Abstract:
A method for testing integrated circuit devices and loading such devices into a test board for further testing and an apparatus therefor is disclosed. The method allows for selection between two modes of operation. In a first mode, the integrated circuit devices are subjected to an electrical test before being placed into the test board for further testing. In a second mode, the integrated circuit devices are tested after being placed in the test board. The apparatus allows for the selection between the first mode and the second mode. In either mode, information about the tested devices and the sockets in the test board is used to load the test boards intelligently. Intelligent loading means that devices under test (DUTs) are not placed in bad sockets and devices that do test bad are removed from the test board, with an option of replacing the failed DUT with another DUT before subsequent environmental testing of the DUTs in the test board is carried out.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to an apparatus and method for testing semiconductor devices. More particularly, the present invention relates to a test apparatus for packaged semiconductor devices that utilizes a pick-and-place mechanism to transfer the devices between and among various locations in the apparatus. 
   Integrated circuit (IC) devices are subjected to a variety of tests after fabrication. These tests occur before the IC is packaged and after the IC is packaged. The tests are designed to determine if the IC will meet performance and lifetime specifications. 
   Apparatus with many different configurations are used to test IC devices. In many apparatus, such as the apparatus described in U.S. Pat. No. 6,323,666 to Ohba et al., the IC&#39;s are loaded into some sort of test board for environmental testing. Typically, the apparatus performs some initial electrical tests on the IC before subjecting the device to the environmental testing. That way, the more expensive, time consuming environmental test is only performed on ICs that pass the initial test. Other apparatus that describe apparatus the perform some electrical testing of ICs prior to environmental testing are described in U.S. Pat. No. 4,902,969 to Gussman and U.S. Pat. No. 6,563,331 to Maeng. 
   Environmental testing is typically referred to as a burn-in test. The test is described as burn-in because it is done at an elevated temperature. Burn-in typically involves placing a large number of integrated circuit (IC) devices on printed circuit boards, referred to herein as test boards. The boards are placed in a chamber in which the environmental conditions, particularly temperature, are controlled. The IC devices are then subjected to electrical tests such as the application of DC current to forward and reverse bias the individual junctions in the IC or actively clocking the ICs to their maximum rated conditions. Running these tests at elevated temperature identifies ICs that do not perform according to their minimum specifications. 
   There are two major objectives associated with such testing. The first and foremost objective is to ensure that ICs that fail or are likely to fail are discovered and kept from being used (at least in the application for which they were identified as likely to fail). The failed ICs, once identified, might be recycled, repaired, retested, etc. The second equally important objective is that the good ICs are not falsely identified as bad ICs. Such misclassification has a number of downsides. First, it wastes an otherwise good IC by preventing it from being used for its intended purpose. Second, a series of false failures can give the impression of an artificially high fail rate. This could lead to an unnecessary, expensive, and time-consuming search for the source or sources of the fail rate. 
   Many apparatus have been proposed to more accurately identify failed ICs and to ensure that the ICs are not improperly identified as failures due to some defect or malfunction in the apparatus itself. One such approach is described in U.S. Pat. No. 6,323,666 to Ohba et al. With reference to  FIG. 1 , a test and burn-in system handler  10  is illustrated schematically. An electronic switch  4  is provided to switch between test signals of the IC test circuit  2  and the test signals of the burn-in board checker  3 . The IC test circuit  2 , the burn-in board checker  3  and the electrical switch  4  are made up as a unit. 
   The IC test circuit  2  is used to perform a pretest of the IC&#39;s  1 A as the devices under test. The burn-in board checker  3  is for testing the burn-in board  1  to detect pattern disconnection, solder failure, short circuits or other defects. The alignment stage  6  is used to straighten the attitude of the IC&#39;s  1 A. 
   The handler  10  is operated in the following manner. The carrier rack  8  has multiple burn-in boards  1 , which, when loaded into the handler  10 , do not contain IC devices. The burn-in boards  1 , are inserted sequentially and each burn-in board,  1 , is tested by the burn-in board checker  3  to determine if the burn-in board  1  contains any bad IC sockets. The burn-in board waits in this position to receive ICs  1 A. 
   The ICs are transferred one at a time from the tray  5  to the alignment stage  6 . After the attitudes of the ICs are straightened at the alignment stage  6 , the ICs  1 A are populated into the burn-in board  1 . If the burn-in board has a defective socket, the loading software is instructed not to populate that socket with an IC. 
   After the burn-in board is loaded, the switch  4  is deployed to activate the IC test circuit  2 . Simplified functional tests are performed on the ICs populating the burn-in board  1 . After the electrical pretest, the ICs that are determined to be defective are removed from the burn-in board  1  while the devices that passed the pre-test remain. Once all of the normal sockets in the burn-in board  1  are loaded with ICs that were determined to be non-defective, the burn-in board is returned to the carrier rack  8 . Once all of burn-in boards  1  in the carrier rack  8  are filled, the carrier rack is transferred to the burn-in apparatus. 
   While the apparatus described above achieves some efficiency and accuracy by testing individual sockets in burn-in boards before loading ICs therein, greater efficiency and flexibility for such test apparatus are sought while still ensuring the IC failures are properly attributed to the IC device, and not actually the result of a bad burn-in board socket or other extraneous reason. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention provides an automated test handler system for testing integrated circuit devices prior to subjecting those devices to an environmental test. The test handler offers at least two modes of operation. In a first mode, individual integrated circuit (IC) devices under test (DUTs hereinafter) are electrically tested before they are placed in a test board. In a second mode, DUTs are electrically tested while in the test board. A test of DUTs when they are in the test board is referred to as a parallel test hereinafter. In yet a third mode, the DUTs are individually tested prior to being placed in the test board and are also subjected to a parallel test. 
   The at least two modes of operation provide certain advantages over prior art methods and apparatus. In operation of the test handler system, according to this aspect of the present invention, the user can select between a mode where a high failure rate is expected (e.g. when new or prototype devices are being tested, the number of hard-failure devices is likely to be higher) and a mode where a lower failure rate is expected (e.g. devices that have been manufactured for some time). In the first mode, an individual electrical test is performed on DUTs before they are placed in the test board. This ensures that, if the DUTs do experience a hard-failure, the test of other DUTs will not be otherwise adversely affected (hard failure can pull down signal lines and stop test execution). Also, the individual electrical test can be performed if the test parameters require very tight tolerances or exacting test standards. As previously noted, in certain embodiments, the DUT will be subjected to both an individual electrical test and a parallel test. 
   The test handler system of the present invention provides an advantageous flexibility in the testing of ICs. By providing two test stations, a parallel test station and an individual test station, the system is able to perform both sophisticated and exacting electrical tests on individual DUTs when required, yet can perform basic electrical tests on multiple DUTs simultaneously. By tracking the response of sockets to electrical tests (sockets can either be tested when empty using a probe-type device or when populated with a DUT as described in detail below) and linking the results of device tests to the sockets in which they were performed, the apparatus according to this aspect of the present invention can intelligently and more effectively manage the loading and unloading of test boards, and the testing of ICs, both individually and when populating sockets in test boards. 
   The test handler system includes a handling apparatus that may consist of a unitary apparatus or as a set of apparatus that are operated in a coordinated fashion. The handling apparatus is used to manage the test and placement of DUTs. The test and handling apparatus typically has a single housing that ensures that the DUTs therein are kept in a sufficiently clean environment. Although the DUTs will be packaged, it is still desirable for the DUTs to be protected, as particles and moisture found in ambient conditions can adversely affect the DUTs. 
   The handling apparatus desirably has a pick-and-place mechanism. The pick-and-place mechanism transports the DUTs from one location to another within the system. Pick-and-place mechanisms are robotic mechanisms that are well-known in the art, and will not be described in detail herein. Pick-and-place mechanisms are also known as suitable for transporting integrated circuit (IC) devices packaged using surface mount technologies (SMT) such as ball grid arrays (BGA) or chip scale package (CSP). While referred to as pick-and-place apparatus herein, the term is intended to encompass all mechanical methods for moving individual integrated circuit devices from one location to another. Also, while the term pick-and-place apparatus is used in the singular form, it is intended to include multiple apparatus that are operated in cooperation with each other. For example, one pick-and-place apparatus can be used to load a test board and a second apparatus can be used to unload the test board. However, because they are working in cooperation with each other to accomplish loading and unloading of test boards, they are referred to as a single apparatus. 
   The handling apparatus also typically has a loader for loading test boards into the automated test handler system. In a preferred embodiment, the loader has a mechanism for storing a plurality of test boards in a storage cassette configuration. Such a configuration enables a plurality of empty test boards to be sequentially introduced into the apparatus. Once the test boards are populated with devices by the apparatus and those devices have been tested, the test board is returned to the storage cassette and another empty cassette is introduced into the system. This process is repeated until all devices have been tested or all cassettes are filled with completely populated test boards. 
   The test boards have a plurality of sockets therein. The sockets are adapted to receive the DUTs. The test boards are configured to electrically interconnect the DUTs to test circuitry of the parallel tester. The test circuitry of the parallel tester is used to evaluate the performance of the DUTs under conditions designed to determine if the DUT is performing/will perform as desired. A number of such tests can be performed and the present invention is not limited to a particular electrical test. 
   In addition to the parallel test described above, the test handler system is advantageously configured so that a DUT can be subjected to other, more rigorous electrical tests depending upon the needs of the manufacturer. These more rigorous electrical tests are referred to as individual device tests herein, because such tests, e.g. DC and/or parametric tests, require fully isolated test circuitry and therefore cannot be performed when the DUT is populated in a test board with other devices. Performing tests that require fully isolated test circuitry on devices populating sockets in a test board is tedious, as the devices need to be removed, tested and replaced one by one. Thus, subjecting DUTs to DC/Parametric tests when the DUTs are populating a test board would cause a significant delay in the testing process. 
   The test system may be configured to conduct individual device tests concurrently with the in-board tests. That is, while one board is being tested (either loaded or empty) the individual device tests are being conducted on DUTs as they are placed into a second board. An individual DUT can be sequentially subjected to both the individual test (on its way from the input tray to the test board) and the parallel test (after being populated into a test board and the board is placed in the parallel tester). This is particularly advantageous if only some of the DUTs being tested require an individual test. In such a situation, the individual test can be conducted as part of the board loading process while another test board (populated with DUTs) is being tested in the parallel tester. 
   The handling apparatus of the test handler system also desirably has a DUT input mechanism adapted to receive DUTs. Typically, DUTs are introduced into a test apparatus in a DUT carrier (e.g. a JEDEC tray). The handling apparatus is configured to receive such DUT carriers and convey the DUT carrier to a location in the apparatus where the DUTs can be removed from the DUT carrier by the carrier heads on a robotic pick-and-place mechanism. For example, multiple DUT carriers can be indexed and fed sequentially into the apparatus. The carrier heads are typically vacuum heads that draw the DUTs from the carrier. The pitch of the carrier heads is such that it corresponds to the pitch of the DUT carrier. This ensures that each head of the robotic pick-and-place mechanism can extract a DUT from the DUT carrier. 
   When the DUT is removed from the DUT carrier, it is subjected to an alignment step where the contacts of the DUT are positioned so that the DUT makes proper electrical contact for the tests performed by the apparatus. Every DUT is subjected to such an alignment step. In one embodiment, the test system is equipped with a precisor station in which the DUTs are placed for alignment. The alignment is performed before the DUT is placed in the parallel test board (or before the individual device test, if performed). 
   The test handler system typically is equipped with an IC test plate for performing the individual device test. The IC test plate desirably is proximate to said input tray/precisor station. The IC test plate is in electrical communication with test electronics for performing the individual device test. The test plate is configured with test sockets adapted to receive DUTs. In a preferred embodiment the IC test plate has a top alignment plate and a test-pin matrix box. The top alignment plate has a plurality of cavities, each cavity adapted to receive an individual DUT. The number of cavities and their pitch are selected to conform to the number of carrier heads and the pitch thereof of the pick-and-place mechanism. The test-pin matrix block is configured to provide electrical interconnection between a DUT and the test circuitry. The test-pin matrix block has a matrix of spring-loaded pins. The matrix is configured to correspond to the pitch (i.e. spacing) of the DUT contacts. For example if the DUT is a BGA, the matrix is configured to correspond to the ball pitch of the BGA. 
   The aforesaid test board carrier is placed proximate to the IC test plate. The carrier is configured to communicate with the loader to receive test boards from the loader. The test board carrier is configured to place the test boards in at least two positions within the housing. The first position is the test board load position. In this position, the pick-and-place mechanism populates DUTs into the test boards. The second position is a parallel test position where either the DUTs, the test board sockets or both the sockets and the DUTs are electrically tested at ambient temperature. 
   The pick-and-place apparatus is configured to transfer the DUTs between and among the input tray, the IC test plate and the test board when the test board is positioned on the test board carrier in the first position. One skilled in the art can configure a pick-and-place apparatus with the requisite range for this purpose. In one embodiment, the pick-and-place apparatus has a rail mounted above the input tray, DUT test plate and test board carrier. The rail allows for the lateral adjustment of the robotic arms&#39; position on the rail. It is advantageous if there are a plurality of robotic arms, so that the various functions performed by these arms (e.g. unloading DUTs from carriers in the input tray, loading the test boards with DUTs, etc.) can be done simultaneously. 
   The parallel test feature of the test system provides electrical communication between the test boards and test circuitry. As such, the parallel tester is capable of testing one of the sockets (using probes), the DUTs in said sockets or both the DUTs and the sockets when said test board is in said second position. The electrical tests to which the DUTs and the sockets are subjected are largely a matter of design choice. Typical tests include functional tests such as clocking or pattern tests. 
   The test handler system desirably includes the aforementioned precisor plate. A precisor plate, as used herein, is a device that performs an alignment function. Preferably, the precisor is adapted to perform alignment for DUTs packaged using surface mount technologies (i.e. packages without a lead frame). When testing such devices, they must be oriented in a particular way when placed in the socket on the test board. Proper positional alignment is critical for surface mount devices. If a DUT is not placed in the test board socket in the proper orientation, the DUT will not test properly. This could lead to the DUT failing a test, because the connection, and not the DUT itself, is faulty. In such circumstances an otherwise good DUT would be discarded, which is an obviously undesirable result. 
   In another embodiment of the present invention, the results of the parallel test, or the results of testing the sockets in an empty test board (i.e. a test board not populated with devices) is used when the pick-and-place mechanism is populating the test boards with DUTs. The test system is equipped with a test board population controller for this purpose. The controller receives test information from the parallel test station. The test information received is test information associated with the sockets in the test board. The test result (i.e. whether the socket tested good or bad, or is otherwise likely to be good or bad) is stored in memory and is associated with that particular socket. Thereafter, when the board that contains that socket is again in place for loading, the controller queries the memory for information about the sockets in that test board. If there are one or more bad sockets in the test board, the controller conveys instructions to the pick-and-place apparatus not to populate those sockets with DUTs. 
   In a preferred embodiment, the test handler system is equipped with a mode controller. The mode controller allows a switch in operation mode of the apparatus from a first mode to a second mode. In the first mode, at least some of the DUTs are subjected to an individual device test before they are populated into a test board. In this mode, DUTs that fail the individual test are still populated into the board in the first place. However, the fact of this failed DUT, and its location in the test board, is stored into memory. The portion of the pick-and-place mechanism that is used to unload/remove IC devices from the test board is instructed to remove the failed DUTs and place them in a carrier on an output tray. The test board is in the first position when the DUTs are tested in the test plate. After known good sockets are populated with known good devices, the test board is moved to the second position for an in-board test, if required. Otherwise, the loaded test board is returned to the carrier. 
   In the second mode, DUTs are not individually tested in the test plate. Rather the test board is populated with DUTs that are transferred from the input carrier tray to the precisor and then to the test board by the pick-and-place mechanism. Once the test board has been populated with DUTs by the pick-and-place mechanism, the test board is moved to the second position where all of the DUTs in the board are subjected to an electrical test simultaneously. In this mode, since bad DUTs are not removed when the test board is in the first position, the output tray used for discarding devices when the apparatus is operated in the first mode can be used as an input tray to accelerate the population of DUTs into the test board. 
   As previously noted, the population controller loads the test board intelligently using the test information obtained about the DUTs and the test information obtained about the sockets that are to be populated with the DUTs. This includes the information from the individual electrical tests on DUTs. Specifically, the results of the individual electrical tests done before the DUTs are loaded into the test board are saved and associated with the socket into which the DUT is placed after it passes such test (DUTs that fail are ultimately separated from the known good devices). When the test board having one or more DUTs that were subjected to an individual electrical test is then moved to the second position and tested again, the results of the individual test are accessed from memory. The results of the individual test are then compared with the results of the board test. If the IC that passed the individual test failed the board test, this indicates that the socket, not the DUT, is bad. This information is stored into memory and the population controller, when executing a query whether or not to load a DUT into this socket during a subsequent load of the test board, will instruct the pick-and-place mechanism not to populate this socket. 
   Bad sockets are also detected by testing a loaded test board and comparing it with a prior electrical test of that same loaded test board. The results of the prior electrical tests are stored in memory and that information is accessed during subsequent electrical test of that same test board. If a given socket is associated with multiple previous tests that DUTs failed, that association indicates that the socket is bad. 
   In yet another embodiment, if a DUT fails the electrical tests performed in the parallel tester, the test board can be moved back to the first position where the failed DUT can be removed and the vacated socket refilled with another device. The test board is then moved back to the second position, where the DUTs are again subjected to an electrical test. This cycle can be repeated until all available sockets are filled with DUTs that pass the electrical tests in the parallel test station. This ensures that all devices sent on for environmental testing at least passed the initial, less rigorous tests at ambient temperature. 
   In yet another embodiment, the test handler system has a sorter mode in which populated boards, returned from environmental testing, are unloaded. The DUTs that failed the environmental are sorted from those that passed the environmental test. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a test handler apparatus for environmental testing of the prior art; 
       FIG. 2(   a )– 2 ( c ) is a flow chart for the IC test process according to one embodiment of the present invention; 
       FIG. 3  is a schematic of test apparatus according to one embodiment of the present invention integrated with other environmental testing apparatus; 
       FIG. 4  is a more detailed schematic of the test apparatus depicted in  FIG. 3 ; 
       FIG. 5  is perspective view a top view of an integrated circuit test plate; and 
       FIG. 6  is a view of a single socket in the integrated circuit test plate of  FIG. 5 . 
   

   DETAILED DESCRIPTION 
   An embodiment of the process of the present invention is described with reference to the process flow diagrams in  FIG. 2A–2C . Referring to  FIG. 2A , the process starts by causing the pick-and place apparatus to pick DUTs from the carrier tray in which the DUT is supplied (e.g. JDEC trays) and place the DUT in an alignment stage referred to as a precisor plate. In the precisor plate, the DUT is rotated (if its orientation is not suited to proper placement in a test socket) and aligned. After alignment the pick-and-place apparatus&#39; next step depends upon an instruction on the mode of test. If the test mode is one that requires the DUT to be subjected to an individual electrical test, the pick-and-place apparatus removes the DUT from the precisor and places it into a test plate for the individual electrical test that requires isolated electrical circuitry. After the individual test is performed, the pick-and-place mechanism takes the DUT from the test plate and inserts it into a socket in the test board. 
   If the apparatus is operated in a mode that does not require the DUT to be subjected to an individual electrical test, then the pick-and-place mechanism takes the DUT from the precisor and inserts it directly into the test board. 
   Referring to  FIG. 2B , the pick-and-place mechanism loads the test board intelligently. That is, the apparatus has a memory that stores certain information about the sockets. In this embodiment, the memory stores information about whether or not the socket has previously tested bad, in which case a DUT is not populated into the socket. The memory also stores information about whether or not a known good DUT (i.e. a DUT that tested good when subjected to an individual electrical test) tested bad when subjected to an electrical test in the socket. If the response to either query is yes, then the pick-and-place mechanism does not populate that socket with a DUT. If the response to both queries is no, the socket is populated with a DUT. 
   Next, a query is run on whether there are any more DUTs to load into the sockets and whether or not there are any more good sockets on the test board to populate. If the response to both queries is yes, the pick-and-place mechanism loads another DUT into the next available socket, repeating the sequence for intelligently loading the sockets that is described above. If the response to either query is no, then another query is run to identify any DUTs that are in the test board but failed the individual electrical test (if performed). If such DUTs are identified, the pick-and-place mechanism is instructed to remove them. If the response to the query is no, then, if all known good sockets on the test board are filled, the test board is moved to the parallel test position. If there are more good sockets to fill, then the loading cycle is repeated for the remaining good sockets. 
   Referring to  FIG. 2C , depending upon the mode of operation, the loaded circuit board is either transported to the parallel test station or returned to the test board storage cassette. If the test board is transported to the parallel test station, the DUTs are subjected to electrical tests at that parallel test station. If all DUTs pass the electrical test, the loaded test board is returned to the test board storage cassette. If one or more DUTs fail the test, then, depending upon the instructions programmed into the apparatus, the DUTs are either removed or not removed. If the instruction is to not remove the DUTs, then the loaded test board is returned to the test board storage cassette. If the instruction is to remove the failed DUTs, then the test board is returned to the DUT load/unload position in the apparatus. There, the pick-and-place mechanism removes the failed DUTs from the test board. The removed DUTs are placed in a storage receptacle for failed DUTs. 
   Depending upon the programming of the apparatus, the emptied sockets are either refilled with untested DUTs, in which case the test sequence restarts. If the sockets are not refilled, then the test board is returned to the storage cassette. This flexibility offers many advantages. If only a very few DUTs fail the test, then the logic may dictate that the test board simply be returned to the cassette and sent on for further burn-in tests without refilling and retesting additional DUTs. If, however, more than a few DUTs fail, the logic may dictate that it is worth the extra time to refill those sockets with additional DUTs and restart the test sequence. 
   When a test board is returned to the cassette, the apparatus is programmed to query whether or not there are empty test boards in the cassette. If the response to that query is yes, then the cassette carousel is rotated to feed another empty test board onto the test board carrier. The test sequence above is then repeated. If all test boards are full, the apparatus has reached the end of its sequence and a signal so indicating is transmitted to an operator or controller. The cassette is then detached, and removed for further processing (typically burn-in test of the DUTs populating the test boards). 
   Referring to  FIG. 3 , a schematic of a test and burn in system  100 , incorporating the test apparatus  101  is illustrated. In addition to the test apparatus  101 , the system has an environmental test (e.g. burn-in test chamber)  102  and a controller  103  for automated control of the various functions of the system  101 . Controller  103  includes mode controller  103   a  (also referred to as mode switch herein) and integrated circuit placement selector  103   b  which is responsive to the mode switch  103   a.    
   This embodiment of the test apparatus has a storage cassette  110  that contains a carousel illustrated schematically as  110   a  that can be loaded with a plurality of test boards (not shown). The storage cassette  110  cooperates with the DUT load and test chamber  115  of the apparatus to load empty test board to and unload filled test boards from the load and test chamber  115 . 
   The load and test chamber  115  manipulates the DUTs for testing. The load and test chamber employs a pick-and-place mechanism for such purpose. The pick-and-place mechanism is a system of sensors, pneumatic cylinders, servo-motors and stepper motors to unload DUTs input into the chamber  115  from their carriers and move the DUTs between and among the precisor, individual device test station and test boards as required. 
   The chamber  115  has two electrical test environments, depicted as blocks  120  and  125 . Block  120  performs more demanding electrical tests that mandate individual testing of the DUT (due to the need for isolated test circuitry and other requirements previously described). Thus block  120  is the schematic block for the individual device test described above. Block  125  is electrical test circuitry for DUTs when loaded into a test board. This electrical test advantageously mimics the electrical tests performed on the DUTs when in the environmental test chamber  102 . By employing the same test hardware, test correlation is more efficient and test discrepancies are reduced, thereby enhancing the accuracy of the test results and ensuring that bad devices are identified without misidentifying good devices as bad. Thus, Block  125  is the schematic block for the loaded board test previously described. 
   The apparatus  101  is also equipped with a sorter  130 . The system server  103 , or other control program uses the results of the tests in both the apparatus  101  and the environmental test chamber  102  to physically separate the good DUTs from the bad ones after testing is complete. 
   Once the test boards are loaded in apparatus  101 , the cassette  110  with the loaded test boards is moved to the environmental test chamber  102 . An autoguided vehicle (not shown) or manually operated trolley (not shown) can be used for this purpose. Movement of the cassette between apparatus  101  and environmental test chamber  102  is controlled by the burn-in server/controller  103  via block  103   c . Server/controller  103  also performs database management functions such as monitoring failure rate, mapping of failures as a function of time, lot, etc. and other information relevant to the burn-in and test process via block  103   d.    
     FIG. 4  is a more detailed schematic of the apparatus  101  in  FIG. 3 . The apparatus has a plurality of pick-and-place heads  140 . The pick-and-place heads  140  are used to transfer DUTs between and among the DUT carrier input  141 , the DUT output tray  142 , the precisor  143 , the test plate  144 , the parallel-test board  145 , the sorting plate  146 , sorting head  147  and sorting tray  148 . The pick-and-place heads  140  are not shown in detail. In one embodiment, the pick-and-place heads  140  have at least three vertically mobile members, each member having a suction device at its moveable end. The suction device is adapted to pick up and retain a DUT. The pick-and-place heads also have a release mechanism (e.g. a mechanism for turning the suction off) for releasing the DUTs. 
   The pitch (i.e. distance) between the pick-and-place heads is preferably adjustable. This adjustability permits the heads can be adjusted to the pitch of a DUT carrier tray or the pitch of the sockets in a test board. 
   In one advantageous configuration, the stroke of travel for the mobile members is controlled in two parts. Most of the stroke is controlled by a stepper motor that moves the member close to the DUTs. The remaining portion of the stroke is controlled by air pressure. This enables control of the amount of pressure placed on the DUTs and ensures that excessive force is avoided. 
   As previously noted, the storage cassette feeds test boards into the test chamber  115 . The test boards are fed, one at a time, onto carrier plate  150 . Carrier plate  150  conveys the test board  145  to a test board socket loading position A and an in-board test position B. In the exemplary process flow described above, the test board  145  is moved from the cassette  110  and to position A where the sockets are populated with DUTs. After the sockets of test board  145  are loaded, the test board  145  is moved to position B where the sockets/DUTs are subjected to electrical tests. 
   The input trays  141 ,  142  are adapted to receive DUT carriers (e.g. JEDEC trays). The input trays are configured to have a receiving end  141 A,  142 A and an output end  141 B,  142 B. The DUT carrier (not shown) enters the test chamber  115  at a receiving end, is moved proximate pick-and-place head  140  for unloading. After unloading, it is moved to the output end where empty DUT carriers are stacked. Multiple trays can be used to speed the loading of the test boards. 
   The pick-and-place heads  140  are used to pick DUTs from the trays,  141 ,  142  and place them into the precisor plates  143 . The precisor plate  143  has cavities that align the devices for placement into either the individual device tester  144  or the sockets in test board  145 . The precisor plates  143  also rotate the devices to ensure proper pin alignment. In the depicted embodiment, there are sufficient pick-and-place heads  140  to have dedicated heads for moving the DUTs from the precisor  143  to the test board  145  and for moving DUTs from the precisor  143  to the individual device tester  144 . 
   It is advantageous if the DUT is brought to an appropriate “drop height” by the pick-and-place mechanism and allowed to free fall into a receptacle in the precisor plate  143 . The receptacles in the precisor plate  143  are self-adjusting so that the pitch of the precisor receptacles matches the pitch of the pick-and-place mechanism. The pitch of the precisor receptacles also matches the pitch of the sockets in the circuit board. 
   The precisor  143  has the capacity to rotate the DUTs to match the pin alignment of the sockets in the individual device tester  144  or in the test board  145 . The rotation of the precisor  143  is controlled by software. For example, the precisor senses the placement of the test pin (e.g. pin- 1 ) in the device. The software then compares the sensed location with the location of pin- 1  in the socket downstream from the precisor (i.e. either the sockets in the DUT tester or the test board). The software, based on this comparison, determines if the DUT can be properly inserted into the socket downstream without rotation, or whether rotation is required. If rotation is required, software determines the degree of rotation (for a square or rectangular DUT the rotation degree options are +/−90 degrees and 180 degrees). 
   Once alignment in the precisor is complete, the DUT is moved to either the individual device tester  144  or the test board  145 , depending upon the particular instruction to the apparatus  101  from the burn-in server  103 . In either case, the pick-and-place heads  140  are used to move the DUTs from the precisor  143  to the next test site. If the next site is the individual device tester  144 , the DUTs are removed from the precisor plate  143  and positioned by the pick and place heads  140  to a testing height above the individual device tester  144 . The pick-and-place heads  140  then press the DUTs into the individual device tester  144 . 
   The individual device tester  144  is depicted in this embodiment as a test plate with three receptacles  161 . The receptacles of the tester  144  are connected with circuitry  120  to perform DC and/or parametric tests on the DUTs. 
   With reference to  FIG. 5 , the tester  144  consists of a top alignment plate  160  with the three receptacles  161  therein. The receptacles are sized to receive the DUT (typically a ball grid array (BGA) or chip scale package (CSP)). It is advantageous if the number of pick-and-place heads  140  ( FIG. 4 ) corresponds to the number of receptacles  161  in the tester  144 . The top alignment plate  160  is on and supported by a test pin matrix block  165 . The test pin matrix block has a matrix of spring loaded test pins  175  ( FIG. 6 ). The pick-and-place mechanism  140  presses DUTs into the receptacles with sufficient force to switch on the electrical test. The control program then causes the specified electrical test to be performed. The test pins are electrically connected to the test circuitry  120  via cables  170 . The spring loaded test pins  175  ensure that the DUTs are properly seated for carrying out the electrical test performed by test circuitry  120 . 
     FIG. 6  is a top view of a receptacle  161 . The matrix of pins  175  is readily observed. The receptacle  161  is readily adapted to the size and configuration of a particular DUT. This is accomplished by changing the size of the opening  176 . For convenience this can be done by simply removing the alignment plate  160  and replacing it with a different alignment plate  160  with openings  161  of the appropriate size. 
   Once the individual DUT test is completed, the DUT is placed in the test board  145  as previously described. If a DUT fails a test, it can be migrated out of the test board  145  and into the tray  148  configured to receive failed devices. The apparatus illustrated in  FIG. 4  is configured to accomplish this migration using pick-and-place head  140  to take the DUT from the test board  145  and place the DUT in precisor  143  to the right of the test board  145 . Another pick-and-place head  140  is then used to move the device first into sort plate  146 . Sort place  146  is a temporary holding stage for the failed DUT to await pick up by pick-and-place sort head  147 . Sort head  147  is then used to move the failed DUT from sort plate  146  to failed DUT tray  148 . 
   Thus, when operated in a first mode in which DUTs are subjected to individual test  120 , DUTs are loaded into chamber  115  via tray  141 . The pick-and-place heads  140  are used to migrate the DUT from tray  141  to precisor  143  proximate to tray  141 . From there pick-and-place head  140  moves the DUT to the individual device tester  144 , where the above described electrical tests  120  are performed. The DUTs are then transferred from the tester  144  to the test board  145 . For this transfer, press bar  180  is used to hold open the sockets in test board  145 . A second press bar  181  is also provided to ensure that the next row of sockets is opened when the row preceding it has been filled with DUTs. The action of pressbars  180 , 181  is controlled by software to avoid delays in the loading of the test board  145  by the pick-and-place heads  140 . 
   As previously noted, DUTs that fail the individual device test  120  are “migrated out” of the test board  145  using the pick-and-place heads  140  and precisor  143  to the right of the test board  145 . From the precisor, these failed devices are moved to sort plate  146  for temporary storage. Pick-and-place sorter head  147  then moves these failed DUTs from the sort plate  146  to the failed DUT tray  148 . 
   In an optional embodiment, the apparatus can also be operated in a sort mode. In sort mode, the sorter  130  of the apparatus figures prominently. In this embodiment, a fully populated test board is placed in position A. In this illustrative embodiment, the fully loaded test board has been returned to the apparatus  101  after environmental testing in the environmental test chamber  102 . Server  103  has retained the results of the environmental test, including which DUTs have passed or failed the environmental test (and their location in the test board). The loaded test board is conveyed to the apparatus  101  using the previously described mechanisms for conveying the loaded test boards from the apparatus  101  to the environmental test chamber  102 . The loaded test boards are then conveyed to position A in the test chamber  115  using the mechanisms previously described. 
   The server  103  then instructs the pick and place apparatus  140  on how to depopulate the test board  145 . The placement of the DUTs from the test board  145  will depend on whether the particular DUT passed or failed a previous test. If the DUT passed, it will be unloaded into a carrier tray (not shown) positioned on tray  142 . If the DUT failed a prior test, then the DUT will be placed on sort plate  146  and held there until sort head  147  can move the DUT from sort plate  146  into failed DUT tray  148 . 
   In yet another embodiment, when the apparatus is operated in the mode in which a DUT is not subjected to an individual test, the burn-in server  103  causes DUTs to be introduced into chamber  115  via trays  141  and  142 . This expedites loading the test board  145 , because DUTs are introduced into the test board from both the left and the right. 
   After the test board sockets are populated with DUTs, the test board is moved from position A to position B. In position B, an electrical connection  185  is used to electrically connect the DUTs in the test board to the parallel test circuitry  125 . As previously noted, if one or more devices fail this test, the test board  145  can be moved back to position A, the failed DUT removed and replaced by another DUT in the manner described above. 
   As previously noted, the apparatus, in the preferred form of the present invention, provides many advantages, chief of which is flexibility for the user. Such an apparatus permits the user to choose between conducting a more time-intensive individual DUT test and a less time-intensive test of DUTs populated into test boards  145 . The more time-intensive electrical tests are typically performed on new device types or prototypes, where a higher failure rate is expected. With these types of devices, there is a greater probability of hard failure. Hard failures, with serious faults can disrupt signal lines and stop the execution of such tests. This causes delays in operation. The apparatus is equipped with a mode switch  103   a  that allows selection between a mode that requires an individual electrical test and one that does not. 
   Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.