Abstract:
A flexible multi-channel trace interconnect array has a head end for electrically connecting write and read trace pairs respectively to write and read elements of a data transducer head, a body formed of a flexible dielectric material and carrying the write and read trace pairs, and at least one circuit end for connecting the write and read trace pairs respectively to write driver and read preamplifier circuits of a data storage device. Each trace pair includes two trace conductors each having a trace width, and an inter-conductor separation space. Adjacent trace pairs are separated by an inter-pair space having a width greater than, and preferably two to twenty times greater than the inter-conductor separation space in order to decouple adjacent channel pairs.

Description:
FIELD OF THE INVENTION 
     The present invention relates to flex circuits for connecting magnetic heads to read and write circuits of a magnetic recording device. More particularly, the present invention relates to a flexible trace interconnect array for a multi-channel tape head which manifests reduced inter-channel cross talk as well as controlled electrical impedance characteristics. 
     BACKGROUND OF THE INVENTION 
     Magnetic tape drives are typically employed to provide data backup and archival storage for user data records and programs. For digital data storage applications, tape drives typically employ either rotating heads, or non-rotating heads. One form of non-rotating head is the streaming tape drive. In a streaming tape drive multiple blocks of user data are typically written to tape in a single streaming operation, rather than in a series of start-stop operations of the tape transport. In the streaming tape drive, a magnetic tape head includes at least one read/write element. The head is typically positioned laterally relative to the tape path by a lead screw, which is controllably rotated by e.g. a stepper motor, or an equivalent arrangement. In this manner a single transducer element, or several spaced-apart elements, may write to, and read from, a multiplicity of linear tracks defined along the magnetic recording tape. 
     In order to permit the head to be moved laterally across the tape in order to confront the multiple parallel tape tracks, a flexible head interconnect arrangement is needed to connect the read/write elements of the head to electronic circuitry conventionally mounted on one or more printed circuit boards affixed to the tape drive base or housing. In the past, flexible wires, twisted together into pairs and gathered into a cable, have been employed as tape head interconnects. 
     Digital linear magnetic tape drives are an improved type of linear streaming magnetic tape drives. One well established digital linear magnetic tape drive is provided by Quantum Corporation as the DLT-7000 drive. This particular tape drive uses a single reel tape cartridge that supplies a stream of half-inch-wide tape via a leader and buckling mechanism. The Quantum DLT7000 tape drive has a four-channel head, with eight write elements and four read elements. A first set of four write elements are placed on one side of the four read elements, and a second set of four write elements are placed on an opposite side of the first set. This particular arrangement enables four data tracks to be written and then read-checked during a single forward tape streaming operation, and a second four data tracks to be written and read-checked during a single reverse tape streaming operation. Azimuth recording is employed to reduce cross talk between adjacent tape tracks. Therefore, the head is not only displaced laterally relative to the tape path, it is also rotated to a forward azimuth angle for forward direction, and rotated to a reverse azimuth angle for reverse direction data recording. Backward compatibility is achieved by orienting the head at a right angle to the tape path such that two non-azimuthal tracks may be simultaneously written and/or read during each tape streaming operation. 
     A flex circuit supporting the eight write elements and the four read elements of the Quantum DLT7000 tape drive product is described in commonly assigned U.S. Pat. No. 5,862,014 to Nute, entitled: “Multi-Channel Magnetic Tape Head Module Including Flex Circuit”, the disclosure thereof being incorporated herein by reference. The described flex circuit permitted the tape head freely to be laterally displaced and also to be rotated to the variously available azimuthal and linear tape confronting positions. In the arrangement described in the &#39;014 patent, approximately 128 linear data tracks were provided on the half-inch recording tape. 
     Data rates and track densities are increasing. One way to increase data rate of a magnetic recording system is to increase the write frequency. Another way to increase data rate is to increase the number of parallel write and read elements of the head and data channels of the tape drive so that more tracks are simultaneously written during each tape streaming operation. A third way to increase data rates is to employ partial response, maximum likelihood signaling techniques of the type known in magnetic disk drives. 
     One way to increase track density is to reduce linear track width and spacing by aligning the write elements/read elements closer together. By employing thin film inductive write elements and magneto-resistive read elements, it is practical to increase the number of data tracks of a one-half inch magnetic recording tape from e.g., 128 tracks to e.g., 1000 or more tracks. Since the head carrying the write and read elements must still be displaced laterally relative to the tape path, a flexible interconnect arrangement is needed in order to connect the write and read elements of the movable head to write and read electronics affixed to the printed circuit board of the drive electronics. 
     A flex trace interconnect array is preferred, because it affords the opportunity to control the electrical impedance characteristics of the traces, as taught for example by commonly assigned U.S. Pat. No. 5,737,152 to Balakrishnan, entitled: “Suspension with Multi-Layered Integrated Conductor Trace Array for Optimized Electrical Parameters”, the disclosure thereof being incorporated herein by reference. Commonly assigned U.S. Pat. No. 5,754,369 to Balakrishnan, entitled: “Head Suspension with Self-Shielding Integrated Conductor Trace Array”, shows an arrangement wherein a read conductor pair is interleaved between two conductors of a write conductor pair in a disk drive flexible trace interconnect. (In disk drive operations, simultaneous writing and reading operations are not usually present, and thus the write traces offer a measure of shielding to the read traces during disk drive data reading operations). The disclosure of the &#39;369 patent is also incorporated by reference herein. 
     Conventionally, the trace conductors connecting the preamplifiers to the read elements of the tape head are interleaved with the conductors connecting the write drivers to the write elements. Because of space restrictions, and the desire to reduce the interconnect mass, the trace conductors have to be placed close to each other. While it is desirable from an electrical viewpoint to space the conductors of any single channel as close to each other as possible, it is equally desirable to increase the spacing between adjacent conductors of separate channels. 
     FIG.  1  and FIG. 3A show a conventional layout of flex conductors on a flexible trace interconnect array  10  which connects two inductive write elements  12  and  14  and two magneto-resistive read elements  13  and  15  of a two-channel tape head  16  to a two-channel preamplifier/write driver circuit  18  of the tape drive. In this example one tape channel (track) is defined by write element  12  and read element  13 , and another tape channel (track) is defined by write element  14  and read element  15 . Further, in the FIG. 1 simplified example a conductor pair  20 A- 20 B of flex interconnect  10  connecting read transducer  15  to its preamplifier in circuit  18  is interleaved between a conductor pair  22 A- 22 B connecting write transducer  12  to its write driver in circuit  18  and a conductor pair  24 A- 24 B connecting write transducer  14  to its write driver in circuit  18 . 
     Flexible trace interconnects are presently available having trace widths as narrow as 75 μm (approximately 3 mils). Thus, in the FIG. 1 multi-channel flex interconnect  10  a flex trace interconnect conductor geometry would have a cross-sectional layout of traces formed on a flexible substrate  11  as shown in FIG.  3 A: - - - 75 μm read trace  26 A - - - 75 μm inter-conductor space - - - 75 μm read trace  26 B - - - 75 μm inter-pair space - - - 75 μm write trace  22 A - - - 75 μm inter-conductor space - - - 75 μm write trace  22 B - - - 75 μm inter-pair space - - - 75 μm read trace  20 A - - - 75 μm inter-conductor space - - - 75 μm read trace  20 B - - - 75 μm inter-pair space - - - 75 μm write trace  24 A - - - 75 μm inter-conductor space - - - 75 μm write trace  24 B, etc. The conductors of trace pairs  20 ,  22 ,  24 , and  26  could be widened or thickened, or both, if required for electrical reasons. 
     Each conductor in the FIG. 1 electrical schematic diagram has a resistance, an inductance and a capacitance associated with it. An equivalent circuit for three of the conductor pairs of FIG. 1 is shown in FIG.  2 . The FIG. 2 equivalent circuit shows the pair of write-channel conductors  22 A,  22 B routed between the adjacent conductors  20 B and  26 A of the two read circuits. There are a number of factors that need to be considered. At higher write current frequencies, the resistance and inductance parameters are governed by skin-effect and proximity-effect phenomena. The resistance is affected by the current distribution in the trace conductors, which tends to move to the surface of each trace conductor at high frequencies. This movement of the current distribution effectively results in a smaller conductor cross section available for the current to flow through, and results in an increase in the high frequency electrical resistance of the trace conductor. The current-density vector and the magnetic flux follow the same movement toward the conductor surface. As the current moves towards the surface, so does the magnetic flux, which for the same current means lesser flux links the conductor. Thus, the electrical inductance of the trace conductor goes down with increasing frequency. The inter-trace capacitances are relatively frequency-independent and may be treated as constant values. 
     In addition to the parameters discussed above, the FIG. 2 equivalent circuit also includes elements that couple the trace conductors  22 A and  22 B of the write circuit with the conductors  26 B and  20 A of the two adjacent read circuits. This coupling is shown in FIG. 2 as coupling capacitors CC FEM .x. The mere presence of the adjacent conductors also affects the flux distribution of the current in the current-carrying conductors. Since this is a frequency-dependent phenomenon, different signals at different frequencies are affected to varying degree. 
     Another impact of the write conductors  22 A and  22 B on the read conductors  26 B and  20 A is due to the fact that the write current amplitude is approximately 40 to 60 milliamperes, whereas the read signals are on the order of one to a few millivolts. During simultaneous writing/read-checking operations of the tape drive, any signals induced by the write current on adjacent read conductors can couple into the read signal at the preamplifier. 
     The cross-coupling of write and read conductive traces and the disparity in write current levels to read signal levels suggests that the read and write signals need to be decoupled. 
     SUMMARY OF THE INVENTION WITH OBJECTS 
     The present invention solves this problem within a flexible trace interconnect array for a multi-channel recording and playback head, such as a digital linear magnetic tape head. 
     One object of the present invention is to provide a multi-channel flexible trace interconnect array of trace pairs in which each trace pair is electrically decoupled from adjacently located trace pairs along the trace array. 
     Another object is to reduce cross talk and eddy current induction between pairs of conductive traces formed on a flexible circuit substrate used to interconnect transducer elements of a positionable head with write driver and read preamplifier circuitry non-moveably affixed to a base of a data storage device, such as a tape drive. 
     In accordance with one aspect of the principles of the present invention, a flexible multi-channel trace interconnect array is provided. The array has a head end for electrically connecting write and read trace pairs respectively to write and read elements of a data transducer head. The array includes a body formed of a flexible dielectric material which carries the write and read trace pairs and leads to a circuit connection end for connecting the write and read trace pairs respectively to write driver and read preamplifier circuits of a data storage device, such as a multi-channel tape drive. Each trace pair comprises two trace conductors, and each conductor has a defined trace width, such as 50 μm. The conductors of each pair are separated along the body by an inter-conductor space having a defined width such as 50 μm. Adjacent trace pairs are separated by an inter-pair space having a defined width which is greater than the defined trace width and greater than the defined inter-conductor space, such as 150 μm-400 μm, or more. Preferably, the inter-pair space has a defined width dimension lying in a range from approximately two to twenty times the inter-conductor defined width. 
     In one preferred arrangement multiple read trace pairs, such as at least twelve read trace pairs, are interleaved with multiple write trace pairs, such as at least twelve write trace pairs, along the trace array body. 
     In an alternative preferred arrangement, multiple write trace pairs are separated into a write pair group and multiple read trace pairs are separated into a read pair group. In this arrangement the write pair group is substantially spaced away from the read pair group along the array body. In a related preferred arrangement trace array body is divided into two elongated segments including a write group segment carrying the write group trace pairs, and a read group segment carrying the read group pairs. In a further related preferred arrangement the write group segment includes a portion leading to the write group circuit connection end which diverges away, preferably perpendicularly from the body and in an opposite direction of divergence away from the body of a portion of the read group segment leading to the read group circuit connection end. Preferably, although not necessarily, the write group segment portion has a length which is approximately equal to a length of the read group segment portion. 
     Furthermore, in this alternative preferred arrangement, each write element and a corresponding read element of the head are aligned to write and read a single storage track of a data storage medium such as magnetic tape. Accordingly, the head end of the array comprises a pattern of plate-through vias and bridging traces formed on an opposite side of the flex body so that a write pair connection location to a write element of the head is placed adjacent to a read pair connection location to the corresponding read element of the head. 
     These and other objects, advantages, aspects, and features of the present invention will be more fully appreciated and understood upon consideration of the following detailed description of preferred embodiments presented in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawings 
     FIG. 1 is an electrical circuit schematic diagram of a flexible trace interconnect array for interconnecting a two-channel write/read tape head with associated write driver/read preamplifier circuitry of a tape drive in accordance with established, conventional principles. 
     FIG. 2 is schematic circuit drawing of an equivalent circuit of a representative portion of the FIG. 1 flexible trace interconnect array. 
     FIG. 3A is greatly enlarged view in elevation and cross section of a trace array in accordance with the FIG. 1 example. 
     FIG. 3B is a greatly enlarged view in elevation and cross section of a multi-channel flexible trace array in accordance with principles of the present invention. 
     FIG. 4A is a family of simulation plots of trace conductor electrical resistance as a function of write current frequency for idealized and real trace arrays which compares the FIG. 3A and 3B examples. 
     FIG. 4B is a family of simulation plots of trace conductor electrical inductance as a function of write current frequency for idealized and real trace arrays which compares the FIGS. 3A and 3B examples. 
     FIG. 4C is a series of graphs of simulation plots of inter-conductor capacitance as a function of layout for idealized trace arrays and for the trace arrays of FIGS. 3A and 3B. 
     FIG. 5 is an enlarged plan view of a twelve-channel flexible trace interconnect array in accordance with principles of the present invention as illustrated in the FIG. 3B example. 
     FIG. 5A is an enlarged plan view of a head connect portion of the FIG. 5 array. 
     FIG. 6 is an enlarged plan view of a twelve-channel flexible trace interconnect array having read channel pairs grouped together and separated from a write channel pairs group in accordance with principles of the present invention. 
     FIG. 6A is an enlarged plan view of a head connect portion of the FIG. 6 array. 
     FIG. 7 is an enlarged plan view of a twelve-channel flexible trace interconnect array having read channel pairs grouped together and separated from grouped write channel pairs, and having oppositely extending group connection ends in accordance with principles of the present invention. 
     FIG. 7A is an enlarged plan view of a head connect portion of the FIG. 7 array. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with principles of the present invention, by employing controlled fine-line (conductive trace width and space) trace interconnect manufacturing techniques used to make hard disk trace interconnect arrays, flexible multi-channel trace interconnects for storage devices such as multi-channel tape drives may be realized which reduce spacing between conductors of the same channel and which increase spacing between adjacent conductors of separate channels. A representative tape drive is disclosed in the Nute U.S. Pat. No. 5,862,014, previously referenced and incorporated herein. 
     For example, by moving to a minimum conductor trace widths and inter-conductor spacing to 50 μm from 75 μm, additional inter-pair separation space between adjacent conductors of separate channel trace pairs can be effectively realized without increasing overall width of the flexible body supporting the trace array. A more desirable spacing can therefore be provided for a multi-channel flexible trace interconnect array over the conventional trace array  10  shown in FIGS. 1 and 3A. 
     In accordance with principles of the present invention and as shown in FIG. 3B, a new trace interconnect array  100  formed upon a suitable flexible dielectric substrate  110  may therefore follow an exemplary geometric cross-sectional layout of: - - - 50 μm read trace  126 A - - - 50 μm inter-conductor space  116  - - - 50 μm read trace  126 B - - - 150 μm inter-pair separation space  118  - - - 50 μm write trace  122 A - - - 50 μm inter-conductor space  116  - - - 50 μm write trace  122 B - - - 150 μm inter-pair separation space  118  - - - 50 μm read trace  120 A - - - 50 μm inter-conductor space  116  - - - 50 μm read trace  120 B - - - 150 μm inter-pair separation space  118  - - - 50 μm write trace  124 A - - - 50 μm inter-conductor space  116  - - - 50 μm write trace  124 B - - -, etc. 
     In the FIG. 3B example the overall width of the trace interconnect array  100  is slightly less than the width of the FIG. 1 trace interconnect array  10 , while the pair-separation distance D between adjacent conductors of separate write and read pairs is increased to at least twice of what was provided in the FIG. 1 conventional approach. Ideally, the inter-pair separation space  118  lies in a range between two and twenty times the inter-conductor separation space  116 . This improved arrangement as shown in the FIG. 3B cross section diagram illustrates the physical separation between adjacent conductor pairs, and results in desired electrical decoupling between adjacent conductor pairs. In the present example such decoupling provides reduced cross talk and eddy current induction between the conductor pairs  126 ,  122 ,  120  and  124 . 
     FIGS. 4A,  4 B and  4 C present a comparison of electrical simulations for the FIG.  3 A and FIG. 3B examples, as well as idealized cases of both geometries. The ideal case simulations consider only two conductors that are located in free space and completely away from any external influences. The ideal case is most desirable as there are no induced effects upon the idealized conductor pair. However, the real cases of the FIGS. 3A and 3B trace array examples  10  and  100  take into account the presence of the conductors of the adjacent channels. 
     The real FIG. 3A inductance plot of FIG. 4B shows the significant difference between the real and ideal cases which follow the conventional FIG. 3A geometry. The real FIG. 3B geometry inductance plot demonstrates the advantages in providing increased, e.g. 150 μm, separation between each conductor pair and the adjacent conductors of other conductor pairs. By following the FIG. 3B example  10  in lieu of the conventional FIG. 3A example  10 , inter-pair capacitive coupling is also greatly reduced, e.g., from 1.7 pF to 1 pF, as shown in FIG.  4 C. 
     FIG. 5 illustrates a practical twelve-channel read and twelve-channel write flexible trace interconnect  100 ′ incorporating the principles of the FIG. 3B example and extending from a head-connecting end  105  to a preamplifier/write driver circuit connection end  115 . In the FIG. 5 example  100 ′ the trace conductor width and spacing within each one of the twelve conductor pairs is 50 μm, while the distance D′ between facing conductors of adjacent pairs is 400 μm, which further reduces the coupling and induced eddy-current effects over the FIG. 3B example  100  of the present invention. A head connection end  105  of the interconnect array  100 ′ is shown in the enlarged plan view of FIG.  5 A. 
     A further improvement  200  is realized by the FIG. 6 example  200 . The FIG. 6 trace interconnect array  200  improves the FIG. 5 trace interconnect array  100 ′ further by separating all of the read conductor pairs from all of the write conductor pairs for a substantial distance between the head and the preamplifier/write driver circuitry. Additionally, the large spacing (e.g. 400 μm) between adjacent pairs of read trace conductors and write trace conductors is maintained. The FIG. 6 trace interconnect array  200  incorporates the advantageous geometry of the FIG. 3B example  100  within each conductor pair as well as between adjacent conductor pairs. Further, the FIG. 6 trace interconnect array separates the twelve read conductor pairs into a read conductor pair group  202  on one side of the array, and the twelve write conductor pairs as a write pair conductor group  204  on another side of the array. Because of a need to place same-channel read and write conductor trace pairs adjacent to each other at a head connection end  205  of the trace array  200 , plate-thorough vias and bridging traces must be defined on a reverse major surface of a flexible substrate  210  of suitable dielectric material at the head connection end  205 , as shown in the enlarged plan view of FIG. 6A of a head connection end of the trace array  200 . 
     In some multi-channel tape drive designs it is desirable to separate physically the write driver circuitry from the read preamplifier circuitry. FIG. 7 shows another example of a twelve-channel flexible read/write trace interconnect array  300  formed on a suitable flexible dielectric substrate  310  which is in accordance with principles of the present invention. In the FIG. 7 exemplary trace interconnect array  300  the read conductor pair group  302  is separated from the write conductor pair group  304  at a head connection end  305 . At a desired location along a longitudinal extent of the array  300 , the conductor pair groups  302  and  304  diverge e.g. transversely and oppositely from each other and terminate at separated circuit connector ends; end  306  being provided for the read pair group  302 , and end  308  being provided for the write pair group  304 . Preferably as shown in FIG. 7, although not necessarily, the write conductor pair group  304  is symmetrical with the read conductor pair group. A head connection end  310  of the array  300  is shown in the enlarged detail plan view of FIG.  7 A. 
     Thus, it will be appreciated by those skilled in the art that the described approaches are simple to implement, provide controlled electrical parameters including greatly reduced cross-coupling between adjacent channels, and thus eliminate one source of cross talk between channels. This approach thereby improves the performance of each channel of a multiple channel linear tape drive. 
     Although the present invention has been described in terms of the presently preferred embodiments of multi-channel flexible trace arrays for interconnecting multi-channel tape heads with respective read preamplifier and write driver circuits in a manner reducing adjacent channel cross talk, it should be clear to those skilled in the art that the present invention may also be utilized in conjunction with, for example, other flexible trace interconnect arrays and storage devices, whether disk or tape, and whether magnetic or optical. Thus, it should be understood that the instant disclosure is not to be interpreted as limiting. Various alterations, adaptations, and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all such alterations, adaptations and modifications as fall within the true spirit and scope of the invention.