6 Critical PCB Design Mistakes That Delay or Fail Your Custom PCBA

While ordering a custom PCBA is more accessible than ever, a simple design oversight can quickly lead to costly production delays, frustrating board failures, or, in the worst cases, a complete and expensive redesign. The transition from a schematic to a physical, assembled board is filled with potential pitfalls that can trap even experienced engineers.

This article will detail the six most common and impactful design mistakes that engineers encounter when preparing a project for custom PCB assembly.

By understanding these issues and how to prevent them, you can ensure your design moves smoothly from your screen to a fully functional prototype. Leveraging a PCB assembly manufacturer's automated Design for Manufacturability (DFM) checks, like the free DFM tool offered by JLCPCB, is your critical first line of defense against these preventable errors.

#1 Incorrect Component Footprints and Land Patterns

This is arguably the single most frequent source of PCB assembly failure. The problem occurs when the copper land pattern on the PCB does not perfectly match the physical dimensions of the component's leads or pads. This critical error often stems from using unverified component libraries downloaded from the internet or failing to cross-reference the footprint against the official component datasheet.

The consequences of an incorrect footprint are immediate and severe. At best, it might cause "tombstoning," where a small two-terminal component lifts on one end during reflow soldering, resembling a tombstone.

At worst, it can lead to misaligned parts, intermittent or non-existent solder joints, or a complete inability for the high-speed pick-and-place machine to even place the component on the board, halting the entire PCB assembly line for your custom project.

The Solutions:

● Always reference every footprint to the manufacturer’s datasheet. The datasheet is the first source of truth for the physical size of a component. Pay particular attention to the recommended land pattern image.

● When designing your own footprints, use the IPC-7351 standard for the land pattern calculations to help provide good solder joints.

● Utilize your EDA tool's 3D viewer. In many cases, putting a 3D model of the component onto its footprint will highlight a mismatch right away.

Correct vs. incorrect component footprint, highlighting the mismatch between a component and its land pattern.

#2 Insufficient Clearances and Violating DFM Rules

A reliable custom PCBA depends on maintaining minimum specified distances between different electrical elements. Placing traces, pads, or vias too close to each other—or too close to the edge of the board—is a direct violation of Design for Manufacturing (DFM) rules. Every manufacturer has a specific set of minimum clearance capabilities dictated by their equipment's precision.

Not following these rules can lead to a variety of problems. Traces that are too close together could lead to solder bridging, which occurs during custom PCB assembly and could cause an unintentional short circuit. Vias that are too close to the edge of the board could break when depanelizing or create delamination. Inadequate clearances could create "acid traps" for the chemistry when etching chemically. When the etch chemistry gets trapped in between clearances, it could corrode through the copper more than intended, leading to an open circuit.

The Solutions:

● Obtain the PCB assembly manufacturer's capabilities document before you begin layout. Reputable manufacturers like JLCPCB publish their PCB capabilities online.

● Set up these specific clearance rules in your EDA software. All professional PCB design tools allow you to define a set of design rules (e.g., minimum trace-to-trace clearance, trace-to-pad, etc.). This allows the software to automatically flag violations as you work.

● Always run your EDA's Design Rule Check (DRC) as a final step before exporting your Gerber files.

Solder bridge and acid trap defects on PCB under the microscope

#3 Poor Thermal Management

New electronics are getting more powerful in a smaller package. If a design does not include sufficient heat dissipation paths for high-power components, such as microprocessors, voltage regulators, or power MOSFETs, it will lead to a failure scenario. Heat is an enemy to any electronic component, and if there is no way to escape, that heat will build quickly.

Poor PCB thermal management can lead to a custom PCB assembly that becomes unreliable even under normal conditions and completely fails under extreme temperatures. Overheating components may suffer performance degradation or catastrophic failure, potentially causing damage to nearby circuits and reducing overall board reliability.

The Solutions:

● Utilize large copper pours (polygons) that are directly connected to the thermal pad of the component. This will functionally convert a part of the PCB into a heatsink.

● Implement a thermal via array directly under the component. The vias act like pipes, conducting heat from the top layer through the board to larger ground or power planes on other layers, where the heat can be dissipated efficiently.

● You should always use wider traces for high-current paths; narrow traces could act like fuses and generate their own heat at elevated load currents.

PCB thermal management comparison, showing a component with and without proper thermal vias and a copper heat-dissipating pour.

#4 Improper Decoupling Capacitor Placement

In digital electronics, decoupling (or bypass) capacitors are often the unsung heroes, providing the instantaneous current demands of integrated circuits (ICs) during rapid switching events and decoupling power supply noise.

But decoupling capacitors can only do their job if they are properly located. One of the biggest traps to fall into when using decoupling capacitors is having them too far away from the power supply pins on the IC you are servicing.

Every millimeter (or inch) of trace length to and/or from the decoupling capacitor adds parasitic inductance in series. This parasitic inductance impedes their speed of current flow and results in a greatly reduced effectiveness of the decoupling capacitor to filter high-frequency power supply noise. All of this leads to IC instability, unexplained logic transients, and increased electromagnetic interference (EMI) that can compromise your ability to pass emissions testing.

The Solutions:

● Place decoupling capacitors as physically close as possible to the IC's VCC and GND pins. This is the golden rule.

● Use short, wide traces to connect the capacitor pads to the IC pins and ground vias.

● Place the via to the ground plane directly adjacent to the capacitor's ground pad. This minimizes the current return path loop area, which is critical for high-frequency performance.

Showing optimal vs. poor decoupling capacitor placement, highlighting the importance of short trace lengths to reduce inductance.

#5 Ambiguous or Inaccurate Bill of Materials (BOM)

While the Gerber files define the physical board, the Bill of Materials (BOM) tells the assembly house exactly which components to put on it. An ambiguous or inaccurate BOM is a guaranteed cause of production delays for any turnkey custom PCBA service.

Common errors include using internal company part numbers, providing incomplete Manufacturer Part Numbers (MPNs), listing incorrect quantities, or having reference designators that don't match the silkscreen.

These mistakes bring the quoting and sourcing process to a halt. The assembly factory's procurement team cannot guess which component you intended to use. This results in production holds while they send emails back and forth with you to clarify the ambiguities, wasting valuable time.

The Solutions:

● First and foremost, always use the full, exact Manufacturer Part Number (MPN). This is the single most important piece of data in your BOM.

● Second, double-check all quantities for components like resistors and capacitors are correct for the number of boards you are ordering.

● Finally, double-check your chosen parts against the manufacturer's in-stock library, like the expansive JLCPCB Parts Library, to confirm that there are enough components in stock. This will alleviate delays due to sourcing parts, since components may be delayed in shipment, the point above.

#6 Unhelpful Silkscreen

Silkscreen might not affect the electrical functionality of your custom PCB assembly, but its importance during testing, debugging, and manual inspection cannot be overstated. A common mistake is treating it as an afterthought, resulting in text that is unhelpful or actively detrimental. Problems include placing reference designators over vias or pads, using fonts that are too small to be legible, or forgetting key polarity markers.

When silkscreen is printed over solder pads, it can interfere with proper soldering. When it's illegible or missing, it makes the task of manually inspecting a board or debugging a circuit with a multimeter an exercise in frustration. Clear markings are essential for orienting polarized components like diodes, electrolytic capacitors, and integrated circuits.

The solutions:

● Most EDA tools have a feature that will ensure silkscreen is a "clipped" from solder pads; make sure this is selected and enabled:

● Use unambiguous polarity markings; use a "+" indication for capacitors, the diode symbol for diodes, and a dot or a number "1" to indicate pin 1 of an IC.

● Place reference designators (R1, C1, U1) in a logical orientation next to their counterparts for easy identification.

PCB silkscreen error example showing text overlapping a solder pad versus correct placement.

Conclusion

Building a custom PCBA is all about the details, and small mistakes made in the design process can have huge ramifications in production. By simply avoiding these six common issues - incorrect footprints, inadequate clearances, improper thermal management, incorrect capacitor placement, BOM inaccuracies, and an unhelpful PCB silkscreen - you can significantly boost your first-pass yield.

Taking the additional time to check your footprints, run your DRC, and clean up your documentation can save you weeks of delays and costly rework. The best practice is to make these loans a part of your design process, so every project you send to manufacture is optimized to be robust, reliable, and assembly-ready.

Are you ready to see if your design is manufacturable? Upload your files to the JLCPCB Instant Quote page and utilize the free DFM automated analysis to catch critical design errors before you place your custom PCBA order!

Frequently Asked Questions

Q: How can I confirm that my component library footprints are accurate?

You should be wary of trusting a downloaded footprint until you have confirmed it is correct. The best way to do this is to verify it directly against the land pattern dimensions recommended in the component datasheet. Additionally, many EDA tools also provide footprint wizards that use IPC standards for land pattern generation based on package dimensions you enter from the datasheet.

Q: Will JLCPCB's DFM (design for manufacturability) catch all of these design errors?

JLCPCB’s automated DFM check is great for verifying physical rule violations like inadequate clearances or silkscreen over rods. It will not be capable of verifying logical design errors in your board design. For instance, it will not know if you are using the wrong footprint for a part, or that you placed a decoupling capacitor too far from the IC. The JLCPCB’s DFM check is a solid safety net, but should not replace a good design practice.

Q. When should I start thinking about thermal management for my custom PCBA?

You should think about thermal management for any component that will dissipate significant power. Always look at the device's datasheet to find the "Thermal Resistance" (θJA), which will help you approximate the temperature rise. As a rule of thumb, assess the thermal design for any linear voltage regulator, switching converter, powerful processor, or high-current MOSFET by implementing copper pours and vias.

Q: Can I correct a BOM mistake after I have submitted my custom PCBA order?

If the order has not yet gone into the procurement of components, you may be able to reach out to customer service and submit a new BOM, but it will delay the order. If they have started procuring components, it will most likely not be possible. It would have been more efficient to review your BOM for accuracy three times before submitting your custom PCBA order.