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Previous flexible circuit materials are not suitable for high speed applications due to their large dielectric loss characteristics (loss tangent). The dielectric constant of these older materials is very reasonable for high-speed materials (in most cases between 3.0 and 3.5). Previously, polyimide was thought to be the key to the problem. However, as shown in Table 1, the dielectric loss of standard flex circuit adhesives (acrylic and epoxy) is much greater than that of polyimide films. In fact, the newly developed polyimide formulation has very low dielectric loss, which means that all-polyimide dielectric layers are a good choice for high-speed circuit materials. However, not all polyimide materials are the same; some materials have higher dielectric loss than others, and for most polymers, dielectric loss is related to moisture absorption, which also applies to polyimide. Therefore, we get a basic rule: dielectric with strong hygroscopicity will also have a large dielectric loss.

Table 1 also lists other dielectric materials that have recently been used to make high-speed flexible materials. Fluoropolymers, the primary material for high-speed rigid sheets, have been added to flexible materials by a variety of material suppliers. Fluoropolymers have a low dielectric constant and low dielectric loss for any polymeric material.

The problem is that the mechanical properties of pure fluoropolymer films are not good enough when used alone. Some suppliers try to use special fiberglass fabrics and fluoropolymer blends in flexible applications. Others improve performance by adding fillers, which are common in rigid, high-speed materials. Another supplier uses a polymer core, which is then coated with a coating of fluoropolymer material and adhesive. These improvements have contributed to increased flexibility and mechanical properties and, in some cases, reduced machining difficulties. However, these improvements also increase the dielectric constant and dielectric loss, so pure polymer properties cannot be achieved in practical products. These new materials are in many cases good candidates for high speed flexible materials.

Liquid crystal polymer (LCP) also has good dielectric properties, low loss, weak hygroscopicity. LC polymers are used for both the foil substrate and the adhesive layer. Fluoropolymer materials are also used in foil substrates and adhesive layers.

The main foil substrates used in high speed flexible circuits are based on fluoropolymers, liquid crystal polymers and low loss all-polyimide structures. LCP and fluoropolymer structures may in some cases act as tie layers and cover layers. But until recently polyimide structures were not able to be used as low-loss coatings and adhesion layers. In fact, low-loss coatings were difficult to develop until they were developed.

The emergence of new low-loss thermosetting adhesives has expanded the range of material options for high-speed rigid sheets. Various versions of low-loss adhesives are already available on the flex circuit material market, providing more options for adhesive layers and overlays. Although the traditional adhesive used in flexible circuits has a large loss, this does not mean that all thermosetting adhesives have a large loss.

In Table 1, the dielectric constants of almost all polymers are relatively low, and in truly flexible materials the dielectric constants of these polymers remain in a low range. Although there are advantages to lower dielectric constants, dielectric loss is the most important dielectric property when choosing high speed flexible materials. The dielectric loss has a much greater effect on the overall flexible circuit material than the dielectric constant.

When talking about dielectric losses and high-speed controlled impedance lines, two factors must be considered: the speed (1 GHz or 20 GHz) and the length of the line design. All all-polyimide foil substrates were used on signal lines with a speed of 20 GHz and a length of 1 ". Many times this is not feasible at 10 "lengths. Therefore, the material for each application should be selected according to the speed and length of the line.

The use of any of these new materials requires trade-offs between the three aspects mentioned in this article: electrical properties, flexibility and mechanical properties, and ease of machinability. All material suppliers have to weigh these three aspects and choose a compromise for the final product. Both manufacturers and end-users must understand these trade-offs and choose materials after fully understanding the trade-offs. The bottom line is that materials should not be selected solely for electrical properties; flexibility and ease of processing are also critical for functional high-speed flexible circuits.

Mechanical properties and flexibility

Most conventional flexible materials are optimized for flexibility and workability at the expense of dielectric properties. Of course, when these early materials were developed, the so-called "high speed" was actually less than 1 GHz. Finding low-loss polymers is a step toward developing new flexible circuit materials.

The fluoropolymer itself does not possess sufficient mechanical properties for use in flexible foil substrates. The strategy used by material suppliers is to reinforce the fluoropolymer by using special flexible fiberglass fabrics, particle fillers, or polyimide cores. All of these materials enhance the mechanical properties of the fluoropolymer. glass fiber fabrics and particulate fillers may limit the flexibility of the foiled substrate or tie layer and are therefore not an option when selecting materials for circuit applications that require bends at sharp angles and/or at low bend rates. but they can also be used to other flexible applications."

Liquid crystal polymers are suitable for freestanding dielectric films on high speed flexible foil substrates. Its mechanical properties meet the needs of most applications. Their bending ability may not be as good as all-polyimide structures, but it is sufficient for most applications.

The all-polyimide dielectric layer is the best candidate material with good mechanical properties and flexibility. If the product requires a high degree of bending, then this material is a good choice. However, even a low-loss polyimide layer is slightly more lossy than most fluoropolymer and LCP structures.

As for the adhesive layer and the cover layer, the material selection is more complicated. Many high speed material foil coated substrates can only be used with specific adhesive layers. Fluoropolymer/polyimide composites are used primarily with adhesive layers of the same material. Similarly, foil coated substrates based on LCP polymers are used primarily with adhesive layers of the same material. Both methods use thermoplastic polymer films that require high temperature lamination. Many manufacturers now use high-temperature laminators to achieve the desired temperatures for new materials (270 ° C to 310 ° C).

Both the low loss adhesive layer and the cover layer are compatible with newly developed all-polyimide foil substrates. Coating the fluoropolymer core with a low-loss thermosetting binder is one of the material options. This approach has been tested with all-polyimide foil substrates and some fluoropolymer foil substrates. One advantage is that thermosetting adhesives can be laminated at more standard lamination temperatures.

Another material choice is very novel--an all-polyimide bond coat and overlay. Initially, this material was developed with all-polyimide foil substrates for use in high temperature applications. However, the loss of this new adhesive film is very low (0.003), and the loss value of the all-polyimide foil substrate is in a range. This new adhesive layer requires high temperature lamination, but this is also the way all-polyimide flexible circuits are currently manufactured. Since this product is relatively new, it has been adopted by relatively few manufacturers, but it is growing. So far, all-polyimide structure is the material of choice for manufacturing high-speed circuits, but this material also needs strong heat resistance.

In addition to fluoropolymer/polyimide composites, all of the bonding layer material options mentioned above can also be used to make the overlay. As mentioned above, low-loss coatings are important only in microstrip line applications. Many of the high-speed flex circuits currently using these new materials are for stripline applications, where only a low-loss adhesive layer is required.

The machinability

Machinability of many new materials has always been an important link affecting their batch production. This should be anticipated when introducing new materials.

Several of these new materials require high temperature lamination, including fluoropolymer/polyimide composites, LCP materials, and all-polyimide bonding layers. Many manufacturers now use presses to raise temperatures to the desired 270 ° C to 310 ° C (520°F to 590°F). However, higher temperatures require the use of new pressure pads and better temperature control. Fortunately, a variety of pressure pads for high temperature lamination are already commercially available.

Some fluoropolymer materials and all LCP materials use thermoplastic films on foil substrates and adhesive layers. For better use, the lamination temperature of the foil-coated substrate must be higher than the lamination temperature of the adhesive layer. If the lamination temperatures of the two layers are too close, it will cause the lines on the imaging layer to move during lamination (also known as swimming). This phenomenon can be prevented by strictly controlling the lamination temperature, but the way to prevent it is to design materials with large lamination temperature differences. Fluoropolymer/polyimide composites were obtained with a difference of 35.d egree. C. between the foil substrate lamination temperature and the bonding layer lamination temperature. For most manufacturers, this temperature difference is sufficient. The lamination temperature difference has increased for many new LCP materials. This difference is always one of the most important factors in comparison with any new thermoplastic material.

size stability is another important factor when considering workability. First, the degree of change in the etched foil-coated substrate may be exacerbated by the use of new materials. Today, all-polyimide structures are widely used flexible materials. Other dielectrics may vary more and require different pattern compensation strategies. But most material suppliers have successfully addressed this issue in their finished products.

During lamination, line movement on the adhesive layer or cover layer is a greater problem. this problem is particularly acute when thermoplastic dielectrics are used and/or high temperature lamination is required. Even if it is an all-polyimide foil substrate, its expansion volume after lamination at 300℃ is larger than that after lamination at 190℃. Such variations can be controlled through proper lamination process control and accumulated predictive experience of the extent of variation.

Drilling and plating processes for these new materials also need to be optimized, especially fluoropolymers and products containing LCP. Both materials can be drilled and plated, but the drilling process, desmearing and hole preparation processes need to be optimized during operation.

All in all, as with any tradeoff in design, products with low losses and in some cases low dielectric constants are harder to handle. From the manufacturer's perspective, if you can master the process technology of new materials first, you can become the choice of end users. From an end-user (OEM) perspective, you should always ask the material supplier to recommend the manufacturer to you. Most material suppliers will work with selected manufacturers to ensure that mature devices can be recommended to early end users. Table 2 shows a comparison of these properties.

The suppliers of high speed flexible materials mainly come from existing suppliers of flexible materials and high speed rigid plate materials. In North America, major developers of high speed materials are DuPont, Rogers and Taconic. There are also suppliers of low-loss polyimide and LCP materials with manufacturing bases outside the United States, such as Panasonic and Ventec International Group. Recently, some new materials have been available for purchase through agents in North American and European markets.

Another option to consider is mixing products from different material suppliers. After the flexible circuit has been tested at the material supplier, a foil-coated substrate from one company is mixed with an adhesive layer or overlay from another company at the manufacturer. Of course, you want to make sure that compatibility is tested and that the process is optimized before ordering new flex circuits.