As the name suggests, flexible PCBs must have one property, which is being bendable. But at the same time, they should provide maximum functionality and reliability.
4-layer flex PCBs offer a blend of these three because they are not very thick but still have enough circuit density to run reliable circuits because they enable things like controlled-impedance routing. Therefore, they can be used in medical, automotive, and power electronics applications.
Here’s all you need to know about 4-layer flex PCBs, including their structure, design considerations, and manufacturing process.
Table of Contents
- What Are the Basics of Flex PCBs?
- What Is the Structure of a 4-Layer Flex PCB?
- Materials Used to Make 4-Layer Flexible PCBs
- What Are the Design Considerations of 4-Layer PCBs?
- What Is the Manufacturing Process of 4-Layer Flex PCBs?
- Advantages of 4-Layer Flex PCBs
- 4 Layer Flex PCB Applications
- What Are the Future Trends and Innovations in Flex PCB Technology?
- Why Choose a 4-Layer Flex PCB Manufacturer Like FlexiPCB?
- FAQs About 4-Layer Flex PCBs
What Are the Basics of Flex PCBs?
The primary factor that defines a flexible PCB is its use of a flexible substrate or core material, such as polyimide. Of course, the other layers, like the copper, must be bendable as well to maintain the desired properties of this board, but its primary defining factor is the flexible core.
These flex boards come in three primary types, which are single-layer, double-layer, and multilayer. This number refers to the copper layer count (excluding ground planes), so 4-layer flex PCBs fall in the multilayer category, which is more sophisticated and challenging to build due to the multiple-layer layout.
Single-layer vs. double-layer vs. multilayer PCB stacks
Generally, flex PCBs are suitable for making tiny or movable electronic parts, such as robotic arms, hearing aids, smartwatches, pacemakers, and smartphones.
4 Layer Vs. 2 Layer Flexible PCB
Here’s a comparison between the 4 Layer and 2 Layer flexible PCB.
Number of Layers
As far as PCBs go, layers (typically copper) are critical.
A circuit board may require a certain number of these layers to function effectively.
For instance, if a circuit board requires more than two layers to perform its function, the designer can double the layers to make it a 4 Layer PCB or even a 6 Layer.
Complexity
Usually, the complexity of a circuit board is the primary determining factor of how many layers a printed circuit board should have.
For example, you may consider yourself an expert or advanced.
So, likely, you’ll require complex circuits that feature RF circuitry, long digital buses, a switch-mode power supply, and various other sophisticated connections and components.
In that case, it’s strongly recommended that you use a 4-layer flex PCB.
Using the 2 Layer would only lead to additional interference and radiation emission.
The other thing is that it would also compromise the signal integrity.
On the other hand, if you want a simple circuit, then the 2-layer flex PCB will suffice.
Density
The 2 and 4-layer printed circuit boards provide the same mounting space.
That’s because you can mount the components on either side.
That means if circuit density is your only concern, then any one of these two PCBs will work just fine.
Cost
With the 4-layer PCB features that guarantee increased functionality, the cost can be three times higher than that of the 2-layer PCB.
This is quite a significant difference, particularly if it’s a DIY project.
Therefore, if budget is one of your primary concerns, you should go with the 2 Layer unless your board needs complex circuitry or capability.
What Is the Structure of a 4-Layer Flex PCB?
The structure of a 4-layer flex PCB can have three different layouts.
Symmetrical Arrangement
This layout has the following arrangement.
- Coverlay
- Signal layer (1)
- Ground plane
- Signal layer (2)
- Flexible substrate/core dielectric
- Signal layer (3)
- Ground plane
- Signal layer (4)
- Coverlay
Such a symmetrical arrangement has similar layouts on either side of the substrate, and it provides good EMI (crosstalk) control due to the ground plane shielding between the signal layers. However, the outer layers can be affected by external layers.
The stackup of 4-layer and 6-layer PCBs
Asymmetrical Arrangement
Asymmetrical 4-layer flex PCBs have this arrangement.
- Coverlay
- Signal layer (1)
- Ground plane
- Power layer (2)
- Flex core
- Signal layer (3)
- Ground plane
- Signal layer (4)
- Coverlay
Such a layout provides maximum isolation between the signals for effective EMI control.
High-Speed Arrangement
Fast, high-speed signal transmission that requires impedance control uses the two outer signal layers in this arrangement.
- Coverlay
- Signal layer (1)
- Ground plane
- Signal layer (2)
- Flexible substrate
- Signal layer (3)
- Signal layer (4)
- Ground plane
- Coverlay
The ground planes provide constant reference planes, with the outer one (number 8) shielding the two inner signal layers from external noise. However, it is not effective at suppressing internal noise (crosstalk).
Materials used in each layer include polyimide for the coverlay and substrate/core, electroplated or annealed copper for the signal, power, and ground plane layers, and adhesives for insulating the copper layers, as well as to join the substrate to the signal/power/ground layers.
Materials Used to Make 4-Layer Flexible PCBs
These materials fall under the following three categories:
Adhesives
The role of the adhesive is to form a bond.
See, if the base material happens to be polyamide, it can cause some issues as far as reliability goes.
Thus, in his case, the adhesive minimizes the effect of the limitations of the polyimide base.
Base Material
Granted, most conventional PCBs comprise a metal or fiberglass base.
However, the flex versions comprise a flexible polymer using polyimide film as a substrate.
Now, this material does not soften when heated.
Rather, you’ll notice that it stays flexible even after thermosetting.
On the other hand, other thermosetting resins tend to turn rigid after exposure to heat.
And this makes PI a superior material for flex PCB construction.
That said, the conventional PI film does not have optimal resistance to elements like tears and humidity.
For this reason, choosing an upgraded PI would be an effective way of eliminating such problems.
Conductive Material
Manufacturers tend to prefer rolled annealed copper foil to regular electrodeposited copper foil.
See, this conductive material makes flex PCBs with layers ideal for industrial control equipment or other applications in the sector.
Also, it has great thickness and electrical properties, plus it’s affordable.
What Are the Design Considerations of 4-Layer PCBs?
When designing 4-layer flex PCBs, it is important to consider these design guidelines.
- Isolate the critical signal lines using ground planes to reduce crosstalk
- For high-speed signals, design impedance-matched traces with enough shielding
- Remember to model the impedance and signal transmission performance while considering the bending conditions
- Broaden the trace width/dimensions to handle flexing without breaking
- Use thermal reliefs on solder joints to reduce stress and strain relief structures if interfacing these boards with rigid PCBs
- Rigid stiffeners are important for connection points
- Increase the component distance from high-flex zones
- Instead of arbitrary flexing, define and specify the bend radius based on the layer count
- Consider the shielding, abrasion resistance, and insulation requirements in the stackup/layout
- Review the layout for acid trap susceptibility during manufacturing (etching)
A rigid-flex PCB (note the inner flexible PCB with 4 copper layers)
What Is the Manufacturing Process of 4-Layer Flex PCBs?
The 4-layer flex PCB manufacturing process follows these steps.
- Flex PCB laminate fabrication
- Flex PCB laminate cutting
- Drilling
- VCP electroplating
- Circuit pattern transfer using photolithography or LDI
- Etching
- Positioning hole drilling
- Electrical testing
- Cleaning
- Layer alignment
- Layer lamination
- Silkscreen printing
- Baking
- Surface finish application
- Finished board electrical testing
- Visual inspection
- Packaging and warehousing
Some of the key technologies involved in this process include:
- Photolithography: This process falls under circuit patterning, and it involves exposing the flex PCB laminate to UV light after photoresist application and photomask layering above the board. The UV light hardens the exposed areas, leaving the PCB ready for chemical etching.
- Laser Direct Imaging: LDI is an alternative to photolithography. It is faster and more precise because it prints the circuit pattern directly on the photoresist without needing a photomask to harden the areas that should not be etched.
- Etching: After photolithography or LDI, the flex PCB is dipped into a chemical etchant (acid), which eats away the non-hardened photoresist and copper underneath it to leave the circuit pattern.
- Lamination: After processing and bonding the polyimide and adhesive layers, they must be laminated to the copper layers to create the final flex PCB. First, these layers must be aligned, and then they go into a lamination machine for bonding at high temperatures.
Advantages of 4-Layer Flex PCBs
The following are some advantages of featuring a 4 Layer flex PCB in your design:
- First, use this circuit board with any connector or component you want. That’s because the compatibility of these boards is quite vast.
- Secondly, the 4 Layer provides higher circuit density, which enables the board to adhere to dynamic flexing requirements.
- Also, the stability and reliability of these PCBs are far greater than their rigid multi-layer counterparts. And the main reason for this is that they possess fewer interconnects or solder joints. In turn, this helps reduce errors that would otherwise occur.
- Besides its improved impedance control, signal quality, and robust operating capabilities, the 4 Layer flex PCB has the edge over other options in the market.
- Further, these types of PCBs have a wider temperature range, which allows them to operate under environments of higher currents. This also helps to lengthen the lifespan of an electronic device.
4 Layer Flex PCB Applications
First, 4-layer flex PCBs tend to be lightweight and have a low bend radius, making them very suitable for medical devices and various control equipment.
For example, they’re applied in interior controlling equipment such as the dashboard and ABS systems in automobiles.
Also, these PCBs serve as one of the top-ranking utilities for the consumer electronics industry.
Thus, you’ll find them in several devices like cameras, calculators, mobile devices, and keyboards.
Caption: 4 Layer flexible circuit and an electronic surface
Secondly, 4-layer flexible PCBs mostly get applied in high-power environments.
Also, they’re thin and stiff, and all of these qualities make them a perfect fit for most industrial applications.
For example, engineers use them a lot for marine and aeronautical equipment.
Another example where 4 Layer Flex PCBs get used is in the medical industry.
Here, they’re employed in various functions, like serving as interconnects with resistors, capacitors, and circuits.
What Are the Future Trends and Innovations in Flex PCB Technology?
Development of Rigid-Flex PCB Hybrids
As the name suggests, rigid-flex PCB hybrids combine rigid and flexible sections to form a hybrid that has the benefits of both worlds.
These include higher component densities than flex PCBs, flexibility/packaging freedom in compact and bendable electronics, and reduced interconnects.
Flexible vs. Rigid-Flex PCBs
HDI Flexible PCB Development
High-density interconnect PCBs are more efficient and reliable because their designs and layouts are highly optimized for better electrical performance.
They also feature reduced package sizes because they have thinner substrates.
Flexible PCB Miniaturization
The demand for compact, high-speed, and low-cost electronics in computers, communication equipment, and consumer electronics segments is on the rise, and innovative flex PCB miniaturization techniques are being developed to cater to these.
This will make it possible to fit high component densities that run at high speeds in compact, flexible boards.
Development of Embedded Component Flexible PCB
With the push for flex PCB miniaturization, one of the enablers of this tech will be component embedment in the PCB. This will allow designers to fit more components in a tiny space.
Multilayer Flexible PCB Manufacturing
4-layer flexible PCBs fall in the multilayer category and provide the perfect balance between flexibility and manufacturing. The more layers the board has, the less flexible it becomes. But, this will change with time as manufacturers develop better materials and construction techniques to make flex PCBs that match the performance of their rigid counterparts.
Sustainable Manufacturing
Emerging sustainable flex PCB manufacturing processes and eco-friendly materials are bound to make these boards more environmentally friendly.
Why Choose a 4-Layer Flex PCB Manufacturer Like FlexiPCB?
4-layer flex PCB manufacturing is a delicate process, so you need a manufacturer who can:
- Etch fine lines, with the trace spacings and widths going as low as 25μm
- Achieve high alignment accuracy of +/- 50μm between the flexible layers
- Bond the multiple layers and attain a 2.5N/mm interlayer peel strength without delamination
- Laser drill microvias (blind and buried) with 1:1 capture pads
- Make 1-mil rings on the buried microvias (annular ring control)
- Apply a uniform thickness surface finish, such as immersion silver, with the variance being less than 10% along flexible contours.
- Achieve tight impedance control to within 5% tolerance of the target value
- Apply overlay/coverlay or a flexible solder mask to optimize flexing (such as LPI)
- Define and characterize the bend radius precisely according to extensive test data
- Monitor and control the manufacturing process using real-time statistical data
- Reliably test the flex PCB for bending/twisting, extreme temperature cycling, drops, and vibrations
At FlexiPCB, we have expert engineers and technicians who can handle and monitor this fabrication process to achieve all these, as well as the equipment to provide the required precision when building these 4-layer flex PCBs.
On top of that, we adhere to the required industry standards, most notably IPC.
We can even extend our expertise and adherence to standards to help you perfect your 4-layer flex PCB design, as well as assemble the fabricated board in-house.
We do all these at reasonable costs, so contact us to get started or to learn more about our 4-layer flex PCB manufacturing services.
FAQs About 4-Layer Flex PCBs
What Are the Typical Costs Associated With Manufacturing a 4-Layer Flex PCB?
The typical costs associated with 4-layer flex PCB manufacturing are related to the order volume, drilling requirements, turnaround time, and inclusion of rigid sections.
Since the number of layers is fixed, you can adjust these other variables to reduce the costs of 4-layer flex PCBs.
Typically, this cost can vary from $150–$300 per square inch, depending on the order quantity.
Can 4-Layer Flex PCBs Be Used in High-Temperature Environments?
Yes, 4-Layer Flex PCBs can be used in high-temperature environments. It all depends on the material used to make the substrate.
For instance, polyimide (the most common in Flex circuits) can withstand up to 300°C.
PEEK (Polyether Ether Ketone) and TPU (Thermoplastic Polyurethane) can also withstand high temperatures because they are thermally stable at temperatures ranging from 250°C–300°C.
In comparison, FR4(the most common in rigid circuits) has a Tg of 130°C.
How Do 4-Layer Flex PCBs Compare in Cost to Rigid PCBs?
Rigid PCBs are cheaper to produce, costing as much as 10x less than Flex PCBs.
This is because rigid PCB materials are more affordable, and their manufacturing process is less complicated.
Typical 4-layer rigid boards can cost $30 per square inch for low-volume orders. But 4-layer flex PCBs can cost $150–$300 per square inch.
How Does the Performance of a 4-Layer Flex PCB Compare to a Rigid-Flex PCB?
The rigid part in rigid-flex PCBs provides durability and stability, which makes it possible to have higher-density interconnects and high component densities in those sections.
This is a significant advantage performance-wise over their flexible counterparts but with limitations in space consumption and flexibility.
So, if your primary issue is performance, 4-layer rigid-flex PCBs make more sense. But if you’re dealing with limited space, 4-layer flex PCBs are the best option.
