Flexible Printed Circuits
In computer peripherals, flexible printed circuits (FPC) are used to connect signals to the moving print heads of printers, to read/write heads on disk drives, and in solar cells. They’re also used in wearable electronics, such as bracelets and watches, and sewn into backpacks and outerwear. FPCs are also an integral part of medical diagnostic devices, where they enable flexible and lightweight portable instrumentation.
By enabling electronic components to be arranged on a flat surface that can bend, flex and twist without damage, flexible circuits enable products to be designed for form, fit and function without losing functionality. By reducing assembly time and costs, they also contribute to product and user safety.
The flexibility of a flexible printed circuit depends on both the substrate material and the design. It is not uncommon to see a flex circuit with multiple layers and varying copper weights, a wide range of conductor widths, and different dielectric thicknesses. Often, the choice of conductor thickness is determined by the circuit’s electrical performance requirements and conductivity needs. A wider copper width provides more conductivity while a thinner dielectric layer allows for closer traces.
Flexible Printed Circuits Enable Conformal Electronics
In general, there are three main types of flexible circuits: single-sided, double-sided and multi-layer. Single-sided circuits have one conductive copper layer on a dielectric. They are the most common and have the broadest application range, including a hinge circuit for a laptop computer or flip open phone.
Double-sided flex circuits have two conductive copper layers and are used for more complex interconnections. They are also used in some dynamic flex applications, such as those that need to be bent or flexed many times. Multi-layer flex circuits are used when specific design requirements, such as shielding or additional component density, require more layers than are available on a single-sided or double-sided flex circuit.
The most common substrate materials for a flex circuit are polyimide, PET and PEN (polyethylene naphthalate). Some higher-performance materials, such as polyetheretherketone (PEEK) and photosensitive polyimide, may be used for some specialized applications.
While a flex circuit’s ability to resist bending and other stresses is crucial, it must be understood that even the most durable flex circuits can fail under certain conditions. To reduce the chance of failure, it is important to use reliable automated manufacturing techniques like flying probe or boundary scan. It is also important to ensure that there are adequate shielding and covers for areas prone to dynamic flexing, and to validate the bond strength and anchor these areas to avoid fatigue and stress.
There are several other factors that can contribute to a flex circuit’s reliability, including the material, design and fabrication. By addressing these issues through best practices that address physical construction and electrical functionality, manufacturers can produce reliable flexible circuits. In addition, using traceability systems that provide unique job travelers for each batch of a flex circuit and utilizing reliable data collection processes help ensure that the flex circuits are fabricated correctly. The most successful flex circuits are the ones that have been designed with these best practices in mind and are properly manufactured by an experienced, quality fabricator.
