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

Mark FinstadA 36" long board will cost plenty. But, there are workarounds.

I have need for a long flex cable (~36"). I sent for quotes, and they all came back as “no-bid.” Are long flex circuits really that much more difficult to build?

Long FPCs are more difficult to build. There are a lot of reasons for this. This month I will cover each, with possible workarounds.

Raw material size limitations. If your bids are from US-based manufacturers, they are probably getting their raw copper-clad materials in 24" x 36" sheets (unless special ordered). So even if the manufacturer makes its processing panel size 36" long, a 36"-long FPC would not fit unless it was run diagonally, which is not practical from a cost perspective. You may want to see if the fabricator is willing to purchase materials from Asia, which typically are delivered on long rolls. This would solve the raw material issue, but not any of the processing issues (covered later).

Read more: Long Flex Boards

Nick KoopCan via-in-pad be used on a flex or rigid-flex circuit with SMT parts?

SMT is successfully implemented on flex and rigid-flex every day. Standard through-hole constructions are the most cost-effective, but in some cases there is no room for through-holes and their larger pad diameters. Via-in-pad is a design strategy that may be required with very tight pitch BGA components and other SMT devices on flex and rigid-flex. The advantage of using a microvia is the hole size is quite small, and the associated pad is small as well. This provides more real estate for routing signals, especially out of BGA patterns. The rules-of-thumb and considerations are different, however, depending on whether the part is purely flex or rigid-flex.

For a pure flex, where all the material is flexible, consider a couple of things. Typically, via depths are much shallower than for a rigid or rigid-flex part. This means less solder may be consumed in the PTH. In some cases, the hole must be plugged, however.

Read more: Microvias on Flex Circuits

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