data transfer lines, clock signals, and differential pairs. Prioritizing these project elements ensures no sacrifices involving their operation down the road.
2. Power circuitry design should include plenty of space around components and copper features to prevent induction in nearby traces, especially around switches. Capacitors must be arranged with capacitance increasing radially away from regulators/converters. Low capacitance nearby helps ensure that the quick charging/discharging elements are closest to the central power components while the less reactive capacitors sit further away.
3. If the board contains an FPGA, the pitch between pins/pads is likely the most limiting factor in determining the spacing between traces and the via size.
4. Routing should be direct and used via transitions at most necessary. One method of organization is to have routing layers organized into alternating vertical and horizontal weaves. This crisscrossing serves two benefits: first, it is an organizational tool to maximize layer routing on dense boards. Secondly, alternating the travel direction minimizes plane-to-plane coupling on adjacent layers, bolsters signal integrity, and keeps actual impedance closer to targeted values.
5. The stackup and any sensitive equipment should be considered, especially regarding split power planes. Traces referencing multiple planes may not possess the characteristic impedance described in the stackup due to poor return paths. Split or mixed planes constrain routing on the design layer and any layers that reference said layer.

• Feature Callouts – Uncommon board features that require attention, such as board cutouts, can be specifically called out in drawings to prevent being overlooked in the chain-of-design processes.
• Silkscreen Legibility– Placing silkscreen text elements is difficult when SMD/TH component density reduces the spacing available for text. While there is value in including as many identifier tags as possible, a greater legibility approach includes as many necessary/valuable silkscreen tags as possible without shrinking the text beyond some established minimum values. Generally, this draws attention to test points (helpful for testers), IC components, and connectors. Resistors and components are usually of less concern than the parts mentioned above and would likely have their silkscreen text hidden away, where necessary.
• Document Readability – This follows from silkscreen legibility: viewing documents from the computer will be significantly different from reading them off a printed page. While it is trivial to zoom in on text in an online PDF with a rich dpi screen, reading small or unclear text is likely to lead to production issues, not to mention several calls and visits from the floor for clarification. Document elements like the drill chart should be quickly digestible with visually distinct legend symbols.
• Fabrication/Assembly Notes– These notes will include general statements on standards and items relevant to the board’s specific processes during manufacturing. For customers, this overviews previously agreed-upon processes that will provide the final board with the highest quality, yield, etc. Tolerances will also establish guidelines for acceptability.