Printed circuit boards (PCBs) serve as the fundamental interconnect infrastructure in electronic devices. Although rigid boards are widely used, flexible PCBs (FPCBs) offer distinct advantages for applications that demand dynamic flexing, bending, or shaping around structures. Among the various FPCB constructions, single-sided flex circuits with conductors on one side are the most straightforward.
This article aims to explore the concept of single-sided flexible PCBs, encompassing their typical applications, associated benefits, materials and construction techniques employed, essential manufacturing capabilities required, and crucial design considerations for implementing single-sided flex circuits.
What is a Single Sided Flex PCB?
A single-layer flexible PCB, utilizing a single-sided polyimide (PI) copper-clad laminate, typically consists of rolled copper foil, chemically etched conductive patterns, and a layer of covering film applied after circuit completion. While it is possible to manufacture a single-sided flexible PCB without a cover layer or protective coating, such layers are often employed to safeguard the circuit, typically positioned as the top layer of the flexible circuit.
The single-sided flexible PCB enables access to component termination functions from only one side, allowing the design of through holes in the base film. These through holes facilitate the passage of electronic component pins, enabling electrical interconnection through soldering.
Insulating substrates commonly used for fabricating single-sided flexible PCBs include polyimide, polyethylene terephthalate, aramid cellulose ester, and polyvinyl chloride.
To enhance the rigidity of the single-sided flexible circuit, reinforcement ribs can be incorporated in specific areas requiring additional stiffness.
Among flexible circuit boards, the single-sided flexible PCB represents the simplest and most cost-effective manufacturing option. When single-sided wiring is sufficient for a particular application, opting for a single-sided flexible PCB is a judicious choice.
Consequently, the utilization of a single-sided flexible PCB offers significant cost-effectiveness.
What Materials Are Used in Single-sided Flexible PCB?
Common materials utilized in the fabrication of single-sided flexible printed circuit boards (PCBs) include:
Base Dielectric: Polyimide is the prevailing choice for the base dielectric material due to its superior flexibility and temperature resistance. Alternatively, materials such as Liquid Crystal Polymer (LCP), Polyethylene Naphthalate (PEN), Polyethylene Terephthalate (PET), or Polyurethane (PU) can also be employed.
Conductor: Electrodeposited copper foil is commonly utilized for the conductor layer, typically ranging in thickness from 12 to 35μm. Rolled annealed copper offers optimal performance in terms of flexure.
Bonding Adhesive: Acrylic or epoxy-based adhesive films are employed to attach stiffeners, connectors, or components onto the flexible PCBs.
Cover Layer: A cover layer, consisting of either polyimide or adhesive materials, can be optionally added to provide insulation or enhance stiffness.
Stiffeners: In areas of the flexible PCB that require rigid mounting surfaces, selective FR4 stiffeners can be incorporated.
Finishes: To ensure solderability and durability, immersion tin or silver over nickel finishes are commonly employed.
What Is The Structure Of A Single-sided Flexible PCB?
The stack-up of a single-sided flexible PCB typically consists of the following layers:
Polyimide Substrate/Cover Layer:
Polyimide film serves as the base material for the single-sided flexible PCB. It is utilized both as a substrate and as a cover layer, positioned on both sides of the circuit board. Polyimide offers excellent flexibility and temperature resistance, making it ideal for the requirements of flexible PCB applications.
Copper Circuit Layer:
The copper layer is formed by chemically etching the copper foil to create the desired circuit pattern. It acts as the conductive layer for the single-sided flexible PCB. The copper layer is positioned in the middle of the PCB, sandwiched between the polyimide substrate and the adhesive layer.
Adhesive Layer:
An adhesive material is employed to bond the copper circuit layer and the polyimide substrate together. This layer ensures the secure attachment of the copper traces to the polyimide film, enhancing the overall structural integrity of the flexible PCB.
Cover Layer Replacement:
In contrast to rigid PCBs that utilize solder mask to protect the circuitry, single-sided flexible PCBs employ polyimide as a cover layer to replace the solder mask. The polyimide cover layer provides both protection and insulation for the circuitry while maintaining the flexibility and durability of the PCB.
Stack-up of A Single-sided Flexible PCB
The stackup of a single layer flex PCB refers to the arrangement of layers and materials employed in its construction. Unlike multi-layer flex PCBs or rigid PCBs, single layer flex PCBs have conductive traces on only one side, making their stackup relatively straightforward. A typical stackup for a single layer flex PCB is as follows:
Stack-up of a single-sided flexible PCB consists of a base dielectric layer, a conductor layer, and optionally, a cover layer. This configuration allows for flexibility, lightweight design, and the ability to withstand dynamic flexing and shape changes required in various applications.
It is important to note that single-sided flexible PCBs have a single conductive layer, which means that components and traces are placed only on one side of the board. The stack-up is relatively simple compared to multilayer flexible PCBs, which have multiple conductive layers separated by dielectric materials.
How To Design a Single- Sided Flexible PCB?
In a one-sided flexible PCB configuration, a single conductive copper sheet is sandwiched between two insulation layers or a single polyimide insulating layer. The exposed side of the copper sheet is utilized for the one-sided flexible PCB design.
Subsequently, the circuit pattern is etched out of the internal copper sheet using a chemical etching technique.
Single-sided flex PCB boards may feature components, cables, pins, and stiffeners. Additionally, certain single-sided flexible PCBs incorporate a dual-access mechanism, allowing access to the conductive material from both sides of the circuit.
This design objective necessitates a compact PCB layout and specialized layers to enable access to the single copper layer through the polyimide layer of the base material.
As a result, single-sided flexible PCBs offer significant cost-effectiveness while still accommodating the required functionality and design considerations.
The Manufacturing Process of Single-sided Flexible PCB
The production process of single-sided flexible PCBs (Printed Circuit Boards) encompasses several sequential steps, ranging from design and material selection to fabrication and assembly. Here is an overview of the standard production process for single-sided flexible PCBs:
Design and Layout:
Design Specification: Establish the electrical and mechanical requirements of the PCB, including dimensions, component placement, and electrical connections.
Schematic Design: Create a schematic diagram of the circuit, specifying the components and their interconnections.
PCB Layout: Utilize PCB design software to develop the layout of the single-sided flexible PCB, encompassing component placement and conductive trace routing.
Material Selection:
Flexible Substrate: Select a suitable flexible substrate material, typically polyimide (PI) or polyester (PET), considering factors such as temperature tolerance, flexibility, and cost.
Copper Foil: Choose the appropriate thickness and type of copper foil to be used for the conductive traces, with copper being a common choice due to its excellent electrical conductivity.
Preparation of Flexible Substrate:
Cleaning and Pre-treatment: Thoroughly clean and pre-treat the flexible substrate material to enhance adhesion and ensure its cleanliness.
Copper Cladding:
Copper Foil Lamination: Laminate the chosen copper foil onto one side of the flexible substrate using an adhesive layer, thereby creating the conductive layer.
Circuit Patterning:
Photoresist Application: Apply a layer of photosensitive material (photoresist) to the copper-clad side of the flexible substrate.
Exposure: Employ a mask and UV light to expose the photoresist, reproducing the PCB layout and delineating the areas where the copper will be etched away.
Etching:
Chemical Etching: Employ an etching solution to chemically remove the exposed copper, leaving behind the desired conductive traces.
Resist Stripping: Eliminate the remaining photoresist, exposing the conductive traces.
Component Placement:
Surface Mount Technology (SMT): Utilize automated pick-and-place machines to accurately position surface-mount components onto the PCB. These components are then attached to the conductive traces using solder paste.
Through-Hole Technology (THT): Manually or automatically insert through-hole components through pre-drilled holes in the PCB and solder them on the opposite side.
Soldering:
Reflow Soldering: In the case of SMT components, subject the entire PCB to reflow soldering, where it passes through an oven to melt the solder paste and establish secure electrical connections.
Hand Soldering: For THT components, manual soldering may be required.
Insulation Layer:
Cover Layer Application: Apply an insulating cover layer, typically made of polyimide, across the entire PCB to safeguard the conductive traces and components. This layer may include openings to expose specific areas or components.
Testing and Inspection:
Conduct electrical testing of the completed single-sided flexible PCB to ensure proper functionality and connectivity.
Perform visual inspection to identify any defects, such as solder bridges or component misalignment.
Cutting and Forming:
Cut and shape the single-sided flexible PCB according to the desired configuration or enclosure requirements, enabling it to conform to specific shapes or fit within designated enclosures.
Final Testing:
Retest the PCB after cutting and forming to verify its correct functioning in its final shape.
Packaging and Shipping:
Package the finished single-sided flexible PCBs and make necessary preparations for shipment to customers or further assembly stages.
It is important to note that the precise details of the production process can vary depending on the manufacturer, complexity of the PCB design, and equipment employed. Nonetheless, the aforementioned general steps provide an overview of the typical process involved in manufacturing single-sided flexible PCBs.
Benefits of Single Sided Flexible PCBs
Single-sided flexible circuits offer several key advantages over rigid boards:
Dynamic Flexing:
Single-sided flex circuits can endure extensive flexing and shape changes required in motion applications, making them highly suitable for applications that involve frequent movement and bending.
Lightweight:
These circuits weigh significantly less, typically 60-80% less, than equivalent rigid boards. This lightweight characteristic is beneficial in weight-sensitive applications, such as portable electronics or aerospace systems.
Thin Profile:
With flex layers as thin as 25μm, single-sided flex circuits allow for tight installations and stacking. Their thin profile enables compact designs and facilitates integration into space-constrained devices.
High Density:
Single-sided flex circuits offer the advantage of finer lines and spaces, allowing for dense interconnections within limited areas. This high-density capability is particularly advantageous in miniaturized electronic devices where space is at a premium.
Three-Dimensional Flexibility:
Unlike rigid boards, single-sided flex circuits have the ability to bend and conform to three-dimensional surface shapes. This flexibility enables their usage in curved or irregularly shaped products and facilitates design versatility.
Embedded Passives:
Single-sided flex circuits can incorporate embedded passive components such as capacitors and resistors. This integration of passive elements directly onto the circuit board enhances compactness and enables space-saving designs.
Cost Efficiency:
By eliminating the need for additional components and connectors, single-sided flex circuits simplify assembly processes and reduce overall manufacturing costs. Their streamlined design and simplified assembly contribute to cost savings.
High-Frequency Performance:
Single-sided flex circuits can maintain controlled impedances and transmit unattenuated signals at high frequencies. This feature makes them well-suited for applications that require reliable signal integrity and high-frequency performance, such as telecommunications or high-speed data transmission.
Durability:
Polyimide materials commonly used in single-sided flex circuits exhibit excellent durability, withstanding millions of flex cycles without compromising their performance. This durability ensures long-lasting functionality and reliability in applications that involve repetitive bending or flexing.
Aesthetics:
Single-sided flex circuits offer the advantage of producing stylish, low-profile products. Their thin and flexible nature allows for sleek and visually appealing designs, enhancing the overall aesthetics of the end products.
Single-Sided Flexible PCB Vs. Single-Sided Rigid PCB
Single-sided flexible circuit boards present a highly advantageous alternative to rigid PCBs, offering numerous benefits over their inflexible counterparts.
Single-sided flexible circuit boards are constructed from a thin plastic sheet that undergoes lamination with copper foil, subsequently coated with an epoxy resin. These boards possess remarkable flexibility, allowing them to serve as the top layer in double-sided boards or as the bottom layer in multi-layer configurations. This inherent flexibility provides several advantages, such as enhanced durability, increased portability, and improved user-friendliness for customers.
In contrast, rigid circuit boards are composed of metal sheets etched with circuits and interconnected by minuscule metal wires, commonly known as “traces.” Their rigid nature makes them particularly suitable for applications like computer cases, where robustness and durability are paramount considerations.
Conclusion
A single-sided flexible PCB represents the fundamental configuration of a flexible circuit. Within the realm of single-sided flex PCB boards, it is possible to incorporate components, wires, adhesive materials, and stiffeners. The application of flexible circuits spans across diverse products, with medical equipment being a particularly notable beneficiary.
In contrast to conventional cabling and rigid PCBs, single-sided flexible PCBs present several advantages, including diminished occurrences of cabling and wiring defects, minimized size and weight, enhanced reliability, and increased durability. These benefits contribute to the overall enhancement of the electronic system’s performance and functionality.
