High-Density Flexible Circuits

Matsuwaki [5] prepared polyimide films, (V), having molecular orientation in the machine direction that were used in high-density mounting of flexible printed circuit boards. 

Relatively little attention has been paid to current-density nonuniformity caused by uneven pattern density. This effect has been discussed by Romankiw and coworkers [36-38]. A theoretical investigation of the active-area-density effect was presented by Mehdizadeh et al. [23]. This was followed by an experimental investigation [39, 40]. Some highlights from the above mentioned publications by Mehdizadeh et al. are presented in the next section. Further discussion of the active-area-density effect appears in a recent publication on electroplated thin-film wiring for high-performance packaging [41] and in a paper on the fabrication of flexible circuits [14]. 

Metallized PI films are used in electronic applications, e.g. for flexible printed circuit boards. Conventional techniques for the fabrication use adhesive bonding to a copper foil. However, the demand for high density packaging of electronic apparatuses requires a further reduction in the thickness of these substrates. It is possible to sputter metal particles into the surface of the PI film at a thickness of 20 nm, which forms the intermediate layer for subsequent formation of a conductive layer of copper or a copper alloy. 

Table 61.3 shows typical requirements for interconnection in the end product, and the preferred specification of circuit parameters for the use of high-density flexible circuits. 

TABLE 61.3 Common New Requirements for High-Density Flexible Circuits... 

Application areas New materials for high-density flexible circuits... 

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. 

Several liquid polyimide resins have been developed as the base materials of high-density flexible circuits. Some of these liquid polyimide resins can be photoimaged, and have been used in high volume as the dielectric layer and coverlay in the wireless suspension of hard disk drives. 

These hquid polyimide resins could be the major dielectric materials of special high-density flexible circuits that demand extremely high density, down to 5 micron pitches with 10 micron via holes. The cost of these materials is higher than that of polyimide film. However, they have broader capability for meeting nonstandard requirements such as ultrathin substrates with microvia holes. The properties are very dependent on the manufacturer. 

Generally, because of poor resolution, a screen-printing process does not satisfy the requirements of high-density SMT assembling on flexible circuits. Also, such as process does not provide good mechanical performance for dynamic flexing.Typical property examples are shown in Table 61.15. 

Managing the conditioning of the process is not difficult. Acryhc or epoxy/dry-film types are good for smaU-volume production because of the flexible manufacturing process. They may need some more improvements in electrical performance and chemical resistance if they are to be used in the general application of high-density flexible circuits. The present version... 

Ultra-high-density flexible circuits have been developed for special apphcations. They have extremely fine circuits that are smaller than 20 m in pitch, as well as micro via hole connections... 

New high-density flexible circuits have supplemental structures such as flying leads and micro bump arrays to complete high-density terminations. As volume production is limited, the designers should consider the mamrfacturers process capabilities seriously. 

High-density flexible circuits, which have been developed since the mid-1990s, have special constructions and therefore require supplemental processes. In addition, high-grade manufacturing technologies are required to generate finer circuits. Sometimes, they are called HDI... 

Furthermore, ultra high-density flexible circuits have been considered recently as the necessary substrate materials for the next generation electronics packaging. Their circuit densities will be under 20 micron pitches with 20 micron diameter via holes. They need completely new manufacturing technologies, such as an additive process instead of traditional subtractive process. Ultra high-density flexible circuits will be considered separately. 

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