A single sided flexible printed circuit (1 layer flex circuit) is a flex circuit with one layer of copper trace on one substrate, and with one layer Polyimide coverlay laminated to copper trace so that only one side copper will be exposed, so that it only allowing  access to copper trace from one side, comparing to dual access flex circuit which allows access from both top and bottom side of flex circuit. As there’s only one layer of copper trace, so it also named as 1 layer flexible printed circuit, or 1 layer flexible circuit, or even 1 layer FPC, or 1L FPC.

Double sided flex circuits consists with double sided copper conductors and can be connected from both sides. It allows more complicated circuit designs, more components assembled. The major material used are copper foil, polyimide and coverlay. Adhesiveless stack up is popular for better dimensional stability, high temperature, thinner thickness.

Dual access flexible circuit board refer to the flex circuit which can be accessed from both top and bottom side but only has only layer of conductor trace. Copper thickness 1OZ and coverlay 1mil, it similar with 1 layer FPC and opposite side FFC. There’re coverlay openings on both sides of flex circuit so that there’re solderable PAD on both top and bottom sides, that is similar with double sided FPC, but dual access flex circuit board has different stack up because of only one copper trace, so no plating process is need to make plated through hole (PTH) to connect between top and bottom side, and trace layout is much more simple.

The multi layer flex circuit refer to a flex circuit with more than 2 layer circuit layers. Three or more flexible conductive layers with flexible insulating layers between each one, which are interconnected by way of metallized hole through the vias/holes and plating to form a conductive path between the different layers, and external are polyimide insulating layers.

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