Advances in PCB fabrication have translated right into new design constraints and guidelines.

For several years flex and rigid flex PCB normally appeared in products as a flexible wire between 2 rigid boards. The past 5 to 7 years have actually brought tighter room restrictions and miniaturization difficulties. Designers should currently position components on the flexible circuit, using it like a rigid substratum. Utilizing both the rigid and the flexible areas for elements, while possible, presents brand-new design restraints that call for more advanced PCB design approaches.

To stop breaking or excessive anxiety on parts, stay clear of positioning parts and vias at the bend locations. A conventional, well-known guideline is that routing have to be orthogonal to the bend line to minimize material stress and anxiety at the bend. Routing on the following layer through the bend location must be balanced out to stay clear of the I-beam impact. Traces that do not follow this regulation may accidentally add tightness to a location that is intended to be flexible. Additionally, the location where rigid and flex PCB zones come together may call for overlap of material and need special spacing for openings and conductive materials. It is useful to think about the transition area a stress-relief area. it reveals a four-layer rigid board linked to a two-layer flex PCB, which on its other side connects to an additional four-layer rigid board.

Rigid and flex PCB usually make use of different materials, and the rigid area usually has more layers compared to the flex section. Numerous sophisticated producers can sustain these styles with more than 2 flex layers. To make sure flex circuits with additional layers work well in all problems, stiffeners that bring strength to these PCBs are positioned near elements or adapter locations or on the other side. Stiffeners are made from materials such as stainless steel or aluminum, with the enhancement of dielectric material like a polyimide build-up. Smaller sized rooms often call for the flexible part to be bent or folded up.

The PCB cross-section editor for a solitary stack-up should now support several cross-sections standing for the different PCB laminates. In addition to sustaining conductor, plane and dielectric layers, cross-section editors need to include new mask and layer layers above and listed below the surface areas of the flex PCB, such as:.

Electroless nickel electroless palladium immersion gold (ENEPIG) for unique plating areas.
Stiffeners– aluminum or stainless steel– that restrict flexing where components are installed, to prevent splitting or peeling.
Material masks that include (precious) steels, adhesives and solder paste masks.
A coverlay (cover layer), which is an adhesive-coated film pushed into the stack-up to shield the circuit.
Advances in fabrication have actually reached materials and the variety of extra mask/conductive layers for flex and rigid flex PCB. New materials– conductive/nonconductive layers, and surface area coatings– need developers to by hand inspect if the design components on the flex circuit are satisfying the maker’s design standards. This includes a substantial amount of time to the design stage.

To stay clear of hands-on checks and make sure the design is created correctly, designers need in-design inter-layer checks to flag problems as they are developed. Checking at the PCB manufacturing sign-off stage is far too late in the design cycle to find errors, and makes the design process unpredictable. Real-time capability could stay clear of taxing steps later on at the same time.

In-design inter-layer checks could lower errors that could be presented during the design process, consisting of the complying with areas:.

– Mask-to-pad, metal-to-coverlay, coverlay-to-pad.
– Gap/overlap between mask layers.
– Edge-to-edge space in areas such as the bend line to the part, via-to-bend line, and stiffener-to-bend location.
– Bend line/area to stiffener, pin and via, component.- Gold mask-to-coverlay, stiffener adhesive-to-stiffener, and pin-to-coverlay.
– Minimum overlap, such as when 2 geometries overlay by a minimum or more (e.g., solder mask overlay right into the transition zone).

Typical Rules for Compound Layouts

Mechanical restrictions.

Rigid flex PCB drive additional policies when the flex circuit is folded up or bent inside an enclosure. Generally, the mechanical engineer provides the bend line, bend radius and bend location to the PCB developer. With contemporary EDMD (IDX) user interface, this information can be automatically imported into the PCB design devices. These bend locations need developers to:.

– Prevent putting pads also near the bend area to stop peeling.
– Do not position vias or pins too close to stiffeners, to avoid shorting.
– Do not overlap bend areas with stiffeners, to stay clear of peeling.
– Prevent positioning vias in bend areas to prevent fracturing the substratum gradually.

Mechanical engineers define the limits for areas– rigid, flex, rigid– where the number of layers and therefore densities are different in each zone. However, they require added details regarding layer structures and density for the areas, layers above top and below bottom to precisely model the thickness of the last PCB assembly, and to perform collision checks before handing the design to PCB manufacturing. Examples of such layers consist of paste mask, coverlay, stiffeners, external copper, and other materials that impact general elevation, thickness, and bend efficiency.

Inter-layer checks.

For sophisticated flex and rigid flex PCB layouts, PCB developers should abide by brand-new design standards from the producers. These brand-new layers and surface finishes need detailed in-design inter-layer checks of nonconductive layers in rigid flex PCBs.With an accurate image, developers could execute a lot more exact DRCs, receive better feedback, and provide better data to the MCAD tool for fabrication. Not having these checks expands the design cycle. In-design inter-layer checks give a correct-by-construction method that stays clear of unnecessary design versions and, sometimes, expensive prototype develops. Tools supplying a photographic view of the stack-ups based upon various substrates allow designers to visualize the layout stack-up intent as it is being defined.

Routing.

Routing on flex wiring usually calls for arcs within the paths. Most of the geometry on the flex portion, consisting of board overview, teardrops and routing, calls for arcs and tapered shifts. Group routing features should lug a group of webs (bus) across the flex, while easily locking to the contour of the flex/board rundown. PCB designers obtain modifications each day; including an added trace to a routed set of nets need to not force rerouting of the entire bus. Shifts in line sizes require tapering and all pad/via entry/exits be tear-dropped to reduce stress and anxiety at the solder joints. A lot of PCB design devices support push-and-shove routing, yet these abilities currently need to sustain push-and-shove with arcs in the traces.

As the intricacy of a flex or rigid flex PCB increases, the quantity of time a developer spends boosts as a result of manual checks. Today’s CAD tools should provide a means for developers to leverage new PCB fabrication methods without prolonging design time. The breadth and depth of in-design checks needs to cover 30 or more new flex and surface coating layers. Customers should likewise have the ability to integrate their own layers for the tool to check, so they do not have to wait for device updates.

The EA Video game company engaged VR Systems to produce a prototype of screen connected to the headgear. This headgear display screen offered a heads-up screen of instrumentation and digital depictions of crucial information. The safety helmet was the heart of the Virtual Reality equipments, enabling the customers to benefit from the whole system for remarkable gaming experience.

PCB requirements. The safety helmet screen supported the rigid flex PCB in its mechanical housing, and an HDMI cord fed into the video clip resource. An optical cord connected to the screen, which carried light to the headset’s LCOS microdisplay. The light illuminated the display and was predicted into waveguides that provided info to the customer’s eyes.

The headset display screen was one of the first reported binocular HMDs in growth utilizing a liquid crystal and silicon microdisplay. This innovative dramatically decreased the price, quantity and weight of standard helmet-mounted displays, replacing cumbersome optics systems with light-weight, slim, translucent diffractive optics. The screen’s physical needs positioned fascinating challenges for the rigid flex PCB.
1. The system had to be little enough to install to the pilot’s helmet and enable activity without causing discomfort.
2. To avoid user injury, it needed to instantly launch all links to the plane in the event the pilot needed to eject from the aircraft.
3. The boards should be flexible enough to twist around the system’s optical components and fit ports at different angles.
4. As a result of size restraints, the design can just break out traces on the eastern and western sides of the main FPGA element, instead of a north-south-east-west pattern.

Xilinx provided valuable info regarding time of trip inside the bundle, as did the IBIS models of the Micro memory modules. The bundle for the memory component was substantially smaller than the Xilinx FPGA, and the Xilinx time of trip info was important. We had the ability to modify out the differences in the memory components after layout was completed.

The design made use of a Xilinx FPGA and a 64-bit wide DDR3 memory bus, where each of 4 parts had a 16-bit vast information bus. Timing was matching to a few picoseconds on the trip times through the board. One of the more tough parts of the design was that the link in between the FPGA and memory called for simulation at a really broadband, so the timing restraints were tight. With such tight margins, it was essential to think about the travel time of flight inside the plans along with on the board. For these factors, die-to-die time of trip was picked, in contrast to simply pin-to-pin time of trip.

There were 2 parts to the simulation. The first was an interesting job: to guarantee that the design satisfied the timing needs of the DDR3 by considering time and length suit. The 2nd part was to guarantee signal stability; factors to consider for impedance matching would make certain no impedance gaps would trigger signal honesty concerns. While the signal rates were high, they were low enough for loss to be a considerable worry, given the trace sizes involved.

Among the DDR3 needs was that the address and control data would certainly exist in a fly-by setting, linking in between the controller and all 4 memory ICs. To comply with the timing constraint that needed the clock course to be longer compared to the data and DQS lines, one needed to include length to the clock. This, naturally, contravened office requirements. Along with this, for the very first and 2nd memory IC, earlier along the path, the information in some cases came from a part of the FPGA, that made the data course rather long.

The trace size in between the memories and FPGA varied between 1.5″ to a number of inches in length. The address and control signals took a trip to all four memory components, while the data were coming from a part of the FPGA that was potentially farther away. It was an obstacle to maintain delays to ensure that the write timing in the DDR3 would certainly function.

To make certain timings matched, standard routing was carried out first then matched the lengths. It was decided which layers would be utilized for each and every of the signals and teams travelled together; for example, each information lane was placed on the very same layer. An effort was made to minimize the number of vias and other features needed to reach the end course.

The FPGA had some versatility with respect to which pins could be used for which objectives. Nonetheless, as rate boosted, it postured constraints due to the fact that particular groups of pins for specific lanes of data were required. The largest trouble came with the address and control lanes, all traveling in large teams on the same layers of the PCB board.

The challenge came when it was time to match sectors. This was challenging because of the lack of area. A basic point-to-point suit for the address and control lanes wouldn’t be enough. Rather, we matched every section: between the controller and the initial IC, first IC to the 2nd IC, and so forth. Luckily, since the ICs were a certain range apart, it was basically a point-to-point match, and the trace sizes were comparable. The biggest obstacle was matching the section from the FPGA to the first memory IC. The lengthiest course defined the length of time the trace needed to be, and sometimes we should boost the trace by a large fraction of an inch to suit this. To include so much length, trombones or accordions were required, which took up space on the board.

The BGA bundles for the HDMI and FPGA controller postured trace breakout concerns. In a similar way, the HDMI controller was a very fine-pitch BGA; it really did not have very many pins, yet it was a 0.5 mm pitch BGA, so breaking out in a traditional pattern would be hard. Although the FPGA wasn’t a particularly huge or thick component, because of the board’s small size, traces might just burst out on the east and west sides instead of the typical north-south-east-west pattern.

Controlled impedance needs. The HDMI video input had a number of various needs for controlled impedance. The size of the trace used was extremely little, and the signals were reasonably slow-moving compared with DDR, so trace size had not been an issue. Nonetheless, the HDMI needed 100Ω differential pairs, while the memory ran at 80Ω. Consequently, it was a fascinating obstacle to ensure controlled impedance, and it was hard ahead up with a rigid flex PCB stackup that would leave appropriate area for 80Ω, as well as 100Ω, without ending up being also slim and hard for PCB manufacturers to make.

HDI PCB and blind and hidden vias PCB were related to break out traces from the HDMI and FPGA controller. The HDMI controller pattern additionally utilized via-in-pads. It served to do some “what-if” scenarios and see exactly how the HDI stackup might potentially come out. We went through a couple of versions on that with a couple of various kinds of materials, considering the impedance control preparation.

At first, FR-4 material was under consideration, but after some screening, we decided to opt for a material that had a lower dielectric constant, attaining reduced loss, signal stability and preferable line-space ratio for impedance controlled traces.

Utilizing HDI PCB lowered the number of layers required in the board generally, and after stabilizing the cost with the benefits of HDI PCB innovation, it was chosen this was the appropriate instructions.

There were 3 rigid sections in the final design:.
1) A main section with the FPGA, DDR3 ICs, and power supply devices;.
2) an area with slower, analog-type components and more power materials;.
3) an area that featured an extremely tiny HDMI receiver with numerous feasible positionings to suit the inbound input cable.

The rigid flex PCB option. As the design moved forward and it became clear that office was an issue, it was chosen to link the rigid boards with a flexible bow to avoid utilizing typical physical adapters that called for even more area.

The final rigid flex PCB stackup was 10 layers. The rigid boards used eight layers and lugged all the impedance-controlled and high-speed traces. The other 2 layers were the flex PCB signal layer, which was also used as the VCC layer generally rigid part of the board. There was some cooperation in between layers of the rigid and flex sections, however, for one of the most part these were dealt with individually.

Rigid Flex PCB allowed the board to fit into the small housing on the helmet-mounted screen system. The flex bow could bend a variety of ways, accommodate various angles, and be rolled up and fully consumed within the quantity of the container, providing options on exactly how the boards would certainly participate in the system and twist around the optical parts.

Anything that needed impedance control was handled completely within among the rigid structures. Simplifying into those sections permitted us to stay clear of any type of need for impedance control on the flex, which was a big win insofar as price goes.

, in just what will certainly be the initial of lots of rigid flex PCB ideas, we will certainly be reviewing optimum rigid flex PCB stackups and materials. One of our clients recently sent us a 6 layers stackup that needed a little tweaking. It’s outstanding just how a few modifications to your rigid flex PCB stackup design could make certain longevity and manufacturability on your rigid flex board.We talked it over with our designers and developed solutions and alternatives to all of the concerns

The Board:
– This was a six-layer rigid flex PCB with adapters calling for impedance control.
– The high-speed ports connected finger areas from the edge to the top side of the PCB board.

The Concerns:
– The board’s flex PCB layers were found on the outside of the stackup, which enhanced the possibility of manufacturing troubles
– Making sure the board met the impedance demands.

The Remedy:
When the flex layers are on the outdoors, panels are more difficult to deal with and more challenging to process. This made the board more sturdy and much easier to manufacture. It also allowed for much better impedance and better control around the flex finger location.
We embedded the flex PCBlayers in the center of the stackup. This safeguarded the layers throughout the PCB manufacturing process and ensured that the less-durable flex layers were not subjected to outer-layer plating. This is how most rigid flex PCB stackups are made.

Because the flex layer is a separate process, putting the flex layers inside enables flex PCB manufacturers the ability to etch far from the design while safeguarding the flex layers. The material used also played a huge component in making this rigid flex PCB instead of flex. Rigid PCB material was utilized, permitting better impedance and reliability. It was a better option than the original FR-4 material.

Putting the rigid PCB material on the outside likewise enables us to produce exactly what is basically a rigid panel. The flex PCB layers are also safeguarded by our surface area plating since it needs to weak the material.

iFastPCB rigid flex PCB manufacturing processes are as adhering to: first to process the flexible layer as a two layers flex PCB. Then to laminate the flex layers in between the rigid PCB layers. The ending action is milling the cover layers so the flex section becomes viewable.

Flex PCB can give several apparent advantages over traditional ribbon cables in particular applications. As an example, a flex circuit can extend in between a board and an adapter on a bulkhead numerous inches away at a right angle in the same plane as the board. That’s not possible with a bow cable.

Flex connections have mechanical advantages over conventional bow cables in numerous applications however in some cases, they likewise have much better chemistry.

A flex circuit can be developed in complex shapes in 3 dimensions with branches to multiple connectors, which would certainly be difficult to accomplish with a ribbon cable. Additionally, flex circuits can be interfaced with rigid boards without the fairly tall and large connectors level cables require, or in the case of rigid flex PCB building and construction, they can be indispensable with the boards and remove external connectors entirely. Furthermore, the conductor thickness of flex PCB can much surpass that of bow cables.

Flex PCB’s Advantages

There are some refined benefits of flex PCB versus traditional ribbon cables past the many clear distinctions. Among the materials typically used for flex circuits, Kapton, has very reduced outgassing in ultra-high-vacuum atmospheres, such as room. Though Kapton-insulated bow cables are offered, they have a limited variety of conductors and could not be transmitted at angles in limited boundaries.

Ribbon cables shielded with Teflon and various other plastic materials outgas fluorine or reactive substances when subjected to high vacuum cleaners, which can attack electronic devices in sealed containers if treatment is not taken to completely duct the gases.

A Kapton Flex PCB

Sinclair Interplanetary integrates a Kapton Class 3 flex PCB made by iFastPCB in optical navigating gadgets the business produces for usage in small satellites. The nine-conductor flex PCB web links a D-type adapter, which secures to the tool unit, with pins on the rigid tool board that bring telemetry information, commands, and power.

The flex PCB in this situation is swaged to the pins on the six-layer rigid PCB board, which is about the size of a credit card. It brushes up the conductors upwards and at a best angle from the pins on course to the adapter and guarantees the port is mechanically isolated from the board.

Sinclair is among several iFastPCB clients that manufacture parts for small satellites or develop the total satellites. Tiny satellites– several of them bit larger than a shoebox– could do very complicated objectives matching those achieved by spacecraft costing loads of times extra.

A straight line is not constantly the shortest route between 2 factors in digital products: Thanks to rigid flex PCB style, circuits can be folded up onto themselves with 180º bends– laid over at minimum elevation– therefore diminishing item measurements. In addition, if a product has moving areas with electronics embedded, rigid flex PCB building is the ticket.

Credit rating rigid flex PCB architecture for the existence of smart devices and various other pocket-sized digital marvels. However, to realize the benefits of rigid flex PCB construction (including lightweight settings up) particular design restrictions apply particularly to the flex PCB layers in a stackup. Do not attempt your initial (or second, or 3rd) rigid flex PCB design prior to you consult your prototype PCB manufacturer.

Using Flex in Rigid Flex PCB Assembly

Unlike a traditional PCB stackup, foil building and construction could not be used for flex layers. The flex layers in a rigid flex PCB assembly are built from unreinforced base substrates normally containing polyimide dielectric film, outfitted with rolled stiff copper.

Consequently, the clothed base material is first pierced, openings are selectively plated, after that the traces and pads are engraved. Bondply, a layer of polyimide film with glue layer on both sides, isolates that conductor layer from the next, and so forth.

Flex PCB materials are flexible under all situations, including processing. During the last lamination of the rigid flex stack, they are much less dimensionally steady than the rigid core and prepreg materials that sandwich them. Vias should be farther from the side of the rigid area adjoining the flex bow than the minimum distance in rigid-only heaps, ideally at the very least 50 mils from the edge, but absolutely no less than 30 mils. This rule is the one most broken in rigid flex PCB designs.

Look for guidance to create your stackup and design guidelines. Differing coefficients of thermal expansion among the flex base material, adhesives, prepreg, and rigid cores needs an extremely careful equilibrium of thicknesses, specifically for impedance-controlled designs. There can be lots of layers of flex in a rigid flex design, depending upon the bend radius of the ribbon section and whether it will stay fixed after assembly. Flex layer count should be restricted in dynamic applications. Consult your PCB manufacturer. If more than four flex layers are required, bonding adhesive must be absent in the sections that are designed to bend. The bend radius will be no less than 12 times more than the circuit thickness.

Successfully Using Trace Routing

Trace routing in the ribbon location should be bent, not angled, to raise peel strength. This referral is other the transmitting technique for rigid PCB boards.

To increase ribbon versatility, airplanes ought to be cross-hatched; nevertheless, the cross-hatch makes complex impedance control. Once more, a cautious equilibrium is required. In some applications, a large, solid strip under critical traces suffices. Traces on various layers should be surprised vertically, not placed atop each other, to boost ribbon flexibility.

Annular rings should be as big as possible in flex-only regions to reduce the threat of peeling, and the change from the annular ring to the trace should be teardrop-shaped for the exact same factor. Including tabs or supports additionally helps to stop peeling.

The stiffeners can be laminated when the cover-coat is bonded and is the recommended approach to avoid tears. The most effective strategy is to avoid utilizing sharp corners in a flex PCB design.

An extremely basic checklist for rigid flex PCB designs consists of these routing factors to consider:
– Stagger flex traces up and down layer to layer
– Transforms should be progressive
– Vias needs to be no closer to the side of the rigid board compared to 30 mils at the flex shift
– Minimize flex PCB layers

Keep in mind, rigid flex PCB designs may be costly to produce, however they can conserve expenses during system assembly. Such design often is the only way to put the required product functions within the target package volume. It’s much better to get in touch with a rigid flex PCB manufacturer during the drawing board.

Rigid flex PCB are special with their incorporated building of both rigid PCB and flex PCB circuits innovations. Being unique includes a number of special needs that should be examined and carried out throughout the rigid flex PCB layout stage of the design process.

The first two requirements relate to minimum area demands, as measured to the Flex Transition Zone within the design, of plated through openings (PTH) and exterior layer copper features. The 2nd two handle the mechanical flexibility and integrity of the flex PCB locations when the components are bent into the required form.

The Flex To Rigid Transition Zones

The “Flex Transition Zone” is specified as the size of the rigid area outline at which the layer framework modifications from a rigid area to a flex PCB area only.

The Flex Change Area are produced by the need to expand the flex PCB location coverlays by a tiny range right into the rigid areas. This allows the flex coverlays to be caught by the lamination of the rigid location layers and make certain a gapless shift in between the flex PCB areas and the rigid locations. The flex coverlays do not expand throughout the rigid locations as called for by IPC 2223C design criterion for flexible PCB.

Layered With Opening to Flex Transition Area min. spacing = 0.050″

– Ensures layered with opening integrity by protecting against any PTH from being drilled via the flex coverlays as they involve and are captured by the Rigid location layer lamination.
– Coverlays are laminated to the flex PCB layers utilizing a flexible adhesive, either acrylic or epoxy based. These adhesives have a very high co-efficient of thermal growth.
– A plated via hole pierced with a coverlay will certainly undergo considerable Z-Axis growth and contraction anxiety during both the assembly re-flow process and possibly throughout the procedure of the finished product. This has actually been determined as a key source of broken opening layering resulting in either instant product failing or long-term latent failing integrity problems.
– This requirement is called out in IPC2223C Sec. 5.2.2.3.

Exterior Layer Copper feature to Flex Transition Zone min. spacing = 0.025″

– Makes sure enough spacing to allow for reliable outside layer imaging processing.
– Rigid layers, while in production panel arrangement and prior to final lamination process, are called for to have the flex PCB areas got rid of. This develops in inner sides, created by the height difference in between the rigid area and the flex location, which the exterior layer image transfer films need to transition.
– Min. spacing 025″ supplies enough area for film attachment and a dependable imaging process.

Flex PCB Location Via Holes

– Not suggested and should be stayed clear of if design permits.
– Includes substantial cost as a result of the extra drilling and layering processes.– Requires blind via manufacturing processes.
Possibly creates mechanical stress concentrators in flex PCB layers which may cause breakage if part is bent in the vicinity of these vias.
– If design does need flex area vias:
-Make certain vias lie away from the certain bend place(s) in flex areas.
-Have PCB supplier review design to assess and determine if any risk aspects exist.

Flex PCB Location Trace Layout

-Traces ought to be maintained directly and parallel, if design enables.
-If trace direction adjustments are required utilized curved corners and minimize as much as feasible.
-Aids get rid of possible mechanical tension concentrators which could bring about breakage when flex PCB area is curved right into placement.
-Stagger traces on surrounding layers, if design permits.
-Improves adaptability and integrity by reducing the “I-Beam” impact of traces placed straight above one another from layer to layer.