Rigid Flex PCB 2

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

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.

 

They 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.

 

 

 

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.

 

They 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.

 

 

 

 

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.

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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