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Many flexible circuit boards (flexible boards) can meet the density requirements of the circuit board by designing the circuit on only one side, but for assembly problems, assembly contacts must be left on both sides.
The simplest solution to this problem is to open a window or a long empty slot in the necessary area of the flexible circuit board, so that the goal of double-sided connection but single-sided circuit production can be achieved. The typical structure and products are shown in the figure below.
The manufacturing method of this kind of soft board structure generally uses the base material to coat the glue resin, then presses to remove the empty area, and then performs the copper skin pressing process. In this approach, copper skins are exposed on both sides of the assembly and joint area, so assembly is not a problem. However, because of the subsequent wiring production, these unsupported areas lose the support of the substrate, and the wet process wiring cannot be etched. Therefore, a protective glue coating process must be added before wiring production. After the circuit is completed, the protective glue must be removed before the metal surface treatment can be performed, so the execution procedure is more complicated. This type of product is currently represented by TAB products, as shown in the figure below.
The idea of making openings afterwards is also adopted by some FPCB manufacturers. The method is to selectively remove part of the flexible circuit board substrate by laser or alkaline etching to achieve the possibility of double-sided connection. Although this production method is simple, the unit price is very different. The most commonly used method in the industry is to open windows with strong alkalis. As the demand for non-adhesive substrates increases, this type of approach may have to be studied more carefully by the industry. Fortunately, laser technology is advancing at a rapid pace, and maybe there will be cheaper solutions in the future.
