Rigid Flex PCB

Flex and rigid-flex circuits have found wide use in electronics packaging. Flexible printed circuits are so useful in part because they can be made to fit where the smallest rigid boards can't.

ASC offers a variety of solutions including Single-sided Flex, Double-sided Flex Multilayer Flex and Rigid-flex.

  • Substitute for bulky wire harnesses
  • Replacement for hardboard / connector / cable assemblies (rigid-flex or flex with stiffeners)
  • Flexible shields or ground planes to reduce noise. Design conductor patterns to block specific types of electrical interference
  • Enhance high-speed signal integrity with matched-impedance flex circuits
  • Miniature jumpers on circuit boards
  • High reliability
  • Repeatable installations
  • Harsh environments
  • High vibration
  • Conductor patterns maintain uniform electrical characteristics. Predictable noise, crosstalk and impedance
  • Reduced assembly cost
  • Replacement for a circuit board and wires
  • Reduced weight and space
  • Dynamic flexing

Your expert for Flex and Rigid-flex is Dave Lackey


Dave has been involved with manufacturing PCBs since 1980 and has worked in various shops, most of which had military certifications and utilized higher technology.

Dave has extensive experience building metal-core boards and PCBs requiring thermal management solutions, as well as flex and rigid-flex boards.

Having worked in most departments throughout the years, Dave has developed a strong engineering background and is knowledgeable in most industry technologies. His background enables him to work not only with buyers but design engineers and quality and manufacturing personnel as well. Most questions can be answered on the spot, without having to deal with multiple visits, e-mails, or calls.


American Standard Circuits (ASC) is one of the fastest growing PCB shops in North America. Technology is a key factor contributing to its growth. A few years ago, when Anaya Vardya became CEO of the company, he decided, along with owner Gordhan Patel, the way to grow and thrive in this economy was to drive the company toward not only high-technology, but a wide variety of technologies, from rigid to flex-rigid boards, metal-backed and RF boards, as well as other new technologies using new materials and laminates.

Vice President of Business Development and Technical Sales David Lackey is the company's resident guru when it comes to flex and rigid-flex technology. I recently sat down with him to discuss the technology and its importance to American Standard’s growth and industry growth in general.

Dan Beaulieu: Dave, good talking to you today. Thanks for taking the time. Tell me about your company, especially when it comes to flex and rigid-flex.

Dave Lackey: ASC has been manufacturing PCBs since 1988 and over seven years ago we began building flex and rigid-flex PCBs, a very good direction for us. We are starting to see a lot of interest from our customers--they are trying to get away from wire and incorporate a more reliable connection using flex circuits. We do simple, single-layer flex with LPI or cover-layer up to high-layer-count rigid flex. We keep a fair amount of flex materials in stock and warehoused locally to allow us to respond to quick-turn opportunities in the flex market delivering product in less than five days on many occasions. Our goal is to make it as easy as possible for our customers to design in rigid-flex boards into their products.

Beaulieu: What do you mean by that?

Lackey: Well, in the past, customers had a hard time finding good rigid-flex suppliers. They considered it a hassle to even try to buy flex boards so they designed around them, even though the ultimate product would have benefited by having flex boards. They told us about this and we set about working on being not only the best, but the easiest flex supplier to deal with in North America.

Beaulieu: Dave, can you tell me about your background in the industry? I know you've been around for a number of years and I am always interested in knowing how people got to where they are.

Lackey: I've been doing this for quite a while. I've been manufacturing PCBs since 1980 and have worked in various shops including the captive shop that used to be at Northrop Grumman. A fair amount of my experience has been in shops with military certifications and higher technology. Apart from flex and rigid-flex, I also have a lot of experience building metal-core boards and PCB’s requiring thermal management solutions. Having worked in most departments throughout the years, I've developed a strong engineering background and am knowledgeable in most industry technologies. I spend most of my time in sales now and my background allows me to work not only with buyers, but design engineers and quality and manufacturing personnel as well. This approach tends to satisfy most of the customers' request or questions without having to deal with multiple visits, e-mails, or calls and give customers answers on the spot.

Beaulieu: So your role is more technical sales or maybe applications engineering?

Lackey: Yes, that is correct. Having been the general manager at many PCB facilities, including ASC, I have a strong understanding of not only what our facility needs to be successful, but also what our customers expect from us a supplier of PCBs.

Beaulieu: So ASC is able to handle just about all flex requirements?

Lackey: Pretty much--although we can’t be everything for everybody. American Standard rarely turns away from an opportunity. One of the challenges with flex and rigid-flex is material handling and processing without damaging the product--this is critical. Since we have dealt with fragile and thin-core RF materials for so many years this was a definite advantage. We have developed way to handle the material making sure that it is not damaged during the process. Even though this seems to be a simple thing, it's not...I can assure you. Another concern is registration. Getting good registration on flex boards is very difficult, but because we have invested in all of the necessary equipment and tools, we are in a position to deal with it. In the end, our success is due primarily to our having committed to extensive training on handling and processing of thin cores and we continue to invest in the people and equipment necessary for serving the flex marketplace.

Beaulieu: How has flex technology evolved over the years?

Lackey: I started building flex circuits in 1983. The biggest difference I see between then and now is that there's a lot more rigid-flex now than in the past, which allows designers to utilize BGA and fine-pitch devices on the rigid area yet have the flexibility to use the flex portion to package their devices in confined areas. As we've seen through technology, packaging of electronic devices tends to get smaller and lighter. Flex circuits are a perfect solution to accomplish both. Other changes are the availability of more flex laminate manufacturers and different materials that meet UL criteria, allowing for adhesive-less builds. Additionally, some of the press pad and release materials used for bonding of cover layers and flex multilayers have improved over the years, allowing for improved conformal adhesion to the circuitry and removal of entrapped air along with reduction of adhesive flow onto pads.

Beaulieu: Can you tell us about your customers? What sort of flex applications are out there?

Lackey: We see a wide variety of requirements from our customers, from military and medical devices to simple, single-layer flexes connecting one device to another.

Beaulieu: What advantages does rigid-flex have over normal rigid boards?

Lackey: Packaging and reliability. Rigid-flex boards allow for tighter packaging and the means of connecting multiple devices together without using bulky connectors and multiple PCBs. You ultimately end up with a more reliable product that can be accommodated in tight spaces. Many designers may feel the overall cost of a rigid-flex PCB verses using multiple rigid boards, connectors, and wires is not worth pursuing; however, in many studies we did for our customers depending on the design and use of a rigid-flex replacement the total overall cost and certainly the reliability is usually justifiable.

Beaulieu: What are the major differences between building a flex or rigid-flex board as opposed to building a traditional rigid board?

Lackey: The major differences, other than some varying processes, are registration concerns and handling. Registration is a concern on all PCBs, but given the nature of flex material in itself having proper registration equipment and tools is a major necessity. Handling of the material in itself is a significant challenge. Special material handling carts or trays and training your operators in how to deal with these fragile layers is a must.

Beaulieu: Where do you see the industry going when it comes to flex technology?

Lackey: It's definitely growing. It comes down to reliability of a flex or rigid-flex circuit used as a connector verses wires and the ability to put together a tighter cleaner package for devices. A well laid out design utilizing flex or rigid flex can not only provide the necessary reliability, but also reduces assembly time and rework.

Beaulieu: Why should customers come to you for their flex and rigid-flex boards?

Lackey: In addition our experience and ability to provide a quality flex product we have the ability to do quick-turn builds when necessary. We offer solutions to our customers and their design and packaging engineers who might need assistance with material selections (what can and cannot be done with flex circuits) and we always keep cost in mind. Our goal is to be a total solutions provider not only with our flex offering, but with all of our technologies as well. No customer is too small for American Standard. We have a well-staffed group of talented engineers and a number of outside resources to assist customers early on in the design phase and oftentimes offer alternative options upon receiving a finished design as a method of reducing cost or providing a solution that can provide increased yields.

Beaulieu: Dave, thanks for talking with me today. I appreciate the time you took out of our busy schedule.

Lackey: No problem, Dan. Thank you.Not many companies can do all that American Standard Circuits can and this is why they are succeeding while other companies are struggling. But there is a lesson for all of us here: If we want to succeed in the PCB business we must put the customer first.

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Flexible / Rigid-Flex PCBs

Flex circuits are printed circuit boards manufactured from material that can bend, fold and twist. Rigid-flex circuits have both a rigid and a flexible component. Rigid-Flex circuits have proven to be among the most versatile and useful electronic interconnection technologies; however, they are also among the most complex.

 

Why Flex / Rigid-Flex Circuits?

Since their introduction, flexible and rigid-flex circuits have been steadily moving from the fringe of electronic interconnection toward its center. Today flex and rigid-flex circuits are found in countless products from the very simple to the highly complex. The reasons for this shift to the center are numerous and most of them are related to the advantages they offer. An examination of some of the benefits and advantages will make this clear.

 

flex-1.pngThey are a remedy to natural product packaging problems. Flexible circuits are often chosen because they help to solve problems related to getting electronics inside the product they serve. They are a true three-dimensional solution that allows electronic components and functional/operation elements (i.e., switches, displays, connectors and the like) to be placed in optimal locations within the product assuring ease of use by the consumer. They can be folded and formed around edges to fit the space allowed without breaking the assembly into discrete pieces.

 

They help reduce assembly costs. Prior to the broad use of flexible circuits, assemblies were commonly a collection of different circuits and connections. This situation resulted in the purchasing, kitting and assembly of many different parts. By using a flex circuit design, the number of part numbers required for making circuit related interconnections is reduced to one.

 

They eliminate potential for human error. Because they are designed as an integrated circuit assembly with all interconnections controlled by the design artwork, the potential for human error in making interconnections is eliminated. This is especially true in the cases where discrete wires are used for interconnection.

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They can reduce both weight and volume requirements for a product. Flexible circuits are appreciably lighter than their rigid circuit counterparts. Depending on the components used and the exact structure of the assembly and final products, they can save perhaps as much as 60% of the weight and space for the end-product compared to a rigid circuit solution. Additionally, their lower profile can help a designer create a lower profile product than is possible with a nominal 1.5mm thick rigid board.

They facilitate dynamic flexing. Nearly all-flexible circuits are designed to be flexed or folded. In some unusual cases, even very thin rigid circuits have been able to serve to a limited degree. However, in the case where dynamic flexing of a circuit is required to meet the objectives of the design, flexible circuits have proven best. 

 

They improve thermal management and are well suited to high temp applications. Not only can they handle the heat, their thinness allows them to dissipate heat better than other thicker and less thermally conductive dielectrics.

 

flex-3.pngThey help improve product aesthetics. While aesthetics may seem a low order advantage, people are commonly influenced by visual impressions and frequently make judgements based on those impressions.

 

They are intrinsically more reliable. Flexible circuits help to reduce the complexity of the assembly and can reduce the number of interconnections that might be otherwise required using solder. Reductions in complexity is a key objective of a reliable design. With respect to the minimization of the number of solder interconnections, reliability engineers know all too well that most failures in electronic systems occur at solder interconnections. It follows naturally that a reduction in the number of opportunities for failure should result in a corresponding increase in product reliability.

 

In summary, flex and rigid-flex circuits have significant advantages. There are many additional advantages which go beyond the short list provided here. What is important to remember is that most of the advantages stem from the versatility and unique integrative abilities these important members of the electronic interconnection family can offer. If you have any concerns about your flex / rigid-flex ASC can consult with you or your customer on design related issues.

 

Materials

Flex and rigid-flex circuits are manufactured using numerous types of materials to meet a wide array of cost targets and performance requirements, both physical and electrical. Because of this variety, relative to the prospective concerns related to each choice, it is vitally important that the designer provide detailed information about the dielectric materials to be used. It is recommended that designers educate themselves about the choices available in terms of cost and performance. The Internet is packed with easily tapped information about flexible circuit materials and how they might be used. The PCB fabricator can also help with this topic. The basic flex material types are:

 

  • Adhesiveless materials, which have no acrylic bonding the copper to the polyimide dielectric.
  • Adhesive materials, which have acrylic bonding the copper to the polyimide dielectric.
  • Flame retardant and non-flame-retardant laminates, coverlayers, and bond plies.

 

 

flex-5.png 

 

Adhesiveless vs Adhesive Flex Cores

 

The type of copper used most often for flexible circuits is rolled and annealed copper (RA copper), which has the best properties for dynamic flex (repeated bending) applications in the flexible section.

 

Coverlayers are a polyimide film with B-staged adhesive used to cover and protect the copper traces of the flex circuit. This is a flexible coating of sorts that protect the delicate surface traces from physical damage and potential wicking of solder along circuit traces. The coverlayer offers protection while leaving open access to design features where interconnections are to be made to components by soldering. Coverlayers are available in various thicknesses of polyimide and various thicknesses of adhesive. For example, one can have a 1 mil polyimide coverlayer with a 1 mil adhesive or 2 mils of adhesive. It is important to determine the thickness of the polyimide and adhesive on the coverlayer as a balance needs to be made between allowing for maximum flexibility while also ensuring there is enough adhesive on it to accommodate the copper weight (the more adhesive, the less flexible).

 

Some one or two-layer flex circuits that will not be subject to multiple flex cycles or extreme radius bends can be coated with an epoxy-based solder mask that is designed to flex without cracking. However, this is not recommended when the design requires any dynamic or extreme flexing. The other option is a laminated flex coverlayer. These are typically materials that have a makeup that is identical to the flex core material and are best suited for dynamic flexible circuit applications.

 

 

Front-End Engineering

It is important to understand that it takes a lot longer to engineer flex and rigid-flex boards than it does rigid boards of similar layer count. While there are many process similarities, there are many other unique design and processing attributes that contribute to added engineering time.

 

Single-Sided Flex

Single-sided flex circuits are manufactured by starting with a pre-pressed and glued sheet of a thin polyimide or polyester with a copper outer layer. The circuit formation is like the single-sided rigid PCBs. The panel is cleaned, dry film resist laminated and then exposed with a negative image of the circuit pattern. Then it is developed to remove unwanted dry film and etched to remove the copper between the tracks. The remaining dry film protecting the tracks is stripped. If required, a coverlayer is pre-drilled and then laminated over the copper side. The coverlayer performs the same function as solder mask does in rigid PCBs. As an alternative, a flexible solder mask may be used but it does have certain limitations. The panel is silk screened with a flexible white marking ink, die cut or laser routed, inspected and shipped.

 

Options for all types of flex circuits include silver ink, which is screened onto the flex circuit to act as an electrical shield and/or the addition of FR4, polyimide or metal stiffeners, which are added after the panel is die cut, using 3M double sided tape or epoxy glue. Stiffeners may also be attached using prepreg under temperature and pressure. Some fabricators of high volume, single-sided, flex circuits have set up all the production machinery in one long line and make thousands of single-sided flex circuits per hour, using flex material in long rolls.

 

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Process Flow for Single Sided Flex

 

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A Single-Sided Board with Different Kinds of Stiffeners

 

Double-Sided Flex

A double-sided flex circuit would consist of flexible polyimide cores usually from 1-5 mils thick and copper weights per the customer specification.

 

The process of forming the circuits is like a double-sided rigid PCB. The bare polyimide, copper-coated sheet is drilled, and any slots or mechanical features are machined into its surface. The surface and holes are made conductive by conventional through hole plating with an electroless copper bath. A layer of photosensitive dry film is laminated onto both the copper surfaces. The image of the top and bottom layers is applied to the dry film using either laser direct imaging or a UV light source. The exposed circuitry design will cross-link and become quite solid in nature. By developing in a mild alkaline solution, sprayed under pressure, the soft unexposed areas will be washed away. The now defined circuit image is plated with copper to the desired thickness. Some plating options are to button plate the via only leaving the special flexible RA (rolled and annealed) copper for the flex portion of the circuit unplated. A thin layer of tin is added to protect the plated copper circuits from the etchant. The dry film is now stripped. Then the panel is cleaned and washed. The non-plated base copper is etched away with ammonia etchant. The traces are protected from the etchant by the tin plating. The protective plated tin surface is removed with a nitric acid-based solution and the board is washed and dried. A copper pattern closely following the desired shape is now prominent on the surface.

 

A pre-drilled (either CNC drilled or laser depending on the size of the openings) coverlayer is laminated to each side with a hydraulic press or a flexible liquid photoimageable solder mask is applied, exposed, developed and cured. A legend ink is screened onto the panel and cured. A surface finish like rigid PCB is then applied to the flex circuit to protect the copper before soldering and assembly. The circuit is die cut, laser cut, or routed to size, with any large unplated holes also drilled/cut at this time. The circuit is then electrically tested, inspected and shipped.

 

 flex-8.jpg  flex-9.jpg
Routed FR4 stiffener Polyimide stiffener

 

Some flex circuit boards may require stiffeners. Stiffeners are typically either FR4, polyimide or in some cases aluminum. The materials are pre-routed on CNC machines. The stiffener can be any desired thickness, either just to strengthen a part or to really secure the circuit to something. The 3M double-sided pressure sensitive adhesive (PSA) used to secure the stiffener, is quite strong and permanent. Also, some stiffeners are bonded to the flex circuit via a press utilizing higher temperature and pressure.

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Process Flow for Double Sided Flex

 

Multilayer Flex

To manufacture multilayer flex boards, the inner layers are made first using standard Polyimide with copper on both sides. Forming the circuit on the inner layers is again like rigid inner layer process. The biggest difference is that these cores are more difficult to handle since they are typically flimsy compared to rigid cores. So, handling and processing through equipment becomes far more challenging and the manufacturing process needs to be set up to handle this.

 

The panel is cleaned and laminated with etch resist dry film.  The dry filmed panels are then exposed using either laser direct imaging or photo tools and developed. The negative dry film image acts as an etch resist, which results in the etch removing the unwanted copper. The dry film is then stripped. The inner layers then go to AOI prior to further processing.

 

The boards are then baked to remove any moisture. The amount of adhesive used to bond the flex layers together is dependent on the inner layer copper weight

 

The layers are then aligned with tooling holes and put on pins that are in the lamination plates and pressed together in a vacuum hydraulic press. The temperature, time, and pressure are determined based on the panel size and the material suppliers’ specifications. The press package typically has special padding that conforms to the flex panels to assist with adhesive conformation and air removal.  After the board is pressed and the flash trimmed, the panel is processed as a normal, double-sided board.

 

The bare copper laminated board is drilled, and any slots or mechanical features are machined into its surface. A cleaning etch plasma cycle is performed to promote copper adhesion to the hole wall. The panel is then cleaned to remove any ash created by the plasma before electroless plating. The holes are made conductive by conventional through hole plating with an electroless copper bath or carbon process. A layer of photosensitive dry film is laminated onto both the outer copper surfaces. The outer layer images are exposed using either laser direct imaging (LDI) or photo tools. By developing in a mild alkaline solution and spraying under pressure, the soft unexposed areas will be washed away. The now defined negative circuit image is plated with copper until the total thickness is 1 oz. or as specified in the drawings. A thin layer of tin is added to protect the plated copper circuits from the etchant. The dry film is removed by a strong alkaline solution and then the panel is cleaned and washed. The non-plated base copper is now etched with ammonia etchant. The circuit trace is protected by the tin plating. The protective plated tin surface is removed with a nitric acid-based solution and the board is washed and dried. A copper pattern exactly following the desired shape is now prominent on the surfaces. The outer layer circuitry is now automated optical inspected (AOI).

 

A pre-drilled polyimide coverlayer is laminated onto each side of the etched panel to protect and seal the circuitry. Alternatively, a flexible liquid photoimageable solder mask is applied, imaged, developed and cured. If required, a flexible legend ink is screened onto the panel and cured. The desired final finish is then applied to the panel.

 

The circuit is die cut, mechanically routed or laser routed to size with any large unplated holes also drilled or cut at this time. The circuit is then electrically tested, inspected, and shipped. Some flex circuits use silver ink to provide electrical shielding or assist in controlling bend areas. It can be applied to the panel on top of the coverlayer. A FR4 or polyimide stiffener can be applied to improve tear resistance and assist assembly.

 

Rigid-Flex Manufacturing

When manufacturing rigid/flex boards, special engineering consideration must be spent on how to section and release the non-circuit areas between the rigid and flex sections. The design calls for a rigid section comprised of FR4 or other rigid material and a section of flex that will extend from the main circuit. How the different areas separate is important. Many methods, such as Teflon blocks or polyolefin release sheets, allow areas of rigid and flex to be separate within the press package. Using a simple example of a 4-layer rigid flex with a double-side flex core. The flex arms that extend off the rigid board cannot adhere the FR4 sheets on top of the flex arms. The low flow prepreg is pre-routed to only cover over the FR4 board and not the flex extension arms. To facilitate final routing, the FR4 is pre-routed with a slot at the join line of the flex and rigid parts. When the board is routed, the slot facilitates the router not passing over or through the join area.

To assist in removing the top FR4 layer, it can alternately be scored on the underside before pressing.

 

Panels can have cut outs in the inner layer filled with a Teflon plug, allowing even pressure from the laminating press but will not stick to either the FR4 or flex adhesive material. Polyimide core bonds to FR4 with prepreg or adhesive sheets providing a superior high strength joint. Expansion difference between flex and rigid sections is a problem for registration. CAD package manipulation is needed to scale and align the layers after processing and pressing.

 

Design time for rigid/flex is considerably more because of the engineering required to ensure the parts will separate and correctly align to the fingers of flex which extend from the main board to the sub boards.

 

The drilling, electroless plating process all works with the materials normally used in rigid flex. A plasma etch cycle is done prior to electroless copper to prepare the hole.

 

Problems exist with trying to obtain a smooth transition from rigid to flex. Some fabricators seal this joint with a strain relief material, to add strength and cosmetically hide the joint. During the design phase, talk with your board shop to determine the best way to produce the board.

 

Typically, the flex circuits are the inner two of the four layers, but it is possible to build flex as the outer layers. In the design of the layup and release sheet management you must solve how to mill out the FR4 inner section that’s not used.

 

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       Sample 4 Layer Rigid-Flex Construction

 

When designs utilize un-bonded multiple flex arms in parallel, special separator sheets or pre-routed adhesive sheets are required to keep the polyimide layers from adhering to one another.

 

 

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Rigid flex joint

 

It is possible to make individual multiple flex leg layers increasingly longer. This creates the proper radius and eliminates the stress. When a 4-6-layer flex arm area bends, it compresses the inner arms and expands the outer arms. By making each flex layer in the lamination package successively longer and pressing the package with expansion areas, the arms will be longer toward the outside of the bend. This is typically called a book binder flex and is much more expensive than a flex where all the layers are the same length.  For more information Rigid Flex PCBs and how ASC can help, Contact Us Today!