How Components Are Mounted on a Coverlay PCB

Mounted on a Coverlay PCB

In most cases, a flexible circuit’s components are mounted onto the surface using a combination of SMT (surface mount technology) and through-hole mounting techniques. This is done to provide a robust assembly process and ensure that the board meets IPC quality control standards. In addition, through-hole mounting provides mechanical support for the copper in the flex portion of the board. This helps prevent detachment from the substrate when the board is bent or flexed, which can cause reliability problems.

Whether you’re using SMT or through-hole mounting, the component mounting process on a flex PCB begins with carefully aligning and affixing durable polyimide coverlay panels to the production panel. This step is done prior to lamination to avoid damage during subsequent manufacturing steps. The coverlay acts as a dielectric layer, protecting the copper and allowing solder paste, adhesives, and other assembly materials to be applied. It also ensures a uniform surface finish across the entire flex portion of the board.

Coverlay is made of a high-temperature, durable polyimide material that provides excellent mechanical strength and chemical resistance. It is available in a variety of thicknesses, with 1 mil being the most common. The flex circuits are then laminated to the coverlay with an epoxy or acrylic based flexible adhesive.

Once the flex PCB has been laminated to the coverlay pcb, it is then subjected to heat and pressure in order to form a strong bond between the two layers. The result is a strong, reliable, and highly flexible printed circuit board.

How Components Are Mounted on a Coverlay PCB

Since conventional soldermask has limited flexibility, coverlay is used to protect the flex circuitry from environmental and manufacturing stresses that would otherwise cause it to detach from the substrate. Unlike solder mask, coverlay is not developed like photosensitive solder resist; it is instead custom-cut and machined to accommodate a range of features such as pads, holes, and other exposed areas of the circuitry. This machining process involves a number of methods including laser cutting, knife cutting, and punch & die sets. As a result, a flex PCB’s coverlay may have larger minimum annular rings to allow for material and manufacturing tolerances, as well as potential adhesive squeeze out during lamination. It also does not contain isolated “island” type features as these are impossible to machine into the film.

It is important to minimize overlapping openings for testpoints and components in the coverlay, which can cause ambiguous or unclear connections. In addition, any dents, bumps, scratches, discoloration, or debris in the coverlay will affect appearance and impact quality perception.

In addition to a minimum of 3 mils between trace and mask openings, Sierra Circuits recommends maintaining a distance of at least 4 mils between ganged coverlay openings. This ensures that there is sufficient clearance between the corresponding traces and solder mask to prevent misregistration. In a rigid-flex PCB, it is common to use LPI solder mask in the rigidized component areas and coverlay in the flex portions of the circuit. However, if you would like to use coverlay on the entire flex circuit, then be sure there are tented or windowed openings in the coverlay.