Mark Finstad

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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Nick Koop

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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Mark Finstad

Flex circuit solderability and life expectancy are influenced by handling and finishes.

What is the shelf life of a flex circuit if it stays in its original sealed packaging? And what is the life expectancy of a flexible circuit?

I assume the first question refers to long-term solderability and the second to the overall functionality of the flex circuit once installed in the final application.

As far as long-term functionality goes, flexible circuits in a static application really don’t have a post-installation “best if used by” date. There are flexible circuits over 25 years old still functioning without issue in military and avionics systems. Flexible circuits have also been used for decades in satellite applications due to their very low mass and high connection density. Considering the cost of a service call to a satellite, only the most reliable interconnects are used in these applications. That flexible circuits are still in use after all these years is a testament to their long-term reliability. The only exceptions to overall life expectancy are flexible circuits operating in very harsh environments, such as continuous temperatures above 275°F, strong acid or caustic exposure, abrasion, etc.

Read more: Why ‘Born on Dating’ is Not a Reliable Means for PCB Shelf Life

Nick KoopDoes it make a difference where the flex on a rigid-flex board resides?

I am working on a rigid-flex design. Does it matter where the flex layers are in the stackup?

Where flex layers are in a stackup does matter. Rigid-flex circuits come in all configurations. Fabricators can make rigid-flex boards with the flex at all different locations in the stackup; each has their reasons and constraints.

As a general rule, we recommend putting the flex in the center of the stackup. This permits the design to have an asymmetric stack. (Symmetry is very important when it comes to managing bow and twist.)

That said, there are a number of reasons why the flex may not reside in the middle of the stackup.

Read more: Flex in the Stackup

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