How are Rigid-Flex PCBs Made?

Rigid-flex printed circuit boards from Rush PCB Inc. have the characteristic flexible sections along with rigid sections in the same board. So far, they were not popular with many designers, as their designs mostly used rigid boards. With electronic equipment design trending toward high-density, more designers are looking for printed circuit boards that can actually bend. This is where our rigid-flex PCB technology is proving to be an advantage over the regular rigid boards.

For our customers new to this technology, designing and using rigid-flex PCBs can be a challenge. Understanding the construction of rigid-flex PCBs can go a long way in handling these boards properly in their design and application.

Rigid-Flex Printed Circuit Board Substrate Materials

All printed circuit boards need a substrate to give them some rigidity. Conventional boards use a base material of fiberglass and epoxy. Although it is fiber, the presence of epoxy gives it the rigidity—especially once the epoxy cures. However, the lack of flexibility does not allow conventional boards to be suitable for many modern products. However, if the board does not have to move or bend after assembly, the conventional rigid printed board is acceptable.

Polyimide

For many applications, the substrate of the board must be more flexible than the epoxy will allow. Most PCB manufacturers replace the epoxy fiberglass combination with PI or Polyimide, as it is soft but strong, and not easy to tear or stretch. It also features high thermal stability, making it suitable for withstanding the high temperatures of the reflow process during soldering. Polyimide exhibits practically no telescopic deformations from temperature fluctuations.

Polyester

PET or Polyethylene terephthalate, commonly known as Polyester, is also a flexible material with somewhat lower heat resistance and temperature deformations compared to that of Polyimide. Rush PCB Inc. uses this material for low-cost electronic devices that require a soft film wrapping. As Polyester cannot withstand high temperatures associated with soldering processes, we use a cold pressing process for forming such soft and flexible circuit boards.

Rush PCB Inc. uses several common materials for building the core of flexible circuits. These include PI films, PET films, thin epoxy resins, and glass fiber. In addition, we also use a protective film of PI or PET film as a solder resist mask. Like the solder mask that we use for rigid boards, the protective film insulates the conductor traces from the outside. This protects them from damage and corrosion. The thickness of the protective films we use ranges from 1 mil to 3 mils. We also use glass fiber and epoxy for not so flexible boards, as their thickness is typically 2 – 4 mils.

Rigid-Flex Printed Circuit Board Conductor Materials

All circuit boards use printed wiring or traces for interconnecting the different components the assembler mounts on them. Conventional rigid PCBs use copper traces. Although rigid-flex boards use carbon film or silver-based inks for forming the traces, copper is also a common material.

We use different types of copper on rigid-flex boards depending on the application. One of the best and most common choice is the electrolytic copper foil. We can increase the current carrying capacity of the traces simply by increasing the copper weight. In high frequency circuits, increasing the copper weight allows us to achieve different copper skin widths, especially for applications such as onboard planar inductors.

However, copper does not perform well where stress fatigue and work hardening are present such as for flexible circuits. RA or Rolled Annealed copper is a better choice where the flexible circuit undergoes repeated bending or folding.

Although rolling and toughening the copper increases its cost, the extended life of the flexible circuit overcomes the extra expense. This is because the RA copper foil can withstand several cycles of bending and folding before fatigue fracture overcomes it. RA copper being more elastic in the Z direction, withstands more bending and rolling, and offers an extended lifetime to the flexible board. The rolling process lengthens the grain structure of regular copper, giving it increased toughness in the planar direction, while making it softer and increasing its elasticity in the Z direction.

Rigid-Flex Printed Circuit Board Adhesive Materials

While fabricating the copper clad, the fabricator must bond the copper foils to the core, such as the PI film or PET film. For a regular rigid FR4 board, this is not an issue as the surface of the copper foil is already rough. After rolling and toughening, the RA copper foil surface becomes very smooth, which does not offer good adhesion even with the application of high temperature and pressure.

Corrodible Copper Clad Laminates

Rush PCB Inc. uses corrodible copper clad laminates from manufacturers like DuPont for making single- and double-sided rigid-flex boards. These copper laminates require only 1-2 mil thickness of epoxy or acrylic glue. Such glue is suitable for flexible circuit boards.

Glue-Free Laminates

Rush PCB Inc. also uses glue-free laminates for making flexible circuit boards. We use new processing techniques like direct deposition and coating of copper on the PI films. This technique is especially useful for flexible HDI circuits that require smaller vias and finer pitches.

Rigid-Flex Printed Circuit Board Supportive Materials

The region where the flexible PCB meets the rigid part often requires support for strengthening it. We typically use silicone, epoxy, or hot melt glue for increasing the tensile strength of the joint. This ensures that even if the flexible PCB undergoes repeated flexing, there is no stress fatigue and no tearing occurs.

Understanding materials that manufacturers use for fabricating rigid-flex boards is important, as it offers the designer to properly choose and suitably select the materials for a specific application. This helps to avoid hidden dangers cropping up in the field leading to catastrophic failures.

Conclusion

By understanding the properties of the material in rigid-flex circuit boards also helps designers in designing, evaluating, and testing mechanical parts of electronic products. For instance, automotive applications require products that can dissipate heat, withstand moisture, chemical corrosion, impact, and many more common situations. Similarly, rigid-flex circuits need careful assessment for achieving the highest reliability with minimum allowable bend radius using the right materials.

Rigid-flex circuit boards are also likely to face harsh environments, primarily in low-cost consumer electronics applications. For instance, personal electronic devices are often subject to vibration, falls, sweat, and more.