Electrically conductive pins for load boards lacking Kelvin capability for microcircuit testing

Abstract

A device under test (DUT) has terminals connected to electrically conductive contacts which are in turn connect to a load board and to a test signal source. A second set of kelvin terminals are likewise connected to the DUT, but by pass the load board for connection to a test signal source. The kelvin terminals extend distally away from the DUT and are bonded to a flex circuit at their distal ends so that they make electrical and mechanical contact with the flex circuit. An intermediary terminal block receives the flex circuit and a ribbon cable or other wire connects to a test signal source. The entire circuit then circumvents the use of the load board.

Claims

1. A kelvin test device for testing a device under test (DUT) by temporarily connecting the DUT to a signal source via the test device connected to a load board, the load board having a peripheral edge, comprising: a. first and second temporary mechanical and electrical contacts between the DUT having a plurality of terminals, the first electrical contacts engaging the terminals at their proximal end and generally extending away from at least one terminal in a first direction to a load board at their distal ends for sending and receiving test signals from a signal source; b. said second contacts being kelvin test contacts and being in temporary mechanical and electrical contact with said DUT at their proximal ends and being generally extending away from at least one terminal in said first direction, parallel to and spaced apart from said first contacts and routed to said signal source beyond the peripheral edge of and without interconnection with said load board at their distal ends, including: a. a flex circuit having electrical traces and distal and proximal ends; b. a portion of said second contacts toward their distal ends overlying and in contact with said flex circuit at its proximal end; c. a connection block having a receiver for the distal end of said flex circuit and being wired to said signal source and bypassing said load board; d. the proximal end of the said flex circuit being electrically and mechanically connected to said connection block; whereby the kelvin contacts are connected to the signal source without passing through said load board.

2. The system of claim 1 wherein said flex circuit and said second contacts are maintained in electrical connection by a retainer plate affixed thereover and applying pressure thereto.

3. A retrofit kelvin test device for testing a device under test (DUT) by temporarily connecting the DUT to a signal source via the test device connected to a load board, the load board having a peripheral edge, said test device not previously configured to accommodate kelvin signals, comprising: a. first and second temporary mechanical and electrical contacts between the DUT having a plurality of terminals, the first electrical contacts engaging the terminals at their proximal end generally extending away from at least one terminal in a first direction and being routed to a load board at their distal ends; for sending and receiving test signals from a signal source; b. said second contacts being kelvin test contacts and being in temporary mechanical and electrical contact with said DUT at their proximal ends and generally extending away from at least one terminal in said first direction, parallel to and spaced apart from said first contacts and being routed to said signal source without interconnection with said load board at their distal ends, including: a. a flex circuit having electrical traces and distal and proximal ends; b. a portion of said second contacts toward said distal ends overlying and in contact with said flex circuit at its proximal end; c. a connection block having a receiver for the distal end of said flex circuit and being wired to said signal source; d. the remaining end of the said flex circuit being electrically and mechanically connected to said connection block; whereby the kelvin contacts are connected to the signal source and bypassing said load board.

4. A method of retrofitting a test system for testing a device under test (DUT), said test system being connected to a load board having a peripheral edge and lacking pads for kelvin contacts on the load board, the test system having non-kelvin and a kelvin test contacts with their proximal ends at the DUT and their distal ends extending away from the DUT, test signals being transmitted to said DUT from a test signal source via said contacts, comprising the steps of: a. aligning said kelvin and non-kelvin contacts in pairs generally parallel to each other and extending the kelvin contacts longitudinally away from the DUT, b. physically and electrically bonding a flex circuit having traces corresponding to kelvin test contact to the kelvin contacts and proximate the distal end thereof, c. compressing overlapping portions of the flex circuit and kelvin contacts together to create an electrical connection and inserting the remaining end of the flex circuit into a contact block and, d. connecting an output of the contact block to a ribbon cable, e. connecting the ribbon cable to a test signal source, thereby retrofitting the test system to include kelvin capabilities.

5. The method of claim 4 wherein said flex circuit and said ribbon cable bypass the load board and have no electrical connection therewith.

6. The method of claim 4 wherein said flex circuit is connected to the test system on an otherwise vacant area of the load board.

7. The retrofit kit of claim 3 wherein said flex circuit and kelvin contacts are electrically connected where they overly each other by a removable retainer plate thereon which applies pressure to the flex circuit and kelvin contacts.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a perspective schematic view of a leaded integrated circuit package and the Kelvin contact system therefore without the load board compliant kelvin capability shown.

(2) FIG. 2 is a side-view cross-sectional drawing of a sample geometry of a sense (voltage) contact in its path from the terminal on the device under test to the contact pad on the load board without the load board compliant kelvin capability shown.

(3) FIG. 3 is a side-view cross-sectional drawing of another sample geometry of a sense (voltage) contact in its path from the terminal on the device under test to the contact pad on the load board without the load board compliant kelvin capability shown.

(4) FIG. 4 is a side plan view of an alternative embodiment with load board compliant capability shown.

(5) FIG. 5 is a view like FIG. 4 with the kelvin circuit extending into a contact block for connection to a load board.

(6) FIG. 6 is an exploded perspective view of a plurality of alternative load board complaint system.

(7) FIG. 7 is a top plan view of FIG. 6.

(8) FIG. 8 is a bottom plan view of FIG. 6.

(9) FIG. 9 is a side plan view of a kelvin structure, with the left side being load board compliant and the right side conforming to FIGS. 1 and 3.

DETAILED DESCRIPTION

(10) A general summary of the disclosure follows.

(11) The terminals of a device under test are temporarily electrically connected to corresponding contact pads on a load board by a series of electrically conductive contacts. The terminals may be pads, balls, wires (leads) or other contact points. Each terminal that undergoes Kelvin testing connects with both a force contact and a sense contact, with each contact electrically connecting to a respective, single contact pad on the load board. The force contact delivers a known amount of current to or from the terminal, and the sense contact measures a voltage at the terminal and draws a negligible amount of current to or from the terminal. The sense contact partially or completely laterally surrounds the force contact, so that it need not have its own resiliency, though it may also be resilient in its own right. This helps keep the force contact in alignment by preventing lateral wobbling. In a first case, the sense contact has a forked end with prongs that extend to opposite sides of the force contact. In a second case, the sense contact completely laterally surrounds the force contact and slides horizontally/laterally to match a horizontal translation component of a horizontal cross-section of the force contact during vertical compression of the force contact. In a third case, the sense contact includes two rods that have ends on opposite sides of the force contact, and extend parallel and laterally away from the force contact. In these cases, the sense contact extends horizontally along a membrane or housing that supports the force contacts. The rods may be housed in respective channels along the membrane or in the housing. While the kelvin feature described above is very effective in improving the throughput and reliability of DUT testing, it is difficult to retrofit existing load boards to include the kelvin feature because the load board must be modified to accommodate the distal end (distant from the DUT) of extensions from the contacts on the board. For purposes of this disclosure, we define a load board which has not been designed with pads for kelvin contacts to be non-compliant or incompatible with Kelvin contacts. In other words, a load board which was not designed initially to work with Kelvin technology and which require modification or replacement to be compatible. Since modification is expensive or impossible without replacement, users may object to the additional cost of modification/replacement of the load board.

(12) Therefore a solution to that problem of noncompliance is set forth herein which routes the kelvin conductors away from the load board to flex circuits or other paths which can interconnect directly to the circuitry behind the load board, or to an unused area on the load board. This makes it possible for an existing (non-kelvin compliant) test system to be retrofit for kelvin capability but without expensive modifications.

(13) In addition to kelvin technology, it may be desirable to monitor other parameters adjacent the DUT, such as heat, humidity etc. Like kelvin circuits, these additional parameters require probes with a path back to a test signal source, via the load board. (A test signal source is defined as a source of power and/or signals transmitted and/or received from the DUT or, kelvin or other sensor probes). The disclosure herein provides an alternative for such probes without using the load board path.

(14) The preceding paragraph is merely a summary of the disclosure, and should not be construed as limiting in any way. The test device is described in much greater detail below.

(15) FIG. 1 shows a leaded device (DUT) 501 with a plurality of leads 502a each having contacts 502. As in the case of pad packages, a force contact 552 makes contact with a lead 502, usually in a central portion thereof. Contact 552 is upwardly biased by element 519. A second bias block 519a is used to apply a downward force on the rocking pin 600. Rocking pin 600 is similar to that shown in U.S. Pat. Nos. 5,069,629 and 7,445,465 and is hereby incorporated by reference.

(16) Contact extensions 544 are formed so that they follow a path to a load board 503, where they make electrical contact. The extension is helpful in reaching available unused areas of the load board layout and facilitates easier trace routing on the load board but ultimately it requires that the kelvin signals be contact to a pad on the load board.

(17) FIG. 2 is side-view cross-sectional drawing of a design 130, showing a sample geometry of a sense (voltage) contact 134 in its path from the terminal 2 on the device under test to the contact pad 4 on the load board 3 which has a plurality of apertures 142 of predetermined gap therein.

(18) The contact 134 extends laterally away from the terminal 2 along a face of the housing 131, bends roughly 90 degrees (orthogonal) to extend through a hole in the housing 131, (portion 134b) and bends (portion 134c) generally equal to or preferably slightly less than 90 degrees to lie roughly parallel to the opposing face of the housing 131 through aperture 142. This generally equal to or preferably less than 90 degree bend provides some bias force to the load board pad 4 assuring a solid connection. When contacting the electrical contact pad 4 on the load board 3, a portion of the contact 134 is longitudinally disposed between the contact pad 4 and the housing 131. Aperture 142 is sized to be greater than the thickness of the portion of the contact passing there through. In the preferred embodiment, the aperture is rectangular or the same shape as the contact passing through, and the gap created between the contact portion 134b and the walls of the aperture should be sufficiently great a turning force (lever action) can be transmitted from the force applied on contact 134c/d by pad 4 (or 2) to contact 134 on pad 2 (or 4). Thus, the gap is wide enough to control the position of the contact through the aperture but still transmit such force. Typically an aperture of twice or three times the thickness of the contact portion will suffice.

(19) In the specific design 130 of FIG. 2, both ends of the contact 134, are bent toward the terminal 2 on the device under test. There are alternatives to this geometry.

(20) For instance, FIG. 3 shows a design 140 similar to design 130, in which the contact 144 extends laterally away from the terminal 2 along a face of the housing 141, bends 90 degrees (portion 134b) to pass through a hole 142 in the housing 141, and bends (portion 134d) roughly equal to or preferably slightly less than 90 degrees to lie roughly parallel to the opposing face of the housing 141. This generally equal to or preferably less than 90 degree bend provides some bias force to the load board pad 4 assuring a solid connection. In contrast with the design 130 of FIG. 2, the design 140 of FIG. 3 has opposite ends of the contact 144 extending in opposite directions, rather than both ends extending toward the terminal 2

(21) In both FIGS. 2 and 3 contacts 134c/d must have a corresponding pad 4 on the load board.

(22) FIGS. 4-9 illustrate embodiments which circumvent this limitation.

(23) FIG. 9 shows a direct comparison between the structure of FIGS. 2-3 and FIGS. 4-8. In FIG. 9, contact 134d requires a contact pad on the load board (not shown) whereas flex circuit 135 does not. FIG. 9 also shows the relative size of decoupling areas 650 and 652. Because of the elimination of space for contact 134d, the void area 650 can be made much larger. This void is a cut away portion between the load board and the alignment plate. This area is needed for decoupling/filter circuits on the load board.

(24) In FIG. 4, the kelvin contacts (sense and force) lead to contact extensions 544 which are either over laid or under laid by a flex circuit 610. In FIG. 4, the flex circuit sits atop the contact extensions. Therefore, the contact extensions are collinearly aligned with exposed trace surfaces on the flex circuit so that a contact trace on the flex circuit abuts an electrode from the kelvin contact. Note that the term flex circuit is intended to be interpreted broadly as any wire or trace on an insulation or substrate.

(25) An alternate embodiment is shown in FIG. 5 where the flex circuit 610 terminates within a contact block 620 which receives the flex circuit line a socket for a plug with the flex circuit being the plug element. The flex circuit may also be reinforced by a rigid or semi-rigid support member 614 or the flex circuit may have solder flow at its distal end to enhance contact in block 620. Connector block 620 may then have a further conductor or flex circuit or ribbon cable 640 which carries the signals to a signal source, or an unoccupied area of the load board. Connector blocks are widely available such as Digi-Key Corporation, Thief River Falls, Minn. 56701 USA.

(26) FIGS. 6, 7 and 8 illustrate the various embodiments at once. The DUT 501 is received within the guide elements 652, 654. In one quadrant, the flex circuit 610 extends directly outwardly to a test signal source 700. In another quadrant, the flex circuit 610 terminates at block 620 and then a second flex circuit or ribbon cable 640 continues to the signal source.

(27) To accommodate block 620, a portion 622 of the retainer is cut away (FIG. 7).

(28) The description of the invention and its applications as set forth herein is illustrative and is not intended to limit the scope of the invention. Variations and modifications of the embodiments disclosed herein are possible and practical alternatives to and equivalents of the various elements of the embodiments would be understood to those of ordinary skill in the art upon study of this patent document. These and other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.