Patent Publication Number: US-9891256-B2

Title: Determining the current return path integrity in an electric device connected or connectable to a further device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Ser. No. 15/088,258, entitled DETERMINING THE CURRENT RETURN PATH INTEGRITY IN AN ELECTRIC DEVICE CONNECTED OR CONNECTABLE TO A FURTHER DEVICE” filed on Apr. 1, 2016; which is a continuation of U.S. Ser. No. 14/796,595, filed on Jul. 10, 2015 and issued as U.S. Pat. No. 9,304,158 on Apr. 5, 2016; which is a continuation of U.S. Ser. No. 13/565,159, filed Aug. 2, 2012 and issued as U.S. Pat. No. 9,134,364 on Sep. 15, 2015; which is a continuation of U.S. Ser. No. 12/523,119, filed Jul. 14, 2009 and issued as U.S. Pat. No. 8,248,082 on Aug. 21, 2012; which is a U.S. National Phase Application of PCT/EP2007/062542, filed on Nov. 20, 2007 and published in English on Jul. 24, 2008 as WO 2008/086908 and claims priority of EP Application No. 07100678.7 filed on Jan. 17, 2007, each of which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to a method for determining the current return path integrity in an electric device with a plurality of signal lines and supply lines, wherein said electric device is connected or connectable to a further device. In particular, the invention relates to a method for determining the current return path integrity in a connector receptacle with a plurality of signal lines and supply lines, wherein said receptacle is connected or connectable to a printed circuit board. Further the present invention relates to a test device for performing the above method. 
     An electric device with a plurality of signal lines and supply lines is usually connected to a further electric device. For example, said electric device may be a connector receptacle and said further electric device may be a printed circuit board. The connector receptacle is connected to the printed circuit board by a plurality of soldering joints. There are at least two types of supply lines, for example VDD lines and GND (ground) lines, where VDD stands for the positive supply voltage of an integrated circuit. If a circuit or device on the printed circuit board requires several voltages, then there are a corresponding number of different voltage lines. For example, an integrated circuit can require an additional negative voltage supply line VSS. 
     The supply lines, in particular the VDD and GND lines are provided for the power distribution in a printed circuit board. The supply lines of the same type are interconnected together and are therefore redundant supply lines. Each redundant supply line of the connector assembly is connected to a corresponding supply trace on the printed circuit board by a soldering joint. If there is an opening in said soldering joint, then the connection is maintained via the other supply lines of the same type. If a direct current is applied to the supply lines, then the single opening has no influence to the electric properties of the supply lines. 
     However, if an alternating current with a high frequency is applied to the supply lines or the signal lines, then the opening in the soldering joint causes a substantial change of the electric properties of the whole electric device. The supply lines, e.g. the VDD and GND lines are coupled capacitive and inductive to the signal lines. The supply lines are utilized as high frequency signal return paths. Losing one or more of those signal return paths causes additional impedance mismatches and an increased signal-to-signal coupling, which may be seriously impact high-speed system performance and reliability. 
     Therefore it is important to test high-speed signal paths after assembly as accurately as possible. In selected cases this can be done on a test bench. However, there is a serious problem, if an online assembly inspection of large connectors with more than 1000 signal connectors has to be done. 
     Known electric online testing methods allow the detection of signal shorts and openings as well as inter-power shorts. In the case of redundant VDD and GND connections it is very difficult to detect one or more openings by an AC (Alternating Current) and/or DC (Direct Current) resistance measurements. Additionally the location of the malicious signal return path opening has to be known to identify the affected signal traces. 
     Further an optical inspection, e.g. by X-ray, is manually possible in some cases. However, said optical inspection is very difficult and time consuming. The optical inspection also depends on human factors. Especially large and complex structured connector assemblies with thousands of signals require online testing methods during the manufacturing process. 
     There is no known method for an automatic testing to electrically localize redundant VDD and/or GND openings. Therefore the test coverage of high-speed links and interfaces is significantly exposed. This leads to data integrity and functionality problems during initial system test and thereby to increased hardware cost and system test delay. 
     BRIEF SUMMARY 
     It is an object of the present invention to provide an effective method for determining the current return path integrity in an electric device with a plurality of signal lines and supply lines. It is further an object of the present invention to provide a test device for performing said method. 
     The above object is achieved by a method as laid out in the independent claims. Further advantageous embodiments of the present invention are described in the dependent claims and are taught in the description below. 
     One idea of the invention is the use of the near end crosstalk signal to localize a defect in a redundant supply line, e.g. a redundant VDD or GND line. An input signal is applied to a signal line and the near end crosstalk signal on at least one neighboring signal line is detected. If there is an opening in a redundant supply line in the neighborhood of those signal lines, then there is a deviation between the actual near end crosstalk signal and a reference signal pattern. 
     The reference signal pattern has been taken from a library with reference signal patterns for a plurality of signal line pairs. The reference signal patterns have been determined on an efficient reference device of the same type, which has been positively tested before by another method. 
     The near cross talk signal is detected within a time window corresponding to the length of the tested signal lines in the electric device, in particular in the connector receptacle. Thus only defects in or on the connector receptacle are detected. The electric device may be tested independently of the properties of the further device. The connector receptacle may be tested independently of the properties of the printed circuit board. 
     The deviation between the actual near end crosstalk signal and the reference signal pattern indicates that there is a defect. The properties of said deviation indicate the kind and the localization of the defect. Return path openings cause increased near end crosstalk signals on the neighboring signal lines. The nature of the time domain of the near end crosstalk signal allows the precise determination of the coupling location. 
     The unplugged connector allows an access to each signal pin. The method may be performed automatically by a test device. The distance to the soldering joints on the printed circuit board to be checked is in the range of some 10 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The above as well as additional objectives, features and advantages of the present invention become apparent in the following detailed written description. 
       The novel and inventive features believed to be the characteristics of the invention are set forth in the appended claims. The invention itself and its advantages are best understood by reference to the following detailed description of preferred embodiments in conjunction with the accompanied drawings, wherein: 
         FIG. 1  illustrates a schematic diagram of a connector receptacle on a printed circuit board provided for a method according to a preferred embodiment of the present invention, and 
         FIG. 2  illustrates schematically an output signal detected by a method according to a preferred embodiment of the present invention and compared by a reference signal pattern. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a schematic diagram of a connector receptacle  10  on a printed circuit board  40 . A method according to a preferred embodiment of the present invention may be applied to the connector receptacle  10  on the printed circuit board  40 . 
     The connector receptacle  10  includes a plurality of pins. A first pin  12  and a second pin  16  as well as a redundant GND pin  14  are explicitly shown in  FIG. 1 . The first pin  12  and the second pin  16  are provided to couple the connector receptacle  10  to a cable or the like. The pins are arranged on the same side of the connector receptacle  10 . Inside the connector receptacle  10  there is a plurality of signal and supply lines. A first signal line  22 , a second signal line  26  and a GND line  24  are explicitly shown in  FIG. 1 . For example, the connector receptacle  10  is built up by a plurality of identical wafers. 
     The connector receptacle  10  is connected to the printed circuit board  40  via a plurality of soldering joints. A first soldering joint  32 , a redundant GND soldering joint  34  and a second soldering joint  36  are explicitly shown in  FIG. 1 . The soldering joints  32 ,  34  and  36  may be provided as SMT (Surface-Mounting Technology) pads, for example. The first signal line  22  connects the first pin  12  to the first soldering joint  32 . The second signal line  26  connects the second pin  16  to the second soldering joint  36 . The GND signal line  24  connects the redundant GND pin  14  to the redundant GND soldering joint  34 . 
     The printed circuit board  40  comprises a plurality of signal and supply traces. A first signal trace  42 , a redundant GND trace  44  and a second signal trace  46  are explicitly shown in  FIG. 1 . The first signal trace  42 , the redundant GND trace  44  and the second signal trace  46  may be provided as conductor paths, for example. The first signal trace  42  is connected to the first soldering joint  32 . The redundant GND trace  44  is connected to the redundant GND soldering joint  34 . The second signal trace  46  is connected to the second soldering joint  36 . 
     The connector receptacle  10  may include a very large number of pins  12 ,  14  and  16 . There are known connector assemblies with more than 1000 pins, for example. Thus the connector receptacle  10  includes also said large number of signal lines  22  and  26  and supply lines  24 . Further the printed circuit board  40  comprises such a large number of soldering joints  32 ,  34  and  36 , signal traces  42  and  46  and supply traces  44 . The printed circuit board  40  may be a part of a high-end server, for example. 
     The method according to the present invention allows for checking of the quality of the signals transferred by the signal lines  22  and  26  in the connector receptacle  10 . In particular, said method allows identifying any openings on the neighboring supply lines  24  within the connector receptacle  10  and on the soldering joint  34 . 
     An input signal  18  is applied to the first pin  12  of the connector receptacle  10 . The input signal  18  is provided as a voltage step, in particular a fast voltage step. The input signal  18  propagates from the first pin  12  through the connector receptacle  10  via the first signal line  22  to the first soldering joint  32 . The first signal line  22  is coupled capacitive and inductively to the supply line  24  and to the second signal line  26 . The coupling strength depends on the geometric relations between the signal lines  22  and  26  and the supply line  24 . 
     The coupling of the signal lines  22  and  26  and the supply line  24  result in far end crosstalk signals and near end crosstalk signals. The far end crosstalk signal is the superposition of all partial coupling contributions along the signal paths. The pattern of the far end crosstalk signal always consists of one single voltage peak. The size and the sign of said voltage peak depend on the inductive and capacitive coupling balance. The near end crosstalk signal contains those partial contributions timely separated. Thus the amount of each single partial coupling contribution can be separated and quantified. If there is an opening in the redundant GND line  24  or in the soldering joint  34 , then the near end crosstalk signal is changed. 
     Separating the redundant GND line  24  and the redundant GND trace  44  by removing the solder joint  34  results in an opening in the redundant GND path. This opening causes a changed output signal  20 . This output signal  20  is a near end crosstalk signal, which is at least partially higher than a corresponding reference signal pattern. For example, the output signal  20  increases from 1 mV to 3 mV due to the opening in the solder joint  34 . 
     The output signal  20  is compared with a reference signal pattern of a reference output signal. The reference signal pattern has been detected with an efficient reference device, i.e. a connector receptacle of the same type. The reference device has been tested before by another method. The reference signal patterns of said reference device has been stored in a library. 
     Each deviation between the output signal  20  and the reference output signal corresponds with an opening in a neighboring signal path. In one preferred embodiment the input signal  18 , the output signal  20  and the reference output signal are the corresponding voltages as a function of the time. 
       FIG. 2  illustrates schematically the output signal  20  and a reference signal pattern  50  from a method according to a preferred embodiment of the present invention. The output signal  20  as well as the reference signal pattern  50  shows the voltage as a function of the time. For example, the output signal  20  and the reference signal pattern  50  are represented by numerical values. The voltages and the corresponding times may be ordered as pairs in a table. 
     The time window has typically a width of about some 100 ps. The length of the signal lines  22  and  26  are known. Further the propagation velocity of the signals  18  and  20  are also known. With this information the opening in the GND line  24  may be localized by the time at which the deviation between the output signal  20  and the reference signal pattern  50  occurs. 
     Since the time window of the output signal  20  is limited to a predetermined range of time, which corresponds with the length of the signal lines  22  and  26 , the connector receptacle  10  may be tested independent of the electric properties of the printed circuit board  40  or any further electric device coupled to said connector receptacle  10 . Within this time window only such deviations may be detected, which occur from an opening in a supply line  26  in the neighborhood of the signal lines  22  and  26 . 
     Due to the direct relationship between time and frequency domain all said analysis of signal  20  and comparison to reference signal  50  can also be applied in the frequency domain. The relationship between time and frequency domain is illustrated by Fourier transformation, for example. 
     A controllable test device is provided for performing the method according to the present invention. The test device allows an automatic execution of the above method. The test device comprises a test head, which may be connected to the connector receptacle  10 . The test head sequentially moves along all pins  12 ,  14  and  16  of the connector receptacle  10 . The test device initiates one or more voltage steps  18  on each pin  12 ,  14  and  16  and records the resulting near end crosstalk signals of the next neighboring lines. 
     Such a test device has a resolution of about 100 μV. If the deviation between the output signal  20  and the reference output signal is greater than a predetermined value, then the test device displays that the test was not successful. If the deviation between the output signal  20  and the reference output signal is smaller than said predetermined value, then the test device displays that the test was successful. For example, these results may be displayed by a red and green LED, respectively. 
     A simple prototype implementation of the test device uses a Tektronix CSA 8000 communication signal analyzer device. The CSA 8000 allows using a TDR (Time Domain Reflectometry) test head. This TDR test head is controlled by a robot arm device. A PC (Personal Computer) controls the CSA 8000 and the robot arm. The library of reference signal patterns is maintained by a computer program executed on the PC, which is performing a method in accordance with the present invention. The computer program displays the test results on a computer monitor connected to the PC. 
     In manufacturing lines specially designed industry robots could be used instead. In order to reduce the test time, the testing can be parallelized by using multiple test heads. For example, industry robots can have multiple robot arms in order to control multiple test heads at once. 
     A modification of the inventive method would be this method with an extended time window in order to detect openings on the printed circuit board  40  or on another electric device coupled to the connector receptacle  10 . 
     Further, the inventive method may be provided for performing a simulation of the behavior of the electric crosstalk signal. 
     The inventive method can be combined with other testing methods. In particular, the inventive method may be combined with a de-embedding method. This allows the test of assemblies with long wires. A de-embedding method is described in the “TDR Primer”, application note published in Printed Circuit Design Magazine, April 2002. This application note contains several further references to publications on de-embedding methods. 
     The method according to the present invention may be used to test cable connections. Further the inventive method may be used to test the conductor paths and/or the other connections on printed circuit boards. 
     Moreover the inventive method may be used to test arbitrary soldering joints. Especially soldering joint in the range of some μm can be tested. 
     In general, the inventive method can be used to test the quality of signals in arbitrary electric devices with several lines. 
     The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein. Further, when loaded in a computer system, said computer program product is able to carry out these methods. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               10  connector receptacle 
               12  first pin 
               14  redundant GND pin 
               16  second pin 
               18  input signal 
               20  output signal, near end crosstalk signal 
               22  first signal line 
               24  redundant GND line 
               26  second signal line 
               32  first soldering joint 
               34  redundant GND soldering joint 
               36  second soldering joint 
               40  printed circuit board 
               42  first signal trace 
               44  redundant GND trace 
               46  second signal trace 
               50  reference signal pattern