Today's electronic industry is constantly looking for ways to improve the reliability of their products. Manufacturers commonly use environmental chambers to check the quality of sample units by simulating extreme operating conditions. One of the most common tests for IC chips, known as THB (Temperature/Humidity/Bias), holds 85�C/85% RH condition while bias loads are applied to the samples. It has become less useful in recent years due to the increased packaging quality of IC chips-reliability tests can run thousands of hours in order to get useful results.
Many users, especially quality control engineers, cannot wait weeks to learn if a batch of product is of acceptable quality. The Highly Accelerated Stress Test (HAST) was devised to decrease the test time needed to get useful results. This test uses a high temperature (over 100�C), high relative humidity (about 85%), under high atmospheric pressure conditions (up to 4 atm). It has also become known as the Autoclave or Pressure Cooker Test (PCT).
HAST is performed to evaluate the non-hermetic packaging of solid state equipment in humid environments. HAST accelerates the penetration of moisture through the external protective material or at the seals around the chip leads. It was developed to be a natural extension of the previous THB 85/85 testing.
HAST testing is a more accelerated stress test than the more traditional THB test for environmental reliability. It loads the specimen with heat and moisture stresses to find the latent defects that could cause failure for the user relying on it. In testing products for reliability, defining the test conditions is the most basic step. If the test conditions do not correspond to the application conditions, then the testing information becomes useless. That is why the modern unsaturated HAST testing has more value to the scientist than previous HAST testing involving saturated humidity, since operating environments for most electronic components are not saturated. The HAST testing corresponds to the older THB test by the fact that failures are caused by the same mechanisms, the resulting failures occur at proportional rates, and there is a correlation between activation energies. Electrical devices and components have become so reliable that several thousand hours of THB testing cannot catch the weaknesses that HAST can in a much shorter time.
By combining temperature, temperature change, and multi-axis vibration, accelerated tests using high rate temperature vibration chambers can rapidly identify process defects and expose design weaknesses in a wide range of electronic products. Using well-defined acceleration factors developed at CALCE, highly accelerated life tests (HALT) provide assessments of lifetime and MTBF, reduce product development cycle time, and increase confidence in the product's life-cycle reliability.
The test combines two elements. Firstly rapid temperature cycling of the test item is introduced to cause different rates of expansion and contraction in the various components of the test item thereby imparting stress and inducing premature failures.
Secondly to achieve meaningful results high energy, broad band random vibration with six degrees of freedom is input through a modally rich table either during temperature ramps or more commonly during dwells.
More information what HALT not is you find in this paper (46k PDF):
WHY HALT CANNOT PRODUCE A MEANINGFUL MTBF NUMBER AND WHY THIS SHOULD NOT BE A CONCERN
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