Patent Publication Number: US-9885650-B2

Title: Accelerated life testing device and method

Description:
RELATED APPLICATION 
     The present disclosure claims priority to and the benefit of U.S. patent application Ser. No. 14/086,750, filed on Nov. 21, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The technical field of this disclosure is qualitative testing devices and methods, particularly, accelerated life testing devices and methods. 
     BACKGROUND OF THE INVENTION 
     Highly Accelerated Life Testing (HALT) is a qualitative test method used to accelerate and identify failures in products, such as medical devices. The products are tested to failure to find failure modes and to identify the root causes of product and sub-system failures. Once failure modes and causes are identified, the product design can be improved to prevent or reduce the identified failures. The improved product design can then be retested to confirm that the identified failures are reduced. The HALT process results in rugged designs and high reliability products. 
     The HALT process can apply any stimulus to a product under test that can accelerate failure in the product, providing an indication of failures likely to occur in the field. One stimulus is temperature stress, which can be used in electronics testing to identify failures due to marginal components, poor timing margins, and poorly mounted heat sinks. Another stimulus is vibration, which can be used in mechanical and electronics testing to identify failures due to poor solder joints, loose hardware, and contact and wear between adjacent parts. Other stimuli used to accelerate failure can include general humidity, over-voltage, and over-current. 
     Unfortunately, present HALT processes are not able to provide a stimulus which accelerates oxidation in the product under test. Oxidation in the field normally occurs over a number of years, so failures in products in the field are too late to contribute to improved product design. The inability to accelerate oxidation during HALT processes prevents product design improvements from determining how oxidation affects the function of the device and identifying areas on which to focus design efforts. This, in turn, prevents achievement of highest product quality, product reliability, and patient safety. 
     It would be desirable to have an accelerated life testing device and method that would overcome the above disadvantages. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides an accelerated life testing method for a test piece within a test chamber, the method including: establishing a first atmosphere within the test chamber; changing the first atmosphere to a second atmosphere to form a deposition layer on the test piece; changing the second atmosphere to the first atmosphere to remove the deposition layer from the test piece; and repeating the changing the first atmosphere to the second atmosphere and the changing the second atmosphere to the first atmosphere to form an oxidation layer on the test piece. 
     Another aspect of the invention provides an accelerated life testing device for use on a test piece, the device including: a test chamber for containing the test piece; and an atmospheric controller operably connected to the test chamber, the atmospheric controller being operable to control temperature and humidity within the test chamber. The atmospheric controller is operable to form an oxidation layer on the test piece by: establishing a first atmosphere within the test chamber; changing the first atmosphere to a second atmosphere to form a deposition layer on the test piece; changing the second atmosphere to the first atmosphere to remove the deposition layer from the test piece; and repeating the changing the first atmosphere to the second atmosphere and the changing the second atmosphere to the first atmosphere to form the oxidation layer on the test piece. 
     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention, rather than limiting the scope of the invention being defined by the appended claims and equivalents thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an accelerated life testing device made in accordance with the invention. 
         FIGS. 2A-2D  are diagrammatic views of a test specimen undergoing accelerated life testing in accordance with the invention. 
         FIG. 3  is photocopy of a photograph of a test specimen showing oxidation structure from accelerated life testing in accordance with the invention. 
         FIG. 4  is a flow chart of an accelerated life testing method in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram of an accelerated life testing device made in accordance with the invention. Changing between a first atmosphere and a second atmosphere in the test chamber of the accelerated life testing device alternately forms and removes a deposition layer on the test piece, causing the surprising and unexpected result that an oxidation layer forms on the test piece. 
     The accelerated life testing device  100  is for use on a test piece  102 . The accelerated life testing device  100  includes a test chamber  110  for containing the test piece  102  and an atmospheric controller  120  operably connected to the test chamber  110 . The atmospheric controller  120  is operable to control temperature and humidity within the test chamber  110 . The atmospheric controller  120  is operable to form an oxidation layer on the test piece  102  by establishing a first atmosphere within the test chamber  110 ; changing the first atmosphere to a second atmosphere to form a deposition layer on the test piece  102 ; changing the second atmosphere to the first atmosphere to remove the deposition layer from the test piece  102 ; and repeating the changing the first atmosphere to the second atmosphere and the changing the second atmosphere to the first atmosphere to form the oxidation layer on the test piece  102 . The accelerated life testing device  100  can optionally include a vibration table  130  to which the test piece  102  can be secured and vibrated for testing response to vibration. The accelerated life testing device  100  can also optionally include atmospheric sensors and control systems to monitor and automatically control the atmospheric conditions within the test chamber  110 . 
     The test piece  102  as defined herein can be any component or assembly of components to which a Highly Accelerated Life Testing (HALT) process is to be applied. In one embodiment, the test piece  102  can be a personal medical device, such as an insulin pump, a continuous glucose monitor, or the like. Other exemplary personal medical devices which can be used as a test piece  102  include pumps, cell pumps, heart-rate monitors, ECG monitors, pulse oximeters, blood pressure monitors, respiration rate monitors, skin temperature monitors, electroencephalography (EEG) monitors, activity level monitors, vital sign monitors, and the like. The surfaces of the test piece  102  on which the oxidation layer forms can be made of metal or any other material desired on which an oxidation layer can form. 
     The test chamber  110  can be any suitable enclosure for establishing an atmosphere around the test piece  102 . In one embodiment, the test chamber  110  can provide a closed atmosphere, i.e., the test chamber  110  is sealed or substantially sealed from the outside environment so that gases and/or materials from the outside environment are not exchanged with test chamber  110 . In another embodiment, the test chamber  110  can provide an open atmosphere, i.e., gases and/or materials from the outside environment are added to or removed from the test chamber  110 . 
     The atmospheric controller  120  controls temperature and humidity within the test chamber  110 , changing between a first atmosphere and a second atmosphere in the test chamber  110  to alternately form and remove a deposition layer on the test piece  102 . This causes an oxidation layer to form on the test piece  102 . The atmospheric controller  120  can be used for temperature stress testing and/or oxidation layer formation. 
     In one embodiment, the atmospheric controller  120  includes a temperature controller  122  and a humidity controller  124 . The temperature controller  122  and the humidity controller  124  can be located within the test chamber  110 , can communicate between the outside environment and the test chamber  110 , or can be located within a loop through which air and/or other gases are withdrawn from and returned to the test chamber  110 . 
     The temperature controller  122  can increase or decrease the temperature within the test chamber  110 . Examples of temperature controllers include a liquid nitrogen source, a cold gas source, a hot gas source, a refrigeration coil, a heating element, and the like. The gas sources (liquid nitrogen source, cold gas source, hot gas source) introduce gases from the outside environment into the test chamber  110 . The sealed sources (refrigeration coil, heating element) do not introduce material from the outside environment into the portion of the test chamber  110  including the test piece  102 . 
     The humidity controller  124  can increase or decrease the humidity within the test chamber  110 . Examples of humidity controllers include a water mister, a water drop injector, a dehumidifier, a desiccant, and the like. The humidity controller  124  can add moisture to or remove moisture from the atmosphere within the test chamber  110 . The water sources (water mister, water drop injector) introduce water from the outside environment into the test chamber  110 . The water removers (dehumidifier, desiccant) can condense or absorb moisture from the atmosphere within the test chamber  110 . 
     The accelerated life testing device  100  can also optionally include an atmospheric sensor  140  to sense atmospheric conditions within the test chamber  110 . The atmospheric sensor  140  can optionally generate an atmospheric signal  150  in response to the sensed atmospheric conditions, and the atmospheric controller  120  is responsive to the atmospheric signal  150  to control the atmospheric conditions within the test chamber  110 . In one example, the atmospheric sensor  140  is a temperature sensor to sense the temperature within the test chamber  110 . The temperature sensor can generate a temperature signal in response to the sensed temperature, and the atmospheric controller can be responsive to the temperature signal to control the temperature within the test chamber  110 . In another example, the atmospheric sensor  140  is a humidity sensor to sense the humidity within the test chamber  110 . The humidity sensor can generate a humidity signal in response to the sensed temperature, and the atmospheric controller can be responsive to the humidity signal to control the humidity within the test chamber  110 . When the atmospheric controller  120  is responsive to the temperature signal and the humidity signal, the temperature signal and the humidity signal can be used separately or in combination to establish or change the atmospheric conditions within the test chamber  110 . 
     The accelerated life testing device  100  can optionally include a vibration table  130  to which the test piece  102  can be secured for vibration testing. Pneumatic or electrodynamic actuators  132  can be used to move the vibration table  130  in the test chamber  110 . Vibration testing can be applied simultaneously or sequentially with temperature stress testing and/or oxidation layer formation applied by the atmospheric controller  120 . 
       FIGS. 2A-2D  are diagrammatic views of a test specimen undergoing accelerated life testing in accordance with the invention. An oxidation layer forms on the test piece from alternately forming and removing a deposition layer on the test piece. 
     Referring to  FIG. 2A , a test piece  202  is located within a first atmosphere  250  within a test chamber (not shown). In this example, the test piece  202  at the start of the accelerated life testing method is bare, without any oxidation layer. 
     Referring to  FIG. 2B , the first atmosphere is changed to a second atmosphere  252  to form a deposition layer  204  on the test piece  202 . The arrows  210  illustrate the moisture coming from the second atmosphere  252  to form the deposition layer  204 . In one embodiment, the temperature of the test piece  202  is below the dew point of the second atmosphere  252  and the deposition layer  204  is liquid water. In another embodiment, the temperature of the test piece  202  is below the frost point of the second atmosphere  252  and the deposition layer  204  is frost or ice. 
     Referring to  FIG. 2C , the second atmosphere is changed to the first atmosphere  250  to remove the deposition layer  204  from the test piece  202 . The arrows  212  illustrate the moisture leaving the deposition layer  204  and entering the first atmosphere  250 . In one embodiment, the deposition layer  204  is liquid water, which evaporates into the second atmosphere  252 . In another embodiment, the deposition layer  204  is frost or ice, which melts into liquid water and runs off the test piece. In yet another embodiment, the deposition layer  204  is frost or ice, which melts into liquid water and evaporates into the second atmosphere  252  (dual phase removal). In yet another embodiment, the deposition layer  204  is frost or ice, which sublimates directly into the second atmosphere  252  (single phase removal). Those skilled in the art will appreciate that in another embodiment, the second atmosphere can be changed to another atmosphere different than the first atmosphere to remove the deposition layer  204 . 
     Referring to  FIG. 2D , the deposition layer on the test piece has been alternately formed and removed a number of times by repeatedly changing between the first atmosphere and the second atmosphere, so that an oxidation layer  206  has formed on the test piece  202 . The oxidation layer  206  introduces an additional failure mode when used in Highly Accelerated Life Testing (HALT). In one example, the oxidation layer  206  forms after the deposition layer on the test piece has been formed and removed approximately 10 times. In another example, the oxidation layer  206  forms after the deposition layer on the test piece has been formed and removed over several hours. In one experimental example as illustrated in  FIG. 3 , the oxidation layer  206  of copper oxide was formed on a printed circuit board made of FR 4  fiberglass reinforced epoxy laminate: the oxidation layer was bluish white and non-conductive, and had a thickness between 0.1 and 35 micrometers. 
       FIG. 3  is photocopy of a photograph of a test specimen showing oxidation structure from accelerated life testing in accordance with the invention. In this example, the test piece is a printed circuit board  302  with an oxidation layer  306 . The formation of the oxidation layer  306  on the circuit board  302  was a surprising and unexpected result. The oxidation layer  306  was formed by alternately placing the circuit board  302  in a first atmosphere at 70 degrees Centigrade and 0 percent humidity and a second atmosphere at −35 degrees Centigrade. The atmosphere was changed from the first atmosphere to the second atmosphere by admitting liquid nitrogen containing moisture into the test chamber. The atmosphere was changed from the second atmosphere to the first atmosphere by heating the inside of the test chamber with an electrically powered resistive heating element. The oxidation layer  306  formed after the deposition layer on the test piece had been formed and removed 10 times over several hours. The oxidation layer  306  was found to be made of copper oxide and have a thickness between 0.1 and 35 micrometers. As illustrated in  FIG. 3 , the oxidation area extended beyond the surface of the copper on the circuit board and extended onto the circuit board and nearby components. 
       FIG. 4  is a flow chart of an accelerated life testing method in accordance with the invention. The accelerated life testing method  400  for a test piece within a test chamber includes: establishing a first atmosphere  410  within the test chamber; changing the first atmosphere to a second atmosphere to form a deposition layer  420  on the test piece; changing the second atmosphere to the first atmosphere to remove the deposition layer  430  from the test piece; and repeating the changing the first atmosphere to the second atmosphere and the changing the second atmosphere to the first atmosphere to form an oxidation layer  440  on the test piece. The method  400  can optionally include vibrating the test piece. 
     The establishing a first atmosphere  410  within the test chamber includes establishing an atmosphere at a desired temperature and/or humidity. In one example, the first atmosphere is established at 100 degrees Centigrade and 0 percent humidity. In other examples, the temperature for the first atmosphere is in the range of 65 to 100 degrees Centigrade and in the range of 0 to 10 percent humidity. 
     The changing the first atmosphere to a second atmosphere to form a deposition layer  420  on the test piece can include decreasing the temperature within the test chamber, increasing the humidity within the test chamber, or a combination of decreasing the temperature within the test chamber and increasing the humidity within the test chamber. This can form a deposition layer of liquid water, frost, or ice. In one example, the second atmosphere is established at 50 degrees Centigrade. In other examples, the temperature for the second atmosphere is in the range of −30 to −60 degrees Centigrade. Those skilled in the art will appreciate that the conditions of the first atmosphere and the second atmosphere can be selected as desired for a particular application as required to form and remove the deposition layer. 
     The changing the second atmosphere to the first atmosphere to remove the deposition layer  430  from the test piece can include increasing the temperature within the test chamber, decreasing the humidity within the test chamber, or a combination of increasing the temperature within the test chamber and decreasing the humidity within the test chamber. The deposition layer can be liquid water, frost, or ice. When the deposition layer is liquid water, the deposition layer can evaporate. When the deposition layer is frost or ice, the deposition layer can sublimate, or melt to liquid water then evaporate from or run off of the test piece. Those skilled in the art will appreciate that the deposition layer can be partially or fully removed as desired for a particular application, i.e., a portion of the deposition layer can be left on the test piece and the next deposition layer formed on top of that portion of the deposition layer. 
     The repeating the changing the first atmosphere to the second atmosphere and the changing the second atmosphere to the first atmosphere to form an oxidation layer  440  on the test piece can be performed as many times or for as long as desired for a particular application. In one example, the oxidation layer forms after the deposition layer on the test piece has been formed and removed 10 times. In another example, the oxidation layer  206  forms after the deposition layer on the test piece has been formed and removed over several hours. In yet another example, the repeating continues for a predetermined number of 15 times, for a predetermined time of a few days, or until the test piece fails. 
     The method  400  can also include sensing an atmospheric condition within the test chamber, such as temperature, humidity, or a combination of temperature and humidity. The method  400  can also include controlling the atmospheric condition within the test chamber based on the sensed atmospheric condition. 
     The method  400  can optionally include vibrating the test piece. Exemplary vibrations can have a frequency of 6 to 10,000 cycles per second within acceleration parameters 5 to 80 gRMS with random vibration energy density profiles, as desired for a particular application. 
     It is important to note that  FIGS. 1-4  illustrate specific applications and embodiments of the invention, and are not intended to limit the scope of the present disclosure or claims to that which is presented therein. Upon reading the specification and reviewing the drawings hereof, it will become immediately obvious to those skilled in the art that myriad other embodiments of the invention are possible, and that such embodiments are contemplated and fall within the scope of the presently claimed invention. 
     While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.