How to guarantee the quality of Rigid-flex Fab

Key Points of the Rigid-Flex Fabrication Process Control

Not only we need to pay attention to key points of PCB design and layout, at the same time, but we also need to care about the rigid-flex fabrication processes.

Graphic Transfer of Inner Slice

Rigid-flex PCB fabrication

Rigid-flex PCB fabrication

Graphic transfer plays a very important role in high-density, thin line PCB, especially for Rigid-Flex Fabrication 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 Rigid-Flex Fabrication 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 an electrolytic cleaning method.
This 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 of the 0.075 mm line width/spacing graphics.
In 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 an auxiliary guide plate and close the suction system on the equipment.

Multi-layer Positioning of Flexible Materials of Rigid-Flex Fabrication

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, an 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 Rigid-Flex Fabrication

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.
Because the base material without the adhesive layer is adopted, the change of the 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 test, 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 shock, 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 use process, which will affect the welding and use. 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 soft and hard combination plate 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 Rigid-Flex Fabrication

The structure of the rigid-flex joint plate 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 the inner copper ring and flexible base material, a sharp 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 drill bit, the bond 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 the 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 the 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

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-flexible 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 of  Rigid-Flex Fabrication

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 the 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 the 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 an 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 the three-dimensional connection, and improve the reliability of metalized 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 whole 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 whole wall.
Process parameters (system pressure 280 more) 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 Rigid-Flex Fabrication, 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 whole wall must be removed.
The treatment methods include the chemical method and mechanical method or a 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 sandblasting and high-pressure water washing. The combination of chemical method and mechanical method is the best.

Electroless Copper and Plating Copper of Rigid-Flex Fabrication

It should be pointed out that the ductility of the 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 metalized holes in rigid-flexible printed boards is much larger than that of rigid multilayer boards. At the same time, the thickness of the copper plating layer has a certain impact on the reliability of rigid-flexible printed boards.
Most manufacturers of Rigid-Flex Fabrication want to improve the reliability of metalized holes by increasing the thickness of a 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 resists 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 the 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.

Shape processing  of Rigid-Flex Fabrication

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 the 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 the laser.