Patent Publication Number: US-2022214396-A1

Title: Thermal conditioning of electronic devices under test based on extensible elements

Description:
TECHNICAL FIELD 
     The present invention relates to the field of the test of electronic devices. More specifically, this invention relates to the thermal conditioning of electronic devices under test. 
     TECHNOLOGICAL BACKGROUND 
     The background of the present invention is hereinafter introduced with the discussion of techniques relating to its context. However, even when this discussion refers to documents, acts, artifacts and the like, it does not suggest or represent that the discussed techniques are part of the prior art or are common general knowledge in the field relevant to the present invention. 
     The electronic devices (for example, each based on one or more integrated circuits) are generally subject to tests to verify their correct operation. For each electronic device under test (also known as Device Under Test, or DUT), the tests are aimed at identifying defects that are either evident (i.e., which occur immediately) or potential (i.e., which might occur after a short period of use of the electronic device). In the latter case, the electronic devices may be subject to a thermal (burn-in) test. For this purpose, the electronic devices may be tested under thermal stress conditions, making them work at very high or very low temperatures (for example, from −50° C. to +150° C.), thereby simulating a long period of operation of the same electronic devices at room temperature (for example, 10-30° C.). 
     Particularly, the electronic devices may be tested in their final form at package level, i.e., with the integrated circuits encapsulated in packages to protect them and to provide terminals of access thereto. In this case, the electronic devices are temporarily housed on test boards (for example, Burn-In Boards, or BIBs, in case of the burn-in test). The test boards are used to interface the electronic devices with a test system. For this purpose, each test board is provided with a plurality of sockets. Each socket mechanically locks the package of an electronic device and electrically connects its terminals to the test system; at the same time, the socket allows removing the electronic device without substantial damage at the end of the test. The sockets are generally arranged in a matrix with high-density, in order to increase a parallelism of the test boards and thus performance of the test. 
     During their operation, the electronic devices generate heat that causes a heating thereof. Particularly, this heating is remarkable in the case of high power electronic devices. In any case, the increasing miniaturization of the electronic devices significantly increases their heating. An excessive heating of the electronic devices reduces their performance and may lead to wear, malfunction or even breakage of the electronic devices. Therefore, heat sinks are generally provided to dissipate the heat as much as possible from the electronic devices. Particularly, in case the electronic devices (at high power and/or miniaturization) generate a large amount of heat, their cooling by air (by either natural convection or forced ventilation) may not be sufficient to ensure their proper operation. For this reason, in the last years there has been a wide diffusion of complex cooling systems, such as of heat-pipe or liquid type, being capable of providing a high cooling capacity. 
     However, the use of such cooling systems during the test is difficult (if not impossible). 
     Indeed, the high density of the sockets in the test boards may prevent the application of the cooling systems to the electronic devices housed thereon. Moreover, the cooling systems may hinder or even prevent the automatic loading/unloading of the electronic devices to/from the test boards (with a deleterious effect on the performance of the tests). 
     In any case, the cooling systems (due to their size) act at global level on all the electronic devices housed on each test board. However, the electronic devices (even if of the same type) are subject to uneven heating (with differences of the order of 40-60%). This makes it difficult to control the temperature of the electronic devices accurately, and in any case it prevents applying the same thermal stress to the electronic devices (with resulting reduction of the reliability of the tests). 
     All of the above has negative effects on the effectiveness of the tests of the electronic devices, which affects the quality of their production process. 
     SUMMARY 
     A simplified summary of the present invention is herein presented in order to provide a basic understanding thereof however, the sole purpose of this summary is to introduce some concepts of the invention in a simplified form as a prelude to its following more detailed description, and it is not to be interpreted as an identification of its key elements nor as a delineation of its scope. 
     In general terms, the present invention is based on the idea of conditioning the electronic devices thermally in an individual manner via corresponding extensible elements. 
     Particularly, an aspect provides a thermal conditioning device, wherein a process heat-conducting fluid is caused to circulate in a plurality of extensible elements; a pressure of the process heat-conducting fluid is regulated to lengthen the extensible elements so that they are pressed against corresponding electronic devices under test to condition them thermally. 
     A further aspect provides a test apparatus comprising this thermal conditioning device. 
     A further aspect provides a corresponding method for conditioning electronic devices under test thermally. 
     A further aspect provides a corresponding method for testing electronic devices. 
     More specifically, one or more aspects of the present invention are set out in the independent claims and advantageous features thereof are set out in the dependent claims, with the wording of all the claims that is herein incorporated verbatim by reference (with any advantageous feature provided with reference to any specific aspect that applies mutatis mutandis to every other aspect). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The solution of the present invention, as well as further features and the advantages thereof, will be best understood with reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings (wherein, for the sake of simplicity, corresponding elements are denoted with equal or similar references and their explanation is not repeated, and the name of each entity is generally used to denote both its type and its attributes, like value, content and representation). In this respect, it is expressly intended that the drawings are not necessary drawn to scale (with some details that may be exaggerated and/or simplified) and that, unless otherwise indicated, they are merely used to illustrate the described structures and procedures conceptually. Particularly: 
         FIG. 1  shows an illustrative representation with phantom parts of a thermal conditioning device according to an embodiment of the present invention, 
         FIG. 2  shows an illustrative representation in partially cutaway view of a test apparatus according to an embodiment of the present invention, 
         FIG. 3  shows a cross-section view of a detail of the thermal conditioning device according to an embodiment of the present invention, and 
         FIG. 4A - FIG. 4E  show the main steps of a test process according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference in particular to  FIG. 1 , an illustrative representation (with phantom parts) is shown of a thermal conditioning device  100  according to an embodiment of the present invention. 
     The thermal conditioning device  100  is used to thermally condition a plurality of electronic devices under test (DUTs) housed on corresponding test board sockets (not shown in the figure). The thermal conditioning device  100  comprises the following components. 
     A plurality of extensible elements  105  (for example, bellow-like) are used to condition the electronic devices thermally in an individual manner, for example, by cooling them during a test thereof. Each extensible element  105  has a variable length, so that it may lengthen and shorten. The extensible elements  105  are arranged in a matrix (for example, with 2-10 rows and 5-20 columns only partially represented in the figure), with a pitch of 3.0-4.0 cm (or more). For each electronic device, a corresponding extensible element  105  is provided (for cooling it when in contact therewith). A process fluid  110  is caused to circulate in the extensible elements  105 . The process fluid  110  is a heat-conducting substance in the liquid state (for example, a mixture of water and glycol), which accumulates and conveys heat from the electronic devices (cooling them). The circulation of the process fluid  110  in the extensible elements  105  is achieved by means of a (process) circulation system  115 , for example, based on corresponding impellers (described in detail in the following). A (process) heat exchange system, for example, a heat exchanger  120  exchanges heat with the process fluid  110  (transferring the heat absorbed from the electronic devices). A pressure regulation system, for example, a piston  125  is used to regulate a pressure of the process fluid  110 . This allows moving the extensible elements  105  between a shortened condition (low pressure) and a lengthened condition (high pressure). As described in detail in the following, at rest the extensible elements  105  are shortened, whereas during the test the extensible elements  105  are lengthened to be pressed against the corresponding electronic devices. 
     The above-described solution allows conditioning the electronic devices thermally (for example, by cooling them) in an effective manner even during the test. 
     Particularly, the thermal conditioning device  100  may be applied to the electronic devices even when the test board whereon they are housed has a high density of the sockets (even with a pitch of a few tens of millimeters). Moreover, the thermal conditioning device  100  does not substantially hinder the automatic loading/unloading of the electronic devices to/from the test board (so that it does not affect the performance of the tests). 
     The pressure exerted by the process fluid  110  significantly improves a mechanical coupling, and thus a heat exchange, between the extensible elements  105  and the electronic devices  205 . 
     All of the above has positive effects on the effectiveness of the tests of the electronic devices, which reflects on the quality of a production process thereof. 
     With reference now to  FIG. 2 , an illustrative representation in partially cutaway view is shown of a test apparatus  200  according to an embodiment of the present invention. 
     The test apparatus  200  is used to test electronic devices, denoted with the reference  205 , at package level; for example, the electronic devices  205  are subject to a burn-in test (wherein the electronic devices  205  are tested under thermal stress conditions). For this purpose, the test apparatus  200  comprises a test board  210  (for housing electronic devices  205  temporarily) and the conditioning device  100  of above (for conditioning the electronic devices  205  housed on the test board  210  thermally). Generally, a test plant (not shown in the figure) accommodates several instances of the test apparatus  200 , together with corresponding driving boards (arranged in a control area maintained at room temperature and connected to the corresponding test apparatus  200  to supply them and to exchange signals) and a loader/unloader (for loading/unloading the electronic devices  205  from the test boards  210 ). 
     Particularly, the test board  210  comprises the following components. 
     A holder  215  of circuitized insulating material (for example, a printed circuit board, or PCB) has a function of mechanical support and electrical connection for the other components of test board  210 . A plurality of sockets  220  (only partially represented in the figure) are mounted on a main (upper in the figure) surface of the holder  215 . The sockets  220  are arranged with a geometry corresponding to the one of the extensible elements  105  (so that when the test board  210  and the conditioning device  100  are coaxial (with test board  210  under the conditioning device  100  in the figure), the sockets  220  are aligned with the corresponding extensible elements  105 . Each socket  220  is used to house an electronic device  205  in a removable manner. Particularly, the socket  220  locks the electronic device  205  mechanically (by acting on its package) and connects the electronic device  205  electrically to the electrical circuit of the holder  215  (by contacting its terminals); at the same time, the socket  220  allows releasing the electronic device  205  mechanical (for removing it from the test board  210  without any substantial damage). For example, the socket  220  is based on a platform for resting the electronic device  205 , with conductive pads for receiving the terminals of the electronic device  205 ; a lid, with a window for leaving the package of the electronic device  205  accessible, is hinged to the base, so that it may be closed and opened (to lock and to release, respectively, the electronic device  205 ). 
     With reference now to  FIG. 3 , a cross-section view is shown of a detail of the thermal conditioning device  100  according to an embodiment of the present invention. 
     The heat exchanger  120  achieves a heat exchange between the process fluid  110  and a service fluid  303  (a similar heat-conducting substance in the liquid state, for example, still a mixture of water and glycol). The heat exchange takes place indirectly through a surface separating distinct compartments wherein the process fluid  110  and the service fluid  303  circulate (so that they are not in contact with each other). Particularly, a (service) chamber  306  contains the service fluid  303  and a (process) chamber  309  contains the process fluid  110 . The service chamber  306  and the process chamber  309  have a (heat exchange) wall  312  in common. For this purpose, the wall  312  is made of thermally conductive material (for example, copper); in addition, the  312  wall is equipped with fins  315  facing the process chamber  309  to further facilitate the heat exchange with the process fluid  110 . A delivery pipe  318  (or more) and a suction pipe  321  (or more) connect the service chamber  306  to a (service) heat exchange system, for example, a chiller  324  being common to all the test apparatus in the test plant (not shown in the figure), which exchanges heat with the service fluid  303  (transferring the heat absorbed by the process fluid  110 ). 
     The following components are provided for each extensible element  105 . A pair of (equal and coaxial) holes  327  and  330  are formed in the wall  312  and in a (working) wall  333  of the service chamber  306  opposite thereto, respectively. A sleeve  336  with a section matching the holes  327 . 330  crosses the service chamber  306  between them; an edge of the sleeve  336  is sealed to an edge of the hole  327  and another edge of the sleeve  336  protrudes beyond the wall  333  (to which a side surface of the sleeve  336  is sealed at the hole  330 ). In this way, the sleeve  336  defines a through-hole  339  that crosses the service chamber  306  between the walls  312  and  333 . A bellow  342  has an edge attached to the edge of the sleeve  336  protruding from the wall  333 . The bellow  342  is flexible (for example, accordion-like foldable), so that it may be lengthened and shortened. A cup  345  has an edge attached to another edge of the bellow  342 . The sleeve  336 , the bellows  342  and the cup  345  define the extensible element  105 . In this way, the extensible element  105  extends from the process chamber  309  across the through-hole  339  of the service chamber  306 , with a base of the cup  345  protruding beyond the wall  333  to define a contact surface  348  with the corresponding electronic device (not shown in the figure). A heating element  351  is arranged at the contact surface  348  to heat the electronic device (for example, made by means of a coil of electrically conductive material, such as constantan, embedded in the base of the cup  345 ). A temperature sensor  354  is associated with the contact surface  348  (for example, arranged on the outside of the cup  345  next to it) to detect a temperature of the electronic device. The heating element  351  and the (temperature) sensor  354  are electrically connected to the control board, for example, via holes made in screw stems for mechanical coupling between the process chamber  309  and the service chamber  306  (not shown in the figure). A delivery pipe  357  extends along the extensible element  105 , with an (upper) inlet protruding above the wall  312  and a (lower) outlet near the base of the cup  345 . In this way, a delivery pipe  360  is defined between the suction pipe  357  and a sidewall of the extensible element  105 . For each extensible element  105 , the circulation system  115  comprises an impeller  363  and an electromagnets crown  366 . The impeller  363  (for example, of tangential type) is arranged in a suction chamber, not shown in the figure, for sucking the process fluid  110  from the process chamber  309  and for conveying it into the delivery pipe  357  (in the direction perpendicular to its longitudinal axis). The electromagnets crown  366  is arranged coaxial with the impeller  363  outside the process chamber  309  to operate (without mechanical coupling) the impeller  363 , which in turn is equipped with permanent magnets (not shown in the figure). A block valve is arranged at the (upper) inlet of the delivery pipe  357  (to block an inflow of the process fluid  110  to the suction pipe  357  when the impeller  363  is stopped). The block valve if formed by a cap, or pin,  369  (with flared profile) and by an electromagnet  372 . The cap  369  is located above the inlet of the delivery pipe  357  in the process chamber  309  (mounted on a corresponding vertical guide, not shown in the figure); the cap  369  has a specific weight higher than the one of the process fluid  110 , so that it is kept lowered onto the inlet of the suction pipe  357  by gravity. The electromagnet  372  is placed over the cap  369  outside the process chamber  309  to lift the cap  369  by acting on a permanent magnet embedded therein (not shown in the figure). A block valve  375  is arranged at an (upper) outlet of the suction pipe  360  (to block an outflow of the process fluid  110  from the suction pipe  360  due to natural convection when the impeller  363  is stopped). The block valve  375  is formed by a ring (with flared profile) fitted on the suction pipe  357 ; the block valve  375  has a specific weight higher than the one of the process fluid  110 , so that it is kept lowered on the outlet of the suction pipe  360  by gravity as well when it is not pushed upwards by the hydraulic head of the flow of the process fluid  110  generated by the rotation of the impeller  363 . 
     With reference now to  FIG. 4A - FIG. 4E , the main steps are shown of a test process according to an embodiment of the present invention. 
     Starting from  FIG. 4A , at the beginning of the test (not shown in the figures), the electronic devices  205  are transported in a tray close to the loader/unloader, which is in front of the test board  210  moved away laterally from the thermal conditioning device  100 . The loader/unloader collects the electronic devices  205  from the tray and deposits them onto the (open) sockets  220  in succession. Once the test board  210  has been filled (totally or partially), the sockets  220  are closed and the test board  210  is moved back under the thermal conditioning device  100 , as shown in the figure, so that each electronic device  205  is aligned with the corresponding extensible element  105 . 
     Moving on to  FIG. 4B , at this point the heat conditioning device  100  and the test board  210  are caused to approach (for example, by lowering the heat conditioning device  100 ). The approaching is such that, where the corresponding electronic devices  205  are present in the sockets  220  (as shown on the left in the figure), each extendible element  105  moves close to the electronic device  205 , with its contact surface  348  at a corresponding approaching distance (for example, 0-1 mm). Therefore, should no electronic devices  205  be present in the corresponding sockets  220  (as shown on the right in the figure), each extensible element  105  remains spaced apart from the bottom of the (empty) socket  220 , with its contact surface  348  at a distance equal to the approaching distance plus a thickness of the electronic devices  205  (for example, for a total of 1-2 cm). 
     Moving to  FIG. 4C , the piston  125  is operated to increase the pressure of the process fluid  110 . As a consequence, where the corresponding electronic devices  205  are present in the sockets  220  (as shown on the left in the figure), each extensible element  105  is lengthened (for example, by 1-2 mm) until it abuts against the electronic device  205 , thereby pressing the contact surface  348  against it (directly or through an elastic insert, of thermally conductive material, not shown in the figure). Particularly, the bellow  342  is capable of adapting even in case the contact surface  348  and the electronic device  205  are not perfectly parallel; this ensures their good mechanical coupling in any situation, further improving the heat exchange. 
     On the other hand, should no electronic devices  205  be present in the corresponding sockets  220  (as shown on the right in the figure), each extensible element  105  would lengthen without any abutment, at least until the contact surface  348  reaches the bottom of the (empty) socket  220 . Therefore, in an embodiment of the invention a limitation system is provided to limit the elongation of the extensible elements  105 , for example, comprising a corresponding limiting element  405  for each extensible element  105 . The limiting element  405  is formed by a vessel fixed under the wall  312  around the extensible element  105 . The limiting element  405  extends slightly beyond the bellow  342  when the extensible element  105  is in the shortened condition, by a length exceeding the approaching distance of above (for example, 1.2-2.0 times if not zero and at least equal to 1-2 mm in any case). A hole is made in the bottom of the limiting element  405 . The hole has a diameter matching the cup  345  to allow its passage (so that the contact surface  348  protrudes beyond the limiting element  405 ); the hole is instead narrower than the bellow  342  (wider than the cup  345 ), so that the remaining part of the bottom of the limiting element  405  (outer edge) defines an abutment for the bellow  342 . 
     Therefore, where the corresponding electronic devices  205  are present in the sockets  220 , each extensible element  105  lengthens as above (with the cup  345  sliding in the hole of the limiting element  405  (so that it does not interfere in any way with the operation of the extensible element  105 ). On the contrary, where no electronic device  205  is present in the corresponding sockets  220 , each extensible element  105  lengthens but only until the bellow  342  abuts against the abutment of the limiting element  405 . This avoids, or at least significantly reduces, the risk of damage of the bellow  342 . 
     Moving to  FIG. 4D , at the beginning all the heating elements  351  are switched off, all the electromagnets crowns  366  are switched off so that all the impellers  363  are stopped, all the electromagnets  372  are switched off so that all the caps  369  are lowered on the suction pipes  357  by gravity force (suction valves  369 , 372  being closed) and all the delivery valves  375  are lowered on the delivery pipes  360  by gravity force (closed). A lower threshold temperature and an upper threshold temperature define a regulation range around a target temperature to be maintained for the electronic devices  205  during the test (for example, ±5-10° C.). 
     For each electronic device  205 , the driving board (not shown in the FIG. monitors its (actual) temperature detected by the sensor  354 . If the actual temperature is lower than the lower threshold temperature, the driving board activates the heater element  351  (with power increasing with a distance of the actual temperature from the lower threshold temperature, for example, by applying a voltage of 12-24 V to generate 0.1-10 kJ of heat by Joule effect). The heating element  351  increases the temperature of the contact surface  348  and thus of the electronic device  205 . As soon as the actual temperature exceeds the lower threshold temperature, the driving board deactivates the heating element  351 . 
     Moving to  FIG. 4E , if the actual temperature (detected by the sensor  354 ) is higher than the upper threshold temperature, the driving board (not shown in the figure) activates the electromagnet  372  to lift the cap  369  in opposition to the force of gravity (delivery valve  369 , 372  being open), thus clearing the inlet of the delivery pipe  357 . The driving board then activates the electromagnets crow  366  to make the impeller  363  rotate (at an angular speed increasing with a distance of the actual temperature from the upper threshold temperature, for example, 5-20 rpm). In this way, the impeller  363  sucks the process fluid  110  from the process chamber  309  and conveys it into the delivery pipe  357  towards the contact surface  348 . The process fluid  110  decreases the temperature of the contact surface  348  and then of the electronic device  205 ; particularly, the cooling of the electronic device  205  depends on the flow rate of the process fluid  110 , which in turn depends on the angular speed of the impeller  363  (with the cooling increasing with the angular speed). The (heated) process fluid  110  goes back up in the suction pipe  360 ; the flow of the process fluid  110  lifts the suction valve  375  against the force of gravity (opening it), thereby clearing the outlet of the suction pipe  360 . The process fluid  110  may thus return from the suction pipe  360  into the process chamber  309 . The process fluid  110  laps the wall  312  (and particularly its fins  315 ) thereby cooling down (at the expense of the service fluid  303 , which heats up and is then cooled by the chiller  324 , which in turn disperses the heat into the external environment). 
     Returning to  FIG. 4D , as soon as the actual temperature falls below the upper threshold temperature, the driving board deactivates the electromagnets crown  366 , so that the impeller  363  stops. Moreover, the driving board deactivates the electromagnet  372 , so that the cap  369  falls by gravity onto the delivery pipe  357  (block valve  369 , 372  being closed), thereby obstructing its inlet. As a result, the flow of the process fluid  110  in the suction pipe  357  and then in the delivery pipe  360  is lacking, so that the delivery valve  375  falls by gravity onto the suction pipe  360  (closing), thus obstructing its outlet. The block valve  369 , 372  and the suction valve  375  block any convective motion of the process fluid  110  in the extensible element  105 , thus avoiding further cooling of the electronic device  205 . 
     In this way, the temperature of each electronic device  205  may be controlled individually with precision (with a hysteresis control); particularly, the same thermal stress may be applied to the electronic devices  205 , even when they are subject to non-uniform heating (with resulting increase of the reliability of the tests). 
     This result is achieved in a simple and effective manner by using a single service fluid  303  for all the extensible elements  105 . Particularly, the service fluid  303  may be conditioned (as a function of the power dissipated by the electronic devices  205 ) to a temperature so as to center, around the target temperature, the control range of the temperature under the action of the impeller  363  and the heating element  351  (so as to limit their intervention as much as possible). 
     In this way, the electronic devices  205  may be tested by the driving board (sending stimulus signals and receiving corresponding result signals) while their temperature is maintained at the desired value. 
     At the end of the test (not shown in the figures), the above-described operations are repeated in reverse order. Particularly, the test board  210  is brought in front of the loader/unloader (moving it laterally away from the thermal conditioning device  100 ) and the sockets  220  are opened. The loader/unloader collects the electronic devices  205  from the test board  210  and deposits them onto the tray in succession. 
     MODIFICATIONS 
     Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply many logical and/or physical modifications and alterations to the present invention. More specifically, although this invention has been described with a certain degree of particularity with reference to one or more embodiments thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible. Particularly, different embodiments of the present invention may be practiced even without the specific details (such as the numerical values) set forth in the preceding description to provide a more thorough understanding thereof; conversely, well-known features may have been omitted or simplified in order not to obscure the description with unnecessary particulars. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any embodiment of the present invention may be incorporated in any other embodiment as a matter of general design choice. Moreover, items presented in a same group and different embodiments, examples or alternatives are not to be construed as de facto equivalent to each other (but they are separate and autonomous entities). In any case, each numerical value should be read as modified according to applicable tolerances; particularly, unless otherwise indicated, the terms “substantially”, “about”, “approximately” and the like should be understood as within 10%. Moreover, each range of numerical values should be intended as expressly specifying any possible number along the continuum within the range (comprising its end points). Ordinal or other qualifiers are merely used as labels to distinguish elements with the same name but do not by themselves connote any priority, precedence or order. Moreover, the terms include, comprise, have, contain, involve and the like should be intended with an open, non-exhaustive meaning (i.e., not limited to the recited items), the terms based on, dependent on, according to, function of and the like should be intended as a non-exclusive relationship (i.e., with possible further variables involved), the term a/an should be intended as one or more items (unless expressly indicated otherwise), and the term means for (or any means-plus-function formulation) should be intended as any structure adapted or configured for carrying out the relevant function. 
     For example, an embodiment provides a thermal conditioning device. However, the thermal conditioning device may be used to apply any type of thermal conditioning (for example, cooling, heating, cooling/heating to any temperature, and so on). 
     In an embodiment, the thermal conditioning device may be used for conditioning a plurality of electronic devices under test thermally. However, the electronic devices may be in any number and of any type (for example, based on integrated circuits and/or discrete components, provided in any type of package, with any number and type of terminals, and so on), and they may be subject to any type of test (for example, reliability, functional, parametric, performed under any thermal stress condition or even simply by keeping their temperature within a predetermined range, and so on). 
     In an embodiment, the thermal conditioning device comprises a plurality of extensible elements with variable length, each for conditioning a corresponding one of the electronic devices thermally in an individual manner. However, the extensible elements may be in any number (for conditioning a corresponding maximum number of electronic devices thermally) and of any type (for example, bellow-like, telescopic, elastic, and so on). 
     In an embodiment, the thermal conditioning device comprises a process circulation system for circulating a process heat-conductive fluid in the extensible elements. However, the process circulation system may be of any type (for example, for controlling the circulation of the process heat-conductive fluid individually in each extensible element or globally, for pushing and/or sucking the process fluid in the extensible elements, and so on) and using any process heat-conductive fluid (for example, liquid, gas, and so on). 
     In an embodiment, the thermal conditioning device comprises a process heat exchange system for exchanging heat with the process heat-conductive fluid. However, the process heat exchange system may be of any type (for example, global for all extensible elements, with distinct components each for a set of one or more of the extensible elements, liquid-based, oil-based, air-based and so on). 
     In an embodiment, the thermal conditioning device comprises a pressure regulation system for regulating a pressure of the process heat-conducting fluid. However, the pressure regulation system may be of any type (for example, mechanical, hydraulic, and so on) for regulating the pressure of the process heat-conducting fluid in any way (for example, by simply activating/deactivating it, setting the pressure to any value selected in a continuous/discrete manner, and so on). 
     In an embodiment, the regulation of the pressure is used to move the extensible elements between a shortened condition and a lengthened condition. However, the shortened condition and the lengthened condition may be defined in any way (for example, with the extensible elements in the shortened condition that are separated from the electronic devices at any distance or are already in contact with them, with the lengthened condition defined by any lengthening with respect to the shortened condition, and so on). 
     In an embodiment, each of the extensible elements in the lengthened condition is pressed against the corresponding electronic device. However, the extensible elements may be pressed against the electronic devices with any force (for example, fixed, adjustable, and so on). 
     Further embodiments provide additional advantageous features, which may however be omitted at all in a basic implementation. 
     Particularly, in an embodiment the process circulation system comprises a plurality of process circulation elements, each for circulating the process heat-conductive fluid in a corresponding one of the extensible elements. However, the process circulation elements may be of any type (for example, tangential, circumferential or axial impellers, suction pumps, and so on, driven by corresponding electromagnets crowns, solenoid coils crowns directly integrated into a multilayer printed circuit board, and so on). 
     In an embodiment, the process circulation elements are controllable individually to regulate a flow rate of the process heat-conductive fluid independently in the corresponding extensible elements. However, the process circulation elements may be controlled individually in any way (for example, on/off, with flow rate regulated according to any law, such as based on any linear or non-linear function of the distance from the target temperature or of the approach rate thereto, and so on) by any control system (for example, corresponding control board, central computer of the whole test plant, and so on). 
     In an embodiment, the thermal conditioning device comprises a plurality of blocking devices each for blocking a convective motion of the process heat-conducting fluid in a corresponding one of the extensible elements in a deactivated condition of the corresponding process circulation element. However, the blocking devices may be of any type (for example, input and/or output, active, passive, and so on) or they may also be omitted at all. 
     In an embodiment, the thermal conditioning device comprises a plurality of heating elements, each arranged in a corresponding one of the extensible elements for heating the corresponding electronic device. However, the heating elements may be of any type (for example, based on Joule effect, magnetic induction, and so on) or they may be omitted at all. 
     In an embodiment, the heating elements are controllable individually to heat the corresponding electronic devices independently. However, the heating elements may be controlled individually in any way (for example, on/off, with power controlled according to any law, such as based on any linear or non-linear function of the distance from the target temperature or of the approach rate thereto, and so on) by any control system (either the same or different with respect to above); in any case, the possibility of controlling the heating elements globally is not excluded. 
     In an embodiment, the thermal conditioning device comprises a plurality of temperature sensors each for detecting a temperature of a corresponding one of the electronic devices. However, the temperature sensors may be of any type and arranged in any position; in any case, the possibility is not excluded of having fewer temperature sensors, each associated with several electronic devices (down to one for all), or even of omitting them completely. 
     In an embodiment, the process circulation elements and/or the heating elements are controllable according to the temperature of the corresponding electronic devices. However, it is possible to control only the process circulation elements, only the heating elements or both of them in any way according to the temperature of the corresponding electronic devices (for example, based on their value or their variation over time, with any linear or non-linear law, and so on). 
     In an embodiment, the thermal conditioning device comprises a thermal coupling element for exchanging heat between the process heat-conductive fluid and a service heat-conductive fluid. However, the thermal coupling element may be of any type (for example, based on plate or tubes, with the fluids in co-current, counter-current or cross-current, and so on) to exchange heat with any service heat-conductive fluid (equal to or different from the process heat-conductive fluid). 
     In an embodiment, the thermal conditioning device comprises a service heat exchange system for exchanging heat with the service heat-conductive fluid. However, the service heat exchange system may be of any type (for example, a chiller, a heat pump, and so on). 
     In an embodiment, the thermal conditioning device comprises a process chamber for containing the process heat-conductive fluid and a service chamber for containing the service heat-conductive fluid. However, the process chamber and service chamber may be of any material, shape and size (equal to or different from each other). 
     In an embodiment, the process chamber and the service chamber are separated by a heat exchange wall. However, the heat exchange wall may be of any material, shape and size (for example, with any number and type of fins, without fins, and so on). 
     In an embodiment, the service chamber is crossed by a plurality of through-holes extending from the heat exchange wall to an operative wall opposite the heat exchange wall. However, the through holes may be of any shape, size and type (for example, defined by the extensible elements, integrated in the service chamber independently of the extensible elements, and so on). 
     In an embodiment, the extensible elements extend from the process chamber each crossing a corresponding one of the through holes, from the heat exchange wall to the operative wall, with a contact surface of the extensible element protruding from the operative wall for contacting the corresponding electronic device. However, the extensible elements may protrude from the operative wall at any distance with any contact surface thereof (for example, flat, convex, rigid, elastic, for direct/indirect contact, and so on). 
     In an embodiment, the process circulation system is configured for circulating the process heat-conductive fluid in each of the extensible elements from the process chamber to the contact surface and from the contact surface to the heat exchange wall. However, the process fluid may be caused to circulate in the extensible elements in any way (for example, along any predefined path, indiscriminately, and so on). 
     In an embodiment, the thermal conditioning device comprises a plurality of suction pipes of the process heat-conducting fluid each extending in a corresponding one of the extensible elements from the process chamber to a position proximal to the contact surface. However, each suction pipe may be of any shape and size, and it may extend in any way in the corresponding extensible element (for example, protruding beyond the heat exchange wall at any distance even zero, stopping at any distance from the contact surface even zero if provided with one or more side windows in correspondence thereto, and so on). 
     In an embodiment, the suction pipe is separated from a lateral surface of the extensible element to define a delivery duct of the process heat-conducting fluid. However, the suction pipe may be separated from the sidewall in any way (for example, symmetrically or non-symmetrically, at any distance, and so on). 
     In an embodiment, each of the blocking devices comprises a suction valve for blocking an inflow of the process heat-conductive fluid from the process chamber to the suction pipe in the deactivated condition of the corresponding process circulation element. However, the suction valve may be of any type (for example, with opening and/or closing control, of cap, butterfly, ball type and so on). 
     In an embodiment, each of the blocking devices comprises a delivery valve for blocking an outflow of the process heat-conductive fluid from the delivery duct to the process chamber in the deactivated condition of the corresponding process circulation element. However, the delivery valve may be of any type (the same as or different from the suction valve). 
     In an embodiment, the suction valve is normally closed by force of gravity. However, the suction valve may have any specific weight (higher than the one of the process fluid) for closing the suction pipe with any force. 
     In an embodiment, the suction valve opens in response to an external command in opposition to the force of gravity. However, the external command may be of any type (for example, magnetic, such as via electromagnet or solenoid coil directly integrated in a multilayer printed circuit, mechanical and so on). 
     In an embodiment, the delivery valve is normally closed by force of gravity. However, the delivery valve may have any specific weight (higher than the one of the process fluid) for closing the delivery duct with any force. 
     In an embodiment, the delivery valve opens in response to said circulating the process heat-conductive fluid in opposition to the force of gravity. However, the flow valve may open in response to any pressure exerted by the flow of the process heat-conductive fluid. 
     In an embodiment, the thermal conditioning device comprises a limitation system for limiting the length of each of the extensible elements when moved towards the lengthened condition in the absence of the corresponding electronic device. However, the limitation system may be of any type (for example, with a limitation element for each extensible element or group of them, unique for all the extensible elements, passive, active and so on) or it may also be omitted at all. 
     An embodiment provides a test apparatus comprising the thermal conditioning device of above and one or more test boards. However, the test apparatus may comprise any number of test boards of any type. 
     In an embodiment, each test board has a plurality of sockets each for housing one of the electronic devices in a removable manner. However, each test board may have any number of sockets (equal to or lower than the number of extensible elements of the thermal conditioning device) of any type (for example, pads/holes for receiving the terminals of the electronic devices, based on lid/latch to lock the packages of the electronic devices, and so on). 
     Generally, similar considerations apply if the thermal conditioning device and the test apparatus each has a different structure or comprises equivalent components (for example, of different materials) or it has other operative characteristics. In any case, every component thereof may be separated into more elements, or two or more components may be combined together into a single element; moreover, each component may be replicated to support the execution of the corresponding operations in parallel. Moreover, unless specified otherwise, any interaction between different components generally does not need to be continuous, and it may be either direct or indirect through one or more intermediaries. 
     An embodiment provides a method for conditioning a plurality of electronic devices under test thermally. The method comprises circulating a process heat-conductive fluid in a plurality of extensible elements with variable length. The method comprises exchanging heat with the process heat-conductive fluid. The method comprises regulating a pressure of the process heat-conducting fluid to move the extensible elements between a shortened condition and a lengthened condition. Each of the extensible elements in the lengthened condition is pressed against the corresponding electronic device for conditioning the electronic device thermally. 
     An embodiment provides a method for testing a plurality of electronic devices. The method comprises housing the electronic devices on corresponding sockets of one or more test boards in a removable manner. The method comprises conditioning the electronic devices thermally as above. The method comprises testing the electronic devices being conditioned thermally. 
     Generally, similar considerations apply if the same solution is implemented with an equivalent method (by using similar steps with the same functions of more steps or portions thereof, removing some non-essential steps or adding further optional steps); moreover, the steps may be performed in a different order, concurrently or in an interleaved way (at least in part).