Flexible-rigid PCB Fabrication

How to Guarantee the Quality of Flexible-Rigid PCB?

Flexible-rigid PCB provides a simple means to integrate multiple PCB assemblies and other elements such as display, input or storage devices without wires, cables or connectors, replacing them with thin, light composites that integrate wiring in ultra-thin, flexible ribbons between sections.

In rigid-flex packaging, a flexible circuit substrate provides a backbone of wiring with rigid multilayer circuit sections built up as modules where needed.

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.

Highfive Electronics has extensive experience, product knowledge and technical competency in this area.

Flex & Rigid-Flex expertise can also be found in various medical and health related test equipment, computing networks, and various scientific and industrial applications.Medical, military, aerospace, artificial intelligence, energy, power, industrial control, Internet of things, security, telecommunications.

Rigid-Flex PCB

Selection of Flexible-rigid PCB Materials

As the saying goes, “if you want to be good at your work, you must first make use of your tools.” so it is very important to make full preparation when considering the design, layout, and fabrication process of a rigid-flex combination board, but it requires certain professional knowledge and understanding of the characteristics of the materials required. The materials selected for the rigid-flex combination board directly affect the subsequent production processes and PCB performance.

Flexible Base Material Selection for Flexible-rigid PCB : Polyimide Flexible Base Material

We selects DuPont’s (AP adhesive free Series) polyimide flexible base material as the copper-clad material of the flexible board. Polyimide is a kind of material with good flexibility, excellent electrical performance and heat resistance, but it has great hygroscopicity and poor alkali resistance.

The reason why the base material without adhesive layer is selected, as the adhesive between dielectric layer and copper foil is mostly acrylic acid, polyester, modified epoxy resin and other materials, among which the modified epoxy resin adhesive has poor flexibility, polyester adhesive has good flexibility but poor heat resistance, while acrylic acid adhesive has satisfactory heat resistance, dielectric performance and flexibility, but it needs to be considered the glass transition temperature (TG) and pressing temperature are relatively high (around 185 ℃), then we currently use Japanese (epoxy resin series) base materials and adhesives to match the rigid-flex PCBs.

Rigid Plate Material Selection for Flexible-rigid PCB

There are also certain requirements for the selection of rigid plates. The choose the epoxy board with low cost, because the surface is too smooth to be adhered, and then choose fr-4.g200 and other base materials with certain thickness to etch the copper. However, due to the difference between fr-4.g200 core material and PI resin system, TG and CTE do not cooperate together, and the warpage of the rigid flex combination part after heat shock is serious.

Therefore, we use rigid material of PI resin series, P95 base material, or it can be simply pressed together with p95 semi curing sheet. In this way, after the rigid and flexible is pressed together, the warpage deformation after heat shocking can be avoided. At present, there are many base material manufacturers who develop and produce some rigid plate materials specially for the combination of rigid-flex boards.

Adhesive Part between Flexible Material and Rigid Plate for Flexible-rigid PCB

No flow adhesive is the preferred for the adhesive part between the flex part and the rigid plate, the No Flow (low flow) prepreg is used for pressing, because its small glue fluidity is very helpful to the soft and hard transition area, and it will not cause rework or functional impact of the transition area due to glue overflow.

At present, many enterprises can provide prepreg raw materials which have been developed as this kind of prepreg sheets and many kinds can meet the all structural requirements. In addition, for customers requirements for RoHS, If there are requirements for high Tg and impact.

It is also necessary to pay attention to whether the characteristics of raw materials can meet the final requirements, such as material thickness specification, dielectric constant, TG value, environmental protection requirements, etc

Highfive Design

Flexible-rigid PCB Types

1: Single side flexible material with or without shield(s) or stiffener (one conductive layer).

2: Double side flexible material with or without shield(s) or stiffener (two conductor layers with plated through holes.

3: Multilayer flexible material with or without shield(s) or stiffener (more than two conductor layers) with plated through holes and HDI.

4: Multilayer rigid and flexible material combinations (more than two conductor layers) with plated through holes and HDI.

Key Points of Fabrication Process Control for Flexible-rigid PCB

Graphic Transfer of Inner Slice

Graphic transfer plays a very important role in high-density, thin line PCB, especially for flexible circuit is very important. Because the flexible single sheet is thin and soft, it is very difficult for surface treatment and other operations, and the cleaning state and roughness of the copper foil surface can be directly affected for the adhesion of anti-corrosion dry film and the production of thin lines.

Due to the high requirements of mechanical wiping plate for equipment, and unsuitable pressure may cause deformation, folding, size expansion of base material, etc., the operation is not easy to be controlled, so we can choose to use electrolytic cleaning method.

method can not only ensure the surface cleanliness, but also ensure the roughness of copper surface by means of micro etching, which is conducive to the production for the 0.075 mm line width / spacing graphics.

addition to the controlling of etching speed to ensure the width and spacing required by the design, acid etching should be paid more attention to prevent the curl and wrinkle of the single sheet. It is better to add auxiliary guide plate and close the suction system on the equipment.

Multi layer Positioning of Flexible Materials

The dimensional stability of the flexible substrate is poor because the polyimide material has strong moisture absorption and passes through the wet treatment.

The shrinkage and deformation of the laminates are serious in different temperature and humidity environment, which makes the alignment of the laminates is very difficult.

In order to overcome this difficulty, the following measures can be adopted: in the design, it is necessary to consider the design of alignment spots and target punching spots, so as to ensure the accuracy of punching alignment holes or rivet holes, and not to cause the deviation of inter-lamination graphics during lamination and lead to scrap.

The positioning hole made after punching can eliminate the error caused by material expansion and deformation in the process of wet treatment.

After lamination, X-ray is used to drill holes and determine the offset to make the holes more accurate. According to the material characteristics and environmental characteristics of polyimide, the outer film is drawn with reference to the drilling offset to improve the coincidence between the outer film and the drilling plate.

In this way, we can meet the requirements of 0.1 mm-0.15 mm ring width for layer alignment, and ensure the accuracy of outer layer graphics transfer.

Lamination of Flexible-rigid PCB

Even if the positioning hole is punched by the ope, the single-chip treatment before lamination has a great influence on the inter-layer alignment.

First of all, because the polyimide material is not resistant to strong alkali, it will swell in the strong alkali solution, so in the process of black and brown treatment, in the strong alkaline process, such as oil removal, black and brown, the temperature and time will be appropriately reduced.

The base material without adhesive layer is adopted, the change of adhesive layer in alkali solution is not needed to be considered, so this method is feasible.

Secondly, the single baking after oxidation treatment should be avoided to be placed vertically, and horizontal baking should be adopted to reduce bending deformation and keep flat as much as possible.

After baking, shorten the mold loading time as much as possible to prevent moisture absorption of the single piece again.

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 semi-cured sheet used for rigid plate lamination, in order to make the adhesive sheet and the single sheet well combined and embedded in the fine line spacing, we choose the material with better covering shape as the lamination pad material, such as polypropylene film, polytetrafluoroethylene (PTFE), silicone rubber The laminating quality of the flexible board can be improved.

After the testings, it is considered that the ideal liner material is silicone rubber, which can ensure the covering shape and reduce the size shrinkage deformation of the pressed part.

Three Aspects of Lamination for the Rigid Boards

One is to pay attention to the same direction of the warp and weft of the glass cloth, whether it is the pressing of the base material or the simple pressing of the semi-cured sheet, and to eliminate the thermal stress and reduce the warpage during the pressing process.

The second is that the rigid boards 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 shocking, its change is different from that of the rigid part.

If the rigid part does not have a certain thickness or hardness, the difference will be obvious, and serious warpage will occur during the usage processes, which will affect the welding and usage. If the rigid part thickness or hardness is fixed, the difference may appear insignificant.

The overall flatness will not change with the change of the flexible part, which can ensure the welding and use. If the rigid part is too thick, it will appear heavy and uneconomical. The experiment proves that the thickness of 0.8-1.0 mm is suitable.

The third is the flexible window processing, usually milling first and then milling, but flexible processing is required according to the structure and thickness of the rigid-flex PCB itself.

If the flexible window is milled first, the milling accuracy shall be ensured, which can not affect the welding or the deflection greatly. The milling data can be made by the project, and the flexible window can be milled in advance. If the flexible window is not milled first, and the waste materials of the flexible window are removed by means of laser cutting after all the previous processes are completed and finally formed, the depth of FR4 that can be cut by laser shall be noted.

The pressing parameters can be optimized by referring to the pressing parameters of flexible base material and rigid plate.

Hole Drilling of Flexible-rigid PCB

The structure of the rigid-flex joint is complex, so it is very important to determine the best drilling parameters to obtain a good hole wall.

In order to prevent nail head phenomenon of inner copper ring and flexible base material, sharpening drill bit should be selected first.

If the number of printed boards processed is large or the number of holes in the processed boards is large, the drill bit shall be replaced in time after drilling a certain number of holes.

The rotation speed and feed of the bit are the most important technological parameters.

When the feed speed is too slow, the temperature rises sharply, which produces a lot of drilling contamination.

However, if the feed speed is too fast, it will easily lead to the breakage of the drilling bit, the bonding piece and the tearing of the medium layer and the phenomenon of the nail head.

Secondly, the drilling machine should be selected and the drilling parameters should be optimized according to the plate thickness and the minimum drilling hole diameter.

At present, there is a drilling machine that can reach 200000 RPM in the industry.

The higher the rotation speed of the hole, the better the drilling quality. At the same time, the selection of cover plate and base plate is also very important. Good cover plate and base plate not only protect the plate surface, but also play a good role in heat dissipation.

It should be noted that the base plate is the most important It is better to use aluminum foil board or epoxy glue board instead of paper board, because the paper board is soft and easy to produce serious drilling burr. When deburring before drilling, it is easy to tear or scratch the hole opening, which will cause trouble to the later process and affect the quality of the board.

Drilling Contamination and Laminating Alignment for Flexible-rigid PCB

Experiments show that the drilling contamination level and thickness of PCB increase with the increase of drilling temperature, and increase faster above the glass transition temperature of resin.

As a result, some manufacturers have tried to reduce drilling contamination by reducing the temperature on the processing plate. The specific method is: first, the rigid-flex printed board is frozen for several hours at low temperature (put in the cold store or refrigerator), and then it is taken out and drilled under the condition of air conditioning and heat preservation.

The hole drilled by this method has only a little bit of drilling contamination, and the effect is very obvious.

It should also be noted that although we have done a lot of work in wet processing, punching OPE holes, laminating alignment and other aspects to ensure the accuracy of inter-layer alignment, due to the influence of moisture and heat on polyimide material itself, it is inevitable to produce uncertain inter-layer deviation and plate deviation.

Therefore, before drilling, the hole should be drilled in X-ray alignment to determine the different expansion and contraction of different plates, and the data should be corrected with reference to the expansion and contraction to ensure the accuracy and effectiveness of drilling.

At the same time, the offset is transferred to the engineering department to handle the scaling of film, and the reference drawing of the outer film ensures the accurate alignment of the outer drawing transfer.

Remove Drilling Dirt and Pitting for Flexible-rigid PCB

Polyimide resin, epoxy glass fiber and epoxy resin are the main contaminants in the holes of rigid-flex boards.

Flexible polyimide resin is inert to concentrated sulfuric acid solution, but it will swell in strong alkaline potassium permanganate solution, so conventional wet decontamination is difficult to work.

We have also tried to use concentrated sulfuric acid or alkaline potassium permanganate solution to decontaminate, change the concentration, temperature, treatment time and other parameters, and many tests have not achieved satisfactory results.

Therefore, we gave up the traditional wet chemical decontamination and changed to plasma method.

Plasma chemical treatment system – plasma drilling and pitting system, is generally composed of five parts: vacuum chamber, vacuum pump, RF generator, microcomputer controller and raw gas.

Different types of plasma treatment equipment are only slightly different in the structure of the electrodes in the vacuum chamber and the input position and mode of the gas.

Plasma refers to the ionized gas, which is the state when the atom completely or partially loses its electronic layer under the action of RF energy generator.

It is composed of ion, electron, free radical, free group and ultraviolet radiation particle, etc.

it is neutral in overall electric display and has high chemical activity. The biggest advantage of plasma decontamination is that there is no selectivity, that is, regardless of the resin type of the treated board, it can be treated as long as the parameters are adjusted.

For example, the contamination of epoxy resin, polyimide, acrylic acid and glass fiber caused by high activity plasma flow can quickly and evenly remove them from the hole wall, and can form certain concave corrosion, effectively realize three-dimensional connection, and improve the reliability of metallized holes.

Three Steps of Plasma Decontamination

(1) To inject high-purity nitrogen and high-purity oxygen into the cavity after the cavity reaches a certain degree of vacuum. The main function is to clean the hole wall and preheat the printed board, so that the polymer material has a certain activity, which is conducive to subsequent treatment. Generally 80 ℃, 10 minutes.

(2) CF4, O2 and N2 are used as raw gas to react with resin to achieve the purpose of decontamination and pitting, generally at 85 ℃ for 35 minutes.

(3) Finally, O2 as the original gas, remove the residue or “dust” formed in the first two steps of treatment, and clean the hole wall.

Process parameters (system pressure 280 mtorr) stage I stage II stage III

CF4 gas percentage (%) 0 25 0

Percentage of O2 gas (%) 0 80 100

N2 gas percentage (%) 100 0 0

Vacuum (mTorr) 110 110 110

RF power (kw) 1.5 2.5 2.5

Processing time (min) 10 35 5

However, it is worth noting that when using plasma to remove the drilling contamination in the holes of multilayer flexible and rigid flexible combination printed boards, the pitting rates of various materials are different, and the order from large to small is: acrylic film, epoxy resin, polyimide, glass fiber and copper.

From the microscope, it can be seen that there are protruding glass fiber heads and copper rings on the hole wall.

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 plasma reaction residues, protruding glass fiber and polyimide film on the hole wall must be removed.

The treatment methods include chemical method and mechanical method or combination of the two.

The chemical method is to immerse the printed board in hydrogen fluoride amine solution, and then adjust the hole wall electrification with ionic surfactant (KOH solution). Mechanical methods include high pressure wet sand blasting and high pressure water washing. The combination of chemical method and mechanical method is the best.

Electroless Copper and Plating Copper of Flexible-rigid PCB

It should be pointed out that the ductility of electroplated copper layer is higher than the thermal expansion of rigid flexible bonding and flexible multilayer printed boards and has higher tensile strength.

The total expansion rate of the substrate is 1.65% higher than that of the copper plating layer in the hole, while this index is only 0.03% in the rigid multilayer. It can be seen that the tensile stress of metallized holes in rigid flexible printed boards is much larger than that of rigid multilayer boards.

At the same time, the thickness of copper plating layer has a certain impact on the reliability of rigid flexible printed boards. Most manufacturers of rigid flexible composite laminates improve the reliability of metallized holes by increasing the thickness of copper layer on the hole wall.

Surface Resistance Welding and Solderability Protective Layer

Due to the flexible requirements of the flexible board in the use process, generally in the flexible window or flexible part, the polyimide protective film crimping method is used to protect the line. However, for the precision line, polyimide is difficult to meet the requirements in the covering and opening window, but it can be coated with solder resist ink. The common solder resist ink is easy to crack and has no flexibility, which can not meet the requirements.

So we can choose a kind of screen printing flexible liquid photosensitive development type solder resist ink, both of which can play the role of solder resist, moisture-proof, anti pollution, mechanical flexure resistance, etc. another method is to stick the development type flexible cover dry film, but the price of raw materials is high, and it needs a vacuum film applicator to finish the coating well.

The solderability protective layer uses organic anti-oxidation protective film to ensure the flat and solderable surface of the pad.

Outline Processing of Flexible-rigid PCB

The shape of rigid flexible PCB should be milled on the milling machine, and the flexible part should be paid attention to, because the flexible part is easy to twist, resulting in uneven and rough shape milled out.

The gasket with the same thickness as the rigid outer layer can be padded on the top and bottom of the flexible window, and it can be pressed tightly when milling the shape, so as to ensure the smooth and even edge of the shape.

If the flexible window is not opened in advance and laser is used to cut off the waste of the flexible window, the shape of the milled flexible part will be more ideal, but not every kind of superposition can use laser.

Highfive PCBAE Service Offerings

1. Innovative Rigid-Flex processes and reliable PCB fabrication.

2. Premium material and on-store.

3. Rigid-Flex with HDI based on our rich experience.

4. Loose Leaf construction can be in accordance with your requirements.

5. Oversized Panels can match your panel size.

6. Layer counts to 40+

7. Heat Sink Application

8. Internal Quality Standards that often exceed the requirements of IPC-6013, MIL-PRF-31032 and MIL-P-50884 for registration, hole quality and overall reliability.

Value-Added Solutions

1.Flex Assembly

2. Product Box-building Engineering

3. Design / Layout

4. Free Engineering supporting

5. Technical Training

6. VMI System

7. Concurrent Engineering

8. Field applications Supporting

For more details , you can join the Rigid-Flex PCB group as you need. Or just contact us now for our free supporting.

Highfive Multilayer PCB Prototype