Patent Publication Number: US-8982512-B1

Title: Low impedance, high bandwidth disk drive suspension circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/623,983 filed Nov. 23, 2009, now U.S. Pat. No. 8,553,364, which claims the benefit of 61/117,441 filed Nov. 24, 2008 and which is also a continuation in part of Ser. No. 12/540,870 filed Aug. 13, 2009, now U.S. Pat. No. 8,395,866. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to disk drive suspensions, and more particularly, to improvements in the manufacture of disk drive suspension interconnects to secure better electrical grounding of suspension components such as copper circuit layers to grounded portions of the suspension such as stainless steel layers including stainless steel layers per se and copper metallized stainless steel layers, to enable increased numbers of copper circuit layers, and further relates to suspension products thus manufactured. The invention further relates to a resilient flying lead and flying lead terminus for disk drive suspensions. Still further, the invention relates to low impedance, low crosstalk signal traces for a disk drive suspension circuit. 
     2. Description of the Related Art 
     Suspension assemblies in hard disk drives (HDDs) include a head gimbal assembly (HGA). The HGA typically includes a gimbal assembly, a head assembly, and an interconnect assembly. The head assembly includes a highly sensitive read/write transducer, commonly referred to as a head, which is integral to the air bearing slider. The suspension assembly positions the head assembly at a generally constant distance from the surface of a rotating magnetic disk to allow the read/write transducer to read data from and write data to the magnetic disk. 
     The head assembly includes electrical terminals configured for interconnection to the interconnect assembly for receiving and relaying data signals. The interconnect assembly includes a plurality of transmission elements, such as wires or traces, for transmitting data to and from a magnetoresistive transducer (MR or read/write head) on the distal end of the head assembly. Amplifying circuits connected at the terminal end of the HGA process, send and receive the data signals to and from the MR head. Transmitted signals are carried between the amplifiers and the MR head by conductors formed along the suspension. 
     Disk drive suspension interconnects, such as Integrated Lead Suspensions (ILS) for hard disk drives typically have three layers, including a stainless steel foil that provides mechanical properties for the suspension, two or more conductive traces comprising gold plated, patterned copper conductive circuits paths that provide the electrical connection between the read/write head slider and the termination pads of the suspension, and a dielectric layer that provides electrical insulation between the stainless steel foil and the conductive traces. 
     In the microelectronic arts, there are two widely known methods for forming conductors on a substrate: the subtractive process and the additive process. The subtractive process utilizes a pre-formed laminate made of layers of insulative and conductive materials. A desired pattern of conductive traces is formed on the laminate by subtracting unwanted portions of the layers using etching and masking techniques. In an additive or build-up process, insulative and conductive layers are formed on a substrate, and a desired pattern of conductive traces are defined on the layers using plating or ion implantation techniques. Additive methods generally provide higher precision vias and improvements in trace density. 
     It is known to be desirable to ground various components of a disk drive suspension such as the body of the read/write head slider. One of the major challenges in the design of hard disk drive suspensions is attaining a suitable, reliable grounding connection between the conductive copper traces connected electrically to the slider and the underlying stainless steel foil layer given the limited space available to make the connection. The difficulty of bonding to stainless steel and dissimilarity of the metals (Cu, Au, SST) pose additional significant challenges to creating a reliable grounding of the slider, but reliable grounding is essential to maintaining the signal fidelity between the read/write head and amplifier. 
     Among the prior art approaches to solving the slider grounding problem is creating a hole in the dielectric between the slider and the stainless steel foil, typically 25 μm deep, and filling the hole with conductive adhesive to provide an electrical connection between the slider and the stainless steel. This approach is deficient, however, since conductive adhesive connections are typified by very high interconnect resistance resultant from the passive (self-healing) nature of the stainless steel and the lack of a conductive, fully metallic bond between the steel layer and the conductive adhesive. High interconnect resistance limits the quality of the electrical connection to ground and thus slider performance dependent on a good grounding is degraded. 
     Another approach to slider grounding uses a spanning lead that extends from the slider, beyond the edge of the dielectric layer and opposite the stainless steel layer where it is subsequently bent over onto the stainless steel layer and electrically and mechanically affixed there, using, typically, a conductive polymer. Spanning leads are very fragile and can be easily mis-bent during manufacture causing lowered manufacturing yields. Further, even if perfectly accomplished, the process of physically bending and adhering leads to the stainless steel suspensions is a very labor-intensive process that does not lend itself to high-volume, low-cost manufacturing, such as simultaneous gang bonding of multiple suspensions. 
     In both of these prior art processes the presence of conductive adhesives can cause increased drive contamination that may adversely affect drive reliability, and their use is environmentally undesirable for workers. 
     Additionally, it is known to provide suspension circuits having tail termination pads at the ends of the circuits, which are flying or unsupported metallic conductors. These structures, are sometimes called flying leads. One purpose of the flying lead region is to allow access to both surfaces of the conductive lead. The flying lead is typically terminated to a rigid or flexible circuit on the suspension actuator using thermosonic bonding. The flying leads have metallic conductors that are unsupported by the dielectric layer that normally separates the conductive signal traces from the other conductive layers and the substrate such as stainless steel below. The flying leads therefore lack the stiffness provided by the underlying dielectric layer. U.S. Patent Publication No. 2005/0254175 by Swanson et al. shows in FIG. 2 a flying lead region 50. 
     Various constructions and metallurgies have been proposed for the flying leads. Swanson et al. disclose, for example, a first embodiment of a flying lead construction in FIGS. 15A-15C in which a flying lead comprises a copper signal conductor on stainless steel with nickel and gold plating, and a second embodiment in FIGS. 17A-17C in which a flying lead comprises a copper signal conductor with nickel and gold plating. U.S. Patent Publication No. 2007/0041123 by Swanson et al. discloses a flying lead portion formed of a noble metal. U.S. Pat. No. 5,666,717 issued to Matsumoto discloses in FIGS. 1 and 2 unsupported flying leads formed by cladding (a subtractive process), sputtering, vacuum deposition, or ion plating. Matsumoto employs a conductor metal formed of copper and nickel, and overplated with a noble metal such as gold which is resistant to corrosion and chemical etching. 
     During the disk drive manufacturing process, the flying leads can be used for test purposes. U.S. Pat. No. 7,110,222 issued to Erpelding describes integrated lead suspensions and tail pad terminations of those suspensions. The tail pads can be electrically connected via soldering or thermosonic bonding, both of which are widely known and practiced in the microelectronics packaging field. U.S. Patent Publication No. 2005/0254175 by Swanson et al. in FIG. 2 shows a test pad portion 46 on the side of the flying leads away from the suspension. Such a test pad portion is typically used to test the completed suspension assembly. If it is found that a read-write head, also referred to as a slider, on a suspension assembly is defective, the head must be replaced by parting the flexure tail bond and replacing the head. On the other hand if the read/write head passes its tests, typically the test pad portion 46 is cut off as no longer necessary, and the suspension is integrated into a completed disk drive unit. The fragile unsupported lead is prone to damage during assembly, test, or when separating the ultrasonic or solder terminal of this terminus for rework. In recent years, as the thickness of the copper conductor layer has decreased from about 12 μm to about 7 μm in the last few years, the lead has become even more fragile, making rework even more difficult. 
     Traditional methods of increasing the strength of these delicate unsupported flying leads include the use of copper alloys such as beryllium copper alloy as taught by Bennin et al. in U.S. Pat. Nos. 5,645,735 and 5,687,479, or less toxic copper alloy alternatives such as NK120 or Olin 7025 alloy. Other efforts have focused on methods of distributing the high stress on the terminations at the point of highest strain, where the unsupported leads emerge from the polyimide. An unintended drawback to increasing the toughness of the copper conductor by substituting a stronger copper alloy for use in the suspension signal traces, is that doing so undesirably increases the gimbal stiffness, whereas decreased gimbal stiffness is desired for the emerging smaller read/write heads. 
     The methods of Swanson et al. and Matsumoto et al. may provide a more rigid tail flying lead terminus than in prior flying leads; however, they require subsequent metal electrodeposition upon dissimilar metals. This is particularly problematic in the case of a copper conductor upon an inert steel, as the presence of the inherent passive amorphous oxide which inherently forms on stainless steel is not readily receptive to acceptable adhesion of subsequent metal deposition. 
     One of the many challenges in HGA design is noise reduction. For example, due to the spatial constraints inherent in microactuator design, the conductive traces formed on HDD suspensions, or flexures, cause cross talk between the read and write transmissions as signals propagate to and from the head assembly. 
     U.S. Pat. No. 7,342,750 issued to Yang et al. discloses an externally wireless laminated suspension for a hard disk drive for reduced noise and crosstalk. In one embodiment, the externally wireless laminated suspension has an insulating layer to electrically isolate a first and second electrical trace from a conductive support layer. The second electrical trace crosses over the first electrical trace. The first electrical trace may be made of a first part on one side of the second electrical trace and a second part on the opposite side of the electrical trace. A conductive island area may be patterned into the support layer. The conductive island area may electrically couple the first part of the first electrical trace to the second part. The number of crossover points that the first electrical trace has may equal the number of crossover points that the second electrical trace has. 
     The hard disk drive industry has traditionally used flexible trace pairs on flexible circuits to make electrical interconnects between the pre-amp that drives and reads the read/write signals and the read/write transducer head. In order to achieve optimal signal transmission between the pre-amp and the head, circuit impedance of the differential pairs have been designed to match the impedance of the heads and the pre-amp, which were in the 50-100 ohm range. The impedance was achieved using a circuit construction that featured the “plus” and the “minus” signals within the differential pair extending in parallel and over a stainless steel ground plane separated from the ground plane by a thin insulative layer such as a 10-20 μm thick polyimide layer. For designs that required a high impedance, such as greater than 100 ohms, the stainless steel layer could be removed under the traces and the trace width could be adjusted to match the target impedance. For low impedance trace pairs, i.e., less than 100 ohms, the stainless steel was used to improve coupling between the trace pair, thus lowering the impedance of the structure. For low data rate (i.e., &lt;1 Gbit/sec) this construction worked well; however, as hard drive data rates continue to increase, parasitic losses in the stainless steel become an issue that limits the circuit bandwidth. 
     In order to achieve circuits of impedance of lower than about 50 ohms and having high bandwidths of 1 GHz or higher, suspension designs have been modified to remove the stainless steel from underneath the signal traces, and the traces have been made very wide (100-200 μm wide) with a very narrow spacing (15-30 μm) between the traces that make up the trace pair. This structure minimized the coupling to the stainless steel ground plane below, which had been causing losses leading to low bandwidth performance; however, in order to achieve lower impedance the width of the traces had to be increased. With very wide traces (150-200 μm) the impedance could be lowered down to 50 ohms with bandwidths of up to 4-5 GHz. Continuing to increase trace widths beyond about 150 μm provides minimal reduction in impedance since the increased width provides minimal increase in coupling between the trace pairs. Some of the newer read/write heads have ultra low impedances of approximately 10-30 ohms, necessitating suspension circuits having even lower impedances than those of the past. 
     U.S. Pat. No. 5,717,547 issued to Young discloses an integrated transmission line array of multiple interleaved trace conductors symmetrically formed in a single plane for electrically interconnecting a read element or a write element of a dual element read/write head to a preamplifier circuit in a disk drive. 
     U.S. Pat. No. 6,762,913 issued to Even et al. discloses a method of adjusting the common mode impedance while enabling maintenance of the differential mode impedance of a pair of traces located with respect to a ground plane formed by a load beam or trace assembly of a disk drive head suspension. The method includes forming a ground plane having apertures with isolated conductive islands in the apertures for setting a desired common mode impedance. The method includes a cut and try approach using sample coupons to adjust the ratio of backed area to island area to adjust the common mode impedance while maintaining the differential mode impedance by maintaining the ratio of unbacked area to the sum of the backed and island areas. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide in disk drive suspension interconnects a highly conductive connection between e.g. copper/gold and stainless steel or copper/stainless steel components of a disk drive suspension. It is a further object to provide novel and improved methods of manufacturing disk drive suspensions having this highly conductive connection using conductive adhesive-free, metallized vias extending between the copper/gold and stainless steel layer components of a disk drive suspension. A further object is to ground a disk drive suspension slider to a stainless steel layer optionally including a copper metallizing layer using the novel vias of the invention. Yet another object is the manufacture of disk drive suspension interconnects with stainless steel layer or copper/stainless steel grounded components using vias having an improved, low resistance electrical connection. A further object comprises a metallizing modification of the vias to enhance copper/stainless steel bonding and electrical connection thereby. A further object is to facilitate addition of further circuit layers through the effective use of the invention via technology. A further object is to provide substantially planar via openings to enhance the bond-affixing of other components. A further object is to provide a suspension circuit having resilient flying leads and bond pads. A still further object is to provide a suspension circuit having low impedance and low crosstalk. 
     These and other objects to become apparent hereinafter are realized in accordance with the invention method of manufacturing a disk drive suspension interconnect comprising a metal grounding layer, a metal conductive layer and an insulative layer between the metal grounding layer and the conductive metal layer, and a circuit component electrically connected to the conductive layer, that includes defining a grounding path from the circuit component and the conductive layer to the metal grounding layer through an aperture in the insulative layer, depositing a tie layer through the insulative layer onto the grounding layer in bonding relation with the ground layer, and depositing a conductor onto both the conductive metal layer and the tie layer in conductive metal layer and tie layer bonding relation, whereby the circuit component is bonded to the grounding layer by the conductor. 
     In one aspect of the invention, an additive manufacturing method is provided to connect the copper circuitry of a component such as a slider with the stainless steel part of the suspension interconnect using vias modified in manufacture to have better electrical connection than available heretofore. In one aspect, the invention provides optimal performance in hard disk drive sliders by improving the electrical grounding thereof through a via having a copper conductor bonded to a stainless steel layer using a tie layer of chromium and optionally a copper flash layer as well. 
     In this and like embodiments, typically, the method includes selecting a stainless steel material as the grounding layer, selecting a copper metallized stainless steel as the grounding layer, selecting a copper material as the metal conductive layer, selecting a synthetic organic polymer as the insulative layer, such as a polyimide, selecting a copper material as the conductor, selecting a chromium material as the tie layer, depositing a seed copper material layer with the tie layer, electrodepositing a copper material layer with the seed copper material layer, and selecting a slider as the circuit component. 
     The invention further provides a disk drive suspension interconnect comprising a metal grounding layer, a metal conductive layer and an insulative layer between the metal grounding layer and the conductive metal layer, and a circuit component electrically connected to the conductive layer, the insulative layer defining an aperture providing a grounding path from the circuit component and the conductive layer to the metal grounding layer, a tie layer bonded to the grounding layer within the aperture, and a conductor bonded to the grounding layer through the tie layer and to the conductive metal layer in circuit component grounding bonding relation. 
     In this and like embodiments, typically, the grounding layer comprises stainless steel or copper metallized stainless steel, the metal conductive layer comprises copper, the insulative layer comprises a synthetic organic polymer, such as a polyimide, the conductor comprises copper, the tie layer comprises chromium, there is also a seed layer comprising copper within the aperture, the grounding layer comprises stainless steel or copper metallized stainless steel, the metal conductive layer comprises copper, the insulative layer comprises a synthetic organic polymer such as a polyimide, and a copper layer electrodeposited onto the copper seed layer, and the circuit component comprises a slider. 
     In another aspect, the invention provides a tougher, more resilient unsupported flying lead in the tail termination area for increased strength in that fragile area, while at the same time allowing a soft metal to be used for the signal trace in the gimbal area for gimbal flexibility, such as pure soft ductile copper. In this aspect, the flying lead comprises stainless steel with a gold plated coating. Prior to gold plating, the stainless steel may be activated by the use of concentrated acids preferably in the presence of reverse polarity current (electropolish). In this embodiment, the signal travels from the soft ductile copper signal trace through a via to the tough stainless steel substrate, through the flying lead which is comprised of the substrate, and back up to the copper signal conductor on the other side of the flying lead through a second via. The stainless steel flying lead may be coated with gold and/or nickel for corrosion resistance, wetability, and/or for thermosonic bonding. The stainless steel flying lead need not include any copper signal conductor. 
     Alternatively, instead of plating gold directly on the stainless steel (SST) flying lead, a layer of copper or nickel may be strike plated upon the stainless steel lead in order to enhance the wetability of the plated gold layer. The nickel may be plated using a “Woods Strike” or variations on the Woods Strike which by themselves are known, such as taught in U.S. Pat. No. 3,645,861 issued to Garvey. 
     The final metallurgy of the flying lead in the unsupported area may be, for example, one of the following: SST/Au; SST/Ni/Au; SST/Cu/Au; SST/Cu/Ni/Au; SST/Ni/Cu/Ni/Au; or other variations. 
     In this aspect, the invention is of a circuit for a disk drive suspension, the circuit having a flexible circuit portion, a flying lead portion being of a relatively resilient material such as stainless steel, with the flexible circuit portion being electrically connected to the flying lead portion through a via. The first flexible circuit portion has a read/write signal conductor of a relatively soft material such as ductile copper, the copper being separated from the generally flat stainless steel foil underneath it via an insulative or dielectric material. The via electrically connects the copper conductor to the stainless steel foil beneath. The flying lead portion is defined by the portion of the circuit that has substantially no insulative material on at least one side and commonly on both sides, separating the flying leads from one another. The stainless steel flying lead is covered by a bond-receptive layer such as plated gold, with the bond-receptive layer being suitable for bonding to the flying lead by, for example, thermosonic bonding or solder ball bonding. The bond-receptive layer renders the flying lead receptive to bonding to an external electrical component such as a wire, an integrated circuit lead, leads on a pre-amplifier, or any other electrical component. 
     In one embodiment the flying leads may be configured and arranged as parallel strips or ribbons. In another embodiment the flying leads may be configured especially for solder ball bonding, with each individual flying lead being generally circular and having a hole therethrough for receiving the solder ball, or using other industry-recognized or otherwise suitable solder ball bonding pad configurations. 
     In another aspect, the invention is of suspension circuit having low impedance and low crosstalk. Each signal trace that defines one polarity of a differential signal trace pair may be split into multiple paths and interleaved with the signal trace of the opposite polarity within that same signal trace pair, to form parallel multiple differential trace pairs resulting in lowered signal path impedance. For each individual differential trace pair, the signals paths repeatedly cross in order to reduce noise and crosstalk. The signal paths may be laid out as two differential pairs, each of the two differential pairs carrying the same “plus” and “minus” pair, and within each differential pair, the “plus” and “minus” crossing. Alternatively, the signal paths may be laid out as a pair of adjacent “plus” paths carrying the same “plus” half of the differential signal, paralleling a pair of adjacent “minus” paths carrying the same “minus” half of the differential signal, with the pair of adjacent “plus” signal conductors crossing the pair of adjacent “minus” conductors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further described in conjunction with the attached drawings in which: 
         FIGS. 1 through 11  are stages of a process flow according to the invention method with the via configurations modified for illustrative purposes; 
         FIG. 12  is a view like  FIG. 11 , but of a more typical configuration of the via realized in accordance with the invention; 
         FIG. 13  is a bottom plan view of a disk drive suspension interconnect according to the invention; 
         FIG. 14  is a top plan view thereof; 
         FIGS. 15-27  are stages of an alternate process flow according to the invention method; 
         FIG. 28  is a top plan view of an exemplary suspension circuit having a flying lead region  250 ; 
         FIG. 29  is a closeup of the flying lead region  250  of  FIG. 28 ; 
         FIGS. 30-42  illustrate the process flow for forming a resilient flying lead according to an embodiment of the invention; 
         FIG. 43  is an oblique view of a solder ball bond pad tail in a disk drive suspension circuit, according to a further embodiment of the invention; 
         FIG. 44  is a top plan view of one of the solder ball bond pads of  FIG. 43 ; 
         FIG. 45  is a simplified sectional view of the solder ball bond pad of  FIG. 44 , taken along section line  45 - 45 ; 
         FIG. 46  is a top plan conceptual view of a reduced impedance suspension circuit having crossing signals according to a first embodiment of a reduced impedance, reduced crosstalk suspension; 
         FIG. 47  is a cross sectional view of the suspension circuit of  FIG. 46  taken along section line  47 - 47 ; 
         FIG. 48  is a top plan conceptual view of a reduced impedance suspension circuit having crossing signals according to a second embodiment of a reduced impedance, reduced crosstalk suspension circuit; 
         FIG. 49  is a top plan conceptual view of reduced impedance suspension circuit according to a third embodiment; 
         FIG. 50  is a cross sectional view of the suspension circuit of  FIG. 49  taken along section line  50 - 50 ; 
         FIG. 51  is a top plan conceptual view of a reduced impedance, reduced crosstalk suspension circuit according to a fourth embodiment; and 
         FIG. 52  is a cross sectional view of a suspension circuit that utilizes a portion of the suspension substrate as the crossover connection. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one aspect, the invention provides a simpler and more economical solution than those criticized above for manufacturing suspension interconnects having a reliable electrical connection between the circuit components to be grounded and the grounding layer of a stainless steel foil by creating a metallized via between them. It is well known that there are inherent difficulties in making electrical connections to stainless steel. In the invention, however, these difficulties are avoided or obviated by using a sputtered tie layer such as chromium that can be used to attach a conductor, e.g. copper, attached to a component, e.g. a slider, to a ground such as stainless steel.  FIGS. 1-11  show the invention process flow for creating a metal via between the copper circuit and stainless steel layers on a hard disk drive suspension interconnect.  FIG. 15-26  show the invention process flow for creating a metal via between the copper circuit and stainless steel coated with conductive copper layer. 
     Suspension interconnects having multiple conductor layers can comprise three layers including top and bottom layers consisting of metal, e.g. stainless steel and copper (preferably copper for traces) and a thin insulating layer such as a plastic layer or film preformed or formed in situ sandwiched between them. These layers are preferably formed by the additive process where the several layers are built up into a laminate assembly. In a less preferred subtractive process, a preformed laminate assembly is treated to selectively remove portions of the layers until the desired reduced (“subtractive”) product is achieved. In a typical case, a pair of upper and lower traces operatively associated on the insulative film layer form a single circuit, be it a read or a write circuit. 
     In various embodiments, an insulating layer, a fourth layer, may be coated over the trace layer; sometimes a further conductive layer is deposited over the fourth layer to provide a five-layer structure that is supported by the load beam. Deposition of the added metal layer can be by sputtering or by electrolytic means, or a metal foil can be used. 
     The invention provides a disk drive suspension interconnect having one or more vias connecting electrical components connected or grounded to a ground layer through an insulative layer. 
     With reference to the drawings,  FIGS. 1-14  show the interconnect  12  product and the process of forming the interconnect or flexible circuit. The additive process begins in  FIG. 1  with a substrate  14  comprising stainless steel  16  and a typically polyimide or polyester electrically insulative layer  18  that can be cast directly onto the stainless steel. In  FIG. 2 , a hole  22  where the via  24  is to be formed is shaped as an aperture  26  projecting not more than about 5 microns photochemically, by laser drilling, by the use of photosensitive polyimide resin or chemically by etching, or otherwise, along with any fiducials (not shown) deemed to be needed into the polyimide layer  18 , e.g. at a wall slope of preferably about 75 degrees and ranging from about 50 to 60 degrees to about 85 to 90 degrees provided the slope is such that sputtering can be effectively performed to metallize the via. In  FIG. 3  the resulting apertured laminate insulative layer  18  is plasma or otherwise cleaned to remove any scum layer left at the bottom of the intended via  24 . Hole  22  is relatively small in diameter enabling placement of several on a typically quite small suspension interconnect  12 . 
     In  FIG. 4 , a chromium tie layer  28  is sputtered or otherwise deposited onto the insulative layer  18  and into aperture  26 . “Chromium” herein includes alloys of chromium in which chromium is the largest single constituent by weight. Monels® are useful. Preferably a copper seed layer  32  is further added on top of the polyimide layer  18  and tie layer  28  and down into the aperture  26  from which the via  24  is to be made. Typically a chromium tie layer  28  will be used and will be between 100 Å and 800 Å thick. The copper seed layer  32  when used typically will be between 500 Å and 1500 Å thick. 
     In  FIG. 5 , a thin layer  34  of copper (including alloys of copper) may be electrodeposited in those cases where subsequent high speed plating will be carried out, as the presence of a flash plated copper layer  34  tends to build up the copper seed layer  32  thickness and provide a copper build-up to a thickness between 1 μm and 5 μm which is desirable for high speed copper plating. 
     In  FIG. 6  a photoresist layer  36  is deposited on top of the tie, seed and electrodeposited layers  28 ,  32  and  34 , respectively. 
     In  FIG. 7 , after reregistering, the photoresist layer  36  is exposed and developed in a manner leaving openings  38  in the resist layer where circuit features are to be connected to ground. 
     In  FIG. 8 , openings  38  in the photoresist layer  36  are finally copper plated, e.g. to form a copper plate  42  having a thickness of about 5 μm to about 20 μm from which suspension interconnect conductive layer comprising conductive traces  44  is formed. 
     In  FIG. 9 , the photoresist layer  36  is stripped to expose the flash plated copper layer  34 . 
     In  FIG. 10 , a flash etch locally removes the exposed chromium tie layer  28 , copper seed layer  32 , and electrodeposited flash copper  34 , where used, so that the remaining copper circuit features  40  are electrically isolated. Suitable etchants for the etch step include potassium permanganate for the chromium tie layer and cupric chloride for the copper layers. 
     In  FIG. 11 , the copper features  40  are plated with gold layer  46  producing the metallized via  38  of the invention. 
     In  FIG. 12 , a more typical configuration of the via is depicted with like numerals to those numerals in  FIGS. 1-11  for like parts. 
     With reference to  FIGS. 15-26  interconnect or flexible circuit  12  is formed by an alternative method, one using a pre-copper coated stainless steel starting material. The additive process begins in  FIG. 15  with a substrate  14  comprising stainless steel  16  that is sputtered with a metallizing layer  17  comprising a seed layer and a thin copper layer. In  FIG. 16  the metallizing layer  17  is electroplated with a highly conductive layer  19  on the surface of the stainless steel metallizing layer  17  that is 1-6 μm thick. In  FIG. 17  a typically polyimide or polyester electrically insulative layer  18  is cast directly onto the now metallized stainless steel  16 . In  FIG. 18 , a hole  22  where the via  24  is to be formed is shaped as an aperture  26  projecting not more than about 5 microns, mechanically by laser drilling, or chemically by etching, or otherwise, along with any fiducials (not shown) deemed to be needed into the polyimide layer  18 , e.g. at a wall slope of preferably about 75 degrees and ranging from about 50 to 60 degrees to about 85 to 90 degrees provided the slope is such that sputtering can be effectively performed to metallize the via. In  FIG. 19  the resulting apertured laminate insulative layer  18  is plasma or otherwise cleaned to remove any scum layer left at the bottom of the intended via  24 . Hole  22  is relatively small in diameter enabling placement of several on a typically quite small suspension interconnect  12 . 
     In  FIG. 20 , a chromium tie layer  28  is sputtered or otherwise deposited onto the insulative layer  18  and into aperture  26 . “Chromium” herein includes alloys of chromium in which chromium is the largest single constituent by weight. Monels® are useful. Preferably a copper seed layer  32  is further added on top of the polyimide layer  18  and tie layer  28  and down into the aperture  26  from which the via  24  is to be made. Typically a chromium tie layer  28  will be used and will be between 100 Å and 800 Å thick. The copper seed layer  32  when used typically will be between 500 Å and 1500 Å thick. 
     In  FIG. 21 , a thin layer  34  of copper (including alloys of copper) is electrodeposited in those cases where subsequent high speed plating will be carried out, as the presence of a flash plated copper layer  34  tends to build up the copper seed layer  32  thickness and provide a copper build-up to a thickness between 1 μm and 5 μm which is desirable for high speed copper plating. 
     In  FIG. 22  a photoresist layer  36  is deposited on top of the tie, seed and electrodeposit layers  28 ,  32  and  34 , respectively. 
     In  FIG. 23 , after reregistering, the photoresist layer  36  is exposed and developed in a manner leaving openings  38  in the resist layer where circuit features are to be connected to ground. 
     In  FIG. 24 , openings  38  in the photoresist layer  36  are finally copper plated, e.g. to form a copper plate  42  having a thickness of about 5 μm to about 20 μm from which suspension interconnect conductive layer comprising conductive traces  44  is formed. 
     In  FIG. 25 , the photoresist layer  36  is stripped to expose the flash plated copper layer  34 . 
     In  FIG. 26 , a flash etch locally removes the exposed chromium tie layer  28 , copper seed layer  32 , and electrodeposited flash copper  34 , where used, so that the remaining copper circuit features  40  are electrically isolated. Suitable etchants for the etch step include potassium permanganate for the chromium tie layer and cupric chloride for the copper layers. 
     In  FIG. 27 , the copper features  40  are plated with gold layer  46  producing the metallized via  38  of the invention. With further reference to  FIGS. 12 and 13 , suspension interconnect  12  comprises a flexible circuit  48  of typically a substrate  14  comprising a stainless steel metal layer  16  (alternatively pre-coated with a metallizing layer  17  and electroplated layer  19  as described in reference to  FIGS. 15-27 ) and a cast coating of a polyimide layer  18  and conductive traces  44  formed from the copper plate layer  42 . Suspension interconnect  12  includes a flexure  50  formed from the substrate metal layer  76  to have a frame  52  and a tongue  54  supporting a slider  60  electrically connected to the conductive traces  44  at  58 . For purposes of grounding a circuit component  56  such as the slider  60  to metal layer  16 , via  24  connects a grounding lead  62  extending from the slider to the via and through the metallized via to the stainless steel metal layer  16  which is itself grounded by means not shown. 
     Thus, the invention method of manufacturing a disk drive suspension interconnect  12  comprising a metal grounding layer  16  (e.g. stainless steel with or without added layers of copper conductor), a metal conductive layer comprising trace conductors  44  and an insulative layer such as polyimide layer  18  between the metal grounding layer and the conductive metal layer, and a circuit component  58  such as a slider  60  electrically connected to the conductive layer traces, includes defining a grounding path  64  from the circuit component and the conductive layer to the metal grounding layer through an aperture  26  in the insulative layer, depositing a tie layer  28  through the insulative layer onto the grounding layer in bonding relation with the grounding layer, and depositing a conductor  42  onto both the conductive metal layer and the tie layer in conductive metal layer and tie layer bonding relation, whereby the circuit component is bonded to the grounding layer by the conductor. 
     In a further embodiment, a disk drive suspension interconnect  12  is provided comprising a metal grounding layer  16  (e.g. stainless steel with or without a copper layer), a metal conductive layer  42  and an insulative layer  18  between the metal grounding layer and the conductive metal layer, and a circuit component  58  electrically connected to the conductive layer, the insulative layer defining an aperture  26  providing a grounding path  64  from the circuit component and the conductive layer to the metal grounding layer, a tie layer  28  bonded to the grounding layer within the aperture, and a conductor  42  bonded to the grounding layer through the tie layer and to the conductive metal layer in circuit component grounding bonding relation. 
     In the foregoing embodiments, the method further includes selecting a stainless steel material with or without a metallizing copper layer as the grounding layer  16 , selecting a copper material as the metal conductive layer  42 , selecting a synthetic organic polymer as the insulative layer  18 , such as a polyimide, selecting a chromium material as the tie layer  28 , depositing a seed copper material layer  32  with the tie layer, electrodepositing a copper material layer  34  with the seed copper material layer, and selecting a slider  60  as the circuit component  56 . Copper, a copper material, chromium, a chromium material, stainless steel or a stainless steel material, etc. herein include materials in which the named element or alloy is the largest single constituent by weight. 
     In other embodiments discussed in detail below, a resilient flying lead comprises stainless steel with a bond-receptive coating such as gold on it, with vias electrically connecting the copper signal traces on either side of the flying lead to the resilient stainless steel flying lead below. The stainless steel flying lead electrically bridges two flexible circuit portions, each flexible circuit portion having a copper signal conductor supported on an insulative material such as polyimide, with vias electrically connecting each of the two flexible circuit portions to the stainless steel flying lead below. In an exemplary embodiment the only circuit path from the first flexible circuit portion to the second flexible circuit portion is down through the first via to the stainless steel flying lead, then across the flying lead including any coating thereon, then back up through the second via to the second flexible circuit portion. This is the only circuit path in the sense that, although tiny amounts of current can always leak across even insulative material, those tiny leakage currents would not be sufficient to allow the circuit to effectively operate as a disk drive suspension circuit. 
       FIG. 28  is a top plan view of an exemplary suspension circuit  200  having a flying lead region  250  that electrically connects first flexible circuit portion  212  and second flexible circuit portion  273 , with second flexible circuit portion  273  having a number of test pads including first test pad  271  and second test pad  272 . First flexible circuit portion  212  is typically connected to the suspension&#39;s slider at the distal end of the suspension, and carries the read signals and write signals, hereafter referred to as the read/write signals, to and from the slider, as well as a ground. The flexible circuit can carry other signals including a microactuator control voltage when the suspension is a dual actuator suspension equipped with a microactuator. Typically, exposed leads within flying lead region  250  are connected to electrical components of the disk drive such as the leads of a pre-amplifier, although they could be connected to any electrical components such as wires, etc. Typically the test pads  271 ,  272  are used to test the suspension assembly once it is completed. If the suspension passes the test, second flexible circuit portion  273  is then severed from circuit  200  by cutting, and the suspension is then integrated into a disk drive. 
       FIG. 29  is a close-up view of the flying lead region  250  of  FIG. 28 . The read/write signals travel through first copper signal conductor  244 , through first via  225 , across flying lead  251 , up through via  224 , and to second copper signal conductor  245 . 
       FIGS. 30-42  illustrate the process flow for forming a resilient flying lead according to an embodiment of the present invention, with a high quality electrical contact being established from the copper conductive traces to the stainless steel flying lead through vias. 
     The process begins in  FIG. 30  with a thin generally flat foil of resilient metal material  216  such as stainless steel. The material should be significantly stronger than pure copper, or whatever alloy of copper or other conductive material is used for trace conductors  245 . 
     In  FIG. 31  a dielectric insulative layer  218  such as polyimide or other suitable material is deposited on the stainless steel  216 . 
     In  FIG. 32 , apertures  226  are then patterned in the polyimide layer  218 . Apertures  226  may be patterned into polyimide  218  by the use of photosensitive polyimide, photoresist and masking, or by a laser such as may be obtained from Electro Scientific Industries, Inc. of Portland, Oreg. The use of photoresist, including laying down of a photoresist layer, masking the photoresist, exposing it to light in order to selectively harden the resist, washing away the undeveloped resist, then etching or plating the washed away areas, and then finally washing away the exposed photoresist, is a well known process that need not be discussed in detail herein. 
     In  FIG. 33 , preferably a chromium tie layer  228  is then deposited by sputtering onto the polyimide  218  and into the exposed portion of stainless steel  216  that lies within aperture  226 . The chromium creates a strong chemical bond to the polyimide and forms a conductive path for the electrodeposition of conductors. Alternatively, tie layer  228  may comprise a Monel® metal. Although chromium tie layer  228  and copper seed layer  232  above it as shown in the following figures is preferably preferentially sputtered onto the areas shown by masking off the areas of polyimide  218  in which the sputtered layer is not shown as being deposited, it is not strictly necessary that chromium tie layer  228  and copper seed layer  232  be kept off of other areas including stainless steel foil  216 , because it is believed that etchants that will be applied to stainless steel layer  216  as described later should sufficiently remove any chromium or copper from the stainless steel. 
     In  FIG. 34 , preferably copper seed layer  232  is then deposited by sputtering onto chromium tie layer  218 . Chromium tie layer  218  and copper seed layer  232  may be deposited as discussed in greater detail above with respect to  FIGS. 4 and 5 . 
     In  FIG. 35  photoresist is applied, masked, exposed, and washed to create photoresist pattern  236 . 
     In  FIG. 36  copper is electrodeposited onto the sputtered copper seed layer  232  to form copper signal conductor  234 . Preferably copper signal conductor  234  is pure ductile copper that is flash electrodeposited, although copper alloys can also be used. Together chromium tie layer  228 , copper seed layer  232 , and copper signal conductor  234  form conductive vias  224  and  225  that electrically connect copper signal traces  234  to the stainless steel  216  that will form the flying lead. 
     In  FIG. 37  photoresist  236  is washed away. 
     In  FIG. 38  a protective and electrically insulating cover layer  260  is applied over the copper signal conductor  234  to protect it from corrosion and electrical shorting. 
     In  FIG. 39  additional photoresist  237  is applied and patterned. 
     In  FIG. 40  an etchant, typically ferric chloride, is used to create the desired pattern in the stainless steel layer. The pattern includes voids  227  in the stainless steel. 
     In  FIG. 41  the photoresist is washed away, leaving bare stainless steel in flying lead region  250 . 
     In  FIG. 42 , a bond-receptive layer  246  such as a gold layer is plated onto the stainless steel flying lead  251 , using processes that will be described in detail below. Bond-receptive layer  246  facilitates both a satisfactory mechanical and electrical connection from an external electrical component to the flying lead. 
     It can be difficult to plate other metals directly onto stainless steel. In order to do so, the stainless steel must be specially prepared or activated. U.S. Pat. No. 4,422,906 issued to Kobayashi discloses a method for directly plating gold onto stainless steel, including activating the stainless steel surface prior to the plating operation. Kobayashi is hereby incorporated by reference for its teachings of preparing a stainless steel surface for gold plating, and for plating gold onto the stainless steel, as if fully set forth herein. Specifically, gold may be plated directly onto stainless steel using a method which comprises the steps of: 
     dipping or otherwise exposing stainless steel in an activating solution; 
     subjecting the activated stainless steel to cathode electrolytic activation; and then, 
     electroplating the cathodically electrolyzed stainless steel with gold. 
     The activating solution used in the first step is preferably an aqueous mixed acid solution containing, based on the weight of the solution: 
     (i) 3 to 20% by weight of hydrochloric acid, 
     (ii) 2 to 30% by weight of sulfuric acid, 
     (iii) 0.1 to 5% by weight of a nonionic or cationic surface active agent and 
     (iv) 0.1 to 20% by weight of 2-pyrrolidone or its N-alkyl derivative. 
     More preferably, the activating solution used in the first step is an aqueous mixed acid solution containing, based on the weight of the solution: 
     (i) 3 to 10% by weight of hydrochloric acid, 
     (ii) 0.5 to 4% by weight of nitric acid, 
     (iii) 2 to 15% by weight of sulfuric acid, 
     (iv) 1 to 5% by weight of acetic acid, 
     (v) 3 to 10% by weight of citric acid, 
     (vi) 0.1 to 3% by weight of a nonionic or cationic surface active agent, 
     (vii) 0.1 to 10% by weight of 2-pyrrolidone or its N-alkyl derivative and 
     (viii) 1 to 5% by weight of an acetylenic glycol. 
     In  FIG. 42 , in the present embodiment gold layer  246  is deposited onto stainless steel layer  216  using any suitable method, including without limitation the methods taught by Kobayashi. Gold layer  246  serves several purposes. First, gold layer  246  provides a low resistance signal path on the skin of the stainless steel, so that once the signal has traveled from via  225  to stainless steel flying lead  251  and through stainless steel flying lead  251  to gold layer  246 , the electrical resistance encountered by the signal as it travels across flying lead region  250  is small. Second, gold layer  246  helps to prevent corrosion of the stainless steel  218  in critical flying lead region  251 . Third, gold layer  246  provides a suitable material and surface for thermosonic bonding or soldering, allowing for relatively easy bonding and relatively easy rework. 
     As an alternative to plating gold directly on the stainless steel flying lead, a layer of copper or nickel may be strike plated upon the stainless steel lead in order to enhance the wetability of the plated gold layer. The nickel may be plated using a Woods Strike bath which by itself is known, being described in U.S. Pat. No. 3,645,861 issued to Garvey, which is fully incorporated by reference as if set forth herein. Furthermore, the nickel may be plated using the variation on the standard Woods Strike taught by Garvey, in which the nickel chloride and hydrochloric acid are replaced with nickel bromide and hydrobromic acid, which Garvey discloses as producing better adhesion of the nickel to the stainless steel than the standard Woods Strike bath. According to Garvey, the preferred plating bath comprises: 
     100-800 g/l, and preferably about 500 g/l, of nickel bromide, and 
     0.2-20% by weight, and preferably about 0.4% by weight, hydrobromic acid with the plating taking place at a current density of 5-200 amp per square foot. The resulting metallization could therefore be: SST/Au; SST/Ni/Au; SST/Cu/Au; SST/Cu/Ni/Au; SST/Ni/Cu/Ni/Au; or other variations that will be obvious to one skilled on the art after receiving the teachings of the present invention. The nickel and/or additional gold layers, when present, would constitute an additional metallic layer between the stainless steel flying lead and the gold bond-receptive layer. 
     The invention has been illustrated above with respect to flying leads that are essentially parallel strips or ribbons of unsupported, generally flat stainless steel foil, with the finished flying leads constituting parallel ribbons of thin rectangular stainless steel foil that are substantially free of insulative material on both sides for electronic access on both sides, and substantially free of any other material that would physically separate the parallel flying leads. 
     In a further embodiment, the invention can be applied to suspension circuit termination pads that are designed for solder ball bonding, which are increasingly replacing unsupported parallel ribbons as termination structures in disk drive suspensions. In prior art solder ball bonding (SBB) terminations for suspension circuits, the circuit tail used to make the electrical connections has an array of solder ball bonding pads. Each pad is generally round in shape with a hole in it, and is made of unsupported copper. In a typical solder ball bonding process the pad is clamped over a bond pad on another circuit that is larger than the hole. In the case of SBB, a ceramic capillary is then brought down to the clamped bond pads and a solder ball is dropped down the capillary to the bond pad and held between the bond pad and capillary. The solder ball is then heated until it flows making an electrical and mechanical connection. An alternative to SBB is solder ball jetting (SBJ), which brings a capillary down to the bond pad, similar to what is done in SBB. Unlike SBB, however, the solder ball is larger than the capillary so when it is dropped into the capillary it gets caught in the capillary. A laser is used to reflow the solder ball in the capillary and nitrogen gas is shot down the capillary expelling the molten solder in the key hole feature in the bond pad. The solder flows both underneath the pad, and also flows through the pad hole and onto the top of the pad. The hole allows the quality of the solder connection to be verified for quality control purposes from the top. The prior art pad itself is generally unsupported copper, with polyimide and possibly a stainless steel substrate defining a matrix that otherwise supports and mechanically connects the pads. In one prior art SBB tail, the shapes of the holes resemble keyholes, similar to the hole shapes shown in  FIGS. 43-44 . Because the SBB bond pads have previously been comprised of copper, which is a relatively soft material, SBB bond pads within the prior art have been somewhat fragile and prone to damage, especially during rework. The prior art SBB bond pads can be improved upon by modifying them according to the structures and techniques of the present invention. 
       FIG. 43  is an oblique view of an SBB termination region  350  in a disk drive suspension circuit, according to an embodiment and application of the invention for solder ball boding. SBB termination region  350  may generally be substituted for flying lead region  250  of the embodiment shown in  FIGS. 28-29  for those suspensions for which an SBB termination is desired. SBB termination region  350  includes a plurality of SBB bond pads. Representative SBB bond pad  351  is electrically connected to copper signal conductor  345  on its test pad side, and to copper signal conductor  344  on its slider side. 
       FIG. 44  is a close-up, top plan view of SBB bond pad  351  of  FIG. 43 . Copper signal conductor  345  is connected to bond pad  351  through via  324  which is shown in dashed lines because it is not visible from the outside. Similarly, copper signal conductor  344  is connected to bond pad  351  through via  325 , which is also not visible from the outside. Polyimide or other suitable insulative material  318  surrounds the top surface  366  of bond pad  351 . Bottom surface  367  of bond pad  351  is electrically isolated from other stainless steel material  317  ( FIG. 45 ) on the bottom surface of SBB termination region  350 . The portion  317  of stainless steel foil that does not form the SBB bond pads, can be an otherwise solid matrix of stainless steel on the underside of the termination region  350 , or can have various patterns etched in it such as alternating strips of stainless steel and no stainless steel in order to control the signal impedance in termination region  350 , or can be etched away altogether. In the exemplary embodiment shown, SBB bond pad  351  has a keyhole shaped hole  353  formed in it, with the hole shape by itself being within the prior art. 
       FIG. 45  is a simplified sectional view of the solder ball bond pad of  FIG. 44  taken along section line  45 - 45 . The stainless steel  316 , covered by gold layer  346 , forms SBB bond pad  351  having hole  353  therethrough. Bond pad  351  has a top surface  366  and a bottom surface  367 , both covered in gold. Bond pad hole  351  has a perimeter whose top surface is defined by top surface  366 , and whose bottom surface is defined by bottom surface  367 . Vias  224  and  225  extending through polyimide  318  electrically connect copper signal conductors  345  and  344  to SBB bond pad  351 , respectively. Cover layer  360  provides environmental protection and insulation against shorting to copper signal conductors  344  and  345 . It will be understood that a chromium or Monel® tie layer and a copper seed layer, analogous to chromium tie layer  228  and copper seed layer  232  in the embodiment of  FIGS. 28-42  are preferably present, but omitted from  FIG. 45  for simplicity of illustration. Typically a chromium tie layer will be used and will be between 100 Å and 800 Å thick, and the copper seed layer when used typically will be between 500 Å and 1500 Å thick. Further, although only a single gold layer  366  is shown over stainless steel  316 , it will be understood that stainless steel  316  may be coated with multiple layers of nickel, copper, and/or gold, as discussed above with respect to  FIGS. 28-42 . 
     Because SBB bond pad  351  is unsupported by polyimide  318  on its top surface or its bottom surface, pad  351  may be referred to as a flying lead or terminal, or flying SBB bond pad. 
     The invention thus provides in disk drive suspension interconnects a highly conductive connection between e.g. copper/gold and stainless steel or copper/gold and stainless steel/copper components of a disk drive suspension, novel and improved methods of manufacturing disk drive suspension interconnects having this highly conductive connection using conductive adhesive-free, metallized vias extending between the copper/gold and stainless steel components of a disk drive suspension for grounding, and in a particular case, a disk drive suspension slider to a stainless steel layer using the novel vias of the invention. The invention further provides a method for the manufacture of disk drive suspension interconnects with stainless steel layer grounded components using vias having an improved, low resistance electrical connection, specifically, metallized, modified vias to enhance copper/stainless steel bonding and electrical connection of suspension components thereby. 
     The invention further provides a resilient flying lead comprising primarily stainless steel, with gold thereon for thermosonic bonding and for soldering of electrical connections thereto, and for corrosion resistance. The copper of the signal conductor does not extend onto the flying lead; rather, the signal travels through the copper signal conductor on one end of the flying lead, through a first via provided in the dielectric layer at a first end of the flying lead and into the stainless steel, through the stainless steel, and back through a second via provided in the dielectric layer at a second end of the flying lead, and into the copper signal conductor at that second end. 
     In another aspect, the invention is of a low impedance suspension circuit, optionally having reduced crosstalk. 
       FIG. 46  is a top plan conceptual view of a reduced impedance suspension circuit having crossing signals according to a first embodiment of a reduced impedance, defining a reduced crosstalk suspension.  FIG. 47  is a cross sectional view of the suspension circuit of  FIG. 46  taken along section line  47 - 47 .  FIG. 46  is a conceptual plan view in the sense that it mainly shows the electrical paths without regard to certain other features. 
     In this embodiment, made by an additive circuit process, the suspension circuit includes a suspension substrate  416 , preferably being a spring metal such as stainless steel or copper metallized stainless steel. Insulative layer  418  is a dielectric material, typically an organic polymer such as polyimide. On top of insulative layer  418  are deposited the signal conductors, typically made of pure copper or a high copper alloy. The signal conductors in this embodiment include first signal conductor  440 , second signal conductor  450 , third signal conductor  460 , and fourth signal conductor  470  formed on insulative layer  418 . Fifth signal conductor  442  continues a first signal path which includes first signal conductor  440 , first cross connection or crossbar  441 , and signal conductor  442 . Sixth signal conductor  452  continues a second signal path which includes second signal conductor  450 , second cross connection or crossbar  451 , and sixth signal conductor  452 . The conductors carry a differential signal pair, for example, a read signal pair (and READ+ signal and a READ− signal) or a write signal pair (a WRITE+ signal and a WRITE− signal). 
     First and second signal conductors  440  and  450  are electrically connected together, preferably at both the slider end of the suspension and near the pre-amp. Similarly, third and fourth signal conductors  460  and  470  are electrically connected together, preferably at both the slider end of the suspension and near the pre-amp. As shown, first and second signal conductors  440  and  450  carry a first polarity signal of a differential signal pair, and third and fourth conductors  460  and  470  carry a second polarity signal of a differential signal pair, the first and second polarities being of opposite polarity from each other. More particularly as shown, first and second signal conductors  440  and  450  carry the “plus” half of the differential signal pair, and third and fourth conductors  460  and  470  carry the “minus” half of the differential signal pair. Taken along a locus defined at the bottom of the figure, the signal paths are arranged in an order of plus, plus, minus, minus when taken from left to right, which is the same as minus, minus, plus, plus when taken from right to left. 
     Cross connection  441  defines the location at which the first signal path crosses the third and fourth signal paths. Similarly, cross connection  451  defines the location at which the second signal path crosses the third and fourth signal paths. At this crossing location, the third and fourth signal paths remain on respective continuous copper conductors  460  and  470  respectively, and it is the first and second signal paths that cross over the third and fourth signals at a different vertical level utilizing what can be thought of as electrical overpasses  441  and  451 . 
       FIG. 46  shows one signal crossing. Typically, the signal paths within a suspension circuit would include multiple crossings. Preferably the first and second signal paths cross the third and fourth signal paths at substantially the same location. That is, where the first and second signal paths cross over the other signal paths, the first and second signal paths are separated by the same distance from each other as they are separated from each other for substantially the rest of the signal path, such as at the bottom of the figure. 
       FIG. 47  shows the construction of one of the cross connections  451  in greater detail. As is conventionally known in additive circuits, first through fourth signal conductors  440 ,  450 ,  460 , and  470  would typically comprise copper electrodeposited as a single continuous sheet onto polyimide  418 , with the unwanted copper then etched away leaving only the desired signal traces  440 ,  450 ,  460 , and  470 . In standard suspension circuit production techniques, a protective topcoat would then be applied over the signal conductors which would all lie on a single plane. In  FIG. 47 , however, an additional insulative layer  419  such as polyimide is applied over the first signal conductor layer. First via  424  and second via  425  are then formed in the polyimide, and an additional layer of copper conductor, which forms crossbar or cross connection  441 , is then electrodeposited. The result is a crossbar  441  by which the first signal path which includes first signal conductor  440  is made to cross over third and fourth signal conductors  460  and  470 . The details of laying down the polyimide, electrodepositing a conductive layer of copper thereon, forming vias at the desired locations in the polyimide, and laying down additional copper, including the necessary masking, photo developing, etching, and washing steps, are well known within the relevant art, or would be obvious from the prior art when combined with the present teachings, and will not be discussed in detail. 
     The figure shows one crossing point. Typically there would be multiple other crossing points (not shown) as well, such that the first and third signal paths repeatedly cross, and the second and fourth signal paths repeatedly cross. Preferably, at the next crossing point the first and second signal paths would remain on respective continuous copper signal paths  442  and  452 , and the third and fourth signal paths would cross over the first and second signal paths at electrical overpasses (not shown). In this way, the plus and minus signal paths would each travel the same or nearly the same distance, and travel the same or nearly the same number of electrical overpasses, thereby keeping the plus and minus signal paths of equal length and impedance balanced to the extent possible. 
     In the embodiment, the suspension circuit includes, in the following vertical order: the suspension substrate  416  at the bottom level; the first insulative layer  418  at the next level up; the various copper conductors  440 ,  450 ,  460 , and  470  that make up the main signal carriers; a second insulative layer  419 ; and the crossbars  441  and  451  that define the electrical cross connections. 
       FIG. 48  is a top plan conceptual view of a reduced impedance suspension circuit having crossing signals according to a second embodiment of a reduced impedance, reduced crosstalk suspension circuit. The physical construction of the portion of the circuit shown in  FIG. 48  is the same as the physical construction of the portion of the circuit shown in  FIG. 46 , but the signal polarities are different. More specifically, instead of the signals being arranged in the order of plus, plus, minus, minus as in  FIG. 46 , the signals are arranged in the order of plus, minus, plus, minus. 
     Preferably the signal paths carrying the “plus” signals cross the paths carrying the “minus” signals at substantially the same location. That is, where the “plus” signal paths cross over the “minus” signal paths, the two “plus” signal paths are separated by the same distance from each other as they are separated from each other for substantially the rest of the signal path, such as at the bottom of the figure. 
     As seen in the figure, all four signal paths extend in parallel; then the plus and minus signal paths for the first differential pair angle away such that they extend in parallel as they cross the plus and minus signal paths for the second differential pair, and the plus and minus signal paths for the second differential pair angle away such that they extend in parallel as they cross the plus and minus signal paths for the first differential pair; then all four signal paths extend in parallel again. 
     With the suspension of  FIG. 48 , suspensions having low trace impedance of approximately 30 ohms or lower can be achieved using copper conductor traces that are in the range of approximately 100-200 μm wide and 10-50 μm spacing between traces and 90% ground plane windowing (90% of the surface area of the stainless steel ground plane removed underneath and near the signal traces), as opposed to the trace widths of 200-400 μm which were previously required in suspensions having a solid ground plane underneath and only a single plus path and a single minus path in order to achieve impedances of approximately 30 ohms or lower. 
       FIG. 49  is a top plan conceptual view of reduced impedance suspension circuit according to a third embodiment in which each polarity signal is split into multiple paths and the paths are interleaved for reduced impedance.  FIG. 50  is a cross sectional view of the suspension circuit of  FIG. 49  taken along section line  50 - 50 . In this embodiment the signal path corresponding to conductor  520  carrying the “plus” half of a differential signal pair is split at one end into three different copper conductors: first conductor  521 , second conductor  522 , and third conductor  523 , and recombined at the other end. Those three conductors extend in parallel and carry the first polarity signal of a differential signal pair. Similarly, the signal path corresponding to conductor  530  carrying the “minus” half of the differential signal pair is split at one end into three different copper conductors: fourth conductor  531 , fifth conductor  532 , and sixth conductor  533 , and recombined at the other end. Typically, the places where the conductors split apart and recombine would be at the slider end and at the pre-amp end. Those three conductors extend in parallel and carry the first polarity signal of a differential pair signal. The conductors are arranged so as to form a horizontal pattern taken left to right along line  50 - 50  of signal polarities of the differential signal pair, the pattern being plus, minus, plus, minus, plus, minus. By splitting each signal into multiple conductors, and then interleaving the “plus” and “minus” conductors to form a pattern of plus, minus, plus, minus, etc., in parallel, the impedance of the transmission line is reduced below what it would have been by merely having one “plus” conductor in parallel with one “minus” conductor. This solves the problem referred to above in which merely increasing the signal trace width beyond about 150 μm provides minimal reduction in impedance and fails to achieve the desired low signal trace impedance values. 
       FIG. 51  is a top plan conceptual view of a reduced impedance, reduced crosstalk suspension circuit according to a fourth embodiment. In this embodiment each signal half is split into three separate traces, the “plus” signal conductors being interleaved with the “minus” signal conductors and extending generally in parallel to the “minus” signal conductors, similar to the embodiment of  FIG. 50 , but with adjacent plus and minus signal traces crossing repeatedly to form three separate crossing signal pairs, each crossing pair extending generally in parallel with the other crossing pairs. The crossings could be at the same point laterally along the suspension, or, as shown, the crossings could be offset. The crossings can be either overpasses constructed from an additional layer of copper traces in electrical communication with the primary copper layer traces below through vias such as shown in  FIGS. 46 and 47 , or could be underpasses constructed from electrically isolated islands of stainless steel substrate in electrical communication with the primary copper layers traces through vias such as shown in  FIG. 52 . 
     The embodiments of both  FIG. 50  and  FIG. 51  could be theoretically extended beyond the interleaved split triples shown in  FIG. 50  and the interleaved and crossing split triples shown in  FIG. 51 , to split N-tuples and crossing split N-tuples, where N is an integer of four or more. Similarly, the embodiments of  FIGS. 50 and 51  could be reduced to only two conductors of each polarity, i.e., split doubles or crossing split doubles. 
       FIG. 52  is a cross sectional view of a suspension circuit that utilizes a portion of the suspension substrate as the crossover connection or crossbar rather than a second layer of copper conductors. The cross sectional slice is taken along the length of the substrate crossbar. The process for forming the substrate crossbar and its features are similar to the process shown in  FIGS. 30-42  and described above for forming the flying lead of  FIG. 29 . The details of that process as applied to forming the substrate crossbar of  FIG. 52 , and the various metal layers and options, apply equally to the process for forming the substrate crossbar of  FIG. 52  and will be readily apparent, and will not be discussed in detail. 
     In this embodiment, insulative layer  518  such as polyimide is formed on top of the suspension substrate which is typically a spring metal such as stainless steel. The suspension substrate includes both substrate crossbar  516  and surrounding substrate material  517 . Substrate crossbar  516  defines a portion of the substrate material that is electrically isolated from the surrounding portions  517  of the substrate via etching of apertures  527  and related processing, as described in greater detail with respect to  FIGS. 30-42 . Substrate crossbar  516  would typically define an electrically isolated generally rectangular ribbon of stainless steel substrate material, but the shape need not be strictly rectangular. Apertures are formed through the insulative material  518  to define first via  524  and second via  525 . Preferably a tie layer of either chromium or chromium alloy, or Monel® metal, is sputtered into the via and onto the stainless steel substrate analogous to tie layer  228  in  FIG. 33 , followed by a copper seed layer analogous to copper seed layer  232  in  FIG. 34 . First conductor  234  which forms a first portion of the signal path and second conductor  235  which forms a second portion of the signal path are then deposited preferably by flash plating. A topcoat  560  is then applied over the circuit to protect it from moisture, oxygen, and electrical shorting. The otherwise exposed portions of the substrate crossbars are prepared for a bond-receptive coating as described above with respect to  FIG. 42 , then preferably coated with copper, nickel, and/or gold, analogous to the various layers including gold layer  246  shown in  FIG. 42 . The final metallurgy of the substrate crossbar in the otherwise exposed areas may be, for example, one of the following: SST/Au; SST/Ni/Au; SST/Cu/Au; SST/Cu/Ni/Au; SST/Ni/Cu/Ni/Au; or other variations. Although a gold layer on substrate crossbar  516  would typically not be used for thermosonic bonding as in the case of the flying leads  250  shown in  FIGS. 28 and 29 , the copper, nickel, and/or gold layers on substrate crossbar  516  would provide a lower resistance path than through the stainless steel itself and thus would help reduce impedance mismatch between copper signal conductors  234  and  235  on the one hand and substrate crossbar  516  on the other hand. A copper or nickel and finally gold layer would also help to reduce potential corrosion of substrate crossbar  516 . 
     The suspension circuit of this embodiment includes the following layers in the following vertical order: the suspension substrate including electrically isolated portions of the substrate that define the crossbars, a first insulative layer having vias extending therethrough, the copper conductor layer that includes the various copper signal conductors, and a protective topcoat. 
     In embodiments using a substrate crossbar as the electrical cross connection, substrate crossbars such as substrate crossbar  516  replace the copper crossbars  441  and  451  in  FIG. 46 , the two copper crossbars shown on  FIG. 48 , and provide all signal path crossings shown in  FIGS. 49 and 51 . 
     When the suspension circuit of  FIG. 46  is implemented using substrate crossbars, the suspension circuit would include first through fourth signal paths, a first cross connection electrically crossing a first signal path over a third signal path, and a second cross connection electrically crossing a second signal path over the fourth signal path, with each of the cross connections including an electrically isolated portion of the substrate defining a crossbar, with vias near the ends of the crossbars extending through the insulative layer to electrically connect the ends of the crossbars to the respective points on the copper conductors that form the main conductors within the respective signal paths. The signal path includes first portion  234  and second portion  235 , both of which are preferably electrodeposited pure copper or high copper alloy. 
     This embodiment therefore has the advantage of enabling the suspension to be thinner and lighter than the embodiments such as  FIG. 47  that utilize a second layer  419  of polyimide over the first copper trace conductor layer, and a second copper trace conductor layer over the second polyimide layer  419  to form the crossbar connections  441  and  451 . 
     Modeling results indicate that a dual interleaved suspension, in which each polarity of a differential signal is split into two conductive paths, and the four total conductive paths are interleaved to form a pattern of plus, minus, plus, minus, can achieve an impedance of around 30 ohms, and that a triple interleaved suspension such as that shown in  FIG. 50  can achieve impedances of all the way down to about 15-20 ohms. 
     It will be understood that the terms such as “approximately,” “about,” and “substantially” as used within the specification and the claims herein allows for a certain amount of variation from any exact dimensions, measurements, and arrangements, and that those terms should be understood within the context of the description and operation of the invention as disclosed herein. 
     It will further be understood that terms such as “top,” “bottom,” “above,” “below,” “over,” and “under” as used within the specification and the claims herein are terms of convenience that denote the spatial relationships of parts relative to each other rather than to any specific spatial or gravitational orientation. Thus, the terms are intended to encompass an assembly of component parts regardless of whether the assembly is oriented in the particular orientation shown in the drawings and described in the specification, upside down from that orientation, or any other rotational variation. Similarly, the terms and phrases “crossing over, “cross over,” and “crossover” referring to electrical connections are to be understood in their broadest sense to a cross connection without reference to any particular spatial or gravitations orientation and therefore include “crossing under,” “cross under,” and “crossunder.” 
     It will be appreciated that the term “present invention” as used herein should not be construed to mean that only a single invention having a single essential element or group of elements is presented. Similarly, it will also be appreciated that the term “present invention” encompasses a number of separate innovations which can each be considered separate inventions. Although the present invention has thus been described in detail with regard to the preferred embodiments and drawings thereof, it should be apparent to those skilled in the art that various adaptations and modifications of the present invention may be accomplished without departing from the spirit and the scope of the invention. Accordingly, it is to be understood that the detailed description and the accompanying drawings as set forth hereinabove are not intended to limit the breadth of the present invention, which should be inferred only from the following claims and their appropriately construed legal equivalents.