Rigid/flexible circuits Materials

FIGURE 65.6 Material constructin for multilayer rigid/flexible circuits. 

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. 

Furthermore, the overall properties of the base materials for circuit boards are determined essentially by the joint between the copper foil and the laminate. This joint can be realized by both an added adhesive and by the laminating resin itself, which additionally, acts as adhesive. An additional adhesive is needed in the manufacture of flexible circuit boards (e.g., polyimide/copper) or of paper-based rigid circuit boards. During this process the adhesive is deposited on the bottom side of the copper foil after... 

Although some substrates may be molded plastic (3-D or molded circuits) or flexible film plastic materials (flexible circuits), the largest group of substrates is of rigid laminate construction. The laminate construction consists of many layers of resin-treated fibers, fabrics, or papers, which are called reinforcements. 

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. 

One of the major differences between flexible circuits and rigid circuit boards is coverlay. In addition to the solder mask used for assembly, coverlay is the mechanical protector for the fragile conductors on flexible circuits. Film-based coverlay and flexible solder mask have been standard materials for traditional flexible circuits. Several types of photoimageable coverlay materials have been developed to satisfy the fine-resolution requirements of FCDI flexible circuits ... 

A screen-printing process similar to that used for rigid circuit boards is available for flexible circuits as the low-cost coverlay process. However, specially conditioned liquid solder mask materials are required to produce appropriate flexibility for the circuits. Standard solder mask materials designed for the rigid boards have small cracks and de-laminations when bending. 

Acrylic or Epoxy/Dry-Film Type. Acrylic or epoxy/dry-fibn types were the first materials developed to act as the flexible photoimageable coverlay for HDI flexible circuits. They have the same product concept as the photoimageable solder mask of rigid circuit boards. The same vacuum laminators and imaging equipment are available for these materials. 

Because of the thiu and fragile materials used, flexible circuits have lower mechanical rehabihty than rigid circnit boards. They have a low conductor bond strength and low base snbstrate tear strength. Nevertheless, most of the flexible circuits are subjected to more mechanical stresses due to movement. This means that special care is required in the circuit design to gain higher circuit rehabihty. 

Figure 63.2 shows standard manufacturing processes of the through holes for the doublesided flexible circuits. A mechanical numerically controlled (NC) drilling process on double-sided flexible circuits can be done similarly to that used for rigid double-sided printed circuit boards. More laminates can be stacked up in the setup because of the thin materials. 

The conveyer systems of the wet processes designed for rigid circuit boards do not work properly for thin flexible materials. They can t make a high process yield generating a lot of wrinkles or scratches, even though the flexible sheet is fixed on a leader board or carrier frame. Appropriated conveyer system should be introduced for the volume production hues of the thin flexible circuits. 

If adhesiveless laminates are used as the major materials, high temperatures can be used with lead-free soldering. In the case of SMT assembling, the flexible circuit should be fixed on a carrier frame. This allows it to be processed by the same mounting and soldering equipment as used for rigid boards. 

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. 

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. 

Polyimides are high-performance polymers, mainly used as films and as substrates for flexible printed circuits, bar code labels, and transformer and capacitor insulation. The rigid structure is responsible for a Tg greater than 300 °C. Since Kapton is flexible, lightweight and withstands very low and very high temperatures, it is an important aerospace material. Potential uses include as a thin film absorber for solar cells114. 

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