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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In some cases, the designer might forego PTHs.

As components continue to shrink, designers are challenged to find strategies to route the supporting circuit to handle all the I/O from those components while using less real estate.

The high I/O count on these devices can cause major heartburn when trying to route out from under the BGA. There is not enough room for pad traces and spaces. And at pitches under 0.8mm, in some cases you cannot route signals between the pads.

This usually drives the designer to use a combination of blind or buried vias and microvias, along with through-holes. All these are fair game in flex and rigid-flex designs. The key is to implement them in a manner that permits the various structures to coexist.

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