Abstract:
A test assembly for testing electrical performance of microcircuits contained in leadless packages has Kelvin contacts. Slider contacts in a plurality of contact assemblies slide compliantly to accommodate lack of coplanarity in terminals on the package. A resilient elastomeric block may be inserted through interior spaces of the contact assembly and in interfering relation with features of a housing that supports and aligns the contact assemblies, to apply force to the slider contacts to force them against the microcircuit terminals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a regular application filed under 35 U.S.C. § 111(a) claiming priority, under 35 U.S.C. § 119 (e) (1), of provisional application Ser. No. 60/555,383 previously filed Mar. 22, 2004 under 35 U.S.C. § 111 (b). 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Testing microcircuits before soldering them onto circuit boards is good manufacturing practice. Defective microcircuits are difficult or impossible to remove from a circuit board, so installing a defective microcircuit typically requires scrapping the entire circuit board. In general, testing of an individual microcircuit involves temporarily connecting test contacts to the microcircuit terminals, and then using special test circuitry connected to the test contacts to operate the microcircuit to test the microcircuit functions.  
         [0003]     Microcircuits are provided in a number of different package types. The means for connecting test contacts to a particular type of microcircuit depends on the type of package enclosing the microcircuit and the type of contacts carried by the package. of course, making good contact between every one of the test contacts and the associated microcircuit terminal is very important, since a bad test connection to even one microcircuit terminal will indicate the microcircuit as defective even though the microcircuit may in fact be fully functional.  
         [0004]     The type of package of interest for this invention is the so-called leadless package, where small connector pads along the edges of one face form solder terminals by which the package is electrically and mechanically connected to the circuit board. Internal wiring connects the internal microcircuit to the solder terminals. Hereafter the term “package” will refer to so-called leadless packages unless otherwise stated or the context clearly indicates otherwise. Further, the microcircuit under test is conventionally referred to as the “DUT”, that is, device under test.  
         [0005]     Such package solder terminals may be 2-5 mm. wide along the edge of the package surface and perhaps 5 mm. long. The spacing between terminals may be 1-2 mm.  
         [0006]     Due to unavoidable variations in the manufacturing process, the connector pad surfaces in leadless packages are not completely coplanar. This does not affect the soldering process because the solder can fill in between the circuit board contacts and the package terminals.  
         [0007]     But when temporarily connecting test contacts to package terminals, the lack of co-planarity may cause poor or even no contact between the test contacts and the package terminals. For this reason, test contacts are usually designed to be compliant, that is shift or move under load so that each test contact makes solid mechanical and electrical contact with the package terminal.  
         [0008]     Not only does the lack of co-planarity cause problems when testing microcircuits in leadless packages, but other problems may as well cause poor electrical connections between the test contacts and the package terminals. For example, oxides may interfere with the electrical connections, particularly because the current involved are often in the μa. or ma. range. In other cases, dirt between the test contacts and the package terminals can cause poor or no electrical connection.  
         [0009]     When these conditions arise, packages that are in fact totally functional may be found to be defective. They will then be discarded unnecessarily, which obviously adds unnecessary cost to the manufacturing process. Accordingly, it is usually cost-effective to take substantial precautions to assure good electrical connections between the test contacts and the package terminals when testing microcircuit packages.  
         [0010]     One approach to detect poor electrical connections between the test contacts and the package terminals places two test contacts on each package terminal. These types of contacts to microcircuit terminals have been given the term of art of “Kelvin contacts”. U.S. Pat. Nos. 6,293,814; 5,565,787; and 6,069,480 are three patents that show various types of Kelvin contact testing systems.  
         [0011]     With two test contacts on each package terminal, the testing system can easily check for good electrical connection between the test contacts and the DUT package terminals. If the sensed connection resistance is too large, this may indicate a problem with the test system itself rather than with the DUTS. At any rate, false failure indications are often reduced significantly using the Kelvin testing system.  
         [0012]     A further issue is with the design of the compliant test contacts. U.S. Pat. No. 5,609,489 shows a type of test contact having a conducting, arctuate slider with a test contact end. The slider is shaped to slide within a conforming arctuate channel in a conducting frame element. The test contact end projects from the channel. A resilient elastomeric spring urges the slider from the channel. A number of these slider/frame units are arranged side by side on a circuit board forming a part of the test system, and in alignment with the spacing of the individual terminals of the DUT.  
         [0013]     In use, the DUT&#39;s terminals are pressed against the aligned test contact ends. The sliders adjust the amount by which they project from the channels and above the test system circuit board, to make good mechanical and electrical contact with the DUT terminals.  
       SUMMARY OF THE INVENTION  
       [0014]     A test assembly for testing electrical performance of microcircuits contained in leadless packages has Kelvin contacts. Slider contacts in a plurality of contact assemblies slide compliantly to accommodate lack of coplanarity in terminals on the package. A resilient elastomeric block may be inserted through interior spaces of the contact assembly and in interfering relation with features of a housing that supports and aligns the contact assemblies, to apply force to the slider contacts to force them against the microcircuit terminals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a test system showing a plurality of contact assemblies assembled in a housing attached to a circuit board with which the contact assemblies make electrical contact.  
         [0016]      FIG. 2  is a perspective view similar to that of  FIG. 1  of a test system, showing elastomeric blocks installed to retain and activate the plurality of contact assemblies assembled in the housing.  
         [0017]      FIG. 3  is a further perspective view similar to that of  FIGS. 1 and 2 , of a test system, showing two sets of installed contact assemblies assembled in the housing.  
         [0018]      FIG. 4  is a bottom perspective view showing how contact feet carried on the contact assemblies make electrical and mechanical contact with contact pads on the circuit board.  
         [0019]      FIGS. 5B and 5B  are elevation views of the two conducting laminations that form a contact assembly.  
         [0020]      FIG. 6  is a perspective view of the two conducting laminations and the insulating lamination, all properly juxtaposed to form the frame of a contact assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Turning first to  FIGS. 1-3 , a portion of a test system  10  or  10 ′ includes a housing  13  made of an electrically insulating material. Housing  13  has a number of spaced apart frame slots as at  20   a  in a first row and a second row as at  20   b , etc. The center-to-center spacing between individual slots  20   a , etc. in the first row and slots  20   b , etc. in the second row, and the number of slots  20   a ,  20   b , etc. must match the center-to-center spacing and the number of the terminals as at  73  ( FIG. 1 ) on the DUT.  
         [0022]     Housing  13  is mounted on a test circuit board  17 . Housing  13  has a series of openings as at  33   b  between each pair of adjacent slots  20   b , and a pair of slots (not visible) also between each pair of adjacent slots  20   a . Openings  33   b  are in substantial transverse alignment with each other, as are also those between pairs of adjacent slots  20   a . The inboard ends of slots  20   a , etc. include a downwardly facing ledge  35   a . The inboard ends of slots  20   b , etc. include a downwardly facing ledge  35   b.    
         [0023]     A first series of electrical contact pads on board  17  are arranged in first and second rows of contact pads  31   a , etc. and  34   a  etc. respectively, below and in alignment with the slots  20   a , etc. A second series of electrical contact pads on board  17  are arranged in first and second rows of contact pads  31   b , etc. and  34   b , etc. respectively, below and in alignment with the slots  20   b , etc. Contact pads  31   a ,  31   b ,  34   a ,  34   b , etc. are to be connected to test hardware, not shown, for testing the DUT.  
         [0024]     The contact pads  31   a , etc. in the first row are offset with respect to the contact pads  34   a , etc. in the second row. The first and second rows of contact pads  31   b , etc. and  34   b , etc. are similarly offset with respect to each other. The reasons for this offset arrangement of the first rows of contact pads  31   a , etc. and  31   b , etc. with respect to the second rows of contacts  34   a , etc. and  34   b , etc. respectively will be explained below.  
         [0025]     A first series of support pads on board  17  are arranged in a first row of pads  25   a , etc. in alignment with the slots  20   a , etc. and with contacts  31   a , etc. The first series of support pads on board  17  further includes a second row of pads  28   a , etc. in alignment with the slots  20   a , etc. and with contacts  34   a , etc.  
         [0026]     A second series of support pads on board  17  is arranged in a first row of pads  25   b , etc. in alignment with the slots  20   b , etc. and with contacts  31   b , etc. The second series of support pads on board  17  further includes a second row of pads  28   b , etc. in alignment with the slots  20   a , etc. and with contacts  34   b , etc.  
         [0027]     Each slot  20   a , etc. holds a contact assembly  30   a , etc. as shown in  FIG. 3 . Each slot  20   b , etc. holds a contact assembly  30   b , etc. as further shown in  FIG. 3 . Contact assemblies  30   a  and  30   b  will typically be identical to each other.  
         [0028]     Each contact assembly  30   a  and  30   b  comprises a frame assembly  60  as shown in  FIGS. 5, 6A , and  6 B, and first and second slider contacts  53   a  and  56   a  as shown in  FIGS. 1-4 . Each contact assembly  30   a  and  30   b  also includes a pair of slider contacts  53   a  and  56   a  as shown for contact assembly  30   a  in  FIGS. 1 and 2 .  
         [0029]     Each frame assembly  60  comprises an electrically conductive sense lamination  42 , a conductive force lamination  40 , and an insulating lamination  70  between the laminations  40  and  42 . The insulation lamination  70  serves to electrically insulate sense lamination  42  from force lamination  40 . Although the three laminations  40 ,  42 , and  70  appear to be unitary in  FIG. 5 , in actuality each lamination can slide freely with respect to the others.  
         [0030]      FIG. 6A  shows force lamination  40  in side elevation. Lamination  40  includes a wall  94   a  defining an opening  95   a  and a slot  91   a . Opening  95   a  has a first stop surface  77   a  and a second stop surface  79   a . At a left or rear side, an external tail  87   a  carries a downwardly projecting support foot  66  near to the rear or left end of tail  87   a . Wall  94   a  also carries a downwardly projecting contact foot  69  spaced from the front or right end of wall  94   a  and an external nose  88   a  projecting from wall  94   a.    
         [0031]     As shown in  FIGS. 1 and 4 , when the test system  10  is completely assembled, contact foot  69  is to rest on and make electrical connection with a contact pad  31   a  or  34   b  on the surface of circuit board  17 . Support foot  66  is to rest on a support pad  25   a  or  25   b.    
         [0032]      FIG. 6B  shows sense lamination  42  in side elevation. Lamination  42  includes a wall  94   b  defining an opening  95   b  and a slot  91   b . Opening  95   b  has a first stop surface  77   b  and a second stop surface  79   b . Wall  94   b  at a left or rear side carries an external tail  87   b . Wall  94   b  also carries a downwardly projecting support foot  80  spaced from tail  87   b . Wall  94   b  carries on an external projecting nose  88   b  on the lower section thereof a downwardly projecting contact foot  83  near to the lower and front or right end of wall  94   b.    
         [0033]     As shown in  FIGS. 3 and 4 , when the test system  10  is completely assembled, contact foot  83  is to rest on and make electrical connection with a contact pad  34   a  or  34   b  on the surface of circuit board  17 . Support foot  80  is to rest on a support pad  25   a  or  25   b.    
         [0034]     Importantly, each contact foot  83  is staggered with respect to the adjacent contact foot  69 . This staggered relationship allows each contact foot  69  to rest on only a contact pad  31   a  or  31   b  and each contact foot  83  to rest on only a contact pad  34   a  or  34   b.    
         [0035]     Each of the laminations  40  and  42  respectively has within the openings  95   a  and  95   b , nearly identical stop edges  79   a  and  79   b , and nearly identical stop edges  77   a  and  77   b . Stop edges  79   a ,  79   b ,  77   a , and  77   b  interact with features on slider contacts  53   a  and  56   a  to restrict movement of slider contacts  53   a  and  56   a  within slots  91   a  and  91   b  respectively.  
         [0036]      FIG. 1  shows this interaction. Slider  53   a  is shown mounted within slot  91   a  with an interior end within opening  95   a . Features on the interior end of slider  53   a  interact with stop edge  77   a  to retain slider  53   a  within slot  91   a  and opening  95   a . At the same time, contact ends  58   a  and  59   a  project upwardly from slots  91   a  and  91   b  as shown in  FIG. 3 . Each pair of projecting contact ends  58   a  and  59   a  presses against a single contact  73  of the DUT. Of course, each lamination  40  and  42  in a contact assembly  30   a  or  30   b  must be electrically isolated from each other except for the contact on the DUT terminal  73  that each of the sliders  53   a  and  56   a  make.  
         [0037]      FIGS. 1-3  show individual contact assemblies  30   a ,  30   b , etc. within slots  20 ,  20   b , etc. The dimensions of contact assemblies  30   a ,  30   b , etc. permit slots  20   a ,  20   b , etc. to closely house contact assemblies  30   a ,  30   b , etc. and at the same time allow slight shifting of contact assemblies  30   a ,  30   b , etc. within slots  20   a ,  20   b , etc. in a plane perpendicular to circuit board  17 .  
         [0038]     Slots  20   a ,  20   b , etc. are loaded with contact assemblies  30   a ,  30   b , etc. before housing  13  is mounted on circuit board  17 . Housing  13  mounts on circuit board  17  so that each contact foot  69  rests on a contact pad  31   a  or  31   b , and so that each contact foot  83  rests on a contact pad  31   b  or  34   b.    
         [0039]      FIG. 2  shows a first elastomeric block  63  that passes through all of the openings  95   a  and  95   b  and openings  33   b . The sizes and shapes of the openings  95   a  and  95   b  and openings  33   b  and of the elastomeric block  63  are such that installation of housing  17  on circuit board  13  compresses block  63 , thereby urging contact feet  69  and  83  and support feet  66  and  80  against the various pads  25   a ,  28   a ,  31   a , etc. carried on circuit board  13 .  
         [0040]     Further, block  63  presses against the interior end of each slider  53   a ,  56   a , etc., to thereby urge sliders  53   a ,  56   a , etc. against an adjacent DUT terminal  73 . The elastomeric block  63  provides consistent pressure against each slider  53   a ,  56   a , etc. to urge the ends  58   a  and  59   a  against the DUT terminals  73 .  
         [0041]     A second elastomeric block  61  is interposed between the individual noses  88   a  and  88   b  carried by the individual contact assemblies  30   a ,  30   b , etc., and the adjacent downwardly facing ledges  35   a  and  35   b . Only one elastomeric block  61  is shown in  FIG. 3 , interposed between ledge  35   a  and noses  88   a  and  88   b  on each of the contact assemblies  30   a ,  30   b , etc. A similar block will of course be interposed between ledge  35   b  and the adjacent noses  88   a  and  88   b.    
         [0042]     As with block  63 , block  61  is distorted and compressed when housing  13  is attached to circuit board  17 . The resilience of blocks  61  and  63  force contact feet  69  and  83  against the contact pads  31   a ,  31   b ,  34   a , and  34   b  etc. This force accommodates variations in contact assemblies  30   a ,  30   b , etc. to assure consistent contact force between contact pads  31   a ,  31   b ,  34   a , and  34   b  etc. and contact feet  69  and  83 .  
         [0043]     During a testing episode, the plurality of terminals  73  carried on a DUT are pressed against the ends  58   a ,  59   a , etc. of the individual pairs of contact sliders  53   a ,  56   a , etc. The resilience of block  63  provides force assuring that slider ends  58   a ,  59   a , etc. make good electrical contact with terminals  73 . The resilience of block  63  allows ends  58   a ,  59   a , etc. to accommodate for deviations in coplanarity of terminals  73 .  
         [0044]     It will be understood that this disclosure, in many respects, is only illustrative. Changes may be made in details, particularly in matters of shape, size, material, and arrangement of parts without exceeding the scope of the invention. Accordingly, the scope of the invention is as defined in the language of the appended claims.