Patent Publication Number: US-2013249566-A1

Title: Kelvin Sense Probe Calibration

Description:
TECHNICAL FIELD 
     This disclosure relates in general to testing electronic components using an automated test system. 
     BACKGROUND 
     Electronic devices of all types, including computing devices, consumer products, telecommunications equipment and automotive electronics, for example, contain electronic components that can be passive or active components. Active electronic components include integrated circuits, multichip packages and semiconductor devices such as transistors and light emitting diodes (LEDs), for example. Passive electronic components include capacitors, resistors, inductors and packages containing multiple components such as multi-layer ceramic capacitors (MLCCs), for example. Both active and passive components require testing before being assembled into electronic devices. Testing can be performed both to insure reliability of the electronic components and to sort the electronic components into groups having similar electronic characteristics. An electronic component being tested is sometimes referred to as a device under test (DUT), and these terms and the term component are used interchangeably herein. 
     BRIEF SUMMARY 
     Disclosed embodiments include methods, apparatuses and systems for calibrating electronic component test systems having a plurality of component carriers. One method includes inserting a first test slug into a first component carrier of the plurality of component carriers and moving the first component carrier with the first test slug into a test position. The first test plug is probed with a Kelvin test probe and a first probe resistance is measured. The method also includes storing a nominal probe resistance set to the first probe resistance, inserting an electronic component in a second component carrier of the plurality of component carriers and moving the second component carrier with the electronic component into the test position. The electronic component is probed with the Kelvin test probe, and an electrical properties of the electronic component is measured using the Kelvin test probe to obtain a measured value. 
     Another aspect of the teachings herein is an apparatus for calibrating an electronic component test system. The apparatus comprises a Kelvin probe, a test station having test electronics, a plurality of component carriers mounted for movement to the test station, the plurality of component carriers including at least a first component carrier and a second component carrier, at least a first test slug, a memory, and a processor configured to execute instructions stored in the memory. The processor can cause the electronic component test system to move the first component carrier holding the first test slug to the test station, probe the first test slug with the Kelvin probe, measure a first probe resistance using the test electronics and the first test slug, store a nominal probe resistance set to the first probe resistance, move the second component carrier holding an electronic component to the test station, probe the electronic component with the Kelvin test probe, and measure an electrical property of the electronic component at the test station using the Kelvin test probe to obtain a measured value. 
     Variations of these embodiments and other embodiments are described hereinafter. For example, in various embodiments, the measured value can be compensated by the nominal probe value and/or the nominal probe value can be updated by processing another test slug. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of an electronic component test system according to disclosed embodiments; 
         FIG. 2  is a schematic diagram of Kelvin probes according to disclosed embodiments; and 
         FIG. 3  is a diagram of a DUT and Kelvin probes according to disclosed embodiments; 
         FIG. 4  is a diagram of a test slug and Kelvin probes according to disclosed embodiments; 
         FIG. 5  is a flowchart of Kelvin probe calibration according to disclosed embodiments; and 
         FIG. 6  is a flowchart of Kelvin probe updating according to disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reliability testing for electronic components can include applying test signals to the components and comparing measured results against predetermined values to decide if the component is good or bad. Sorting electronic components can include applying test signals to the components and using the measured results to determine the performance qualities of the component and thereby determine how the component will be rated and marketed, for example. Both types of testing can use equipment in the form of test systems to handle large volumes of components at high speed without damaging the components while producing accurate test results over long periods of time relative to the amount of time required for testing a single component. For example, testing a single electronic component can take less than one second, while the test system can be expected to run continuously for many hours. As used herein the term signal refers to any electrical or electronic voltage, current, waveform, data, information or electromagnetic radiation supplied or received in any form, including wired or wireless. 
     Both reliability testing and sorting can be performed by probing, which means temporarily applying one or more conductive test leads to conductive areas on the electronic component, sometimes called “pads,” and applying an electronic test signal to the electronic component. The system can then measure electrical properties of the electronic component in response to the test signal. Electrical properties measured with test probes, for example, can include measuring the resistance of the component, which can involve applying a known voltage and measuring the current flowing through the component. Capacitance, for example, can be measured by applying a known voltage and measuring the rate at which current flows into the device. A measurement can also be made in cooperation with other equipment, for example when testing LEDs, a known voltage and current can be applied to the LED and the light output measured by a photometric device. 
     The accuracy of electrical properties measured by a test system can depend upon the accuracy with which the voltage or other signal applied to the component is known. For measurement involving small differences in voltages, for example, the test probe resistance can be a significant part of the total resistance measured. A Kelvin test probe can reduce the effect that test probe resistance has on the measurement, sometimes called parasitic resistance, by applying two probes in close proximity to a single pad. The first probe, called the force probe, carries the test voltage and current to or from the device while the second probe, called the sense probe, measures the applied voltage. In this way the voltage drop across the probes carrying the test current can be minimized, since the sense probe can be part of a much higher impedance circuit that carries less current and therefore suffers less voltage drop. Using Kelvin probes can result in more accurate test results with higher resolution and sensitivity than non-Kelvin probes. 
     According to the teachings herein, measuring Kelvin probe resistance during operation of a test system can permit changes in test values to be tracked over time by monitoring possibly changing parasitic resistance. The resistance can desirably be used for contact verification, probe wear characterization and/or compensation of measured values for a DUT. 
       FIG. 1  is a block diagram of an example electronic component test system  100  according to disclosed embodiments. An example of an electronic component test system that can be adapted to accomplish disclosed embodiments include the ESI Model 3800 manufactured by Electro Scientific Industries, Inc., Portland OR. Electronic component test system  100  includes a track  102  having a plurality of component carriers  104  operating under control of a controller  130 . Track  102  can be arranged as a disk, belt, or any other means of maintaining recirculating or reciprocating motion wherein electronic components can be loaded, tested and unloaded using track  102  or component carriers  104  attached to track  102 . Component carriers  104  can be attached or applied to the track  102  and are operative to receive one or more electronic components  114 ,  118 ,  124 , for example, temporarily hold components  114 ,  118 ,  124  in a pose that permits testing and permits components  114 ,  118 ,  124  to be unloaded while being indexed by track  102 . 
     Component carriers  104  can be indexed in the direction of arrow  106  from a load station  112 , to a test station  122  and to a sort station  116  in an intermittent or continuous fashion under control of controller  130 . Controller  130  can be a computing device having a memory  132 . The term “computing device” includes any device or multiple devices capable of processing information including without limitation: servers, hand-held devices, laptop computers, desktop computers, special purpose computers, and general purpose computers programmed to perform the techniques described herein. Memory  34  can be read only memory (ROM), random access memory (RAM) or any other suitable memory device or combination of devices capable of storing data, including disk drives or removable media such as a CF card, SD card or the like. In one implementation, controller  130  includes a central processing unit (CPU) that performs in accordance with a software program stored in memory  132  to perform the functions described herein. In another implementation, controller  130  includes hardware, such as application-specific integrated circuits (ASICs), microcontrollers or field-programmable gate arrays (FPGAs), programmed to perform some or all of the functions described herein. 
     As shown in  FIG. 1 , test system  100  under control of controller  130  loads electronic components at load station  112  with loader  110 , indexes components to test station  122  and unloads components at sort station  116  using sorter  115 . Indexing refers to a type of start/stop motion wherein track  102  can be stopped to hold component carriers  104  momentarily still at load station  112 , test station  122  or sort station  116  to permit loading, testing or unloading and then can be re-started to move component carriers  104  positions at load station  112 , test station  122  or sort station  116 , where track  102  is again momentarily stopped to permit loading, testing or unloading and then re-started. The indexing movement between stations can optionally be performed in a series of smaller steps. Indexing proceeds continuously at to permit numbers of components to be loaded, tested and unloaded/sorted efficiently and at high speed. Continuous movement is possible in some test systems. At load station  112 , a loader  110  has, for example, a bulk load of electronic components to be individually loaded on to a component carrier  104 . Track  102  is indexed to position an empty component carrier  104  proximate to loader  110  at load station  112 . Loader  110 , under control of controller  130 , loads an electronic component  114  into component carrier  104  at load station  112 . Track  102  indexes a component carrier  104  with a loaded electronic component  124  to test station  122  under control of controller  130 . At test position  122 , a tester  120  can test component  124  by probing with Kelvin probes  126  under control of controller  130 . In this example, probing is accomplished by moving one or more Kelvin probes  126  in the direction of arrow  128  through an opening  108  in track  102  and component carrier  104  to contact component  124 . 
     Tester  120  contains test electronics  134  that can send signals though Kelvin probes  126  to component  124  and can receive signals from component  124  through Kelvin probes  126  to measure electrical properties of component  124 . An example of test electronics  134  is the ESI Model  820  source/measurement unit, manufactured by Electro Scientific Industries, Inc., Portland, OR. The measured electrical properties and other signals generated by additional testing, for example photometric data from electro-optical components, can be sent to controller  130  for further processing or storage in memory  132 . Following testing, tester  120  can retract Kelvin probes  126  to permit track  102  to index the next electronic component to be tested to test station  122 . 
     At sort station  116 , an electronic component  118  can be unloaded from a component carrier  104  using a sorter  115 . Sorter  115  can remove component  118  using, for example, compressed air, vacuum or mechanical means. Sorter  115  can include one or more bins and one or more channels or tubes for conveying component  118  to one of the bins under control of controller  130  depending upon the results of testing of component  118 . Sorting by sorter  115  can include simple “go/no go” sorting where electronic components that have measured electrical properties indicating that they have failed testing are separated from electronic components that have passed testing based on their measured electrical properties. More elaborate sorting schemes where the measured electrical properties of electronic components are separated into multiple bins depending upon values of the measured electrical properties are also possible. 
     Note that although this description describes loading, testing and unloading one component resting in each component carrier  104 , it can be desirable for multiple components to be loaded into each component carrier  104  for subsequent testing and unloading to speed processing. In this case, tester  120  could include a plurality of Kelvin probes  126 . When referring to a Kelvin probe  126  and a measurement of the probe resistance herein, more than one Kelvin probe  126  and more than one corresponding measurement is not excluded. 
     Disclosed embodiments can calibrate and track Kelvin probes  126  by replacing component  124  with a test slug and measuring Kelvin probe resistance with tester  120  and storing the measured Kelvin probe resistance in memory  132 .  FIG. 2  is a diagram showing a Kelvin probe circuit  200  applied to a test slug  208  to measure probe resistance values. A Kelvin probe circuit  200  can include two probes, a force probe  206  to supply the electrical signal to test slug  208  and a sense probe  210  to acquire the electrical signal as it is applied to test slug  208 . Since the sense circuitry to which the sense probe is attached can be a high impedance circuit while the force circuitry to which force probe  206  is attached can be a lower impedance circuit to deliver the current required to test slug  208 , the cumulative effects of the resistances of the sense circuit due to circuitry internal to the test electronics, cabling, connectors and probes can be lower for sense probe  210  than the force circuitry to which force probe  206  is attached, thereby improving the accuracy and sensitivity of measurements performed using sense probe  210 . 
     Kelvin probe circuit  200  as shown has a voltage source  202  that supplies voltage v 1 , which causes current i 1  to flow through Kelvin probe circuit  200  in the direction of an arrow  211 . While voltage source  202  is represented by a battery symbol and hence represents a DC source, any circuit operative to supply a signal that permits testing of test slug  208  can be used. Resistor  204  represents the combined resistance of the force circuit, which can include printed circuit board (PCB) trace resistance, PCB component resistance, connector resistances, cable resistances and the resistance of force probe  206  (collectively, “parasitic resistance”). Current i 1  flows through force probe  206  to the point where it contacts test slug  208 , and then flows through test slug  208  to the point where sense probe  210  contacts test slug  208 . Test slug  208  can be made of very low resistance materials, for example a solid piece of copper. Other configurations of test slug  208  are possible, as long as test slug  208  appears as a low or substantially zero resistance to the test electronics and matches the size, shape and weight of an electronic component closely enough to be able to be held and tested using component carrier  104 . Resistor  212  represents the combined resistance of the sense circuit, which can include PCB resistances, PCB components resistances, connector resistances, cable resistances and the resistance of sense probe  210 . Resistor  216  can have a low resistance, for example 22 Ohms, to permit the majority of current i 1  to flow to ground in the direction of an arrow  214 , since the input at an analog-to-digital converter (ADC)  218  can have a relatively high impedance. ADC  218  can measure voltage v 2 , which indicates the voltage drop caused by resistors  204  and  212 , and thereby determine the probe resistance values of Kelvin probe circuit  200 . Although not shown, a buffer may optionally be coupled to the input of ADC  218 . 
     Once the combined probe resistance values of the Kelvin probes, called nominal contact resistance, is known, any increase in the combined resistance is representative of Kelvin probe wear, since typically only the probe resistance values of resistors  204  and  212  change over time. The nominal contact resistance or nominal resistance R nominal  can be calculated by the formula: 
         R   nominal =( V   1   −V   2 )/ i   1    (1)
 
     Measuring the contact resistance R contact  at points in time after nominal resistance R nominal  is measured can permit an increase from nominal resistance R nominal  to be calculated by the formula: 
         R   contact   =[v   1   −v   2 −( R   nominal   *i   1 )]/ i   1    (2)
 
     Knowing nominal resistance R nominal  and contact resistance R contact  of Kelvin probe circuit  200  can permit electronic component test system  100  (such as through the use of controller  130 ) to compensate for the Kelvin probe resistance and thereby improve the accuracy and sensitivity of DUT measurements. By comparing the measured nominal contact resistance of newly installed Kelvin probes to subsequent contact resistance measures using equation (2), wear on the Kelvin probes can be estimated and tracked, permitting, for example, the Kelvin probes to be replaced, cleaned or otherwise serviced in a timely fashion. 
       FIG. 3  shows a DUT  252  in a component carrier  254  being probed by Kelvin probes  260 ,  262  according to disclosed embodiments. A first contact  256  on DUT  252  can be probed by Kelvin probe  260 , which includes force probe  264  and sense probe  266 , and a second contact  258  can be probed by Kelvin probe  262 , which includes force probe  268  and sense probe  270 . Kelvin probes  260 ,  262  can be moved up and down in the directions indicated by the arrows in order to, for example, move up to probe DUT  252  through holes  272 ,  274  provided in component carrier  254  and then move down to permit component carrier  254  holding DUT  252  to index to a next position. A tester  276  includes test electronics that supply and receive signals from Kelvin probes  260 ,  262  to measure electrical properties of DUT  252 . The up and down motion of Kelvin probes  260 ,  262  can be accomplished mechanically, for example through mechanical linkages that detect component carrier  254  being indexed into position, or electrically, for example through solenoids or voice coils operating in response to detecting component carrier  254  being indexed into position, or a combination of both. 
     In operation, testing an electronic component or DUT  252  can employ two Kelvin probes  260 ,  262 , each having a respective force probe  264 ,  268  and sense probe  266 ,  270  to send and receive signals to DUT  252 , where one Kelvin probe  260  can be used to supply a signal to DUT  252  and one Kelvin probe  262  is used to receive a signal indicating the measurements. Other configurations can use a Kelvin probe  262  to supply the signal and a conventional probe to receive the signal. 
       FIG. 4  shows a test slug  278  held in component carrier  254  being probed by Kelvin proves  260 ,  262  while supplying and receiving test signals from test electronics included in tester  276 . Note that test slug  278  replaces DUT  252  in component carrier  254  and can be probed by Kelvin probes  260 ,  262  without modification to tester  276 , Kelvin probes  260 ,  262  or component carrier  254 . Test slug  278  presents a low or substantially zero resistance to signals from tester  276 . 
       FIG. 5  is a flowchart showing a method of operation  300  for measuring an electronic component with Kelvin probes using an electronic component test system according to disclosed embodiments. Using test system  100  as an example structure to implement steps of method of operation  300 , a test slug  208  can be loaded into a component carrier  104  at step  302  using loader  110  of load station  112 . At step  304 , the loaded test slug  208  can be indexed along track  102  into position at test station  122 . At step  306 , the test slug is probed using Kelvin probes  126 , and the nominal contact resistance of test slug  208  is measured according to, for example, equation (1) in step  308 . At step  310 , the measured nominal contact resistance is stored, for example at memory  132 . At step  312 , an electronic component is loaded into a component carrier  104  at load station  112 . Then, component carrier  104  and its supported DUT are indexed into position at test station  122  at step  314 . At step  316 , the DUT is probed using Kelvin probes  126 , and a resistance value of the DUT is measured at step  318 . At step  320 , the stored probe nominal resistance is read from memory  132 , for example, and the measured resistance of the DUT is adjusted or compensated for the resistance of Kelvin probes  126  at step  322 . This could involve, for example, subtracting R nominal  from the measured resistance value for the DUT. Other compensation techniques are possible. Method of operation  300  continues performing steps  312  through  322  for a number of cycles corresponding to a set of components, generally but not necessarily decided by a user. For example, the set could conform to testing of a single tested component type. As another example, the set could conform to a known performance of the Kelvin probes, such as an average number of cycles to breakdown. In yet another example, the set could form a predetermined number of components based on a defined maintenance protocol. 
     During the processing of steps  314  through  322 , such as between one set of electronic components and the next, the Kelvin probe resistance can be updated.  FIG. 6  is a flowchart showing a method of operation  400  for updating the nominal contact resistance of Kelvin probes using an electronic component test system according to disclosed embodiments. Again using test system  100  as an example structure, method of operation  400  starts at step  402 , where test slug  208  is inserted into a component carrier  104  at load station  112 . At step  404 , component carrier  104  and test slug  208  are indexed to test station  122 . At step  406 , test slug  208  is probed, while its Kelvin probe resistance is measured at step  408 . At step  410 , the nominal contact resistance measure stored in memory  132  is updated to reflect the new resistance measure R contact  using equation (2), for example, or controller  130  can indicate that the contact resistance has deviated from acceptable levels. The updated nominal contact resistance can then be used for resistance compensation for the next set of components in step  322  of  FIG. 5 , for example. 
     The sensitivity of Kelvin probes can permit probe contact verification, where the test system verifies whether or not a probe is actually making contact with the DUT by comparing the output with the Kelvin probe nominal resistance, for example. Measuring Kelvin probe resistance during operation of a test system, such as periodically, can permit changes in test values to be tracked over time by monitoring possibly changing parasitic resistance. This permits probe wear characterization, which can determine how probes are performing and tracks wear over time by measuring probe resistance when contact is made with a DUT. The tracked test values can be used to monitor the expected wear or contamination of Kelvin probe tips to, for example, permit replacement of tips before the wear or contamination is significant enough to present erroneous test results or detect anomalous wear or contamination conditions. 
     Tracking test values can also be used to dynamically adjust test values, wherein compensation by the probe contact resistance can improve the accuracy of the measurement of the DUT. In another example, the tracked test values can be logged for storage by the test system to permit statistical analysis of the performance of the test system, such as test system  100 . 
     Measuring and tracking Kelvin probe resistance can require calibration. As described herein, calibrating a test system using Kelvin probes can include applying the Kelvin probes to a calibration device that has an accurately known resistance and performing a measurement. One type of calibration device is the described test slug, which can be a metallic object, sometimes copper, in the shape of an electronic component that can be assumed to have, for testing purposes, low or substantially zero resistance. Any resistance measured by the test system as a result of testing the test slug can be attributed to the test system itself, including the Kelvin probes. This resistance can be stored by the test system and can be used to compensate and track this resistance when making measurements of DUTs and be used to compensate for the Kelvin probe resistance, thereby making the measurements more accurate and sensitive. 
     Another issue with testing electronic components using Kelvin probes can be maintaining calibration. Electronic test systems can be in use for long periods of time testing many electronic components. As Kelvin probes are used, the resistance of the probes can change, for example due to buildup of material transferred to the tip of the Kelvin probes from the pads of the DUTs. This change in contact resistance can cause the measurements made using the Kelvin probes to change or drift over time and eventually require the Kelvin probes to be replaced. Calibrating Kelvin probes periodically during the testing period as described herein can improve the accuracy of the measurements and thereby improve the accuracy and sensitivity of the testing. 
     Employing a test slug according to disclosed embodiments can permit calibration of test equipment in clean room environments. Calibrating test equipment in clean room environments can be difficult if the calibration requires additional test equipment to be brought into the clean room. Test slugs are inexpensive and small, therefore test slugs representing the types of components to be tested on the test system can be qualified for clean room use at the same time the test system is installed and kept with the test system in the clean room. Therefore, there is no requirement for additional equipment to perform testing during the testing period. 
     Some electronic components can be combined with other components in to electronic assemblies before testing. For example, electronic components can be attached to substrates or interposer devices so that the contacts cannot be accessed directly. In these cases a test slug can be attached to the substrate or interposer device in the same way as the component, thereby permitting calibration of the test system in the same configuration as the component will be in during testing. 
     Disclosed embodiments can permit calibration of electronic component test system while requiring minimal changes to the test equipment by employing a test slug. A test slug is defined as an article manufactured to mimic the size, shape and weight of a DUT while providing substantially zero ohm resistance to the test system. In this way, a test slug can be substituted for a DUT in a test system without requiring any changes in the operation of the test system. The test slug is designed to provide a low or substantially zero ohm resistance when probed by the test system using Kelvin probes in the same manner and at the same position as the contact pads of the DUT. 
     While this disclosure includes certain embodiments, it is to be understood that the disclosure is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.