The highly accelerated stress test (HAST) method was invented by Nihal Sinnadurai while working as a research engineer at British Telecommunications Research Laboratories in 1968 in order to perform highly accelerated reliability testing of electronics components that are likely to encounter humid environments during normal (ambient) operation. The method uses the principle of a non-saturating autoclave designed and engineered with no moving parts, to deliver close temperature (<1°C) and humidity (<2%RH) control and a high reliability of the stress equipment. Nihal Sinnadurai and his team carried out many millions of device-hours of reliability stress testing in HAST chambers and arrived at a clear correlation with a humidity exponent as the stress-accelerating agent.
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 Boltzmann's constant, To is the operating temperature in kelvins, and Ts is the stressed temperature.
Therefore the total acceleration factor for unbiased HAST testing is