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

${\displaystyle AF_{\text{H}}=e^{{\text{const}}\cdot (RH_{\text{s}}^{n}-RH_{\text{o}}^{n})},}$

${\displaystyle {\text{const}}}$ is a value which normally goes from 0.1 to 0.15

where RH_s is the stressed humidity, RH_o 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

${\displaystyle AF_{T}=e^{(E_{\text{a}}/k)(1/T_{\text{o}}-1/T_{\text{s}})},}$

where E_a is the activation energy for the temperature-induced failure (most often 0.7 eV for electronics), k is the Boltzmann constant, T_o is the operating temperature in kelvins, and T_s is the stressed temperature.

Therefore the total acceleration factor for unbiased HAST testing is

${\displaystyle AF_{\text{HAST}}=AF_{\text{H}}\cdot AF_{T}=e^{{\text{const}}\cdot (RH_{\text{s}}^{n}-RH_{\text{o}}^{n})}e^{(E_{\text{a}}/k)(1/T_{\text{o}}-1/T_{\text{s}})}.}$