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A Material Mix & Match

Tue, 06/19/2007 - 6:30am
Modern flexible circuitry requires durability, conductivity, and flexibility. The challenge is finding the right combination.

The choice of materials used in the composition of a flexible circuit can have a direct impact on the circuit’s ability to bend or flex. The thicker the flexible circuit, the less it will be able to bend without cracking. It is important to identify the end use of the flexible circuit before beginning the design so that materials can be chosen accordingly.

When possible, the circuit should be designed without copper plating on the conductors in the flexing area. Copper deposited electrolytically may exhibit much lower ductility than rolled-annealed copper. The vertical grain structure of plated copper is much more susceptible to fracturing when it is stretched during flexing. Note that there are substrates made with additives that produce a very fine grain copper comparable in flex life to rolled-annealed in low-strain, high flex cycle applications, but copper plating added during the fabrication process is not typically this same fine grain copper.

Trimming the fat
Circuits that require high bend ratios need flexible materials. RA copper and polyimide insulation are usually the best choice.
In general, reducing the thickness of a circuit will aid in the circuit’s ability to flex. Efficient use of plating will not only help reduce thickness, but also minimize the need for other materials in the fabrication process.

Utilizing selective (pads-only) plating or adding outer (pads-only) layers to the circuit can reduce or eliminate copper plating on the flexing conductors. Eliminating the copper plating will also reduce the overall thickness of the circuit (only in the flexing area for pads-layer alternative) by removing the thickness of the plating, often allowing the manufacturer to also reduce the cover adhesive thickness. Adding other types of additional plating such as gold, nickel, or both, should also be avoided in the flexing area for the same reasons.

The overall thickness of a flexible circuit can be lowered by reducing the base copper weight, which also lowers the corresponding adhesive thickness. The dielectric thickness can also be reduced depending upon the application. Another possibility is to start with adhesiveless base materials. Subtracting the need for adhesive will usually reduce the starting thickness of each substrate by 25.4 to 50.8 µm when compared to adhesive-based substrates. Reducing the thickness by a few tens of µm may seem trivial, but if the net result pushes the bend ratio over 10:1, it is well worth the effort. The dielectric type will also be a contributing factor to the flexibility and ultimately the reliability of the circuit. Various dielectrics of the same thickness will have considerably different flexibility properties. Naturally, if stiffer materials are used in the construction, the result will be a stiffer finished product.

Packaging alternatives
Circuits typically go through a full-panel plating operation which adds copper plating over the entire copper conductor. Many circuits will mask the flexing areas during the plating process to ensure maximum flexibility where the circuit bends. Images: Minco
The choice of cover materials can also affect the properties of a flexible circuit, and a few useful alternatives have emerged on the commercial market.

Teflon has been used as a cover material for close to 20 years and is frequently employed when developers are looking for an insulation material. It usually appears on the outer layers in the form of covers, but it can also be used internally as an adhesive. Because Teflon is a thermoplastic material, which melts with temperature, it has a relatively high melting point when compared to an acrylic adhesive and is more difficult to process. For that reason, Teflon covers are not widely used. The covers are typically bonded to the circuit with a thermosetting adhesive that sets or cures with temperature, such as an acrylic adhesive at temperatures well below the Teflon melting point.

Acrylic adhesive has been the flexible circuit industry mainstay for bonding circuits together and has been in use for more than 30 years. An alternative to acrylic adhesive, however, is all-polyimide (AP) film. AP construction is typically employed only for high temperature applications. When a flexible circuit is exposed to elevated temperatures, the acrylic adhesive is typically the weak link. The thermal properties of acrylic adhesive are inferior to anything else in the stack. However, it does outperform AP in typically every other category, including bond strength and flexibility, so the application must be evaluated by case. For high temperature applications, AP construction is preferable. For lower temperatures, acrylic adhesive is preferable because of reduced cost.

Some assemblers opt for a liquid photo-imageable coverlay (LPI or PIC) cover layer to allow tighter bends. The advantage of this material depends on the type of photo image cover layer that is used. Some are fairly brittle and LPI will give worse performance than a polyimide cover. Some are more flexible. Typically, the best flexibility comes from a standard acrylic and polyimide cover.

Benefits of RA copper
Circuits typically go through a full-panel plating operation which adds copper plating over the entire copper conductor. Many circuits will mask the flexing areas during the plating process to ensure maximum flexibility where the circuit bends. Images: Minco
In most situations, copper is the material of choice for flexible circuit applications. It has many advantages, especially in rolled-annealed (RA) form. But developers also face more specific decisions, such as whether to use one-half ounce RA copper or one-ounce RA copper. One-half ounce RA copper is thinner—18 µm versus 35 µm for one-ounce copper, so the overall thickness of the circuit can be reduced, making it more flexible. At the same time, however, it is much more fragile than one-ounce RA copper, which boasts reliability despite driving up circuit thickness. This thickness increase is negligible, however, adding less than 25 µm. A little more weight in the copper will provide a more durable conductor.

Copper is not the only choice, however. Virtually any metal that can be supplied in a thin sheet, and that can be chemically etched with standard chemistry, could be used. Minco, Minneapolis, Minn., has a division that specializes in manufacturing flexible heaters, and fabrication processes there use Inconel, cupro nickel, and other metals as the conductive materials. These metals are ideal for this application because they heat up when current is run through them. In this situation, copper, which has low resistivity, is suitable only for making interconnects.

Still, other metals may not tolerate bending and flexing as well as copper. Fully annealed copper is highly ductile and has low resistance. If heat is desired in a select area of the flexible circuit, copper can be mixed with resistive metals to provide that heat while also maintaining the desirable flexible properties of copper. If shields and/or ground planes are required on the circuit, a crosshatched pattern should be used rather than solid copper. Another shileding option is a screened-on conductive coating such as silver epoxy, which is much more flexible than copper.

For applications requiring a tougher material than pure copper, Inconel is an option because it is a hard metal. However, it also has high resistance and less tolerance to bending or flexing than copper, which makes it more prone to fracturing when flexed. The ductility, or lack of hardness, usually gives copper the advantage for use in flexible circuitry. Minco can supply thicknesses as low as 9 µ, and as high as 356 µm. Other materials have a narrower range of useful thicknesses—15 µm to 58 µm for cupro nickel, and 75 µm or 100 µm for beryllium copper.

Meeting bend ratio goals
Click to enlarge.
High bend ratios are desirable for a circuit developer—the circuit will be open to a wider range of applications. Achieving reliability can be problematic, but circuit builders with specific design goals can achieve these high ratios. For example, a double-layer circuit bent to a 90° angle that will not be required to move after installation can achieve a 4:1 or 5:1 bend ratio without reliability problems. However, if the circuit is being handled, and portions of the circuit are being plugged and unplugged, or the joint is exercised by even a few degrees, the reduced bend radius can reduce reliability. Also, a double-layer circuit will have plating on the outer layers with variation from vendor to vendor. If a manufacturer is running less than 15-20% elongation on its plated copper, reliability could suffer at 5:1 or 4:1 bend ratios.

Options for finishing
The plating process for bare copper traces is also open to a variety of choices, though most alternatives are specifically intended to fulfill special design considerations. Partly because of the disadvantages of these custom finishes, and the increased cost, the list of choices quickly narrows.ENIG (electroless nickel immersion gold), immersion silver, and electrolytic gold over nickel are the most common plated finishes. ENIG and immersion silver can be applied without the need for a buss bar to provide electrical connection. Immersion tin is popular but can cause problems in some applications because of tin whiskers. Other plating options are available, but, again, specific design considerations are usually involved.

An alternative to ENIG and immersion silver finishes are organic solderable preservatives (OSP). Commonly used for ball grid array assemblies, OSP is not as widely accepted as other finishes. The most common lead-free finishes, ENIG and immersion silver, hold the advantage because they allow the assembler to simply solder paste, place components, and reflow.

—Mark Finstad
Principal Applications Engineer,
Flex Circuit Division, Minco, www.minco.com
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