Dad worked at a firm that produced high-reliability capacitors (they went into the IBM System/360, etc) and they did 100% testing. IBM would do their own 100% testing upon receipt, and they would still find a few rejects each month. They investigated these of course, but never really found a cause. They marked it up to delayed yield problems from the production line.
The funny thing was Delphi was also a customer (the electronics subsidiary of General Motors). They wouldn't pay for more than normal statistical sampling. When he visited them, they had large bins of defective car radios that wouldn't turn-on, etc. To Delphi it was cheaper to run the production line flat-out and deal with the failures, rather than find problems earlier by inspection of received parts.
The funny thing was Delphi was also a customer (the electronics subsidiary of General Motors). They wouldn't pay for more than normal statistical sampling. When he visited them, they had large bins of defective car radios that wouldn't turn-on, etc. To Delphi it was cheaper to run the production line flat-out and deal with the failures, rather than find problems earlier by inspection of received parts.