The Highly-Accelerated Temperature and Humidity Stress Test (HAST)
is performed to evaluate the reliability of non-hermetic packaged solid-state devices that are likely to encounter humid environments during normal (ambient) operation. Also known as Pressure Cooker Test (PCT) or Unsaturated Pressure Cooker Test, the purpose of HAST is to accelerate moisture penetration into the internal parts of the specimen by raising the water vapour pressure inside the test chamber to a level that is drastically higher than the water vapour pressure inside the specimen to evaluate the specimen’s resistance to humidity.
FIGURE: Principle of HAST test
HAST test reduces the time it takes to complete the typical humidity 85 ºC / 85% RH testing for semiconductors (96 HAST hours are equivalent to 1000 THB hours). By elevating temperatures above 100°C (usually up to 130°C) and increasing the pressure, simulation of normal humidity tests can be made while maintaining the same failure mechanisms.
FIGURE: HAST test diagram
The test should be considered as failed if the devices under testing exceed the parametric limits or their functionality cannot be demonstrated under nominal conditions or in accordance with the applicable acquisition document.
FIGURE: ESPEC EHS-411M for HAST or Pressure Cooker Test.
The applicable test methods are the following:
- – IEC 60068-2-66 (1994-6) Environmental testing. Part 2: Test Methods-Test Cx; Damp heat, Steady state (unsaturated pressurized vapor).
- – IEC 60749 AMMENDMENT 1 (1991-11) Semiconductor devices Mechanical and climatic test methods 5C Damp heat, steady-state-highly accelerated.
- – JEDEC STANDARD (1988-6) No.22-110 Test Method A110 Highly-Accelerated Temperature and Humidity Stress Test (HAST).
- – EIAJ (Electronic Industries Association of Japan) ED-4701 (1992-2) (Japan) Environment and Duration test methods for Semiconductor Devices Method B-123 Unsaturated vapor pressure test.
HAST has been demonstrated to be an extremely efficient tool for screening and testing of components for space applications, reducing the execution time and ensuring the results are obtained in an efficient and timely manner.
The increasing use of COTS (Commercial Off The Shelf) parts in space applications requires also a fast response time should the product screening be performed. HAST can be considered a significant advantage on the two key elements of the screening flow: time and the representativeness of the results.