By Joseph C. Fjelstad, Pacific Consultants LLC, Mountain View, California The short definition of a flexible circuit is "a patterned arrangement of printed wiring utilizing flexible base material with or without flexible cover layers." While the above is a reasonably functional definition, it fails to portray adequately the depth and breadth of this extraordinarily versatile interconnection technology. Today, flexible circuits can be found in nearly every type of electronic product, from simple toys and games up to the highly sophisticated electronic instrumentation found in space hardware. 1898 Patent While the technology appears new to many, flexible circuits (or flexible printed wiring), as the technology is also known, has actually been in use-conceptually, if not in fact-for more than a century, based on a patent issued to Albert Hanson of Berlin, Germany, in 1898. Hanson's patent describes the pro-duction of flat conductors on a sheet of paraffin-coated paper. What could be more flexible? Moreover, the concept was amenable to multilayer construction-a technology clearly ahead of its time (Figure 1). A few years later, a laboratory notebook exchange between Thomas Edison and one of his subordinates yielded concepts that look amazingly like today's polymer thick film-on-membrane technology.
Novel Methods Edison apparently foresaw the use of graphite powders coated over a pattern of liquid adhesive that would be cured later. Other researchers and scientists in the first half of the 20th century followed. They conceived and described many other novel methods for producing and using flexible electrical interconnections. Today, flexible circuits are used in nearly every imaginable type of electrical and electronic product. They represent one of the fastest growing interconnection product segments, and the technology should enjoy continued growth and increasing numbers of participants among both users and manufacturers, especially in the area of IC packaging. According to a recently published report from Techsearch International, last year standard flex circuit technology represented about a $3.4 billion market worldwide, with perhaps another $2.5 billion market, accounting for high-density flex, which will be required for IC packages. 15% Growth Rate With an anticipated average growth rate of about 15% per year, the future of the technology looks very bright. Currently, Japan leads the world in both production and application of this versatile interconnection technology; however, flex manufacturing is rapidly spreading throughout Asia. Techsearch International also notes that there are more than 40 top and mid-tier manufacturers of flex circuits in Asia. Entry into flex circuit manufacture requires a fair amount of care, and-in some cases-capital. It is not a "drop in" replacement for rigid PWBs, either in fabrication or assembly. The difficulties have been recorded by many flex circuit novices who have been bloodied by hard experience. These intrepid companies, both manufacturers and users alike, were often lacking the specific design and production knowledge required to prevent the types of problems they encountered. Lessons Learned Others repeated the mistakes many times over until the lessons learned came to the attention of a broader electronics manufacturing industry, and the experience was formalized through standards, specifications and design guidelines. The common language of these documents enhances the possibilities for better communication, further improvement and continued growth of this vital technology. Common Types and Constructions Flexible circuits are most commonly manufactured using one of two base materials, either polyimide or polyester. Polyimide is favored where soldering of the assembly is required, while polyester is generally used in low-cost applications. Polyimide is also the material of choice for nearly all CSPs and flex BGAs; however, polyester has been successfully used in the creation of Smart Cards, which are arguably a large format chip package and due some recognition (see Figure 2).
Flex circuits are produced in several basic forms that generally parallel rigid PWB constructions. The product is normally supplied with a cover layer. Cover layers are typically polymer films of the same family as the base film with an integral adhesive layer. It is also possible, however, to use other processes and materials. An important example used in chip- packaging substrates is photo-imageable cover films. There are advantages to using cover films, but there are also potential performance differences, as well, which need to be understood (see Figure 3).
'Flexing Life' The primary concern is related to the flex-ing life of the product. It is important to know that flexible films tend to perform better in extended flexing applications. Single-Sided Flex Circuits Single-sided flexible circuits are the most common type in production today. They are also the construction most often employed and best suited to dynamic flexing applications. Their construction consists of a single patterned conductor layer on a flexible dielectric film. Termination features on these circuits are accessible only from one side. Single-sided flex circuits can be fabricated with or without such protective coatings as cover layers or cover coats. While many different metal foils can be used as the conductor, copper foil is the most common. Back-Sided Flex Circuits Back-sided flex circuits (also known as double-access flex circuits), contain only a single conductor layer, but are processed to allow access to the conductors from both sides. This construction is often employed in IC packaging. Tape automated bonding (TAB), for example, relies almost exclusively on this construction (see Figure 4). Similarly, most flex-based CSPs employ this construction.
Double-Sided Flex Circuits Double-sided flex circuits contain two conductor layers and can be produced with or without plated-through holes, depending on design requirements. Two-metal layer flex circuits will likely see increased use in chip packaging, as operating frequencies continue to rise and the need for controlled impedance constructions increases. Multilayer Flex Circuits Flex circuits that have three or more conductor layers are referred to as multilayer flex. The layers of the circuit are interconnected with plated-through holes. Newer methods of constructing multilayer flex circuits may limit the need for high aspect ratio plating. Newer multichip packages (MCPs), with their higher interconnection density, may employ such structures in the future. Rigid-Flex Circuits Rigid-flex circuits are a hybrid construction, consisting of rigid and flexible substrates laminated together into a single package and electrically interconnected by means of plated-through holes. Such flexible circuit types have also enjoyed tremendous popularity among military product designers, but, in recent years, this type of construction has made gains in the commercial world, as well. Rigid-flex boards are normally multilayer designs, but double-sided (two-metal layer) constructions are possible, as well, and, in fact, have been selected for certain microelectronic chip-packaging applications, most notably in the construction of hearing aids. Technology Applications Flexible circuits are as diverse in their application as they are in their design, and while IC packaging is currently one of the more interesting applications for flexible circuit technology, there are many other products that benefit from them. As increasing numbers of engineers become familiar with the technology, new applications are bound to evolve. Beyond IC packaging, there are only two very basic use categories for flex circuits. These are "flex to fit" uses and dynamic flexing applications. Flex-to-fit applications represent the vast majority of flexible circuits in use today. These are flex circuits designed to be flexed or formed one time as they are placed into their application. Dynamic flex circuits are self-defining and represent the typical vision most people have of how a flexible circuit is used. Such circuits are designed to be flexed during their useful life, either intermittently, as would be in the case of flex circuits used as hinges in electronic products or drawer pulls, or continuously, as in the case of disc drive read-write heads. Application Drivers There are numerous motivational reasons for using flex circuit technology. As an interconnection methodology, they are unmatched in terms of their versatility. In some cases, such as dynamic flex applications, the choice of flexible circuits is an obvious one, driven strictly by the lack of viable alternatives. However, there are many other subtle areas of opportunity to employ flexible circuits. Following is an examination of some situations where flex circuits can solve packaging problems:
The Next Generation The newer flex circuit IC packaging constructions are showcasing the technology's ability to meet the demand for increased circuit density. These new packages often occupy a small fraction of the volume of more traditional approaches. The result is a continuation of the electronics industry's familiar rubric of smaller, faster and lighter. As pointed out earlier, flex circuits have been used for semiconductor packaging since the late 1960s, when substrates for TAB were first introduced. However, TAB never really took advantage of the full range of properties flex circuit materials have to offer. In fact, some TAB was made on thin glass epoxy substrates, adding testimony to the lack of true concern about flexibility, beyond the material's ability to be wound and unwound from large film reels. Illustrated in Figure 6 and described below are the three primary ways of employing flex circuits to package integrated circuits:
Over the course of the last few years, there has been a surge of activity geared to the advancement of these approaches. A key feature of flex circuits that has spurred this new activity is its physical compliance as a substrate. The advantages of flexible substrates for mounting SMT devices was first pointed out some years ago, when engineers at IBM, Sheldahl and elsewhere noted that surface-mounted devices assembled onto flexible substrates survived many more thermal cycles before failure than did the devices on their rigid substrate counterparts. Solder Joint Failures The experiments were carried out during the early, painful period that followed the introduction of surface mount technology. It was during the time when solder joint failures were the most common mode of failure of a surface mount assembly. Meanwhile, other engineers, especially those in the military and aerospace industries, were busy trying to find other ways to mitigate the fundamental problem of mismatched coefficients of thermal expansion. The problem stems from the fact that ceramic die packages had a CTE of ~6 ppm/°C, while common printed wiring substrates possessed CTEs of ~18 ppm/°C for FR-4. Such strains can result in significant deformation of the package and lead to solder joint failure or cracking of the package itself. Finite element modeling and analysis methods, had they been widely employed, could have helped packaging and assembly engineers visualize the impact of their choices (see Figure 7).
Choices Reduced Ultimately, the choices were reduced to two, either 1: develop new materials that matched the CTE of ceramic or 2: form the leads of the SMT device to allow the leads to absorb the strain of the thermal cycling. Because of the expense of new materials, the lead shaping became the most common solution. The familiar gull wing shape of leads on peripherally leaded packages provides visible testimony to this effort; however, peripherally leaded packages will have a limited roll for higher pin counts or when size reduction is required. Because solder balls offer very limited compliance, new area array packages are forcing new solutions. Flex circuit based IC packages will likely become a big part of future IC packaging. The demand for fine line features from the flex circuit with 50 µm (2-mil) line and space plays to one of the strong suites of flex circuit manufacturing technology. Summary Flexible circuit technology offers many viable solutions for those challenged with packaging electronic products. The technology has survived its infancy and become a strong contender in the arena of electronic packaging technologies. The limiting factors in the advance of electronics packaging with flexible circuits rest in the imagination of those assigned the task. Hopefully, the concepts shown here will spark new ideas in the mind of the reader, helping them see a clearer path to the solution of an interconnection problem through the use of flexible circuits. Further Reading
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