Number 304 - September 2008

USB Flash Drive Data Retention
By Carl Tenning, Tacoma Open Group For Microcomputers
Data retention refers to the ability of a memory bit to retain its data state over long periods of time regardless of whether the part is powered on or powered off.     The question of USB flash drive data retention came up at two of the recent TOG meetings. Advertisers are proclaiming data retention of ten years or better, but it was stated at a recent club meeting that data retention was only reliable up to 30 days. Searching the web, the most reliable article I could find, outside of advertising, was from one of the manufactures, Imation. They state that the flash technology stores data in ones and zeros in individual cells made up of two transistors each. One transistor is a floating gate and separated from the other transistor (a control gate) by a thin oxide layer. In the free condition, electrical charge flows freely from the floating gate to the control gate, producing a binary "1". A binary "0" is created by applying a small voltage to the floating gate. The electric charge is then pushed through the thin oxide layer and trapped on the control gate side. To restore the cell to the normal "1" state, an electric field is applied to the cell and the electric charge is restored to the floating gate. So, as stated at our most recent club meeting, data retention is a function of holding an electrical charge. Anyone who knows anything about electrical charge, knows that electrical charge can leak away. So, the stored data depends on the quality of the insulating material. Well, Imation is also stating a data retention of ten years.	But what about repeated usage or external influences like airport x-ray machines, hot or cold temperatures? A technical paper published by Spansion says that these devices are subject to physical device degradation. One factor is endurance, i.e. the cumulative program/erase cycling capability of any individual sector within a device. They state that their single-bit-per-cell floating-gate flash devices are designed to withstand 1,000,000 erase and reprogram cycles prior to any cell failure that results from the erase operation. Another manufacturer, Freecale Semiconductor, generally guarantees a minimum data retention life in its product specification of 10 to 20 years, but that in practice, their technologies are capable of producing much longer lifetimes than the specified minimum. Typical data retention at room temperature might be 100 years, but one hour at 250 degrees C (482 degrees F), is equivalent to 100 years at room temperature. So baking one in a hot oven would not be recommended. However, one person posted a note that one of his flash drives had been through the washing machine twice and the dryer once with apparently no ill effect.        Personally I have had no trouble from the airport x-ray machines. My conclusion is to not worry about it.
Number 304 - September 2008