Highly Accelerated Stress Test (HAST)

The acronym "HAST" stands for "Highly Accelerated Temperature/Humidity Stress Test." It was developed as a shorter alternative to Temperature Humidity Bias (THB) Testing. If THB testing takes 1000 hours to complete, HAST results are available within 96-100 hours. Because of this, the popularity of HAST has continuously increased in recent years, to the extent that some companies have totally replaced THB testing with HAST. Figure 1 shows an example of a HAST system - the Trio Tech 6000X.

Like THB testing, HAST accelerates corrosion, particularly that of the die metal lines and thin film resistors. HAST requires preconditioning and is conducted with electrical bias at 130 deg C and 85% RH for 96-100 hours.

Fig. 1. Example of a HAST system: the Trio Tech 6000X

Electrical bias during HAST stressing must be defined based on the following guidelines: 1) power dissipation must be minimized to ensure that moisture is always present at the die surface; 2) alternate pins must be subjected to opposite bias (low voltage versus high voltage) as much as possible; 3) potential differences between the various metallizations on the die must be maximized; 4) the operating voltage range for the device must also be maximized, as long as the power dissipation is kept under control.

The samples for HAST stressing are loaded into HAST boards prior to loading into the HAST chamber. HAST boards are designed to withstand the severe test conditions of HAST and are, therefore, relatively expensive. The HAST boards are then inserted into board racks (see Fig. 2) inside the chamber of the HAST system.

Fig. 2. A look inside the chamber of the Trio Tech 6000X

HAST is recommended for the qualification of any change that can potentially affect the corrosion resistance of the product. Thus, a new fab process, a new package, or a new fab/assembly site definitely requires HAST data. HAST is also recommended for changes in the die glassivation, metallization or thin film resistors, as well as changes in the molding compound. It is also used for the reliability assessment of lots suspected to be prone to corrosion due to ionic contamination.

One may refer to JEDEC JESD22-A110 for more details on this reliability test.

Reliability Tests: Autoclave Test or PCT; Temperature Cycling; Thermal Shock; THB; HAST; HTOL; LTOL; HTS; Solder Heat Resistance Test (SHRT); Other Reliability Tests

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