In quality management there is a saying: "Don't drive after drinking." It means any operation that may put quality at risk must be prohibited.
The relationship between accidents and drinking is this: drinking does not necessarily cause an accident, but it greatly raises the probability. When we investigate accident cases, the driver has often been drinking. Our countermeasure: if you've had a drink, you don't drive. So why isn't this rule obeyed? Because most people who drive after drinking don't crash — they form a fluke mentality.
Our quality management philosophy is: because traffic accidents caused by drinking have happened, we ban driving after drinking outright. That is the difference between the absoluteness of quality management and the contingency of an accident. In manufacturing we cannot rely on luck.
A case from the shop floor: a rigid-flex PCB required coverlay film, so we used laser cutting on the coverlay. The laser process produces carbide residue. Because this residue settled between the PCB traces, we saw short-circuit defects. Investigation showed the order quantity was small, so the team took the convenient route of laser cutting. Laser cutting inevitably produces carbide; short circuits were therefore essentially inevitable.
Someone might argue that if we tune the laser energy to the optimal point the problem goes away. Although the probability of a short would then be very low, it still exists. From a quality management standpoint: as long as laser cutting is used, carbide will be produced, and shorts can follow.
Combining both points: no driving after drinking, no laser cutting of coverlay film.
