Flexible circuit materials Copper-clad

This type of coil was prepared from copper cladded printed circuit board material by applying photolithographic techniques. The p.c. board material is available with difierent copper thicknesses and with either a stiff or a flexible carrier. The flexible material offers the opportunity to adapt the planar coil to a curved three dimensional test object. In our turbine blade application this is a major advantage. The thickness of the copper layer was chosen to be 17 pm The period of the coil was 100 pm The coils were patterned by wet etching, A major advantage of this approach is the parallel processing with narrow tolerances, resulting in many identical Eddy current probes. An example of such a probe is shown in fig. 10. 

The basic difference between flexible circuits and rigid circuit boards is the thin and flexible materials used in the substrates of flexible circuits. Furthermore, because the flexible circuits have complicated constructions, supplemental materials other than copper-clad materials have been required to build a whole circuit. 

See Table 61.6 for a list of new materials for HDI flexible circuits. There is a significant difference between traditional flexible circuits and HDI flexible circuits. For example, more than 80 percent of traditional flexible circuits were covered by adhesive-based copper-clad laminates that use the traditional polyimide films, Kapton H or Apical AV . Conversely, the majority of high-density flexible circuits used in large-volume apphcations utilize all new materials. 

The majority of flexible circuit manufacturers start the process with copper-clad materials. The properties of these materials depend on the capabilities of laminate manufacturers, even though the same base films and copper foils are used. To choose the right materials, manufacturers must carefully consider the basic properties of each laminate material. 

Historically, copper-clad laminates with acrylic or epoxy adhesives have been the major materials for flexible circuits. Each manufacturer has developed a special resin grade or special additives to ensure reliable flexibility and bond strength. Other adhesive materials such as phenol resin or silicon resin have been developed however, they have not become standard adhesive materials in flexible circuits. 

The adhesive-based copper-clad laminates stiU represent more than 50 percent of the traditional flexible circuit market. The major properties of the materials are shown in Table 61.12. 

The same process is repeated for double-sided copper-clad laminates. (Some laminate manufacturers, however, have developed simplified manufacturing processes that can make the double-sided laminate in one single process to reduce manufacturing cost.) A well-conditioned aging process is important to achieving reliable bond strength and flexible characteristics of the laminates as the raw material of flexible circuits. 

Several laminates without adhesive layers have been developed as the advanced materials for the next generation of flexible circuits. Lamination technology using epoxy resin or acrylic resin has been almost eliminated from HDI flexible circuits even though it uses new high-performance polyimide films as the substrates. Three types of adhesive less copper-clad laminates have been developed (see Fig. 61.5) ... 

The sLCP has excellent dielectric characteristics at high frequencies and low moisture absorption as shown in Table 2.1(C). The dielectric properties of the sLCP film were formd to be constant up to a frequency of 25 GHz. Trials of sLCP as a substrate material of flexible printed circuit boards for higher frequency applications have been performed. Interestingly, the sLCP itself has a relatively high thermal conductivity. Its low viscosity allows the incorporation of a larger quantity of fillers to even further improve thermal conductivity. Trials have been continued in order to make use of sLCP as a substrate for metal based copper clad laminates (MCCL). 

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