Flexible circuits Multilayer

In 1990 the majority of U.S. PCB production resulted from subtractive or print-and-etch processing additive processes were less than 6% of the total multilayer boards accounted for 55.8%. The ratio of rigid to flexible surface areas plated is about 15 1. High performance plastics including polyimide. Teflon, and modified epoxy comprised 6% of the market ( 324 million) flexible circuits were 6.6% ( 360 million) (42). 

Exemplary developments of microsystems as pre-integration platform in smart structure technology are described in the present paragraph. There are two promising approaches seen allowing for the integration of piezoceramic units and electronic circuits, flexible circuits boards and active ceramic multilayer devices. 

PEI resins have been extruded into film and sheet. Since they are thermoplastic, both sheet and film can be formed and drawn in secondary operations. PEI film can be used in many electrical applications, such as the manufacture of capacitors and flexible circuits. These films can also be laminated or coextruded to make various multilayer structures. PEI films are often laminated with metals, such as copper, which are then made into electric circuits. 

Several different types of adhesive materials can be used in flexible circuits depending on the application, except for the adhesive layer of copper laminates and fihn coverlays. The major applications are the bonding of stiffeners and multilayer constructions. 

Multilayer flexible circuits with through-holes... 

A combination of rigid and flexible circuits can be called rigid/flex, flex/iigid, orrigidfllexible. However, they are called multilayer flex when flexible materials are used instead of glass/epoxy. The term rigid/flex is used throughout this chapter. 

Multilayer rigid/flex is basically a combination of rigid multilayer and flexible circuit processes. 

There are many different concepts in the construction design of multilayer rigid/flexible circuits. 

TABLE 65.1 Materials for Multilayer Rigjd/Flexible Circuits... 

There are many varieties of manufacturing processes for multilayer rigid/flexible circuits because of their complicated structures. Figures 65.5 shows a typical layer construction using... 

FIGURE 65.7 Example of typical multilayer rigid/flexible circuit with via holes. (Source Toshiba.)... 

FIGURE Cross section of a through-hole in a multilayer rigid/flexible circuit. 

Several adhesive resins, especially acryhc-based adhesives developed for flexible circuits are not stable with the permanganate solution that is common for rigid multilayer boards, which means that a very narrow window is allowed for the desmearing of the rigid/flex process. If the materials are dipped too long in the solution, the adhesive layers swell and the rehabihty of the through holed will be damaged. 

The use of supplemental plasma etching, called the Etch Back Process, is recommended to achieve high reliability of through-hole multilayer rigid/flexible circuits. The plasma gas provides further etching on both of polyimide films and adhesive layer in the drilled holes. The plasma gas does not etch metal materials and it clears the surface of the copper foils for reliable copper plating as shown in Fig. 65.9. 

FIGURE 65.9 Etchback of through-hole in multilayer flexible circuit. 

FIGURE 65.11 Micro blind via hole on a multilayer rigid/flexible circuit made by a build-up process using resin-coated foil. The hole diameter is 150 micrometer. (Source Photo Machining.)... 

A great variety of resia formulations is possible because other thermosets, such as epoxies or acrylates, and reactive diluents, such as o-diaUyl phthalate [131-17-9] triaUyl cyanurate [101-37-17, or triaUyl isocyanurate [1023-13-6J, can be used to further modify the BT resias. The concept is very flexible because bismaleimide and biscyanate can be blended and copolymerized ia almost every ratio. If bismaleimide is used as a major constituent, then homopolymerization of the excess bismaleimide takes place ia addition to the copolymerization. Catalysts such as ziac octoate or tertiary amines are recommended for cure. BT resias are mainly used ia ptinted circuit and multilayer boards (58). 

The major applications of LCP films in electronics are multilayer boards and multichip modules, and flexible printed circuits. Current materials used in the.se applications include polyimide film, high performance thermoplastic film such as polyphenylene sulfide and polyetherether ketone, and fiber reinforced composites such as quartz fiber-polyimide. The price of these materials for electronic packaging is in the range of. 50 to. l(K)/lb, so LCPs can compete very effectively at their current prices. Based on a two to three times increase in price... 

Bismaleimide resins are extensively used in multilayer printed circuit boards. They can also be used in insulation fibres for protective clothing. Due to their ability to adhere and retain adhesive strength at high temperature, bismaleimides are widely used as a base material for flexible printed circuit boards in the electrical and electronics industries. Bismaleimides are often linked with rubber chains for use as flexible linking molecules to reinforce the rubber compound for tyre applications. The double bond of bismaleimides readily reacts with all hydroxy or thiol groups found on the matrix to form a stable carbon-sulfur, carbon-nitrogen or carbon-carbon bond. 

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