Abstract:
Disclosed is an environmental test apparatus having a thermal chamber for stress testing electronic products. The apparatus has a carrier for supporting the products in the chamber and equipment for monitoring product characteristics during stress testing. The improved apparatus has an auxiliary chamber in which ambient temperature and humidity conditions prevail. A flexible partition is interposed between the thermal and auxiliary chambers, the monitoring equipment is in the auxiliary chamber and the carrier having products under test mounted thereon is in the thermal chamber. A conveying mechanism is in the auxiliary chamber and the support platform is in driven engagement with such mechanism, thereby causing movement of the platform along the auxiliary chamber.

Description:
RELATED APPLICATION 
     This application is a division of application Ser. No. 08/841,216, filed Apr. 30, 1997, now U.S. Pat. No. 6,005, 404. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to electrical measuring and testing and, more particularly, to product measuring and testing involving temperature cycling. 
     BACKGROUND OF THE INVENTION 
     Environmental test chambers are widely used to test electrical products such as printed circuit boards and other electrical/electronic products which include a printed circuit board as a component part. Such testing involves monitoring certain electrical operating characteristics of the product while it is undergoing extreme changes in temperature. Temperature cycling over a range of +125° C. to −65° C. (about +255° F. to −85° F.) is not uncommon and is often accompanied by extreme changes in humidity and/or by vibrating the product under test. In the industry, testing of this type is often referred to as “stress testing.” 
     A primary reason that product stress testing is undertaken is to identify particular products (within a larger group of products) which exhibit characteristics evidencing probable premature failure. And such testing is intended to cull out those products which actually fail during test. In the vernacular of the industry, such products are said to exhibit “infant mortality.” Those products which do not exhibit infant mortality are much more reliable in the automotive, aircraft, military or other application in which they are used. U.S. Pat. No. 3,656,058 (Leathers); U.S. Pat. No. 4,683,424 (Cutright et al.); U.S. Pat. No. 4,949,031 (Szasz et al.) and U.S. Pat. No. 5,021,732 (Fuoco et al.) all disclose apparatus used for environmental stress testing. 
     Designers of environmental test chambers must deal with a number of engineering considerations. One is the rapidity with which the temperature in the product-containing chamber can be changed. In a test chamber having refrigeration and heating systems of a particular size, the rate at which the temperature can be changed is, in significant part, a function of the mass (e.g., the mass of the products and product carriers) in the chamber. This is so because the heat absorbed by the chamber contents and the heat which must be removed therefrom is a function of such mass. 
     A household refrigerator is a good analogy of the foregoing. For a particular refrigerator, two pounds of foodstuffs in the refrigerator are more quickly cooled to a particular temperature than twenty pounds of foodstuffs. And as a corollary, cooling twenty pounds of foodstuffs to a particular temperature within a stated time requires a larger refrigeration unit than is required to cool two pounds of foodstuffs to the same temperature within the same time. 
     Another engineering consideration involves the equipment used to monitor the electrical operating characteristics of the products under test. Such equipment is temperature sensitive and must be maintained nominally at room ambient conditions. In other words, such equipment should not be in the chamber with the products undergoing test. 
     Still another engineering consideration is whether to configure the test chamber for batch-process or continuous- process testing. The apparatus of above-noted Szasz et al. patent is for batch testing in that a number of products are placed on a pallet which is inserted into the chamber. All the products on the pallet undergo test simultaneously and after such test is completed, the pallet and its “batch” of products is removed and another pallet loaded with products to be tested is inserted. 
     On the other hand, the vibration chamber disclosed in U.S. Pat. No. 5,226,326 (Polen et al.) may be referred to as a type of continuous testing arrangement. Such chamber uses a conveyor having spaced pairs of rollers to grasp respective edges of flatwise-oriented printed circuit boards to be tested. Similarly, U.S. Pat. No. 5,397,998 (Soeno et al.) discloses several different arrangements of a conveyor and products to be tested carried atop such conveyor. In one arrangement, feeder apparatus along the conveyor supply electric power to the products during burn-in and the “burned-in” product are then tested after exiting at the end of the conveyor. 
     While these earlier arrangements are thought to have been generally satisfactory for their intended uses, they are not without disadvantages for some types of applications. For example, the conveying arrangements shown in the Soeno et al. patent apparently do not permit instrumented product testing while the product is moving through the chamber. In other words, such instrumented testing is carried out after the product leaves the burnin chamber. The “failure mode” characteristics exhibited by the products while in the burn-in chamber and after they leave such chamber may differ markedly. 
     Yet another disadvantage of the conveying arrangements of the Soeno et al. patent is that the mass of the conveyor (as well as that of the product to be tested) is in the burn-in chamber. Chamber temperature changes can be accomplished and maintained only by adding heat to or removing heat from the conveyor components. 
     Still another disadvantage of prior art arrangements is that they seemingly have not appreciated how to configure test chambers so that the size and capacity thereof can be selected or changed to suit a particular application. For example, the arrangement shown in FIG. 5 of the Polen et al. apparently has a fixed length which cannot be changed. At least, there is no suggestion to the contrary. 
     A new environmental test apparatus which addresses certain shortcomings of earlier apparatus would be an important advance in the art. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide an environmental test apparatus which overcomes certain problems and shortcomings of the prior art. 
     Another object of the invention is to provide an environmental test apparatus which is useful for stress testing electrical and electronic products. 
     Another object of the invention is to provide an environmental test apparatus which permits simultaneous product electrical testing and temperature stress testing. 
     Still another object of the invention is to provide an environmental test apparatus which minimizes the mass contained in the thermal chamber. 
     Another object of the invention is to provide an environmental test apparatus which implements continuous process testing. 
     Another object of the invention is to provide an environmental test apparatus by which continuous process testing may be carried out while maintaining the test instrumentation substantially at room ambient conditions. 
     Yet another object of the invention is to provide an environmental test apparatus which may be “custom-configured” for any one of a variety of applications. How these and other objects are accomplished will become apparent from the following descriptions and from the drawings. 
     SUMMARY OF THE INVENTION 
     The invention involves an environmental test apparatus of the type having a thermal chamber for stress testing electronic products. Such apparatus includes a carrier for supporting the products in the chamber and instrument-type test equipment for monitoring product characteristics during stress testing. 
     The improved apparatus has an auxiliary chamber and a flexible partition interposed between the thermal and auxiliary chambers. The carrier is in the thermal chamber (where the temperature environment is made intentionally harsh) and the test equipment is in the auxiliary chamber which is nominally at room ambient temperature and humidity. A highly preferred embodiment of the carrier includes a flat, sheet-like product such as a feedthrough card having attached thereto a fixture for releasably mounting an electronic product, e.g., a printed circuit board, on the carrier. 
     The carrier includes first and second opposed, parallel carrier surfaces and the partition includes first and second partition members bearing against the first and second carrier surfaces, respectively. A preferred partition member is a strip-mounted brush with carbon-bearing bristles for reducing static electricity. Separate partition members are mounted at either side of the carrier and resiliently bear against a respective carrier surface to prevent significant air transfer between the chambers. 
     It is particularly desirable to prevent warmer, more humid air in the auxiliary chamber from migrating or transferring to the thermal chamber since such transfer speeds the rate at which frost accumulates in the thermal chamber. In a highly preferred apparatus, there are plural partitions, one being a primary partition. One (and preferably two) auxiliary partitions are spaced from one another and from the primary partition and are interposed between the primary partition and the auxiliary chamber. 
     While the partition members perform acceptably to prevent air transfer between chambers and, particularly, to prevent such transfer from the auxiliary chamber to the thermal chamber, it is preferred to take yet additional measures to inhibit air transfer. The spaced-apart partitions define a flow path between them. Dry air is caused to flow along the flow path, thereby substantially preventing moisture from migrating from the auxiliary chamber to the thermal chamber. 
     The new apparatus has yet other features which facilitate movement of the carrier along the thermal chamber and which facilitate product monitoring during temperature stress testing. The apparatus includes a support platform in the auxiliary chamber. The carrier is attached to the support platform and extends downwardly through the partition. An exemplary support platform is a square or rectangular slab made of DELRIN™ plastic or the like. 
     A conveying mechanism is also in the auxiliary chamber and the platform is in driven engagement with the conveying mechanism to cause movement of the platform along the auxiliary chamber. A specific conveying mechanism has two spaced-apart sections, each such section comprising an endless belt supported by rollers and, typically, driven by one roller. The spaced-apart edges of the platform rest on respective conveying sections. 
     It is also highly desirable to be able to electrically operate the products under stress test as they move along through the apparatus. Thus, the auxiliary chamber contains a power bus embodied as two parallel, spaced-apart rails. The support platform includes collector shoes in electrically-conductive relationship to the power bus, thereby providing power to products mounted on the carrier. In one preferred embodiment, a test board or a board computer is in the auxiliary chamber, is supported by the platform, moves with such platform and is electrically connected to both the power bus and to the product(s) under test. 
     To better accommodate several product carriers with their associated platforms, the thermal chamber is horizontally elongate and an air plenum is coextensive with the thermal chamber. The air plenum is isolated from the thermal chamber along most of the length of both but there are openings at either extreme end of the plenum and thermal chamber so that the plenum is in air flow communication with such chamber. A motor-driven blower, e.g., a centrifugal blower, urges air along the plenum and the thermal chamber. A refrigeration evaporator is in the air plenum so that the air being circulated along the plenum and the thermal chamber may be cooled for stress test purposes. 
     The new apparatus is thoughtfully configured so that it can be applied in any one of several application requirements. The apparatus includes a plurality of modules attached to one another. The modules cooperatively function in “building block” fashion in that a portion of the thermal chamber is in each of the modules. Stated in other words, the modules can be attached to one another end to end to form an apparatus of the desired length and product testing capacity. The plurality of modules includes at least one module having a door mounted for movement between an open position for placing products in the thermal chamber and a closed position for stress testing the products. Typically, a door module is placed at either end of the assembly of modules. And, conveniently, each module has levelling feet, thereby permitting the modules to be aligned with one another. 
     Because the new apparatus has thermal and auxiliary chambers which are isolated from one another and because the auxiliary chamber is essentially always at room ambient temperature, the chambers preferably have differing wall configurations. The auxiliary chamber has a plurality of first walls, the thermal chamber has a plurality of second walls and each of the second walls is substantially thicker than each of the first walls. And each of the second walls includes an insulation layer making up at least one-half of the wall thickness. 
     Other details of the new apparatus are set forth in the following detailed description and in the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representative front elevation view of the new test apparatus using plural modules. Separating panels and air flow paths within the apparatus are shown in dashed line. Parts are broken away. 
     FIG. 2 is a top plan view of the apparatus of FIG.  1 . Parts are broken away. 
     FIG. 3 is an elevation view of plural modules used in the apparatus of FIGS. 1 and 2. One module is broken away. 
     FIG. 4 is a sectional elevation view of a module taken along the viewing axis VA 4  of FIG. 2 or FIG.  3 . 
     FIG. 5 is an elevation view of the product carrier and the product mounted thereon as shown in FIG.  4 . 
     FIG. 6 is an elevation view of a portion of FIG. 4 enlarged to show additional detail. Parts are broken away. 
     FIG. 7 is an elevation view of a flexible partition shown in conjunction with a feedthrough card of the product carrier. 
     FIG. 8 is a perspective view of a typical partition member used in the partition of FIG.  7 . 
     FIG. 9 is a view of a conveyor section used in the apparatus of FIG.  1 . Parts are broken away. 
     FIG. 10 is a top plan view of a portion of the product carrier shown in FIGS. 4,  5  and  6 . Such view is along viewing axis VA 10  of FIG.  6 . 
     FIG. 11 is a representative elevation view of a door module of the apparatus of FIG.  1 . Such elevation view is along viewing axis VA 4  of FIGS. 2 or  3 . 
     FIG. 12 is an elevation view of the door module of FIG. 11 taken along the viewing axis VA 12  thereof. 
     FIG. 13 is a representative elevation view of a portion of another embodiment of the new test apparatus which incorporates a movable test probe. Parts are broken away. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Before describing details of the new test apparatus  10 , it will be helpful to have an overview description of its general arrangement and operation. Referring first to FIGS. 1,  2  and  3 , an exemplary embodiment of the new environmental test apparatus  10  has first and second sections  11  and  13 , respectively. The first section  11  has several modules  15  coupled to one another and the section  11  is terminated by an entry door module  17  at one end and by an exit door module  19  at the other end. 
     It is to be appreciated that either section  11 ,  13  may be used alone to conduct only cold stress testing or only hot stress testing, respectively. When a section  11  is used as an apparatus  10  for cold stress testing only, it is highly preferred to configure the exit door module  19  to have two spaced-apart doors  21 . In that way, the doors  21  can be operated sequentially to provide what might be described as an air lock. (It will be apparent from the specification that the doors  21  may be used to remove products from the section  11  and that the right-hand door  21  may be used to place products into section  13 .) 
     Each of the modules  15  has an access door  23  for convenient entry into the thermal chamber  25 . And, conveniently, each module  15 ,  17 ,  19  has levelling feet  27 , thereby permitting the modules  15 ,  17 ,  19  to be aligned with one another. Referring also to FIG. 4, an auxiliary chamber  29 , described in more detail below, is between the walls  31 ,  33 . 
     The thermal chamber  25  is horizontally elongate (at least by virtue of two or more modules  15  being attached to one another) and a part of such chamber  25  is within each module  15  behind (to a viewer of FIG. 3) a panel  35 . An air plenum  37  is below the thermal chamber  25  and along most of the length of both, the chamber  25  and the plenum  37  are isolated from one another by an imperforate panel  39 . But the panel length (measured left-right in FIG. 1) is less than the length of the plenum  37  and chamber  25 , thereby providing openings  41 ,  43  at respective extreme ends of the plenum  37  and chamber  25 . A blower  45 , e.g., a centrifugal blower, driven by a motor  46  urges air along an elongate, “race track shaped” path  47  through the plenum  37  and the thermal chamber  25 . A heat exchange device is embodied as a refrigeration system  49 , the evaporator  51  of which is in the air plenum  37 . The air being circulated along the plenum  37  and the thermal chamber  25  may thereby be cooled for stress test purposes. 
     From the foregoing, it is apparent that the first section  11  is configured for use only in depressed-temperature (i.e., depressed from ambient temperature) stress testing. The second section  13  is configured for use only in elevated-temperature stress testing. 
     The second section  13  differs from the first in that a heating system  53  rather than a refrigeration evaporator  51  is in the air plenum  37 . In a specific embodiment, the system  53  includes electrical resistance heaters. 
     Referring also to FIGS. 5,  6  and  7 , the general interior arrangement of the apparatus  10  includes a flat, vertically-oriented carrier  57  on which a product  59  to be tested is mounted. The carrier  57  is suspended from a platform  61  which is urged along the auxiliary chamber  29  by a conveyor  65  and flexible partitions  67  isolate the auxiliary chamber  29  and the thermal chamber  25  from one another. These features will now be described in more detail. 
     The new test apparatus  10  has an enclosure  69 , within which is the auxiliary chamber  29 , the thermal chamber  25  and the air plenum  37 . The auxiliary chamber  29  is bounded in part by first walls  31 ,  33 ,  71  which are substantially free of insulation and the thermal chamber  25  is bounded in part by second walls  73 ,  75 ,  77 , each comprised in significant part of an insulation layer  79 . In a specific embodiment, the thickness of each layer  79  is over one-half of the total thickness of a wall  73 ,  75 ,  77 . 
     Referring also to FIG. 8, opposed barrier walls  81 ,  83  extend from the rear and front, respectively, of the apparatus  10  and each wall  81 ,  83  terminates in a mounting piece  85 . The pieces  85  are spaced slightly from one another and serve to positionally hold a flexible primary partition  67   a,  the purpose of which is described below. Such primary partition  67   a  is comprised of first and second partition members  87 ,  89 , respectively, and a preferred partition member  87 ,  89  includes a brush  91  affixed to an extruded aluminum strip  93  and having flexible bristles  95  which are carbon-bearing bristles for reducing static electricity. Appropriate partition members  87 ,  89  are available from Sealeze Corporation, Richmond, Va. 
     A highly preferred apparatus  10  has plural partitions  67 , the first auxiliary partition  67   b  also being comprised of a separate set of members  87 ,  89 , and the second auxiliary partition  67   c  further being comprised of yet another separate set of members  87 ,  89 . The partitions  67   b,    67   c  are spaced from one another and as is apparent from FIGS. 4 and 6, are interposed between the primary partition  67   a  and the auxiliary chamber  29 . As shown in FIGS. 6 and 7, the brushes  91  of the members  87 ,  89  are angled with respect to one another and to the carrier card  107  and each such brush  91  defines an acute angle A 1  with the carrier  57 . Such angled orientation is preferred since, when there is no carrier card  107  between the members  87 ,  89 , such members straighten somewhat and enmesh to form a seal through which air does not easily pass. 
     A support platform  61  has standoffs  97  mounted atop it for securing a test bed  99  having exposed electrical contact points. An exemplary support platform  61  is a square or rectangular slab about one-half inch thick (about 1.27 cm.) and made of DELRIN™ plastic or the like. 
     Instrument-type test boards such as a load board  101  or a computer board  103  may be mounted on the test bed  99 . A load board  101  applies a load to the product  59  under test to simulate the load or aspects of the load the product  59  will experience in actual application. A computer board  103  periodically “interrogates” the product  59  under test and stores the information for later retrieval and analysis. Points on the test bed  99  and points on the carrier  57  are in electrical contact with one another using edge connectors  105  or other known means. 
     And it may be desirable to “exercise” a product  59  undergoing stress test by carrying out an operating regimen substantially like that the product  59  will experience in an actual application. To that end, an embodiment of the apparatus  10  shown in FIG. 13 includes a probe  106 . The probe  106  is configured to be lowered and raised to electrically connect with the load board  101 , board computer  103 , the test bed  99  or some other “interface” board supported by the platform  61  and connected to the product  59 . The probe  106  is further described below. 
     A preferred carrier  57  includes what is known as a “feedthrough card  107 .” A typical card  107  is thin, generally flat, rectangular and has a substantial number of electrical conductors laminated between two insulating layers bonded together. Each of the conductors is “brought out,” i.e., exposed at opposite card ends, for making electrical connections thereto. Attached to the carrier  57  is a fixture  109  for releasably mounting an electronic product  59 , e.g., a printed circuit board, on the carrier  57 . 
     The carrier  57  is coupled to the support platform  61  and extends downwardly through the partitions  67   a,    67   b,    67   c.  The partition members  87 ,  89  of each partition  67   a,    67   b,    67   c  bear against respective surfaces  111 ,  113  of the card  107 . As is now apparent, the partitions  67   a,    67   b,    67   c  coact to prevent significant air transfer between the chambers  25 ,  29 . 
     It is particularly desirable to prevent warmer, more humid air in the auxiliary chamber  29  from migrating or transferring to the thermal chamber  25  since such transfer speeds the rate at which frost accumulates in the thermal chamber  25 . To the end of further inhibiting transfer of humid air to the thermal chamber  25 , the primary and first auxiliary partitions  67   a,    67   b,  respectively, define a flow path  115  between them. Dry air is caused to flow along the flow path  115 , thereby substantially preventing moisture from migrating from the auxiliary chamber  29  to the thermal chamber  25 . Good results are obtained by flowing dry air along either one of the flow paths  115 ,  117  or along both. 
     Referring next to FIGS. 4,  6  and  9 , the conveyor  65  has two spaced-apart sections  119 ,  121 , each such section  119 ,  121  comprising an endless belt  123  supported by rollers  125  and driven by one roller  125 . The spaced-apart edges  127  of the platform  61  rest on respective conveying sections  119 ,  121 . (For clarity, FIG. 6 shows the platform edges  127  spaced slightly above respective conveying sections  119 ,  121 .) 
     Referring now to FIGS. 4,  5 ,  6  and  10 , it is also highly desirable to be able to electrically operate the products  59  under stress test as they move along through the apparatus  10 . Thus, the auxiliary chamber  29  contains a power bus embodied as two parallel, spaced-apart rails  129 . The support platform  61  includes collector shoes  131  in electrically-conductive relationship to the power bus rails  129 , thereby providing power to products  59  mounted on the carrier  57 . 
     It is highly preferred that electrical continuity between the rails  129 , and the support platform  61  and board  101  or  103  mounted thereon be maintained. To that end, the support platform  61  includes a first set of shoes comprising shoes  131   a,  which are electrically connected in parallel. Similarly, there is a second set of shoes comprising shoes  131   b,  which are electrically connected in parallel. 
     In a specific embodiment, each shoe  131  pivots about an axis  135  and is urged toward its respective rail  129  by springs  137 . When the platform  61  is so configured, a temporary “bounce” of less than all of the shoes  131   a  or  131   b  away from the respective rail  129  will not interrupt the continuity of power to the platform  61 . 
     Referring to FIGS. 1,  2 ,  4 ,  11  and  12 , the apparatus  10  has at least one module  17  with a door  21  mounted for movement between an open position for placing products  59  in the thermal chamber  25  and a closed position for stress testing the products  59 . Door movement is by pneumatic cylinders  139 . When the door  21  is closed, it is sealed by an air-inflated seal of a known type. And frost formation on the doors  21  is inhibited by a door heater. 
     FIGS. 11 and 12 show a single door entry module  17  for purposes of explanation. And the door  21  is broken away to show the interior of the module  17  which is also equipped with conveyor sections  119 ,  121 , bus bars  129  and the like. After appreciating the specification, an exit module  19  and a double door module as shown in FIG. 1 may readily be constructed. 
     In a highly preferred embodiment, the modules  15 ,  17 ,  19  are made using fiberglass sheets  141  for the outer surfaces and aluminum sheets for the inner surfaces  143 . Aluminum structural extrusions  145  available from 80/20, Inc. of Columbia City, Ind. are very useful for joining sheets  141 ,  143  at corners and at sheet edges. 
     Referring to FIG. 13, the probe  106  includes a stiffening plate  149  having electrical boards  151  mounted thereon. Spring-loaded pins  153  (sometimes referred to as “pogo pins”) extend through the plate  149  and the boards  151 . (The boards  151  and pins  153  are sometimes referred to in the industry as a “bed of nails.”) 
     When the probe  106  is lowered, the pins  153  contact respective points on the bed  99 . Guide rods  155  extend through the plate  149  and the plate  149  and the boards  151  move upwardly and downwardly under the control of a pneumatic cylinder  157 . It is to be appreciated that the probe  106  may be mounted for vertical movement only rather than for both vertical movement (i.e., up/down in FIG. 13) and horizontal movement (i.e., left/right in FIGS. 1,  2  and  3 ). But given the specification, persons of ordinary skill will readily appreciate how to configure the probe  106  to move horizontally along the auxiliary chamber  29 . 
     As used herein, the phrase “dry air” means air, the relative humidity of which is in the range of 5-10% or less. As used herein, the term “thermal chamber” means a chamber, the interior of which exhibits wide excursions of temperature, e.g., temperature “swings” of +125° C. to +23° C. (about +255° F. to +73° F.), +23° C. to −65° C. (about +73° F. to −85° F.), or even +125° C. to −65° C. (about +255° F. to −85° F.) during stress testing. The term “stress testing” means testing electrical products using one or more wide temperature excursions. 
     While the principles of the invention have been shown and described in connection with a few preferred embodiments, it is to be understood clearly that such embodiments are by way of example and are not limiting. As other examples, the new apparatus  10  may be configured to provide “hot only” stress testing, i.e., stressing testing involving temperatures above room ambient. In that instance, the module  15  terminating the section  13  may not require a door  21 . A hot air “curtain,” or “knife” as it is sometimes called, is adequate. 
     And cold stress testing may be carried out using a refrigeration unit separate from and ducted to the apparatus  10 . Such unit preferably has two redundant evaporators  51  so that one evaporator  51  may be disabled and defrosted while the other evaporator  51  is cooling air flowing through the thermal chamber  25 .