Double-Sided Flexible PCB : Double-sided flex PCB feature two conductive layers with a layer of polyimide insulation between them. The conductive layer’s outer sides can be either exposed or have covers, like a copper pad. Layers are connected typically by plated through-holes, but other methods can be used. Like the single-sided flexible circuits, the double-sided flexible PCB can support additional elements such as pins, connectors and stiffeners.
Double Sided Flexible PCB Stack Up
Introduction
Flexible printed circuits (FPCs) enable unique capabilities like dynamic flexing and conforming to surfaces which are not possible with rigid boards. 2 layer flex PCBs with conductors patterned on both sides of a dielectric offer more interconnect density versus single sided flex, while retaining excellent flexibility.
This article examines 2 layer flex stackup configurations, critical design considerations, the manufacturing challenges involved, advanced capabilities needed for quality 2 layer FPCs, and guidelines for optimizing performance.
What is a 2 Layer Flex PCB?
A 2 layer flex PCB consists of:
- Flexible base dielectric material such as polyimide. This forms the core.
- Copper foil conductors patterned on both sides of the base material.
- Cover layers laminated over the etched traces for insulation, abrasion resistance and stiffness.
- Plated through holes (PTHs) for interconnecting between conductors on each side.
- Edge contacts or stiffeners to provide external connections.
The 2 metal layers bonded to a thin, flexible core dielectric provide a robust flex circuit construction with enhanced interconnect density versus single sided FPCs.
Applications of 2 Layer Flex Circuits
Some common applications of 2 layer flex PCB technology include:
- Displays – Flexible OLED displays, curved touch sensors.
- Automotive – Seat control panels, steering wheel electronics.
- Consumer Electronics – Foldable phones, wearables, VR headsets.
- Medical – Catheters, implants, transdermal patches.
- Robotics – Flexible cabling for manipulator joints.
- Defense/Military – Rugged flexible electronics, avionics systems.
- Industrial – Flexible sensors, actuators, kontrollers.
The compactness, dynamic flexing and extended cycle life of 2 layer FPCs make them well suited for these applications.
Benefits of 2 Layer Flex PCBs
Some key advantages of 2 layer flex PCBs are:
- Double routing density – Traces and componenets on both sides doubles layout area.
- Integrated shielding – Ground planes can shield signals from EMI/RFI.
- Embedded components – Passives can be embedded within flex layers.
- Impedance control – Better impedance matching with sandwiched microstrips.
- Higher interconnection density – More interconnects with fine traces on two sides.
- Rigid sections – Can incorporate stiffened sections for component mounts.
- Smaller product size – High density flex packing enables miniaturization.
- Lower assembly cost – Fewer discrete wires and connectors to assemble.
- Three-dimensional – Dynamic shaping and folding around structures.
- High frequencies – Controlled impedances benefit high frequency performance.
2 Layer Flex PCB Stackup Configurations
Typical stackup structures used in 2 layer flex PCBs include:
Coverlayer Based Stackup
Coverlayer 1
Signal Layer 1 Flexible Core Dielectric Signal Layer 2 Coverlayer 2
This offers good abrasion resistance and insulation with coverlayers enveloping the conductors.
Adhesive Based Stackup
Signal Layer 1 Bonding Adhesive Flexible Core Dielectric Bonding Adhesive Signal Layer 2
Here the core dielectric is sandwiched between conductors using adhesive bonding films. Provides ultra-thin profile.
Copper Clad Stackup
Signal Layer 1 Flexible Core Conductor Signal Layer 2
In this case, the flexible copper clad dielectric itself functions as the core, removing the need for bonding. Can be cost-effective.
Shielded Stackup
Coverlayer 1 Signal Layer 1 Flexible Core Dielectric Ground Plane 2 Signal Layer 2 Coverlayer 2
The embedded ground plane provides shielding between signals and blocks EMI.
Challenges in 2 Layer Flex PCB Manufacturing
While providing many benefits, fabricating 2 layer flex PCBs poses manufacturing difficulties including:
- Achieving and maintaining fine line resolution on both sides.
- Tight layer-to-layer registration across two flexible sides.
- Preventing delamination or separation between layers.
- Producing plated through holes with adequate annular rings.
- Controlling impedances across dynamically flexing layers.
- Avoiding rigid sections from detaching on flexing.
- Managing thermomechanical stresses from flexing.
- Maintaining solder joint integrity across flex cycles.
- Handling, processing and assembling extremely thin circuits.
- Ensuring flexibility and performance with embedded passives.
- Qualifying and modeling impedance shifts under various flex configurations.
Advanced Manufacturing Capabilities Needed
High quality 2 layer FPC production requires manufacturers to demonstrate several advanced capabilities:
- Fine line etching – Trace/space down to 25μm on thin flex cores reliably.
- Registration accuracy – Alignment around 50 to 75μm between flex layers.
- Annular ring control – Producing 1 mil annular rings on small PTHs.
- Flexible solder masks – Halogen-free liquid photoimageable (LPI) masks.
- Reliable multilayer bonding – Interlayer peel strengths above 2 N/mm without delamination.
- Surface finish – Uniform plating thickness of immersion Ag or Sn across dynamic contours.
- Fine space via tenting – Protecting ultra-fine trace gaps during PTH drilling.
- Flex fold engineering – Highly controlled folding with extensive test data.
- Plated through holes – Smooth, void free copper plating of PTHs with 1:1 aspect ratio.
- Process control – Real-time Statistical Process Control (SPC) for stability.
Reliability testing – Dynamic bend cycling, twist, vibration, thermal shock, drop testing.
2 Layer Flex PCB Design Considerations
Some key design aspects when working with 2 layer flex circuits include:
- Modeling electrical performance under different bend configurations and cycles.
- Assigning critical signals requiring impedance control or low skew to outer layers.
- Using wider traces than rigid PCBs and allowing adequate spacing between traces.
- Adding shielding planes or ground fills if EMI is a concern.
- Using linear routing for traces along the bend axis and minimizing perpendicular traces.
- Watching for impedance variations at transitions between rigid and flex sections.
- Accounting for registration shifts between layers during dynamic flexing.
- Providing sufficient annular ring margins around plated through holes.
- Ensuring vias have 1:1 capture pad aspect ratios for reliability.
- Incorporating thermal reliefs to reduce thermomechanical solder joint stresses.
- Adding EMI shielding vias around components if needed.
- Utilizing stiffeners, encapsulation and strain relief structures at stress points.
Conclusion
2 layer flexible PCBs enable increased routing density and the ability to integrate shielding planes while retaining excellent dynamic flexing capabilities. Harnessing these benefits requires mature flex PCB manufacturing processes and disciplined design techniques tailored for flex. When designed properly, robust 2 layer flex circuits provide electronics engineers an invaluable technology for developing innovative, compact and motion-tolerant products.
