Modes for Flexible PCBs
PCBs are subject to a variety of stresses during the manufacturing, assembly, and shipping processes. These stressors can cause a number of failure modes that impact the overall quality and reliability of the circuit board. These stressors include mechanical, thermal, chemical, environmental, and electrical overstress. Fortunately, these failure modes can be mitigated through effective design considerations, material selection and manufacturing techniques.
Broken tracks occur when the conductive material is ripped off the circuit board, often due to mishandling. This can cause an electrical short, which is typically easy to diagnose by using a continuity tester or multimeter. The best way to prevent this type of failure is to properly resolder the track, however, in some cases, replacing the entire flexible pcb may be necessary.
When pads and holes are smaller than recommended, the solder is unable to flow as easily. As a result, the solder joints are weak and susceptible to failure. To avoid this, ensure that each pad and hole has a minimum of 1 square millimeter of copper surface area.
Failure Modes for Flexible PCBs
Insufficient surface finish is a common problem that can lead to a variety of issues, including contamination and corrosion. This issue can occur when the adhesive used to assemble the components is not strong enough or when the components are not thoroughly cleaned before being placed on the PCB. In addition, an excessive or repetitive application of soldering level temperatures can also result in a thin layer of copper detaching from the underlying surface.
Rigid-flex circuits are designed to be able to bend and flex, but in some cases, these curves can become too pronounced, creating mechanical stress concentration points within the circuit. This type of failure is typically the result of a poorly-designed layout in which rigid areas are located directly adjacent to flex circuits. These areas can be created by a number of factors, including the shape of the flex circuit outline, the location of stiffeners, via or PTH holes in the flex area and the configuration of the coverlay layers.
A significant concern in rigid-flex circuits is the potential for chemicals to leak out of the board. These chemicals can cause corrosion, which is typically accelerated by humidity and abrasive environments. The good news is that these problems can be prevented by ensuring that all chemicals and cleaning agents are used appropriately, and that the boards are well-sealed with conformal coatings.
Over time, all PCBs succumb to the entropy of the universe, and while it’s impossible to completely eliminate this process, taking steps to reduce heat transfer is a surefire way to keep your PCBs running long-term. To do this, make sure that your designs utilize the appropriate CAD tools to minimize thermal excursions and that your board manufacturers have the experience and technology to execute these designs successfully. In addition, making sure that your traces are routed at the proper thickness is a good way to limit thermal stress.