In the future, FPCs will not only increase in number but also undergo significant quality improvements. Moving from historically single-sided circuits to a rising prevalence of double-sided or multilayer rigid-flex circuits, the mass production at BOE’s Chengdu plant has brought flexible display screens to the forefront. Many envision phones that can be rolled up and placed in pockets, or foldable pads. However, numerous technical challenges must be overcome for such developments, including creating flexible batteries and circuit boards…
Today, I will discuss the technology of Flexible Printed Circuit boards (FPC) and examine the developmental trends in both FPC technology and materials.
In recent years, the global demand for FPCs in consumer electronic devices has surged, particularly in portable electronics such as mobile phones and thin video devices like flat-panel TVs. The quantity and area of FPCs used in a typical mobile phone with integrated digital camera circuits far surpass those of rigid PCBs. FPCs in flat panel displays (FPDs) are arranged vertically and horizontally, and their size and usage have expanded rapidly.
Looking ahead, the evolution of FPCs involves not only increased quantity but also substantial quality improvements. Initially focused on single-sided circuits, FPC technology has advanced to incorporate more double-sided and multilayer rigid-flex circuits, steadily increasing circuit density. Consequently, manufacturing techniques have seen continuous enhancement. The traditional subtractive method (etching) has inherent limitations necessitating new manufacturing technologies and the development of high-performance materials.
### Basic Structure of FPC
The basic structure of a single-sided FPC traditionally involves fixing a copper foil conductor onto a base film like polyimide, sandwiched with an adhesive such as epoxy resin. The circuit, etched onto this setup, is then covered with a protective film. Despite the mechanical reliability offered by this adhesive-based structure, its heat resistance, constrained by materials like epoxy or acrylic resin, acts as a bottleneck limiting the FPC’s maximum operating temperature.
To overcome these limitations, adhesive-free copper clad laminates comprising solely polyimide and conductor layers have been adopted, reducing overall thickness and significantly enhancing properties like bend resistance. This advancement also facilitates the creation of fine or multilayer circuits, broadening the selection of materials suitable for diverse applications.
### FPC Technology Trends
Driven by increased diversification and miniaturization, FPCs employed in electronic devices demand not only higher circuit densities but also superior qualitative performance. Recent advancements include achieving conductor pitches of 30μm or less for single-sided circuits using the subtractive method, and even smaller pitches for double-sided circuits, made feasible by technologies such as CO2 lasers and chemical etching.
Manufacturing high-density circuits is broadly classified into three categories based on circuit pitch and via hole diameter: traditional FPCs, high-density FPCs, and ultra-high-density FPCs. While traditional methods can handle pitches down to 150μm and via hole diameters of 15μm, technological improvements now allow processing of 30μm pitches and 50μm via holes, with many high-density FPCs employing these techniques.
However, achieving pitches below 25μm and via holes under 50μm with high yield rates necessitates novel processes and materials. Among proposed methods, the semi-additive approach utilizing electroforming technology emerges as most suitable, differing not only in basic processes but also in materials and ancillaries employed.
Advancing FPC assembly technology demands heightened reliability and performance, correlating closely with circuit processing technologies and material choices.
### FPC Manufacturing Process
Historically, almost all FPC manufacturing has relied on the subtractive method. Typically, a copper clad board serves as the base material, with a resist layer applied via photolithography. This layer defines the circuitry pattern, with unwanted copper etched away. However, challenges like undercutting limit the method’s applicability to fine circuits.
Recognizing these challenges, the semi-additive method has gained traction for its effectiveness in micro-circuit processing. Various semi-additive techniques have been proposed, leveraging processes such as coating liquid polyimide resin on a suitable carrier, sputtering to seed layers, and electroplating to form conductor circuits. This method supports the creation of multilayer circuits through successive application and removal of resist and seeding layers.
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These revisions aim to enhance clarity, coherence, and technical accuracy while maintaining the original structure and flow of the article.
