HAST stands for Highly Accelerated Stress Test, also known as High Accelerated Temperature and Humidity Stress Test in Chinese. It is a testing method used to evaluate the reliability and lifespan of products under high temperature, high humidity, and high pressure conditions.
HAST can quickly trigger specific failures in PCBs and chips, such as delamination, cracking, short circuits, corrosion, and popcorn effects.
The causes of faults caused by moisture include water vapor infiltration, polymer material depolymerization, decreased polymer bonding ability, corrosion, voids, detachment of wire solder joints, leakage between leads, detachment of chip to chip adhesive layers, pad corrosion, metallization or short circuit between leads.
The way water vapor enters IC packaging:
·The moisture absorbed by the silver paste used in IC chips, lead frames, and SMT;
·Moisture absorbed in the plastic sealing material;
·When the humidity in the plastic packaging workshop is high, it may have an impact on the components;
·After packaging, water vapor penetrates through the plastic packaging material and the gap between the plastic packaging material and the lead frame. Because there is only mechanical bonding between the plastic and the lead frame, small gaps are inevitable between the lead frame and the plastic.
Note: As long as the gap between the seals is greater than 3.4 * 10- ¹⁰ m or more, water molecules can pass through the protection of the seals. Airtight packaging is not sensitive to water vapor, and generally does not use accelerated temperature and humidity tests to evaluate its reliability. Instead, its airtightness and internal water vapor content are measured.
Corrosion process in aluminum wire: water vapor penetrates into the plastic casing → water vapor penetrates into the gap between the resin and the wire; Water vapor seeps into the surface of the chip, causing an aluminum chemical reaction.
Factors that accelerate aluminum corrosion:
·The connection between the resin material and the chip frame interface is not good enough (due to differences in expansion rates between various materials);
·During packaging, the packaging material is contaminated with impurities or impurity ions (due to the presence of impurity ions);
·High concentration phosphorus used in non active plastic encapsulation films;
·Defects present in non active plastic encapsulation films.
Chip and PCB delamination: Due to the inconsistent thermal expansion coefficients of the packaging body, disk, and lead frame materials, stress concentration will occur at the connections of different materials inside the plastic encapsulated device under thermal stress. If the stress level exceeds the yield strength or fracture strength of any of the packaging materials, the device will be delaminated. Moreover, generally speaking, the glass transition temperature of epoxy resin for plastic sealing materials is not high, and its thermal expansion coefficient and Young's modulus are very sensitive to temperature changes in the vicinity of the glass transition temperature. Under extremely small temperature changes, the thermal expansion coefficient and Young's modulus of epoxy plastic sealing materials will undergo particularly significant changes, making it more prone to reliability issues for plastic sealing gifts.
Plastic encapsulated devices are semiconductor devices packaged with resin based polymers. Resin based materials themselves are not dense and have the characteristic of adsorbing water vapor. Moisture is also introduced into the plastic encapsulated device at the bonding interface between the package and the lead frame. When the water vapor content in the plastic encapsulated device is too high, it can cause corrosion on the chip surface and dissociation of the resin at the interface between the package and the lead frame, which in turn accelerates the entry of source gas into the interior of the plastic encapsulated device, ultimately leading to delamination.
In harsh environments with high temperature, high humidity, and bias voltage, accelerating the penetration of moisture through the external protective layer or along the interface between the metal and the external protective layer can cause the failure of the sample.
Popcorn effect: Originally referring to ICs packaged in plastic outer bodies, the silver paste used for chip installation absorbs water. Once the package is not properly protected and subjected to high temperatures during downstream assembly and welding, the moisture will cause the package to burst due to vaporization pressure, while also making a sound similar to popcorn. Therefore, it is named after the popcorn phenomenon. When the water vapor content absorbed is higher than 0.17%, popcorn phenomenon will occur. Recently, P-BGA packaging components have become very popular. Not only does the silver glue absorb water, but the substrate of the carrier board also absorbs water. Poor management often leads to popcorn phenomenon.
Moisture causes internal corrosion of the package: The cracks caused by moisture during the packaging process bring external ion contamination to the surface of the chip. Through surface defects such as pinholes, cracks, and poor coating, it enters the semiconductor components, causing corrosion and leakage current problems. If a bias voltage is applied, faults are more likely to occur.
Corrosion: Corrosion failure (water vapor, bias voltage, impurity ions) can cause electrochemical corrosion of aluminum wires in ICs, leading to wire opening and migration growth. Due to their low prices and simple processing techniques, aluminum and aluminum alloys are commonly used as metal wires for integrated circuits. From the beginning of the integrated circuit plastic packaging process, water vapor will penetrate through epoxy resin, causing corrosion of aluminum metal wires and making it difficult to generate open circuits, becoming one of the most headache inducing problems in the chip industry. Although various efforts have been made to improve the quality, including the use of different epoxy resin materials, improvement of encapsulation technology, and enhancement of non active encapsulation films, with the rapid development of miniaturization in semiconductor electronic devices, corrosion of encapsulated aluminum metal wires remains a very important technical issue in the electronics industry.

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