Abstract:
A surrounding structure of a probe card is described, and particularly an isolator unit, which is formed by filling in a wire leading region with epoxy material to substitute for an originally clear portion, so as to improve high impedance characteristics of the cross-over portion by using the resultant parasitic capacitance effect, and improve the compensation for the purpose of impedance matching. Additionally, the energy loss is reduced and the application frequency range is widened.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a surrounding structure of a probe card, and more particularly to an isolator unit, which is formed by filling in a wire leading region with epoxy material to substitute for an originally clear portion, so as to improve high impedance characteristics of the cross-over portion by using the resultant parasitic capacitance effect, and improve the compensation for the purpose of impedance matching. 
   2. Description of the Prior Art 
   After the completion of wafer manufacturing, a wafer test is required to guarantee the functions of the products. The wafer test is performed by using a test device and a probe card to test each die on a wafer, in order to confirm that the electrical characteristics and performances of the die are in accordance with the design specifications. As chip function becomes more sophisticated, the high-speed and accurate test requirement is more significant. 
   In the circumstance, a probe card is used for the wafer test of semiconductor integrated circuits, which is very critical especially in the test stage of RF (radio frequency) wafer-level mass production. The application is to perform functional tests on bare die by the probe before packaging of integrated circuits, to screen out the defectives and then proceed to further packaging fabrication. 
   The test process begins with locating a die on a wafer on the test device. The probe card is mounted on the test device, so as to have the contact pads on the die aligned and touched with the probe. 
   Reference is made to  FIGS. 1 and 2 , which are respectively top and cross-sectional views of a commercially available epoxy probe card. A probe card  9  comprises a circuit board  92 , and hundreds of probes  90 . The circuit board  92  has an isolation locking ring  94  which is extending along the lower surface thereof. The probes  90  are fixed on the isolation locking ring  94  of the probe card  9  by means of epoxy element  96 . The probes  90  are electrically connected to the circuit board  92 , and electrically connected individually to the metal pads  84  on the die  82  of a wafer. The probes  90  are used to contact the pads  84  on the die  82 , so as to directly input signals or read the output values to or from the die  82 . 
   During the one-by-one detection of wafer testing, if any die  82  does not pass the test, it is marked as defective. Consequently, during chip dicing and separation, these dies which are marked as defective are screened out and excluded from further packaging fabrication. As a result of the wafer test, the dies that pass the test proceed to the next stage of packaging fabrication. 
   However, in the design of a conventional epoxy probe card  9 , since the wiring portion is far away from the ground wire  93  (distributed over the surface of the circuit board  92 ), the loop inductance produced becomes large and the problem of impedance mismatching becomes worse. Meanwhile, the dielectric medium  98  between the signal wire  91  and the ground wire  93  is air, the electrical field dissipates in the air, and thus power consumption becomes large when the frequency is increased. 
   Reference is made to  FIG. 3 , which is a frequency response graph of a simulation test for the conventional epoxy probe card;  FIG. 3A  is a frequency response comparison table of S 11  and S 21  against 3 dB. The simulation test makes comparison between two cases; Case A is a case where the signal wire is far away from the ground wire and Case B is a case where the signal wire is near the ground wire. S 11  illustrates the fact that test signals are rebounded due to impedance mismatching, and a value thereof is closer to minus infinite dB is better. S 21  illustrates the fact that all testing signals completely pass without any loss, and a value thereof is closer to 0 dB is better. The values are obtained in dB corresponding to the frequency response S 11  and S 21  of the simulation test to give the graph of  FIG. 3 . From the graph, it can be found that once the leading wire crosses over that wire, the corresponding return loss decreases with the increase of the frequency, thus resulting in serious signal reflection. In a high frequency band, the corresponding insertion loss will become large; the frequency width corresponding to 3 dB is only about 560–720 MHz. With this structure, under high frequency, the signal loss is quite serious, such that no energy is delivered. In particular, as the speeds of electrical products are increasingly refreshed and there is increased need for high speed, this condition more urgently requires improvement. 
   Accordingly, there is a need to improve the above inconvenience and disadvantage of the conventional probe card in the actual application. 
   Accordingly, this invention is provided to improve the above disadvantages with a reasonable design. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a top view of a conventional epoxy probe card; 
       FIG. 2  is a cross-sectional view of the conventional epoxy probe card; 
       FIG. 3  is a frequency response graph of the conventional epoxy probe card; 
       FIG. 3A  is the frequency response comparison table of S 11  and S 21  against 3 dB according to  FIG. 3 ; 
       FIG. 4  is a top view of a probe card according to the present invention; 
       FIG. 5  is a cross-sectional view of the probe card according to the present invention; 
       FIG. 6  is a simulation frequency response graph showing the cross-over curves S 11  and S 21  of the invention and the conventional structure; and 
       FIG. 6A  is the frequency response comparison table of S 11  and S 21  against 3 dB according to  FIG. 6 . 
   

   SUMMARY OF THE INVENTION 
   The present invention provides a surrounding structure for a probe card which improving the assembling way of epoxy probe card for enhancing the characteristics of signals passing this test interface, such as the electric characteristics and the impedance matching. 
   The present invention provides a surrounding structure for a probe card having a circuit board, an isolation locking ring, a plurality of signal wires, a plurality of probes, and a dielectric medium. The isolation locking ring is extending downward along a lower surface of the circuit board. The signal wires are electrically connected to the circuit board. The probes are electrically connected individually to corresponding signal wires. The dielectric medium is fixed on the isolation ring of the probe and surrounding the signal wires and a predetermined portion of the lower surface of the circuit board. 
   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Reference is made to both of  FIGS. 4 and 5 , top and cross-sectional views, respectively, of one embodiment of a probe card according to the present invention. The probe card  1  comprises a circuit board  12 , an isolation locking ring  14  which is extending downward along the lower surface of the edge of the circuit board  12 , and a plurality of probes  10  which are electrically connected to the circuit board  12 , and a dielectric medium  16  which is fixing the probes  10  on the isolation locking ring  14 . The circuit board  12  has ground wires  13  and signal contacts  15  on a lower surface thereof. 
   In order to improve the connecting mode of the epoxy probe card, the connection path from the test device to the die  82  to be tested begins with the signal route of the probe card  1 , and then goes through the connecting wire (generally made of wolfram or wolfram alloy), i.e., signal wire  11 , and finally arrives at the die  82 . The structure can be divided into two parts: one is the circuit board  12  of the probe card  1 , and the other is the portion of the signal wire  11 . In the structure portion of the circuit board  12  of the probe card  1 , impedance control and high-speed and high-frequency signal transmission can be achieved with an appropriate lamination and ground/power layer design. But in the portion of the signal wire  11 , the signal contacts  15  of the probe card  1  extends to the contact pad  84  of the die  82 . For the signal quality of this portion, it is required that the impedance matching should be designed well to alleviate the signal reflection between both ends due to the problem of impedance mismatching, thereby avoiding the influence on signal transmission. 
   According to the above analysis, this invention is characterized by improving the surrounding structure of the probe card, and particularly providing the dielectric medium  16  which is fixing a large portion of the signal wires  11  of the probe  10 , and covering the isolation locking ring  14  and a ground wire portion  13  of the circuit board  12 . In place of air used as the dielectric medium in the conventional probe card, the dielectric medium  16  with a higher dielectric constant than that of the air (for example, epoxy resin) is used, thereby avoiding the electrical field dissipating in the air and making it possible that the loss does not increase as the frequency is rising. Therefore, when the signal wire crosses over the area without wires, the resultant impedance mismatching condition can be compensated by the dielectric medium surrounding the signal wire and ground wire. The dielectric medium can be an epoxy or a similar isolation material. Thus the parasitic capacitance effect of the portion is applied to improve the high impedance effect of the cross-over portion, thereby achieving impedance matching and alleviating the energy dissipation to the air. 
   The electrical field dissipation to the air can be prevented effectively, as long as the dielectric medium  16  according to the invention covers a slightly larger area than that of the conventional structure coverage. For example, covering a portion of the circuit board  12  partially or wholly. When the dielectric medium  16  partially covers the circuit board  12 , the probes  10  can still be replaced since the signal contacts  15  are not surrounded. As a result, the coverage area can either extend from the lower surface of the isolation locking ring  14  to the end of the signal wire  11 , i.e., in front of the signal contacts  15 , or extend to cover the signal contacts  15 . 
   Reference is made to  FIG. 6 , which is a simulation frequency response graph showing the cross-over curves S 11  and S 21  of the invention and the conventional structure.  FIG. 6A  is the data comparison table according to  FIG. 6 . This invention carries out the improvement of the probe based on the structure of  FIGS. 4 and 5 . The impedance is calculated by the equivalent approximate formula (L/C) 1/2 , where L is the equivalent inductance and C is the equivalent capacitance; therefore to reduce the impedance, it is necessary to reduce L value and increase C value. The dielectric constants of the air and epoxy are 1 and about 3.5, respectively; this significantly increases the value of C. 
   This invention is characterized by use of an isolation material with a high dielectric constant, for example epoxy resin, as a dielectric medium to surround the signal wire and ground wire, to enhance an useful capacitance effect. With the parasitic capacitance effect, this facilitates cross over of its own high impedance to achieve the purpose of impedance matching. Additionally, with epoxy resin or a similar isolation material surrounding the signal wire and ground wire, the distribution of the electrical field is suppressed and prevented from energy loss due to dissipation in the air. 
   According to this invention, the dielectric medium with a higher dielectric constant is used in place of air as the dielectric medium of the conventional probe card, thereby avoiding the electrical field dissipating in the air and making it possible that the loss does not increase as the frequency is rising. In this way, the return loss is improved, the condition of impedance mismatching is alleviated, the insertion loss is reduced, and the frequency bandwidth corresponding to 3 dB is widened to have an effective application frequency bandwidth thereof doubled, so as to facilitate the transmission of high-frequency and high-speed signals. 
   This invention thus has the following advantages: 
   1. The return loss during signals passing through the wires is improved, thus alleviating the problem of impedance mismatching. 
   2. The insertion loss of high-frequency is reduced and the effective frequency bandwidth corresponding to 3 dB is widened, thereby enabling the high-frequency signal to be transmitted entirely and reducing the energy loss. 
   3. The application range of test frequencies of the epoxy resin probe card is widened and the quality of the transmitted signal is improved. 
   In conclusion, this invention fully meets the requirements for the patent application. Therefore, The application is proposed according to the patent law. While the preferred embodiments of this invention have disclosed above, they are not indented to limit the scope of this invention. Therefore the appended claims cover all such changes or modifications as fall within the spirit and scope of this present invention.