Flex circuit assembly is a delicate process that assemblers continue to optimize to make these boards cheaper and easier for electronics manufacturers to adopt in the long run. This assembly has come a long way in terms of cost because it has dropped significantly over the years, making it possible to build miniaturized and ergonomic electronics for various use cases cheaply.
In this article, we’ll look at what flex circuit assembly is all about, including the commonly used materials, key techniques, and how the process differs from traditional rigid circuit assembly.
What Is Flex Circuit Assembly?
Flex circuit assembly is the process of placing and soldering components on a flexible PCB to form a PCBA for the final electronic product. This process presents unique challenges from the regular rigid PCB assembly process due to the flexible nature of these boards. You have to handle them with care and secure each board to avoid tearing the thin layers.
What Materials Are Commonly Used in Flex Circuit Assembly?
You would think that materials aren’t important during flexible PCB component assembly because the board has already been fabricated. But this isn’t true. These boards require specialized carrier materials to ensure accurate PCB and pad alignment during SMT component placement and soldering.
The carrier materials can include:
- Aluminum plates
- Synthetic stone
- Magnetized high-temperature-resistant steel plates
- Silicone sheets
Each specialized carrier and positioning template must be based on the board’s design (CAD) files and machined carefully to form a flat surface during placement and solder paste printing because the board’s features can vary. Overall, these carriers must be made using lightweight and high-strength materials that are resistant to deformation and heat absorption (after multiple soldering cycles).
Some flex PCBs also need stiffeners to provide support and prevent warping on sections of the flexible board. These are usually made of FR4, which can be peeled off after assembly if temporary.
How Is Flex Circuit Assembly Different From Traditional Circuit Assembly?
Besides using a supporting material, flex circuit assembly differs from traditional circuit assembly primarily in pre-baking.
Flex PCBs are normally not vacuum sealed during storage or delivery, and since they absorb moisture, they must be preheated before assembly to eliminate it. Otherwise, the layers will delaminate during reflow soldering due to the expanding water (steam) inside. Moisture can also cause blistering.
However, these issues occur during reflow, which bonds the components at high temperatures. So, pre-baking at high temperatures won’t be any different. The key here is to heat the board at relatively low temperatures of 80–100°C for a long time (4–8 hours) to get the water out gradually without causing damage.
Also, avoid stacking too many flexible boards together during this process, and inspect the boards afterward to ensure they are warp-free, free from discoloration, and don’t have any other defects before proceeding to assembly.
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 flex circuit assemblers must contend with is heat sensitivity. Because of the flex circuits’ much thinner size, the heat applied cannot dissipate during 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.
What Are the Key Techniques in Flex Circuit Assembly?
Connection Methods
Connection basically refers to fixing the flexible PCB to the carrier, which can have spring-loaded positioning pins. If it doesn’t have them, you’ll need a separate positioning template for alignment and fixation. This complicates the process because you’ll need adhesive tape (single-sided or double-sided) to secure the connection. The tape should provide moderate sticking strength to avoid flexible board tearing after soldering or leaving residue on the surface.
Solder Paste Printing
The solder paste used in assembling flexible circuit boards should provide superb thixotropy for easy application and release. As for the squeegee, the printer should have a polyurethane squeegee (with an 80–90° hardness) instead of a metal one to handle the uneven flex PCB surface. The printer should also have an optical alignment system for quality control in the printing process.
Pick-and-Place Technology
The speed of the pick-and-place machine can vary depending on the required placement efficiency and component count. Like rigid boards, the flex PCB should have mark points to help with placement, but the pick-and-place machine’s adjustable parameters should be set to match this task to avoid component damage or misalignment due to the board and carrier’s flexible nature. So besides speed, other parameters to adjust including nozzle height and air pressure.
Soldering
Flex circuit boards rarely have thru-hole components, so the primary soldering technique used is reflow. The recommended oven is the forced hot air convection infrared type because it distributes heat evenly, which reduces soldering defects, such as bridging and voids.
However, we recommend doing precise temperature profiling to match the thermal properties of the components and the carrier material.
But the general rule of thumb is to use RSS (Ramp-Soak-Spike), which reduces thermal shock.
Simply set the oven’s temperature to the lower end of the specification provided for the solder paste you’re using and use the lowest airflow settings.
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 are 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
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.
How Can You Ensure Quality in Flex Circuit Assembly?
Quality assurance is fundamental in flex circuit assembly because these boards are delicate to deal with, meaning anything can go wrong during assembly. So they must be tested to ensure they leave the factory at the required quality. Similarly, flex boards must be tested to ensure they will survive in their application area.
Some important tests to consider include:
- Tension testing
- Button strike life testing
- Bend testing
- Hand sweat testing
- Solder joint pull strength testing
- Bridge testing (measures capacitance, resistance, and conductance)
- Environmental protection testing
- Fuse testing
- Metallographic Microscope Inspection (MMI)
AOI won’t work in inspecting flex circuits because they have poor dimensional stability, so use MMI.
Besides these internal quality control processes, you should consider industry standards like IPC to make the board as highly functional and reliable as possible.
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%.
Why Choose FlexiPCB As Your Flex Circuit Manufacturer?
When outsourcing flexible PCB manufacturing, consider these factors for the evaluation process.
- Quality and adherence to industry standards
- Manufacturing capabilities
- Production lead times
- Cost/pricing
- Experience (reputation)
- Communication/customer service
At FlexiPCB, we have an expert team of engineers who are up to date with the latest IPC certifications.
Combined with our ISO-certified facilities, you can expect the best flex PCB manufacturing and assembly production quality.
This team can help you to develop your product’s concept, optimize the circuit design, select the materials, design the stack, do DFM, signal, mechanical, thermal, and power integrity analysis, and assemble the board to ensure your flexible circuit meets all our internal and required quality standards from the start.
Our turnkey solution is beneficial to our customers because we streamline the entire production process from design to assembly to ensure a short lead time without compromising on quality.
Part of this turnkey solution includes packaging, shipping, and after-sales support, so we’ll be with you even after deploying the flexible boards in your products.
And we provide these services whether you want a quick-turn prototype, low-volume, medium-volume, or high-volume order.
Interested in flex circuit PCB assembly? Contact us for inquiries or further details about our flex circuit solutions.
