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Material and Build-Up Construction of Flex-Rigid PCB

To implement multiple functions of electronic devices, PCB (printed circuit board) tracing becomes more dense with the improvement of component I/O density, leading to the increase of build-up layer count of PCB from one layer, two layers to three layers or even more, which results from interconnect density uprising and pitch downsizing. To eliminate fine lines and pitch, stacked via technology is developed using plated filled microvia. All the stacked via technologies make PCB designers feature larger extent of freedom in terms of component layout with fine pitch and optimal build-up layers. To obtain best manufacturing capabilities, stacked via technology is capable of allowing for agreeable tracing/spacing. However, this type of build-up PCB not only features issues on size and performance, but its manufacturing cost hardly meets customers' requirement.

Progress of interconnect technology also affects the demands on material characteristics. In the process of PCB fabrication, dielectric material has to conform to manufacturing requirement and all the material characteristics feature rigorous tolerances. Dimensional stability of substrate that is used for PCB fabrication and assembly is such a significant property. To make a series of different dielectric materials effectively applicable for build-up manufacturing, it's important to meet the demands of both OEMs (original equipment manufacturers) and designers before determining which type of special material.

Market Background

Demands on both flexible PCB and flex-rigid PCB has rocketed up along with the fast development of a series of electronic products such as cell phones, digital cameras, tablets, plasma display etc. Rigid-flex circuits hold extremely large advantages: containing no connectors, cables and reducing assembly procedures; lighter weight, excellent flexibility and 3D assembly, all of which can never be achieved by rigid PCBs. Naturally, each coin has two sides. Compared with rigid PCB, flexible circuits feature low mechanical intensity and reliability. Moreover, it's difficult and complicated to administer and manufacture such thin and light flexible circuits. Nevertheless, up to now, the essential merit of flex-rigid PCB is foreseeable that it is advantageous in cost, quality and reliability.

Recent years have witnessed obviously expansion of application fields of flexible circuits and technical solutions and constant innovation have been able to meet the requirement of cost challenge. Although manufacturing cost of flex-rigid PCB will never be lower than that of rigid circuit board and cables, it will definitely bring forward more advantages to EMS (electronic manufacturing service) and OEM in terms of technology and capability. As far as the whole supply chain is concerned, the merits flex-rigid PCB brings forward is even more valuable based on requirement set by material suppliers and OEM.

New Solutions of Flex-Rigid PCB

An ordinary method to fabricate flex-rigid PCB starts with flexible material that will be covered on rigid board. This technology requires that all the equipment should be capable of controlling flexible material, including massive manual operations or reel-to-reel operations. Complicated manufacturing definitely leads to cost improvement. Since flexible material features dimensional instability in terms of build-up manufacturing, connection density is therefore limited on design regulations. Majority of traditional build-up layer adhesion depends on unsupported adhesive bondplys so vias containing adhesive layer has to eliminate desmear through plasma. Due to high CTE (coefficient of thermal expansion) of adhesive, to achieve the reliability of PTH (plated through hole) and lasered through hole, plated copper thickness should be very high as well. It's known that because unsupported adhesive bondplys features high CTE, deformation will take place on and on, finally leading to cracks occurring within copper plated hole, especially at the corner of via.

Up to now, almost all the cell phones, digital cameras, LCD and plasma display take advantage of flexible HDI (high density interconnect) PCB containing HDI build-up layers. All the new technologies call for technologies with low complexity and it should be manufactured by ordinary equipment for manufacturing rigid PCB.

Snap-Off Flex-Rigid PCB

Snap-off flex-rigid PCB fabrication starts from rigid core layer manufacturing and rigid core layers range from 2 to 12 containing PTH and other vias. Flexibl layer composed by flexible PI or epoxy resin prepreg should be stuck to rigid core layers and pure flexible area should not. As a result, prepreg has to match with images and goes through milling. Adhesion is carried out by applying ordinary lamination and lithography is done after build-up manufacturing and rigid core layers are quite stable, dimensional stability of flexible material won't lead to protruding problems, allowing ordinary drilling or laser drilling to be leveraged. Since epoxy resin prepreg lamination doesn't get adhesion involved, ordinary desmear elimination and plated copper technology can be applied. Due to application of technology without the application of adhesion, PCB without thick copper plated features high reliability as well.

Selective cover layer should be applied before solder mask coating, which can be completed by ordinary multi-layer lamination, applicable for flex-rigid PCB for dynamic flexible situations.

Material Selection

Different from flexible material calling for dynamic flexibility, flex-rigid PCB calling for static flexibility should take advantage of a new terminology.

Semi-flex flex-rigid PCB refers to a type of rigid-flex PCB that needs to be bent only during assembly or for static applications. Flexibility only needs to be achieved a couple of times. Flex-rigid PCB refers to circuit boards calling for dynamic flexibility.

When it comes to semi-flex applications, flexible substrate material doesn't need to be required and it's sufficient to implement bendability. Limitations of semi-flexible boards lie in times of flexibility circle and flexibility radius. Different material selection depends on their application conditions. FR4 material works suitably for semi-flex boards and its bendability time and radius are limited. Generally speaking, flexible FR4 material depends on thin glass fiber (1080) or special material belonging to modified resin system.

Aramid material features better flexibility than FR4 material and its applications depend on bendability times. Compared with PI, aramid features better CTE, therefore leading to better manufacturing window. Aramid has to be stably used for applications without dynamic flexibility demands.

Copper thickness and quality will affect the reliability of flexibility test and application. Generally speaking, copper foil for multi-flexibility requirement should pick up RA copper foil.

Let The Professionals Do Their Job.

PCBCart has rich experience to manufacture flex-rigid printed circuit boards. If you have new Flex-rigid PCB design ready for prototyping or production, please do not hesitate to reach us to discuss your project! We will provide practical and cost-effective solution asap.

Helpful Resources
Flex-rigid PCB Manufacturing Capabilities
Advanced PCB Assembly Services
Streamline Assembly and Improve Reliability with Flexible and Flex-rigid PCBs

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