Stability in PCB Flex Circuits

Compared to their rigid counterparts, PCB flex circuits offer a host of advantages. They can be shaped and folded to optimize usable space within the product, and they are also extremely durable, making them ideal for military and medical applications. Moreover, they can provide excellent thermal management, as their materials dissipate heat faster than the material used in rigid PCBs. Despite their many benefits, they also come with a few challenges that are unique to this type of technology.

Dimensional stability is a major issue in pcb flex manufacturing. As the name suggests, these PCBs are far more flexible than rigid ones and they are very hygroscopic—they absorb moisture like a sponge. This problem can lead to catastrophic delamination or blistering during the reflow process, and it is exacerbated by the fact that these materials are more sensitive to high temperatures than FR-4 or polyimide prepregs.

The solution to this is the use of what is called a stiffener, which is a piece of rigid material that is added to specified areas of the flex circuit. This allows the circuit to withstand higher levels of stress and can also improve its handling during assembly processes. Stiffeners add cost to the flex circuits, but they do reduce the number of errors during production and can also increase the reliability of the final product.

How to Handle Dimensional Stability in PCB Flex Circuits

As with any other printed circuit board, a flex circuit starts with a core substrate. These are often a thin layer of PI or PET film, but they can be more robust substrates such as glass fiber or epoxy. They are then covered with an additional film—often a flexible solder mask ink or a coverlay—which protects the copper traces from corrosion and damage.

These materials are subsequently cut to shape, which is done using lasers and/or routers. The holes are then drilled and the layers of the circuit are connected by depositing copper. The edges of the flex circuit are typically rounded to improve its durability and resistance to cracking, but this is not mandatory if a customer wants to save on costs.

In the medical field, flexible circuits are utilized in medical implants, diagnostic equipment, and wearable health monitors, offering flexibility and biocompatibility for patient comfort and safety. In aerospace and defense, PCB flex is deployed in aircraft avionics, satellite communications, and military electronics, where reliability and performance are paramount.

As a result of these special requirements, the fabrication process for flex and rigid-flex PCBs is more complex than that of rigid PCBs. In addition, these circuits are more expensive to manufacture because they require more steps and more materials. Fortunately, a detailed calculator can help you determine how much your design will cost before releasing it to a manufacturer. For an even more accurate estimate, try the CAD tools in Altium Designer that automatically generate Gerbers and assembly drawing documents.