Complaints with unexpected failures
Statement 7:
Field returns show unexpected failure modes or no defect founds
65% experienced this problem.
No defects found (NDF)
The consequence of:
- unjustified DOA's (Dead On Arrival)
- mismatch of parameters
- intermittent behavior
For the unjustified DOA's see <ST01 High warranty costs>
Another cause is a mismatch of properties between this product and the other parts around. The real cause can be difficult to find certainly if you have no knowledge on the other parts and its application. A correct failure description could help. This by the way is a quality problem.
Another nasty part are those defects which are the intermittent. Often temperature and/or shock sensitive. A correct failure description would help.
These last two types of failures can often be analyzed with a HALT (Highly Accelerated Life Test).
An extra problem is, especially with the more expensive products, that these products with NDF goes back onto the market or service channel.
There were a few examples that the same product (same serial number) was returned as a failure from Japan, Australia en USA. At the home base it was categorized as NDF. Thanks to a good registration system this effect was found. Something must be wrong with that product. Later on the procedure was changed in a way that after the second return this product will not be used anymore.
So, what do we do with it? At least store them. Until we know more. Unfortunately most of them were destroyed. Development had no time for investigation anyway.
Unexpected failure modes
The "thrilling"defects were those products with totally unexpected failure modes. "Thrilling" in the sense of learning something new in reliability engineering. Stories are known about the discovery of whiskers, electro migration, purple plague, all being failure modes never expected but explained afterwards. They were all of a technological nature. Design rules were adapted accordingly. To prevent future problems all this knowledge was present within the reliability engineering department. But with no such department...
Repaired products came back quickly
Also products who were repaired came back soon with a similar failure. The repaired product was not as good as new. In this case the second repairs were stored until we had enough samples for an analysis. Because of that we saw finally that a small design error caused these problems.
Can we prevent this type of failures?
In a lot of cases the answer is yes. But only by a thorough reliability investigation. But for that you need a reliability department. See <ST10 No reliability department>. And sometimes you can not discover them in time and the customer will find out. The reliability world is constantly improving in finding methods and tools and sharing knowledge for this quest.