Flex circuits: Innovations and processes

Flex PCBs have been a key enabler of modern high density electronics, but achieving this density requires thinner layers and finer lines. Conventional three-layer flex circuits comprised of copper, polyimide, and bonding adhesives are giving way to thinner, smoother two-layer flex circuits that forego the adhesive layer – the copper is instead deposited directly on the polyimide. These two-layer circuits may be as thin as 30 µm, with line spacing as fine as 15 µm (0.6 mils). It’s imperative, therefore, that the processed panels are handled extremely carefully to avoid causing wrinkles, tension, or scratches.

Folded flex circuit connecting electronics in a mobile phone

The inherent physical delicacy of flex circuits poses some key manufacturing challenges that can negatively affect yield and potentially impact a design’s viability. These challenges are being addressed by flex-supporting technologies that enable large-scale FPC (flexible printed circuit) production while ensuring quality yield and output. More flex circuit suppliers are adopting advanced flex manufacturing techniques to enhance manufacturing efficiency, improve yield, and maintain low costs and market competitiveness.

Production design and manufacturing of FPCs is different from rigid PCBs, and over the last ten years, new solutions throughout the production cycle were developed to support the delicacy of flex PCB production. Improvements in traditional sheet-to-sheet material handling and lately, automated roll-to-roll (R2R) processes are bolstering flex circuit production to meet growing market demands. If you’re working with flex, here are some technologies you should know about.

Multiple flex circuits in a smartphone design

Flex CAM and CAD solutions: Better control of the process from design to production

Special design for manufacturing (DFM) software tools for flex circuits help neutralize production problems during the design stage. These advanced tools are used to fully automate manual editing sessions, reducing errors and critical cycle time. Among today’s available flex DFMs are automatic joint curving and surface smoothing, and automated coverlay and solder mask optimization that make design faster, higher quality, and more accurate.

Panel of FPCs analyzed and production-optimized (source: Frontline PCB Solutions)

Typical multilayer rigid flex design (source: Frontline PCB Solutions)

Tool-based flex circuit design analyzers provide additional control by enabling engineers to review designs before, during, and after tooling. They can be used to check construction constraints for flex boards, and report problems related to stiffeners, air gaps, pre-bend areas, frequent moving parts, trace overlaps and joints, and conductive masks.

Special dedicated CAD and CAM tools such as Xpedition from Mentor Graphics and GenFlex and InCAM Flex from Frontline are available for flex printed circuit design.

Laser drilling of flex panels for high density & yield

Laser drilling and routing are common in flex printed circuit production. Ultra-violet laser drilling technology is utilized in high-density flex manufacturing to drill vias under 70 µm directly through the copper and polyimide layers. High-precision laser drilling has the capability to acquire targets on the panels and accurately align the drilling location to ensure the best registration of vias. Laser machinery is also used for accurate fine routing and slot ablation which are common in PCBs. Laser drilling supports sheet-based production and has lately been adopted for roll-to-roll production mode.

Laser Direct Imaging: Accuracy and distortion compensation for non-planar flex materials

To date, advanced flex circuit suppliers have relied primarily on laser direct imaging (LDI) equipment to use with their sheet-based imaging for double-sided flex, rigid flex, and multi-layer flex materials. LDI helps overcome flex production challenges with:

  1. Highly accurate depth of field (DOF) optics: Imaging fine line features on flex materials where a surface is not always flat and for surface height variations of 100 µm to 300 µm. LDI with Large Scale Optics (LSO) technology provides depth of focus of over 300 µm ensuring optimal line quality and uniformity on any flex surface. Other less accurate imaging optics architectures that have DOF of below 100 µm result in low line quality and lack of uniformity, impacting yield.
  2. Distortion Compensation: Polyimide flex material is deformed and stretched during production. To compensate for this deformation, each individual flex sheet is measured and then imaged with a compensated corrected image, ensuring accurate alignment of the pattern with the drilled vias. Only a digital direct imaging solution with high registration accuracy can modify and correct the pattern image per each measured sheet. This supports accurate pattern to vias registration, enabling small capture pads and high density with high-yield FPC production.

LDI is used in over 80% of sheet format mass production of fine line flex printed circuits. The need for LDI working in roll-to-roll production infrastructure continues to grow, and LDI for flex roll is being developed.

Automated Optical Inspection (AOI): Achieving higher levels of quality inspection

The majority of FPC products are either double- or single-sided. Traditionally, these did not always undergo AOI inspection. In the past five years, fine-line flex has become a major part of the smartphone interconnect, resulting in integrated device manufacturers demanding higher quality control of the single- and double-sided FPCs, and making AOI-level inspection mandatory.

Polyimide base material is transparent and presents an inspection challenge. New inspection AOI tools have been developed with multiple imaging capabilities, enabling the FPC to be scanned in a manner that ensures full detection with no false overcalls from the bottom pattern layers.

AOI typically involves manual handling of the panel sheets in the inspection and verification stages. This manual method presents handling challenges and frequently causes handling damage for the delicate and thin FPC sheets, creating a substantial increase in scrap.

Automated handling of thin cores of flex sheets is a major technical challenge and therefore the need to move to a roll-to-roll operation mode for inspection and verification has grown.

Automated flex roll AOI system

Today around 100 AOI systems use roll-to-roll for inspection and verification mode processing in the Asia Pacific region, mainly for smartphone production. These AOIs are used for single- and double-sided flex circuits, as well as the inner layers of rigid-layer and multi-layer flex circuits.

Automated Optical Shaping: Yield improvement by restoring FPC scrap

Due to the 30 µm core of the fine-line double-sided FPC, manual repair or rework of defects has not been an option in the past – defective FPCs were immediately scrapped. But in the past three years, fully automated copper shaping solutions have been developed for shaping and saving fine FPCs. This automated optical shaping solution uses advanced fluorescent-based imaging and laser ablation tools working together in a closed-loop shaping system, removing fine shorts with virtually no penetration, and no damage to the FPC. These damaged FPCs are restored and no longer need to be scrapped, increasing final yield and saving significant production costs. Automated optical shaping was recently implemented in roll-to-roll mode.

Direct imaging (DI) for soldermask flex layer

Thin flex products that have no reinforcing glass fibers tend to move and deform along the production process. These deformations are accumulated throughout the production process and need correction at the solder mask stage. This explains why the use of DI at the solder mask layer for smartphone FPCs is increasing, and why DI is becoming a solution of choice for high yield, high-volume FPC production.

Roll-to-roll processing: Eliminating damage

Whereas sheet-to-sheet processes are hindered by multi-step batch handling procedures and small substrate sizes, roll-to-roll processing enables the high speed, continuous processing of a long flexible web that’s typically 100m long. With this method, production efficiency is improved dramatically, producing tens of thousands of small FPCs on one long continuous production web. Roll-to-roll can accommodate single- and double-sided flex circuits, and the processing of the inner layers of multi-layer flex circuits.

High-speed continuous R2R processing

Roll-to-roll processing infrastructure requires a significant upfront capital investment in customization of all the described production equipment as well as for the other processes including the chemical lines. This investment is far larger than what’s required for sheet-to-sheet processing, so flex circuit suppliers have been understandably conservative with roll-to-roll adoption. However, by eliminating flex handling damage, the benefits of using the roll-to-roll process for higher quality and higher yields make it cost effective.

Conclusion

FPCs are invaluable for a wide range of applications – none more so than the modern smartphone, where they provide a high level of pattern density together with interconnection folding capabilities. This enables efficient thin product designs that simply cannot be achieved with conventional rigid PCBs. But producing these ultra-thin, flexible, and delicate interconnects comes with many challenges. Extra care must be taken throughout the production process to ensure that the technology benefits that these circuits enable aren’t compromised by low yield and manufacturing inefficiencies that ultimately drive up the cost of end devices.

Leveraging highly efficient roll-to-roll processing with advanced laser drilling, AOI, and DI capabilities, complemented by flex-optimized software tools, flex circuit suppliers are achieving newfound economies of scale and providing designers with the highly reliable, highly versatile flex circuits they need to differentiate their products in a hotly competitive marketplace.

Written by: Micha Perlman, Senior Marketing Manager, Orbotech
Published by: EDN

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