Abstract:
A tester for a testing a Hard Disk Drive (HDD) flex circuit prior to electrical installation of a Head Gimbal Assembly (HGA) includes a shorting block that makes electrical contact to the bondpads on the sample. The shorting block includes one or more electrical contacts that are electrically grounded and have a size and/or configuration to contact the bondpads as well as the surface of the sample around the bondpads to accommodate positioning tolerances of the sample under test, without need for optics, precise probes, or precision stages. The electrical contacts of the shorting block may be, e.g., a matrix of pogopins or a flexible electrically-conductive material. During testing, the bondpads are shorted together and to ground with the shorting block while it is determined whether Short failures are properly detected. While the shorting block is not engaged with the bondpads, it is determined whether open failures are properly detected.

Description:
CROSS-REFERENCE TO PENDING PROVISIONAL APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/255,803, filed Oct. 28, 2009 and entitled “Testing Flex and APFA Assemblies for Hard Disk Drives”, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     As technology advances, it becomes increasingly difficult and costly to test or verify the operation of electronic devices. One example of this is found in storage systems within the Hard Disk Drive (HDD) industry. There is pressure to reduce cost in the manufacturing of HDD subcomponents while not increasing the cost of scrap that occurs if failed subcomponents are not screened early enough and are not detected until later in the manufacturing process. 
     It is therefore desirable to test HDD subcomponents, such as the Flip Chip on Flex (FCOF) subassemblies used on the HDD Head Stack Assembly (HSA) early in the manufacturing process to increase yield and reduce cost. The FCOF is an electrical flex circuit assembly including an HDD preamplifier chip, such as products supplied by, e.g., Texas Instruments, a connector, and various other electronic components all electrically bonded to the flex circuit. This flex circuit will also have several bondpads available for electrical connection to one or more magnetic recording heads, referred to as Head Gimbal Assemblies (HGAs). For each HGA that will be electrically bonded to the flex circuit, the flex circuit will have a corresponding set of individual bondpads matching both the quantity and geometry of the bondpads of the HGA. Today&#39;s HGAs typically have 8 bondpads, for example, thus the flex circuits will contain matching sets of eight bondpads (i.e. bondpad sets) for each HGA to be bonded. As an example, considering an HSA designed for 8 HGAs, where each of these HGAs requires 8 bondpads, there will be 8 bondpads/HGA×8 bondpad sets, i.e., 64 individual bondpads per FCOF. For purposes herein, individual bondpads will be referred to as bondpads, and a bondpad set will be a set of these individual bondpads (for example, 8 bondpads) corresponding to the quantity and geometry used to match to the mating HGA. 
     During the HDD manufacturing process the FCOF will be installed onto an HSA Arm and this new assembly of FCOF plus HSA Arm is generally referred to as an Actuator Pivot Flex Assembly (APFA). Then, the one or more HGAs will be mounted to this APFA, through the combination of being mechanically swaged to the HSA arm and electrically bonded to the FCOF by bonding the HGA contact pads with the corresponding bondpad set on the FCOF to provide electrical connection. Then this HSA assembly will be installed into the hard disk drive for actual operation. Because the HGAs add significant cost to the HSA assembly, if the FCOF has any sort of failure, such as any opens or shorts in the traces on the flex circuit, any failures of the preamplifier chip, or any improper electrical bonding of any one of the bonded electrical components, it will be very costly to find the failure in the FCOF at later stages in the manufacturing process. 
     For illustration, reference is made to  FIG. 5 , which shows an example of an APFA  180 , with an FCOF  182  and an HSA Arm  184 , just prior to bonding 3 HGAs  186 . In this example, each HGA  180  has 6 bondpads, so the illustrated FCOF  182  is shown with 18 bondpads  112  in total, in three sets of 6. Also illustrated on the FCOF  182  are a preamplifier chip  114  and a connector  111  bonded to a flex circuit  183 . 
     The 6 or 8 HGA pad connections of each bondpad set provide unique functions to the magnetic recording head. One pair of connections will be the +/− connections for the magnetoresistive Reader element of the head. Another pair will be the +/− connections for the Write element of the head. Other individual or pairs of connections per each HGA will have other purposes, such as for flying height control, microactuation, or other features. As the hard disk drives become more complex the HGAs are designed with additional devices to increase performance, and so the quantity of individual bondpad connections increases because each of these new devices must be electrically coupled to and controlled by the preamplifier chip or primary circuit board on the hard disk drive. Thus, there is a tendency for the quantity of bondpads in each bondpad set to increase. Further, as the quantity of bondpads increase there is a tendency to make each individual bondpad smaller, for size reduction. 
     Current testers for FCOFs have test circuitry that can exercise the FCOF plus a means of electrically coupling this test circuitry to the FCOF, generally through the use of small probes. One set of larger probes, such as pogopins, will contact to the fairly large connector on the FCOF sample, and another typically smaller probe set will contact the individual bondpads sets on the flex circuit. Generally, due to the small size of the bondpads, these smaller probe sets use individual cantilever needle-probes, such as those provided by SV Probe, Inc., of Gilbert, Ariz., or other typical semiconductor probe card suppliers. After making electrical contact to the bondpads (input) and connector (output) of the FCOF, testers can measure the signals from the connector to determine whether the FCOF is operational by electrically simulating different head conditions on the bondpads. Testing is generally done by exercising the preamplifier chip through a series of test conditions. A typical test sequence is to simulate a Short condition across the +/− Reader bondpad pair, using the circuitry on the head simulation board, then measure if the preamplifier chip properly detects this Reader Shorted failure by looking at the resulting signals through the connector. Continuing the test sequence, the head simulation circuitry may then electrically float (open) these two Reader bondpad connections and confirm the preamplifier chip detects this Reader Open failure. This sequence may then be repeated for the Writer bondpad pair, and all other device bondpad connections. The head simulation circuit may further apply various typical loads to the bondpads, to simulate different possible Writer, Reader, and other device resistances, and verify that the preamplifier chip detects these normal conditions properly. This overall measurement practice is commonly referred to as Open Faults, Shorted Faults, or No Faults Detection. As an example, if the head simulation circuitry configured an Open condition on one pair of Reader bondpads, but the preamplifier chip responded as either Shorted or Normal condition, then this particular FCOF sample would be separated as having a failure. Various Fault and Resistance measurements are available through the preamplifier chip and are known in the art today. 
     Overall this system requires precision circuitry for simulating the different head conditions and also a precise means of probing each of the individual bondpads, generally in the form of needle-probes, which are quite small, delicate, and tend to dent, scratch, or otherwise damage the bondpads on the flex circuit. Therefore, it is desirable to make a simplified tester that can still detect these failures in the FCOF, but does not require precision probing or specialized head simulation test circuitry. 
     It is also desirable to test at the APFA level. Whether or not testing had been done at the FCOF level, the APFA testing will again purge failures that may have occurred before or after FCOF assembly, including any damage caused by mounting of the FCOF onto the HSA Arm. Per above the next step in the manufacturing process is to bond the HGAs to the APFA. The HGAs are a very expensive component of the HSA Assembly, so it is desirable to confirm the APFA and FCOF are functioning properly before the expensive HGAs are installed. Fundamentally, APFA testing is the same testing as FCOF testing, since they are both testing the same FCOF, except that the positioning requirements for probing the FCOF bondpad sets while the FCOF is mounted to the HSA Arm are very difficult. The difficulties are that the HSA Arm partially obstructs these bondpads, making access via cantilever needle-probes difficult, and the position of the FCOF is now more variable due to the mechanical tolerances of the HSA Arm that the FCOF has been mounted to, making tolerances higher and alignment much more difficult. No testers are known to exist today for this more complex application. 
     SUMMARY 
     A tester for a testing a Hard Disk Drive (HDD) flex circuit prior to electrical installation of a Head Gimbal Assembly (HGA) includes a shorting block that makes electrical contact to the bondpads on the sample. The shorting block includes one or more electrical contacts that are electrically grounded and have a size and/or configuration to contact the bondpads as well as the surface of the sample around the bondpads to accommodate positioning tolerances of the sample under test, without need for optics, precise probes, or precision stages. The electrical contacts of the shorting block may be, e.g., a matrix of pogopins or a flexible electrically-conductive material. During testing, the bondpads are shorted together and to ground with the shorting block while it is determined whether Short failures are properly detected. While the shorting block is not engaged with the bondpads, it is determined whether open failures are properly detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a tester that incorporates a shorting block that contacts portions of the FCOF flex circuit that do not contain bondpads. 
         FIG. 2A  illustrates a shorting block composed of a matrix of pogopins that match the HGA bondpads contacts on the flex circuit. 
         FIG. 2B  illustrates a shorting block composed of a flexible material with low contact resistance. 
         FIG. 3  shows a flowchart of a typical test sequence whereby a sequence of Open and Short failure detection is achieved 
         FIG. 4A  and  FIG. 4B  illustrate a means of alignment of the shorting block to the test sample and show a means for a compliant contact. 
         FIG. 5  shows a typical APFA Assembly and three HGAs just prior to bonding, as known in the art today. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a tester  100  that incorporates a shorting block  140  and a test sample  110  mounted onto tooling  120 . The shorting block  140  is mounted to an arm  132 , which is then mounted to a stage  133 . The stage  133  may be a linear bearing assembly that allows the arm  132  and shorting block  140  to be slid into contact or away from the test sample  110 . The bearing assembly is driven by an actuator  134 , such as a pneumatic actuator. The arm  132  is electrically conductive and may be electrically coupled to the chassis  131  of the tester  100 , e.g., via a wire  135 . Other methods of actuation can be used, such as motors or solenoids, hydraulic, or manual, if desired. The shorting block  140  may use force to ensure proper contact with the test sample  110  and, thus, the actuator may be capable of providing a force of e.g., 3 Kg. Alternatively, the shorting block  140  may be in a fixed position and the test sample  110  mounted on the actuation stage  133  (or both the shorting block  140  and the test sample  110  are mounted on actuation stages) to provide respective motion. While this automated engagement is desirable, it is also possible that the shorting block  140  be engaged to the sample  110 , e.g., through the use of a clamp, prior to installation of the sample  110  onto tooling  120 , negating the need for the actuation stage  133 . 
     The tooling  120  in  FIG. 1  is illustrated as holding an APFA test sample  110 , but the tooling  120  may be configured to hold test samples such as an FCOF or similar HDD flex circuit subassembly prior to bonding of the one or more HGAs. The APFA tooling  120  utilizes a post  124  to which the test sample  110  is mounted, and a clamp  121  with pusher  125  that holds the test sample  110  onto the post  124 . The clamp  121  also has a pusher  123  that presses the connector  111  of the sample  110  against a corresponding connector  127 , such as pogopins, contact pads, or a suitable mating connector, on the tooling  120 . The clamp  121  also includes a latch  126  that can engage to the base of the tooling  120  to latch the lid in place, thereby clamping the sample  110 . The tooling also uses a pin or similar mechanical element  122  that holds the sample  110  in place when the shorting block  140  is engaged. The tooling may have an electrical circuit board  136  that is coupled to the mating connector  127 , which will perform the measurements, e.g., in response to signals provided by a processor. The tooling  120  and circuit board  136  may be electrically grounded to the chassis  131  of tester  100 . 
     When the shorting block  140  is engaged against the HGA bondpads (not shown in  FIG. 1 ) of test sample  110  then there is a closed circuit where the bondpads are electrically grounded to the ground of the circuit board  136 . The circuit board  136  then tests the sample  110  through conventional means to confirm that the sample properly detects that the bondpads are shorted. 
     Because the shorting block  140  is mounted to a movable stage it may also be disengaged from the HGA bondpads of sample  110 . In this case the circuit board  136  may then test the sample  110  through conventional means to confirm that the sample properly detects that all pads are Open. Of course this open testing may occur before or after the Shorted testing. The Normal testing, which is performed by conventional testers, is not performed through the shorting block  140  with tester  100 . As nearly every failure of the test sample  110  is related to Open/Short recognition, the simplification to remove the Normal configuration testing, outweighs the benefits of Normal configuration testing. 
     A PC or similar processor  138  can be connected to tester  100  in order to sequence the Open/Short testing and to display and record the results. The processor  138  includes a memory storing a computer-usable medium having computer-readable program code embodied therein for causing the processor to control the tester and to perform a desired analysis, as described herein. The data structures and software code for automatically implementing one or more acts described in this detailed description can be implemented by one of ordinary skill in the art in light of the present disclosure and stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system such as processor  138 . This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, compact discs, and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). The processor  138  may include storage and a display for storing and/or displaying the results of the measurement. 
       FIG. 2A  illustrates one embodiment of the shorting block  140  in the form of a matrix of pogopins  141   a  illustrated in cross section with respect to bondpads  112  on a test sample  110 . The test sample  110  may have multiple HGA bondpads  112  on the flex circuit. As an example, a flex circuit with 6 bondpads per HGA and just 1 HGA will have 6 bondpads in total. The layout of the pogopin matrix is designed with a quantity of pogopins sufficient to match these corresponding HGA bondpads  112  on the test sample  110 , while also contacting some additional surface of the flex circuit around the bondpads. While variations are possible, one embodiment of the pogopin diameter is roughly 1.5× the smallest dimension of the HGA bondpad  112 . In this example the width of each bondpad  112  is 0.007″, so a pogopin of diameter 0.010″ is used, for example Qualmax PN DBD10CAR-TSK. When installed in a staggered-configuration as shown in  FIG. 2A , the pogopins have a center-to-center spacing of 0.005″, suitably smaller than the 0.007″ minimum dimension of the bondpad  112  it is intended to contact. In this way one or more pogopins will contact the bondpad, to assure electrical contact, while some of the pogopins will also contact areas of the flex circuit that do not contain bondpads. 
     Because the test sample  110  has multiple bondpads  112 , where the bondpad set is 6 bondpads in total, the matrix of pogopins  141   a  will include enough pogopins to span across the bondpads and with suitable additional tolerance. As shown in sub-matrix  113  there are two pogopins that are not contacting the HGA bondpads at all when the pogopin sub-matrix  113  is positioned to be centered along the centerline of the bondpad set, where one extra pogopin is towards the upper end of sub-matrix  113  and another extra pogopin is towards the lower end. Since the diameter of these pogopins is 0.010″, assures electrical contact with these bondpads is assured even if the test sample  110  is vertically displaced by more than +−0.010″ vs. pogopin sub-matrix  113 . Such a matrix of pogopins  141   a  allows the convenience of handling variations in assembly tolerances, machining tolerance, and operator installation and alignment, because the pogopins will still make contact even if there is a large variation in positioning from sample to sample. If further contact variation tolerance is desired then additional pogopins can be added to the rows and columns of the sub-matrix  113 . 
     In conventional probing and flex testing, each bondpad must be electrically coupled to its own independent signal. Therefore, in a conventional tester, each individual bondpad must be precisely probed, and the probe cannot be allowed to electrically short-out an adjacent probe. Similarly, the probe cannot be allowed to bridge the bondpads and short them together. As a result, to make each probe and bondpad set electrically isolated each probe must be precisely placed with respect to the target bondpad, and the probe must be small in size. This is complicated by the fact that the size of bondpads tends is to decrease as technology progresses towards smaller form factors and the quantity of bondpads increases. 
     In tester  100 , electrical isolation of each individual bondpad  112  is not required; in fact, shorting of the bondpads together is used. As a result each of the pogopins  141   a  may contact each other and the pogopins  141   a  may bridge bondpads. Accordingly, the size requirements of the pogopins are simplified considerably, allowing for usage of larger and more durable pogopins as opposed to needle-probes. Moreover, the tip diameter of each pogopin may be relatively large (ex. 0.010″), as opposed to the typical tip diameter of needle-probes (ex. 0.001″), which will reduce the impact of denting on the bondpads  112 . After testing, the HGAs will be bonded to the bondpads  112  on the sample, and thus, damage to the bondpads during testing is undesirable. 
       FIG. 2B  illustrates another embodiment of the shorting block  140  that uses a flexible electrically-conductive material  141   b . One possibility is to use material cho-seal #1501 from Parker-Chomerics. Similar to the matrix of pogopins  141   a , the flexible electrically-conductive material  141   b  is cut or otherwise manufactured into a layout that matches the size and configuration of the HGA bondpads  112 , and flexible electrically-conductive material  141   b  is mounted to the arm  132 . The flexible electrically-conductive material  141   b  should be cut to size that is adequate for contacting the target bondpads  112  but large enough to also contact some additional surface of the flex circuit around the bondpads  112 . 
     The shorting block  140  is electrically grounded, e.g., via the arm  132 . By way of example, if pogopins  141   a  are used within the shorting block  140 , then the side of the pogopins  141   a  facing the arm  132  are installed to mechanically contact with the arm  132 , shorting the pogopins  141  and the arm  132  together and to the measurement ground (as an example through the chassis  131  as shown in  FIG. 1 ) to electrically couple the shorting block  140  to the measurement ground during testing. 
     Thus, the shorting block  140  includes one or more electrical contacts, e.g., pogopins  141   a  and/or flexible electrically-conductive material  141   b  (collectively referred to herein as electrical contacts  141 ) that are electrically grounded and have a size and/or configuration that is sufficient to contact at least one bondpad  112  on the sample while also contacting some surface of the sample that does not include a bondpad  112  so that contact with the bondpad is assured. 
     The tester  100  includes a shorting block  140  that is configured to short the bondpads  112  on the sample  110  by physically contacting the bondpads  112 , and placing the bondpads  112  in an open condition by removing the physical contact with the bondpads  112 . Thus, the tester  100  tests for an open condition when there is no physical contact between the one or more electrical contacts  141  of the shorting block  140  and the bondpads  112  on the sample  110  and tests for a Short condition when the one or more electrical contacts  141  of the shorting block  140  contact the bondpads  112  on the sample  110 . MOVE? 
       FIG. 3  is a flowchart  200  illustrating a typical test sequence with tester  100 . The operator loads the test sample  110  onto tooling  120  of tester  100  and starts the test ( 202 ). Through the circuit board  136 , the tester  100  exercises the preamplifier chip within sample  110  to detect that the sample properly recognizes all Open conditions ( 204 ). If Open conditions are not properly detected ( 206 ), the sample will be reported as a Failure ( 212 B). If Open conditions are properly detected ( 206 ), the tester engages the actuator  134 , bringing the shorting block  140  into contact with the HGA bondpads  112  of test sample  110 , and through circuit board  136  the tester exercises the preamplifier chip within sample  110  to detect if the sample properly recognizes all Shorted conditions ( 208 ). If Shorted conditions are not properly detected ( 210 ), the sample will be reported as a Failure ( 212 B). If Shorted conditions are properly detected ( 210 ), the tester reports the sample as Pass ( 212 A). The tester disengages the shorting block ( 214 ) and the operator can remove the sample ( 216 ). Note that the sequence described is typical but other variations may apply. For example the Shorting test may occur before the Open test, and the failure reporting may be reported at the end to allow both Open and Short testing to occur even if the first test produces a failure. The results can then be recorded and displayed by processor  138 . 
       FIG. 4A  shows a perspective view of a test sample  110  with 18 bondpads  112  and a side view of a shorting block  140  prior to engagement with the bondpads  112 , where the shorting block (in this case composed of pogopins  141   a ) are in free state.  FIG. 4B  shows a side view of the test sample  110  engaged against the shorting block  140  such that the pogopins  141   a  are compressed. Also shown in  FIGS. 4A and 4B  is the mechanical compliance element  142  that contacts the test sample  110  once the desired compressions is achieved. Further shown is an alignment element  143  that may be used for alignment purposes and removed or left in place during normal testing, as desired.  FIGS. 4A and 4B  collectively will be referred to as  FIG. 4 . 
       FIG. 4  illustrates a means for aligning the shorting block  140  to the HGA bondpads  112  of test sample  110 . While only coarse alignment is required it may be desirable to allow the user to easily align the shorting block  140  to a typical test sample. This may be helpful if the user needs to replace the shorting block  140  for cleaning or repair, or to exchange the shorting block  140  with a different configuration for a test sample  110  of a different design. One means for aligning is the alignment element  143 , which may be a two pins or similar mechanical pieces, that may be temporarily installed onto the shorting block  140 . The alignment element  143  is designed to fit over and under, or otherwise register, against the test sample  110 . The shorting block  140  may be mounted to the arm  132  via loose screws, and then the shorting block is engaged to the test sample. Once engaged, the alignment element  143  guides the shorting block  140  into alignment against the test sample  110 , at which time the loose screws fixing the shorting block  140  against the arm  132  may be tightened. The alignment element  143  may be removable or may remain on the shorting block  140  throughout normal testing for automatic alignment with the test sample  110  during actuation. 
     Also, as shown in  FIG. 4 , the shorting block  140  and arm  132  may also have a compliance means that limits the contact force against the test sample  110  and also allow for variability in positioning tolerance of various test samples. This compliance means could be in the form of a mechanical compliance element  142  that mounts to the arm  132  and contacts the test sample  110  once the desired compression of the shorting block  140  is achieved. The mechanical compliance element  142  may be a portion of the alignment element  143  if desired. Alternatively, the compliance means could also be in the form of a controller on the actuator  134 , such as controlled air pressure or motor force. Alternatively, the compliance means could be a spring assembly installed between the actuator  134  and the shorting block  140 . In general, the advantage of using a compliance means is that the actuator  134  can automatically engage the shorting block  140  against the sample  110  with the desired contact force. 
       FIG. 5  illustrates an example of a typical APFA  180 , which may be used as the test sample  110  with tester  100 , and set of three HGAs  186  prior to electrical bonding and mechanical swaging to the APFA  180 , as known in the art today. Also shown are bondpads  112 , in this example, eighteen in total, connector  111 , and preamplifier chip  114 , which are all part of the APFA. While  FIG. 5  illustrates an APFA  180 , which may be used as the test sample  110 , the test sample  110  may be an FCOF sample or any HDD flex circuit test sample prior to bonding the HGAs  186 , which may all be tested in a similar manner. Other possible examples of the test sample that may be used with tester  100  include the Actuator Flex Assembly (AFA), which is similar to APFA except the pivot bearing is not yet installed, or any number of other variations of flex circuit testing prior to the bonding of HGAs  186 . For such various sample testing it may be preferable to rearrange the tester  100  such that the test sample  110  is mounted with the bondpads  112  facing up or down, such that the shorting block  140  would be engaged vertically, as opposed to the horizontal actuation shown in  FIG. 1 . 
     It should also be noted that the shorting block  140  may be designed to contact any one or more bondpads  112  on the test sample. If a test sample is equipped with multiple bondpads, then the shorting block  140  may contact any one or more of these individual HGA bondpads, as long as the shorting block is sufficiently large enough to also contact a surface on the sample that does not include a bondpad. 
     Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.