In this video, Microchip explains their EEPROM manufacturing test flow that creates the highest quality (lowest failure rate) products in the market.
Before shipping, every bit on every EEPROM are tested hundreds and sometimes thousands of times, on which they call as the Microchip Triple Test. The first test is called the wafer probe where any dead parts are quickly eliminated. Every die that's still good gets up to 5000 erase write cycles on every bit on the memory. The wafers are then baked at 250 degrees Celsius up to 24 hours using heat to over stress the memory cells. For test two, the wafer is probed wherein the chart strength of the memory cell is measured. If the chart strength has changed noticeably on any memory cell, that part is marked as bad. The IC is then packaged and test three is done on the final part right before it's shipped.