Material choices are often based on the planned assembly.
Many different materials are used to rigidize flexible circuits. Likewise, the reasons for stiffening an area on a flex board are many. The “best” stiffener material is tied to exactly why you are stiffening your flex circuit.
Rigidized SMT or through-hole component areas. Providing a rigid, stable surface for mounting components is probably the most common reason for stiffening an area on a flexible circuit. If components are mounted on a flex, which is then bent in that area, there is a very good chance the solder joints or solder pads will be damaged. The industry standard is to rigidize any area on a flex that has soldered components. If components are all SMT, install the stiffener on the side opposite the components. If through-hole components or connectors are used, mount the stiffener on the same side as the components. If components are on both sides, rigid-flex construction is probably needed, but that is a topic for a future column. By far the most common (and least expensive) stiffener material is epoxy-glass laminate (FR-4). This inexpensive sheet material comes in a range of thicknesses and is machined to size and shape by the flex circuit manufacturer. The machined stiffeners are then applied with either a pressure-sensitive or thermosetting adhesive (see below). Another material for stiffening a component area is 0.003" to 0.005" polyimide film. This material is common and cost-effective, since these stiffeners can often be added in panel form. This option is typically specified when overall thickness is a concern. The material is a bit more expensive than FR-4 but offers significant time savings during stiffener mounting. This material will not provide the same level of stiffness as a thicker FR-4 stiffener, so operators must exercise care in handing and forming during installation.
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