Rigid-flex board is a new type of printed circuit board that has both the durability of rigid PCB and the adaptability of flexible PCB. Among all types of PCBs, the combination of rigid and flexible is the strongest resistance to harsh application environments. Favored by manufacturers of medical and military equipment, Chinese companies are also gradually increasing the proportion of rigid-flex boards in total output.
Rigid-flex board is a kind of interconnection technology that combines the durability of rigid PCB and the characteristics of light, thin, flexible and can meet the needs of the three-dimensional assembly. It is increasingly widely used in the electronics and communication industries. Pay attention. The design and production of the flexible board and the design and production of the connection between the rigid-flex board are the key points in the production of the rigid-flex board. This article discusses the production process, design principles, and process control points of the rigid-flex board, so that reasonable design and accurate process control can be achieved in the design and production of the rigid-flex board.
Classification of rigid-flex board
If it is classified according to the manufacturing process, the way the flexible board and the PCB are joined can be divided into two categories of products: rigid-flex composite board and rigid-flex composite board. The difference lies in the technology of the rigid-flex composite board. The flexible board and the PCB can be used in the manufacturing process. Combination, in which there are common blind and buried hole designs, so higher-density circuit design can be achieved, while the technology of flexible and PCB is to separate the rigid-flex boards and then press them into a single circuit board. , There is a signal connection but no through hole design. But at present, the commonly used “rigid-flex board” collectively refers to all rigid-flex board products without subdividing the two.
Physical characteristics of the rigid-flex board
Rigid-flex boards are different from the original flexible boards and rigid boards in terms of materials, equipment, and manufacturing processes. In terms of materials, the material of the PCB is FR4 of PCB, and the material of the flexible board is PI or PET. There are problems such as bonding and different shrinkage rates between the two materials, which are stable for the product. In terms of the degree of difficulty, the rigid-flex board due to the three-dimensional space configuration characteristics, in addition to the consideration of the stress in the XY-axis direction, the stress-bearing in the Z-axis direction is also an important consideration.
At present, some material suppliers have Board manufacturers provide improved materials suitable for rigid and flexible boards, such as epoxy or resin, to meet the bonding problems between PCB rigid boards or flexible boards. In terms of equipment, due to the difference in material characteristics and product specifications, the equipment of the pressing and copper plating parts must be corrected for the rigid and flexible board. The applicability of the equipment will affect the product yield and stability, so it has entered the combination of rigid and flexible. The applicability of the equipment must be considered before the production of the board.
Advantages of rigid-flex board
01 Advantages of the rigid-flex board compared to general PCB:
- The interposer is thin.
- The transmission path is short.
- The through-hole is small.
- Less noise and high reliability.
02 Advantages of the rigid-flex board compared to the rigid board:
- It is flexible and can be wired three-dimensionally, and the shape can be changed according to space constraints.
- High and low-temperature resistance, flame resistance…
- Can be folded without affecting the signal transmission function.
- Can prevent electrostatic interference.
- Stable chemical changes, stability, and high reliability.
- Facilitate the design of related products, reduce assembly man-hours and errors, and increase the service life of related products.
- The volume of the application product is reduced, the weight is greatly reduced, the function is increased, and the cost is reduced.
Application of rigid-flex board
- Industrial use-Industrial use includes rigid-flex boards used in industry, military, and medical. Most industrial parts require precision, safety, and resistance to damage. Therefore, the required characteristics of rigid-flex boards are high reliability, high precision, low impedance loss, complete signal transmission quality, and durability. However, because of the high complexity of the manufacturing process, the output is small and the unit price is quite high.
- Mobile phone-the application of rigid-flex boards in mobile phones, such as folding mobile phone hinge (Hinge), image module (Camera Module), keys (Keypad), and radiofrequency module (RF Module), etc. are common. The advantages of using a rigid-flex board in a mobile phone are the integration of parts in the mobile phone and the consideration of the amount of signal transmission. At present, mobile phone products use a rigid-flex board to replace the original combination of two connectors and a flexible board. Its greatest significance in the product is to increase the durability and long-term use reliability of the movable point of the mobile phone. Its products are highly stable and highly valued. On the other hand, due to the popularity of camera phones and the integration of multimedia and IT functions in mobile phones, the amount of internal signal transmission in mobile phones has increased, and the need for modularity has arisen accordingly.
- Consumer electronic products-Among consumer products, DSC and DV are representative of the development of rigid-flex boards. They can be discussed in two main axes: “performance” and “structure”. In terms of performance, rigid and flexible boards can connect different PCB rigid boards and components three-dimensionally, so under the same circuit density, the total use area of the PCB can be increased, the circuit load capacity can be increased, and the signal transmission limit of the contacts can be reduced. Assembly error rate. On the other hand, since the flexible and rigid board is lighter and thinner, it can bend the wiring, so it is helpful to reduce the volume and reduce the weight.
- Automobile-the use of rigid-flex board in automobiles, commonly used are the buttons on the steering wheel connected 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 exterior detection systems, etc.
Design and production process of rigid-flex board
As the saying goes: “Workers must first sharpen their tools if they want to do their jobs well.” So when considering the design and production process of a rigid-flex board, it is very important to make full preparations, but this requires a certain degree of professional knowledge and The understanding of the required material characteristics, the material selected for the rigid-flex board directly affects the subsequent production process and its performance.
As the copper-clad material for flexible boards, our company selects DuPont’s (AP no-adhesive series) polyimide flexible substrate. Polyimide is a kind of flexible substrate with good flexibility, excellent electrical properties, and durability. Hot material, but it has greater hygroscopicity and is not resistant to strong alkalis. The reason for choosing a substrate without an adhesive layer is that the adhesive between the dielectric layer and the copper foil is mostly acrylic, polyester, modified epoxy resin, and other materials, among which the modified epoxy resin adhesive is flexible Polyester adhesives have good flexibility but poor heat resistance. Although acrylic adhesives are satisfactory in terms of heat resistance, dielectric properties, and flexibility, they need to be considered. The glass transition temperature (Tg) and pressing temperature are relatively high (around 185°C). At present, many factories use Japanese (epoxy resin series) substrates and adhesives to produce rigid-flex boards.
There are also certain requirements for the choice of rigid boards. We first choose the lower cost epoxy glue board, because the surface is too smooth to stick firmly, and then choose to use FR-4.G200 and other substrates with a certain thickness to etch away Copper, but ultimately due to the difference between FR-4.G200 core material and PI resin system, Tg and CTE are not compatible. After the thermal shock, the rigid-flex joint part warped seriously and cannot meet the requirements, so the rigid material of the PI resin series was finally selected. It can be formed by pressing with P95 base material or simply by pressing with P95 prepreg. In this way, the rigid-flexible board of the matched resin system can be pressed together to avoid warpage and deformation after thermal shock. At present, many substrate manufacturers have developed and produced some rigid board materials specifically for the rigid-flex boards.
For the adhesive part between the flex board and the PCB, it is best to use No flow (low flow) Prepreg for pressing, because its low fluidity is very helpful to the rigid-flex transition area, and will not cause Overflow of glue causes the transition zone to be reworked or the functionality is affected. At present, many companies that produce raw materials have developed this kind of PP sheet and there are many specifications that can meet the structural requirements. In addition, for customers in ROHS, High Tg, Impedance and other requirements also need to pay attention to whether the characteristics of the raw material can meet the final requirements, such as the thickness of the material, the dielectric constant, the TG value, and the environmental protection requirements.
The protective material of the outer layer pattern, that is, the solder mask generally has three types to choose from. The first type is the traditional cover film (Coverlay), which is a polyimide material and an adhesive directly connected to the etching after etching. The protected circuit board is laminated by lamination. This kind of cover film requires pre-formed before pressing to expose the parts to be welded, so it cannot meet the requirements of finer assembly. The second type is the photosensitive and developing type cover dry film, which is used by the film laminator. After pressing, the soldering part is leaked through the photosensitive development method, which solves the problem of assembly fineness. The third type is liquid screen printing type covering materials. The commonly used thermosetting polyimide materials, such as solar PSR-4000 and Photosensitive and developing type flexible circuit board special solder resist ink, this kind of material can better meet the requirements of fine-pitch, high-density assembly of flexible boards.
02 Production process flow and design, process control focus:
The development of the rigid-flex board is based on the flexible board and the high-density multilayer rigid board. It has many similarities with the rigid board in terms of process manufacturing. However, the rigid-flex board material and its structural and particularity of application determine that it is different from ordinary rigid boards and flexible boards from design requirements to production processes. Almost every production link must be tested and adjusted, and the entire process and parameters are finally optimized.
1 Production process
2 Interference board inner layer wiring design and graphics transfer production control point
1). Interruptable board inner layer wiring design
Since FPC generally transmits low-frequency signals, the main influencing factor is DC impedance, so unnecessary turns should be avoided and the transmission distance should be shortened.
The principle of wiring modification: does not affect the function, and the graphics are beautiful.
❶ Try to use short traces to avoid excessive corners.
❷ Try to use a circular arc connection.
❸ Try to use line shifting and PTH hole shifting to meet the process requirements.
❹ Try to avoid moving the solder pads of the parts.
- FPC circuits usually need to be arc treated.
- It is generally necessary to add teardrops where the line is connected to the PAD.
- Can help improve the yield of circuit forming
- Smooth line can reduce stress concentration and increase product reliability
❸ In order to facilitate the design of CVL OPEN, try to avoid wiring between PADs. When necessary, partial or even most rewiring can be carried out.
❹ For double-panel lines, avoid overlapping upper and lower lines, and staggered design to make the conductor as close as possible to the central axis of bending, which is beneficial to reduce bending stress and increase the number of bending.
❺ The PTH hole must be at least 2mm away from the broken line position.
❻ The direction of the line should be perpendicular to the broken line as far as possible, or at an angle of 45 degrees.
❼ Try to avoid the line width change at the position of the broken line, and try to make the line spacing evenly distributed.
⑵.Disturbing board graphics transfer production control points
Graphic transfer occupies a very important position in high-density, thin-line printed boards, especially for flexible circuits. Because the flexible monolith is thin and flexible, it brings great difficulties to operations such as surface treatment, and the clean state and roughness of the surface of the copper foil directly affect the adhesion of the resist dry film and the production of fine lines. Since mechanical wiping requires high equipment, and unsuitable pressure may cause substrate deformation, curling, size expansion, etc., the operation is not easy to control, so we can choose to use the electrolytic cleaning method. This method can not only ensure surface cleanliness but also use the micro-etching method to ensure the roughness of the copper surface, which is conducive to the production of line patterns with a line width/spacing of 0.1mm~0.15mm. In acid etching, in addition to controlling the etching rate to ensure the line width and spacing required by the design, attention should be paid to preventing the single chip from curling and wrinkling. It is best to add an auxiliary guide plate and close the ventilation system on the equipment.
3 Multi-layer positioning of flexible materials
The dimensional stability of flexible substrates is poor. This is because polyimide materials have strong moisture absorption. After wet treatment or in different temperature and humidity environments, they shrink and deform seriously, resulting in the lamination of multi-layer boards. Difficulty in alignment In order to overcome this difficulty, the following measures can be adopted: in the design, the design of the alignment motifs and target punching spots should be considered, so as to ensure the accuracy when punching the alignment holes or rivet holes, and not to cause problems during stacking. The deviation of the graphics between layers leads to scrapping. Positioning holes after OPE punching can eliminate errors caused by material expansion and contraction during wet processing. After lamination, use an X-ray to drill the holes to determine the offset to make the drilling more precise. According to the material characteristics and environmental characteristics of polyimide, the outer film is drawn with reference to the drilling offset to improve the overlap between the outer film and the drilling board. In this way, we can meet the requirement of 0.1mm~0.15mm ring width for interlayer registration, and ensure the accuracy of outer layer graphics transfer.
Even if the positioning holes are punched with OPE, the single-chip processing before lamination has a great influence on the alignment between layers. First of all, because the polyimide material is not resistant to strong alkalis, it will swell in the strong alkali solution. Therefore, in the process of blackening and browning, the strong alkaline process such as degreasing, blacking, and browning should be appropriately reduced. Temperature, reduce time. Since the base material without an adhesive layer is used, there is no need to consider the change of the adhesive layer in the lye, this method is still feasible. Secondly, the single-chip baking after oxidation treatment should be avoided to be placed vertically, and horizontal baking should be adopted to reduce bending deformation and keep it as flat as possible. After baking, shorten the molding time as much as possible to prevent the single piece from absorbing moisture again. Pre-stacking heat fusion is a very important link before pressing. It must be checked and confirmed during the operation process to ensure accuracy. The precautions for the process inspection are as follows:
- Before fusing the flexible board and NF-PP, pay attention to prevent the PP powder from falling onto the cover film or copper surface of the flexible board area or the copper surface or the gold surface during pre-stacking, which may cause defects in the flexible board area of the finished board.
- Before fusing, use 10x eyepieces to check the four standard inspection holes on the four corners of the board (near the inner layer of the board) to check the alignment of the inner core board and PP. Acceptance standard: ≤4mil, under ten times the lens The copper ring after browning can be seen and the PP below can not be seen in the hole, that is, the X/Y plane only allows PP to be tangent to the hole, and each board must be inspected.Before fusing the flexible board and NF-PP, pay attention to prevent the PP powder from falling on the cover film of the flexible board area or the copper surface or the gold surface during the pre-stacking, resulting in defects in the flexible board area of the finished board.
Because the flexible single sheet is easy to deform, the flatness before lamination is poor, and the resin fluidity of the adhesive sheet used is much lower than that of the prepreg used for rigid board lamination. Therefore, in order to make the adhesive sheet and the single sheet bond well and embed In the fine line spacing, we choose to use materials with better-covering shape as the laminated gasket material, such as polypropylene film, polytetrafluoroethylene (PTFE), silicone rubber sheet, etc., which can improve the lamination quality of the flexible board. After the test, it is believed that the ideal gasket material is a silicone rubber material, which can ensure its shareability and relatively reduce the shrinkage and deformation of the pressed part.
The following three aspects should be paid attention to in the processing of the PCB part:
⑴. Regardless of whether it is substrate pressing or pure prepreg pressing, it is necessary to pay attention to the same warp and weft directions of the glass cloth and to eliminate thermal stress and reduce warpage during the pressing process.
⑵. The PCB should have a certain thickness because the flexible part is very thin and has no glass cloth. After being affected by the environment and thermal shock, its change is different from the rigid part. If the rigid part does not have a certain thickness or Hardness, this difference will be very obvious, and serious warpage will occur during use, which will affect welding and use. If the rigid part has a certain thickness or hardness, this difference may appear insignificant. The flatness will not change with the change of the flexible part, which can ensure welding and use. If the rigid part is too thick, it will appear heavy and uneconomical. The experiment proves that the thickness of 0.8~1.0mm is more appropriate.
⑶. It is the processing of flexible windows, usually by milling first and milling later, but it needs to be processed flexibly according to the structure and thickness of the rigid-flex board itself, if it is milling the flexible window first The accuracy of milling should be ensured, neither the welding nor the deflection should be affected too much. The milling data can be produced by the engineering, and the flexible window can be milled in advance. If you do not mill the flexible window first, and then use laser cutting to remove the waste material of the flexible window after completing all the previous processes and finally forming, you should pay attention to the depth of the FR4 that the laser can cut.
The pressing parameters can be optimized comprehensively with reference to the pressing parameters of the flexible substrate and rigid board.
The structure of the rigid-flex board is complex, so it is very important to determine the best process parameters for drilling to obtain a good hole wall. In order to prevent the nail head phenomenon of the inner copper ring and the flexible base material, a sharp drill bit must be selected first. If the number of printed boards to be processed is large or the number of holes in the processed board is large, the drill bit must be replaced in time after a certain number of holes have been drilled. The speed of the drill bit and the feed are the most important process parameters. When the feed is too slow, the temperature rises sharply and a lot of drilling dirt is generated. If the feed is too fast, it is easy to break the drill bit, the adhesive sheet and the tearing of the media layer, and the nail head phenomenon.
Secondly, the drilling machine should be selected and the drilling parameters should be optimized according to the plate thickness and the minimum drilling diameter. At present, there are drilling machines that can reach 200,000 revolutions per minute in the industry. For small holes, the higher the speed, the better the quality of the drilling. At the same time, the choice of cover and backing board is also very important. Good cover and backing board not only protect the board surface but also play a good role in heat dissipation. It should be noted that the backing board is best to use an aluminum foil board or epoxy glue board. , Do not use the paper backing board, because the paper backing board is flexible, it is easy to produce more serious drilling burrs, and it is easy to tear or scratch the holes when deburring before boring, which will bring trouble to the subsequent work and affect the board. quality.
It should also be noted that although we have done a lot of work in wet processing, punching OPE holes, and lamination alignment to ensure the accuracy of interlayer alignment, the polyimide material itself is exposed to moisture and heat. The impact is large, and uncertain interlayer deviation and inter-board deviation will inevitably occur. Therefore, before drilling holes, an X-ray should be used to drill small holes to determine the different expansion and contraction amounts of different boards and to calibrate the data with reference to the expansion and contraction amounts to ensure accurate and effective drilling. At the same time, the offset is handed over to the engineering department to process the scaling of the film, and refer to the drawing of the outer film to ensure the accurate alignment of the outer graphics transfer.
6 Desmear and pitting
The contaminants in the holes of the rigid-flex board are mainly polyimide resin, epoxy glass fiber, and epoxy resin. Flexible polyimide resin is inert to a concentrated sulfuric acid solution but will swell in a strong alkaline potassium permanganate solution. Therefore, conventional wet decontamination is difficult to be effective. We have also tried to use concentrated sulfuric acid or alkaline potassium permanganate solution for decontamination, changing the concentration, temperature, treatment time, and other parameters. Many tests have not received satisfactory results, so we gave up the traditional Wet chemical decontamination, using the plasma method instead.
Plasma chemical processing system-plasma decontamination and hatchback system generally consists of five parts: vacuum chamber, vacuum pump, RF generator, microcomputer controller, and original gas. Different types of plasma processing equipment are only slightly different in the structure of the electrode in the vacuum chamber and the gas input position and manner. Plasma refers to the ionized gas, which is the state when atoms completely or partially lose their electronic layer under the action of the radiofrequency energy generator. It is composed of ions, electrons, free radicals, free radicals, and ultraviolet radiation particles. It is electrically neutral and has high chemical activity. The biggest advantage of plasma decontamination is that it has no selectivity, that is, regardless of the resin type of the board being processed, it can be processed as long as the parameters are adjusted. For example, the high-activity plasma stream can quickly and uniformly remove the contamination caused by epoxy resin, polyimide, acrylic, glass fiber, etc. from the hole wall, and can form a certain amount of etching. Effectively realize the three-dimensional connections and improve the reliability of metalized holes.
Plasma decontamination is generally divided into three steps:
(1). After the equipment cavity reaches a certain degree of vacuum, inject high-purity nitrogen and high-purity oxygen into it in proportion. The main function is to clean the whole wall and preheat the printed board to make the polymer material have a certain activity. Conducive to subsequent processing. Generally, it is 80°C for 10 minutes.
(2). Use CF4, O2, and N2 as the original gas to react with the resin to achieve the purpose of decontamination and etch, generally 85°C, 35 minutes.
(3). Use O2 as the original gas to remove the residues or “dust” formed during the first two treatment steps and clean the hole walls.
However, it is worth noting that when plasma is used to remove the dirt in the holes of the multilayer flexible and rigid-flexible printed board, the etch rate of various materials is different, and the order from large to small is: acrylic film, epoxy resin, Polyimide, glass fiber, and copper. It can be clearly seen from the microscope that there are protruding glass fiber heads and copper rings on the wall of the hole. In order to ensure that the electroless copper plating solution can fully contact the hole wall so that the copper layer does not produce voids and cavities, the residue of the plasma reaction, the protruding glass fiber, and the polyimide film on the whole wall must be removed. The treatment methods include chemical Method and mechanical method or a combination of both. The chemical method is to soak the printed board with a hydrogen fluoride amine solution and then use an ionic surfactant (KOH solution) to adjust the changeability of the hole wall. Mechanical methods include high-pressure wet sandblasting and high-pressure water washing. The combination of chemical and mechanical methods has the best effect.
7 Electroless copper plating, copper electroplating
It should be pointed out that the elongation rate of the electroplated copper layer is required to be greater than the thermal expansion rate of the rigid-flexible combination and the flexible multilayer printed board and has a higher tensile strength. When subjected to thermal shock, the total expansion rate of the rigid-flexible multilayer printed board substrate is 1.65% larger than that of the copper-plated layer in the hole, and this index is only 0.03% in the rigid multilayer board. It can be seen that the tensile stress of the metalized holes in the rigid-flex printed board is much greater than that of the rigid multi-layer board. At the same time, the thickness of the copper-plated layer also has a certain impact on the reliability of the rigid-flex printed board. Most manufacturers of rigid-flex multilayer boards rely on increasing the thickness of the copper layer on the hole wall to improve the reliability of the metalized holes.
8 Surface solder mask and solderability protective layer
Since the flexible board has a requirement for flexing during use, the polyimide protective film is usually used in the flexible window or the flexible part to protect the circuit by crimping, but for the precision circuit, the polyimide is in the cover type. It is difficult to meet the requirements on the window and the window, but it can be coated with solder mask ink. Ordinary solder mask ink is easy to be brittle and has no flexibility and cannot meet the requirements, so we can choose a screen printing flexible liquid photosensitive Development type solder mask ink, both can play the role of solder mask, moisture-proof, pollution prevention, mechanical deflection resistance, etc. Another method is to paste the development type flexible cover dry film, but the raw material price is higher and it needs The vacuum filling machine can complete the coating well.
The solderability protective layer uses an organic anti-oxidation protective film to ensure that the surface of the pad is flat and solderable.
9 Shape processing
Rigid-flex printed boards need to be milled on the milling machine, and the flexible part should be mainly paid attention to because the flexible part is easy to be twisted and the milled shape is uneven and rough. Pads with the same thickness as the rigid outer layer can be placed on the upper and lower sides of the flexible window, and pressed tightly when milling the shape, to ensure a smooth and uniform shape edge. If the flexible window is not opened in advance, and the laser is used to remove the waste of the flexible window at the end, the shape of the milled flexible part will be more ideal, but not every type of stack can be lasered.
There are two general processes for shape processing, and the distribution is:
1). The forming process of the Rigid-Flex board is as follows:→Pre-gong→Laser decap→Uncovering→Laser cutting→the Second gong.
2). UV laser production process: process arrival → parameter design, program call → first article inspection → mass production → self-inspection monitoring → IPQC sampling.
The precautions for the uncovering process are as follows:
- The first board section inspection must be done to confirm the uncovering parameters: the cutting depth must be ≥15µm away from the Coverlay layer, and the upper limit of the remaining thickness depends on the specific structure to ensure that the appearance after uncovering meets the quality control requirements; the residual rigid-flex board bonding area must be ≤0.5 mm.
- In addition to the slice confirmation depth, the uncovering parameters have also been determined by the operator’s ease of uncovering operation. After uncovering, the residue at the R-F junction must be repaired neatly with a small blade.
- The first plate must be sliced to confirm the laser parameters when uncovering, to ensure that the manual uncovering is easy to operate.
- When manually uncovering, you must gently tear up along the cutting line. Brute force should not be used when it is not easy to uncover. It is necessary to determine whether it is necessary to readjust the laser cutting parameters.