SSD Endurance and Temperature
SSD endurance is defined by the amount of data that can be written to a flash-based drive before it becomes unreliable, where reliability is dictated by being able to read data back from an unpowered drive after a set duration of unpowered storage. For most drive endurance specifications reliability is defined as 12 months of storage at 40°C at the end of usable drive life.
The SSD endurance specification can change dramatically when used at extreme temperatures such as those commonly seen in industrial and embedded applications. While operating temperature (particularly at cold temperatures) can have a negative impact on drive endurance, it is storage at hot temperatures that is the focus of this technical document.
Flash data retention is dependent on the ability to determine the level of charge stored in each flash cell. Over time this charge deteriorates and must be refreshed else the data stored in the flash cell will be lost. When the flash is powered, cells are periodically refreshed to “top off” and restore lost charge. However when the flash is not powered (e.g. in storage) cells are not refreshed and charge degrades with time. The rate of charge loss (detrapping) is related to the storage temperature, accelerated with higher temperatures, and slowing with lower.
The acceleration factor for storage at various temperatures can be calculated using the Arrhenius equation:
Ea = Activation energy (1.1 eV for SSD flash)
k = Boltzmann’s constant (8.623 x 10-5 eV/°K)
T1 = Baseline temperature (°K), typically 313°K
T2 = Application storage temperature (°K)
Which gives us the following acceleration factors for various storage temperatures:
||Max storage duration at end of drive life
One observation from this equation is that the rate of charge loss is greatly accelerated as storage temperatures go above 40°C. For instance, at storage temperatures of 85°C the loss is 2 orders of magnitude faster than at 40°C. This effectively shortens the storage duration from 1 year to 2 days at the end of drive life.
For more information on how to determine drive endurance and impact of storage temperature and duration to SSD life see Understanding SSD Life Expectancy.