Abstract:
In a first slot of a plurality of adjacent slots in alignment with traces on a load board of a tester, first and second conductor layers, each to make electrical contact with both a load board trace and a DUT lead. Each of the first and second contacts receives force from a resilient element extending across the slots and that urges a contact point on the contact against at least one trace and a DUT lead. Insulation between said first and second contacts in the first slot electrically insulates the first and second contacts from each other within the first slot.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is an international application filed under 35 USC §363, claiming priority under 35 USC §119(e) of U.S. Prov. Appl. No. 60/883,128 filed Jan. 2, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention pertains to improvements to equipment for testing microcircuits. This is important because the manufacturing processes for microcircuits cannot guarantee that every microcircuit is free of defects. Dimensions of the contacts or leads for the individual microcircuits are microscopic, on the order of tenths of a mm., and process steps very numerous and complex, so small or subtle failures in the manufacturing process can often result in defective devices. 
         [0003]    Usually (but not always) a microcircuit is mounted within a small plastic housing or package that protects the microcircuit within from damage. The individual microcircuit contacts are connected within the package to leads external to the package. The pitch, or center-to-center spacing of adjacent leads, may be as small as 0.4 mm. for some packages. 
         [0004]    A number of different designs exist for the housing or package of the microcircuit. Some have cantilevered leads projecting from the bottom edge of the package. Others have small contacts that are flush with or project only slightly from the package surface. The leads of others are small solder balls that are melted during installation on a circuit board. In any case, the usual practice is to solder each of the many leads of a package to a larger circuit board that provides for connections to switches and other discrete components, transducers such as speakers, etc. A circuit board often interconnects a number of microcircuits as well. 
         [0005]    Mounting a defective microcircuit on a circuit board is relatively costly. Once soldered to a circuit board, removing a microcircuit is problematic because the very act of melting the solder for a second time may ruin the circuit board. Thus, if the microcircuit is defective, the circuit board itself is probably ruined as well, meaning that the entire present value of the circuit board is lost. Even if it is possible to remove a defective microcircuit, detecting which of perhaps several is defective is difficult and expensive. For all these reasons, a microcircuit is usually tested before installation on a circuit board. 
         [0006]    Each microcircuit must be tested in a way that identifies a very high percentage of the defective microcircuits, but yet does not improperly identify more than a small percentage of good microcircuits as defective. Failure to identify defective microcircuits in particular adds substantial overall cost to the final product. Wrongly identifying good microcircuits as defective if frequent, can also add significant costs to microcircuit production. 
         [0007]    The need to accurately test microcircuits has led to the development of dedicated equipment for testing microcircuits. Reliable test equipment faces a number of challenges that makes the test equipment itself quite complex. First of all, the test equipment must make accurate, low resistance, temporary electrical contact with each of the closely spaced microcircuit leads without damaging either the leads or the microcircuit. Because of the small size of microcircuit leads and the spacing between the leads, even small alignment errors in making the contact will result in incorrect connections. Connections to the microcircuit that are misaligned or otherwise incorrect will cause the test equipment to identify the device under test (DUT) as defective, even though the reason for the failure is the incorrect electrical connection between the test equipment and the DUT rather than defects in the DUT itself. 
         [0008]    Test equipment in current use has an array of test contacts that match the pattern of the microcircuit lead array. The array of test contacts is supported in a structure that precisely maintains the alignment of the contacts relative to each other. An alignment template or board aligns the microcircuit itself with the test contacts. The test contacts and the alignment board are mounted on a load board having conductive pads that make electrical connection to the test contacts. The load board pads are connected to circuit paths, also known as traces, that carry the signals and power between the test equipment electronics and the test contacts. 
         [0009]    A certain type of common test contact has the form of a thin, elongate lever or arm in a stylized S shape. U.S. Pat. No. 5,967,848 shows test contacts of this type, see  FIG. 6  therein for example. In use, a point near one end of each test contact rests on a load board contact. A point near the other end of each test contact rests on a DUT lead. A guide has walls that define a set of side-by-side slots that holds these test contacts in alignment with the DUT leads and the traces. The pitch of the slots is chosen to align the individual slots with the DUT leads. 
         [0010]    The levers are held in place within the slots by retaining pins or bars made of a resilient material such as an elastomer. The retaining bars pass through holes in the walls defining the guide&#39;s slots to fit inside the curves defined by the S shape of the levers. The &#39;848 patent shows how the elastomeric bars intercept the levers. The resiliency in the retaining bars provides compliance in the positioning of the lever contacts when a presser foot presses an array of DUT leads against the lever contacts. The retaining bars deform in shear slightly to provide consistent force between each contact lever and its associated load board contact and DUT lead. 
         [0011]    One problem that arises in microcircuit testing is the potential for electrical resistance between the lead and its test contact. When this resistance is too high, signals that the microcircuit produces during the test may appear to be too low. That DUT may then be rejected as faulty even though the problem is actually a poor contact between a DUT lead and the test contact. This poor contact is strictly an artifact of the testing and is unlikely to be present once the DUT has been soldered onto a circuit board. 
         [0012]    “Kelvin” testing refers to a process where the test equipment provides two test contacts, often referred to as “force” and “sense” contacts, for each DUT lead. The force contact carries the signal to or from the DUT for testing DUT operation, and can also be called the signal contact. The sense contact carries the Kelvin signal for assuring that the contact to DUT lead is good, and can also be called the Kelvin contact. 
         [0013]    A preliminary part of the test procedure measures the resistance between the two test contacts. If this value is high, one or both of the two test contacts are not making good electrical contact to the microcircuit terminal. If the possibility of high resistance at this interface will affect the accuracy of the actual testing of the microcircuit performance, then the issue can be addressed according to the provisions of the testing protocol. For example, a Kelvin test failure may suggest that the operator should adjust the test equipment or replace the test contact. 
         [0014]    Kelvin testing provides additional assurance that each test contact has made good electrical connection with a DUT lead. When the Kelvin test shows good contact at each DUT lead, then it is reasonable to conclude that a failure in the remainder of the test resulted from a defective DUT. Thus, a Kelvin test often eliminates falsely detecting that a DUT is defective. 
         [0015]    U.S. Pat. No. 5,967,848 shows lever-type contacts that have on the sides thereof, electrical components such as capacitors, inductors, transistors, and even microcircuits. These components can for example be used to match impedances of a microcircuit lead and the load board circuitry to which it is connected. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0016]    The invention is for use in a microcircuit tester of the type having i) a load board supporting a plurality of traces arranged in a predetermined pattern and for connection to circuitry for testing operation of a microcircuit comprising a device under test (DUT), ii) a test contact guide having a plurality of adjacent planar slots in alignment with the traces, each for receiving a contact element for electrically connecting a lead on a DUT to a trace on a load board, and iii) a resilient element within the slots. 
         [0017]    The invention itself comprises in a first slot, first and second conductor layers each to make electrical contact with both a load board trace and a DUT lead. Each of the first and second contacts receives force from the resilient element that urges a contact point on the contact against at least one of a trace and a DUT lead. Insulation between the first and second contacts in the first slot electrically insulates the first and second contacts from each other within the first slot. 
         [0018]    The insulation layer may attach to either contact. Or, two contact layers may be provided, one attached to a facing surface of each contact. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIGS. 1 and 2  are front and side section views of portions of a guide with pairs of contacts in the slots of the guide. 
           [0020]      FIG. 3  is a front elevation view of a pair of contacts in the position relative to each other they will occupy in a guide slot. 
           [0021]      FIG. 4  is a side elevation view of a contact in horizontal alignment with  FIG. 3 . 
           [0022]      FIG. 5  is a variation on the structure of the contact pair. 
           [0023]      FIG. 6  is a wire frame perspective of a variation of the invention of  FIGS. 1-4 . 
           [0024]      FIG. 7  is an exploded side view of the variation of the invention shown in  FIG. 6 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIGS. 1 and 2  should be considered together, and show portions of the interconnection between a microcircuit package  37  (the DUT) and traces  13  and  25  on a load board  10  for temporary use during testing of the microcircuit. For ease of replacement, traces  13  and  25  may be carried on a membrane  31  lying on and electrically interconnecting with load board  10 . 
         [0026]    The microcircuit package  37  has a number of leads  41  projecting outwardly from the walls of package  37 . These leads will be soldered to pads on a circuit board when package  37  is installed.  FIGS. 1 and 2  are not to scale so as to better show the structural relationships among the various features. Note also that clearances between various elements of the invention are exaggerated to allow better understanding of the invention. 
         [0027]    Contact elements  40  and  40 ′ are supported by a contact guide, relevant portions of which are shown in  FIGS. 1 and 2  as including walls  43  defining between them individual slots  47 . Each slot  47  holds a pair of contact elements  40  and  40 ′. A presser bar  34  shown in phantom in  FIG. 1  applies pressure to stabilize the position of each lead  41  against force provided by individual contact elements  40  and  40 ′. 
         [0028]    The structure shown in  FIGS. 1 and 2  forms a Kelvin contact configuration. Contact elements  40  and  40 ′ have conductor layers  26  and  26 ′ that make the electrical connections between a single lead  41  and traces  13  and  25  on load board  10 . Contact element  40  may function as a test contact, and contact element  40 ′ may function as a Kelvin contact. Traces  13  and  25  respectively make the test and Kelvin connections from contact elements  40  and  40 ′ to the test circuitry. 
         [0029]    Resilient elastomeric cylinders  52  and  55  are held in parallel holes or passages extending transversely through all of the individual walls  43  and slots  47 . The size and stiffness of cylinders  52  and  55 , their positions within slots  47 , and the shape and dimensions of contact elements  40  and  40 ′ cooperate to cause a slight distortion or deflection of cylinders  52  and  55  within walls  43  and slots  47 . 
         [0030]    One can see that the result of this distortion is a pair of oppositely directed forces, in effect a couple, urging contact elements  40  and  40 ′ to rotate in a counterclockwise direction (as viewed in  FIG. 2 ). This counterclockwise-directed rotation resiliently presses each pair of contact points  57  against a lead  41  and each pair of contact points  65  and  65 ′ respectively against traces  13  and  25 . Lead  41  and traces  13  and  25  urge clockwise rotation of contact elements  40  and  40 ′, to thereby create contact force for good electrical connection. Presser bar  34  limits the deflection of individual leads  41  from the force applied by contact elements  40  and  40 ′. 
         [0031]      FIG. 3  shows an edge view of a pair of contact elements  40  and  40 ′ having in this particular configuration insulating layers  19  and  19 ′ attached respectively to conductor layers  26  and  26 ′. Many other configurations are possible, such as omitting one of the insulating layers  19  and  19 ′. Similarly, the projective shape and dimensions of a contact element  40  or  40 ′ as shown in  FIGS. 2 and 4  can vary considerably depending for example on the force required for proper electrical contact to the traces  13  and  25  and the leads  41 . 
         [0032]    Each conductor layer  26  and  26 ′ has lead contact points  57  and  57 ′ that make the electrical and mechanical contact with a lead  41 . Each conductor element  26  and  26 ′ also has trace contact points  65  and  65 ′ that make electrical and mechanical contact with a set of traces  13  and  25 . 
         [0033]    Insulating layers  19  and  19 ′ preferably have low friction facing surfaces to allow independent sliding or shifting of contact elements  40  and  40 ′ with respect to each other. The surfaces of leads  41  facing contact points  57  and  57 ′ typically vary slightly in terms of elevation or spacing from contact points  57  and  57 ′. The ability of each contact element  40  and  40 ′ to slip freely with respect to each other in a slot  47  accommodates these variations in the elevation of an individual lead  41  surface. 
         [0034]    Insulating layers  19  and  19 ′ preferably are recessed slightly relative to conductor layers  26  and  26 ′ in the vicinity of lead contact points  57  and  57 ′ so that say, contaminants between conductor layers  26  and  26 ′, will not interfere with the electrical connections to lead  41 . 
         [0035]    Insulating layers  19  and  19 ′ have projections  30  and  30 ′ that extend past the conductor layers  26  and  26 ′ in the vicinity of contact points  65  and  65 ′. Projections  30  and  30 ′ form features on contact elements  40  and  40 ′ that cooperate with a feature on load board  10  to provide an alignment function that holds contact elements  40  and  40 ′ in proper position on each set of traces  13  and  25 . 
         [0036]      FIG. 1  shows one form of a cooperating feature on load board  10  as comprising a slot or void  16  between traces  13  and  25  into which projections  30  and  30 ′ fit. Projections  30  and  30 ′ hold contact elements  40  and  40 ′ in proper relationship to traces  13  and  25  so that conductor layers  26  and  26 ′ make good electrical contact with traces  13  and  25 . 
         [0037]    It is trivial to reverse the placement of the void and projection. The void may comprise a recess between contacts  40  and  40 ′, with the projection extending from between traces  13  and  25  into that recess between contact element  40  and  40 ′. The two embodiments are conceptually fully equivalent. 
         [0038]      FIG. 5  shows such an arrangement. A projection  42  carried on membrane  31  extends into a recess between conductive layers  26  and  26 ′, to improve alignment of contact elements  40  and  40 ′ with traces  13  and  25 . 
         [0039]      FIG. 5  also shows contact elements  40  and  40 ′ as having only a single insulating layer  19 , which is often valuable because slot width is not large. 
         [0040]    The wire frame view of the invention in  FIG. 6  and the exploded side view in  FIG. 7  add a feature of providing an alternate connection from the load board  10  to individual contacts  40  and  40 ′. This may have value for example if contact elements  40  or  40 ′ carry active electrical components. The reference numbering is consistent throughout the Figs. of this description. 
         [0041]    In  FIG. 6 , a membrane or ribbon connector  70  makes electrical connection to load board contacts  75  through conducting paths  72 . Elastomeric cylinder  52 ′ fits within a passage  89  extending transversely through all of the slots  47  holding the pairs of contact elements  40  and  40 ′.  FIG. 7  shows cylinder  52 ′ in phantom outline  52 ″ and illustrates how membrane fits around elastomeric cylinder  52 ′. Connector  70  fits around a portion of the periphery of passage  89  and extends between cylinder  52 ′ and contact elements  40  and  40 ′. 
         [0042]    Referring to  FIG. 7 , contact element  40  includes a contact pad  80  on the edge thereof and adjacent to membrane  70 . Contact element  40  makes electrical contact through contact pad  80  with an individual path  72  that cylinder  52 ′ presses against element  40 . Path  72  may conduct voltage through a path  83  on contact element  40  to an active component  86  carried by the contact element  40 . 
         [0043]      FIGS. 6 and 7  are conceptual, in that they show only one connection through membrane connector  70  to contact element  40 . However, providing for two or more paths  72  to a single contact element  40  is simple. This will provide the option of power for an active component  86  directly on the contact element  40 , and directly in the path carrying the signal from lead  41  to load board contact  13 . 
         [0044]    In a structure such as  FIGS. 6 and 7  shows, contact elements  40  and  40 ′ may comprise a silicon substrate having photolithographically formed electrical components and electrical connections. Thus, active component  80  may comprise a transistor or even a microcircuit.