Flexible Printed Circuit Boards (PCBs) are perfect for the current electronic needs. They are lightweight, can be compact, and with proper design, can offer extremely robust solutions. However, although flexible PCBs can bend, they still must fulfill some specific requirements that traditional rigid PCBs do not.
The design process of flexible PCBs must consider the number of layers, placement of features, circuit architecture, and materials. The designer must also consider the frequency of circuit flexure, and the method of formation of the bend, including the bend tightness and the degree of the bend. The designer can take advantage of the full potential of the technology by working within the requirements of flexible PCBs. These include recognizing the unique demands made upon the flex PCBs, while carefully defining the application and design priorities.
Designers must ensure flexible PCBs are robust and capable of effectively functioning for a long time. A variety of factors can influence the performance of a PCB when it flexes. In addition, they must also factor in the risk from environmental damages and take necessary steps for mitigation.
Manufacturers use the latest equipment when manufacturing PCBs to maintain accuracy and reduce the risk factor for damage to boards in use. For instance, the Pluratec Inspecta X-drill optimizes tooling holes for subsequent pinning at the drill. It can measure panel shrinkages for accurate drill scaling, and measure individual layer shrinkages for subsequent layer scaling. All this substantially improves drill and layer-to-layer registration, while building reliable HDI designs with small designed annular rings.
Critical Design Factors
The distance of the neutral bend axis from the center of the PCB’s material stack is a critical design factor. This distance must remain small to distribute the forces evenly among all layers of the PCB when it flexes.
The risk of damage increases if the PCB is thick and has to flex more—a low bend angle reduces the risk, while thin PCBs face a lower risk of damage when flexed. The risk of damage reduces if the bend radius is large.
A higher frequency of flexing enhances the risk of failure dramatically. The risk is further compounded if the factors above are also added to the high flexing frequency.
A proper selection of material is very important for accommodating flex and the way these forces travel to other layers in the bend area. The risk of damage reduces with the use of materials allowing greater flexibility.
Presence of stiffeners within or near the bend area increases the risk of PCB failure. Designers should avoid placing stiffeners and similar features in or near the bend area, as these make the PCB vulnerable to forces generated in the bend area. Additionally, they can weaken the surrounding circuit structure when the PCB flexes.
Placing discontinuities in the bend area enhances the risk of damage when the PCB has a high frequency of flexing. Forming techniques and conductor routing are other factors affecting the risk of damage during PCB flexing.
Among the environmental factors affecting flexible PCBs are the presence of humidity, dust, gas or liquid chemicals, static electricity, and temperature.
Moisture condensing or presence of water on a PCB can electrically short two neighboring tracks, rendering the entire gadget non-functional. Similarly, PCBs operating in damp conditions may lead to mold formation, and subsequent circuit failure.
Electro Static Discharge (ESD) reaching the board may affect the circuit while it is in operation, which increases the risk. Presence of liquid chemicals giving off fumes or the presence of gas and dust may lead to build up on the PCB, leading to corrosion of the linkages.
Temperatures, both high and low, can also have negative effects of flexible PCBs. In the presence of moisture, heat can cause the board to warp, leading to breakage of linkages and PCB malfunction.
It is possible to reduce the risk factors above through careful planning and design considerations. The objective should be to achieve specific goals to obtain all benefits of flexible PCBs at the lowest possible costs.