The flexible circuit board FPC board is a highly reliable and excellent flexible printed circuit board made of polyimide or polyester film as the base material. Referred to as soft board or FPC, it has the characteristics of high wiring density, lightweight, and thin thickness.
Prenatal treatment
There must be a complete and reasonable production process to make a good-quality FPC board. From pre-processing before production to final shipment, every procedure must be strictly implemented.
In the production process, in order to prevent excessive opening and short circuits from causing low yield or reducing process problems such as drilling, calendering, cutting, etc., FPC board scrap and replenishment problems, and evaluate how to select materials to achieve customer use the best-effect flexible circuit board. Prenatal pretreatment is especially important.
Pre-natal pretreatment, there are three aspects that need to be dealt with, and all three aspects are completed by engineers.
The first is the FPC board engineering evaluation, which is mainly to evaluate whether the customer’s FPC board can be produced and whether the company’s production capacity can meet the customer’s board-making requirements and unit cost;
If the project evaluation is passed, the next step is to prepare materials immediately to meet the supply of raw materials for each production link;
Finally, the engineer processes the customer’s CAD structure drawing, Gerber line data, and other engineering documents to suit the production environment and production specifications of the production equipment, and then transfers the production drawings and MI (engineering process card) and other data to the production department and document control, Procurement and other departments enter the regular production process.
1. FPC board cutting
Except for some materials, the materials used for flexible printed boards are basically rolls. Since not all processes must be processed by tape-reel technology, some processes must be cut into sheets to be processed, such as the drilling of metalized holes of double-sided flexible printed boards, which can only be drilled in sheet form at present. So the first process of double-sided flexible printed board is cutting.
Flexible copper-clad laminates have the extremely poor bearing capacity for external forces and are easily injured. If it is damaged during cutting, it will have a serious impact on the pass rate of subsequent processes. Therefore, even if it seems to be a very simple cutting material, in order to ensure the quality of the material, it must be paid enough attention. If the quantity is relatively small, a manual shearing machine or a hob cutter can be used, and for large quantities, an automatic shearing machine can be used.
Whether it is single-sided, double-sided copper foil laminate, or cover film, the accuracy of the opening size can reach ±0.33.
The cutting reliability is high, and the opened materials are automatically and neatly stacked, and no personnel is required to collect the materials at the exit. The damage to the material can be controlled to a minimum, and the material has almost no wrinkles or scars by using the change in the size of the feeding roller.
Moreover, the latest device can also automatically cut the flexible printed circuit board after the reeling process. The optical sensor can detect the corrosion positioning pattern and carry out automatic opening and positioning, and the opening accuracy is up to 0. 3mm, but the frame of this cutting material cannot be used as the positioning of the subsequent process.
2. FPC board drilling through holes
The through-holes of flexible printed boards can also be drilled by numerical control like rigid printed boards, but they are not suitable for hole processing of double-sided metalized hole circuits in tapes. With the increase in the density of circuit patterns and the smaller diameter of metalized holes, coupled with the limitation of the diameter of CNC drilling, many new drilling technologies have been put into practical application. These new drilling techniques include plasma
Body etching, laser drilling, punching with small apertures, chemical etching, etc., these drilling technologies are easier to meet the hole-forming requirements of the tape process than CNC drilling.
The through-holes of flexible printed boards can also be drilled by numerical control like rigid printed boards, but they are not suitable for the hole processing of double-sided metalized hole circuits in tapes. With the increase in the density of circuit patterns and the smaller diameter of metalized holes, coupled with the limitation of the diameter of CNC drilling, many new drilling technologies have been put into practical application.
These new drilling technologies include plasma etching, laser drilling, punching with small apertures, chemical etching, etc. These drilling technologies are easier to meet the hole-forming requirements of the tape process than CNC drilling.
2.1. CNC drilling FPC board holes
Most of the holes in the double-sided flexible printed board are still drilled with a CNC drilling machine. The CNC drilling machine and the CNC drilling machine used in the rigid printed board are basically the same, but the drilling conditions are different. Because the flexible printed circuit board is very thin, it is possible to overlap multiple pieces of drilling. If the drilling conditions are good, 10 to 15 pieces can be overlapped for drilling.
The backing board and cover board can use paper-based phenolic laminate or glass fiber cloth epoxy laminate or aluminum plates with a thickness of 0.2 to 0.4 mm. Drills for flexible printed boards are available on the market, and drills for drilling rigid printed boards and milling cutters for milling shapes can also be used for flexible printed boards.
The processing conditions for drilling, milling, and the shape of the reinforced board are basically the same. However, since the adhesive used in the flexible printed board material is soft, it is easy to adhere to the drill bit, and it is necessary to frequently check the state of the drill bit. And it is necessary to appropriately increase the speed of the drill bit. The drilling of multilayer flexible printed boards or multilayer rigid-flex printed boards should be particularly careful.
2.2. Punching FPC board holes
Punching small apertures is not a new technology, and has been used in mass production. Since the reeling process is continuous production, there are many examples of using punching to process the through-holes of the reel.
But batch punching technology is limited to punching diameter O. Compared with the drilling of the CNC drilling machine, the 6~0.8mm hole has a longer processing cycle and requires manual operation. Because the size of the initial process is large, the punching mold is correspondingly large, so the mold price is very expensive. Although mass production is beneficial to reducing costs, the burden of equipment depreciation is large, and small batch production and flexibility cannot compete with CNC drilling, so it is still not popular.
However, in recent years, great progress has been made in both the precision of punching technology and numerical control drilling. The practical application of punching on flexible printed boards has been very feasible.
The latest mold manufacturing technology can produce holes with a diameter of 75um that can punch 25um thick base material without adhesive copper-clad laminates. The reliability of punching is also quite high. If the punching conditions are suitable, it can even punch a diameter of 50um of the hole.
The punching device has also been numerically controlled, and the mold can be miniaturized, so it can be well used for punching flexible printed boards, and neither CNC drilling nor punching can be used for blind hole processing.
2.3. Laser drilling FPC board holes
The smallest through holes can be drilled with a laser. The laser drilling machines used to drill through holes on flexible printed boards include excimer laser drills, impact carbon dioxide laser drills, YAG (yttrium aluminum garnet) laser drills, and argon gas. Laser drilling machine, etc.
The impact carbon dioxide laser drilling machine can only drill the insulating layer of the substrate, while the YAG laser drilling machine can drill the insulating layer and copper foil of the substrate. The speed of drilling the insulating layer is significantly faster than the speed of drilling the copper foil. Fast, it is impossible to use the same laser drilling machine for all drilling, and production efficiency is very high.
The copper foil is etched to form the hole pattern, and then the insulating layer is removed to form the through-hole so that the laser can drill holes with extremely small apertures. However, at this time, the position accuracy of the upper and lower holes may restrict the hole diameter of the drilled hole.
If it is to drill blind holes, as long as the copper foil on one side is etched away, there is no problem with up and down position accuracy. This process is similar to the plasma etching and chemical etching described below.
At present, the holes processed by the excimer laser are the smallest. The excimer laser is ultraviolet light, which directly destroys the structure of the resin of the base layer, makes the resin molecules discrete, and generates very little heat. Therefore, the degree of heat damage to the periphery of the hole can be limited to a minimum, and the hole wall is smooth and vertical.
If the laser beam can be further reduced, holes with a diameter of 10-20um can be processed. The larger the thickness-to-aperture ratio, the more difficult it is too wet copper plating.
The problem with excimer laser technology drilling is that the decomposition of polymer will cause carbon black to adhere to the hole wall, so some means must be taken to clean the surface before electroplating to remove the carbon black.
When laser processing blind holes, the uniformity of the laser also has certain problems, which will produce bamboo-like residues.
The biggest difficulty of the excimer laser is that the drilling speed is slow and the processing cost is too high. Therefore, it is limited to the processing of high-precision and high-reliability micro-holes.
The impact carbon dioxide laser generally uses carbon dioxide gas as the laser source, which radiates infrared rays. It is different from the excimer laser which burns and decomposes resin molecules due to thermal effects. It belongs to thermal decomposition, and the shape of the processed hole is worse than that of the excimer laser.
The aperture that can be processed is basically 70-100um, but the processing speed is significantly faster than the excimer laser speed, and the cost of drilling is much lower.
Even so, the processing cost is much higher than the plasma etching method and the chemical etching method described below, especially when the number of holes per unit area is large.
The impact carbon dioxide laser should pay attention to that when processing blind holes, the laser can only be emitted to the surface of the copper foil, and the organic matter on the surface does not need to be removed. In order to clean the copper surface stably, chemical etching or plasma etching should be used as a post-treatment.
Considering the possibility of technology, the laser drilling process is basically not difficult to use in the tape process, but considering the balance of the process and the proportion of equipment investment, it does not have an advantage, but the ribbon chip automatic welding the process TAB(Tape Automated Bonding) has a narrow width, and the use of tape technology can increase the drilling speed. There have been practical examples in this regard.
3. FPC board hole metallization
The hole metallization of flexible printed boards is basically the same as that of rigid printed boards.
In recent years, there has been a direct electroplating process that replaces electroless plating and adopts the technology of forming a carbon conductive layer. The hole metallization of flexible printed circuit board also introduces this technology.
Due to its softness, flexible printed boards need special fixing fixtures. The fixtures can not only fix the flexible printed boards but also must be stable in the plating solution. Otherwise, the thickness of the copper plating will be uneven, which will also cause disconnection during the etching process. And the important reason for bridging. In order to obtain a uniform copper plating layer, the flexible printed board must be tightened in the fixture, and work must be done on the position and shape of the electrode.
Hole metallization outsourcing processing should be avoided as much as possible to factories without flexible printed board drilling experience. If there is no dedicated electroplating line for flexible printed boards, the quality of hole forming cannot be guaranteed.
4. The cleaning of the surface of the FPC board copper foil
In order to improve the adhesion of the resist mask, the surface of the copper foil must be cleaned before coating the resist mask. Even such a simple process requires special attention for flexible printed boards.
Generally, there are chemical cleaning processes and mechanical polishing processes for cleaning. For the manufacture of precision graphics, most occasions are combined with two kinds of clearing processes for surface treatment. Mechanical polishing uses the method of polishing. If the polishing material is too hard, it will damage the copper foil, and if it is too soft, it will be insufficiently polished.
Generally, nylon brushes are used, and the length and hardness of the brushes must be carefully studied. Use two brushing rollers, placed on the conveyor belt, the rotation direction is opposite to the belt conveying direction, but if the pressure of the brushing rollers is too large at this time, the substrate will be stretched under great tension, which will cause dimensional changes one of the important reasons.
If the surface treatment of the copper foil is not clean, the adhesion to the resist mask will be poor, which will reduce the pass rate of the etching process. Recently, due to the improvement in the quality of copper foil boards, the surface cleaning process can also be omitted in the case of single-sided circuits. However, surface cleaning is an indispensable process for precision patterns below 100μm.
5. FPC board coating of resist
Now, the resist coating method is divided into the following three methods according to the precision and output of the circuit pattern: the screen printing method, the dry film/sensitization method, and the liquid resist sensitization method.
Now, the resist coating method is divided into the following three methods according to the precision and output of the circuit pattern: the screen printing method, the dry film/sensitization method, and the liquid resist sensitization method.
The anti-corrosion ink uses the screen printing method to directly print the circuit pattern on the surface of the copper foil. This is the most commonly used technology and is suitable for mass production at a low cost. The precision of the formed circuit pattern can reach line width/spacing 0.2～O. 3mm, but not suitable for more sophisticated graphics.
The dry film method can produce 70-80μm line-width patterns as long as the equipment and conditions are complete. At present, most of the precision patterns below 0.3mm can be formed by the dry film method to form resist circuit patterns. Using dry film, its thickness is 15-25μm, conditions permit, the batch level can produce 30-40μm line width graphics.
When choosing a dry film, it must be determined according to the compatibility with the copper foil board and process and through experiments. Even if the experimental level has a good resolution ability, it does not necessarily have a high pass rate when used in mass production.
The flexible printed board is thin and easy to bend. If a harder dry film is selected, it will be brittle and have poor follow-up properties, so cracks or peeling will also occur, which will reduce the pass rate of etching.
The dry film is in roll form, and the production equipment and operations are relatively simple. The dry film is composed of a three-layer structure such as a thin polyester protective film, a photoresist film, and a thicker polyester release film.
Before attaching the film, firstly, peel off the release film (also called the diaphragm), then press it on the surface of the copper foil with a hot roller, and then tear off the protective film (also called carrier film or cover film) before developing. Generally, there are guiding and positioning holes on both sides of the flexible printed board, and the dry film can be slightly narrower than the flexible copper foil board to be pasted.
The automatic filming device for rigid printed boards is not suitable for the filming of flexible printed boards, and some design changes must be made. Due to the high linear speed of dry film lamination compared with other processes, many factories do not use automatic lamination but use manual lamination.
After sticking the dry film, in order to make it stable, it should be placed for 15-20 minutes before exposure.
If the line width of the circuit pattern is less than 30μm, and the pattern is formed with dry film, the pass rate will be significantly reduced. Generally, dry film is not used in mass production, but liquid photoresist is used.
Depending on the coating conditions, the coating thickness will vary. If a liquid photoresist with a thickness of 5-15μm is coated on a copper foil with a thickness of 5μm, the laboratory level can etch line widths below 10μm.
Liquid photoresist must be dried and baked after coating. Since this heat treatment will have a great impact on the performance of the resist film, the drying conditions must be strictly controlled.
6. The formation of FPC board conductive patterns
The photosensitive method is to use an ultraviolet exposure machine to form a circuit pattern on a resist layer that has been pre-coated on the surface of the copper foil.
In the previous sections, we introduced some related FPC technologies for making double-sided FPC. In this section, we will introduce the formation of conductive patterns in the FPC manufacturing process.
The photosensitive method is to use an ultraviolet exposure machine to form an FPC circuit pattern from a resist layer that has been coated on the surface of the copper foil in advance.
If it is a single-chip FPC board for exposure, the equipment is the same as that used for rigid printed boards, but the fixtures for overlapping positioning are different. A special pattern mask positioning fixture for flexible printed circuit boards FPC boards is available on the market.
However, many FPC manufacturers make them independently, which is very convenient to use. With positioning pins, due to the shrinkage and deformation of the flexible printed circuit board FPC, it is generally resistant to the spray pressure of the developer. Therefore, the nozzle structure, nozzle arrangement, and pitch spray direction.
The pressure is very critical. Since the developer is recycled and will gradually change, it is necessary to check and analyze the developer frequently and perform regular updates at an appropriate frequency according to the analysis results.
7. FPC board etching, resist stripping
Many of the process conditions described above are prepared for etching, but the process conditions of etching itself are also very important.
The etching process is not exactly the same as the wet treatment process mentioned above. During the etching process, the mechanical strength of the flexible printed board will change greatly. This is because most of the copper foil is etched away, so it is softer, so it is designed for Special attention should be paid to this when the etching machine of the flexible printed circuit board.
A reliable flexible printed circuit board manufacturing assembly line means that the etched flexible board must be able to be transported smoothly without guiding the traction board.
When considering the etching of precise patterns, attention should be paid to the etching device and other process conditions, and the original image should be compensated and corrected according to the etching coefficient. That is, all the process conditions are the same, and the etching coefficients of the parts with high line density and the parts with low line density are also different.
Compared with the parts with large line spacing, the parts with small line spacing have a poor exchange of new and old etching solutions, so the etching speed is slow.
For circuits with the same design width, if the circuit density is different in the same board, the wires in the high-density area may not be etched and separated during etching, and the circuits in the low-density area may be deformed due to side erosion. Thin or even broken wires, it is difficult to solve this situation completely relying on etching equipment and etching process conditions.
In order to solve this problem, the etching coefficient can be obtained by experimenting with each line density in advance, and compensation and correction can be made when the original image is made. For precision lines with large densities, the actual etching process must be performed and the etching conditions can be corrected. Sometimes need to go through 2 to 3 corrections.
Although this is trivial, it is very important to ensure the etching accuracy and stable process of high-density circuits. From the perspective of improving material utilization, the smaller the width between the plate and the graphics, the better, but for stable transmission, the edge width of the plate should be as wide as possible, and the interval between the impositions should also be larger.
From the above description, it can be seen that there are many factors that affect the etching performance of the circuit. Any project is not a separate process, but a process of mutual coordination and complicated relationship. To carry out high-precision precision circuit etching and obtain satisfactory results, it is necessary to continuously accumulate data on these factors affecting etching in the daily production process.
8. Processing of FPC board cover film
One of the unique processes of the flexible printed circuit board manufacturing process is the processing procedure of the cover layer. There are three types of processing methods for the covering film, screen printing of the covering layer, and photo coating layer. Recently, newer technologies have been developed to expand the range of options.
The processing of FPC board cover film is divided into three parts.
8.1. FPC board cover film
Cover film is the earliest and most used technology for flexible printed board cover layer applications.
The same film as the base film of the copper-clad laminate is coated with the same adhesive as the copper-clad laminate to make it a semi-cured adhesive film, which is sold and supplied by the copper-clad laminate manufacturer.
At the time of delivery, a release film (or paper) is attached to the adhesive film. The semi-cured epoxy resin adhesive will gradually solidify at room temperature, so it should be stored in low-temperature refrigeration. Printed circuit manufacturer before use, it should be kept in a refrigerated warehouse at around 5°C or sent by the manufacturer before use.
The general material manufacturer guarantees a use period of 3 to 4 months if it can be used for 6 months under refrigerated conditions. Acrylic adhesives hardly solidify at room temperature. Even if they are not stored under refrigerated conditions, they can still be used after storage for more than half a year. Of course, the lamination temperature of this adhesive must be very high.
One of the most important issues for the processing of the cover film is the fluidity management of the adhesive. Before the cover film leaves the factory, the material manufacturer adjusts the fluidity of the adhesive to a specific range. Under the conditions of refrigerated storage at an appropriate temperature, the service life of 3 to 4 months can be guaranteed, but within the validity period, the adhesive fluidity of the agent is not fixed but gradually decreases over time.
Generally, since the adhesive is very fluid when the cover film is just shipped from the factory, the adhesive is easy to flow out during lamination and contaminates the terminal part and the connection plate. At the end of the service life of the adhesive, its fluidity is very small or even no fluidity. If the lamination temperature and pressure are not high, a covering film that fills the pattern gaps and has high bonding strength cannot be obtained.
The cover film needs to be processed by opening windows, but it cannot be processed immediately after being taken out of the refrigerator. Especially when the ambient temperature is high and the temperature difference is large, the surface will condense water droplets. When the base film is polyimide, it will also be processed in a short time. Will absorb moisture and affect the subsequent processes.
Therefore, the general roll cover film is sealed in a polyethylene plastic bag. The sealed bag should not be opened immediately after being taken out from the refrigerator but should be placed in the bag for several hours. When the temperature reaches room temperature, it can be removed from the sealed bag. Take out the cover film for processing.
The cover film opening window uses a CNC drilling and milling machine or a punching machine, and the rotation speed of the CNC drilling and milling should not be too high. This kind of operating cost is high, and this method is generally not used for mass production.
Lay 10-20 sheets of cover film with release paper together and fix them with upper and lower cover pads before processing. The semi-cured adhesive is easy to adhere to the drill bit, resulting in poor quality. Therefore, it should be inspected more frequently than when drilling copper foil plates, and the debris generated during drilling should be removed.
A simple die can be used when processing the window of the cover film by the punching method, and the die is used for the processing of batch holes with a diameter of less than 3mm. When the hole of the window is large, use a punching die, and small and medium batches of small holes are processed by CNC drilling and punching together.
After removing the release film from the cover film with the opened window holes, paste it on the substrate of the etched circuit. Before lamination, the surface of the circuit should be cleaned to remove surface contamination and oxidation. Chemical methods for surface cleaning.
After removing the release film, there are many holes of various shapes on the cover film, which completely becomes a film without a skeleton. It is particularly difficult to operate. It is not easy to use the positioning hole to overlap the position on the line.
At present, mass production plants still rely on manual alignment and stacking. The operator first accurately locates the cover film window hole and the connection plate and terminal of the circuit pattern and temporarily fixes it after confirmation. In fact, if the size of either the flexible printed board or the cover film changes, it cannot be accurately positioned.
If conditions permit, the cover film can be divided into several pieces before lamination positioning. If the cover film is forced to be stretched for alignment, the film will be more uneven and the size will change more. This is an important cause of wrinkles on the board.
Temporary fixing of the cover film can be done with an electric soldering iron or simple pressing. This is a process that completely relies on manual operation. In order to improve production efficiency, various factories have thought of many methods.
The fixed cover film must be heated and pressurized to completely cure the adhesive and integrate the circuit. The heating temperature of this process is 160-200℃, and the time is 1.5-2h (one cycle time).
In order to improve production efficiency, there are several different solutions, the most commonly used is to use a heat press. Put the printed board with the cover film temporarily fixed between the hot plates of the press, overlap in sections, and heat and press at the same time. Heating methods include steam, thermal media (oil), electric heating, etc. Low cost of steam heating,
But the temperature is basically 160°C. Electric heating can be heated to above 300°C, but the temperature distribution is uneven. The external heat source heats the silicone oil. The heating method using silicone oil as a medium can reach 200°C, and the temperature is evenly distributed. Recently, this heating method has gradually increased.
Taking into account that the adhesive can be fully filled into the gaps of the circuit pattern, it is ideal to use a vacuum press. The equipment is expensive and the pressing cycle is slightly longer. However, it is cost-effective to consider the qualification rate and production efficiency. The introduction of vacuum presses is also increasing.
The laminating method has a great influence on the state of the adhesive filled between the lines and the bending resistance of the finished flexible printed board. Laminated materials are commercially available general products. Taking into account the cost of mass production, each flexible board factory has self-made laminated materials. According to the structure of the flexible printed board and the material used, the material and structure of the laminated board are also different.
8.2. Screen printing of FPC board overlay
The missing cover layer has worse mechanical properties than the laminated cover film, but the material cost and processing costs are lower. The most used are civilian products and flexible printed boards on automobiles that do not require repeated bending.
The process and equipment used are basically the same as those for solder mask printing on rigid printed boards, but the ink materials used are completely different.
It is necessary to select inks suitable for flexible printed boards. There are UV-curable and heat-curable inks in the market. The former has a short curing time and is convenient, but the general mechanical properties and chemical resistance properties are poor.
If it is used for bending or under harsh chemical conditions, it is sometimes inappropriate. In particular, it is necessary to avoid electroless gold plating, because the plating solution will penetrate from the end of the window to the cover layer, which will cause the cover layer to peel off.
Since the curing of thermosetting inks requires 20-30 minutes, the drying tunnel for continuous curing is relatively long, and intermittent ovens are generally used.
8.3. FPC board photo coating
The basic process of the photo-coating layer is the same as that of the photoresist film used for rigid printed boards. The materials used are also dry film type and liquid ink type.
In fact, the solder mask dry film is still different from the liquid ink. Although the coating process of the dry film type and the liquid type is completely different, the same device can be used for the exposure and subsequent processes. The specific process conditions will vary.
The dry film must be pasted first, and all the circuit diagrams should be covered with dry film. The ordinary dry film method is likely to have air bubbles between the lines, so a vacuum filming machine is used.
Ink type is to use screen printing or spraying method to coat the ink on the circuit pattern. Screen printing is the use of more coating methods, the same as the rigid printed board process.
However, the thickness of the ink coated by a missed printing is relatively thin, basically 10-15um, due to the squareness of the circuit. Orientation, the thickness of the ink in one printer is not uniform, and even skip printing occurs. In order to improve reliability, the missing printing direction should be changed and then the second missing printing should be performed.
The spraying method is a relatively new technology in the process of printed boards. The spraying thickness can be adjusted by the nozzle, and the adjustment range is also wide, the coating is uniform, there are almost no parts that cannot be coated, and the coating can be continuously applied for mass production.
The ink used for screen printing is epoxy resin and polyimide, both of which are two components. Mix with the curing agent before use. Add solvent to adjust viscosity as needed. Drying is required after printing. Double-sided lines can be lighted. After the coated side is temporarily dried, the other side is coated on the other side and temporarily dried, and then dried and cured after exposure and development.
The pattern exposure of the photo-coating layer requires a positioning mechanism with a certain accuracy. If the size of the disk is about 100um, the position accuracy of the covering layer is at least 30-40 m. If the mechanical capability of the device is guaranteed, this precision requirement can be achieved. However, after the flexible printed circuit board has been processed by multiple processes, it is difficult to meet higher requirements due to its own size expansion or partial deformation accuracy.
There are no major problems in the development process. Pay attention to the development conditions for precise patterns. The developer is the same sodium carbonate solution as the resist pattern developer. Avoid sharing the same developer with pattern development even in small batches.
In order to completely cure the developed photo-coating layer resin, post-curing must also be performed. The curing temperature will vary depending on the resin, and it must be cured in an oven for 20-30 minutes.
9. FPC board surface electroplating
There are many types of electroplating for flexible printed boards, and only general electroplating is introduced in this chapter.
9.1. FPC board plating
(1) Pretreatment of FPC board electroplating
The surface of the copper conductor exposed by the flexible printed circuit FPC after the coating process may be contaminated with adhesive or ink, and there may also be oxidation and discoloration due to high-temperature processes. If you want to obtain a dense coating with good adhesion, you must contaminate an oxide layer on the surface of the conductor are removed, it so that the surface of the conductor is clean.
Some of these contaminants are very strong in combination with copper conductors, and cannot be completely removed with weak cleaning agents. Therefore, most of them are often treated with a certain strength of alkaline abrasives and brushing.
The coating adhesives are mostly epoxy Resins that have poor alkali resistance, which will lead to a decrease in bonding strength.
Although it will not be clearly visible, in the FPC electroplating process, the plating solution may penetrate from the edge of the covering layer, and the covering layer may peel off in severe cases.
In the final welding, the solder penetrates under the covering layer. It can be said that the pre-treatment cleaning process will have a significant impact on the basic characteristics of the flexible printed circuit board FPC, and full attention must be paid to the processing conditions.
(2) Thickness of FPC board plating
During electroplating, the deposition speed of the electroplated metal is directly related to the electric field intensity. The electric field intensity changes with the shape of the circuit pattern and the position relationship of the electrode.
Generally, the thinner the wire width, the sharper the terminal at the terminal and the distance from the electrode. The closer the electric field strength is, the thicker the plating layer at that part.
In applications related to flexible printed boards, there is a situation where the width of many wires in the same circuit is very different, which makes it easier to produce uneven plating thickness. In order to prevent this from happening, a shunt cathode pattern can be attached around the circuit. , Absorb the uneven current distributed on the electroplating pattern, and ensure the uniform thickness of the coating on all parts to the greatest extent.
Therefore, efforts must be made on the structure of the electrode. A compromise is proposed here. The standards for parts that require high coating thickness uniformity are strict, while the standards for other parts are relatively relaxed, such as lead-tin plating for fusion welding, and gold plating for metal wire overlap (welding). High, and for the lead-tin plating used for general anti-corrosion, the plating thickness requirements are relatively relaxed.
(3) Stain and dirt of FPC board plating
The state of the plating layer that was just electroplated, especially the appearance, is not a problem, but some surface stains, dirt, discoloration, etc. soon after, especially when the factory inspection did not find any abnormalities, but it is waiting for the user to accept the inspection, found that there is an appearance problem.
This is caused by insufficient drifting, and there is a residual plating solution on the surface of the plating layer, which is caused by the slow chemical reaction after a period of time. In particular, flexible printed boards are not very flat due to their softness. Various solutions are prone to accumulate in the recesses, which will then react and change color in this part. In order to prevent this from happening, not only must be fully drifted but also needs to be fully dried. The high-temperature thermal aging test can be used to confirm whether the drift is sufficient.
FPC surface electroplating-double-sided FPC board manufacturing process
There are many types of electroplating for flexible printed boards, and only general electroplating is introduced in this chapter.
9.2. FPC electroless plating
When the line conductor to be electroplated is isolated and cannot be used as an electrode, electroless plating can only be performed. Generally, the plating solution used in electroless plating has a strong chemical effect, and the electroless gold plating process is a typical example.
The electroless gold plating solution is an alkaline aqueous solution with a very high pH value. When using this kind of electroplating process, it is easy for the plating solution to drill under the covering layer, especially if the quality management of the covering film lamination process is not strict and the bonding strength is low, this problem is more likely to occur.
Due to the characteristics of the plating solution, the electroless plating of the displacement reaction is more prone to the phenomenon that the plating solution penetrates under the covering layer. It is difficult to obtain ideal plating conditions for electroplating with this process.
FPC surface electroplating double-sided FPC board manufacturing process
There are many types of electroplating for flexible printed boards, and only general electroplating is introduced in this chapter.
9.3. FPC board hot air leveling
Hot air leveling was originally a technology developed for the rigid printed board PCB coating with lead and tin. Because this technology is simple, it has also been applied to the flexible printed board FPC board.
Hot air leveling is to immerse the board in a molten lead-tin bath directly and vertically and blow off the excess solder with hot air.
This condition is very harsh for the flexible printed board FPC board. If the flexible printed board FPC cannot be immersed in the solder without any measures, the flexible printed board FPC must be clamped between the screen made of titanium steel and then immersed in molten solder. Beforehand, the surface of the flexible printed circuit FPC board must be cleaned and coated with flux.
Due to the harsh conditions of the hot air leveling process, it is easy to cause the solder to drill from the end of the cover layer to under the cover layer, especially when the bonding strength of the cover layer and the copper foil surface is low, this phenomenon is more likely to occur frequently.
Since the polyimide film is easy to absorb moisture, when the hot air leveling process is used, the moisture absorbed will cause the cover layer to bubble or even peel off due to the rapid heat evaporation. Therefore, the FPC board hot air leveling process must be dried and moisture-proof before the FPC board hot air leveling process management.
10. FPC board shape and hole processing
Most of the processing of the hole and shape of the flexible printed board is processed by punching. However, it is not the only method. Depending on the situation, various methods or combinations can be used for processing. Recently, new processing technologies have been introduced with the demand for higher precision and diversification.
10.1. FPC board shape and hole processing technology
At present, punching is the most used for batch processing of FPC boards, and small batches of FPC and FPC board samples are mainly processed by CNC drilling and milling. These technologies are difficult to meet the future requirements for dimensional accuracy, especially position accuracy standards, and now new processing technologies are gradually being applied, such as laser etching, plasma etching, chemical etching, and other technologies.
These new shape processing technologies have very high position accuracy, especially the chemical etching method not only as high position accuracy but also have high mass production efficiency and low process cost. However, these techniques are rarely used alone and are generally used in combination with the punching method.
The purpose of the use is classified into FPC board contour processing, FPC board drilling, FPC board groove processing, and trimming of related parts. The simple shape and accuracy requirements are not high, all are processed by one-time punching.
For substrates with particularly high precision and complex shapes, if the processing efficiency does not meet the requirements with a pair of molds, the FPC can be processed in several steps. Specific examples are the plug parts of the narrow-pitch connector and positioning holes for high-density mounting components, etc.
10.2. FPC board guide hole
It is also called a positioning hole. Generally, the processing of the hole is an independent process, but there must be a guide hole for positioning with the line pattern. The automated process uses a CCD camera to directly identify positioning marks for positioning, but this kind of equipment is expensive and has a limited scope of application, so it is generally not used.
Now the most used method is to drill positioning holes based on the positioning marks on the copper foil of the flexible printed board. Although this is not a new technology, it can significantly improve accuracy and production efficiency.
In order to improve the precision of punching, the positioning hole is processed by the punching method with high precision and less debris.
10.3. FPC board punching
Punching is to use a pre-prepared special mold to perform hole and shape processing on a hydraulic press or crank press. Nowadays, there are many kinds of molds, and molds are sometimes used in other processes.
10.4. FPC board milling processing
The processing time of milling processing is in seconds, which is very short and low in cost. The production of molds is not only expensive but also requires a certain cycle, which makes it difficult to adapt to the trial production and design changes of urgent parts.
If the numerical control data of the numerical control milling processing is provided together with the CAD data, the operation can be carried out immediately. The length of the milling processing time of each workpiece directly affects the level of processing cost, and the processing cost is also high for a long time, so the integrated adjustment processing is suitable for products with a high price and low quantity or short trial production time.
11. Processing of FPC Reinforced Board
Reinforced boards are unique to flexible printed boards, and their shapes and materials used are also diverse.
Adhesives are generally film-like, and the two sides are protected by a release film. The adhesive film with one side of the release film is attached to the reinforcement board, and then the shape and hole are processed, and then it is laminated with the flexible printed board by the hot roll lamination method.
The material used is different, and the dimensional accuracy of the shape is also different.
The rigid board of epoxy glass cloth laminate and paper-based phenolic laminate can be processed by CNC drilling and milling machines or molds. Polyester and polyimide films can also be processed with a knife mold for simple shape processing. Generally, the film-like reinforcing plate does not need to be processed with fine holes and can be processed by CNC drilling and molds.
If it can be processed or automated in a relatively short period of time, it will reduce manufacturing costs.
Aligning and positioning the reinforcement board on the flexible printed circuit board with the processed shape and holes, this process is difficult to automate, and it accounts for a large proportion of the processing cost because this operation has to be carried out manually if a flexible printed board requires multiple reinforced boards of different materials, which increases the cost.
On the contrary, if the design is simple or the easy-to-operate jig is used, the production efficiency will be significantly improved, thereby reducing the cost.
All factories are working hard to improve this process, but still, need personnel with certain production skills to operate.
There are pressure-sensitive (PSA) and thermosetting types for the bonding of reinforcing plates, and the labor required for their processing is also very different. Using the pressure-sensitive type is very simple, tear off the release film on the pressure-sensitive type, and after aligning with the position on the flexible printed board, it can be pressurized in a short time, even as long as it is pressed by hand. When a certain bonding strength is required, a simple press can be applied for a few seconds or through a hot press roll.
Using thermosetting adhesives is not so simple. Generally, a pressure of 3～5MPa (30～50kg/cm.) and a high temperature of 160～180℃ are required, and it must be pressed for 30～60min. In order to prevent the flexible printed board from being affected by stress, the pressure on the reinforced board must be uniform. If the reinforced board is simply pressurized, the end of the reinforced board may be broken due to the stress.
In addition, the double-sided adhesive film with release film on both sides can be used to bond flexible printed boards, flexible printed boards, or flexible printed boards with rigid printed boards, which is equivalent to half of the rigid printed boards. The solid sheet, it is processing, and curing process are the same as the laminating process of the reinforced board.
12. FPC board inspection
There are many inspection items for flexible printed board FPC because the rigid printed board PCB board only serves as an electrical connection, and the flexible printed board FPC not only has the same function as the rigid printed board PCB board, but also has foldable, The function of bending motion, so this mechanical performance must also be checked to ensure quality.
At present, 100% of inspections are carried out on flexible printed circuit boards and FPC boards. Of course, in addition to the FPC disconnection and short circuit must be checked and there is inspection equipment, there are also many other items for visual inspection.
General lines can be inspected manually or with a magnifying glass magnified 2 to 3 times, but a high-power microscope should be used to inspect high-density lines.
Lines with a size of about 100μm should be inspected with a magnifying glass of 5-10 times, lines with a size of 50-100μm should be inspected with a magnifying glass of 10-20 times, and lines below 50μm should be inspected with a stereo microscope with a magnification of 20 times or more.
It is not that the higher the magnification of the microscope, the better. To be able to perform inspections efficiently, a wide field of view is also very important. Although it is a high magnification, the inspection efficiency cannot be improved without the electronic image magnification function.
The use of automatic optical inspection (AOI) to inspect the defects of the flexible printed circuit board is still only a part of mass production.
The automatic optical inspection instrument has been used in the tape reel process, but the automatic optical inspection instrument can only inspect the defects of the circuit, and can only partially replace the inspector, and the flexible circuit is different from the usual digital circuit, and the conventional automatic optical inspection instrument cannot be used. , A special program must be attached. It is difficult to adapt to the rapid development of miniaturized circuits.
With the high density of circuits, the magnification of the microscope used will increase accordingly, and the inspection time per unit area will also be extended. The proportion of man-hours required for the inspection of flexible printed boards is not small. With the further development of circuit density, this proportion will increase further. If the defect rate drops, the inspection speed will increase, but the density of the circuit continues to move forward. From the inspection standpoint, the pass rate of the precision pattern circuit will not increase significantly.
All inspection items of the flexible printed board are not carried out in the final process, especially the defects of the circuit, and coverage are better to be inspected in the process. The reality is that the inspection in the process cannot completely replace the final inspection, but it still has a certain effect on improving the efficiency of the entire production.
13. FPC board packaging
Special attention should also be paid to the packaging of the finished flexible printed circuit board. It is not a matter of arbitrarily and simply stacking an appropriate number of flexible boards together. Due to the complex structure of the flexible printed circuit board, it is easy to be damaged by a slight external force, so the packaging of the flexible printed circuit board must be extra careful.
The commonly used packaging method is to stack 10-20 pieces of flexible printed circuit board FPC together and use paper tape to roll each part and fix it on the cardboard. Avoid using tape, because the chemical substance contained in the tape adhesive should penetrate It is easy to cause oxidation and discoloration of the terminal.
When the base film is a polyimide film, because it is easy to absorb moisture, the flexible printed board FPC should be put into a polyethylene bag together with a desiccant such as silica gel and sealed at the mouth of the bag. Then put it and cushioning materials into a corrugated box. Due to the unique shape of the flexible printed circuit board FPC board, different packaging methods should be adopted according to different shapes.
In some cases, the flexible printed board FPC board is attached to a polyester support sheet coated with a weak adhesive before punching the shape, and then the half-cut shape processing (embedded punching) is performed with a die, and it is left intact. To the user, the user can take the flexible printed circuit board FPC off and assemble it, or assemble it first, and then remove it from the polyester carrier film after the assembly is complete.
This method can only be used for small-size products, which can greatly improve process efficiency both for flexible printed circuit board FPC manufacturers and users.
The safest and most reliable method is to use special trays. First of all, pallets should be equipped according to the variety. Although the management is troublesome, the quality is guaranteed and the use is convenient, which is beneficial to the user’s assembly. The cost is not high and can be discarded after use.
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