Abstract:
Apparatus and 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 ground plane has 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.

Description:
BACKGROUND OF THE INVENTION 
     In the past, it has been known to use printed circuit electrical traces on a head suspension for disk drives. The head suspension itself is typically formed of steel, and serves as a supporting structure for the traces, separated therefrom by a thin insulator layer. The metallic nature of the supporting structure or layer affects both the differential mode impedance and the common mode impedance with respect to the electrical traces which are used to carry signals between a read/write head carried on the suspension and related circuitry off the suspension. 
     To address these issues, it has been known to remove one or more portions of the metallic layer to alter the impedances affecting the traces. Such removed portions have been informally known as “windows.” 
     The present invention relates to an apparatus and method for controlling the common mode impedance, typically increasing the value thereof, while allowing independent control of the differential mode impedance. This is accomplished by providing at least some windows with electrically isolated islands or “doors” therein to enable an increase in the common mode impedance while maintaining a desired level of differential mode impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a head suspension assembly according to the present invention, illustrating a conductive trace mounted to the load beam. 
     FIG. 2 is an isometric view of the reverse side of the head suspension assembly of FIG. 1, showing discontinuities in the load beam surface spaced beneath the conductive trace. 
     FIG. 3 is a cross section view taken along line  3 — 3  of FIG.  2 . 
     FIG. 4 is an enlarged plan view of a detail portion of FIG. 1, partially cut away to illustrate certain aspects of the present invention. 
     FIG. 5 is a sample coupon useful in the practice of the present invention. 
     FIG. 6 is an enlarged plan view of a portion of a fully backed coupon similar to that of FIG.  5 . 
     FIG. 7 is an enlarged plan view of a portion of a completely unbacked coupon similar to that of FIG.  5 . 
     FIG. 8 is an enlarged plan view of a portion of a coupon similar to that of FIG. 5, except with windows present in the metallic layer of the coupon. 
     FIG. 9 is an enlarged plan view of a portion of a coupon similar to that of FIG. 5, except with doors present in the windows of the metallic layer of the coupon. 
     FIG. 10 is a top plan view of a test setup to measure differential and common mode impedance using a test coupon. 
     FIG. 11 is a side elevation view of the test setup of FIG.  10 . 
     FIG. 12 is a waveform showing a differential mode impedance measurement for a fully backed coupon. 
     FIG. 13 is a set of waveforms showing differential mode impedance measurements for four windowed coupons. 
     FIG. 14 is a simplified model of a section of transmission line useful in illustrating aspects of the present invention. 
     FIG. 15 is a set of waveforms showing common mode impedance measurements for four coupons having varying percents of windows and doors in the coupons. 
     FIG. 16 is a plan view of a head suspension and trace assembly with the present invention located in a trace assembly of the suspension. 
     FIG. 17 is an enlarged detail view of region  17  of the trace assembly of FIG.  16 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the Figures, and most particularly to FIGS. 1 and 2, a head suspension assembly  86  useful in the practice of the present invention may be seen. An electrical insulation layer  12  is used between each of a pair (or more) of conductors or printed circuit traces  14  and the load beam  10 . Load beam  10  is preferably made from sheet stainless steel. The electrical insulation layer  12  is preferably a polyimide material. Traces  14  may be copper or a beryllium copper alloy such as BeCu 172 or other similar suitable materials. 
     The load beam  10  may be etched or otherwise processed to create apertures  16  under the traces  14  to reduce capacitive coupling which is generally detrimental to signal transmission at high frequencies. 
     One or more apertures  16  may have one or more islands  18  located therein. It is to be understood that island  18  is electrically isolated from the load beam  10  by a gap  20 . Furthermore, island  18  is electrically isolated from the traces  14  by insulation layer  12 , even though island  18  is positioned under traces  14 . It has been found that providing islands in at least some of the apertures enables control of both the differential mode impedance of the traces  14  and the common mode impedance of the combination of the traces  14  and the ground plane formed by the load beam  10 . Generally, it is desirable to have a predetermined differential mode to match impedance with, for example, a read/write transducer head  32  at a distal end  34  of the assembly  8  and driver circuitry (not shown) connected to the traces  14  at a proximal end  36  of the assembly  8 . At the same time, it has been found desirable to have a relatively high common mode impedance between traces  14  and the conductive layer  10  for noise suppression. 
     Referring now also FIGS. 3 and 4, an island  18  may be formed in one or more apertures  16 , by etching the conductive layer  10  after application of the insulating layer  12 . It may be found desirable to have some apertures without islands and some apertures formed with islands, as will be described in more detail infra. As can be seen most clearly in FIGS. 3 and 4, the islands are held in position by the insulating layer  12  which also supports conductive traces  14 . The traces  14  are juxtaposed over the insulating layer  12  which is located over the load beam and apertures  16  and one or more islands  18 . 
     Referring now also to FIGS. 5-9, it has been found desirable to use a number of test coupons, an example of which is shown as coupon  40  in FIG. 5, to characterize both the differential mode and common mode impedances of structures to be formed into assembly  8 . It is to be understood that each coupon has at least a pair of traces  22  mounted on an insulating layer  24  attached to a stainless steel back plane  26 . The pair of traces is 50 mm long, with a short  28  between the pair of traces at a distal end, and has a pair of terminals  30  at a proximal end. The width of individual traces and spacing between the pair of traces is normally held constant for a given pair of traces, but it is to be understood that width and spacing values may be chosen as desired. For the coupons shown herein the trace width is 50 microns and the spacing is 40 microns. A fully backed structure  42  as shown in FIG. 6 will characteristically have low differential and common mode impedances, typically in the range of 50 to 60 ohms. A completely unbacked structure  44  as shown in FIG. 7 will characteristically have high differential and common mode impedances, typically in the range of 90 to over 100 ohms. A structure  46  with windows  50 , as shown in FIG. 8 will characteristically have intermediate differential and common mode impedances. It is to be understood that a “windowed section” is a section containing a backed and an unbacked portion. The “percent windowing” is the length ratio along the direction of the traces  14  of the unbacked portion to the length of the windowed section. 
     Referring now also to FIG. 9, a structure  48  having both windows  50  and islands  52  permits control of both differential and common mode impedances. Although islands  52  are shown in every window  50  of structure  48 , it is to be understood that the present invention contemplates alternating sections of higher and lower characteristic impedance (by alternating backed and unbacked sections) a line approximating an intermediate characteristic impedance can be obtained, provided that the length of the section is much less than the highest wavelength of interest. Keeping the section length to less than 1 mm will allow operation up to about 20 GHz, by following the rule of thumb of keeping the section length less than or equal to λ/10. It also has been found desirable to keep the impedance changes from section to section relatively low; stated another way, the larger the impedance change from section to section, the smaller the section length should be. 
     The characteristic impedance is determined using time domain reflectometry on a coupon having the structure of interest thereon. A test setup for taking TDR measurements is shown in FIGS. 10 and 11. A pair of probes  60  are connected to a TDR Generator sampling head  62 , available from Tektronix in Beaverton, Oreg., which in turn is connected to a digital sampling oscilloscope  64  such as a model 11081C, also available from Tektronix. 
     An example measurement  66  taken by the test setup of FIGS. 10 and 11 is shown as waveform  68  in FIG.  12 . FIG. 12 shows impedance in ohms on the ordinate plotted against time in picoseconds on the abscissa. In this test, a pair of traces 50 microns wide and 50 mm long, separated by a 40 micron gap was excited with a step input waveform with a 35 ps (picosecond) risetime. The thicknesses of the various layers were as follows: stainless steel layer: 20 microns, insulating layer: 18 microns, and copper trace layer: 18 microns. 
     By convention, the initial starting point is the time  70  of inflection point  72  after the first peak  74  of the waveform, indicated by line  76 . Similarly, the ending point is the time  80  of the inflection point  82  after the waveform drops off at  84 , indicated by line  86 . A best fit linear sloped line  88  is matched to the overall slope of the waveform between the starting and ending times, and the intersection  90  of the start line and the sloped line gives the characteristic impedance, in this instance, 77 ohms. 
     Referring now also to FIG. 13, five waveforms  68 ,  94 ,  96 ,  98 , and  100  are shown for 0, 25, 50, 75 and 100 percent windowing with the length of one section being 1 mm in the longitudinal direction of the traces, it being understood that the trace related dimensions are the same as those for FIG. 12, and that 0% windowing is for a fully backed conductive layer beneath the traces, and 100% windowing is for a fully unbacked conductive layer. As explained in more detail infra, a section is made up of a fully backed subsection, a fully unbacked subsection, and a partially backed section having a door or island therein. Waveform  68  in FIG. 13 is the same form from FIG. 12, except on a different scale. 
     Table 1 gives the parameters corresponding to the waveforms in FIG.  13 . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Zo 
                 Zo 
                 Time 
                 Time 
               
               
                 WAVE- 
                 PERCENT 
                 Measured 
                 Calculated 
                 Delay 
                 Delay 
               
               
                 FORM 
                 WINDOWING 
                 Ohms 
                 Ohms 
                 Measured 
                 Calculated 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 68 
                 100 
                 137 
                 137 
                 180 
                 180 
               
               
                 94 
                 75 
                 114 
                 115 
                 208 
                 200 
               
               
                 96 
                 50 
                 97 
                 99 
                 226 
                 216 
               
               
                 98 
                 25 
                 86 
                 87 
                 237 
                 227 
               
               
                 100 
                 0 
                 77 
                 77 
                 237 
                 237 
               
               
                   
               
             
          
         
       
     
     Impedance Design Technique 
     In order to size islands to be placed in windows of a head suspension assembly, it has been found desirable to obtain data using test coupons. The process generally is as follows. First, measure multiple coupons with different arrangements of the conductive layer (i.e., fully backed, fully unbacked, and coupons with “full” windows (i.e., with islands substantially filling the windows, but electrically isolated from the window “frames”) to establish baseline data. The parameters of differential and common mode impedances, and differential and common mode time delays are derived from the measurements (as described with respect to FIG. 12, above). The parameters are then inserted in a mathematical model, using equation (1): 
     
       
           Z   in   =Z   o   [Z   R  cos β1 +jZ   o  sin β1)/(Z o  cos β1 +jZ   R  sin β1)]  (1) 
       
     
     where 
     
       
         β=2π/λ  (2) 
       
     
     and Z R  is the impedance of the prior section, and Z o is characteristic impedance, λ is the wavelength in the medium of interest, and l is the length (in meters) of the section under consideration. It has then been found desirable to iterate the above steps, varying the amount of backed area present including the islands and any other conductive layer area under the traces until a desired characteristic common mode impedance is obtained. 
     In order to hold differential mode impedance constant, the percentage of unbacked to total backed area of the conductive layer under the traces is held constant. The total backed area is made up of electrically isolated islands and electrically connected portions of the conductive layer. To vary the common mode impedance, the ratio of the island areas to the electrically connected portions is varied. 
     More particularly, the steps in designing a structure according to the present invention are as follows: 
     1. Measure a 100% backed coupon using time domain reflectometry (TDR) as described above. Then extract the backed unit parameters (BUP) from the measured data. 
     2. Measure a 100% unbacked (fully unbacked) coupon using TDR. Then extract unbacked unit parameters (UUP) from the measured data. 
     3. Measure a 100% door coupon (i.e., a fully windowed coupon having islands in each window) using TDR. Then extract door unit parameters from the measured data. 
     All unit parameters (BUP, UUP, and DUP) will be in the form: 
     *UP 
     differential impedance (Zo (diff)) as measured at very high frequency in ohms. Normalized to 1 meter. 
     differential time delay (TD (diff)) as measured at very high frequency in ohms. Normalized to 1 meter. 
     common mode impedance (Zo (cm)) as measured at very high frequency in ohms. Normalized to 1 meter. 
     common mode time delay (TD (cm)) as measured at very high frequency in ohms. Normalized to 1 meter. 
     4. Determine the maximum frequency of the wave that will travel through the interconnect, f(max). 
     5. Determine the wavelength of f(max) using the highest permittivity of the materials being used, l(max). 
     6. The maximum section length should be &lt;(1/10)*l(max). 
     7. A section will be made of a combination (subsections) of backed transmission line, unbacked transmission line, and door transmission line. Each individual subsection&#39;s length, or the sum of the lengths of like subsections, divided by the section length will be the percent length for that subsection. The sum of the percent length of these structures will equal 100% for each section. The percent length of any subsection can range from 0-100%. 
     All subsection percent lengths shall be labeled as 
     %* 
     *=B for a fully backed transmission line subsection 
     *=W (window) for a fully unbacked transmission line subsection 
     *=D (door) for a transmission line subsection having an isolated island in a window 
     All subsection lengths shall be labeled as 
     L* 
     *=B for a fully backed transmission line subsection 
     *=W for a fully unbacked transmission line subsection 
     *=D (door) for a transmission line subsection having an isolated island in a window 
     8. Arbitrary %B, %W, and %D are picked. 
     9. Differential impedance and time delay will be determined using standard transmission line theory on a circuit with discrete subsections connected together to form (n) series sections, where n is equal to designated interconnect length divided by the section length. 
     For the fully backed subsection: 
     
       
           Zo=BUP ( Zo (diff)); and time delay= BUP (( TD (diff))* LB   
       
     
     For the fully unbacked subsection: 
     
       
           Zo=UUP ( Zo (diff)); and time delay= UUP (( TD (diff))* LW   
       
     
     For the door subsection: 
     
       
           Zo=DUP ( Zo (diff)); and time delay= DUP (( TD (diff))* LD   
       
     
     10. In the case where the differential impedance is lower than the design target, the %W should be increased and the %B and %D both decreased. 
     11. In the case where the differential impedance is higher than the design target, the %W should be decreased and the %B and %D both increased. 
     12. Steps 9-11 may be repeated until convergence to the design target is achieved. 
     13. Common mode impedance and time delay will be determined using standard transmission line theory on a circuit with discrete subsections connected together to form (n) series sections, where n is equal to designated interconnect length divided by the section length. 
     For the fully backed subsection: 
     
       
           Zo=BUP ( Zo (comm)); and time delay= BUP (( TD (comm))* LB   
       
     
     For the fully unbacked subsection: 
     
       
           Zo=UUP ( Zo (comm)); and time delay= UUP (( TD (comm))* LW   
       
     
     For the door subsection: 
     
       
           Zo=DUP ( Zo (comm)); and time delay= DUP (( TD (comm))* LD   
       
     
     14. In the case where the common mode impedance is higher than the design target the %B should be increased and the %D decreased. 
     15. In the case where the common mode impedance is lower than the design target the %B should be decreased and the %D increased. 
     16. Steps 13-15 will be repeated until convergence to the design target. 
     17. Slight changes in differential impedance and time delay may occur when converging to a common mode impedance target. For better refinement, steps 9-13 and 14-15, may be repeated. 
     Referring now most particularly to FIG. 14, a visual representation in the form of a section  102  of transmission line is shown. Section  102  has a first subsection  104 , a second subsection  106  and a third subsection  108 . It is to be understood that the transmission line segments  110  correspond to the transmission line parameters, Zo and TD, with segment  110  in subsection  104  corresponding to a fully backed subsection (B), and with segment  110  in subsection  106  corresponding to a fully unbacked subsection (U), and with segment  110  in subsection  108  corresponding to a door subsection (D). The characterizing parameters for each subsection are in the following format. 
     For subsection  104 : 
     
       
           Zo=BUP ( Zo ( xxxx )); and time delay= BUP (( TD ( xxxx ))* LB   
       
     
     For subsection  106 : 
     
       
           Zo=UUP ( Zo ( xxxx )); and time delay= UUP (( TD ( xxxx ))* LW   
       
     
     For subsection  108 : 
     
       
           Zo=DUP ( Zo ( xxxx )); and time delay= DUP (( TD ( xxxx ))* LD   
       
     
     where “xxxx”=either “diff” or “comm,” as the case may be. 
     Utilizing conventional transmission line theory, this notation refers to a repeating structure or section  102  of a series of 3 transmission line subsections  104 ,  106 , and  108  with each subsection having respective parameters Zo and time delay TD, for differential or common mode characteristics. 
     Referring now most particularly to FIG. 15, four waveforms for determining common mode impedance for various percentages of window and door combinations may be seen. The common mode impedance in ohms is plotted along the ordinate and time in picoseconds is plotted along the abscissa. Waveform  112  is for a coupon having a 90.6% window and a 9.4% door. Waveform  114  is for a coupon having a 58% window and a 42% door. Waveform  116  is for a coupon having a 31% window and a 69% door. Waveform  118  is for a coupon having a 10% window and a 90% door. 
     The data for FIG. 15 is shown in Table 2 below: 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Time 
                 Time 
               
               
                   
                   
                   
                   
                 Z 0 (comm) 
                 Z 0 (comm) 
                 Delay 
                 Delay 
               
               
                   
                 Percent 
                 Percent 
                 Percent 
                 Measured 
                 Calculated 
                 Measured 
                 Calculated 
               
               
                 Waveform 
                 Backed 
                 Window 
                 Door 
                 Ohms 
                 Ohms 
                 ps 
                 ps 
               
               
                   
               
             
             
               
                 112 
                 0 
                 90.6 
                   9.4 
                 86 
                 85 
                 160 
                 165 
               
               
                 114 
                 0 
                 58   
                 42 
                 82 
                 81 
                 170 
                 170 
               
               
                 116 
                 0 
                 31   
                 69 
                 70 
                 72 
                 175 
                 175 
               
               
                 118 
                 0 
                 10   
                 90 
                 66 
                 65 
                 170 
                 178 
               
               
                   
               
             
          
         
       
     
     It can thus be seen that the common mode impedance can be adjusted between about 66 to about 86 ohms over the range of window and door percentages shown. It is to be understood that one may select other percentages of window and door combinations while still remaining within the scope of the present invention. 
     Referring now to FIGS. 16 and 17, an alternative embodiment of the present invention may be seen. In this embodiment, a head suspension  130  has a flexure or trace assembly  132  with a “tail”  134  extending from a mounting region  136  of the head suspension  130 . It is to be understood that the flexure or trace assembly  132  has a ground plane layer  138  (usually formed of stainless steel), an insulator layer  140  (usually formed of polyimide) and a layer made up of a plurality of conductive traces  142  (usually formed of copper or a copper alloy). The present invention may be embodied in such a trace assembly by forming a window  144  and islands  146  in the ground plane layer  138 . It is to be understood that more than one window may be formed in the ground plane, even though in this embodiment there is only one opening or window  144  extending the length of the traces  142 . In this embodiment, a separate pair of traces  148  do not have the structure of the present invention associated therewith, but are shown as spaced from the ground plane  138  by the dielectric or insulator layer  140 . 
     This invention is not to be taken as limited to all of the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention.