Rigid-Flex PCB - 8-layer Rigid-Flex PCB

Rigid Flex PCB Design and Manufacturing Standard

1.0 Purpose:

Develop rigid flex PCB design and manufacturing standards, clarify the precautions in the rigid flex PCB design and manufacturing process

2.0 Scope of Application:

Suitable for Rigid Flex PCB products used

3.0 Responsibilities:

3.1 Process Department: Develop and follow up on the revision of process flow and technical parameters;

3.2 Production Department: Execute operations according to the process parameters formulated by the Process Department;

3.3 Quality Department: Supervise operations according to the process parameters formulated by the Process Department;

3.4 Engineering Department: Implement the design and optimization of data according to the standards formulated by the Process Department

4.0 Definition:

The explanation of Rigid Flex PCB is: a printed board made by combining a flexible substrate with a rigid substrate in different areas. In the rigid flexible bonding area, the conductive patterns of the flexible substrate and the rigid substrate are usually interconnected

5.0 Rigid Flex PCB Classification

If classified by process, the bonding method between rigid and flexible boards can be divided into two categories: rigid flex composite boards and rigid flex PCBs. The main differences are as follows

The technology of rigid flex composite board can combine rigid flex boards in the manufacturing process, with common blind and buried hole designs, thus allowing for higher density circuit design

The technology of combining rigid and flexible boards is to separate them and then press them together into a single circuit board, with signal connections but no through holes.

But currently, the term "Rigid Flex PCB" is commonly used to refer to all Rigid Flex PCB products without subdividing them.

6.0 Rigid Flex PCB Structure

6.1 Rigid Flex PCB Structure (Part 1)

Rigid Flex PCB is a flexible board with one or more rigid layers attached, and the circuits on the rigid layer are connected to the circuits on the flexible layer through metallization. Each Rigid Flex PCB has one or more rigid areas and one flexible area

6.2 Rigid Flex PCB Structure (II)

The combination of a flexible board and several rigid boards, and the combination of several flexible boards and several rigid boards, using drilling, plating holes, and laminating processes to achieve electrical interconnection. According to design needs, the design concept is more suitable for the installation and debugging of components and welding operations, ensuring that the installation of assembled parts is more flexible

7.0 Rigid Flex PCB Process Flow Design

7.1 Process Design

The production process of Rigid Flex PCB

7.2 Material Description

7.2.1 Substrate: PI, PET, PTFE

7.2.1.1 PI: It has excellent high temperature resistance, with immersion welding resistance up to 260 ℃ and 20sec. It has high dielectric strength, good electrical and mechanical properties, but is prone to moisture absorption. Commonly used substrate for FPC.

7.2.1.2 PET: Many properties similar to polyimide but with poor heat resistance can only be used at room temperature.

7.2.1.3 Polytetrafluoroethylene (PTFE): Only used in high-frequency products with low dielectric constant

7.2.2 Coverlay: The covering film is equivalent to solder mask ink for rigid circuit boards, serving as a solder mask. The covering film is composed of PI and adhesive

7.2.3 Bonding part: mainly used for bonding and insulation between layers, divided into two types: pure adhesive and non flowing semi cured sheet No Flow PP (NF)

7.2.3.1 No Flow PP: Composed of epoxy resin, glass cloth, and filler, with minimal overflow, it is generally used for selective compression and products with high compression TG (Rigid Flex PCB)

7.2.3.2 Adhesive: commonly known as "acrylic adhesive", scientific name "acrylic", TG below 100 degrees, good flexibility, but relatively high thermal expansion coefficient, generally used for multi-layer flexible board compression

7.2.4 Stiffener: A hard material that is pressed onto a local area of the FPC board for the purpose of welding parts or adding reinforcement for installation. There are generally three types of materials, such as polyester (PET), polyimide (PI), and FR-4.

7.2.4.1 Polyester (PET) - commonly used in areas without welded parts

7.2.4.2 Polyaniline (PI) - commonly used on FPC boards with soldered components.

7.2.4.3 FR-4- commonly used in thicker areas. Under normal circumstances, PI is bonded using thermal sensitive adhesive (TSA), while FR4 is bonded using pressure-sensitive adhesive (PSA).

7.3 Introduction to Key Material Characteristics

7.3.1 Dielectric materials (including semi cured sheets)

Semi cured sheets are commonly used for processing and manufacturing Rigid Flex PCBs. The adhesive rigid layer usually uses "no flow" or "low flow" semi cured sheets. Half cured films with high glass transition temperature (Tg) have the characteristics of high working temperature and low Z-axis expansion coefficient. Low Z-axis expansion coefficient is also beneficial for controlling the reliability of plated through holes with ≥ 8 layers. Its disadvantages are low medium strength and low flexibility. When semi cured materials are used for flexible and Rigid Flex PCBs, they should comply with the requirements specified in IPC-4101, and the areas where adhesive is used and areas where adhesive is not used should be indicated on the main drawing.

7.3.2 Flexible adhesive film (casting adhesive or bonding layer)

Flexible adhesive films are commonly used to bond multiple layers of flexible materials and for attaching devices for thermal control or structural support. This material has high bonding strength to flexible media. Flexible adhesive films can be formulated using resins with low glass transition temperatures (Tg) to enhance adhesion and flexibility. When designing rigid flexible circuit boards, the use of this material in the rigid area should be minimized or avoided as much as possible to avoid excessive expansion of the Z-axis

7.3.3 Flexible metal plate (FCCL)

Flexible metal plate is a composite material of dielectric film and metal foil. Metal foil can be attached to a medium using various methods, such as resin adhesive method or direct deposition method. The medium (such as the reaction monomer solution of PI) can be coated on the metal foil. Coated medium laminates and directly deposited laminates are called adhesive free soft copper foil substrates. A flexible copper foil substrate with adhesive is made by bonding a dielectric film to a metal foil using an adhesive. The glass transition temperature (Tg) of resin adhesives is usually lower than that of dielectric films. When designing high layer rigid flexible circuits, non adhesive soft copper foil substrates are often used to reduce the influence of adhesives with low Tg. Therefore, it is recommended to avoid using adhesive soft copper foil substrates for all type 4 circuit boards and type 3 circuit boards with more than eight layers in high-temperature applications.

7.3.4 Coverlay

The PI thickness in the cover film is generally 12.5um, 25um, and 50um, and the corresponding adhesive thickness varies. The function of glue is to adhere to the board. The selection of covering film should not only meet the requirements of plate thickness, but also consider the relationship between the copper thickness of the plate and the adhesive thickness of the covering film. Because the adhesive does not flow and only fills the gaps left by copper etching, the film thickness should be roughly the same as the copper thickness when selecting. For products with high flexibility requirements, it is recommended to choose a thinner covering film.

8.0 Precautions for Engineering Design of Rigid Flex PCB

8.1 Design of Inflation and Shrinkage Control

8.1.1 Design of the number of rivet holes

The key technology of Rigid Flex PCB involves the lamination process of composite materials, including FPC, FR-4, Coverlay, and No Flow PP. To eliminate the differences in material expansion and contraction during the lamination process, 8 rivet holes must be designed at the edge of the board during engineering design, with two rivet holes on each side.

8.1.2 Design of Rivet Hole Form

The riveting holes are divided into hard board riveting holes and FPC riveting holes. During the pressing process, FR-4 and FPC are riveted using rivets with a diameter of 3.175mm. The hard board riveting holes are processed using a CNC drilling machine and a 3.20mm drilling tool is used to drill down. Considering the special nature of FPC material, copper should be retained at the edge of the FPC rivet hole punching to increase strength and prevent hole breakage and misalignment when riveting the PIN sleeve.

8.2 Graphic design of flexible board FPC circuit

8.2.1 Design of alignment and fitting reference lines

The inner soft board bonding reinforcement sheet, adhesive tape, single PCS or strip bonding cover film need to be made with identification lines or copper wires outside the forming area. The identification lines or copper wires should be 10mm and attached to the center of the identification lines; The bonding of the entire PNL or SET board requires the production of a bonding alignment mark point, and a hole with a diameter 0.1mm larger than the mark point should be drilled in the cover film.

8.2.2 Design of FPC Circuit

8.2.2.1 Avoid sudden expansion and contraction between lines, and use tear lines between thick and thin lines.

8.2.2.2 The maximum value of solder pads should be taken to meet electrical requirements. Smooth design should be adopted at the connection between solder pads and wires to avoid right angles

8.2.2.3 Increase the design of copper as much as possible, and design as much solid copper as possible for waste materials

8.2.2.4 Copper laying design in bending area

1) Vertical to the curved area;

2) Uniformly distributed throughout the entire curved area;

3) Maximizing the entire bending zone;

4) No additional electroplated metal;

5) Uniform width;

6) If possible, the central axis should be located at the center conductor of the laminated board.

7) The wires in a double-sided circuit should not directly cross each other, resulting in the "I-beam" effect. This situation may need to be considered from the perspective of electrical performance, but it should also be considered from the perspective of mechanical installation requirements,

8) The number of layers in the curved area should be kept within the lowest number of layers;

9) In the curved area, conductive and plated through holes should be avoided;

10) When using rolled copper materials, the lattice direction parallel to the bending direction can improve flexibility

11) Bend and bend to reinforce copper design.

8.2.2.5 Distance from hole to edge (flexible and rigid zones)

The minimum distance between the outer edge and the inner hole edge and the cutting edge should not be less than 0.5mm. When designing the distance, positioning accuracy, dimensional tolerances, and outer processing tolerances should be considered.

8.2.2.6 Distance from plated through-hole to edge (soft hard bonding area)

The minimum distance between the soft hard composite zone and the edge of the gap hole should not be less than 0.6.

8.2.2.7 Cover film window design

The process edge and board edge cover film need to be designed with windows to increase the bonding force between the rigid board and the flexible board after compression. The flexible board cover film needs to be designed with a single-sided extension of 0.4mm into the hard board. At the same time, ensure that there is a gap of at least 0.5MM between the plated through holes and the shrunken cover film inside the unit

8.2.2.8 No Flow PP Window Design

The PP used for internal compression of rigid plates needs to be designed to shrink 0.25mm inward from the edge of the rigid plate

9.0 Precautions for Rigid Flex PCB Production Control Process

9.1 Material cutting

Rigid Flex PCB and FPC that require single-sided design are not allowed to have a single panel directly, and must be etched from both sides to form a single panel. To ensure interlayer bonding and avoid bursting and delamination.

9.2 Inner Circuit

9.2.1 Inspection of incoming materials: FPC is flat without folds or wrinkles, and the board surface is free of adhesive stains.

9.2.2 All FPCs must be attached to a drag board for development etching, with one FPC attached to each drag board to prevent bending and wrinkling of the FPC due to etching transmission issues.

9.2.3 All boards must pass through AOI, and FPC is not allowed to be wired

9.3 Compression

9.3 Compression Level

9.3.1 Rigid Flex PCB lamination must use a dedicated program, which cannot be mixed with other boards for lamination.

9.3.2 Precautions for riveting

9.3.2.1 When the FPC is excessively browned, a drag board must be attached. If there are wrinkles on the FPC after browning, it must be scrapped and disposed of. Before riveting, it is necessary to ensure that the FPC is browned properly.

9.3.2.2 Before pressing No Flow PP, the PP must be stored in a dehumidifier for at least 12 hours. Before riveting, check if there are any folds in the PP, and dispose of PP with severe wrinkles.

9.3.2.3 After riveting FR-4, FPC and No Flow PP, first check for insufficient or excessive PP placement, and then use X-ray to check for layer deviation.

9.3.2.4 During the pressing process, in order to ensure sufficient filling of the adhesive, the top and bottom of the riveted plate must be covered with silicone gel, with 6 layers per box.

During the riveting process, pay attention to cleaning the PP powder on the table with a dust-free cloth to avoid PP residue after pressing, which may cause poor appearance.

9.4 Drilling

9.4.1 The tool used is an MO drilling tool

9.4.2 The fixed machine is produced using a dedicated drilling rig.

9.4.3 During the hole inspection process, the focus is on checking for any residual glue, burrs, or defects on the soft board at the T-position of the soft and hard joint. If such abnormalities are found, promptly notify the process to follow up and handle them

9.5 Copper Plating

9.5.1 Rigid Flex PCB adhesive residue removal program is a dedicated program. Please pay attention to the switching of the adhesive removal program before copper deposition

9.5.2 After removing adhesive residue, the Rigid Flex PCB is copper plated according to the double-sided board program after PI adjustment. The PI solution is prepared at a volume concentration of 40%, with a working temperature of 40 ℃ and an adjustment time of 4-8 minutes. Liquid analysis control table

9.5.3 The requirement for copper backlight detection is greater than level 9.5

9.6 Rigid Flex PCB Opening Technology

There are three ways to open the lid of Rigid Flex PCB: laser opening, gong machine controlled deep opening, and V-CUT opening.

9.6.1 1R+2F+1R structure opening technology

The selection of the lid opening method for Rigid Flex PCB is closely related to the thickness of FR-4 core board, and the overall lid opening adopts blind gong controlled deep docking technology.

Before pressing, first control the depth of the blind groove in the inner layer of FR-4, leaving about 1/2 of the residual thickness. After pressing, control the depth of the blind groove again to achieve the opening of the rigid flex joint area.

Based on this technology prototype, when the thickness of the inner FR-4 core board changes due to rigid bending, the opening method will also change. The general technical requirements are as follows:

When d ≤ 0.40mm, use two laser blind groove depth control techniques to open the cover;

When d>0.40mm, two blind grooves can be used to control the depth of the cover or blind grooves can be used first and then V-CUT can be used to open the cover. When the rigid joint area is irregular, only two blind grooves can be used to control the depth of the cover opening; When the soft hard combination area presents a regular straight line shape, taking into account production efficiency and cost factors, the blind groove first and then the V-CUT cover opening method can be prioritized.

9.6.2 2F+2R structure opening technology

FPC is directly connected to the hard board through compression bonding. Considering the reliability of the circuit at the soft hard junction, this type of product structure uniformly adopts laser blind cutting to control the depth first, and then gong controlled depth or V-CUT controlled depth opening.

9.7 Inspection of Rigid Flex PCB

Rigid Flex PCB quality inspection and testing are divided into the following categories:

A: Electrical performance, B: size, C: appearance, D: reliability

9.7.1 Testing Standards

The relevant inspection, testing, and acceptance standards refer to the following specifications:

IPC-A-600 Printed Circuit Board Acceptance Criteria

IPC-6012 Rigid Printed Circuit Board Identification and Performance Scale

IPC-6013 Identification and Performance Specification for Flexible Printed Boards

9.7.1.1 Dimensional Inspection Items

9.7.1.2 Reliability testing items