Introduction
Rigid-flex PCBs integrate rigid PCB substrates and flexible circuits into a single interconnected assembly. They provide solutions for products requiring dynamic flexing, 3D configuration, high density, and miniaturization.
The manufacturing process for rigid-flex PCBs encompasses many steps to transform raw materials into completed boards. It draws upon techniques used for both rigid boards and flexible circuits, with additional steps to laminate and interconnect the sections.
This article provides an overview of the rigid-flex PCB manufacturing process covering:
- Raw materials used
- Fabrication process outline
- Details on key process steps
- Advanced manufacturing capabilities
- Quality testing and inspection
- Factors impacting cost
Understanding the manufacturing methods helps designers, engineers, and procurement teams specify viable rigid-flex boards and make informed sourcing decisions.
Raw Materials Used in Rigid Flex PCB Fabrication
Rigid-flex PCBs utilize similar base materials as individual rigid and flex PCBs during fabrication. Key raw materials include:
Rigid substrates
- FR-4 – Glass reinforced epoxy laminate
- Polyimide – Flexible at higher temp than FR4
- Ceramics – Alumina, aluminum nitride, etc.
- Metal cores – Aluminum, copper, etc.
Flexible dielectric films
Copper foils – Rolled copper, electrodeposited copper
Bonding films – Acrylic, epoxy, nitrile phenolic resins
Cover layers – Solder mask, legend, stiffeners
Plating finishes – ENIG, immersion tin, silver, gold
Outline of Rigid Flex PCB Fabrication Process
Overview of rigid-flex PCB fabrication process flow – Image source: ResearchGate
The manufacturing process for rigid-flex PCBs involves the following major steps:
- Fabrication of rigid sections – Individual rigid substrate layers are processed similar to standard PCB lamination, drilling, and metallization steps.
- Fabrication of flexible layers – Flexible dielectric films are patterned, layered, and imaged using conventional flex PCB processes.
- Lamination – Rigid layers and flex layers are strategically aligned and bonded together using adhesive films.
- Drilling – Aligned mechanical and laser drilling form interconnections between sections.
- Metallization – Plated hole walls interconnect circuit layers and bond rigid-flex junctions.
- Patterning – Circuits are imaged on individual layers using lithography and etching.
- Assembly – Components can be assembled onto rigid portions of the PCB.
- Testing and inspection – Electrical, mechanical, and optical inspection verifies quality.
- Finishing – Routng, edge treatments, stiffener application, and protective coatings.
Details on Key Rigid Flex PCB Fabrication Steps
1. Rigid Layer Processing
The inner conductive layers of the rigid sections consist of 1oz or 2oz copper foil that is bonded to insulating dielectric substrates. Common substrates are FR-4, polyimides, ceramics, and metal cores.
Layers are laminated together under heat and pressure in a press. Alignment is critical to achieve layer-to-layer registration. Drilling provides vertical interconnects between layers known as plated through holes (PTHs).
Copper is plated along hole walls to form electrical connections between embedded layers. Layers are circuitized using lithographic patterning and etching to form the conductive traces and pads.
2. Flex Layer Processing
The flex layers consist of thin copper foil on top of a flexible dielectric film substrate. Common flex materials are polyimides, PEN, PET, PAR, and Teflon PTFE.
The base metal can be rolled annealed copper or electrodeposited copper foil. Flex layers do not require drilling but use a subtractive process to image circuits. Flex layers are bonded together using adhesive or thermoplastic bonding films.
3. Layer Lamination
The individual rigid and flexible layers are precisely aligned and stacked up in the desired configuration. Bonding films join the layers together into a monolithic circuit assembly when heat and pressure are applied.
Curing ovens with air flotation technology assist in gently handling thin flexible layers. Accuracy at this stage ensures reliable plated joints between sections.
4. Drilling
The laminated board then undergoes drilling to create interconnecting holes between board layers and circuit pathways between rigid and flex sections.
Mechanical drilling and laser drilling techniques are used. Laser drilling achieves very fine hole sizes down to 0.10mm in diameter. Depth control creates “blind” and “buried” vias.
5. Metallization
A copper layer is plated onto exposed hole walls to electrically interconnect board layers. Electroless copper followed by electrolytic copper builds hole conductivity. At the board perimeter, strips are masked off to delimit plating extents.
6. Patterning
Photolithography forms the circuit pattern on each layer by selectively removing metal regions. Liquid photoresist is applied, exposed through a mask, developed, and washed. Bared copper is etched away, leaving a copper pattern.
Fine lines down to 2-3 mils are achievable. Soldermask defines solderable areas and provides insulation. Legend printing identifies components and polarity.
7. Assembly
For boards with mounted components, surface mount pads on rigid areas can be populated using solder paste printing and reflow soldering techniques.
8. Testing and Inspection
Testing validates electrical connectivity on the finished boards. It includes in-circuit testing (ICT), flying probe, and boundary scan. Automated optical inspection (AOI) checks features.
9. Finishing
Finished boards undergo routing, stiffener application, edge treatments, and protective organic solderability preservative (OSP). Conformal coating can be applied to aid flex durability.
Advanced Manufacturing Capabilities for Rigid Flex
Rigid-flex PCBs demand tight tolerances and advanced processes to meet quality and reliability requirements, including:
- Fine line patterning – Lines/spaces down to 2/2 mils
- Laser processing – Precise laser drilling, ablation, cutting
- Advanced materials – Metal core substrates, ceramic cores, exotic flex materials
- High layer counts – 20+ layers for complexity
- HDI technology – Dense microvias and fine interconnects
- Flex-on-rigid – Embedding thin flex layers into rigid sections
Quality Testing and Inspection
Multiple testing and inspection steps validate rigid-flex PCB quality:
- Electrical testing – Tests for opens, shorts, continuity. ICT fixture testing or flying probe.
- Automated optical inspection (AOI) – Checks trace widths, spacing, hole quality, pad shapes, etc.
- X-ray inspection – Detects hidden defects and verifies layer alignment
- Cross-sectioning – Inspects plated-through holes and layer interfaces
- Flexibility testing – Qualifies minimum bend radius for flex areas
- Dimensional analysis – Measures circuit features for adherence to specifications
- Visual inspection – Examines boards for defects in all process steps.
Factors Impacting Rigid Flex PCB Cost
Several factors influence cost for rigid-flex PCB manufacturing:
- Board complexity – Layer count, flex-rigid transitions, density of interconnects
- Materials – Flex dielectrics, bonding films, metal core substrates
- Tolerances – Tighter tolerances require more controls
- Board size – Larger boards consume more base materials
- Testing – Extensive testing and inspection adds cost
- Volume – Low volume prototyping has higher cost than mass production
- Lead times – Rush service can increase cost
- IP considerations – Some applications require strict IP protection
Proper design for manufacturability and partnering with qualified manufacturers helps balance cost against performance.
Frequently Asked Questions
Q: How are layers aligned in a rigid-flex PCB?
A: Advanced lamination presses use precision tooling pins for layer-to-layer registration. Laser drilled or x-rayed tooling holes provide alignment guides. Vision systems also assist alignment.
Q: What are some key tests performed on finished rigid-flex PCBs?
A: Testing involves electrical validation, flying probe testing, boundary scan for components, automated optical inspection, x-ray imaging, cross-sectioning, and mechanical flexibility testing.
Q: Can components be mounted directly onto rigid portions of a rigid-flex PCB?
A: Yes, surface mount components can be assembled onto rigid board areas using standard SMT assembly processes of solder paste printing, component placement, and reflow soldering.
Q: What are some ways to improve flexibility durability for the flex areas?
A: Polyimide coatings, special adhesive treatments, and mechanical shielding such as metal stiffeners help improve flex life and prevent conductor cracking.
Q: How are very dense interconnects achieved on rigid-flex boards?
A: Rigid-flex boards utilize stacked microvias, fine line circuitry, and advanced HDI processes to create high density interconnections especially within the rigid sections.
