Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1998-01-27
2001-03-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S755090
Reexamination Certificate
active
06208155
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to testing of integrated circuit (IC) devices, and more particularly to the design of pogo-pins used in socket test probes, contactors and the like for achieving reliable electrical contact with the solder ball contacts of an IC device. The invention has particular application in the testing of or interfacing with ball grid array (BGA) devices wherein electrical contact must be achieved with an array of densely packed solder ball contacts having spherical or hemispherical contact surfaces.
When testing IC devices a reliable electrical interface must be achieved between the test probes and the IC device in order to produce reliable test results. Testing devices typically utilize an array of individual conductive probes called “pogo pins”, which provide a compliant contact array to electrically interface with the I/O contacts of the IC device. Generally, the tips of the pogo pins are designed to achieve efficient electrical contact between the pogo pin tip and the tested or contacted device, and to minimize the introduction of contact resistance in the signal path. However, little or no regard is given to the possible distortive effects of the probe on the I/O contacts themselves.
As IC packaging has become smaller, and I/O densities higher, ball grid arrays (BGAs) have emerged as an alternative IC packaging for integrated circuit devices having extended pin contacts. In BGA devices, the I/O contacts are provided in the form of an array of densely packed solder balls, which permit the device to be surface mounted to a circuit board. In addition to the efficiencies of surface mounting, BGAs permit smaller packaging for higher circuit densities and greater functionality with reliable electrical and thermal performance. On the other hand, with smaller BGA sizes come problems with testing. Specifically, in very small devices, known as micro-BGAs, solder balls are very small and have greatly reduced surface areas. In these devices the deformation of the solder balls caused by the probe contact is relatively large compared to the overall size of the balls, and this can detrimentally affect both the inspection and assembly of the device onto a printed circuit board. For example, excessive deformation of the solder balls can cause devices to be rejected during quality control procedures which may employ laser inspection techniques. Also, surface deformations can lead to problems in the integrity of the solder joints between the micro-BGA and the circuit board when the micro-BGA is mounted to the board.
Heretofore, probes used in the testing of BGA and micro-BGA devices have had probe tips with single or multiple points which dig into and around the apex of the BGA's solder balls. Such probe tips generally leave deep incision marks in the solder ball contacts and also deform the solder balls from their original shape. Furthermore, with such tip designs separation of the solder balls from the probe tips requires a degree of force that is generally undesirable. One reason for this is the relatively deep penetration of the points of the probe tips into the solder balls which causes the solder balls and probe tips to come into contact over a substantial surface area. Also, such probe tips are typically coated or plated with metallurgy, such as gold, that readily fuses or migrates into the soft solder material of a solder ball contact.
The present invention overcomes the problems associated with surface deformation of the solder ball contacts of a BGA device caused when the BGA's solder ball contacts are contacted by external conductive probes. The present invention further overcomes problems associated with the release force necessary to separate the BGA devices from external probes used to make electrical contact with such devices.
SUMMARY OF THE INVENTION
Briefly, the invention involves an improved probe tip for making electrical contact with the solder ball contacts of an IC device and particularly a BGA device. (As used herein, “BGA” is intended to refer to any BGA device, including mini-BGAs and micro-BGAs.) The invention also involves a method for making electrical contact with the circumferential surface of a solder ball in a manner that produces minimum distortion of the solder ball surface. A further aspect of the invention involves a socket employing probes having an improved probe tip in accordance with the invention.
The improved probe tip of the invention generally comprises a base portion and at least two point structures which extend from the tip's base portion. Each of the at least two point structures have a point end lying substantially on a defined perimeter circle which is centered about the center axis of the probe tip and which is sized to exceed the defined diameter of solder ball contact of a BGA or other IC device to be contacted by the probe tip. Each point structure provides an interior edge or edge contact facing the center axis of the probe tip and each of these interior edges provides a medial contact zone which is angled relative to the center axis for tangentially contacting the solder ball of the IC device below the apex of the solder ball. By contacting a solder ball below its apex with angled interior edge contacts rather than by the point contacts, and by limiting the medial contact zone of the interior edges which come into contact with the solder ball, distortion of the solder ball surface can be minimized. Further, any distortion that does occur occurs away from the apex of the solder ball where the solder ball first contacts the pad of a mother board to which the IC device is mounted. With minimal disturbance of the solder ball of the IC, the planarity of a solder ball array will be maintained, thereby decreasing the likelihood of a poor electrical connection at any solder ball joint.
Preferably, the solder ball contacts the interior edges of the probe tip point structures approximately 45 degrees below the apex of the solder ball. If the solder ball is contacted substantially above this angle, that is, closer to the apex, excessive distortion of the solder ball can result and will occur in a region of the ball having the greatest effect on solder ball planarity. Conversely, inefficient electrical contact with the solder ball is likely to result if contact is made too far down on the solder ball, that is, on surfaces near the solder ball's lateral circumference. To produce an optimum tangential contact, the angle of the medial contact zone of the interior edges of the point structures is preferably within a range of approximately 40-50 degrees relative to the center axis of the probe tip, and optimally, approximately 45 degrees. Further, to limit the medial contact zone to a portion of the overall interior edge of each point structure, each of the interior edges is provided with a receding portion at at least one of its extremities so as to limit the extent of the contact between the solder ball and the interior edge's medial contact zone. In one aspect of the invention, such receding area is provided in the form of a radius along the point end extremity of the point structure's interior edge, and in another aspect of the invention, such recessed area is provided in the form of a central cavity in the base portion of the probe tip into which the interior edge of the point structures terminate.
In the illustrated embodiment of the invention, the probe tip is shown as having four point structures for providing four interior edges which contact the solder ball at 90 degree intervals. It will be understood, however, that probe tips in accordance with the invention can be fabricated having different point structure counts, provided there are at least two point structures. Preferably, the point structures would have an equal angular spacing between them so as to provide a balanced structure which contacts a solder ball at edge contacts of equal angular spacings around the solder ball.
In accordance with the invention, the improved probe tip can additionally be
Barabi Nasser
Jonaidi Siamak
Beeson Donald L.
Cerprobe Corporation
Metjahic Safet
Nguyen Vincent Q.
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