General purpose flexible circuit boards are those with electronic components that can be bent to fit in tight spaces. Most are just one or two layers thick and are meant for "flex to install" applications, as they will tolerate limited flex cycles. Circuit boards like this are often found in a variety of medical and consumer products.
The more difficult flexible circuits have three or more layers and are based on specifications that require, for instance, high flex cycles, or boards that must be bent or flexed to fit into unusual packaging. What constitutes high flex cycles? It's is hard to say? There have been tests done in the commercial industry where they are well into 100K bend cycles, but it all depends on the length, type of materials, copper weight on the boards, type of bends etc. The government, on the other hand, can and does use many flexible circuits that are in one-time bends and high flex cycles. Electrical engineers are getting so creative that many new flex circuits are specified with unusual features that can take a little research and experimenting before they are manufactured in quantity. This article tells of one such flex circuit, and although it was not for a medical product, it could be used for medical applications.
A Few Unusual Circuits
Most people think of a flex circuit as a board consisting of conductors sandwiched between layers of insulating material. Although true, that description fits many types of flex circuits. A few examples would include:
- Single-sided flexible circuits that have one conductive layer of copper on a dielectric (insulating) material. These circuits are simple circuits that could be used in a hinge circuit on a laptop computer or maybe on the hinge of a flip open phone. These can be built by a number of different fabricators.
- Double-sided circuits with copper on both sides of the dielectric. These have similar applications as above, but are used where more interconnections are required in a small space.
- Flexible circuits with several layers. Each is registered to the other layers and bonded together with a suitable adhesive cured layer. The layers are all interconnected by plated through-holes. The number of flex layers can be up to the engineer's imagination, but too many layers and too much copper will turn it into a rigid board.
- Sculptured flexible circuits are single or double-sided designs with thicker copper to allow connections such as fingers or pads as rigid extensions of the flexible conductors. These circuits have thicker copper plated on exposed areas where the conductors could be inserted into a mating connector.
- Molded flex are flex circuits that are thermally molded into simple shapes. A good example of this would be a formed dome on a flexible circuit.
- Multi-layer rigid flex circuit boards combine flex circuits and rigid boards where the flex circuit extends from the rigid board area. These boards are the most complex and are used to interconnect two or more rigid or multilayer boards together.
Fermi National Accelerator Laboratory is home to the Tevatron, a four-mile circumference high-speed particle accelerator. The lab needed a flex circuit for an unusual application – a particle detector inside the accelerator. The circuit had to work as a controlled impedance board with a low loss material because the circuit would be detecting low amplitude signals. Also, the circuit could tolerate no out-gassing because the researchers at Fermi didn't want the detector finding particles that had been out-gassed off the circuit. In addition, the circuit had to be flexible enough to be inserted into an enclosure.
The lab needed a controlled impedance board that was flexible, even though the design required four copper layers — two layers of signals and two layers of shielding. What's more, the circuit would have to withstand soldering to make all the electrical connections.
A Few Materials
There is no such thing as a perfect flex circuit material. Each material has it's pros and cons, so the design trick is to make or find a material that meets most of your design or use needs. The most common flex material used today is DuPont's Kapton. It is a dimensionally stable polyimide compatible with most printed circuit board processes. The material has high tensile strength, it is non-flammable, and it has about the same coefficient of thermal expansion as copper. The biggest downside is that it has a high moisture-absorption value.
Mylar or polyester films are also used in flex circuits. These materials are highly flexible, dimensionally stable, and have a low cost. Another advantage is their low moisture absorption. However, polyester films don't tolerate soldering temperatures.
Nomex, an aramid material, is dimensionally stable, has good tensile strength, and can stand up to soldering temperatures. Its drawback is its propensity to pick up process chemicals and moisture.
All of the common flex-circuit materials considered for the Fermi job had some shortcoming that disqualified them. The biggest problem was that most of the materials had high moisture-absorption values and similar high chemical absorption. If a material can pick up moisture or chemicals during manufacturing, it can then give off or out-gas those particles later in the application. The polyester material, however, looked as if it could meet the low outgassing requirements needed because of its low moisture absorption spec.
Polyesters presented a few challenges, first with trying to build a four copper layer board and, second, with the high temperatures required for soldering. These two problems made the polyester materials difficult to work with in this application.
Brigitflex Applications Engineer Chuck Lawrence worked with Taconic's Applications Engineering Manager, Ed Sandor, to find a solution. Taconic develops and produces woven glass PTFE (Teflon) materials. The company has been making Teflon materials since the mid 1960s, and Teflon laminate materials since the mid 1980's. Taconic makes laminate materials with Dielectric Constants (Dk) ranging from 2.1 up to 10.2 with thicknesses from 1.5 mils to over 0.5-in. Taconic engineers have produced copper clad materials down to 1.5-mils thick. They have grouped some of their thinner PTFE materials into a family of flex materials called Hyrelex. These come in two different Dk values: Taconic TF-260 has a Dk of 2.6 and TF- 290 has a Dk of 2.90. They are available in thicknesses from 1.7 to 3.5 mils. The Dielectric Constant, also known as the Permittivity, or Er, of the material, is an important property. Knowing the Dk of the material, an engineer can easily calculate the correct etched line width to get the controlled impedance you need for a particular material thickness.
Brigitflex engineers examined Kapton and other polyimide films, and compared their moisture absorption properties to those for Hyrelex. They chose a polyimide laminate and built samples to compare them to the PTFE (Hyrelex) materials. Taconic's TF-290 appeared to be the best material to meet the electrical specs. It's Dk of 2.90 gave a good line width and spacing to get the right impedance for the board.
The next step was to see if the polyimide laminate or the Taconic's TF-290 material would meet the application's out-gassing requirements. Because TF- 290 has a moisture absorption of less than 0.02%, Lawrence and Sandor were confident the material would not pick up moisture or plating chemistries during processing. Also, Sandor pointed out that Taconic's TF-290 material has been tested against NASA requirements for outgassing in space applications and it meets them easily. Proof would come from tests at the customer's facility. Circuits made with Kapton and other polyimide films failed the out-gassing test, but the Hyrelex materials passed easily.
Brigitflex engineers had the right material but still faced the challenge of making a four copper-layer board flexible enough to meet the application's bend requirements. The original design, with solid ground planes on the outer layers, proved to be a stiff board. The team solved the problem by making the outer ground planes a cross-hatched pattern. This eliminated almost half the copper on the outer layers but did not affect the electrical integrity of the board. It also made the board more flexible. The boards were built and shipped on time, then tested and installed, and are now working in the accelerator at Fermi National Labs