The highly accelerated stress test (HAST) method was first proposed by Jeffrey E. Gunn, Sushil K. Malik, and Purabi M. Mazumdar of IBM.
[1] The acceleration factor for elevated humidity is empirically derived to be where RHs is the stressed humidity, RHo is the operating-environment humidity, and n is an empirically derived constant (usually 1 < n < 5).
The acceleration factor for elevated temperature is derived to be where Ea is the activation energy for the temperature-induced failure (most often 0.7 eV for electronics), k is the Boltzmann constant, To is the operating temperature in kelvins, and Ts is the stressed temperature.
Therefore the total acceleration factor for unbiased HAST testing is