Google said in a recent report that 30% - 80% of the SSDs used in its data centers gave been identified to have bad blocks within four years of use. Bad block can be defined as a phenomenon in which a physical or electrical damage in a memory storage system makes certain memory cells irrecoverable.

According to experts, SSD durability deteriorates and bad block occurs when reading and writing are repeated thousands of times in an MLC NAND flash memory in wide use. Then, cells subject to the bad block lose their data. According to Google, an SSD storage system is normally less error-prone than a hard disk drive (HDD) storage system but has a higher uncorrectable bit error rate. It added that this problem is more frequent in NAND products that have a floating gate structure.

Samsung Electronics, which is the largest NAND supplier in the world, recently adopted charge trap flash (CTF) for 3D NAND production in order to deal with this SSD NAND reliability issue. CTF is a technique for keeping an electrical signal in a non-conductor and can ensure a higher level of stability than the floating gate structure.