Rigid Flex PCB: Design guildelines and Manufacturing Solution

rigid-flex_pcb

 

Rigid flex PCB have been used in the military and aerospace industries for more than 20 years. In most rigid flex circuit boards, the circuitry consists of multiple flexible circuit inner layers selectively attached together using an epoxy pre-preg bonding film, similar to a multilayer flexible circuit. However, a multilayer rigid flex circuit incorporates a board externally, internally or both as needed to accomplish the design.

 

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Rigid flex PCB combine the best of both rigid boards and flexible circuits integrated together into one circuit. The two-in-one circuit is interconnected through plated thru holes. Rigid flex circuits provide higher component density and better quality control. Designs are rigid where extra support is needed and flexible around corners and areas requiring extra space.

 

Flex and Rigid-Flex applications also provide increased reliability. Mean time between failure rates (MTBF) typically exceed those of standard PCB sets with discrete wires and connectors, often becoming the choice of companies and engineers alike for its dependable and consistent performance.

 

Rigid-Flexible Circuit Board Manufacturing, & Assembly at RayMing Technology .Flexible Printed Circuit Boards are one of the most popular types of circuit boards used in a variety of industrial and commercial applications. At RayMing, we manufacture flexible printed circuits keeping the needs of our customers in mind. Our circuit boards provide quality and durable performance while being cost-effective in terms of manufacturing.

 

Our Standard Offerings We specialize in single and double-sided circuits, as well as multi-layer rigid and flex circuit assemblies. The flex circuits are built to the exacting specifications of our customers.

 

  1. Flexible Printed Circuit Boards with 8 layers rigid PCB
  2. Single-Sided Flexible with rigid PCB board up to 6 layer
  3. Double-Sided Flexible Printed Circuit Boards  with 4 layer to 12 layer rigid pcb
  4. Multilayer Flexible Printed Circuit Boards with multilayer rigid PCB

 

Application of Rigid-Flex PCB

 

rigid flex pcb manufacturers

Rigid Flex PCB Manufacturers

 

1.Industrial application

Industrial use includes rigid-flex boards used in industry, military, and medical. Most industrial parts require the characteristics of accuracy, safety, and resistance to soil damage. Therefore, the characteristics required for rigid-flex PCB are high reliability, high precision, low impedance loss, complete signal transmission quality, and durability. However, due to the high complexity of the manufacturing process, the output is small and the unit price is relatively high.

 

2.Mobile phones

The application of rigid-flex PCB in mobile phones, such as folding mobile phone hinges, camera modules, keypads, and radiofrequency modules, are common.

 

3.Consumer electronic products

Among consumer products, DSC and DV are representative of the development of rigid-flex PCB, which can be discussed in terms of performance and structure. Rigid and flexible boards can connect different PCB rigid boards and components three-dimensionally. Under the same circuit density, the PCB total usable area and the circuit carrying capacity can be increased. The signal transmission limit of the contacts can be reduced, and reduced Assembly error rate. On the other hand, because the rigid-flex board is lighter and thinner, the wiring can be bent easier, so reducing the volume and weight will be helpful.

 

4.Automotive

Rigid-flex PCB is commonly used in the automotive industry. Here is a list of examples: the buttons on the steering wheel which connects to the motherboard, the connection of the car video system screen and the control panel, the operative connection of the audio or function keys on the side door, and the reversing radar imaging system, sensors (including air quality, temperature, and humidity, special gas regulation, etc.), vehicle communication systems, satellite navigation, rear-seat control panels, and front-end controller connection boards, vehicle detection systems, etc.

 

Rigid-flex PCB reduces the assembly size and weight of electronic products, avoiding wiring errors and increasing assembly flexibility. It also improves reliability and realizes three-dimensional assembly under different assembly conditions.Flexible Printed Circuits (FPC) flexible structure, small size, light weight, and flexibility can meet the needs of 3D assembly. Interconnection technology has been widely used and valued in the electronic communication industry. In recent years, the trend towards rigid-flex boards further shrinks the entire system, the volume, and enhance its function.

 

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Advantages of Rigid-Flex PCB:

  • Space-saving: Save space without connectors
  • Flexible design: Flexible with 3D design
  • Enhancing product reliability (without connector)
  • Simple assembling process

 

Disadvantages of Rigid-Flex PCB:

  • Design complexity
  • Less Rigid-Flex PCB design company

 

Comparison and analysis of different materials vs. Rigid-flex PCB

Design Options FPC Boards Build Up FPC Thin Laminate circuit board Rigid-flex Circuit boards Mini Cable Wire Harness Coaxial Cable
Material Polyimide Thin Laminate Fr4 Laninate and Polyimide Cable/wire and connectors Cable and Connectors
Applications Dynamic Static Dynamic Dynamic Dynamic
Examples

Key Message:

  • Rigid-Flex PCBcan eliminate some connectors in the 3D assembly

 

Common structures of Rigid-flex PCB

 

rigid flex board

 

  1. The rigid-flex board is to glue one or more rigid layers on the flexible board. The circuit on the rigid and flexible layersare connected through metallization. Each rigid-flex board has one or more rigid areas and a flexible zone

 

The circuit on the rigid and flexible layers

 

  1. The combination of one flexible board and several rigid boards, and the combination of several flexible boards and several rigid boards,

The electrical interconnection is realized by drilling, plating holes, and laminating processes. According to the design requirements, the design concept is more suitable for installing and debugging the device and the welding operation. The installation of the assembly is more flexible.

 

one flexible board and several rigid boards

 

Design Suggestions of Rigid-flex PCB

 

Rigid-flex PCB board is a product segmentation that promotes technical level and application field of flexible circuit boards to help system products move to a broader space. Not only can the product be more miniaturized, but it also can solve many problems in assembly and wiring.

 

Structural considerations are the most critical factor in the design of Rigid-flex. It is necessary to make the process simple, highly reliable, and achieve low cost and application.

 

1.The thickness should be reduced as much as possible, and the types of materials must also be reduced.

 

Rigid-Flex that is too thick adversely affects the miniaturization of assembled thick products. It also causes inconvenience to the manufacturing process, especially pressing. In addition, the types of materials used by Rigid-Flex include copper foil, polyimide film, and acrylic glue. Different materials, such as different materials, will cause considerable challenges in dimensional accuracy. Simultaneously, due to the large difference in thermal expansion coefficient, the adhesion between the layers after thermal shock also needs attention.

 

2.Stress prevention at bending points

 

In addition to the process of hot pressing, it is necessary to reduce the stress on the contact edge of the flex and rigid board or provide additional reinforcement. The best policy is to avoid bending points.

  1. Consideration of folding resistance and shock resistance. Wiring deployment must meet folding resistance. Prior consideration is needed if the product is being used in a high-vibration environment.
  2. Process considerations Pre-imagine the problems that may occur in the process, simplify the process to reduce costs and increase yield

 

 

Design Tips for Rigid-Flex PCB

 

rigid flex circuit

Rigid Flex Circuit

 

There are many differences between the flex and rigid board in CAD design of rigid-flex PCB

 

1) Design requirements for flexible zone circuit:

1.1 Avoid sudden expansion or contraction of the line and adopt a tear shape between the thick and thin lines.

 

 Design requirements for flex pcb hole

 

 

Use rounded edges to avoid sharp corners:

Use rounded edges to avoid sharp corners:

 

 

1.2 When the pad meets the electrical requirements, the maximum value should be taken. A smooth transition line is used to connect the pad and the conductor to avoid right angles. The independent pad should be added with a toe to strengthen the supporting effect.

 

connect the pad and the conductor to avoid right angles

2) Dimensional stability: Add copper design as much as possible.

 

Add copper design as much as possible

Design as many solid copper ponds as possible in the waste area.

 

3) Design of cover film window

 

  1. a) Add manual alignment holes to improve alignment accuracy.
  2. b) The window design considers the range of glue flow;usually, the window opening is larger than the original design,

The specific size is provided by the ME design standard.

  1. c) Small and dense windows can use a special mold design: rotating punch, jump punch, etc.

 

4) Design of rigid-flex transition zone

  1. a) For the smooth transition of the line, the direction of the line should be perpendicular to the direction of bending.
  2. b) The wires should be evenly distributed throughout the bending area.
  3. c) The wire width should be maximized in the entire bending area.

Try not to use PTH design in the transition zone,

Design of Coverlay and No flow PP in the rigid-flexible transition zone

 

 Design of Coverlay

 

 

5) Design of flexible zone with air-gap requirements

 

  1. a) There must be no through holes in the part to be bent.
  2. b) Additional protection copper wires are added to the two sides of the line. If the space is insufficient, choose the inner R corner of the bent part.

Additional protection of copper wire.

  1. c) The connecting part of the line needs to be designed as an arc.
  2. d) It is better to have a larger bending area, which lessens the effectin

 

6) Other

 

The flex board's tool holes cannot be shared, such as punch hole, ET, SMT positioning hole, etc.

 

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Manufacturing Capability of Rigid-Flex PCB

 

Manufacturing Capability of Rigid-Flex PCB

 

Item Standard Limit
1 Min.Line/space 3/3mil 2/2mil
2 Min.PTH/LAND 150/450mm 100/300um
3 Min.Blind Via/Land 100/300um 80/250um
4 Min.BGA Pitch 500um 400um
5 Min.PTH Aspect ratio 3:1 5:1
6 Min.Via Aspect ratio 0.75:1 1:1
7 Available Plus I Plus II

 

rigid-flex pcb rigid side desig

 

Part SYM

ITEM

Standard (um) Limit (um)
On the rigid side a Cu(P/G0 to boarder 600 400
b Pad edge to border 800 500
c Parts edge to border 800 500
d S/M opening to border 200 100
e Non-PTH to border 800 500
f Legend to border 500 400
g Adhesive squeeze out 1500 800
On flex Side h Distance of rigid edge 6000 4000

 

rigid-flex PCB flex side pcb design

 

Item Standard Limit
1 Min:outline tolerance ±125um ±100um
2 PTH Diameter tolerance ±75um ±50um
3 NPTH Diameter tolerance ±50um ±50um
4 CVL opening to conductor (a) ±200um ±150um
5 Center to center tolerance (b) ±100um ±50um
6 SMD edge to outline tolerance ±100um ±75um

 

 

Material Characteristics of a Rigid-Flex Board

 

 Typical Rigid-Flex PCB Material Feature

 

Material Description Specification
Flexible Laminate Polyimide (adhesive-less or adhesive)
Rigid Laminate FR-4 or glass-polyimide C-stage laminate
Coverlay and Bond Ply Adhesive coated polyimide film
Prepreg No-flow FR-4 or polyimide prepreg
Build-up Material Resin costed copper foil or build up dielectrics

 

Rigid-flex material selection

 

Rigid PCB Flex PCB
Base substrate Phenolic/Paper

Epoxy/Glass

Polyimide/Glass

Metal Core

Polyimide

Polyester

Adhesive layer Expoxy

Phonolic

Polyimide

Acrylic

Epoxy

Conductor ED Copper

High ductility

Copper

RA copper

High Ductility

Copper

ED copper

 

Introduction to Flexible PCB Materials

 

Introduction to Flexible PCB Materials

 

Base Material

1).     FCCL ( Flexible Copper Clad laminate)

 

  • 1.Polyimide: KaptonÔ (12.5 mm/20 mm/25mm/50mm/75mm)
  • 2.High flex life, good thermal management, high moisture absorption, and good tear-resistant
  • 3.Polyester (25mm/50mm/75mm)
  • 4.Most cost-effective, good flex life, low thermal resistivity, low moisture absorption, and tear-resistant

 

2).For FPC Material Status 2L FCCL

 

FPC Material Status 2L FCCL

 

Suppliers Material Characteristics Current Status
Taiflex 2lps High Flexibility HVM
Rogers 2 series High Flexibility HVM
Nippon steel MC Low Dimension HVM
Dupont AC High Flexibility HVM

 

3).For FPC Material Status 3L FCCL

 

FPC Material Status 3L FCCL 

 

Supplier Material Characteristics Current Status
Taiflex I X N High Flexibility HVM
Rogers 7 series Low Dimension HVM
Microcosm PSBR Low Dimension HVM
Arisawa LASW High Flexibility HVM

 

1.Dielectric Substrates dielectric film: polyimide (PI), polyester (PET)

 

Characteristics of PI:

  1. Good heat resistance: The long-term use temperature is 260℃.In the short term, it can withstand high temperatures above 400℃.
  2. Good electrical and mechanical properties.
  3. Good weather resistance and chemical resistance.
  4. Good flame retardancy.
  5. The water absorption rate is high, and the size changes after moisture absorption. (Defect)

 

IQC must regard the dimensional change rate as an important acceptance index for PI incoming materials. The environmental control requirements of the production process are also stricter than rigid boards.

 

Polyester film PET has good mechanical and electrical properties such as tensile strength, water resistance, and dimensional stability after moisture absorption. However, it has a large shrinkage rate when heated and has poor heat resistance. It is not suitable for high-temperature soldering (Now the lead-free soldering temperature is 235+/-10 ℃), and its melting point is 250 ℃. Less used

 

Polyimide (PI) is the most widely used, and 80%  is made by DuPont in the United States.

 

2.Cover Layer

 

Function: Protect circuit, insulation, electrical demand, and board deflection.

Features: 1. Good electrical characteristics, 2. Good processability, 3. Excellent flexibility

 

Cover Layer from ½ mil to 5 mils (12.7 to 127µm)

Polyimide: (12.5 mm/15 mm/25mm/50mm/75mm/125mm)

High flex life and high thermal resistivity.

 

Cover Layer

 

1) Other protective films and covering film materials

2) Flexible solder mask

 

The most cost-effective, lower flex life, and better for registration.

 

3) PIC—Photo Imaging covercoat

Lower flex life, better for registration.

 

3.Adhesive Sheet

Bond-ply insulation combination layer with bonding effect.

 

Cover layer Glue overflow

Cover layer Glue overflow

Pressing conditions: 190℃/85(kg/cm2) gauge pressure/forming pressure 60sec

 

4.Conductive Layer

Rolled Annealed Copper (9mm/12mm/17.5mm/35mm/70mm)

  • High flex life, good forming characteristics.
  • Electrodeposited Copper (17.5mm/35mm/70mm)
  • More cost-
  • Silver In
  • Most cost-effective, poor electrical characteristics. Most often used as shielding or to make connections between copper layers.

 

 

5.SF-PC5000 Electromagnetic Wave Protective Film Thickness Characteristics

 

1) Ultra-thin - Total thickness is only 22 microns.

The hot-melt process is used for compound processing between the two insulating films.

The inner insulating layer is extremely flexible, and the outer insulating layer has excellent wear resistance.

2) The sliding performance and flexural performance greatly improved.

Since it has a better sliding performance than silver paste, it further promotes the thinning of the slide-type mobile phone.

3) Adapt to moisture-resistant reflow soldering.

Due to the improved insulating resin, the thinning is realized, and the gas penetration ability is greatly improved.

It is fully suitable for lead-free reflow soldering.

4) Good dimensional stability.

 

Compared with previous materials, the thermal shrinkage rate of the insulating resin in this product is less than one-tenth of the original.

As a single-sided shielding material for thin FPC and COF, it can significantly reduce the warping problem caused by material shrinkage.

 

6 .Additional Material & Stiffeners

 

The hard material is additionally pressed on the local area of the soft board for welding parts or adding reinforcement for installation.

Stiffeners types: FR4, Aluminum, PI

 

7.  No / Low Flow PP

 

TYPE (usually very thin PP) is used for lamination of rigid and flexible boards.

  • 106(2mil)
  • 1080(3.0mil / 3.5mil)
  • 2116(5.6mil) w/o micro-via

Flow PP Manufacturers:       TUC, Panasonic, Arlon, Hitachi, Doosan

 

No / Low Flow PP

 

How are rigid flex PCBs made?

 

rigid flex pcb design

 

1.Drilling

When drilling a single-sided soft board, pay attention, and ensure the glue side is up to prevent nail heads. If the nail head faces the glue surface, the bonding force will be reduced.

 

2 Desmear

Generally, there are four methods for removing drilling pollution: sulfuric acid method, plasma method, chromic acid method, and potassium permanganate method.

PI produced less drilling stains in the rigid-flex board, while modified FR4 and acrylic acid produced more drilling stains.

 

Modified epoxy drilling dirt can be removed with concentrated sulfuric acid, while acrylic acid can only be removed with chromic acid. Polyimide is inert to concentrated sulfuric acid and is not resistant to strong alkalis (potassium permanganate). PI will swell in strong alkalis. The same chemical treatment method cannot remove the drilling dirt of the rigid-flex board.

At present, the ideal method for removing glue from rigid and flexible boards is the plasma method.

 

Plasma uses a radiofrequency energy generator to make ions, electrons, free radicals, free radicals, etc., lose their electrical properties under vacuum, and show neutrality. At this time, the drilling of various resin types can quickly and evenly pass from the hole. The wall is removed, and a certain bite is formed to improve the reliability of the metalized hole.

 

When using Plasma to remove the hard and soft board hole drilling dirt, the biting speed of various materials is different, from large to small:

Acrylic, epoxy, polyimide, glass fiber, and copper,

From a high-power microscope, it is evident that there are prominent glass fiber heads and copper rings. In order to remove the fiber heads and copper rings, it is usually adjusted with a very low concentration of alkali after degreasing of PTH (usually KOH), of course. Rinse with high-pressure water. (PI is not resistant to strong alkalis)

 

3.Chemical copper deposition:

 

PTH of flex board commonly used black hole process or shadow process (Shadow)

The electroless copper of the rigid-flex PCB is the same as the principle of the chemical copper of the rigid board.

 

The flexible material polyimide is not resistant to strong alkalis. As a result, the pretreatment of copper sinking should use acidic solutions. Acid colloidal palladium should be used for activation rather than alkaline ion palladium.

 

At present, most chemical copper deposits are alkaline, so the reaction time and the concentration of the solution must be strictly controlled. If the reaction time is long, the polyimide will swell. The insufficient reaction time will cause cavities in the holes and poor mechanical properties of the copper layer. Although the board can pass the electrical test, it often fails to pass the thermal shock or the user's assembly process.

 

4.Copper plating:

 

To maintain the flexibility of the flex board, sometimes choose the plated copper, called Button Plate. (The pattern transfer of the plated hole is done before the selective plating)

The electroplating principle is the same as the hard board.

 

5.Graphics transfer:

Same process as the rigid board.

 

6.Etching and film removal:

Etching: The etching solution mainly includes acidic copper chloride and alkaline copper chloride etching solution. Since there is polyimide on the flexible board, acid etching is mostly used.

 

Film removal: same process as rigid PCB

Pay special attention to the infiltration of liquid at the rigid-flex joint, causing the rigid-flex joint board to be scrapped.

 

7.Laminating:

Laminating is to press copper foil, P sheet, inner flexible circuit, and outer rigid circuit into a multilayer board.

The lamination of the rigid-flex board is different from the lamination of the soft board or rigid board. It is necessary to consider the problem of easy deformation of the flexible board during the lamination process. Also, the smooth surface after the rigid board is laminated is of concern as well. For the issue of sex, we must also consider the protection of the flexible window at the junction of the two rigid areas.

 

Lamination control points:

1)The amount of glue flowing in No-Flow PP prevents excessive glue from

 

2) Because No-Flow PP has an opening, there will be a loss of pressure during lamination, so use a conformal sheet and release film when laminating.

No Flow's PP needs to open a window at the rigid-flex joint (using a gong or punching method). After the outer green oil is completed, the rigid part of the rigid-flex joint is uncovered and laminated during the appearance processing.

 

3) Before lamination, the rigid outer layer and flexible inner layer must be baked to eliminate latent thermal stress and ensure the quality and dimensional stability of the hole metallization.

 

4) Appropriate cushioning materials should be selected. The ideal cushioning material should have good conformability, low fluidity, and non-shrinking characteristics during cold and hot processes to ensure that the lamination is free of bubbles and flexible materials during the lamination process. This is to make sure no deformation can occur.

 

Quality inspection after lamination: Check the appearance of the board to see if there are quality problems such as delamination, oxidation, and glue overflow. Also, the peel strength test should be carried out to ensure quality.

 

8.Surface Finish

After the protective film (or solder mask) of the flexible board is laminated, the bare copper surface to be soldered must be made of organic soldering preservatives (Organic Solderability Preservatives; OSP), hot air leveling (HASL), nickel gold or Electric nickel gold)

 

Quality control points on the surface of the rigid-flex board

  1. Thickness,
  2.  Hardness,
  3. Porosity,
  4. Adhesion

Appearance: exposed copper, copper surface pinhole/dent/scratch/yin and yang color.

 

9.Profile processing:

Most of the profile of high volume flexible board is mold-cutting.small volume is laser. The process is as follows:

Mold design → mold making → test beer → (first board) measurement of size → production.

 

When CNC the rigid-flex board, special attention should be paid to the uneven shape and rough edges caused by the easy distortion of the flexible part.

 

In order to ensure the accuracy of the outline processing size, the process method of adding the gasket and the thickness of the rigid plate is adopted and should be fixed or compressed when CNC.

 

In addition, low feed and high speed will cause the edges of the board to be burnt. Meanwhile, the high feed and low speed will break the knife and the edge of the board.

 

Rigid-flex board and flexible board are glued to open windows, cover film open windows, base materials open windows, and reinforced processing with CNC and punching methods can also be used.

 

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Manufacturing  Flow for different Rigid-Flex PCB Project

 

Case 1: Motorola 1+2F+1 Mobile Display & Side Keys

1+HDI design, BGA Pitch: 0.5mm RIGID FLEX PCB

 

PCB features: 1+HDI design, BGA Pitch: 0.5mm

Flex board thickness: 25um with IVH hole design, PCB thickness: 0.295 +/- 0.052 mm

Inner LW/SP: 3/3mil

Surface treatment: ENIG

 

 

Process flow of Rigid-Flex PCB 1

Process flow of Rigid-Flex PCB 1

 

Case 2: Motorola 1+2F+1Motorola 1+2F+1 rigid flex pcb

PCB features: 1+HDI design

BGA Pitch: 0.5mm

Flex board thickness: 25um without IVH hole design, the whole

PCB thickness: 0.275+/-0.028mm

Inner LW/SP: 3/3mil

Surface treatment: ENIG + Silver Paste

 

Process flow of Rigid-Flex PCB 2

 

Process flow of Rigid-Flex PCB 2

 

Summary of Rigid-Flex PCB

 

8 layer rigid flex pcb

  1. The PCB design trend is the development of light and thin. In addition to high-density circuit design, there are also three-dimensional connection and assembly methods of rigid-flex PCB.
  2. The design structure of the flexible board and rigid-flex board is complicated, and the production process is difficult.
  3. The Cover-lay, No-Flow PP,and finished boards of the rigid-flex board need to be stamped with a mold. The mold size design and the size expansion control of the material are very critical.
  4. The material variety of the rigid-flex board is expensive.
  5. It is difficult to control the alignment accuracy of the rigid-flex board, and it requires high dimensional stability of the material.
  6. The manufacturing process of the rigid-flex PCB has strictrequirements for the operation workers.