How to Avoid Pitfalls in PCB Design

Designing a printed circuit board (PCB) requires careful attention to various factors to ensure a smooth manufacturing process and avoid potential pitfalls. From hole size and slot design to tra ce width and copper pour considerations, understanding these aspects is crucial for creating reliable and functional PCBs. In this article, we will explore some common pitfalls in PCB design and provide recommendations to overcome them.

Hole Size in Via Design

In PCB manufacturing, a 0.3 mm hole is considered standard, while holes smaller than 0.3 mm are classified as small holes.

Small holes can have several negative impacts on production:

Difficulty in Plating: Smaller holes are more likely to result in unplated or poorly plated holes. For small holes, JLCPCB uses a four-line low resistance process to ensure reliability.

Lower Processing Efficiency: Smaller holes require slower drilling speeds and shorter drill bits. Subsequently fewer boards are drilled at a time. Therefore, when designing, it's advisable to use holes larger than 0.3 mm, considering the use of small holes only when space is limited.

JLCPCB's minimum process capabilities:

Single/double-sided boards: 0.3 mm (inner diameter) / 0.45 mm (outer diameter)

Multilayer boards: 0.15 mm (inner diameter) / 0.25 mm (outer diameter)

Outer diameters should be 0.1 mm larger than the inner diameters, with a recommended difference of 0.15 mm or more.

Slot Holes in Via Design

Short Slots in PCB Drilling: Slots with a length less than twice their width are termed short slots. The optimal length-to-width ratio for short slots is length/width ≥ 2.5 (with a limit of ≥ 2).

Using Hot Air Solder Leveling for Long Slots: For slots selected for hot air solder leveling, it's recommended that one side of the slot hole has a minimum width of 0.4 mm (with an absolute minimum of 0.3 mm) or consider using gold immersion technology.

Hot air solder leveling involves high-pressure and high-temperature processing. Small slot widths can lead to plating peel-off.

Slots with copper plating on one side but no copper ring on the other side, particularly single-sided copper ring slot holes, are more prone to peel-off due to high-temperature stress.

For densely arranged slots, it's advised to use gold immersion technology.

Hole Attributes in Via Design

During design, hole sizes are categorized as VIA holes or PAD component holes, and it's crucial not to mix them.

VIA Holes: These typically serve the purpose of providing electrical connections between circuitry on both sides. During manufacturing, VIA holes are usually treated with soldermask, and JLCPCB does not specify VIA hole size tolerances during production.

PAD Component Holes: These are generally designed as component holes for soldering components.

Mixing them can lead to:

Mistakenly treating VIA holes as PAD component holes and selecting solder mask treatment can result in solder mask covering or clogging component holes, making it unsuitable for soldering. This also makes it difficult to control their size effectively.

Using PAD component holes as VIA holes will prevent the software from giving them soldermask, resulting in vias requiring soldermask treatment remaining uncovered.

Trace Width and Spacing in Trace Design

Thinner traces are not necessarily better; wider traces are preferable whenever conditions allow. Thinner traces have lower conductivity, a higher likelihood of breakage, and lowered yield rates.

Standard trace width and spacing: 4 mils for both. Anything narrower than 4 mils is considered thin.

JLCPCB's minimum trace width/spacing capabilities:

Hatched Copper Filling in Large Area Traces

When filling large areas with copper, avoid using small hatched grids as this can severely impact production. Choose solid copper fill or large grids instead of small ones. Small grids can lead to several issues:

Inadequate solder mask coverage, as illustrated.

AOI (Automated Optical Inspection) may not detect small grids and may result in errors.

Small grids can cause solder mask peeling, affecting conductivity.

If you must use grid copper filling or need to separate networks, ensure:

Grid line width/spacing is at least 0.254 mm.

There is at least 0.254 mm spacing between traces from the same net.

Uneven Copper Pour Distribution

Uneven Outer Layer Copper Pour: Uneven outer layer copper pouring affects plating current balance and may lead to excessive copper thickness or short circuits due to resin gathering.

Uneven Inner Layer Copper Pour:

In cases where the void area is too large, the resin glue on the PP accumulates and flows toward the copper-free area. This results in thinner boards, copper wrinkling, missing resin causing white spots, and delamination issues.

For products with strict thickness requirements in key areas like gold fingers, if the corresponding inner layer area is too spacious, it can lead to thinning in the gold finger region, poor contact with connector slots, and other problems.

Inconsistent copper coverage area ratios on different layers can also risk board warping.

Copper Pour Design

Avoid Leaving Void Areas: Try to fill copper in void areas whenever possible.

Keep Copper Away from Normal Trace Pads: Ensure that when routing traces, the distance between traces, copper areas, and drill holes is at least 0.5 mm or more. Use solid copper fill rather than small grid copper fill.

Copper pours must be present on all layers where gold finger connectors are located to ensure the thickness of the final board. Care should also be taken to avoid laminate structures with insufficient thickness.

Copper Under Antennas: Follow product design guidelines for copper placement in antenna areas to prevent interference.

Consistent Copper Pour on Both Sides: Ensure that copper is consistently poured on both sides and not left empty on one side.

These design considerations are essential for a smooth PCB manufacturing process and to avoid common pitfalls.

Also Read: Top 6 Custom PCBA Defects Engineers Must Avoid

Conclusion

A well-designed PCB will not only function reliably but also contribute to the overall success of your electronic product. So, take the time to familiarize yourself with these design considerations, incorporate them into your workflow, and enjoy the benefits of a smooth and efficient PCB manufacturing experience.

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