Flexible PCBs Fabrication Guide

The electronics market is ever-changing, with smaller, lighter and more powerful components being the norm. At the core of smartphones, cars, and even medical implants is the flexible printed circuit board. Flexible PCBs can be bent, folded and shaped to fit over or within different objects – which rather than conforming to a design, the design conforms to what is possible with flexible PCBs. Designing a flexible PCB is more complicated and labor-intensive than a rigid PCB, as it involves blending material science with advanced manufacturing techniques through multiple phases. In this article, we will focus on the core processes of flexible PCB manufacturing where raw materials are converted into sophisticated flexible electronic circuits.

Substrate Preparation and Layer Patterning for Flexible PCB

A flexible PCB journey starts with its foundational: the flexible substrate. Generally, it is a thin film of polyimide which is known for its exceptional thermal stability, chemical resistance and mechanical durability. A thin coat of copper is laminated onto this substrate, providing the base for the conductive pathways.
The first big step is to get the circuit design onto the copper clad laminate. This is mainly done by photolithography. A photosensitive dry film resist is laminated on the copper surface. The circuit pattern is on a photographic film called a photo tool, and the photo tool is laid on top of the resist and then exposed to ultraviolet light. The regions of the resist that are exposed to harden and the areas that are not exposed are still soluble. They are then chemically removed in a developer solution, leaving a very exact negative image of the circuit pattern in the copper.

Etching and Layer Alignment of Flexible PCB

With the resist pattern serving as a protective blanket, the panel goes to the etching process. There, the board is immersed in a chemical etchant, such as ferric chloride or ammonium persulfate, to strip away the unprotected copper. The hardening of the resist has left only the copper traces you want protected. Then, the resist itself is stripped off with another chemical, exposing the delicate circuitry of copper.

This step is repeated for each inner layer of a multilayer flexible PCB. Registration holes are used to align these layers perfectly during the lamination process. The internal layers are stacked with pre-impregnated adhesive and outer copper foils and then laminated with heat and pressure. This joins the layers into a single unified panel, the adhesive melts and cures to become a solid dielectric core. Final finished create electrical connections provide the necessary electrical connections to the module in a manner consistent with the module type, such as board-level module, component insert module, and so on. Lamination is finished and connections between those layers should now be made. This is done by drilling microvias and through-holes with high-precision mechanical or laser drills. Industry reports indicate that laser drilling is the preferred method for smaller, denser vias in high end flex PCB and multilayer PCBs. These holes are then plated with copper through an electrodes and electrolytic plating process, resulting in a conductive barrel that is used to connect one circuit layer to another. Then, these external layers are also use the same pattern, etch, strip, to get the circuit.

A surface finish is added to protect the copper from oxidizing and to make it easier to solder. A standard fabricating option for a flexible PCB is Electroless Nickel Immersion Gold (ENIG), which is a flat surface that is solderable, or a simple protective solder mask layer, often a flexible epoxy or coverlay, a pre-cured polyimide film that contains adhesive. Test, Cut, and Final Inspection Check, slice and inspection at the same time. The last and most important step is the electrical testing. Each flex PCB is testing for continuity and shorts by flying probe or Bed-of-Nails. Once approved, the separate boards are scribed or routed from the larger production panel. A final full inspection for physical defects, dimensional accuracy, and surface finish integrity. This stringent process guarantee that every flex PCB can be tailor-made with high quality for your needs, making it a must have part in the world of electronics now and forever.

FPCB for mobile phone repairing