If there is insufficient data one can proceed with additional experiments and analyses. Then one can clarify the failed mechanism effectively and economically in a shorter period of time. It is possible to develop the accelerated testing in the following procedure by using FTA as a basic tool.
1] Define top event and construct a fault tree.
2] Causes and factors affecting the top event come to the second tier and the next level causes and factors come to the third tier and the like.
3] Choose the causes and factors actually affecting the top event by using the information such as design, process, and field data and others.
4] If there is insufficient data one can proceed with additional experiments and analyses.
5] Select the most probable failed mechanism through the Fault Tree Analysis described above.
6] Design the accelerated testing to detect the top event earlier according to the failed mechanism described in item 5].
7] Prepare samples with design weakness such as the products from a lot which include field failure.
8] Confirm the effectiveness of the accelerated testing with samples described above appling "Design of Experiment".
9] Caluculate the acceleration factor of the accelerated testing by comparing the plotted data from field and the test on Weibull probability paper.
10] Introduce the new test method as tentative specification and follow the field data.
11] If the data obtained above is what you expect and the failed machanism is verified, establish the specification as standards.
3. Application of the procedure to develop the accelerated test for "Termistor Temperature Sensor'
3.1 Structure of resin-molded thermistor sensor
The sensor is constructed as shown in Fig. 2. An epoxy-coated element with fired silver electrodes on both sides of a 0.5x0.5x0.25mm thermistor chip with lead soldered to the electrodes is connected with vinyl-coated wire and further coated with epoxy resin for protection.
Fig. 2 Sensor structure
3.2 Description of the failure and the FT diagrm
The 95% of failures are decrease in resistance and 5% are increase. The range was broad as indicated in the change rates from 5% to 95%.
Since the identification of failing locations was difficult, possible causes were listed in the FT diagram as shown in Fig. 3 to 5.
3.3 Conducting FTA
The primary contents of analysis as to each branch of the FT diagram are as summerized in Table 1 (including Fig. 6〜12 and Photo. 1〜2). Respective general descriptions are provided in the FT diagram.
Based on the results of analysis, possible routes which the failed mechnism may have followed are shown with bold line in Fig 3 to 5.
The FT diagram and Table 1 imply that the resistance decrease is attributable to ionic conduction due to the presence of moisture and ions, and that the resistance increase is due to electrode separation accompanying chemical changes caused by thermal distortion and moisture.
3.4 Identifying the mechanism of failure
By analysing the Fault Tree described above, the failed mechanism is identified as shown in Fig. 13 with respect to the phenomena considered in the actual field use and the result of the failure analyis of the sensors.
