Anyone who has used a gadget, such as a laptop, smartphone, or portable media player regularly over the past three decades or who has simply looked up has probably seen some striking aesthetic shifts in these products. One notable change is the diminutive size of many modern electronic gadgets.
With the help of innovations like flex PCB, designers now have more leeway to create products that are not only strong and long-lasting but also easy to assemble as part of a streamlined, highly efficient economy of scale. Let’s take a look at the design, applications, and benefits of flex PCBs.
Flex PCB Applications
- Utilization in Aerospace
As employed in aircraft, the heads-up display (HUD) is a well-known piece of technology that prevents pilots from taking their eyes off the road to view crucial operational data by projecting it directly into their field of vision. An emerging trend in wearable technology utilizes the HUD concept to project 3D holographic pictures from a distance onto a visor attached to a helmet.
Liquid Crystal on Silicon (LCOS) display technology is used in holographic waveguide helmet-mounted display (HWVD), which allows for high-resolution genuine 3D imagery using flex PCB cables. The HWVD’s minimal mass makes the installation of the display doable. The flex PCB cables’ dependability, flexibility, and performance make them effective in real-time use for avionics.
- Medical Relevance
A medical device manufacturer relies heavily on flex PCB designs for a new category of hearing aids, which include improved frequency response and fidelity compared to conventional hearing aids. A tiny micro-actuator and photoreceptor are implanted in the ear canal.
A microphone is placed outside the ear, and the sound is converted to digital signals by a digital signal processor (DSP) before being transmitted to the inner ear, just as in traditional hearing aids. The intriguing part, however, is that the digital impulses trigger a laser housed within the ear canal that excites the photoreceptor, converting the digital audio.
Engineers used flex PCB to install the DSP, microphone, and battery in a compressed pack that can fit behind the patient’s ear, allowing the laser to supply signal and power to the micro-actuator and passive photoreceptor.
The product harnesses the miniaturization facilitated by flex PCB design to convert sound into laser signals that operate a micro-actuator inside the ear, thereby turning the user’s eardrum into a speaker.
Flex PCB is used in the following:
- Pacemakers
- Electronic hearing aids
- Devices for heart diseases
- Activity trackers
- Electronic Marketing Consumption
Flex PCBs have too many possible uses and applications to enumerate in the consumer electronics market. However, there is a good probability that it contains flex PCB whether you carry it, wear it, or drive it.
Most people’s first mental image of a flex PCB is the connector between a laptop’s keyboard and display. However, in actuality, flex PCB designs are used to join the two parts of a flip phone. They have replaced ribbon connectors in mobile print heads in modern printers and have also improved the reliability and cost-effectiveness of disc drives.
Flex PCB is used in the following:
- Laptops
- Cameras
- Calculators
- Convertible mobile phones
- Keyboards for Computers
- Robotic arms in the production sector
- Machines for Processing Sensor Data
- Aspects of Bar Code Technology
- HMI gear
- Automotive Industry Usage
In the automotive industry, flex PCB designs have several uses because of the weight reductions they provide compared to a traditional PCB and wiring harness, as well as the normal dependability benefits. Lighter weight means better fuel economy.
- Flex PCBs significantly simplify the process of making an automobile’s wiring harness.
- Flex PCB designs incorporate numerous control units, sensors, and signals into a compact, durable device for use under the dashboard of a modern car.
- Flex PCB designs are lightweight and durable circuit boards that incorporate several sensors, control units, and communications hidden away under the dashboard of a modern car.
- Flex PCB shines in connecting the different components of an electronic system, whether for durability, economy, increased quality, or enhanced performance.
Flex PCB is used in the following ways:
- Air Bags
- Breathable Pads
- Systems with Global Positioning Satellites
- Mechanical Handling of the Motor
Dynamic Flex PCB Application Examples
Mechatronic Gantry
A mechatronic gantry is an example of a common dynamic flex application. It can be found in machines like CNC machine heads and 3D printers. Separate rigid boards would be used in physically bigger systems when electrical components must follow the same movement as a mechanical element. They would be linked together through cables. Flex ribbons are preferable for low-profile assembly and the necessary mobility in smaller, sleeker designs.
The advantage of flex PCB is that it allows us to make folds with a small radius for the installation process, provided you use the proper materials and carefully organize the whole assembly. In some cases, rather than using curved flex PCB, use a static flex portion with a fixed crease.
In light of the panel, the fold is immediately put to good use. This crease eliminates the need to incorporate a bend in the board design when making flex PCB. As an alternative, you can utilize a straight section of flex PCB to align a group of flex ribbons on a single panel. This approach results in a massive boost in yield. The lower overall cost per board is a result of the higher yield per panel and fewer manufacturing requirements for pick-and-place assembly. This advantage may be offset, however, if the fold at one end of the assembly necessitates the placement of components on the opposite side.
Assemblies often require motors to be installed off-axis from their shafts so that just the motor and its control printed circuit board rotate. To facilitate movement in both directions, the flex PCB is intended to be folded into a cylinder form and terminated at either end to a stationary base assembly.
Flexible Printed Circuit Design Best Practices
The following are the best practices for flexible printed design:
- Be Aware of Electrical Parameters
The electrical parameters of the system are:
- Present limits
- Voltages
- Modulations of Signals
- Discrepancies in capacitance
- Behaviors in the face of impedance
- Safety measures
- Details on the variety and placement of electronic parts and connections
- Make a Comprehensive Net Wiring Diagram and Listing
Create a schematic, which is a simplified diagram of your flex PCB’s layout and components. Consider your PCB’s pliability at this early stage. Remember that in contrast to a static board, which may only bend a few times, a dynamic board must be able to withstand repeated bending over the course of a lifetime.
- Design Your Flex PCB Using a Schematic Capture Tool
You can see how your board will look and function down to the individual components thanks to the efforts of the correct layout provider and principal engineers who create a layout on a software platform. Your layout will then be sent to a mechanical engineer who will load it and assess how it will fit in the final product. Consider flex PCB materials, which call for more lax manufacturing outline tolerances than standard boards.
- Plan the Stackup of Your Flex PCBs
Planning is crucial given the significance of impedance—the resistance to the flow of current in a conductor—during the early stages of flex PCB design. The stackup affects the EE and mechanical engineer’s ability to design the flex PCB and fit it into the device. Put the flexible layers in the middle of the stack to maximize their protective effects.
- Determine the Parameters for the Design
The electronics manufacturing industry sets the standards and acceptance criteria that must be met at this stage. Finding a flex PCB designing service that is well-versed in IPC standards is crucial if you want to keep your project on schedule and prevent substantial changes from being needed.
- Set up Your Parts
Component placement is typically discussed between the customer and PCB/layout provider during the design and layout phase. Some components may be prohibited from being placed in close proximity to one another due to electrical interference requirements. Every part will have a data sheet that the layout service will use to place the part in the mechanical layout for client review and approval.
- Create New Holes with a Drill
Specifically, the components and the link are what motivates this action. About half of flex PCB sold today are double-sided, meaning they link from the top to the bottom layer of a drilled hole.
- Put the Wires Properly
After the holes have been drilled and the components have been positioned, you may begin routing the traces.
- Identify and Label
The time to add any necessary IDs, labels, or marks has arrived. Component placement on a circuit board might be complicated, but reference designators can help.
Precautions to Take Before Assembling
There’s a lot to consider when you’re really putting together your flex PCB, but here are a few guidelines to help you along the way:
- Before subjecting the circuit to high temperatures, you should ensure all moisture has been eliminated.
- The flex PCB should be processed immediately after baking.
- Your circuits should be kept in a nitrogen chamber or a sealed dry box with desiccant.
Physical Constraints in Flexible Printed Circuit Design
It takes careful consideration of many details, from materials to vias, to create a flex circuit. Consider these factors as you plan your flex PCB layout:
- Bend Ratio
The term “bend ratio” describes the relationship between the bend radius and the flex circuit’s thickness. The risk of breaking while bending increases with decreasing bend radius.
- Electricity Transmission
It is the path that electricity takes as it travels from one location. It is also important to examine the conductor pattern to see if it will be modified by the bending. Every effort should be made to keep conductors as perpendicular to the surface of the bending area as practicable.
- Tear Relief
A relief slot and a big corner radius are two of the most prevalent methods for preventing tears in flex PCB.
- Protection and Flat Layers
Having a reference plane layer and shielding is crucial for impedance management and signal integrity. Solid copper, the standard form of shielding, should be factored into the thickness-to-bend-radius analysis since it increases circuit stiffness. The versatility of shielding techniques like cross-hatching and Silver ink can be increased by employing these alternate strategies.
The following are the best physical constraints in flexible printed circuit design:
- Multiple Flex Sub-Stacks
Although it is theoretically promising to construct any conceivable stack using rigid and flex sections, the cost can quickly skyrocket if the production methods and material qualities aren’t carefully considered. It’s crucial to remember that flex circuits experience strains in their materials due to the bending of the circuit. Repeated flex cycling at small radii will lead to fatigue cracks in copper because this non-ferrous metal work hardens under stress. Using only single-layer flex circuits can help with this problem, as the copper will be located in the middle of the median bend radius, putting the coverlay and the film substrate in the most tensed and compressed position.
- Adhesive Beads, Stiffeners, and Terminations
In some cases, you may want to use reinforcements at the point where for the rigid board. However, dispensing and curing these liquids can be time-consuming processes that drive up production costs.
You can employ automated fluid dispensing, but you need to work closely with the assembly engineers to avoid spilling glue or other fluids under the assembly. Manually applying the glue might increase production time and material costs.
If not the main rigid board assembly, flex circuits often end at a connection. Therefore, it’s most practical to leave the flex’s ends stowed away inside the rigid-flex pieces and put a stiffener on end.
- Rigid Flex PCB Panels
Components are connected to the rigid termination parts of the rigid flex circuit while it remains in its panel throughout the assembly process. Some products call for mounting components on flex PCB in addition to rigid areas. In this case, the manufacturer will assemble the panel with added stiff areas to hold the flex. Most of these sections are not glued to the flex PCB; therefore, they must be removed via routing and then punched out by hand.
- Do Keep Flex Flexible
The V-grooves on the stiff sides allow them to be detached easily. Assembling it into the housing will go more quickly.
Remember that flex circuits feature some seriously annoying material oddities. Inconsistencies such as copper’s work hardening and fatigue, the comparatively high z-axis extension coefficients of adhesives, and the lower adherence of copper to coverlay and PI substrate. Most of the negative effects can be mitigated by remembering a few simple rules.
- Don’t Bend at Corners, and Use Curved Traces
It’s important to determine how much give your design needs right off the bat, as well as whether or not that give needs to be repetitive or if it can get away with a static bend. If the manufacturer folds flex-circuit parts in your device during assembly and then leaves them in a rigid position, you have a lot more leeway. However, if the flex-circuit portions will constantly be bending, shifting, or rolling, you should decrease the overall layers for the sub-stack of flex PCB and select substrates without adhesives.
- Don’t Abruptly Change Widths
There is a fragile spot that can affect the copper. It might deteriorate over time anywhere a track enters a pad, especially in a flex-circuit terminator. The best approach is to taper off from above the pads unless a stiffener is applied, or there will be a crease lasting only one time near the trace width transition. Weak spots can be caused by trace width changes and pad entries.
- Do Add Support for Pads
The low adherence of copper to polyimide increases the likelihood that copper on a flex circuit will detach from the substrate during repetitive bending. Therefore, protecting the exposed copper is crucial. Because through-hole plating provides a reliable mechanical anchor from one flex layer to another, vias do not require any additional support. Many fabricators propose through-hole plating in addition to the standard plating used in rigid circuit boards. Unsupported pads, such as those used for surface mounting or that aren’t plated through require extra protection to stay put. Flex through-hole pads are supported by plating, anchoring stubs, and smaller coverlay access holes.
The coverlay mask apertures can be used to securely anchor pads on two sides. To do this while still providing adequate solder, the pads must be larger than those found on conventional rigid-board layouts. Since flex circuits are inherently less dense than rigid ones, this obviously affects the mounting density of components.
- Allow for Squeeze-Out
Laminating a coverlay over the copper and substrate will cause some adhesive to squeeze out any coverlay gaps surrounding the pads. Squeeze-out can only occur if the access opening and pad land are big enough to permit adhesive leakage while still exposing enough copper for a sturdy solder fillet. 360 degrees of solder wetting roughly over the hole is applicable to high-reliability designs, while 270 degrees is sufficient for intermediate flex designs. Cut holes in the pads and coverlay big enough for the glue to squeeze out.
- Double-Sided Flex Routing
Avoid placing traces over each other in the same direction for dynamic double-sided flex circuits. Alternate the placement of traces across levels to prevent overlap. When copper is placed uniformly between copper layers, strain stress on the traces is reduced. If the traces are overlapping, bending stress will be concentrated on one layer due to the layers pushing against each other. Staggering helps distribute the tension throughout the flex substrate, making it more consistent along the traces. Avoid using copper traces that cross over onto an adjacent layer. Instead, layers of traces should be staggered to lessen stress on the traces during bending.
- Do Use Hatched Polygons
Power and ground planes are sometimes required to be transported through a flex circuit. Solid copper can be used, but only if you don’t mind the metal becoming quite stiff and possibly buckling over sharp turns. Use hatching polygons whenever possible to keep your options open.
- Via Placement
Vias will need to be used as a layer transition for multi-layer flex zones. It can quickly tire from flexing motion and should be avoided if possible. Keep at least 20 mils of space between the rigid-to-flex board contact and the copper annulus of the nearest via.
If a flex circuit must have vias, the PCB editor’s design criteria can be set to restrict via a placement to only the rooms in which you are confident there will be no bends. Alternately, rigid parts can be defined using the layer stack manager.
- Defining Flex Cutouts and Corners
The precise terminal of a cutout is essential if it is to be placed in the board’s flex area. To avoid tearing the flex substrate materials at the corners, finish with circular sections with radii greater than 1.5 mm. The rule of thumb is to always employ a peripheral coiled corner with a radius of more than 1.5 mm for inside corners. A circular hole should be punched out of a corner that is less than ninety degrees.
Final Word
Looking for some assistance with your next flex PCB design? Hemeixin to assist you. We have been making flex PCBs for various businesses around the world for a while now, including nearly every major player in the technology sector. Get in touch with us right away if you have any questions about our services or would want to speak with one of our engineering professionals about your upcoming flex PCB project.
