Challenges for Flexible and Rigid PCB Boards

The world of electronics has become very compact and mobile over the past 30 years. PCB innovations like flexible and rigid-flex circuits have made it possible for products to be curved and folded into tight spaces, reducing size and weight while increasing functionality and reliability. The benefits of these types of circuits are significant for a variety of applications. However, these boards have their own unique challenges that must be taken into account when designing and manufacturing them. These include thermal stability, component compatibility, and assembly and repair.

The most common base raw material for flex PCBs is Polyimide (PI). PI is very similar to the fiberglass used in standard printed circuit boards, but it retains its flexibility even after being thermoset (cured with heat and pressure). This allows flex circuits to be bent and twisted without breaking, which is important for many devices that are frequently handled and moved.

Another feature of PI that makes it useful for flex pcb board is its resistance to humidity and tears. Many flex PCBs are exposed to harsh environments and must be able to resist extreme temperatures, moisture, chemicals, and shock and vibration. Choosing upgraded PI film can mitigate these issues and increase the reliability of a flex circuit.

Rigid flex PCBs are also well suited to high-heat applications. They can be designed with copper planes that are more conductive than traditional PCBs and offer superior heat dissipation. These advantages, coupled with the fact that the copper is deposited on both sides of the board, mean that rigid-flex circuits can handle a wider range of power requirements and higher operating frequencies than their traditional counterparts.

5G Requirements and Challenges for Flexible and Rigid PCB Boards

With the advent of 5G, however, there are new demands on the electrical performance of circuits. Massive MIMO (5G technology) uses multiple antennas to transmit and receive signals, which requires RF transceivers with very high amplitudes and phase variations. This is very demanding on the components that make up an RF unit, including power amplifiers and PAs. These generate a lot of heat, and they must be located in areas that can dissipate the heat effectively.

These new requirements also put a strain on the materials that can be used in the construction of rigid-flex circuits. The materials that are typically used in these types of circuits can only support signals up to 3 or 5 Ghz. For full 5G functionality, the RF circuits will need to operate at 25 Ghz or more. This requires more expensive and difficult to manufacture materials, which can extend the lead times for production.

For these reasons, it’s essential to work with a PCB fabricator that has experience working with a wide range of materials and knows how to optimize the stack-up for maximum performance and reliability. For example, the layer stack of a flex PCB should always include at least one stiffener made from FR-4 fiberglass and a thin layer of pressure-sensitive adhesive (PSA). This helps ensure that the circuit is able to bend or fold at the same points without breaking.