Deciding what to test and how to test it
PoF provides the basis for drives testing programs devised according to the product type and the failure mechanism under investigation (see figure 3). Testing is carried out at all stages: in R&D the aim is to verify that the design and component selection meet both the specifications and customer expectations; in production the purpose is to verify the quality of the product and ensure it continues to perform as designed; and when the product eventually fails, failure analysis can be carried out to identify what went wrong or if the failure was caused by natural wear-out.
Type testing carried out as part of R&D focuses on nominal samples so relatively small sample sizes can be used. Because the samples in the test are nominal products, they should all fail the same way. An important step at the end of the testing process is to analyze the failure to ensure it is indeed a nominal product and the failure was not caused by a defective component or production error. If a defect is found in such a small sample it indicates that there could be a severe quality problem in production.
When testing for defective products, it is vital to select the correct sampling rate. In most cases only a small percentage of the products is defective, so it is necessary to test a large number to verify their true proportion. Statistical theory tells us that if 99% of products are good and the required confidence level is 99%, for example, then we need to test 459 units without a single failure to confirm that the proportion of defective products really is less than 1%.
Highly Accelerated Life Testing (HALT)
The purpose of HALT testing is to probe the product’s weakest links and determine how much overstress it can withstand, i.e. to verify the overstress margins. HALT tests often focus on temperature and vibration, both separately and in combination, with typical test temperatures of -55°C to 150°C and vibration levels up to 50 g. Other stresses commonly used in HALT testing are voltage, current, mechanical shock, over-torqueing of terminals, moisture, etc. Figure 4 shows an example stress profile for HALT testing with temperature and vibration.
HALT testing is most commonly performed on components and sub-assemblies. When the product fails the root causes are analyzed to determine whether a similar failure could occur in a real life situation. In many cases, it turns out that the same type of failure could occur in a real application if certain abnormal conditions arose – such as an accident during transportation or malfunctioning of a cooling system. If necessary the design is improved and HALT testing is repeated to verify that the improvements have had the desired effect.