Have you ever been overwhelmed while shopping to the point of inaction? A phenomenon known as decision paralysis may be to blame. While many people make shopping lists as a memory aid, some use them to help cut through the mental clutter of choices. Even if the stakes may not be particularly high, a list provides unambiguous step-by-step instructions that help wrap up a shopping trip in the shortest possible time.
Similarly, PCB development can benefit from a checklist for production appropriateness. The goal of any PCB DFM checklist is to minimize costs associated with poor manufacturing results (including time for design revisions) to enhance yield and performance. DFM follows general best practices but also needs to be geared to the design intent of a particular board, such as the IPC Classification and Producibility Level. Additionally, the sophistication of the manufacturer’s toolset will also be of importance. Most shops will cover all basic PCB processes. However, a board’s design requirements may be beyond their abilities or so difficult to produce that a job is only first accepted in an exploratory capacity.
PCB DFM Checklist At-a-Glance
The Stackup Forms the PCB Framework
DFM needs to fully capture every aspect of the board’s design to enhance production outcomes. Therefore, the best place to start is at the stackup. The stackup provides the layer-by-layer build of the board as an alternating conductor and dielectric construction. The purpose here is twofold:
- Mechanically meet the thickness requirement of the board (extremely thin or thick boards become more difficult to process, and the latter incurs extra materials cost).
- Electrically plan out the distribution of signal/plane layers to promote signal integrity and build the impedance profiles of single or double-ended traces (i.e., differential pairs found in high-speed transfer protocols like USB or HDMI).
The stackup should be a balanced construction about its center for both conductor and dielectric materials. During lamination, warpage can occur due to different thermal expansion rates when the board materials are fused with high heat and pressure. Boards that exhibit minimal warpage are likely to encounter issues during assembly due to the lack of coplanarity. Keeping the distribution of layer types and thickness mirrored about the vertical center can mitigate any bow and twist of the board while also fulfilling electrical requirements. Boards with especially thick copper foil layers will also want to consider the distribution of copper throughout the board to avoid having any vertical regions that are relatively poor in copper.
Fabrication and Documentation Best Practices
Fabrication will cover the general layout constraints of the board. Here, designers will want to define the size of central features such as trace width, clearances, hole sizes, and more. Because the board’s requirements will dictate the producibility, product development will need to balance performance against cost. Smaller features will be more difficult to produce and reduce yield, but they may be the only way to fit an HDI circuit on a size-restricted board.
The design rules are central to the layout and are unlikely to cause issues or defects, assuming they have been implemented as discussed between the different teams. It is far more likely that poor documentation undermines production due to ambiguous or omitted instructions. Ensure artwork and notes contain the following elements to prevent errors:
- Drill chart: The drill chart indicates the different sizes and occurrences of the holes drilled during fabrication. Whenever possible, the number of distinct hole sizes should be minimized to prevent slowdown during drilling. Additionally, drill symbols should be simple and distinct for ease of readability.
- Component indicators: Indicators will be used as a reference for component placement, especially in manual soldering. They should be large enough to be reasonably viewable without excessive magnification; avoid overly muddling the indicators by crowding and shrinking to fit. Instead, prioritize the most important indicators – test points, polarized components, connectors, ICs, etc. – and hide bidirectional indicators.
- Scale: Designers with the freedom to zoom in and out of documentation may end up outputting artwork that is small and difficult to read once printed. Check to ensure any documentation headed to the manufacturing floor is appropriately sized.
For Assembly, A PCB DFM Checklist Improves Solderability
The assembly is, in many ways, a split process. While the actual placement and bonding of discrete components will occur following fabrication, component sourcing will be one of the first steps once a schematic iteration is finalized. While simulations are a foundational part of design competency, procurement allows for verifiable board testing. Production runs of any size will want to ensure they have at least the minimum number of per-board components to fulfill the lot and likely some spares to account for rework and design revisions. With component lead times still experiencing a significant hangover due to manufacturing shutdowns and facilities running at less than full capacity, it is imperative to establish sourcing channels at the earliest opportunity.
A secondary advantage of placing the BOM order is the allowance for replacement parts in the case of obsolescence or end-of-life (EoL) production status. Replacement parts may confer benefits or drawbacks depending on the part they’re replacing. For example, a newer packaging style may reduce the land pattern and free up routing space but have an increased per-unit cost.. Larger component replacements may force a more extensive revision depending on the assembly density or can slow overall production time due to lengthier processing from manual soldering. The earlier the BOM list can be verified and components obtained, the faster design can respond to any changes in design.
Designers themselves will have a profound impact on the success of the assembly. Still, it can be more subtle than the fabrication:
- Land patterns: This task may fall to the designer or the library manager, depending on the team size. Regardless, parts cannot solder down successfully without a correctly designed land pattern. Land patterns also need indicators for orientation, such as polarized components or reversible connectors.
- Manufacturers’ layout guidelines: Often found for power circuitry, component datasheets will offer the suggested layout for best performance and how the adjustment of certain passives may be used to alter functionality. Designers should heed these suggestions without fail unless there is a compelling DFM reason based on a board’s specific implementation.
- Placement density: Designers must fit many copper features and components within a small area without sacrificing circuit performance. While components must be tightly packed in HDI cases, signals still need room to fan out (unless via-in-pad is used). Additionally, components with large size discrepancies placed too close together may cause soldering issues: tall components can create a solder shadow during wave soldering that prevents solder from accessing small components behind them in the direction of the traveling wave, and manual rework of solder defects may be confined.
- Placement distribution: Making space for components often requires both sides of the board. Single-side placement (if possible) reduces the number of soldering steps a board has to withstand, improving turnaround time and limiting heat-related aging.
Your Contract Manufacturer Checks All the Right Boxes
A PCB DFM checklist is a necessary preparation for NPIs or design revisions to minimize cost and accelerate production. Engineers, designers, and manufacturers can lend their particular expertise to development to provide a seamless transition from schematic to final product.
The best DFM methodology comes from those with extensive experience in PCB manufacturing, and VSE has built its sterling reputation over 40 years of producing high-quality electronics. As a team of engineers committed to building electronics for our customers, we dedicate ourselves to defect-free designs that improve users’ livelihoods.