Underfill Play in Rigid Flex Circuits

Rigid flex circuits are combinations of rigid PCB sections with a flexible printed circuit board attached. They are often used in applications where a small footprint is required, space optimization, or unique form factors are needed. Rigid-flex boards also offer EMI protection and shielding, and signal integrity.

To make a rigid-flex board, first the rigid section is formed using FR4 or other standard rigid material. Then, the flex circuit is attached to the rigid board with an adhesive or mechanical fasteners like bolts and screws. The flex circuit may be one or more layers, depending on the design. Stiffeners are often added along the flex sections to help with torsional stress.

In addition to the stiffeners, the flex-to-rigid transition areas require a special layer called the flex-to-rigid interconnection. This is where the flex circuit connects to the rigid sections of the board and where signals are transmitted between both. This is where the flex circuit must be placed carefully, with a generous flex-to-rigid bend radius (typically 10-12x the thickness of the flex) to reduce stress and failure points.

Once the flex and rigid circuits are joined, copper is plated on both surfaces through holes in the rigid section. The flex circuit then undergoes a series of electrical and mechanical tests to verify performance. Upon successful completion of the flex-to-rigid tests, the flex circuit is etched, allowing the final through-plated holes in the rigid section to be exposed and etched as well.

What Role Does Underfill Play in Rigid Flex Circuits?

Rigid flex circuits can be very complex to floorplan, but the advantages they provide in terms of weight reduction and assembly speed far outweigh the additional engineering effort required. To ensure success, it is important that the design team understands the fabrication process and the mechanical constraints. It is also essential that the fab and assembly teams collaborate closely to avoid miscommunications and delays in the production process.

The flex circuit is then covered with an adhesive or, in some cases, a rigid flex circuits interconnect is placed on top of the copper. This flex-to-rigid interconnect consists of a single or double-sided copper foil bonded to an adhesive-free polyimide film and, in some instances, a solder mask. This allows the flex-to-rigid area to be populated with components and assembled to the rigid sections of the board.

During the etching process, the photosensitive etch resist on the flex-to-rigid interface is exposed and developed to reveal the copper, which is then chemically plated. The holes in the rigid-to-flex interconnect are drilled, plated and etched with the same steps as rigid PCBs. A single or double-sided copper core is a common choice, but other materials such as Cuposit are also available.

Perform electrical tests, including continuity and isolation tests, at various stages of production to identify defects early. Conduct bend and flex tests to ensure the circuits can withstand the mechanical stresses they will face in application. Use automated and manual inspections to check for surface defects, proper solder mask application, and overall assembly quality. Ensure the final product undergoes rigorous functional testing to verify performance under operational conditions.

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