Flex Circuit Assembly

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Oakley Mae

About Flex Circuit Assembly, Before the early 2010s, flex circuit assembly was rather expensive in terms of both person-hours and materials needed to complete a design.

As a result, it was not extensively adopted. In addition, the early flex circuit designs rarely functioned as well as their rigid counterparts.

But as the costs came down, flex circuit dominance in everyday products steadily rose. 

Table of Contents

What Is Flex Circuit Assembly?

It’s an assembly process to reduce size and weight while improving reliability.

And FCBs can achieve this by utilizing thin layers of polymer film that serve as a dielectric existing between conductive circuit patterns seamlessly etched into the layers of copper foil cladding. 

What Makes Flex Circuit Assembly Different?

Usually, devices with an on/off switch have a circuit board inside. Therefore, it’s common to see the traditional flat, rectangular circuit boards in laptops, computers, TVs, and other devices. 

But things are different with the new ergonomic product designs and the proliferation of wearable tech.

Today, engineers build circuit boards to fit in compact, three-dimensional spaces and still can resist mechanical issues like wear and vibrations.

Now, thanks to flex circuits, manufacturers can be more creative.

Further, because of the reduced assembly costs we’re seeing today, their presence in everyday items has significantly risen.

Flex Circuit Assembly Challenges

Great as they are, flex circuit boards present manufacturers with the following challenges: 

More Complex Technique

The standardized dimensions of rigid PCBs make it easier for assemblers to produce highly engineered conveyor belt-type processes ideal for various boards.

But unlike rigid boards, engineers assess each flex circuit individually.

For one, the pallet supporting the flex circuit has to fit a unique and flexible shape.

Otherwise, an air cushion could form underneath the circuit, leading to mechanical resistance during ongoing processes like the pick-and-place soldering paste screening.

Why? Because the air cushion serves as a trampoline and forces the components to start bouncing off the assembly instead of sticking to the circuit’s membrane.

But getting the pallet to sit flat on the surface is challenging. That’s because ultra-light flex circuits typically don’t sit flat on a pallet surface.

And as for the double-sided ones, assemblers have to design a pallet capable of accommodating top and bottom configurations of the circuit. 

Therefore, the entire process requires specialized tools and backup features that one can calibrate or alter to allow for unique and flexible circuit designs.

This tailored pallet geometry helps ensure the circuit remains flat regardless of thickness at different stages.

Heat Constraints

Another problem that flexes circuit assemblers must contend with is heat sensitivity. Because of the flex circuits’ much thinner size, the heat applied cannot dissipate during soldering. 

Image of flex circuit soldering

(Caption: Image of flex circuit soldering)

Therefore, soldering of flex circuits requires highly precise soldering tools.

In addition, because there’s a much lower margin for error, the technician must be very experienced to cut costs and optimize productivity.  

Flex Circuit Assembly Process

Prepare the PCB Material

To prepare the material, clean the CCL (Copper-Clad Laminates). Then cut the PI (Polyimide), prepreg, coverlay, and chemically-washed CCL into the correct sizes.

Create the Inner Core of the Flex Section

First, you need an internal flex board. Also, for a double-sided flex, you must generate the inner core of the flex to create the flex board.

But if it’s a single-sided flex circuit, wrap it in thin copper foil.

Create the Inner Circuits of the Flex

In this step, you want to leave a certain pattern of copper traces on the foil and eliminate any leftover copper compositions.

To do that, coat the copper foil with a curable photoresist, then use a non-transparent film to draw the PCB connections present on the copper foil. 

Next, expose it to UV light, causing the dry film to cover and protect the copper traces in the PCB circuit pattern, and using a chemical solution, wash the uncured photoresist.

Lastly, eliminate any exposed copper by dissolving it using some sodium hydroxide (Na0H) solution. 

Laminate the Flex Section

Usually, the lamination of PI layers depends on the number of flexible layers found in a flex design.

So, you can laminate alternating PI layers and copper foil on the flex inner core for more than two flexible layers.

Otherwise, you can proceed to the last step, generating the circuits. 

In this case, you’ll remove the copper traces in the PCB. Doing so involves electroplating the circuit region with copper and then with tin.

Next, remove the photoresist and wash away any copper outside the circuit area using NaOH solution.

Then remove the tin and drill holes on the plate using a laser before you laminate the coverlay on the flex layers.

Laminate the Rigid Layer

Engineers laminate the rigid layer with alternating prepreg layers tightened with copper foils if through-holes’re plating on the rigid-flex PCB.

Drill Holes

The next step is to drill holes in the vias and pads. For multi-layered plates, you can combine them to form several reels and drill them simultaneously.

But if you want ultra-small holes or have some HDI requirements, it’s best to employ laser drilling for enhanced accuracy. 

Through-Hole Plating

It is a technology manufacturer used to insert leads on the components into holes in the circuit board. So after drilling, you’ll insert copper into the holes. 

Testing

Engineers inspecting a flex circuit

(Caption: Engineers inspecting a flex circuit)

To test, apply a silkscreen surface finish and solder on the rigid-flex PCB, then treat the panel’s ½ edge holes and V-Cut. The main tests include the flying probe and nail-in-bed.

But for components intended for the medical, military, automotive, and aerospace industries, it’s necessary to do an additional four-terminal sensing test.

Flex Circuit Assembly Application

Flex-rigid circuits have numerous advantages, including reliability, low weight, and compact size. Also, it’s possible to build these boards to fit different devices precisely.

For this reason, manufacturers wanting to fit high-end technology into tiny spaces are turning to rigid-flex circuits. 

Flex circuits also allow for a much denser device population through the use of lighter conductors while still maintaining a relatively high level of flexibility. 

Some of the other areas of application include the following;

  • Medical wearables for tracking or identifying heart rates and breathing patterns.
  • Industrial applications such as RF devices and power distribution circuits
  • E-commerce applications related to tracking and scanning shipments. 

FAQs

What is Flex in Circuit?

Flexible electronic circuits, sometimes called flex circuits, are a type of technology used for assembling electronic circuits.

It works in different devices on flexible plastic substrates such as transparent conductive polyester film, PEEK (polyether ether ketone), or polyimide. 

How Do You Make a Flex Circuit?

Designers compose a flexible printed circuit out of metallic layer traces, usually made of copper bonded to a dielectric layer, such as polyimide.

What Is Flex PCB?

It’s a patterned arrangement consisting of printed circuits and different components that utilize flexible materials with or without a flexible coverlay.

What Does a Flexible Printed Circuit Do?

Unlike most rigid designs, the flex PCB comprises materials that can bend, providing the boards with improved resistance to movement and vibrations.

In addition, their miniaturized design means they can decrease package weight by as much as 75%.

Conclusion

Now you know the process involved in flex circuit assembly and the challenges and applications of the technology.

With the continued advancements in the supporting technology needed for creating FCBs, we can expect to see more state-of-the-art tools to increase the productivity of this assembly process.