Printed circuit boards (PCBs) help power many of today’s technologies, from consumer products to highly sophisticated medical and aerospace equipment. PCB fabrication often occurs quickly and at high volumes to keep up with increasing demands. As with any design and manufacturing process, there is always a possibility of small errors that can cause circuit failure.
In this guide, we discuss common issues that can occur during printed circuit design and assembly, and how you and your manufacturer can prevent them.
What Factors Can Contribute to PCB Failure?
PCB failure occurs for many reasons. Sometimes, design factors are the cause. In other instances, the complexity of the PCB fabrication process introduces the chance for errors. Although you can never completely eliminate the risk of issues that cause a PCB to fail, there are measures you and your fabricator can take to increase the quality ratio of your production order.
Plating Voids
Though rare, plating voids can cause failure in printed circuit boards. Plated through-holes, also known as vias, are holes drilled through the circuit board and coated with copper to allow electricity to move across the circuit. Plating voids occur when the copper fails to coat the through-hole thoroughly, resulting in gaps (voids) that impact electrical conductivity.
This issue can occur when the design specifies through-holes that are too small to allow proper flow and coverage. Your through-holes should be large enough to handle copper plating while leaving sufficient space for mounting and wiring.
Plating voids that occur during PCB fabrication are typically caused by:
- Dull drill bits, which can tear the fiberglass, leaving “strings” in the hole
- Contamination from improper cleaning
Inadequate Copper-to-Edge Clearance
Part of the PCB fabrication process involves trimming the board. If your design does not follow proper clearances, part of the coating could be cut during this process, exposing the copper and potentially affecting circuit functionality.
Your fabricator will have standard rules for copper-to-edge clearances and tolerances. Typically, this is ten thousandths of an inch, or 10 mils. As long as you keep 10 mils between copper to edge, you will have a robust circuit design.
Starved Thermals
Thermals are the small traces surrounding the pads. They connect pads to the plane and are critical to carrying current to the pad and ultimately the component. Starved thermals occur when there are fewer than two spokes connected to the plane. Typically, there are four spokes per pad. If this happens, the component may not be properly grounded or carry enough current through the power connection to properly run the component.
To prevent issues, your design should specify at least two thermal spokes on every ground or power pad. Sometimes, fabricators will expand thermals to make the circuit easier to manufacture. However, if you expand it too much, you round off the ground plane, and then there is not enough room for the spokes.
When your fabricator sends your design to you for final review, check to make sure they did not expand the spacing in a way that would leave less than two thermals on any pads requiring grounded power.
Acute Angles in Circuits
Acute angles in a circuit can trap acid during the etching process, causing buildup that can erode connections and affect PCB performance. This issue is known as “acid trap.” To prevent acid traps, avoid 90-degree angles in your design. Ideally, you want rounded traces or a 45-degree chamfered corner on traces. In addition to preventing acid traps, you will reduce the chance of corner fractures during PCB assembly.
Missing Solder Mask Between Pads
The solder mask is the layer applied over the circuit board’s copper layer. Its purpose is to protect copper traces from corrosion and prevent accidental short circuits caused by contact with metal or conductive materials during assembly. A missing solder mask between pads exposes sensitive areas of the circuit.
When reviewing your design, make sure there is sufficient space between pads for the solder mask. If there is not enough room for a solder mask between the pads and you “gang open” the pads, it creates the possibility of solder bridges from pad to pad.
If your design specifies “gang open” pads, keep in mind that this increases the chance of solder bridges in those areas. You will want to conduct a thorough inspection after receipt of your PCB assemblies to identify any affected areas of the circuit.
Poor Quality Soldering
Improper soldering during PCB assembly can lead to quality issues in your printed circuit assembly. One problem is “cold soldering,” which occurs when the solder is not heated enough. This can result in weak electrical connections that impact circuit performance.
As long as your fabricator is soldering and assembling to J-STD-001 standard and IPC Class II or III specifications, the likelihood of poor soldering issues is quite rare.
Environmental Contamination
If your circuit is not functioning and you have checked each element of the design to ensure it meets specifications, the malfunction may be due to environmental contamination during the fabrication process. This could be from the etch chemical wash, the copper plating, the solder mask, or other issues. Fabricators typically have stringent quality standards for production to maintain cleanliness; however, there is a rare chance of contamination in any manufacturing process.
The Final Word: How to Prevent Common PCB Design and Fabrication Issues
Despite your and your fabricator’s best efforts, human error can always be a factor during design review and the printed circuit assembly process. Check everything twice (at least) and ask your fabricator questions about the measures they take to ensure quality during printed circuit manufacturing and assembly.
- Always check your CAD files before sending them to the fabricator, using this PCB design checklist as a reference.
- Conduct a thorough DFM review with your fabricator to identify potential problems with your design. If your fabricator suggests any modifications, make sure the changes do not alter the circuit’s performance in your application. The design recommendations should also comply with all IPC Class I, II, and III specifications.
- Review the final CAD files with your design team, again referencing the design checklist and common issues mentioned above.
Contact the experts at Tramonto today to find out how we can support you in developing circuits you can depend on.