Design for Testability:Reliability of Chips.

Reliability of Chips

Of course a chip should not only behave correctly immediately after production but should also work correctly for a long period of use. Thus one re- quires a high reliability of chips. In figure 15.69 the typical time of failure during operation is sketched. Since for any kind of technical products  one can determine the average span of life, the failure rate rises when at the end of the expected span of life. But more interesting is that relatively many chips fail during the first period of operation. This is because of physical defects which do not immediately cause an erroneous behavior but cause increasing deviations from the optimal behavior during operation.

Design for Testability-0341

Therefore burn in tests are used to test chips under extreme conditions. For example, one uses high temperature and an increased voltage of the power supply. Other possible stress factors are, for example, mechanical vibrations or extreme variations in temperature. Using such extreme environmental conditions one can simulate a longer period of operation such that early failures will already occur during the burn in test and not during the period of use. After the burn in test the usual chip tests are repeated to eliminate chips which have failed in the meantime. In particular, an IDDQ test is useful  after burn in, because during burn in IDDQ faults may become more obvious.

Since burn in tests are relatively time consuming and expensive one is interested in detecting unreliable chips by a simpler test. One such possibility is the IDDQ test. Indeed, experimentally it can be shown that chips with increased quiescent current will fail earlier [15.29]. In particular, many of the chips which do not pass an IDDQ test will not even pass a functional test after applying a burn in phase. As a consequence applying an IDDQ test will improve the reliability of chips, but un- fortunately the burn in test can not be completely replaced by an IDDQ test.

When using an IDDQ test to improve reliability, then in accordance with the burn in test one should use a supply voltage as high as possible. In this way defects may become more obvious. On the other hand, one should not increase temperature because this causes an increase of quiescent cur- rent that does not correspond to a shorter span of life.

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