For most of us, telling the difference between a flex board and a rigid-flex board design needs to be effortless. Well, it all is until it is not. So let us try this: enumerate all the differences you can think of – can you? I thought not.
However, understanding the distinction between flex and the rigid-flex board can make or break your PCB design. You must navigate all the intricacies and master the ruses involved if you want a functional and efficient PCB for your application. But first things first, what is a flex board design? What is a rigid-flex PCB design?
This article seeks to exhaustively tackle the difference between flex board design and rigid-flex design.
What is a Flex Board Design?
Like you, most electronic engineers have gotten accustomed to rigid PCB designs. However, a significant trend shift has increased the flex circuit design. While not necessarily a new concept, you will find most contemporary electronics possessing flex circuits. Your smartwatch, printers, mobile phone, keyboards, etc., represent some of the most common electronic examples with flex PCBs.
Flex board designs in other scenarios also possess stiffeners. Such instances happen when your application area demands it. However, unlike many other people, you need to avoid confusing this with rigid-flex PCB design. The two represent two entirely different things.
What is a Rigid-Flex Board Design?
A rigid-flex PCB implies a board that uses rigid and flexible board technologies when deployed. You will find such a board comprising several layers of flex circuit substrate that get attached to more rigid boards (externally or internally).
Your application area often determines the design type, though the flex substrate by design constantly bends. Because of this, you will find it formed in the flexed curve during fabrication or installation.
Another vital point to consider entails its extra challenging design environment compared to the conventional rigid boards. In most cases, you must design a rigid-flex PCB in a 3D environment that provides more spatial efficiency. As a result, you as the engineer or designer can twist, roll, and fold the flex substrate to realize the intended shape for the anticipated application.
Differences Between Flexible PCB Design and Rigid-Flex PCB Design
Considering that the days of rigid printed circuit boards as the sole PCB design option no longer exist, understanding both the flex and rigid-flex PCB design becomes imperative as an engineer. You have to grasp the details about each and the points of departure for each to successfully design a suitable printed circuit board for your unique application.
So, do you want to design a flexible PCB or a rigid-flex PCB but unsure of where to begin? Consider the following guidelines.
Flex PCB Design Guidelines
Knowing the Bendability of Your Flexible PCB
Flexible circuits offer the unique benefit of bendability for challenging application areas requiring constant flexibility. However, as the designer, you should understand how many times the flex PCB will bend, the extent of the bend, and the number of bend times in the application. Such information determines whether to settle for a static or dynamic flex board.
A static flex board implies a flexible circuit that needs to bend upon installation, though it will only bend or flex not more than a hundred times post-installation. On the other hand, a dynamic flex circuit proves more robust. Unlike the static flex board, it must constantly bend and withstand more than tens of thousands of flexes or bends. You will mostly find dynamic flex circuits applied in military and spacecraft applications.
Another crucial aspect to consider concerning the flexibility or bendability of your flex PCB entails the bend radius. The bend radius implies the minimum bend amount of the flex area. Therefore, you must always identify the bend radius early during the design phase to allow for the design to accommodate any amount of bends without subjecting the copper to any damage. So how do you calculate the bend radius?
Consider the number of layers in your flex PCB to calculate your bend radius. For instance, a single layer needs to have an x6 flex thickness, a double amount of thickness for a double layer, and x24 flex thickness for a multilayer flex board.
Consideration for Laying the Bend Radius
- Avoid 90-degree bends that otherwise cause high strain
- Avoid placing plated through-holes in the bend area
- In multilayer flex circuits, stagger the conductors to achieve greater effectiveness
- Conductors measuring less than ten mils need to get placed in the neutral bend axis because of the absence of compression or tension during flexing
Heat-Forming Flexible Printed Circuit Boards
In most cases, the need for heat-forming requires a steel jig. The steel jig forces the PCB board to lie in one specific way before getting into an oven. Heat-forming has a significant advantage in producing a tighter bend radius. However, you can only use it primarily for a more straightforward installation process as much as the tighter bend radius achieves.
Slots and Cutouts in the Bend Area
You can always minimize the bend area or region when it lacks traces. However, the bend radius needs to get minimized by inserting slots or cutouts. Cutouts reduce the number of materials for the bend. It is also possible to seek a different alternative by removing some flex sections without circuitry. However, it would help if you carried it out lengthwise, also needing a routing afterward.
Understand your Flexible PCB Materials
Most flex PCBs use polyimide as the main material for the core and cover-lay layers. Flex materials provide enhanced properties than rigid printed circuit boards. Another crucial benefit of flex material is its uniform thickness with an enhanced dielectric constant of 3.2 to 3.4. The absence of woven glass reinforcement eradicates Dk variations. Similarly, you will get polyimide with a uniform thickness owing to its unique manufacturing process (cast) – a typical layer has a thickness ranging from 0.5 mils to 4 mils.
It is also important to note that polyimide cores (flex) possess either rolled or electro-deposited annealed copper. The copper comes thin and proves ideal for both static and dynamic applications. One of the most common electro-deposits of copper measures 0.5 and 1 ounce, though the limit is 2 ounces. You tend to get the best mix of the thinnest construction.
Flex material types can come in either adhesive-based materials or adhesive-less materials. Adhesive-based materials use acrylic adhesive to bond the copper to the polyimide. Conversely, the adhesive-less type features the copper cast directly to the polyimide. Adhesives, in general, get used to laminating the layer of copper together with the core (polyimide). But in using an adhesive, you can quickly get cracks forming in the copper plating, especially within the vias. It arises because the acrylic adhesive tends to become soft upon heating. As such, you need to integrate tear-drops and anchors when designing.
Drawbacks of Utilizing Adhesive-Based Materials
- It forms cracks within the copper plating, as explained above.
- Incorporating adhesive thickens the copper-clad laminate though eliminating it also leads to thinner laminates.
- All adhesive-based materials tend to absorb environmental moisture. Because of this, it proves best for a system that exposure to the outside environment.
- The core’s thickness can be reduced post the fabrication process, leading to errors in dimensions.
In trying to address the issues outlined, the idea of adhesive-less construction ensued. Some of the critical features of this material include the following.
Adhesive-less Material Features
- Enhanced flexibility because of reduced flex thickness
- A diminished flex thickness owing to the eradication of the layer of adhesive
- An enhanced controlled impedance, especially on signal attributes
- It is well-suited for application in extreme or harsh environments
- It features a better temperature rating than an adhesive-based material
Another crucial point for you to consider regarding understanding your flex materials for your flex PCB entails its looser outline tolerance to other boards. As a result, flex materials have lesser dimensional stability compared to rigid ones. Further, based on the tolerance profile, it may become necessary to use a laser or hard tool cutting, which may prove too expensive.
Similarly, chemicals within the adhesive can become soft upon heating. Therefore, it becomes essential to enlarge your pads as much as possible. Here you can use anchors, spurs, and tear-drops to achieve stability of the outer layer besides reducing stress in your design.
Monitor the Flex Trace Routing
The layout of your circuitry can break or make your printed circuit board. For instance, when you consider the bend radius dimensions, a larger one becomes preferable to the sharp angles, which otherwise shorten the board’s lifespan. Additionally, it would help if you avoided the I-beaming to reduce the copper circuit-thinning stress effect. Remember, curved traces results to lower stress compared to angled traces. Also, keep your traces at a perpendicular angle to that of the general bend. Further, stagger the traces when you place them in two-plus layered PCBs (bottom and top staggering always works)
Position the Flex Layers in the Stack-Up’s Center
It primarily applies to rigid-flex PCBs where you must place the flexible layer between two rigid layers. However, milling the part becomes necessary to expose the flex part. Such an arrangement offers protection to the flex part from exposure to plating of the outer layer. It is an arrangement that also simplifies the manufacturing process besides enhancing control and impedance in the flex region.
However, it is possible to etch away the flex layer as a segment of a different process. It will allow for extra protection.
Mitigate Risks in the Flex Vias
Vias tend to break peel or sometimes crack in flex PCB designs. However, you can always mitigate this by tear-dropping the vias while incorporating the tabs and the anchors. Further, enlarging the annular rings also come in handy.
Benefits of Flex Circuit Boards
If you want to design and use a flex circuit board, you can enjoy a myriad of advantages. It includes ease-of-use, function, fit, etc. Below, some key benefits of flex PCBs.
Ease of Use
- Flex PCBs have few design constraints, with design flexibility to fit any shape or type of device.
- Its motion range permits printed circuit boards to suit every application.
- The reduced mass of flex PCBs limits risks in rough environments like one with consistent vibrations.
- Flex PCBs also have reduced errors otherwise found in most standard printed circuit board assemblies.
- You also get to enjoy a limited weight aspect when using flex PCBs as no extra cables, connectors, and wires exist.
The overall installation costs are low
Flex PCBs eliminate a myriad of steps in production, thereby shortening the total turn-time and reducing the cost.
Rigid-Flex PCB Design Guidelines
As one of the fascinating PCB types in the contemporary world, rigid-flex PCBs fuse both elements of flexible circuits and rigid PCBs. The hybrid’s fabrication process mimics that of a conventional hardboard circuit. However, some layers are flexible circuitry and run along through rigid or hardboards. As a designer, you need to consider that the fabricator will have to incorporate plated through holes to ensure a compelling connection between flexible and rigid regions of the circuitry.
Once you factor in such configurations, you can develop a rigid-flex circuit capable of getting assembled as a hardboard PCB. Further, it will also fold besides fitting into the anticipated electronic product without constraints.
A rigid-flex PCB also proves ideal for dynamic flex situations or applications. It can handle a hundred thousand plus flex cycles without any mishaps if well-designed. What is more? You can integrate both the flexible and rigid substrates into a unit capable of further getting manipulated into a three-dimensional subassembly.
Design Guidelines for Rigid-Flex PCBs
A rigid-flex PCB design closely resembles that of a hardboard or rigid design. However, with an experienced eye, you will notice that the flexible layers extend into the board’s rigid areas. Everything notwithstanding, however, a rigid-flex design will require a similar set of requirements to a rigid PCB when it comes to fabrication. For instance, you must submit the Gerber file, nomenclature, solder mask layers, cover-layer, rout files, etc. However, other distinct points of departure exist as well.
But what should you consider when designing a rigid-flex PCB?
Material layup can heavily influence a rigid-flex PCB’s total costs, performance, and manufacturability. As a designer, spending a considerable amount of time determining the best material becomes essential. For instance, resistance, controlled impedance, and other requirements like current-carrying can, in turn, affect both material selection and copper weight.
It would help to always collaborate with us at RayMing PCB and Assembly or any other ideal PCB fabricator for material selection deliberations. For instance, variables like costs vis-à-vis cost implication and performance can influence your rigid-flex design. Another essential aspect to consider entails the accepted standard of 20 or fewer layers for rigid-flex boards. However, you can also have rare occasions to design the PCB with more layers. Further, the rigid sections can vary in layer count, provided the material layup and thickness prove similar.
Plenty has happened recently when it comes to design rules. Fresh rules for component placement on a rigid-flex PCB design allow increased freedom, unlike before. For instance, you can now place components within the flexible area of the PCB. Together with a multilayer approach, such an approach allows for more circuitry buildup in your rigid-flex PCB design. However, with greater freedom comes more challenges of holes and routing that you must contend with.
For instance, you cannot place vias or components near the bend line as the flexible segment of the circuit due to the likelihood of material stress. It is also imperative to utilize more thru-hole plating besides bolstering the pad’s support with extra cover-lay to anchor the PCB pads.
It would help to think of the potential electromechanical factors that can influence both the rigid and flex parts of the PCB. Here, aspects like the bend radius to thickness ratio become important. Always keep the bend radius at least ten times the flex-circuit material’s thickness. It is also vital to develop a “paper doll” to know the bend areas.
Another crucial aspect entails avoiding any stretching of the flex section of the PCB along its inner bend or outer bend. For instance, enhancing the bend angle over 90 degrees also increases its stretching from one end and compression on another flex circuit point. It would be best to consider the type and thickness of the conductor within the bend area. It is possible to reduce the mechanical stress and thickness by reducing the plating on conductors and utilizing pads-only plating. Additionally, heavy gold, nickel, or copper plating reduces flexibility at the bend region, allowing for mechanical stress and potential fracturing.
Recent printed circuit board design tools provide aspects that allow for the management of several layer stacks, checking design rules, visualizing the three-dimensional electromechanical designs, and simulation of the flex circuit operations. However, the enhanced aspects of the tools notwithstanding, you must incorporate teamwork (fabricators and your team) exceedingly early in the project’s design phase to become successful.
Differences between Flex Circuit and Rigid-Flex PCB and their Production
|Flex Circuit||Rigid-Flex PCB|
|It only has flexible circuitry and thus only flexible substrate or polymer film containing the conductive circuit.||It incorporates both flex and rigid materials by layering flexible substrates within the rigid material.|
|Moderately low cost of production||The high cost of production|
|The basis for the development procedure of rigidized flex fabrication||Requires an FPC overlay while the FR-4 material loads up during its manufacturing|
|Application areas rest on consumer electronic products like cell phones, etc.||Application areas feature satellites, the military, and other high-quality request areas.|
|A simpler quality control process compared to rigid-flex PCB||A more troublesome quality control process|
|Mostly has a flexible film created from polyester, polyimide, and PTFE||Possesses flexible cement film and a flexible dielectric film|
|Offers high flexibility though with differing longevity based on the type (dynamic and flex)||Lesser flexibility though durable|
IPC Flex Circuit Standards
Quality verification proves an important aspect of the manufacturing process of flex and rigid-flex PCBs. However, various industry standards exist as benchmarks for any new PCB product. As a PCB designer or engineer, you can either test or use other guidelines to check the quality of your flex and rigid-flex PCBs. But why not use the IPC or Association Connecting Electronic Industries guidelines? Below are some of the best IPC guidelines to start acquainting yourself with.
IPC-6013 (December 2013)
It first came to the fore in 2013 and gave the performance and qualification specification for flexible circuits. It supersedes some of the earlier IPC standards, including the first IPC-6013 established in 1998. The IPC standard specifies diverse test methods, including bend, thermal, and impedance examination or testing. It also entails quality assurance like sample test guidelines and coupons for quality conformance and acceptance tests.
It offers guidance on picking rigid-flex interface and adhesive materials. You also get tips about flex vias and plated thru-hole.
It is a quality assurance standard that offers insights regarding PSAs or pressure-sensitive adhesives for flex PCB assembly. The guide gives information on the types of adhesives available besides suggested processes for their correct use. It also provides their weaknesses, strengths, and limitations.
IPC-2223 (Sectional Design Standard for Rigid-Flex/ Flexible Printed Boards)
It is a quality assurance guideline document commonly used in conjunction with IPC-2221. IPC-2223 outlines the design specification for flex printed circuits and offers information on interconnecting structures and component mounting.
IPC test methods apply to circuit boards. Such circuit board testing comes as an environment and application-specific process. It often proves a solid foundation for designing reliable, flexible circuits. But as a design or engineering beginner, starting with IP-FC-234 and IPC-2223 can prove a decent place to start from. You will have assurances of reliability for your flexible PCB.
Both flexible PCBs and rigid-flex PCBs prove crucial in diverse application areas of this contemporary world. Therefore, a proper design of the two can make or break the potential successes of your project. However, considering the insights on each besides the differences deliberated will kickstart your flexible or rigid-flex PCB design journey.