HGA Technology Driver The Need for Speed With Control Robert Dodsworth and George Hare, 3M Microinterconnect Systems Division Presented at the 21 Head/Media Conference and published in DataStorage, September. 21. Reprinted with permission. Abstract: Rotating memory devices continue to be the most cost effective means of gigabyte size storage. They maintain that reputation based on systematic, continuous improvement of technology once obstacles are identified. A new area of concern is the interconnect between the channel chip and the slider on the HGA. It has been assumed that 2 metal rather than single metal flexures were needed for high data rate HDDs. 2 metal flexures may add unnecessary costs where single metal flexures demonstrate excellent performance. Single metal layer flex is compared to conventional 2 metal flexures used in HGAs. Work at 3M shows that significant improvement in electrical properties can be obtained using fine pitch circuit fabrication to create tightly, electrically coupled single metal layer trace pairs. When properly designed, these pairs mimic classical 2 metal layer structures. Single metal layer structures have been shown to achieve controlled 5 ohm impedances and have low common and differential mode crosstalk that is acceptable in most HDD applications for data rates up to and exceeding 1 gigabit/sec. Advancements in all components of the hard disk drive make the need for speed pervasive. Recent network memory system advances assure that faster drives will not be technical demonstrations but will find viable and sizable applications. In PCs, the 133 MHz and wide bus backplane designs (64-bit or double 32-bit word sizes) will be coupled to wideband networks. In server farms, high capacity Hard Disk Drives (HDDs) will dump data directly to switch processors at GHz rates, as was recently demonstrated by APT Technologies using Seagate ATA HDDs and Vittesse Semiconductor transceiver chips. The fundamental problem is how to read and write data reliably at these extremely high data rates. An enabling component is the interconnect between the pre-amp and slider. While higher bit densities are causing the suspension mechanics to be reconsidered (both static and dynamic mechanical characteristics), the new, high data rate requirements are causing the conductor path characteristics to be reconsidered. The interconnect is now being classified as a transmission line or even as an active component. Current suspension design improvements are aimed at reducing track pitch and pos itioning accuracy. Suspension design avenues are: 1) Extended base plate (4 piece) suspension designs (to increase stiffness to raise key resonance frequencies) 2) Costly dual actuator suspensions, either the milli- (loadbeam) or micro- (head) actuator designs. Either approach means that the mechanical design of the signal conducting structure (TSA (Trace Suspension Assembly) flexure or FSA (Flex Suspension Assemblies)) face new constraints. In the gimbal region there is need for 1) reducing the absolute values of RSA (Roll Static Attitude) /PSA (Pitch Static Attitude) and 2) reducing circuit impedance near the slider. The obvious approach of implementing thinner leads will satisfy #1 but conflicts with #2. In the loadbeam
region, the circuit stiffness must be minimized (by reducing the trace thickness) to avoid introducing offsets in the RSA/PSA or adding variability to the gram loading process. An example of the such a compromise is in TSA design. Due to the subtractive process used in TSA fabrication, the traces must be offset from the gimbal metal to reduce stiffness, but the key electrical attribute of the TSA, having a inherent ground layer, is lost. The result is that the traces in the body of the flexure are at 4-5 ohms impedance, but the gimbal area exceeds 8 ohms, preventing a close impedance match with the GMR (Giant Magnetoresistive) read sensor on the slider. Thus, although a single metal layer circuit structure would be best for mechanics, current circuit fabrication capability is thought to have significant limitations in electrical performance compared to future HDD needs. READ signal paths must have lower impedances to closely match the mean slider output impedance. Such matching couples more energy from the milli-volt level GMR or TMR (Tunneling Magnetoresisitive) sensor into the pre-amp. The Writer IC must output a fast risetime current pulse to drive the Write Coil. Controlled impedance with low inductance is ideal for maintaining the sharp signal pulse slope to maximize the change in flux from the coil. Extensive electrical modeling and sample part measurement confirms that constant impedance paths can be designed using single metal flex attached to suspensions, (Flex On Suspension or FOS) with complex shapes. 3M has the capability to design and manufacture Controlled Impedance FOS circuits (CI FOS) which have lower nominal impedances and impedance tolerances which exceed standard FSA and STTSA/BFC (Short Tail TSA / Bridge Flex Circuit constructions. At a 1 GHz data rate, the transition (rise and fall) time is typically 3ps or less. A transmission line is considered any circuit path that is longer than 1/5 of a signal s typical rise/fall time. For the HGA, the transitions (rise time and fall time) define quality of the WRITE pulse. At high data rates, typically having 3 ps or faster rise/fall times, controlled impedance transmission lines are necessary to maintain the integrity of the WRITE signal transitions and the circuit must analyzed as such. See Figure 1.
FIG. 1 Anatomy of a Data Pulse Shows Need For Transmission lines In HGAs Write Pulse Graphic 6 Electrical Length of HGA 18-24 ps Volts 5 4 3 2 Internal Data Rate 2 GB 1 GB 5 MB 25 MB 1-1.5 1 1.5 2 2.5 3 3.5 In HDD design, the objectives are to either write smaller bits or write bits faster. Either objective requires short, well-defined current transitions from the Write Driver to the Write Coil. Because a Write Coil is excited by a large change of current in the shortest time span (di/dt), the shape of the WRITE current must be maintained along the transmission line. Because energy transmission is the purpose of the circuit, it should not have any abrupt changes in impedance, especially inductance. Such changes distort the signal shape or cause loss of signal amplitude through attenuation or reflection, reducing di/dt. Current state of the art HGA constructions use an STTSA mated to a solder bump or ultrasonically welded BFC. The impedance along the HGA can be viewed using a TDR (Time Domain Reflectometer, Tektronics 1181B w/sd24 sampling heads) as shown in Fig. 1. This instrument injects a very short risetime pulse into a DUT (device under test) and calculates local impedance of the conductors based on amplitude of reflections of the input pulse and the timing of the reflection compared to the initial pulse s launch. An instrument, such as the Tektronix 1181 Digital Sampling Oscilloscope with a TDR sampling head, outputs pulses with a risetime of ~35 ps, and at small impedance changes has a distance resolution of a several millimeters. Fig. 2, below, shows the difference in local impedances between a conventional HGA (STTSA + BFC) and a LTTSA (Long Tail Trace Suspension Assembly). For WRITE pulses of ~5 ns. (~2 Mbits/sec), the STTSA/BFC appears to be a lumped ~8 ohm impedance, although the local impedance varies by?3 ohms. At 7MB to 1.2 GB data rates the WRITE risetime will be ~ 3 ps. and the HGA must be treated as a transmission line. Thus, smoother trace profiles with? 1 ohm tolerances will become the norm.
Fig. 2 TDR traces Show Instantaneous Impedance Along An HGA LTTSA & BFC/TSA Impedance Profiles TDR Probe Outside Tail Bond Window, Open Circuit 3M 8/ Ohms 14 12 1 8 6 4 2 PCC Bond Window STTSA/BFC BFC/TSA U/S Bond 1 2 3 4 5 6 Measurements Open Circuit LTTSA 7 Ohm Trace 5 Ohm Trace Modeling and measurement of common HGA constructions with copper traces and ground layers of various metal shows that electric fields are contained between them, effectively controlling circuit impedance. Approximate field distributions are shown in Fig. 2 for various HGA crosssections. Recently, 3M has been experimenting with single-metal layer and ground plane circuit forms to achieve specific impedances. Analysis of circuit structures shows that it is possible to achieve desirable electrical properties with a single layer structure. The key is to create tight coupling between trace pairs by reduction of the absolute distance between them. This tight coupling contains the electric field in a confined region between the traces, which reduces inductance and reduces the influence of adjacent conductors, especially if the trace widths are small (such as required in the gimbal area). Adjustment of the trace layer geometry must include the effects of the suspension and E-block. Adjustments to the circuit shape (trace width, trace pitch, thickness, modification of dielectric layers) can be made to keep the single metal layer flex circuit s electrical properties within narrow electrical property ranges over its entire length. A carefully designed FOS with accurate assembly to a suspension provides an appropriate transmission line for use in HGA applications. In the use of fine pitch circuits (spacing between traces of 2-3 um), the actual shape of trace sidewalls has an effect on the absolute lowest attainable impedance value. In semi-additive circuit fabrication, photoresist is used to control the width and sidewall shape of the traces. Traces are then plated up with their final shape determined by the resist. Subtractive circuits are defined by imaged resist on the surface of the copper layer. Etchant then removes the unwanted copper between leads and produces a sloped sidewall because of the flow pattern of the etching bath.
Fig.3 Typical HGA Cross-sections Needed For Electrical Modeling 1 2 3 4 COVERCOAT CU TRACES SUBSTRATE ADHESIVE FLEXURE COVERCOAT CU TRACES SUBSTRATE COVERCOAT CU TRACES SUBSTRATE AIR GAP FLEXURE TSA (no Adhesive) or FOS ATTACHMENT TO FLEXURE FOS IN E-BLOCK REGION FOS IN PCC BOND AND OVER LOAD BEAM REGION Subtractive Circuit Traces Semiadditive Circuit Traces For example, at a target impedance of 7 ohms, a semi-additive trace (vertical sidewalls) will have ~7 ohms lower impedance than subtractively processed trace pairs (~15? sidewalls) at the same nominal spacing (Fig. 3(4)). At trace spacings ~2x the copper thickness actual trace shape is not a major factor in electrical performance. However, as the spacing decreases, the effect becomes stronger as shown and the target impedance is reduced from 8-1 ohms, typical of standard FSA and STTSA/BFC HGAs to controlled impedance ranges of 5-7 ohms.
Samples using both fine pitch traces in a 1 metal layer construction and 2 metal layer parts (Trace + groundplane) were designed to a target 5 ohm impedance. An existing FOS circuit (which originally had 8-12 ohm impedance sections) was used as the basis for the 5 ohm design. The finished parts were tested using the TDR to measure the impedance along the circuit. Both part constructions achieved the target as shown in Fig. 3. Fig. 4 Experimental HGA Using Ground Place and Fine Pitch FOS For Impedance Control on Same Suspension Low Impedance FOS Circuit Constructions TDR Trace (35 ps Risetime) Leaded FOS/No Slider Impedance 12 11 1 9 8 7 6 5 4 3 2 Distance/Time Along HGA (output shorted) Fine Pitch, 1 Metal Traces w/ground Plane Although single metal layer FOS may be appropriate for most applications (down to ~ 5 ohms), 2 metal layer constructions can reach lower absolute impedances. As usual, there are trade-offs. Normal 2 metal layer flex is ~1.3-2. X the cost of single layer flex. Alternate ground plane constructions are being evaluated at 3M to reduce the cost differential. Use of high power pulses needed to write fast to take advantage of the increasing data density capability of the disk also creates crosstalk. Ground planes reduce crosstalk through control of the field between the conductor and ground plane. The results of modeling for both differential and common mode (CM) crosstalk provide estimates of the peak voltage levels of crosstalk for various HGA structures. Modeling of simple structures (i.e., parallel lines) can be misleading when compared to modeling of actual HGAs. It was found that, when comparing structures, simple parallel trace models would produce much higher (4X) peak crosstalk voltage values than found in actual HGA designs. Much of the crosstalk does come from specific and sometimes very short areas of the circuit. Usually there is some traditional trace shapes where traces which are constrained by traditional mechanical requirements such as tail forming or gram loading. However, in the gimbal/suspension area, the READ and WRITE signal pairs have additional separation and crosstalk is correspondingly reduced due to reduced coupling in this area. Use of fine pitch, semiadditive FOS allows control of impedance in the gimbal which is not possible with the usual subtractive circuit construction. Figure 4 below shows the TDR (impedance) trace along an FSA. Note the reduced impedance in the trace area. With FSA, this is possible with no change in RSA/PSA parameters since the electrical properties are inherently de-coupled from the electrical properties.
Fig. 5 Actual HGA Using Fine Pitch FOS 14 FSA Impedance Profile - 8 Ohm HGA Target Note Gimbal Impedance 13 Impedance, Ohms 12 11 1 9 8 7 Body Swage Plate Gimbal FSA1 Write 6 FSA1 Read 5 4 FSA2 Write FSA2 Read 1 Distance/Time Writer crosstalk to the READ circuit is a concern for designers. A Read Head is typically biased at 5 ma and can tolerate maximum continuous current of about 1 ma. For a 5 ohm READ circuit, this translates to a bias voltage of 25 mv leaving a margin of 25 mv rms for crosstalk. Comparison of an 8 ohm, 1 metal layer FOS to an 8 ohm TSA-type structure (or 2ML FSA) with full groundplane for a 3.5 inch format HGA, for differential and common mode crosstalk: The levels shown are peak as shown in Figure 6A. Simulations were run on complete flexure models (from bond pads to head connection points) with a typical head model at the output and typical Reader IC input model. Simulations were run for Writer output impedances of 8 ohms and 2 ohms. From a crosstalk standpoint, these are worst case waveforms for currently available READ/WRITE preamp/driver circuits. In both HGA constructions, 2ML FSA/TSA and 1ML FSA, the levels of crosstalk are well within 25 mv margin for protection of the Reader element. The source pulse waveforms are shown in Fig. 6B. Fig. 6A Crosstalk Simulation, Peak, Far End (at GMR Head) Differential and Common Mode Components Far End X-talk Peak Voltage @ GMR Head, 8 & 2? Write Drivers 25 GMR Damage Threshold 2 Read Line X-talk, mv 15 1 Diffential Mode FE 1ML FSA 2ML FSA/TSA 5 Commom Mode FE @ 8? @ 2? @ 8? @ 2?
Fig. 6B Differential and Common Mode Stimulus 12 Diff Pulse 3 ps rise/fall time 6 CM Pulse 3 ps rise/fall Time 1 5 Amplitude (V) 8 6 4 Amplitude (V) 4 3 2 2 1 1 2 3 4 5 6 Time (ns) 1 2 3 4 5 6 Time (ns) Summary: Economical, single metal FOS-based HGAs can provide the necessary performance for today s, as well as tomorrow s, high data rate HGA designs. Single metal FOS offers controlled impedance with low levels of crosstalk. For HDD applications with data rates up to and exceeding 1 gigabit/sec, use of single metal FOS meets and can typically exceed design requirements by providing superior de-coupling of the mechanical components while maintaining electrical design freedom. Authors: Robert Dodsworth, 3M, Product Development Specialist, with a 2 year background in advanced interconnect technology. Recent experience in designing interconnects for flip chips, LCD modules and portable electronics as well for HDD applications. BSME, University of Missouri, MBA, College of St. Thomas. George Hare, 3M, Product Development Specialist, with over 25 years of experience in product development, test engineering, and cable, connector and system modeling experience. BSEE, University of Texas, MBA, University of Texas.