Automated optical inspection and direct imaging are ensuring higher yields for flexible circuit makers.
Most 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.
Figure 3: 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: Restoring FPC scrap
Due to the 30µm 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 solder mask flex layer
Thin flex products that have no reinforcing glass fibres 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.
Figure 4: 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.
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.
- Micha Perlman is Senior Marketing Manager at Orbotech.