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62.3.7 Flying-Lead Construction (Double access construction) A specialty construction for flexible circuits is the double-electrical access capability on even single conductor layer circuits, achieved by removing the base insulation layer under the conductors as shown in Fig. 62.10. It can be an alternative construction instead of double-side circuits with through holes for the low cost 3-D wiring. When the insulation layers are removed from both sides of the conductors, they can then be partially free from the organic materials. Unsupported conductors, also known as flying leads or flying fingers, are not a new idea in the construction of flexible printed circuitry. (Fig. 62.10) The biggest advantage of flying-lead technology is its extremely high heat resistance and thermal conductivity when compared to standard conductors insulated by dielectric substrate materials. Based on the lack of insulation, flying leads can be treated as a bare conductor material similar to bare wire. There are many high-density flexible circuit applications that require flying-lead terminations. Details are discussed in Chap. 64.
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Coverlay film Conductor
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FIGURE 62.10
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Flying lead construction (Double sided construction).
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Consideration must be given to specific surface treatments required on the flying leads to make each of the various application terminations. Previously, tin/lead plating was the most common treatment for various soldering processes; however, it is expected to be replaced by lead-free technologies, which need higher temperatures for the soldering. Soft gold plating with a nickel under-plating is the standard surface treatment for the wire bonding and direct bonding to the other devices. The purity and the thickness of the plated gold, along with the bond strength of the plated layer, are important factors in bond performance. Copper thickness is a key consideration when developing a new circuit design involving flying leads.Although it is easier to manufacture high-density circuits utilizing thinner copper, the resultant flying lead may be extremely fragile and will not have the mechanical strength of the thicker copper circuits (see Fig. 62.11). Manufacturing yield plays an important role in the overall circuit costs, and thinner copper foils typically result in more damage during processing. It is important to understand this correlation as poor circuit design can affect manufacturing yields. Because the flying leads are basically bare conductors, suitable surface treatments should be conducted according to termination technologies.
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FIGURE 62.11 Flying lead capability of copper foil, showing potential line width versus thickness of the copper foil.
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DESIGN OF FLEXIBLE CIRCUITS
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62.3.8 Micro Bumps and Dimples Many kinds of micro bump arrays and dimple arrays have been developed for high-density terminations of flexible circuits other than solder ball grid arrays. Recent technologies allow for a wide variety of shapes, materials, sizes, and pitches; down to 50 microns. The solder ball arrays are built by similar processes of the rigid BGA substrates. The combinations of the electrical and electroless-plating process provide a lot of choice for the shapes and sizes of the micro bump arrays on flexible circuits. The double-access capabilities of flexible circuits provide more choices for the locations of the micro bumps, as shown in Fig. 62.12, other than through coverlay. The micro bumps can be built on the bottom side of the conductors through base layers. They can be also built on both sides of the flying leads. An appropriate design should be chosen according to the requirements of the terminations.Whether it s a permanent connection or non-permanent connection is one of the key points needed for designing the basic constructions of the micro bumps. Bump density and reliability are other factors needed to design the micro bump structures. Details of the manufacturing processes are described in Chaps. 64 and 67.
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Through base film
Through coverlay On flying lead
FIGURE 62.12
Locations of microbumps.
Dimple structure of flexible circuits is a low cost solution for the non-permanent termination. Multiple dimple arrays can be formed on the polyimide base or PET base flexible circuits by simple pressing processes. (Details are shown in Fig. 62. 13) A typical example of the dimple termination is the connection of the disposable cartridges of ink jet printers. Over 60 dimples with hard gold plating are formed at the end of flexible circuit for the connection with the ink cartridge. The dimple arrays can make reliable connections over one thousand times.
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