rigid-flex pcb

Rigid Flex PCB

What is Rigid-flex PCB?

Rigid-flex PCB are the combination for flexible circuit boards and rigid circuit boards that are pressed together. It utilizes the hardness of FR4 PCB and deflection of PI flex PCB, so it is not a two-dimensional space but a three-dimensional internal linkage, which can be arbitrarily bending and folding.
rigid-flex PCB
The Features for Rigid-flex PCB
Rigid-flex PCB provides a new way to connection between electronic components. With the development of electronic information technology and customer demand trend in thin, light, short and multifunctional electronic devices, Rigid-flex printed circuit board is right on trend.

Advantages:


- 3D wiring assembly
- Dynamic use, arbitrarily flexibility
- High density circuit design for HDI
- High reliability, low impedance loss, and complete communication transmission
- Reduced installation time, cost, and easy operation
- Rigid board strength for support.

Disadvantages:


- Difficulty in production; it not only follow rigid board the production process, but also follow the flexible board production process, so production process for rigid-flex board is much more complicated than that of single rigid board or single flexible board. -High cost at one time, large investment in equipment, both rigid board production equipment and flexible board production equipment are facilitated.
- Quality issue, it is impossible to repair the damage after assembly and disassembly, resulting in other parts being scrapped.

The material for Rigid-flex PCB


Rigid-flex PCB is made of flexible circuit board (FCCL) and rigid circuit board (FR4) combined together by adhesive.
1、Flex printed circuit par is composed of coverlay film and flex board substrate. And Substrate is categorized by adhesive and non-adhesive.
●The adhesive substrate consists of adhesive, copper foil and PI, which is less reliable and less flexible.
rigid-flex PCB

●The non-adhesive substrate consists of PI (polyimide) and copper foil, and has good reliability and flexibility.
rigid-flex PCB

●Coverlay film consists of PI and adhesive. The adhesive mainly uses acrylic adhesive and epoxy resin adhesive series.
rigid-flex PCB
2. For the adhesive part, it use No Flow PP (abbreviation: NF) and pure adhesive (acrylic adhesive).
● NF, consisting of epoxy resin, glass fiber and filler, is used for selective lamination. We currently use SO401N, SP170N, 49N and TU-84P.
● Pure glue, commonly known as "acrylic glue", the scientific name is "acrylic", good flexibility, but expansion shrinkage the coefficients is large, so it is mainly used for multi-layer PFCB board lamination.
rigid-flex PCB

3、Rigid board part, FR4, Taikou EM827, Lianmao IT180, Shengyi S1000-2, Isola, etc.

The Stack up for Rigid-flex PCB


1. 4 layer rigid-flex pcb stack up (1R+2F+1R)
4 layer rigid-flex pcb stack up

2. 6 layer rigid-flex pcb stack up (2R+2F+2R)
6 layer rigid-flex pcb stack up

3. 8 layer rigid-flex pcb stack up (3R+2F+3R)
8 layer rigid-flex pcb stack up

How to process rigid-flex PCB?


Here are the manufacturing process to produce rigid-flex PCB after the FPC flex board is finished,

1、Hole punching
Drill holes on top of FR4 and PP film, and the design on top of the positioning holes should not be the same as the PTH holes. After the punching is completed, it is necessary to browning process.

2、Riveting
The FR4, PP adhesive, FPC circuit board will be stacked and placed correctly in the right place, If follow the traditional process to stack and press materials layer by layer, it wastes a lot of time. After several attempts, it is possible to stack all materials and press them together at one time.

3、Lamination
This is a relatively complete step in the production of hard and soft combination board, most of the materials are integrated for the first time, first of all, the bottom layer of copper laminate and PP film, above the FPC soft board made in the previous process, in the FPC soft board on top of a layer of PP film, followed by the placement of the last layer of copper laminate. All the materials to be laminated are placed in order to complete, pressed together.

4、Milling board edge (also known as the edge removal)
It is to remove the part of the circuit board edge position that has no lines and is not going to make lines later. Then measure whether the material has extension and shrinkage, because PI material used for flex board has extension and shrinkage, which has a great impact to produce rigid-flex circuit boards.

5、Drilling
This step is to conduct holes on the whole circuit board. the production parameters need to adjust according to the design parameters.

6、Glue slag removal, plasma cleaning
First of all, remove the slag after drilling, then use plasma cleaner to clean conducting holes and the board surface.

7、Plating Through Holes
It is plating through holes process, also known as hole metalization, to achieve through-hole power conduction.

8、Copper Plating
plating the surface around holes is to increase the copper thickness to be a little higher than the copper-clad plate surface.

9、The outer positive dry film
Production process of dry film is same as that of flexible boards, the lines need to be etched on the copper-clad plate. After developing the film, it is necessary to inspect all the lines.

10、Pattern plating
Copper is electrodeposited onto the copper foil that is outlined by the dry film. We should follow the design requirements to calculate current time and choose copper plating lines so that we can make sure the desired plating area.

11、Alkaline etching
Prepare the alkaline etching solution, add the alkaline etching solution into the etching tank, Put the developed PCB board into the etching machine for etching; wash the etched PCB board several times, and then check whether the PCB board is qualified.

12、Print soldermask
Green oil is a liquid photoresist, an acrylic oligomer. a protective layer for PCB board wiring is coated on substrate or wiring without soldering on the printed circuit board, or as a solder resist. The purpose is the long-term protection of the PCB etched line. Of course, it can be other color, red, yellow, black, white,ect.

13, Routing De-cap
Routing De-cap focus on flexible board area. Using laser cutting to the unnecessary area on rigid board get the soft board exposed.

14、Curing is also a process of baking

15、Surface treatment
Generally soak the circuit board in a chemical solution so that the metal elements of solution can cover on the circuit board lines. The purpose is to metalize the lines of the circuit board and protect them from wear and oxidation. EING, HASL, Immersion Silver, Immersion Gold are available.

16、Printing silkscreen 
Use silkscreen printer to print component shape, serial number and other descriptive text on the component surface or solder surface in order to assembly all components on the boards and maintenance.

17、Testing
This is an inspection process whether the circuit board is qualified including electrical testing and visual inspection,electrical testing generally have impedance test, open and short circuit test, etc.
18、Final inspection
visual inspection is to check the surface quality,such as soldermask printing, silkscreen printing, stain,ect.

19、Packaging and shipping
Mostly, it use sealing bags and compartments to separate, and then use the vacuum packaging machine to vacuum pack rigid-flex boards.
rigid-flex PCB prodction flow chart

Why we need rigid-flex PCB?


Nowadays, the products are designed to be miniaturized, low cost and high speed, especially in the mobile device market. In recent years, a large number of high-performance, multifunctional devices have emerged, and the market has grown particularly rapidly, which lead to high density design for electronic devices in the market. Circuit board plays an important role in electronic device, so high-density pcb design with low cost and short time are taken into account. All engineer try to rigid-flex PCB design to meet these challenge.

In the rigid-flex PCB design , it is the replacement for Connector or Hotbar process to reduce cost for connector or Hotbar manufacturing and get two boards connected tightly. At the same time, no need to solder connector in the boards, which shorten the production time and save space to make the whole product smaller. What’s more, rigid-flex circuit boards can help greater speed and shorter distance for signal transmission. In the past, signal pathway like "circuit board → connector → flexible board → connector → circuit board", now for rigid-flex printed circuit board, signal pathway like "circuit board → flexible board → circuit board", it is obviously that the rigid-flex PC board enable signal transfer to travel shorter distance and low attenuation between different media. And it greatly improve the reliability for the whole product. That’s why rigid-flex circuit boards are widely used in different industries.

Application for Rigid-flex PCB
Rigid-Flex PCB has both hardness of FR4 circuit board and softness of FPC printed circuit, so it would have a better prospect in the global. Rigid-Flex PCB boards is gradually used in various application, such as communication, automotive, medical, industrial, military, Aerospace, ect.

Consumer Electronics:
smartphone
wireless headsets
Smart Glass
cameras
VCRs
DVDs
micro radios
calculators

Communication Use:
multi-functional phones
cell phones
video phones
fax machines

Automobile Use
Control Instrument
exhaust hood controllers
guard plate circuits
circuit breaker systems

Industrial Use
laser measurement and control instruments
Sensors
Heating coils
Photocopiers
Electronic scales

Instrumentation Use
nuclear magnetic analyzers
X-ray devices
infrared analyzers

Medical Use
physiotherapy instrument
cardiac pacemaker
endoscope
ultrasonic control instrument

Aerospace Use
artificial satellites
radar seconds radio
communications