What is a Flex PCB?

A flex PCB, short for flexible printed circuit board, is a type of PCB made from flexible plastic substrates like polyimide or PEEK. Unlike rigid PCBs made from fiberglass, flex PCBs can bend, twist, and fold to fit into tight spaces or shaped enclosures. This makes them ideal for applications that require small, lightweight, or uniquely shaped circuit boards such as wearables, medical devices, aerospace systems, and more.

Flex PCBs offer several advantages over rigid boards:

However, flex PCBs also present some unique manufacturing and assembly challenges compared to rigid boards. The flexible substrates are more delicate and prone to damage during handling. The copper traces can crack if bent too sharply. And the materials and processes used are generally more expensive.

Flex PCB Manufacturing Process

Substrate Selection

The first step in making a flex PCB is selecting the appropriate flexible substrate material. The most common choices are:

• Polyimide (PI) – A strong, lightweight, heat-resistant polymer film. Kapton is a well-known PI film made by DuPont. Offers excellent electrical properties but is more expensive than PET.

• Polyethylene terephthalate (PET) – Another polymer film that is lower cost than PI but not as heat resistant or durable. Typically used for low-cost consumer electronics.

• Polyether ether ketone (PEEK) – A high-performance thermoplastic with excellent mechanical strength, chemical resistance, and thermal stability up to 260°C. Used in harsh environment applications but very expensive.

The substrate is chosen based on the end application requirements for cost, flexibility, environmental resistance, and dielectric properties. PI is the most widely used for its balance of performance and price.

Circuit Design

Next, the circuit schematic is designed using EDA software, just like a rigid PCB. However, some additional design rules and guidelines must be followed for flex circuits:

• Use teardrops or curved traces instead of 90° angles to prevent stress concentrations
• Maintain larger than normal spacing between traces and pads
• Avoid plated through-holes if possible to increase flexibility
• Use hatched polygons instead of solid copper fills
• Add stiffeners and anchors to connector areas
• Define the bend locations and directions
• Specify the number of flex layers needed (single, double, or multi-layer)

The EDA design is used to generate the printouts, photomasks, or files needed for the subsequent fabrication steps.

Printing and Etching

With the circuit pattern defined, it is printed onto the flex substrate using photolithography, similar to making rigid PCBs. A photoresist coating is applied to the substrate, exposed to UV light through the photomask, and chemically developed to transfer the circuit image.

Then the unwanted copper is etched away using wet chemical baths or plasma etching, leaving only the desired circuit traces behind. The photoresist is stripped off, leaving the bare copper circuits on the flex substrate.

For multi-layer flex PCBs, this imaging process is repeated to build up additional circuit layers, with laminated or adhesive insulation layers in between. Blind and buried vias can be used to interconnect the layers as needed.

Coverlay and Finish

To protect the delicate circuits, a final insulating layer called coverlay is laminated on top, like a permanent solder mask. Coverlay is made of polyimide or polyester film with a heat-activated adhesive (acrylic, epoxy, silicone) on one side. It is cut to shape with openings for the contacts and laminated onto the flex PCB under heat and pressure.

Additional surface finishes may be applied to the exposed contacts for solderability, wire bonding, or contact reliability:

The choice of surface finish depends on the component types, assembly processes, and reliability requirements. ENIG and IAg are the most popular for flex PCBs.

Singulation and Test

Lastly, the multi-up flex panels are cut into individual circuit units using steel rule dies, laser cutting, or blanking. The singulated flex PCBs are 100% electrically tested to check for shorts and opens before being packaged for shipment.

Visual inspection and other non-destructive testing may also be performed to check for material and workmanship defects. Highly reliable flex PCBs for aerospace, defense, or medical use require additional testing and qualification to ensure the product meets specifications.

Flex PCB Assembly Methods

Once the raw flex circuit is fabricated, it needs to be populated with electronic components to become a functional product. The flex PCB assembly process is arguably more challenging than rigid board assembly, due to the unique material properties and handling requirements. However, many of the same SMT methods and equipment can be used for flex assembly, with some adaptations.

Stiffeners and Stabilizers

Before any components are attached, the flex circuit should be stabilized in the assembly area to prevent shifting and movement. Temporary stiffeners can be applied to the bottom side using pre-cut polyimide pieces with pressure-sensitive adhesive. This holds the flex PCB flat and rigid throughout the assembly process.

In addition, cauls, frames, or pallets may be used to hold the flex circuit in place, provide structural support, and aid in transporting between machines. These stabilizing materials are removed after assembly.

Solder Paste Printing

Solder paste is printed onto the flex PCB’s SMT pads using standard stencil printing machines. Pneumatic or hydraulic clamping is used to hold the flex flat against the tooling plate or pallet to prevent warping.

Due to the flexible nature of the substrate, a thinner stencil (3-4 mils) and harder squeegee blade (90 Shore A durometer) are recommended to minimize distortion during printing. The print parameters (pressure, speed, separation) should be optimized for a clean, consistent paste transfer.

Pick and Place

Next, the SMT components are picked up and placed onto the solder paste deposits using automated pick-and-place machines. Like in the printing step, the flex PCB must be firmly supported and held down during placement to ensure positional accuracy.

A pallet or custom vacuum tooling plate can be used to keep the flex PCB flat and immobile. The pick and place machine’s component feeders, nozzles, and placement force must be compatible with any overhanging flex material to avoid damaging the circuit.

Standard SMT chip components can be placed on flex PCBs without issue. However, larger or heavier components may require additional adhesive to prevent them from shifting or tombstoning during reflow. The component placement file should be programmed to recognize any flex-specific fiducials or board features.

Soldering

After placement, the populated flex PCB is soldered using standard convection or IR reflow ovens. The flex material can tolerate typical lead-free reflow profiles up to 260°C peak temperature without degradation.

For double-sided flex assemblies, the process is repeated to print solder paste and place components on the other side, taking care to protect the first side during handling.

If any through-hole components are present, they can be soldered using a selective soldering machine or manual hand soldering afterward. Wave soldering is not recommended for flex PCBs due to the risk of damaging the thin materials.

Cleaning and Inspection

Flux residues should be removed from the soldered flex assembly using an appropriate cleaning method and chemistry. Ultrasonic cleaning is not advisable as it can delaminate the coverlay or internal adhesive layers.

Automated optical or X-ray inspection is used to verify the solder joint quality, component placement, and overall assembly integrity. The flex PCB may need to be flattened or constrained to get clear images for inspection.

Applications and Examples

Flex PCBs are used in a wide range of industries and products where traditional rigid PCBs are inadequate, such as:

• Medical devices – Hearing aids, pacemakers, defibrillators, surgical tools
• Wearables – Smartwatches, fitness trackers, AR/VR headsets
• Automotive – Instrument clusters, sensors, camera modules
• Aerospace – Avionics, satellites, missiles, UAVs
• Industrial – Robotics, automation, controls
• Consumer electronics – Smartphones, laptops, gaming systems

Some specific examples of flex PCB assemblies include:

In each case, the flex PCB enables a compact, lightweight, and unique form factor that would be impossible with a rigid PCB assembly. As electronics continue to shrink in size and grow in complexity, flex PCBs will become an increasingly important packaging and interconnect solution.

FAQ

How much do flex PCBs cost compared to rigid?

Flex PCBs are typically 2-5 times more expensive than equivalent rigid PCBs, due to the higher material costs and specialized processing required. The price depends on the number of layers, size, complexity, and quantity ordered. Simple single-sided flex circuits may only be marginally more expensive than rigid, while complex multi-layer or rigid-flex combos can be significantly pricier.

What are the most common causes of failure in flex PCBs?

The most frequent failure modes for flex PCBs are:

• Cracking or fatigue of the copper traces due to repeated bending
• Delamination of the coverlay, bonding adhesive, or between circuit layers
• Solder joint fractures under high strain or vibration
• Conductor corrosion or dendrite growth in humid environments

Proper design, material selection, and strain relief can mitigate these risks and ensure a reliable flex circuit assembly.

How tight can you bend a flex PCB?

The minimum bend radius depends on the thickness and number of layers in the flex PCB. Generally, the minimum radius is 6 times the total thickness to avoid damaging the copper traces. For example, a 0.2 mm thick flex PCB should not be bent tighter than a 1.2 mm radius. Multi-layer flex PCBs are often designed with “bend-to-install” sections that can handle a one-time crease but are not meant for continuous flexing.

Can rigid and flex PCBs be combined?

Yes, rigid-flex PCBs are a hybrid construction that incorporates both rigid and flexible substrates laminated together into a single assembly. The rigid sections mount the components while the flex sections act as interconnects between the rigid boards. Rigid-flex provides the best of both worlds in terms of mechanical support and flexibility, but are also the most complex and expensive to fabricate.

What files are needed to get a flex PCB quote?

To get an accurate quote for a flex PCB, you should provide:

• BOM (bill of materials) with all the component types, values, and quantities
• Gerber files or ODB++ package with the circuit layer stackup and images
• Dimensioned drawing or DXF showing the flex PCB outline, bend locations, and component placements
• Assembly and test requirements (SMT, through-hole, test points, programming, etc.)
• Any special certifications or standards to be met (UL, IPC class, etc.)

The more detailed information you can give the fabricator and assembler, the better they can assess the manufacturability and cost drivers of the design. Ideally, involve them early in the design process for DFM feedback and cost optimization.