Name: 6Layers Rigid-Flex PCB
Brand: PCB Printed Circuit Board and PCBA manufacturing
Rating: 5 (1 reviews)

Description
Data Sheet

1. Outer Layer Pattern Protection Materials (Solder Mask) for Rigid-Flex PCB

Three types of solder mask materials are commonly used for the outer layer pattern protection of Rigid-Flex PCB, each with distinct process characteristics and application scenarios, as detailed below:

• Traditional Coverlay: Composed of polyimide base material and adhesive, it is laminated onto the etched circuit board for protection. This type of coverlay requires pre-forming before lamination to expose the soldering areas, which limits its adaptability to fine-pitch assembly requirements.

• Photosensitive Developable Cover Dry Film: After lamination via a laminator, the soldering areas are exposed through photosensitive development technology. This effectively solves the precision bottleneck of traditional coverlay, meeting the requirements of fine assembly processes.

• Liquid Screen Printing Cover Materials: Thermosetting polyimide materials are commonly adopted, such as Solar PSR-4000 and photosensitive developable flexible circuit board special solder resist ink. These materials exhibit excellent adaptability to fine-pitch, high-density assembly of flexible layers, ensuring reliable protection and process compatibility.

Rigid-Flex PCB production process

2. Rigid-Flex PCB Manufacturing Process & Key Process Control

Rigid-Flex PCB is developed based on flexible PCBs and high-density multilayer rigid PCBs, sharing certain process similarities with rigid PCBs. However, due to the particularity of its materials, structure, and application scenarios, it differs from ordinary rigid and flexible PCBs in both design requirements and manufacturing processes. Almost every production link requires strict testing and parameter adjustment to optimize the overall process and ensure product quality.

2.1 Inner Layer Single-Sheet Pattern Transfer

Pattern transfer is a core link in the production of high-density, thin-line Rigid-Flex PCBs, especially critical for flexible single sheets. The thin and soft nature of flexible single sheets brings great challenges to surface treatment operations; the cleanliness and roughness of the copper foil surface directly affect the adhesion of the resist dry film and the formation of fine lines.

Mechanical wiping is not recommended due to its high equipment requirements—improper pressure can cause substrate deformation, curling, and dimensional expansion, which are difficult to control. Instead, electrolytic cleaning is preferred: this method not only ensures surface cleanliness but also achieves the required copper surface roughness through micro-etching, facilitating the formation of line patterns with a line width/line spacing of 0.1mm~0.15mm.

In the acid etching process, in addition to controlling the etching rate to meet the designed line width and spacing, measures must be taken to prevent single-sheet curling and wrinkling. It is advisable to add auxiliary guide plates and close the ventilation system on the equipment to maintain process stability.

2.2 Flexible Material Multi-Layer Positioning

Flexible substrates have poor dimensional stability due to the strong moisture absorption of polyimide materials, which can cause severe shrinkage and deformation after wet processing or under varying temperature and humidity conditions, leading to inter-layer alignment difficulties in multilayer PCBs. The following measures are adopted to address this issue:

• During the design phase, alignment patterns and target punching positions should be fully considered to ensure accuracy when drilling alignment holes or rivet holes, avoiding inter-layer pattern misalignment and product scrapping during lamination.

• Positioning holes drilled via OPE (Optical Positioning Equipment) can effectively eliminate errors caused by material expansion and contraction during wet processing.

• After PCB lamination, X-ray drilling is used to determine offset values, ensuring higher drilling precision. Based on the material and environmental characteristics of polyimide, the outer layer film is drawn with reference to drilling offsets, improving the overlap between the outer layer film and the drilled board. This meets the inter-layer registration requirement of 0.1mm~0.15mm ring width and ensures the accuracy of outer layer pattern transfer.

2.3 Rigid-Flex PCB Lamination

Even with OPE-drilled positioning holes, single-sheet processing before lamination has a significant impact on inter-layer alignment. Key control points for the lamination process are as follows:

• Flexible Single-Sheet Control: Polyimide materials are not resistant to strong alkalis and will swell in strong alkaline solutions. Therefore, during the blackening/browning process, the temperature and duration of strong alkaline processes (such as degreasing, blackening, and browning) should be appropriately reduced. For adhesive-free substrates, this method is feasible as there is no need to consider adhesive layer changes in lye.

• After oxidation treatment, flexible single sheets should be baked horizontally (not vertically) to reduce bending deformation and maintain flatness. The molding time after baking should be shortened as much as possible to prevent reabsorption of moisture by the single sheets.

• Lamination Gasket Material Selection: Flexible single sheets are prone to deformation and have poor flatness before lamination; the resin fluidity of the adhesive sheet used is much lower than that of prepregs for rigid PCB lamination. To ensure good bonding between the adhesive sheet and the single sheet, and to fill fine line spacing, gasket materials with excellent conformability are selected, such as polypropylene film, polytetrafluoroethylene (PTFE), and silicone rubber sheets. Tests have shown that silicone rubber is the ideal gasket material, as it ensures formability and relatively reduces shrinkage and deformation of the laminated part.

• Rigid Board Lamination Control: For the rigid board part of Rigid-Flex PCB, the following three aspects should be noted during lamination:

1. Whether laminating PCB substrates or pure prepregs, the warp and weft directions of the glass cloth must be consistent; thermal stress should be eliminated during lamination to reduce warpage.

2. The rigid board should have an appropriate thickness: the flexible part is thin and lacks glass cloth, leading to different deformation characteristics from the rigid part under environmental and thermal shock. Without sufficient thickness or hardness, the rigid part will exhibit obvious deformation differences, resulting in severe warpage during use and affecting welding and performance. Excessive thickness, however, leads to increased weight and poor economy. Experiments have confirmed that a thickness of 0.8~1.0mm is optimal.

3. Flexible window processing can be achieved via pre-milling or post-milling, depending on the structure and thickness of the Rigid-Flex PCB. For pre-milling, precision must be ensured to avoid excessive impact on welding and deflection; milling data can be prepared by engineering teams for advance processing. For post-milling (laser cutting of flexible window waste after all pre-processes and final forming), attention should be paid to the laser cutting depth of FR4 materials.

Lamination parameters can be comprehensively optimized with reference to the lamination parameters of flexible substrates and rigid PCBs.

2.4 Rigid-Flex PCB Drilling

The complex structure of Rigid-Flex PCB makes the determination of optimal drilling parameters crucial for achieving high-quality hole walls. Key control points are as follows:

• Drill Bit Selection: To prevent nail head phenomena on inner copper rings and flexible substrates, sharp drill bits must be used. For large-volume processing or boards with a high number of holes, drill bits should be replaced in a timely manner after a certain number of holes have been drilled.

• Drilling Parameter Control: Drill speed and feed rate are critical process parameters. Excessively slow feed rates cause sharp temperature rises and excessive drilling debris; excessively fast feed rates easily lead to drill breakage, adhesive sheet tearing, medium layer separation, and nail head phenomena.

• Drilling Machine & Auxiliary Material Selection: The drilling machine and parameters should be selected based on board thickness and minimum drilling diameter. Currently, industrial drilling machines with speeds up to 200,000 revolutions per minute are available; higher speeds are preferred for small holes to ensure drilling quality. The selection of cover plates and backing plates is also important—high-quality cover plates and backing plates protect the board surface and provide effective heat dissipation. It is recommended to use aluminum foil boards or epoxy resin boards as backing plates; paper backing plates should be avoided, as their softness causes severe drilling burrs, which may tear or scratch holes during pre-boring deburring, affecting subsequent processes and product quality.

Model : 6Layers Rigid-Flex PCB

Material : FR-4+PI

Layer : 2+2+2

Color : Green/White

Finished Thickness : 1.0mm

Copper Thickness : 1OZ

Surface Treatment : ENIG 2U"

Minimum line width / distance : 0.15/0.15mm

Application : Medical treatment Rigid-Flex PCB

6Layers Rigid-Flex PCB

Description