Abstract:
An on-board tester (referred to as a “test fixture”) for testing integrated circuit chips, particularly ball grid array (BGA) chips. The test fixture of the present invention eliminates many of the problems associated with presently available test fixtures, particularly the lack of control in mounting the chips to the test fixture, and the unpredictable testing results. The present test fixture has an upper assembly and a lower assembly. A circuit board containing the BGA chip to be tested is mounted between the upper and lower assemblies. The lower assembly has guide pins extending toward the upper assembly which allows any circuit board having alignment holes that match the configuration of the guide pins to be mounted to the lower assembly. Moreover, the present test fixture has a unique latching mechanism which uses rotational movement to latch and unlatch the test fixture. Particularly, a collet assembly is used which allows rotation of a shaft to compress other plates in the assembly so that the upper and lower assembly are properly secured together. Rotational movement is also used to secure the test pins of the test fixture to the BGA chip. The test fixture has a knob that is rotated to linearly draw the upper and lower assemblies together until the probe test pins have an effective electrical connection with the BGA chip.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Provisional Patent Application No. 60/203,664 filed May 12, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to test fixtures, particularly on-board testers, for integrated circuit chips, such as a ball grid array (BGA) chip. The on-board tester (referred to herein as the “test fixture”) includes an upper assembly and a lower assembly that attach through a circuit board mounted between the upper assembly and lower assembly. The circuit board contains the BGA chip to be tested and an electrical engagement between the test fixture and the BGA chip is achieved by rotational movement that linearly compresses probe test pins of the lower assembly against the BGA chip. 
     BACKGROUND OF THE INVENTION 
     Testing of integrated circuits, such as those contained within ball grid array (BGA) packaging, is accomplished through the use of what is commonly referred to in the art as a test fixture. BGA test fixtures typically include a housing mounted to a load board which interfaces with the test electronics. The load board is generally a circuit board for transferring test signals from the integrated circuit in the BGA to the test electronics. 
     Previous methods of attaching the test fixture to the load board include through hole techniques and surface mounting techniques. In the surface mounting connection, the test fixture includes test pads which make contact with the solder balls on the bottom of the BGA as the BGA is compressed against the test pads to transfer the test signals to the load board. A problem associated with the surface mounting test fixture arrangement is that the solder balls on the bottom of the BGA can vary in height and good electrical contact between each solder ball and the test pad cannot always be assured. A second problem associated with surface mounting is that once the test pads become contaminated from the solder balls, the entire fixture assembly must be replaced. 
     Through hole techniques for connecting the fixture to the load board include holes drilled through the load board for the passage of spring loaded contact pins which contact the solder balls on the BGA and transfer the test signals to the load board through the contact between the test pins and the holes in the load board. A problem associated with the through hole fixture arrangement is that the test pins extending up through the load board could easily be bent or damaged which would negatively impact the test results. To avoid this problem, a receptacle can be positioned between the fixture and the load board to protect the test pins extending through the load board. The result of incorporating a receptacle requires the length of the test pins in the fixture to be increased which creates a problem for testing high speed integrated circuits. To address this problem spring probes have been incorporated which have a short travel length, however with short travel springs, the spring life is short requiring constant replacement. In addition, the use of spring probes in the fixture can create an impedance problem for the transfer of the test signal from the BGA to the load board. 
     Consequently, a need exists for a new test fixture for BGA packages which reduces the problems associated with prior art test fixtures. Particularly, a new test fixture that can be easily mounted on a circuit board and produce accurate results. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an on-board tester (referred to herein as a “test fixture”) for testing integrated circuit chips, particularly ball grid array (BGA) chips. The test fixture of the present invention eliminates many of the problems associated with presently available test fixtures, particularly the lack of control in mounting the chips to the test fixture, and the unpredictable testing results. 
     The present test fixture has an upper assembly and a lower assembly. A circuit board containing the BGA chip to be tested is mounted between the upper and lower assemblies. The lower assembly has guide pins extending toward the upper assembly which allows any circuit board having alignment holes that match the configuration of the guide pins to be mounted to the lower assembly. This makes the test fixture versatile as compared to many existing test fixtures which have a “clamshell” latching mechanism that will only allow a circuit board having exactly the same dimensional properties as the test fixture to be tested. Moreover, the present test fixture has a unique latching mechanism which uses rotational movement to latch and unlatch the test fixture. Particularly, a collet assembly is used which allows rotation of a shaft to compress other plates in the assembly so that the upper and lower assembly are properly secured together. 
     The test fixture of the present invention also provides increased testing accuracy by using rotational movement to secure the test pins of the test fixture to the BGA chip. The test fixture has a rotatable knob that is rotated to linearly draw the upper and lower assemblies together until a plurality of probe test pins have an effective electrical connection with the BGA chip. This controlled compression eliminates the problems of previous methods which tend to damage the test pins and provide unpredictable contact with the test pins, and thus produce unreliable results. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional side view of a test fixture of the present invention; 
     FIGS. 2 a  and  2   b  are partial cross-sectional side views of the test fixture of FIG. 1 shown in the assembled, non-actuated position, and the assembled, actuated position respectively; 
     FIG. 3 is an exploded perspective view of a portion of the upper assembly of the test fixture of FIG. 1; 
     FIG. 4 is an exploded perspective view of the complete upper assembly of the test fixture of FIG. 1; and 
     FIG. 5 is an exploded perspective view of the lower assembly and an assembled upper assembly of the test fixture of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     The test fixture  10  of the present invention is used to test integrated circuit chips that have been mounted to a circuit board  50 . In a preferred embodiment, the test fixture  10  is designed to test a ball grid array (BGA) chip  51  that is mounted to the circuit board  50 . The novel design of the test fixture  10 , however, is equally applicable for testing other types of integrated circuit chips. For clarity, the detailed description will be limited to describing the test fixture  10  as used to test the BGA chip  51  that has been mounted to the circuit board  50 . 
     Generally, FIGS. 1,  2   a  and  2   b  depict the test fixture  10  during testing of the BGA chip  51 . The test fixture  10  includes an upper assembly  8  and a lower assembly  9 . When the BGA chip  51  is being tested, the circuit board  50  is placed between the upper assembly  8  and the lower assembly  9 . The lower assembly  9  has a plurality of guide pins  14  extending upwardly, and the circuit board  50  having supplied holes is placed over the guide pins  14  until the circuit board  50  rests on the lower assembly. Preferably there are four guide pins  14  arranged in a square pattern. The guide pins  14  extend beyond the circuit board  50 , and the upper assembly  8  is then aligned over the guide pins  14  which pass through holes in the upper assembly until the upper assembly is seated on the circuit board  50 . The upper assembly  8  and lower assembly  9  are connected by a collet assembly  16  located in the upper assembly. The collet assembly  16  serves to either compresses the upper assembly and lower assembly together, or to separate the connected upper and lower assemblies. When the upper and lower assemblies are connected the test fixture  10  is said to be “latched”, and when they are separated the test fixture is “unlatched”. 
     The collet assembly  16  comprises an upper collet plate  16   a , a plurality of collets  16   b , a lower collet plate  16   c , and a rotatable shaft  16   d.  The upper collet plate is positioned vertically above the lower collet plate  16   c , and both the upper and lower collet plates have a plurality of apertures wherein the collets  16   b  and the rotatable shaft  16   d  are inserted. The collets  16   b  are generally disposed between the upper collet plate  16   a  and the lower collet plate  16   c . The rotatable shaft  16   d  is vertically disposed through apertures in the upper and lower collet plates and has a portion extending above the upper collet plate while being engaged in the lower collet plate. When the collets  16   b  and the rotatable shaft  16   d  have been inserted into the upper and lower collet plates, the upper collet plate  16   a  and lower collet plate  16   c  are approximately parallel to each other in horizontal relation, and the collets  16   b  and the rotatable shaft are approximately parallel to each other in vertical relation. 
     Clockwise rotation of the rotatable shaft  16   d  draws the lower collet plate  16   c  towards the upper collet plate  16   a , thereby compressing the collets  16   b  against the guide pins  14 , thus making a secure connection between the upper and lower assemblies. At this point, the test fixture is said to be latched. Counterclockwise rotation of the rotatable shaft  16   d  will loosen the collet assembly  16 , thus removing the collets  16   b  from being disposed over the guide pins  14  and returning the test fixture to the unlatched position. 
     To test the BGA chip  51 , an effective electrical engagement with the BGA chip  51  and the probe test pins  60  of the lower assembly  9  needs to be made. Generally, a rotatable knob  17  is positioned around the rotatable shaft  16   d  and above the upper collet plate  16   a , and clockwise rotation of the rotatable knob  17  draws the lower assembly up toward the upper assembly, thereby creating the necessary electrical engagement. After testing of the BGA chip  51  has been completed, the circuit board  50  is removed from the test fixture  10  by using the same motions as described, but in the reverse order. 
     Referring to FIG. 5, the majority of the upper assembly  8  is comprised within a modular frame  7  that is formed by a top plate  21 , front plate  30 , rear plate  29 , two side plates  28 , and bottom plate  34 . The side plates  28  and front plate  30  and rear plate  29  are connected to each other by a plurality of dowel pins  31 . In a preferred embodiment, each of the plates comprising the modular frame  7  are made of aluminum or any other dimensionally stable, strong, machinable material. 
     As shown in FIG. 3, a flanged guide  18  passes through the top plate  21  and is attached to the top plate by a plurality of threaded fasteners  22 . The flanged guide  18  serves as a linear bearing for the rotatable shaft  16   d , as the rotatable shaft passes through a bore in the center of the flanged guide. Further, the flanged guide  18  has a threaded exterior onto which the rotatable knob  17  is threaded. Preferably the flanged guide  18  is formed of phosphorous bronze, but any other material suitable for this purpose can be used. 
     The rotatable shaft  16   d  passes through a washer  19 , the rotatable knob  17 , a washer  20 , the center recess of the flanged guide  18 , and an accurately placed hole in the top collet plate  16   a . The rotatable shaft can be formed from any suitable material that is resistant to corrosion, gauling and the like, such as hardened stainless steel. The upper collet plate  16   a  is captured on the rotatable shaft  16   d  by a plurality of snap ring retainers  23 . Further, a plurality of washers  24  are placed between the upper collet plate  16   a  and the snap ring retainers  23 . Preferably the washers  19  and  24  are made from black delrin, but any suitable low friction material can be used. 
     The upper collet plate  16   a  of the upper assembly  8  preferably has seven accurately placed holes which are adapted to receive four collets  16   d , two dowel pins  25 , and the rotatable shaft  16   d . The two dowel pins  25  are pressed into the upper collet plate  16   a  and pass through the flanged guide  18  and the top plate  21  and rest against the base of the rotatable knob  17 . This configuration keeps the collet assembly  16  captured on the rotatable knob  17 , thereby causing the collet assembly  16  to move in direct relation to the vertical position of the rotatable knob  17 , which is threaded onto the flanged guide  18 . 
     Referring to FIG. 4, the lower collet plate  16   c  also has seven accurately placed holes which are adapted to receive the four collets  16   b , two drill bushings  26 , and the rotatable shaft  16   d . The threaded portion of the rotatable shaft  16   d  is threadingly received within a threaded hole in the lower collet plate  16   c . Rotation of the rotatable shaft  16   d  in a clockwise direction causes the lower collet plate  16   c  to move upward towards the upper collet plate  16   a , which results in the collets  16   b  being compressed between the upper collet plate  16   a  and the lower collet plate  16   c , thereby “latching” the test fixture  10 . As noted above, counterclockwise rotation of the rotatable shaft  16   d  will loosen the collet assembly and “unlatch” the test fixture. 
     The lower collet plate  16   c  and the bottom plate  34  are connected by the two drill bushings  26  and two tooling pins  27 . Specifically, the drill bushings  26  are received in bores of the lower collet plate  16   c  and serve as linear bearings for the tooling pins  27 , thereby preventing the lower collet plate  16   c  from rotating when a rotational force is applied to it by the rotatable shaft  16   d . Additionally, the heatsink riser  15  is attached to the bottom plate  34  by threaded fasteners  37 . The heatsink riser  15  surrounds the area of the circuit board  50  where the BGA chip  51  is mounted and serves to allow the BGA chip  51  to cool while being tested and provide a protective space between the BGA chip  51  and the test fixture  10 . 
     With respect to the lower assembly  9 , it has been noted that positioning the circuit board  50  over the lower assembly  9  is accomplished by placing the circuit board  50  over the guide pins  14 . The circuit board  50  has alignment holes that were designed to match the guide pins  14 . In a preferred embodiment, the circuit board  50  is manufactured with four alignment holes and the lower assembly  9  has four guide pins  14 . It should be noted, however, that any plurality of guide pins  14  (i.e. three guide pins) can be used in the lower assembly so long as the circuit board  50  is manufactured with a matching number of alignment holes. 
     As shown in FIG. 5, the probe plate  11  is the upper most component of the lower assembly  9 . The probe plate  11  has a surface that faces the circuit board  50 , which will be deemed a top surface, while the opposite side will be deemed a bottom surface. The probe plate  11  has counter bored holes in which the guide pins  14  are upwardly disposed through the bottom surface of the probe plate, so that the heads of the guide pins  14  are flush against the bottom surface. The guide pins  14  extend through the top surface of the probe plate  11 , thereby allowing the alignment holes of the circuit board  50  to be disposed over the guide pins. The probe plate  11  also has an array of accurately drilled holes in which a plurality of probe test pins  60  are mounted. 
     The probe test pins  60  each comprise a tube shaped body having a spring mounted inside the body and a pair of plungers placed partially inside each end of the body, where a plunger extends from each end of the body. The plungers at each end of the body of the probe test pin  60  press against the spring in such a manner that the probe test pins  60  are considered “spring loaded”. When the probe test pins  60  are pressed into the probe plate  11  the desired distance, each probe test pin  60  will have a plunger extending beyond the top and bottom surfaces of the probe plate  11 . The plunger extending from the top surface contacts the BGA chip  51  mounted on the circuit board  50 , while the plunger at the opposite end of the probe test pin  60  contacts an analyzer card  13 . The probe test pins  60  serve to transfer signals that are received from the BGA chip  51  to the analyzer card  13 . The analyzer card  13  is connected to appropriate monitoring equipment (not shown) which displays the results from the testing. 
     A spacer plate  12  is mounted immediately below the probe plate  11 , so that the top surface of the spacer plate  12  is adjacent to the bottom surface of the probe plate  11 . The spacer plate  12  has a window  12   a  milled through it that allows a portion of the analyzer card  13 , which is positioned immediately below the spacer plate, to be exposed. Specifically, the analyzer card  13  has a plurality of gold fingers  13   a  which are the portion of the analyzer card that contact the probe test pins  60  and serve to receive the signals being transmitted by the probe test pins. The spacer plate  12  ensures that a proper distance between the probe plate  11  and the analyzer card  13  is maintained. The probe plate  11 , spacer plate  12 , and analyzer card  13  are all connected by a plurality of should screws  36  and washers  35 . Once the probe plate, spacer plate and analyzer card have been connected, the probe test pins  60  and gold fingers  13   a  be in contact with each other and capable of transfer signals received from the BGA chip  51 . 
     Novel features of the present invention relate to the manner and method by which the BGA chip  51  is positioned for testing in the test fixture  10 . The test fixture  10  provides increased reliability and accuracy due to the means of connecting the circuit board  50  with the lower assembly  9  and upper assembly  8 , and the controlled contact created between the probe test pins  60  and the BGA chip  51 . In addition, the test fixture  10  exhibits increased efficiency and flexibility over conventional test fixtures due to its compact size and weight. 
     The use of the guide pins  14  to mount the circuit board  50  to the lower assembly  9  allows the test fixture  10  to be used with any circuit board that has alignment holes that correspond to the guide pins  14 . This method of testing the BGA chip  51  eliminates many of the problems associated with conventional testing fixtures that use a “clamshell” latching mechanism. Alignment of the circuit board  50  with the test fixture  10  is accurate and consistent. Unlike the “clamshell” approach which can only be used with circuit boards having exactly the same dimensional properties as the testing fixture, the test fixture  10  of this invention can be used with any size circuit board. Moreover, the test fixture  10  uses rotational movement connect the circuit board  50  to the lower assembly  9  in a manner that will not damage the probe test pins  60  and ensures accurate testing of the BGA chip  51 . 
     The contact required between the BGA chip and the probe test pins  60  is achieved by a unique design that uses rotational movement to produce linear motion. The upper assembly and lower assembly are “latched” and “unlatched” by the collet assembly  16 , which uses rotational movement of the rotatable shaft  16   d  to compress the upper and lower collet plates together which results in the collets  16   b  being compressed onto the guide pins  14 . The upper assembly  8  and lower assembly  9  are drawn together to create contact between the probe test pins  60  and the BGA chip  51  by rotation of the rotatable knob  17 . Most conventional test fixtures achieve linear motion by using cams, vacuums or air pressure. The use of rotational movement to achieve linear motion, however, has several advantages including an improved mechanical advantage (torque), it requires less parts than conventional test fixtures, and it ensures an effective electrical engagement between the test fixture  10  and the BGA chip  51  because the compression is accurately controlled. 
     Although the present invention has been described and illustrated with respect to preferred embodiments thereof, it is to be understood that it is not to be so limited because changes and modifications maybe made which are within the full intended scope of this invention as hereinafter claimed.