Flexible circuits Flying-lead

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. 

FIGURE 66.1 Basic constructions for Flying-leads for flexible circuits. 

Figure 66.1 indicates two basic structures of dual-access conductors, in both the single sided and double-sided types of flexible circuits. Figure 66.2 shows actual examples of the fine flying leads built on the single side circuits and double side circuits. Basically, a coverlayer is applied to the side of the conductive layer opposite the substrate.The coverlayer can be processed easily to create the top-side exposure. The flying lead is created when the base substrate is also removed in the same area. 

FIGURE 66.4 Manufacturing process for flying lead flexible circuit construction using (a) laser ablartion and (b) plasma etching or chemical etching. 

FIGURE 66.5 Comparison of opening sizes of flying-lead flexible circuit manufacturing... 

TAB S manufacturing capabilities are very different from traditional flexible circuits and they cover different apphcation area as shown in Fig. 66.10.TAB has been focusing single side fine traces with flying lead constructions. It has advantages for the volume productions of the small size circuitries because of special reel-to-reel manufacturing. 

There are several choices of places to bnild micro bnmps on flexible circuits, as described in Chap. 64 (see Sec. 64.3.8). The same processes nsed to build ball grid arrays on rigid circuit boards are appropriate for flexible circuits with flexible photo-imageable solder mask. It is possible to build micro bumps on bare conductors using plating masks. Many micro bumps are built on fine flying leads without solder masks. A valuable construction of a flexible circuit is micro bumps built on the other side of the circuit through the dielectric layer.This can provide a reliable micro bump array because of the stable construction. 

FIGURE 66.12 Various micro bump arrays on flexible circuits (a) Column shape copper bump array on the bare traces (b) Column shape Ni/Au bump array built through covelay (c) Solder ball array built though base sub-strate(d) Combination of flying leads and solder ball array. 

The basic quality assurance concept for high-density flexible circuits is the same as for other circuit types. However, a difference for high-density flexible circuits is the acceptable defect size, which is one order smaller than that for traditional flexible circuits, necessitating higher inspection capabilities. Furthermore, new high-density flexible circuits contain additional structiues such as flying leads and microbump arrays that require additional inspection capabilities for reliable termination. Exact 3-D accuracy and uniform surface conditions are required. Dimensional allowances are smaller than 0.3 percent, and sometimes 2-ftm accuracy is required. 

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