Process innovations enable versatile flex circuits

Article By : Micha Perlman

Roll-to-roll processing with advanced laser drilling capabilities have realised economies of scale for flex-circuit suppliers, allowing wider use in such products as smartphones.

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 30µm, with line spacing as fine as 15µm (0.6 mils). It’s imperative, therefore, that the processed panels are handled extremely carefully to avoid causing wrinkles, tension, or scratches.

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.

Flex CAM and CAD solutions

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.

FlexPCB_Orbotech_Figure1 (cr)
*Figure 1: Panel of FPCs analysed and production-optimized (source: Frontline PCB Solutions).*

FlexPCB_Orbotech_Figure2 (cr)
*Figure 2: Typical multilayer rigid flex design (source: Frontline PCB Solutions).*

Tool-based flex circuit design analysers 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

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 70µm 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

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 100µm to 300µm. LDI with Large Scale Optics (LSO) technology provides depth of focus of over 300µm ensuring optimal line quality and uniformity on any flex surface. Other less accurate imaging optics architectures that have DOF of below 100µm 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.

Next: AOI, R2R boost flex circuit viability »

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