Single Sided Flexible PCB
single sided Flex PCB are the most basic type of flexible circuits. They consist of a flexible dielectric film laminated to a single sheet of copper. The copper layer is then chemically etched according to the specified circuit pattern design. Polyimide cover lays can be added to the circuit board for additional insulation and protection, if desired.
Single-Sided Flexible PCB : This design includes a single conductive copper layer that can be bonded between two layers of insulation or built with one polyimide insulating layer and an uncovered side. The interior copper layer then goes through a chemical etching process, producing the circuit design. Single-sided flex PCB board designs support the inclusion of electronic components, connectors, pins and stiffeners.
Introduction
Printed circuit boards (PCBs) provide the interconnect foundation in electronics products. While rigid boards are most common, flexible PCBs (FPCBs) offer unique advantages for applications requiring dynamic flexing, bending or shaping around structures. Single sided flex circuits with conductors only on one side are the simplest FPCB construction.
This article will examine what single sided flexible PCBs are, their typical applications and benefits, materials and construction methods used, critical manufacturing capabilities needed, and key design considerations when implementing single sided flex circuits.
What is a Single Sided Flex PCB?
A single sided flexible PCB consists of:
- A flexible dielectric base made of polyimide or other flexible polymer substrates.
- Conductive copper traces etched on one side of the base material.
- No conductors or dielectric on the reverse side.
This provides a thin, lightweight and highly bendable circuit layer with traces on only one side. Connections to the conducting layer are made using:
- Plated through holes (PTHs) drilled through the base substrate.
- Edge contacts formed along one or more edges of the circuit.
- Metal stiffeners or connectors soldered to pads on the flex layer.
Applications of Single Sided Flex Circuits
Some common applications of single sided flex PCB technology include:
- Wearable devices – Fitness trackers, smart watches, health monitoring devices.
- Displays – Flexible displays, electronic signage and indicators.
- Medical – Catheters, implants, transdermal patches.
- Automotive – Seat heaters, capacitive switches, sensors.
- Consumer Electronics – E-readers, headphones, VR headsets.
- Robotics – Flexible cabling for joints and manipulators.
- IoT devices – Flexible sensor nodes and connectors.
The dynamic flexing and continuous motion capabilities of single sided FPCBs suit them for these applications.
Benefits of Single Sided Flexible PCBs
Some key benefits of single sided flex circuits versus rigid boards are:
Dynamic Flexing – Can withstand extensive flexing and shape change required in motion applications.
Lightweight – Weigh 60-80% less than equivalent rigid boards.
Thin profile – As thin as 25μm flex layers permit tight installations and stacking.
High density – Finer lines/spaces allow dense interconnections in small areas.
Three-dimensional – Can bend and conform to 3D surface shapes.
Embedded passives – Capacitors and resistors can be screened for compactness.
Low cost – Eliminates components, connectors and simplifies assembly.
High frequency – Controlled impedances and unattenuated signals at high frequencies.
Durability – Polyimide materials withstand millions of flex cycles.
Aesthetics – Can produce stylish, low profile products.
Single Sided Flex PCB Materials
Typical materials used to fabricate single sided flex PCBs include:
Base Dielectric – Polyimide is most common. Can also use LCP, PEN, PET, PU. Polyimide provides the highest flexibility and temperature durability.
Conductor – Electrodeposited copper foil, typically 12-35μm thickness. Rolled annealed copper offers optimal flexure performance.
Bonding Adhesive – Acrylic or epoxy based adhesive films attach stiffeners, connectors or components.
Cover layer – Optional polyimide or adhesive covers can be added for insulation or stiffness.
Stiffeners – Selective FR4 stiffeners may be added in flex areas requiring rigid mounting surfaces.
Finishes – Immersion tin or silver over nickel are typically used for solderability and durability.
Critical Manufacturing Capabilities
Producing robust, reliable single sided flex PCBs requires advanced manufacturing expertise:
- Fine line etching – Ability to reliably produce and hold trace widths/spaces down to 50μm.
- Registration accuracy – Tight process control to meet registration tolerances around 50-75μm .
- Surface finish uniformity – Consistent immersion silver or tin plating thickness across flex circuits.
- Quality laminating – Eliminating delamination or separation failures through process controls.
- Flex fold engineering – Highly controlled folding based on extensive modeling and testing data.
- Conductor adhesion – Adhesion strength of copper traces exceeding 1.5N/mm on polyimide.
- Plated through holes – Smooth, void free copper plating of small through holes.
- Panel handling – Special carriers, tooling and protocols to avoid damage to flex layers.
- Reliability testing – Environmental testing, dynamic flex cycling, vibration and shock exposure.
Process control – Statistical control and continuous improvement for process stability.
Single Sided Flex PCB Design Considerations
Key design aspects when implementing single sided flex boards include:
- Trace widths and spacing – Use wider traces than rigid boards as narrow traces may break on flexing. Space traces adequately apart for flexibility.
- Trace routing – Route traces linearly in the direction of bending rather than perpendicular to the flex axis. Avoid sharp turns.
- Bend areas – Use fewer traces in sections that undergo maximum bending. Add stiffeners if needed.
- Bond pad openings – Allow for adequate bonding material around component pads for shear strength.
- Annular rings – Maintain sufficient annular rings around plated through holes for adhesion.
- Fold compensation – Account for swelling of outer side and shrinkage of inner side when folded.
- Adhesives – Select flexible adhesives optimized for flexing rather than rigid bonds.
- Reinforcements – Add stiffeners and backing structures selectively in high stress areas.
- Termination – Use robust, flexible solderable edge connector contacts.
- Impedances – Model electrical performance under various flex configurations.
Conclusion
With their unmatched dynamic flexing capabilities, light weight and thin profile, single sided flexible PCBs enable many new innovative applications not possible with rigid boards. When implementing single sided flex circuits, working with an expert manufacturer and following sound design practices allows harnessing the unique benefits of flex PCBs successfully. As electronics strive for greater motion tolerance, flexibility and structural integration, single sided flex PCB technology provides an enabling interconnect platform.
