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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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.
Does 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.
A 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).