Failure Modes of Flexible PCBs
Rigid-flex circuit boards offer numerous advantages over traditional PCBs, but they aren’t immune to failure. Understanding the three main failure modes of flexible PCBs can help manufacturers, engineers and designers avoid these issues and make better decisions on how to best use these types of boards.
The most common type of flexible PCB failure is cracking and fractures in the traces. These fractures can result from the mechanical stress of flexing and bending, as well as other factors like vibrations and thermal excursions. This type of failure can be very difficult to diagnose and resolve, requiring the board to be disassembled, reworked and reassembled, or even replaced completely.
Another issue with flex circuits is the possibility of corrosion. The solder flux used in the assembly process can leave corrosive residue on the flex that can eat away at any metal it comes into contact with. Various cleaning agents can also cause corrosion on a PCB by leaving conductive residues or fumes.
A third common issue with a flexible pcb is the possibility of peeling. This can happen when the solder mask or coverlay isn’t properly matched to the flex layer and the rigid section of the board. This type of error can be avoided by ensuring that the vias on the rigid section are well spaced from those in the flex area. Keeping these vias at least 3 mils apart (for coverlay) and 5 mils apart for solder mask is recommended.
Understanding the Failure Modes of Flexible PCBs
Other common issues with flex PCBs are dielectric failure and brittle fracture. Dielectric failure can occur when the insulator between two conductors fails. This can lead to an electric arc that burns the surface of the conductive material and the surrounding material, causing damage to the equipment or even a fire if the equipment is turned on.
Brittle fracture is a condition that can be caused by repeated bending of the flex, often with small bend radiuses. This can be due to mishandling by unauthorized personnel or simply the repeated stress of flexing and bending. This can cause the flex to become thinner over time and eventually fail in terms of its ability to hold up to electrical stresses.
The good news is that many of these problems can be prevented with careful manufacturing processes and attention to detail. The use of a PCB FMEA can help identify the potential causes of these failure modes and then implement corrective actions to ensure quality. Additionally, inspection technologies such as X-ray, optical SEM and micro-sectioning analysis can help to diagnose the root cause of a faulty PCB. Once the cause has been identified, a manufacturer can work to eliminate it from future productions.
Furthermore, electrical overstress (EOS) and electrostatic discharge (ESD) pose significant risks to the reliability of flexible PCBs. EOS occurs when excessive voltage or current is applied to the circuit, leading to insulation breakdown, component damage, or catastrophic failure. ESD events, on the other hand, result from the sudden discharge of static electricity, which can cause immediate or latent damage to sensitive electronic components. Implementing proper ESD protection measures, such as shielding, grounding, and incorporating transient voltage suppressors, is essential for safeguarding flexible PCBs against these electrical threats.
