Nowadays, flexible PCB develops fast, and many people apply it in various fields, such as electronics and aviation. Thereby, many people fear the detailed performance of flexible PCB material. The answer is that flexible PCB still has many unknowns, and there are a lot of areas to catch up with.
Finding a good flex PCB board material that we can use to develop a new product is not easy. Flexible circuit board materials belong to two categories: one category is plastic, and the other is epoxy resin. It is difficult to determine which one of these two materials will be the best for developing a new product since they have their performance characteristics.
We can find the dielectric constant of a given material by measuring the capacitance of the material. The flex PCB dielectric constant is the number of coulombs needed to charge up one mole of electrons inside one mole of the substance.
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Primary Flexible circuit materials
The fundamental properties of PCB depend on the type of material and its processing. Currently, the most widely used materials are FR-4, glass epoxy, and ETFE.
Its development has been fast in recent years because of the emergence of printed wiring boards. At present, RayMing PCB and Assembly is mainly using the FR-4 material to produce a variety of PCBs. Although FR-4 is sometimes called “fiberglass,” this kind of material contains no fiberglass. Instead, it is a heat-resistant plastic with good strength and durability.
The flexible PCB material is mainly composed of copper foil and epoxy resin. Epoxy resin adheres to the copper foil and adheres to the copper foil, called flexible. Flexible PCB stiffeners are the most used stiffeners for flexible circuit manufacturing. They are easy to handle and can be applied either by hot air lamination or using solvents in solvent-based assembly.
Some people use polyurethane as the primary material of flexible PCB. However, because polyurethane is a plastic material, it is not easy to produce a good product. This is because the circuit network of polyurethane will break due to heat.
Ordinary Film Substrate Material Performance Improvement
We use film substrate material in the horizontal and vertical offset printing of PCB. The film substrate consists of photoresist, glass, and stainless-steel materials.
Flexible circuit board material mainly uses photoresists as film substrate materials, so AZ’s most used photoresist.
We process AZ film substrate in various thicknesses, and the thicker it is, the higher its performance. However, we can only process this kind of film in thicknesses of 0.35mm to 2.5mm, so it has a certain limitation in the choice of thickness and board design.
On the other hand, the glass substrate is a more common material used for flexible circuit board material development. This is because it does not generate much waste when manufacturing PCBs.
However, the material is challenging to process, mainly because it is hard to produce a large area. In addition, we can only process it in thicknesses of 0.4mm or 2mm.
We also use stainless steel substrate for flexible circuit board material development. The stainless-steel substrate has good mechanical strength and resistance. We process it in different thicknesses. So far, there have been two kinds of stainless-steel substrate: thinner (0.2 – 0.4mm) and thicker (0.4-0).
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Two-Layer PI Substrate Material
To save processing time, many people are beginning to use the material of the two-layer PI substrate. This flexible circuit material is easy to process, but there is a specific limitation. We can only process it in thicknesses of 1.6mm and 2mm. There are two kinds of PI film substrate: silver and gold plated.
The dielectric layer that uses silver cladding will improve the electric performance because it will have better conductivity. However, the cost of silver is higher than gold. In general, people continue to use gold for the dielectric layer, so the overall performance of this material is not as good as silver. In addition, again, there are many unknowns about this kind of flexible circuit board material.
LCP Substrate Material
The substrate of the LCP substrate is glass, but it uses a special coating to improve its characteristics.
This kind of material is mainly used for flexible circuit board material development, and not only that, but it is also easy to process. Before application, we must cut the film which makes up this material into different sizes to develop the products according to the requirements. So far, there are two kinds of LCP substrate: silver and gold plated.
Like silver, gold also has good conductivity. However, it is more cost-effective. By contrast, silver is more expensive but better. Glass substrates coated with silver or gold are the main electric circuit boards.
The silver-coated glass substrate has a better performance than gold. However, its performance is not as good as the above glass substrate material. This is especially when the temperature is high, or the humidity is large
We have been using copper coating materials in recent years. In addition to the electrical properties, it also has a better color effect than ordinary substrate materials. Because of its good performance, more and more people are using copper coatings to produce various flexible PCB materials. The processing technology is easy to get started in, but we still must explore. At present, we can achieve the copper coating by vacuum evaporation or electroplating. The vacuum evaporation method produces thin foil above 0.1mm, and the thickness can reach 0.2mm.
On the other hand, electroplating produces thicker film substrate materials. In recent years, the processing technology of copper coating is improving, and its cost reduced significantly. So, it has become a highly competitive material in recent years.
Halogen-Free Flexible Substrate Material Compatible with Environmental Requirement
With the development of technology, environmental protection awareness improves. Additionally, more people are paying attention to environmental protection. And the halogen-free material is an ideal choice for environmental protection. Therefore, this material is widely used, especially for flexible circuit board material production.
Halogen-free materials do not contain bromine and chlorine elements, so it has no toxicity to humans. However, these two elements are still used in some materials to improve the product’s performance. At present, the most used material for flexible circuit board material is polyimide.
Halogen-free and high-performance polyimide are in demand. However, it isn’t easy to achieve both simultaneously because they are challenging to process and produce a large area. In addition, it is also costly when compared with normal materials with halogen elements. For example, one layer of halogen-free and high-performance kapton flex circuit is four times more expensive than the material that contains bromine and chlorine elements.
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New-Type Copper Foil
We use copper foil for SMT and other processes in electric circuit boards. Therefore, the quality and performance of copper foil are essential to the high-quality production of electronic products.
We extensively used copper foil in electric circuits and high-tech products in the past. As a result, there are more applications for copper foil. For example, they are also widely applied in wearable medical equipment and other medical devices. We mainly use copper foil for SMT and through-hole processing in electric circuit boards. By contrast, it is challenging to process thick copper foil to produce flexible PCB material.
It is mainly because the thickness of copper foil should be 10-20um, which is too big for the thickness of normal flexible circuit material. In addition, we can also process it at a lower temperature or with high pressure. Copper foil’s normal pressure and temperature are much lower than flexible PCB material. More people are beginning to use thinner copper foil in polyimide flex PCB production to reduce production costs.
Conductive Silver Paste
Although silver paste is also a kind of flex PCB material, it has characteristics of a conductive paste. In the past, we did not use this kind of material in the production process. But now more people are beginning to pay attention to its development.
Silver paste has good electrical conductivity, and we can use it for SMT processing. However, its application area has limitations. Its thickness is insufficient for the flexible circuit board material and cannot withstand high-temperature changes.
We mainly use Silver Paste as a conductive paste, but we can also process it into a thick film by adding sulfur. The use of the silver paste has limits on SMT and solder processing and many other flexible circuit board material applications.
Silver Film Gold-Plated Substrate Material
The main application of this material is in the production of flexible circuit board material; however, we can use it in other fields. A silver film by itself has good electrical conductivity and electrical conductivity. We mainly use it as a material in the high-temperature molding process. The thickness of the silver film is very thin, and we can process it at a relatively low temperature. The electrical performances of silver film are superior to ordinary substrate materials. However, its price is still high enough. We mainly use it to produce electronic components and equipment, such as inductors.
In recent years, gold-plated substrate material has become a widely used material in producing electronic components and electronic equipment. Because of its high performance has become a highly competitive material in recent years. The gold-plated substrate material is not limited to single-layer processing. However, we can also process into multi-layer processing by adding other materials. Currently, we mainly apply it in the production of high-end electronic components and electronic equipment.
In this kind of material, there is usually only one layer of gold, but we can also process it into two or three layers. The electrical performance and the reliability of the individual layers are also more than ordinary materials.
Gold-plated substrate material has also become a flexible circuit board material. However, it cannot withstand high-temperature changes and relatively low-pressure processing.
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Photosensitive PI Coverlay
The PI coverlay is a suitable material to produce flexible circuit board material. It has good processing performance, and the mechanical strength of the overall product is not affected. In addition, the electrical performance of PI coverlay is also high. Additionally, we can process it into multi-layer production by adding other materials.
In many applications, we use PI coverlay to process large-scale areas such as 30mm * 15m or 30mm * 10m. The size of the entire process area is much bigger than that of the flexible circuit board material. In this case, we can process PI coverlay into large-scale circuits.
PI coverlay is also a layer of flexible circuit board material. In addition, it has good electrical performance, and it is also suitable for multi-layer processing. However, because there are many restrictions on its use and it is easy to change color during the production process, people usually do not use this kind of material for SMT or flex PCB production.
Gold-Foil Film
In addition to becoming a kind of flexible circuit board material, we can also process it into producing electronic components and electronic equipment. We mainly use it in the production of inductors. The main reason that this material is popular in the production of high-end electronic components and electronic equipment is that it has good electrical performance and the best mechanical strength. Gold-foil film has good conductivity, but its resistance to wear is relatively high.
Flexible circuit board Performance characteristics
Flexible circuit board performance characteristics consist of electrical, mechanical, and packaging. We primarily use it to produce wireless electronics and wearable medical equipment. In addition, we can also use the flexible circuit board in other applications such as GPS and GNSS. Typical applications of flexible PCB materials are:
1 Responsive to temperature changes
The above features are mainly affected by heating speed, temperature, and heating time. To a certain extent, the heating time will affect the temperature change of the flexible circuit board material. If you require a specific temperature, it will take a long time to reach it. The heating process for flexible circuit board materials mainly uses infrared radiation. However, there is a limit to the heating speed of infrared radiation. Also, it is not suitable for processing flexible circuit board material. Therefore, you need to adjust the design of this kind of product accordingly.
2 High-temperature resistances
It is mainly affected by temperature resistance. We can see this in the case of materials primarily characterized by resistance to high-temperature oxidation treatments (e.g., resin-filled boards). Such materials help process flexible circuit board material in some cases (e.g., PCB core). Therefore, continuous processing is critical so that the temperature does not exceed a predetermined limit.
The above two points are also related to one another. In the case of a material that can withstand high-temperature oxidation processes, its resistance to high-temperature oxidation treatments is not very good. Additionally, it cannot withstand high-temperature materials.
3 Mechanical strength
Unlike ordinary substrate materials, flexible circuit materials have relatively high elastic deformation resistance. In general, this kind of material has high elastic deformation resistance and low strength.
This material is not suitable for processing many layers, but it is suitable for processing one or two layers.
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4 Burnout temperature
The burnout temperature of the flexible circuit board material affects the length of the heating zone and the heating speed. In general, the burnout temperature will be lower if you can make the heating zone length extremely short. However, because there are many restrictions on this processing technology, we cannot achieve this easily.
The above features are mainly affected by the heating speed, heating time, and cooling time. We can see the following details in the heating zone length and the cooling rate.
5 Electrical performances
Flexible circuit board materials are mainly sensitive to the electrical performance of their material structure. It is different from ordinary substrate materials. In general, we need advanced technology to obtain good electrical performance.
Electrical parameters of electronic components such as inductors or resistors often affect electronic components that use these materials greatly.
6 Aging characteristics
Since we use flexible circuit board material to process electronic components and equipment, it needs good aging properties. Aging properties include good mechanical strength, thermal resistivity, and electrical performance.
These features are mainly affected by the heating speed, heating temperature, and cooling time of the material. In addition, it is also affected by the thickness of the heating zone.
7 Radiofrequency performance
The above features mainly affect the frequency characteristics of the antenna and the electrical performance of the PCB material processing. This PCB requires a process that can quickly conduct with high frequency, so it is convenient to use RF laser processing technology. If we use a conductive layer to process PCB materials, it isn’t easy to meet the requirements for radiofrequency performance. Therefore, we often use non-conductive layers such as polymer films.
In the case of a non-conductive layer, we can see it in both types.
8 Packaging characteristics
It is mainly affected by the packaging performance and moisture permeability properties. Compared with ordinary substrate materials, flexible circuit board material has more excellent moisture permeability to ensure moisture never enters the package. The mechanical structure of its film mainly causes this thickness mainly causes it. It is different from ordinary substrate materials. The above features affect the material’s heating speed, temperature, and cooling time. In addition, it is also affected by the heating speed, heating temperature, and cooling time of the material. Additionally, it is also affected by the heating speed, heating temperature, and cooling time of the material.
In addition, it is also affected by the packaging performance and moisture permeability properties. Compared with ordinary substrate materials, flexible circuit board material has greater moisture permeability to ensure moisture never enters the package.
9. Flame retardancy
We use a flexible circuit board material to process electronic equipment, so it must have good flame retardant performance. The main reason flexible circuit board materials are highly flammable. It can also burn easily is that they contain organic substances. Some PCB material manufacturers also use halogen-containing flame retardants when producing flexible circuit board materials to improve their flame retardant performance.
10. Low-temperature performances
The low-temperature performance of flexible circuit board materials affects the electronic component process before packaging. If the flex PCB uses a cold seal, its minimum operating temperature decreases. It significantly influences the processing performance of electronic components. The PCB material processing technology is also affected by its low-temperature performance. We mainly use it to process circuit boards with a thickness of more than 2 mm, and its ability to withstand low temperatures is better.
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Flexible circuit board Classification
We can make Flexible circuit board material into various types, such as the printed circuit board, single-sided PCB, and double-sided PCB.
The printed circuit board is essential in processing electronic components with different functions. PCB has a variety of shapes, sizes, and specifications. This is special processing machines can process.
We process the single-sided printed circuit board from one side only, and the components are on the same surface on both sides. Its surface consists of glass or PVC, and the components are on both sides of the board.
The double-sided printed circuit board (usually referred to as two-side PCB) is processing from both sides. For example, we process it from a single piece of plastic film thickness 1–3 mm. We then place the components on both sides of the board, processed through special machines.
There are three different classes of flexible circuit board materials.
Class one (General Electronic Products)
We mainly use this class of flexible PCB materials in household appliances and small electronic products. It has excellent flexibility, heat resistance, hot resistance, cold resistance, and aging properties. It only consists of one layer. They produce High-voltage double-sided circuits with printed circuits on both sides of the insulating substrate.
It can process more than ten electronic components 10 mm in diameter or less.
Class two (Dedicated Service Electronic Products)
This class of flexible circuit board materials is essential in the electronic components attached to electronic products with high precision. It has excellent cold resistance and aging properties but is slightly inferior to class one.
Its temperature range is from -40 degrees C to +80-degree C.
It can process more than 100 electronic components of 10 mm diameter or less on both sides. We can use this class of flexible PCB in high-precision equipment or instruments such as watches, calculators, digital cameras, etc.
Class three (High-Reliability Electronic Parts)
This flexible circuit board class is essential in electronic components with high reliability.
Its temperature range is from -55 degrees C to +125 degrees C. The use of this material makes it possible to withstand long-term use at high temperatures and is suitable for multiple connections, short-circuit, and thermal stresses. We can use this class of flexible PCB in high-reliability equipment or instruments such as computers, printers, mobile phones, etc. It is important to note that PCB stiffener must be compliant with all design rules of the PCB manufacturing process.
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Flexible circuit materials processing operations
Flexible circuit board processing is a critical process developed in flexible circuit board material processing. We can divide the whole process into six stages. They include film formation, plating and coating, resin bonding processing, cleaning, drying, heat treatment, and packaging.
The first stage of the film formation process is to integrate a film with printed circuits and components on both sides. We can make this film out of plastic or glass with a thickness of 0.025 mm–0.5 mm and consists of silk-printed circuit film.
The second stage is to paste the printed circuits and components and heat them to form a coating layer.
The third stage is to melt the plastic or glass film under high temperature, applying a hot-melt extrusion coating film.
The fourth stage is to apply a passivation layer on both sides, forming a double-sided flexible circuit board material that is uniform in thickness.
The fifth stage is the resin bonding process, in which the flexible circuit board material is wet with a hot-melt adhesive. Then we stack the electronic components on both sides of the circuit board. We can carry out the whole process under pressure to form a certain thickness of a flexible circuit board.
The sixth stage is to heat or air cool after packaging. It forms a completed product with multiple integrated electronic components and printed circuits.
Professional equipment must carry the whole process for high-quality flexible circuit board materials. If you do not perform the process according to the standard procedure, many defective products will have lower efficiency.
Conclusion
International research institutes have researched and developed flexible circuit board material processing technology. It has met users’ needs and can form a new trend in flexible circuit board materials processing.
The overall performance of flexible circuit board materials in terms of flexibility, heat resistance, cold resistance, wear resistance, aging, and so on is better than that of rigid circuit board materials. The material is easy to process, and machines can process it with automation. It has excellent mechanical properties and processing performance for electronic components.
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