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Non-Layered Rigid-Flex PCB

  • Non-Layered Rigid-Flex PCB
  • non-layered-flexible-pcb-2
  • non-layered-flexible-pcb-3

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OVERVIEW

Item Rigid-Flex PCB
Max Layer 36L
Inner Layer Min Trace/Space 3/3mil
Out Layer Min Trace/Space 3.5/4mil
Inner Layer Max Copper 6oz
Out Layer Max Copper 3oz
Min Mechanical Drilling 0.15mm
Min Laser Drilling 0.1mm
Aspect Ratio(Mechanical Drilling) 12:1
Aspect Ratio(Laser Drilling) 1:1
Press Fit Hole Ttolerance ±0.05mm
PTH Tolerance ±0.075mm
NPTH Tolerance ±0.15mm
Countersink Tolerance ±0.15mm
Board Thickness 0.4-3mm
Board Thickness Tolerance(<1.0mm) ±0.1mm
Board Thickness Tolerance(≥1.0mm) ±10%
Impedance Tolerance Single-Ended:±5Ω(≤50Ω),±10%(>50Ω)
Differential:±5Ω(≤50Ω),±10%(>50Ω)
Min Board Size 10*10mm
Max Board Size 22.5*30inch
Contour Tolerance ±0.1mm
Min BGA 7mil
Min SMT 7*10mil
Surface Treatment ENIG,Gold Finger,Immersion Silver,Immersion Tin,HASL(LF),OSP,ENEPIG,Flash Gold;Hard gold plating
Solder Mask Green,Black,Blue,Red,Matt Green
Min Solder Mask Clearance 1.5mil
Min Solder Mask Dam 3mil
Legend White,Black,Red,Yellow
Min Legend Width/Height 4/23mil
Strain Fillet Width 1.5±0.5mm
Bow & Twist 0.05%
Table of Contents
Primary Item (H2)

With rigidity and flexibility rolled into one, non-layered rigid-flex PCB offers the best of both worlds. You get a sturdy circuit that can bend and play different roles in different industries. This article will shed some more light on non-layered rigid-flex PCB, its features, benefits, applications as well as limitations.  

What is a Non-layered rigid-flex PCB?

It is a printed circuit board that has one conductor layer nestled between two polyimide layers. This implies that the circuit traces are etched directly onto a single layer of flexible insulation material. The copper layer in a non-layered rigid-flex PCB generates a circuit pattern for which it is designed.

Key Features of Non-layered Rigid-Flex PCB

Some distinguishing factors of the rigid-flex PCBs with no layers include the following three. 

  • Shielding: This is a way used extensively to put a limit on electrostatic interference. 
  • Controlled Impedance: With the need to develop PCBs with higher speed requirements, it's good that interference traces are constructed. These assist in lowering electrical resistance. It is also helpful in transitioning between interconnections and the track. 
  • Panelization: Many circuits are cut after fabrication. These circuits may be cut from edges, but they stay connected to the panel through small tabs. This is why they stay unmoved when component assembly is carried out and regardless of the method used: wave soldering or pick-and-place tabs. 

Benefits of non-layered rigid-flex PCB

This kind of circuit board offers many advantages over other PCBs. 

Saves space as well as time

Non-layered rigid-flex PCB does not require cable assemblies or connectors. This makes it cost-efficient and leaves out ample empty space on the circuit. Also, as there are only a few parts, which makes non-layered rigid-flex PCB lightweight and compact.  

Quick Assembly

Since only one assembly line is needed, the non-layered rigid-flex PCB is easy to build. The assembly is quick and easy as there are fewer components to combine. This is a significant advantage when compared to rigid PCBs. The latter demands much more time in assembly and production.  

Higher Reliability

Non-layered rigid-flex PCBs have exceptional reliability, even better than cable assemblies and connectors. While connectors can cause problems, non-layered rigid-flex PCBs offer fewer instances of poor connections and, hence, failure. These properties make them highly dependable in extreme operating conditions. 

Resistance to vibrations

Many industries need parts that can bear shock and vibrations when working effectively. This is where non-layered rigid-flex PCBs work well. The flexible part can handle these shocks easily without the connections breaking due to shock. This qualifies them for use in applications where shock and vibrations are unavoidable. 

Easy to test

Testing is an important procedure that helps confirm the PCB's efficacy and efficiency. The best part about non-layered rigid-flex PCBs is that, in the absence of connections and assemblies, testing is easier and faster.

Limitations of non-layered rigid-flex PCB

Low current carrying capacity

Since non-layered rigid-flex PCB has a single layer, it can only carry a limited amount of electricity, making it a limited option in some applications. 

Prone to damage

On the flexible substrate, there are some exposed traces. This makes this type of PCB more susceptible to damage from external elements, such as from the environment. 

Restrictions in routing

The single layer of traces is incapable of accommodating complex circuitry, which also differentiates it from the layered rigid-flex PCB. 

How to Assemble a Rigid Flex PCB Board?

Before proceeding, it is important to learn about the various materials to be used in a non-layered rigid-flex PCB. 

Materials to be Used

  • Cover/substrate: Polyimide film, liquid photoimageable coverlay, epoxy glass or polyimide glass  
  • Conductor: Copper
  • Adhesive: polyimide prepreg, Acrylic, epoxy, flame retardant acrylic, epoxy prepreg.
  • Stiffener: Epoxy-glass, polyimide, polyimide-glass, copper, and aluminum

Now let's go over the main steps:

Prepare the materials

Clean the copper film laminates first with chemicals and then cut them into appropriate sizes. Also, cut overlay, polyimide, stiffeners, and prepreg. 

Create the inner core of the flex part

Begin with an internal flex board and the generation of its inner core. For a non-layered rigid-flex PCB with a single flex layer, cover it with thin copper foil. For two or more layers, more copper coils are used.  

Make the inner core circuits for the flexible part

While creating this PCB, it is important to ensure that there are still some copper traces on the foil while removing the rest. Then cover the copper foil with a special material that hardens up when exposed to UV radiation. Then, place a film on its top to draw the desired connections. 

When UV light falls on it, the hardened material protects the copper traces. The remaining material is cleaned with a chemical solution. The copper exposed to sunlight is submerged in a sodium hydroxide solution. This leaves the copper circuit traces behind. 

Drilling holes

Use a laser beam to make holes in the board. This should be done before laminating the coverlay to complete the flex circuit.

Make the rigid layer

When the rigid-flex PCB has plated-through holes, a lamination technique that utilizes alternate copper foil and prepreg layers is used. This is followed by the creation of circuits and drill holes. Again, a laser is used for the purpose of determining if the PCB has HDI features. Last, as you finish laminating the alternate flex layers, you end up creating circuits on the rigid part.  

Cut extra prepreg

Remove the extra prepreg material just exterior to the flex region with a laser.

Test to verify

Finish the process with a surface finish, solder mask, and silkscreen. Carry this out before cleaning the V cut and holes. After this, two main tests are conducted - nail in bed and flying probe to confirm the operability of the PCB. 

This aside, some industries require different tests, such as four-terminal sensing tests for the military, medicine, and automotive sectors. 

Applications of non-layered rigid-flex PCB

Being more reliable, these PCBs have found application in several industries. Here are a few of them.

Aerospace and defense

These are lighter in weight and are also resistant to harsh external environments. This makes them suitable for use in military equipment such as radars, satellites, and also in avionics. 

Automotive industry

The dynamic capabilities of non-layered rigid-flex PCBs and space efficiency work perfectly for cars that need higher functionality with fewer electrical parts. This is perhaps the reason they are widely used in making advanced automotive sensors, control systems, and displays.

Consumer electronics

As already discussed, they are small in size and are flexible. That is why they are used to make wearable devices, smartwatches, smartphones, and VR/AR headsets.

Industrial automation

The rigid part can handle shock, and the flexible part makes the PCB bendable. These reasons are enough to make them work for industrial sensors and robotics. 

Medical devices

They are used in implantable medical devices, surgical instruments, and diagnostic tools due to their ability to miniaturize and integrate well.

How to Avoid Making Mistakes in Non-layered Rigid-Flex PCB

It is not easy to construct a rigid-flex PCB. You may say that the single layer is the easiest of all, yet it requires meticulous attention to detail so as not to lose sight of the final application.  

While constructing a non-layered rigid-flex PCB, the manufacturers do not have to deal with the electrical parts only. The mechanical aspect is as important as the electrical. If ignored, it can cost them heavily.

Here's what one can do to navigate the challenge:

Do not create holes in the parts that bend.

The vias and pads should never be placed on the bending areas. This will exert mechanical stress close to the bending line, and unfortunately, this might lead to structural alterations in the plated through holes. 

What to do: Place vias and pads in safe places. The best place is where they do not bend at any stage of their operation. Ideally, this is never suggested, so to be safe, use anchors. This will also make the plated holes sturdy. You can try your hands at a teardrop to attach the plated hole to the trace. Larger pads or vias are also a good idea. 

Make routing over the bending section carefully.

All the traces on the bending line should be in a straight line and at equal distances. To avoid making mistakes, add dummy traces. Doing so will help boost mechanical sturdiness and prevent traces from facing damage like breaking. 

Additionally, stay away from creating corners with traces right where the bending section is. Even if the traces have to move in another direction, opt for curved pathways instead of sharp corners. 

Implement a ground plane patterned with cross-hatching.

Do not fill the ground plane with solid copper, as this will make the area too rigid and less flexible. This will also put immense pressure on the area. A safe bet would be to try your hands at a ground area with a criss-cross pattern on the flex part of the PCB.

Or to combat this problem, use PCB design software. A well-built tool will ensure that you adhere to all the guidelines of the non-layered rigid-flex PCB design. Or use CAD. This helps create zones and connections appropriately, making it easy to design circuits that are both rigid and flexible.

The Future of Non-layered Rigid-Flex Technology

While the PCB technology has progressed really well in the past decade, don't expect it to get slower. With the introduction of newer and better technology every now and then, expect several surprises. Some of them comprise the following: 

More applications: It is expected that the flexibility and reliability of non-layered rigid-flex PCB will enable its use in more industries. Some of them include wearables and IoT. 

Reduction in manufacturing cost: The improvements in manufacturing techniques will continue. This means that we can expect the production cost to go down, making it more affordable. 

Development of sophisticated tools: Technology advancement is making strides. Future CAD tools with new and improved updates will allow the design process to be simpler and faster. Since prototyping will be quick, expect the entire procedure to finish quicker.  

3D integration: In the coming years, flex circuits may include curved 3D shapes. This might help develop interesting shapes in consumer goods, which may be guided appropriately by additive manufacturing. 

Better materials: The reliability of non-layered rigid-flex PCBs will get a boost from new materials that will make the PCB more heat-resistant and also stable. 

Combine all the above, and you can picture the future: dynamic, higher performance, and more applications. 

Parting Thoughts

All in all, non-layered rigid-flex PCB has several benefits and has thus found a place in different industries. Though they are not used as frequently as layered rigid-flex PCBs, they are still considered to be of great value and use. In case you are interested in learning more about the non-layered rigid-flex PCBs, contact our team.

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