The manufacturing of circuit boards has existed for many years. Circuit boards are also known as Printed Circuit Boards (PCBs)s. A rigid flex circuit board is an electronic device that needs to withstand different conditions. That is why rigid flex circuit boards, such as temperature variation, humidity, and vibrations. The main advantages of using rigid flex circuit boards are the low cost and flexibility. The most common materials used to manufacture these circuit boards are glass epoxy, FR-4, paper phenol, and TGIC.
Understanding Rigid-Flex Printed Circuit Boards
Rigid flex circuit boards are not flexible at all. We use this type of circuit board to withstand different conditions and resist heat, humidity, and dust. There are different ways in which one can manufacture rigid flex circuit boards. One of the methods used to manufacture these rigid-flex PCBs is the liquid-based printing technique. It uses heatsinks to transfer the components on the board.
Additionally, other manufacturing techniques include screen printing and gravure printing. These rigid-flex circuit boards do not have a solder mask, and we do not use flux. The main advantage of using the rigid flex PCB is that they have a high heat tolerance to withstand extreme conditions.
Flex Circuit Types
There are two different types of flex circuits; one is flex, and the other is rigid flex. The difference between rigid and flexible circuits has to do with their flexibility. There is no difference in their design or cost as they make both from the same materials. The main difference between these two flex circuits is their flexibility.
The rigid flex circuit board has no bend, and the flex circuit can bend easily. However, the Flex circuit is not so rigid as it cannot withstand any shock or impact. One of the disadvantages of using rigid flex PCBs is that their flexibility does not work with high-density circuits. In addition, it will affect the appearance of the circuit board.
Factors that affect rigid and flexible PCB Cost
Many factors do not affect rigid flex circuit boards. These factors include material costs, process costs, and time. The main factors that affect the cost of rigid flex PCB in a manufacturing company are its cost, process cost, and manufacturing time. In other words, the rigid flex circuit boards have some variations in their fabrication process. Therefore, one must produce them in different manners.
1. Material Choice
The material used to produce rigid flex circuit boards is also affected by many factors. They include flexibility, thermal stability, and strength. Thickness is also a factor. The main materials used to fabricate rigid flex circuit boards are flexible epoxy laminate, fiberglass laminate, and FR-4. We use these materials in producing rigid flex PCBs because they allow the board to be flexible and strong. factors that impact material include:
a. Mechanical Properties:
Mechanical properties majorly affect the property of flexible PCBs. Some examples include flexural strength, bending strength, and shear strength. The thickness of the board also plays a significant role in the flexibility of the rigid flex PCB. Ease of use affects mechanical properties and, therefore, material choice.
b. Signal Performance:
The signal performance of rigid flex PCBs gets adverse effects from the various properties of the materials used to produce the rigid flex PCBs. Some of these properties include Raman scattering, infrared radiation, and coupling.
c. Heat Transfer:
Heat transfer also affects materials. Without heat transfer, the rigid flex PCBs will not have functionality. The right material should have good thermal conduction.
High temperature is also one of the factors that affect heat transfer. However, the materials used to produce rigid flex PCBs could withstand high temperatures. That is why rigid flex PCBs are essential in some applications that require high temperatures.
d. Temperature Reliability:
Most rigid flex PCBs are essential in applications that require temperature variation. The circuit board should be able to withstand those conditions. Rigid flex circuit boards can handle temperature variation well using the right material.
e. Thermal Reliability:
Temperature variation is one factor that affects rigid flex circuit boards. A rigid flex circuit board should be able to withstand temperature variations. That is why rigid flex PCBs are essential in applications that require temperatures from -40c to +140c.
2. PCB Size
The process cost depends on PCB size since one must produce it correctly. Manufacturing a circuit board is not something that you can do anyhow. They do different processes to produce the circuit board correctly. They also bear the changes in temperature, humidity, and other conditions. The process cost should not be high because of the PCB size. A rigid flex PCB is usually larger than a standard PCB manufactured through expensive processes. The size must be right for the rigid flex circuit board to have flexibility and function. Size also affects the flexible PCB price since a smaller PCB takes less time to produce.
Common dimensions include:
21 X 24″ (533 x 610mm)
18 X 21″ (457 x 533mm)
18 X 24″ (457 x 610mm)
3. Number of Layers
A rigid flex PCB has multiple layers taped together. Producers should tape layers together to form a circuit board for flexibility and strength. Therefore, the number of layers must suit the rigid flex circuit board to be flexible and strong. The thickness of each layer also affects the number of layers that need production in a circuit board. The number of layers affects the manufacturing cost because we must build the circuits from the bottom upwards. The board should withstand flexibility, strength, and we require long-lasting materials.
With each layer added to the rigid flex PCB, it will increase the flex PCB quote . This is because circuit boards containing more layers are more expensive than those having fewer layers.
Here is a list in descending order on the price increase of PCB based on the number of layers:
20 – 30%: 10L to 12L
20 – 30%: 8L to 10L
30 – 35%: 6L to 8L
30 – 40%: 4L to 6L
35 – 40%: 2L to 4L
35 – 40%: 1L to 2L
4. Finish (HASL, ENIG)
One must finish rigid flex PCB before using it. The finish affects the overall properties of a rigid flex PCB. For example, a manufacturer should finish a flexible flex circuit board with a transparent layer to allow the transmitting of light. The finish affects the cost of the circuit board because they must make it from multiple layers.
The finishing process includes:
- Etching: Etching is one of the common methods used in most circuit boards.
- Electroless Plating: We use this process in rigid flex circuit boards. It refers to electroplating without the use of an electrical current.
- Decoupling: Decoupling refers to the process of rubbing (disrupting) and applying a coating onto a circuit board’s surfaces.
- Encapsulation: Encapsulation of circuit boards refers to the process of sealing them inside a solid flexible coating medium for protection.
- Immersion Gold: Immersion Gold refers to plating using very thin layers of gold over a circuit board’s surfaces.
We need more materials to achieve a more transparent finish starting from the top and down. The material used will depend on the above processes. One can apply less process on the rigid flex PCB to reduce cost. Less finishing processes will reduce flex PCB cost since we need fewer materials to produce these PCBs.
The surface treatment involves unique features that include:
HASL:
It is the most used surface treatment for rigid flex circuit boards. HASL stands for “Hot Air Solder Leveling.” The process is simple in that we heat the PCBs and then apply an organic acid (phosphorous acid) and solder to the surface of the rigid flex circuit board. We also coat the top side with a lacquer to stay shiny and smooth.
LFHASL:
LFHASL stands for “Low-Frequency HASL.” The process involves applying phosphorus acid and organic acid to the PCB. First, we spray it with solder paste and then heated it to create a smooth surface on the top part. Then we wash off the surface of the PCB with water, dried, and then coat it with lacquer.
OSP:
We use this process in rigid flex circuit boards. It’s a process that separates and removes the copper traces from the surface of a rigid flex PCB. It is also called the “low-k” process in the industry.
Encap:
We submerge the circuit board in a liquid that allows the coating to wrap or protect the film from airborne contaminants, rain, snow, sleet, and ice.
IMM Ag:
This is the process of plating the circuit board with a super thin layer of gold over the entire surface.
SAP:
SAP stands for “Surface Attached Polyimide.” It is also called “Conductive Polyimide” by some manufacturers. We use the polyimide layer as a heat sink to absorb the heat from circuits and manage the excessive temperature.
This process is also called “Enamel Solderable.” We plate the rigid flex PCB with a thin tin layer over all the circuit board surfaces. Then it’s coated with a conductive lacquer
ENIG:
ENIG stands for “Electroless Nickel Immersion Gold.” Manufacturers plate most rigid flex circuit boards with ENIG finish. This type of surface treatment is very common and cost-effective.
ENEPIG:
This is a variation of ENIG, where one does the entire process in the air. This process helps prevent the formation of copper oxides at the surface and improves solderability.
Other surface treatments include Sn-Pb, silver immersion, immersion tin, and tin/lead.
Elec Au:
This process involves electrophoresis using gold as the plating material.
Lead:
While we do not use this process in rigid flex circuit boards, it’s still good to include it here since it’s another common surface treatment for PCBs. In addition, lead is usually used in high-reliability circuit boards and requires high-quality, low impedance connections.
5. Size of Hole
The holes in a rigid flex PCB should be big enough to allow components and other parts to pass through. The hole size will affect the manufacturing cost since we do some processes manually. The overall quality of a rigid flex PCB will not influence depending on the size of holes. However, when manufacturing many PCBs, then larger hole makes it easier to assemble them.
The size of the hole depends on:
– Hole Drilling: We can use Circular or Rectangular holes. The hole drilling process uses a drill bit to drill holes in the PCB.
– Laser Cutting: Laser cutting is another method used to cut holes in rigid flex circuit boards. We use it when making a small number of PCBs.
Punching holes in rigid flex boards refers to punching holes out using a mechanical tool like a press or punch. Punching holes can be circular, rectangular, or square.
– Holes made manually: Making holes manually is usually the most common method for rigid flex PCBs. It is a practice that one can easily learn, and it’s not difficult to learn about it.
The manufacturers of rigid flex PCBs need a certain amount of time to make holes in them and make sure that you cut them precisely.
6. Minimum Trace and Space
Trace and space of a rigid flex PCB refer to the distance between each copper layer and the size of the copper lines. The space is the distance between the layers, while a trace is the width of the copper lines. Trace width is usually 90% of the available signal width to prevent ghosting effects. Also, if the trace width is 90% of signal width, then it means that you will waste 8% of the wire.
Trace and Space are:
- Trace Width: Trace width depends on the size of the signal and its distance from each other.
- Signal Width: Signal Width refers to the minimum trace width used for signal routing.
- Minimum Tracespace refers to how much space we can define between two traces to ensure signal continuity.
- Clearance: Clearance refers to the physical space required between two tracks.
7. Aspect Ratio and The Thickness of the PCB
The aspect ratio of rigid flex PCBs is between PCB length and PCB width. Therefore, we can consider it the length divided by the width of a rigid flex circuit board. The Aspect Ratio depends on the following factors:
- Board Thickness: The thickness of the rigid flex PCB should ensure that the circuit board’s overall rigidity is sufficient. We can measure the thickness using a gauge or using a ruler. Also, thick rigid flex PCBs are more stable since they maintain their shape much better than thinner ones.
- Board Thickness’s Effect on Quality: One must consider a few factors while choosing the right size for rigid flex PCBs. They are mainly the manufacturing cost, process, and final quality. The board thickness can convert the signal from high to low signal quality signals. Also, we can use it to reduce the overall distortion in the circuit board.
The standard thickness should be around 3.2 mils to 3.4 mils. However, it is essential to consider the manufacturing costs. Additionally, we should consider the production tools. This is because smaller thicknesses are helpful when they are more economical or cheap flexible PCB than large thicknesses. Also, most rigid flex PCB manufacturers can supply smaller rigid flex PCBs. They can do this if they produce them in a different process, which will cost more than those made in standard processes.
8. Unique or Custom Specifications
Manufacturers can design the rigid flex PCB to fulfill many different applications. However, some components are essential in most cases. Some of these components include:
- Connectors: We can find these elements on most PCBs to connect them to other PCBs or the outside world via the pins and feet.
- Voltage Divider: This electrical circuit element is essential to reduce voltage using a ratio between resistances while keeping the same current flow through both resistances.
- Power, Ground, and Signal (PGS): These components are essential in some applications. They provide power, ground, and signal connectivity to PCBs.
- Inductors: Inductors are essential to control current flow in a circuit.
- Capacitors: These act as capacitors to reduce or eliminate ripple current. At the same time, they store energy that we lose during high-frequency switching operations. A good example is solid-state power supplies or active interference canceling transmitters.
- Resistors: Resistors are essential to lower the voltage applied to a component or provide a voltage divider.
- Varistors: These elements are often used in high-voltage or high-power applications to protect sensitive equipment.
The above components are commonly used in rigid flex PCBs, although it is possible to use other ones. The selection of components depends on the output of the design phase, which will depend on the requirement given by the customer.
How to ensure you get cheap flex PCB
These types of boards are the most cost-effective for IC manufacturers. In addition, these boards are cheaper to make, allowing them to pass through less quality control processes. One can apply numerous standards on a flex PCB and rigid flex PCB to make them more cost-effective and easier to manufacture.
1. Use of Various Aspects on Top of Each Other
A common method to save on flexible PCB costs is to place the various parts mostly used on many PCBs on top of each other. This will allow for more space which means a more workable area for testing and other processes. In addition, these flexible PCBs are easier to manufacture and cheaper than their rigid counterparts.
2. Smaller Sizes to Save on Materials
We can make Rigid and Flex PCBs into small sizes. This means that we reduce the amount of material needed by a significant amount. The use of smaller-sized boards reduces the amount of copper laminates needed. Therefore, if we produce them in smaller sizes, they will cost less.
3. Considerations on Mounting
Most Flex and Rigid PCBs are essential for their functionalities and appearance or aesthetic value. According to their requirements, they might use different types of connectors, feet, or a display of some kind that we can incorporate in them. This will affect the rigid flex PCB cost factor and the overall cost savings obtained from these flexible PCBs compared to their rigid counterparts.
4. Fewer Parts During Mass Manufacturing Runs
Although it is not possible to save on the parts used for PCBs during mass manufacturing runs, there are some options that one can try. For example, using single-sided boards rather than double-sided ones will improve the yield factor. There will be fewer parts since double-sided boards require more copper and more time to handle.
5. Simplified Assembly Run Requirements
The PCB assembly procedure will depend on the type of rigidity and flexibility that the PCBs have. There is a significant difference between flexible and rigid PCBs, especially in the soldering process. The flexible covers one’s whole work area, while the rigid end covers only a limited portion of the board. It means we need to assemble more boards before soldering them together. This will influence the cost factor and the requirements that vary concerning soldering.
6. Longer Lasting Printed Circuit Boards
To avoid any damage to the PCBs, we apply a protective coating to the boards, which will make them last longer. However, these coats also increase the costs of making and using these PCBs. Therefore, we can provide them without these protective coats when we make them. So, they will only cost a small amount. Additionally, they will not last long and can only use them in cases where they do not need usage for an extended period.
7. Reduction of Human Errors
One of the most critical factors influencing manufacturing costs is human errors. The PCB manufacturers will have to ensure no damage from human errors. There are different cases in which one can make a mistake, increasing the cost of making and using these PCBs. More strict supervision of their processes should decrease the cost of producing and costing for flexible PCBs.
8. Less Testing Costs
During the manufacturing process, manufacturers will make most of the progress by the workforce. It depends on their skills. Therefore, these companies will want to ensure that they avoid mistakes during this process for flexible PCBs because most of the mentioned costs were human errors.
9. Use rigid board laminates to achieve overall thickness
It is not easy to achieve the thickness of a PCB that you are looking for. There are rigid board laminates that we can use to do this. These laminates allow for the creation of thicker boards without any problem. At the same time, they remain flexible and will be able to handle most of the bending and twisting requirements when they use them in their normal applications.
10. Limit controlled impedance requirements
Companies make rigid PCBs with a limited range of impedance requirements required for a PCB to meet. To ensure that we will meet these requirements regardless of the environment and surroundings, different types of circuit models can help calculate this.
11. Specialized processing
Many companies produce different types of PCBs. To ensure that they will be able to produce the same quality of PCB each time once they have achieved a certain level of experience, there are intelligent processes that these companies exercise.
12. Smart characteristics
If you look at the rigid PCBs, you will find out that you cannot flex most of them more than 1mm due to the limitations of the process used in their creation and capabilities.
Conclusion
Companies have been using flexible PCBs in many different applications. They have proven to be a good option for the manufacturing of PCBs. They are less expensive, smaller, easier to use, and more flexible.
However, with the advancement of technology, these new flexible PCBs are becoming more complex due to imposing much wider requirements. The most common ones are the REACH directive, EMC requirement, and the new standards.
To achieve these new changes in the requirements, we require specialized testing and modification of the PCBs. You should not forget these costs when mass-produced since they will directly influence the overall flexible PCB price.
However, all these improvements have their advantages and disadvantages, which one should consider if you want to minimize your costs and maximize your savings.
