Rigid Flex Circuit board Resources

Implementing a rigid-flex PCB design allows for a lightweight and compact design that reduces overall packaging size. Rigid-flex PCBs can be used in smaller areas and have contributed to product miniaturization over the years. More importantly, they can be bent and folded to fit into smaller devices without sacrificing performance.

The technology has evolved over the years and is heavily deployed in the military and aerospace industry. Today, it allows for usage in lightweight and small form factor electronic products, such as medical devices, wireless products, and wearables.

However, there are a few challenges that designers face when switching over to rigid-flex PCBs. These include:
● The cost of fabrication and assembly
● How can the critical folded states and range of motion be modeled and tested?
● How will the different flex and rigid materials be managed and communicated to a supplier?

These are some of the major roadblocks facing new designers. It is important to understand how flex and rigid-flex circuits are designed. Just as important is the use of materials and the fabrication processes used by most rigid-flex PCB vendors.

Materials Used for Rigid-Flex PCB

Most rigid flex PCB boards are made from glass fiber impregnated in epoxy resin woven into fabrics. In real applications, this material is fabric but we use the term ‘rigid’ to describe a single laminated layer once it has achieved a minimum threshold of elasticity. Properly curing the epoxy allows the material to achieve rigidity.

Some people also describe the circuits as organic rigid printed circuit boards because of the use of epoxy resins.

Most applications involving flex circuits require flexible plastic compared to epoxy resin. The main material used is polyimide due to its durable and flexible structure (it cannot be stretched manually, making it ideal for use in assembly processes), and incredible resistance to heat. This allows the material to undergo several cycles of solder reflow while staying reasonably stable during expansion and contraction despite the temperature fluctuations.

Another commonly used material for flex-circuits is polyester (PET), but it does not tolerate the high temperature needed in soldering. They are often used in low-cost electronics that often don’t require soldering - rather, the contact is made with the help of crude pressure using an isotropic conductive elastomer.

Conductors
Conductors differ based on the application of the circuit board. However, copper is the default choice for most manufacturers because of its superior electrical conductivity, allowing heat transfer along the circuit. In addition, copper is one of the most abundant metals in the world and a large percentage is utilized in electronics.

Of course, the application of the circuit rigid flex circuit board largely dictates the material that will be used. Secondly, you may need to consider a rolled copper foil (manufactured from thick ingots) if the final application involves the movement of the flex circuit or repeated creasing.

With that said, the extra step of adding foils can increase the cost to the manufacturer. However, annealed copper is well suited to stretching under extreme strain without cracking. This is because annealing increases the grain structure of copper in the planar direction.

Adhesives
Adhesives are used in circuit board printing because they meet the requirements for processing and materials. They commonly include epoxy, acrylics, cyanoacrylates, silicones, and urethane acrylates for PCB.

Single Layer Flex Circuits
These circuits feature one conductive copper layer bonded between two insulating polyimide layers. This is very similar to most flexible flat connectors (cables). Copper is pre-laminated onto the polyimide film by the material supplier, then etched and drilled with a powerful backing plate before being treated with an adhesive based coverlay.

Designers should pay special attention to the materials used in flexible and rigid-flex circuits. This could be the difference between a successful product and one that fails.

Knowing more about the material will also help in the design, evaluation, and testing of the product. For instance, if the person is designing a rigid flex circuit board for the aerospace industry, they will need to model the product by factoring in heat, chemicals, shock, and moisture, to determine reliability and minimum allowed bending radius.

Flex Build Ups
The design of most flex or rigid-flex PCB boards is not straightforward. They require additional steps including:
● Application of adhesive
● Addition of copper foils
● Drilling
● Thru-hole plating
● Etching and stripping
● Etch-resist printing
● Cutting out the flex
● Covercoat or coverlay

Physical Constraints
It may be possible to build most stack-ups with flex and rigid sections. However, the process can get difficult if the designer does not evaluate the material properties and production steps. You must understand flex PCBs’ stress ranges within the materials due to circuit bends.

For example, copper can suffer from work-hardening with fatigue fracture occurring due to small radii and repeated flex cycling. One way to prevent this is to only use single-layer flex circuits, where the copper traces occupy a central location within the bend radius, and therefore, the coverlay and substrate are subjected to greater tension and compression. This won’t be a problem for them because polyimide is much more elastic and will allow the product will last longer under all the repeated movement compared to multiple copper layers.

Adhesive Beads
Designers may want to use strengtheners if the flex circuit is exciting the rigid board. In this case, the addition of a holt-melt, acrylic, or epoxy can improve the board’s durability. However, dispensing these liquids can be laborious and increase overall costs. It is possible to use automate fluid dispensing, but this should be done carefully.

Never Bend at Corners
It is important to avoid right angle traces in a flex-circuit board. However, this may not be possible in certain design situations. In these cases, try to keep the trace work as gently curved as possible and use conical radius bends wherever possible.

Add Support for Pads
Copper can easily detach from its polyimide substrate because of repeated stresses related to bending and the lower adhesion. This is why it is important to support the exposed copper. Vias are suitable for this job because the thru-hole plating offers a convenient mechanical anchor for flex layers.

Learn more about rigid-flex PCB with Hemeixin here.