Section: PCB technology

The growth in the application of flexible printed circuits now exceeds any other aspect of interconnection for electronic systems. At the same time, the diversity of types, manufacturing techniques and innovation of flexible PCBs are proceeding at an even greater rate, yesterday's possibilities are today's realities. This review explores the progress.

What types of modern flexible circuits should a designer be considering? The simple definition of a flexible circuit is a thin dielectric material with a network of electrical conductors. In practice, them are numerous options and many technical aspects to be considered.

The construction can be single or double sided and may have stiffeners or rigidised sections to allow for mounting of discrete or surface mount components and interfaces. The conductor pattern is encapsulated within a covercoat or coverlay. A coverlay is normally made of the same dielectric material as the base laminate, whilst the covercoat can be either a liquid or photo-imageable film. These latter materials afford a lower cost product, in addition to being advantageous for applications where surface mount components are applied.

Where single or double sided circuits cannot achieve the required connectivity then Multilayer Flexible Circuits (MFCs) are the next choice. These consist of multilayers of circuit patterns stacked and bonded together to form a high density flexible or formable interconnection system. Constructional materials are the same as those used for single and double sided flexible circuits however, it will be appreciated, the more layers the stiffer the circuit will become. Typical circuits of this type consist of no more than 4 to 5 layers. For more complicated circuits and for higher layer counts then Multilayer Flex-rigid is the approach to take, see figure 1. As the name implies these circuits are made by combining flexible and rigid materials The rigid part is used for component mounting and the flexible section employed to allow folding or hinging to meet the physical characteristics of the final installed shape. This technique is relatively expensive but provides considerable technical advantages that offset the price difference. Reliability is generally somewhat enhanced as the use of connectors between the rigid elements is eliminated altogether. Not only does reliability improve considerably but the connection costs are eliminated. This technology is best suited to circuit constructions up to 8 layers without the materials having an effect on product reliability.

In avionics and aerospace applications there is a call for: critical levels of reliability, low weight and high density circuits. In this case, there is a solution with a technology called Regal-flex (Rigid Epoxy Glass Acrylic Laminate). This technology yields high integrity flex-rigid multilayer circuits with layer counts above 8. Due to the constructional techniques employed the rigid element is made entirely of FR4 epoxy glass laminate materials (no polyimide or acrylic adhesive is used as in the conventional constructional methods). Examples of the construction are shown in figures 2, 3 and 4. Flexible materials are only utilised on the required flexible area. Therefore, the "Z" axis expansion problems associated with high layer flex-rigid circuits, utilising Polyimide and Acrylic adhesive in the rigid element, is totally eliminated. Today, this technology is also being employed in a wider variety of applications such as in computers, communications equipment, the automotive and medical industries. Actual examples are shown in figure 5.

A technology that has proved itself to be reliable and cost-effective in many applications ranging from cellular telephones to missiles is that of sculptured circuits, see figure 6. These are normally manufactured from 0.25mm (0.01 in) thick copper which, as the name implies, is selectively reduced in thickness to enhance flexibility where required. The manufacturing technique retains its original thickness in areas where rigidity or high current carrying capacity is necessary.

As the process is effectively "chemical milling" all features in the copper geometry are produced whilst the conductor pattern is being etched. These include holes through the copper in positions where components are to be attached. This eliminates the need for drilling or piercing operations, apart from those in the dielectric material. In addition, the robustness and extended use of conductors up to 0.25mm thick means that the material can be preformed without expensive tooling and used to provide an alternative to terminals or connectors.

An alternative and modular approach is utilising ModuleFlex. As the name suggests this is a composite of the various other technologies that combine to provide a complete interconnection packaging solution. For example, it is possible to integrate sculptured circuits with flex-rigid multilayer circuits, or flexible circuits attached to rigid printed circuit boards. ModuleFlex is generally employed where a single item solution has many complex interconnections and would otherwise be cost-prohibitive, or physical size and shape constraints restrict the design.

Design considerations

When embarking on the design of a flexible circuit, or indeed any of the associated circuit technologies, there are a number of factors to consider to ensure that the final design meets all the mechanical, electrical and manufacturing requirements necessary for a fully compliant, cost-effective solution.

It is important that the design engineer understands that the commonly known design rules for rigid PCBs differs from flexible technology in a number of ways.

Firstly, minimise the design constraints. What does this mean? Simple point to point connections make for simple circuits, which in turn generally reflect in a lower cost solution. If all the terminating points are predetermined it is inevitable that the interconnections will be more complex.

Teamwork from the outset of design is paramount and the more everyone "buys" into this stage of the process the better the end result. Not only must designers be aware of what they need to achieve from a circuit diagram, but how the product is manufactured and how the fabricators assembly process is handled. It is also important that the "team" involves the flexible circuit manufacturer at an early stage as good choice of technology and materials can have a beneficial effect on the design and cost of the resulting circuit.

From the artwork design standpoint an important aspect is understanding that unlike a rigid PCB, flexible materials "move" and therefore allowances in the design must be incorporated. In addition, the shape and size of solder pads, methods of board and component anchorage and the thermal relief points need to be considered in line with the general physical parameters of the various components.

Covercoat or coverlay apertures are a feature that also has a bearing on the overall robustness of the circuit and it is generally advisable to leave the selection to the fabricator. Their extensive knowledge and experience means they are best placed to ensure maximum available solder land conditions based around process and manufacturing restrictions.

Changing the rules

When it comes to conductor width and spacing, again, the "rules" are somewhat different to those generally adopted in the rigid board industry. Because the laminate is flexible and the bond strength of copper to the base laminate is not as high, it is good design practice to make the conductors as wide as practical. Despite common belief, fine line widths can be produced on flexible substrates, with special attention. Dynamic applications do present a problem for fine line widths as there is less copper applied, compared to the insulator material and this does result in a circuit more susceptible to failure. Therefore, balancing the copper to insulator ratios makes for good, reliable design.

In choosing materials, as mentioned previously, flexible circuit fabricators are probably the best source of information. Ultimately, material choice generally reflects the application, the environment and the price structure. Some of the materials include Polyimide (trade name is Kapton), Polyester and PEN.

Polyimide is still the most widely used material when is comes to harsh environments, component assembly and safety critical applications. Polyester (Mylar) is the lowest cost material which is most often used in less critical applications and where cost is of paramount importance. Newer materials have started to appear over the last few years and PEN (Polyethelnapthalate), to name just one, is a product with a price/performance somewhere between Polyimide and Polyester.

There are a significant number of other design considerations to be born in mind and these cannot be covered in detail in this article, for example EMI/RFI specifications and limitations, electrical requirements, current carrying capability, performance characteristics, ease of manufacture and individual cost analysis, in this respect, [1] is useful.

Communicating the design

With a satisfactory design in place, communicating accurate and detailed plans to the flexible circuit manufacturer for prototypes or production is the next stage. The most cost-effective method of transmitting this information is electronically via digital data. The vast number of software formats makes this area a potential minefield for error. As mentioned earlier, it is recommended that a compatible digital format be agreed at the outset of design, rather than left until this latter phase of the process.

Although many software packages use a digital language that is unique to the functions performed by that program, a series of filter interpreters are usually supplied to import and export files. This is fine if the board manufacturer has the same version of the software package or suitable interpreter. In practice, it is impossible to cater for every version. Some of the software formats that are used for designs include, Infinite Graphics IGI2100 Proflex (CAD software), PADS PowerLogic and PowerPCB Windows95 version (PCB software), Tibor Darvis Planmaster and surprisingly, word processing and graphics software like Microsoft Office 97, Lotus Amipro and CorelDraw. Most engineers understand that many software programs are backwards compatible, to an extent. As the software and files get ever more complex there are still likely to be problems with incompatibility. In the case of PADS, this program cannot read files from earlier versions.

With so many versions of CAD/CAM software available it is worth looking a little deeper into the special requirements for these packages. One of the most popular formats is Gerber. This language is used to directly drive a photo plotter and although there are several versions of Gerber, they fall into two styles - Gerber and Embedded Gerber. When sending files in Gerber it is recommended that the files are supported by documentation such as decimal point positioning, ie, format such as 2.4 meaning two places before the decimal point and four after. In addition, confirming end of block characters, units of measurement (imperial or metric) and provision of an aperture list containing all D-codes employed. The advantage with Embedded Gerber is that more information is included within the file, such as decimal point and end of block characters. However, it is still recommended that the format and units of measurements be confirmed.

DXF language is used as an output from CAD software and has the advantage that it can be translated into almost any CAD and graphics program. The scale can usually be established as long as some dimensions are contained within the file. Again, to avoid problems, it is recommended that the units of measurement be supplied separately. In addition, although outlines of tracks can be converted back into Gerber via the CAD program, it is desirable to export DXF files with polylines.

Hewlett Packard's HPGL language is designed to drive a pen plotter and can be opened in a graphics program, but not a CAD program. Data supplied in this form is for information only and an accurate image cannot be obtained through conversion to other formats.

Within word processing and graphics programs BitMaps or BMP files are images that can be read by most software packages. Although a conversion to a CAD format is possible, the resulting file can only be used as a guide. An accurate image will need to be created using drawings supplied either as paper plots or digital files.

As more programs come onto the market and new digital formats are created, the need to integrate files across programs will mean more compatibility with new programs and less with older ones.

Because of their flexibility, Gerber and DXF are the two main formats that should be used wherever possible. In the case of Drill formats, the final Drill program is dependent upon panel layout and it is recommended these are supplied as graphics files and not in Drill languages.

The experts are the answer

Whilst there is a growing awareness of flexible circuit technology and its use, the ever increasing demand in many market sectors means that circuit fabricators should be considered one of the main sources of knowledge. This knowledge resource should not be left untouched until late in the design stage, if design constraints leading to over complicated and unnecessarily costly solutions are to be avoided. In addition, new materials are becoming available that lend themselves to a wide variety of differing applications and improved manufacturing techniques. New equipment capability means that, with the help of the circuit fabricator, almost all types of interconnect solutions can be realised and produced economically.

PHOTO (COLOR): Figure 1: Examples of Flex-rigid and Regal-flex circuits

DIAGRAM: Figure 2 (above): Example of a typical Regal-flex construction.

DIAGRAM: Figure 3 (above right): Exploded view of the construction elements of a Regal-flex circuit

DIAGRAM: Figure 4: An example of a double sided inner layer of a regal-flex assembly

PHOTO (COLOR): Figure 5: An 8 layer regal-flex for an automotive application

PHOTO (COLOR): Figure 6: Examples of sculptured circuits

Reference/sources of information

[1] Flextronic, the company with which the author is associated have produced a Designers' Guide. This comprehensive document takes the designer from first principles through to design completion and addresses more fully all aspects of good design practice. This is available as an interactive CD-ROM or a book and incorporates a check list of questions to ensure that at all stages of the design the designer has access to the most up to date information on flexible circuits.

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By Paul Thomas, Flextronic (+44 (0)1243 784516)