Semiconductor back-end process: Reliability testing and standards for semiconductor packaging (Part 2)
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Semiconductor back-end process: Reliability testing and standards for semiconductor packaging (Part 2)

Posted Date: 2024-01-13

Reliability testing under various external environmental conditions

There are many causes of external environmental conditions that lead to semiconductor product failure. Therefore, products need to undergo reliability testing under specific environmental conditions before being shipped to their destination to ensure that they can withstand different environmental conditions.

Preprocessing test

After the product is shipped and stored, pre-processing testing can be used to evaluate problems that may arise during customer use, such as factors that affect product reliability such as hygroscopicity (Hygroscopic)4 and thermal stress. Pretreatment evaluates the reliability of the product under humid conditions by simulating the state of the product during the process of selling, shipping to customers, opening vacuum packaging, and system installation.

Pretreatment is a prerequisite for environmental condition reliability testing, including Temperature Humidity Bias (THB) test, Highly Accelerated Stress Test (HAST) test and Thermal Cycle (TC) test.

4Hygroscopic: The phenomenon of absorbing moisture from the air.

Within the semiconductor industry, this phenomenon can lead to semiconductor device failure. The order of evaluation is thermal cycle, bake, endotherm, and reflow soldering. Figure 3 shows the process of applying preprocessing testing to packaging, transportation and system installation links.



▲ Figure 3: The relationship between production, transportation and use and pretreatment test conditions (ⓒ HANOL Press)

Thermal cycle testing

Thermal cycle (TC) testing is to evaluate the product's resistance to transient temperature changes that may occur in different user environments. Semiconductor packages and modules are composed of different materials with different coefficients of thermal expansion (CTE)5, which can lead to fatigue failure due to stress, which generally occurs after thermal changes. , caused by expansion and contraction.

*^5^*Coefficient of Thermal Expansion: A material property used to express the extent to which a material expands when heated.

The primary purpose of thermal cycling testing is to measure the ability of a semiconductor package to withstand stress as temperature changes, but high and low temperature stress can also cause many other failure issues. Long-term thermal shock can be used to verify the possibility of interface delamination (Interfacial Delamination) 6, internal and external packaging cracks, and chip cracks caused by various semiconductor packaging materials due to stress and thermal expansion factors. In addition, due to green product regulations restricting the use of hazardous substances such as lead, and the expansion of application fields such as portable mobile devices, the importance of solder joints is increasing day by day, and thermal cycling is also an effective test method to evaluate the reliability of solder joints.

*^6^*Interfacial Delamination: Refers to the separation of interfaces from each other in semiconductor packaging.

Temperature and humidity storage test and temperature and humidity bias test

Temperature Humidity Storage (THS) testing is used to evaluate the resistance of semiconductor products to high temperature and high humidity conditions. In order to determine the appropriate exposure time, it is recommended to simulate the actual use environment by measuring the amount of moisture absorption after opening the moisture-proof packaging. At the same time, the temperature and humidity bias (THB) test evaluates the moisture-proof performance by applying an electrical bias (Electrical Bias) 7 to the product. Although most failures are caused by aluminum corrosion, temperature stress can also cause other potential problems. This test can also be used to detect other package reliability issues, such as pad metal corrosion caused by moisture penetrating into small gaps between leads or molding holes, and failure problems caused by moisture penetrating through the gaps in the protective film.

*^7^*Electrical Bias: Applying direct current (DC) between two points to control the circuit.

High pressure furnace test

The Pressure Cooker Test (PCT) is an ideal way to early evaluate moisture resistance. Its test standards are more stringent than the temperature and humidity storage test and the temperature and humidity bias test. The high-pressure furnace test is also known as the Autoclave 8 test. This test evaluates the moisture resistance of the molded material and the reliability of the molded structure through moisture penetration under 100% relative humidity and high pressure. Additionally, the test can be used to detect product failures caused by moisture penetration between leads and molded vias.

*^8^*Autoclave: a high-pressure appliance. When water is added to a high-pressure vessel while it is sealed at high temperature, the water will evaporate, thereby increasing pressure and humidity, creating necessary conditions for the samples in the high-pressure vessel.

Similar to temperature and humidity storage testing, high-pressure oven testing was once an important method for reliability testing of thick semiconductor packages. However, judging from the current JEDEC evaluation results and the latest international trends, the high-pressure furnace test shows that the stress amplitude is too large for the current package. Therefore, this test method needs to be used selectively depending on the package type. High voltage furnace testing is mainly used for lead frame products, while unbiased highly accelerated stress testing (UHAST) is mainly used for substrate products.

Unbiased highly accelerated stress testing, highly accelerated stress testing and highly accelerated life testing

The Unbiased Highly Accelerated Stress Test (UHAST) evaluates product reliability by applying stress similar to the high-pressure furnace test on thin-packaged substrate-type products, such as fine-pitch ball grid array package (FBGA) products. The two tests also have similarities in identifying and finding types of product failure. The high-pressure furnace test uses saturated humidity or 100% relative humidity to apply stress, while the unbiased high-acceleration stress test uses a relative humidity similar to the user environment. For 85% unsaturated humidity conditions. This test method mainly uses galvanic corrosion (Galvanic Corrosion) 9 or direct chemical corrosion.

*^9^* Galvanic Corrosion: An electrochemical process in which the more active anode metal is in contact with the more corrosion-resistant cathode metal in the electrolyte solution, and the more active metal is easily corroded.

Another evaluation is the Highly Accelerated Stress Test (HAST), which evaluates the reliability of non-hermetic packages in humid environments. This test uses the same method as the temperature and humidity bias test. The pins continue to be subjected to temperature, humidity, and pressure stress while in a static bias state. Finally, there is Highly Accelerated Life Testing (HALT), a rapid stress test that helps identify and correct design flaws during the product design phase.

Mechanical Factor Reliability Test

During the handling, storage, transportation and operation of semiconductor products, they are subject to environmental stress caused by mechanical, climatic and electrical factors. These loads can seriously affect the design reliability of the product. Therefore, it is necessary to evaluate products under development or in mass production to monitor for such anomalies. During the evaluation process, manufacturers may subject the product to physical stresses such as vibration, shock, or drops.

Impact test

Impact testing evaluates the impact resistance of a product by simulating the impact that the product may receive during handling and transportation. Typical impact tests include hammer testing and drop testing. During the hammer test, the test sample is fixed in place and then struck with a hammer; the drop test refers to letting the product fall freely. Hammer testing is used to evaluate the hammer force and impulse resistance a product can withstand, as well as the number of impacts. In the drop test, the test sample needs to fall freely from a height of 1-1.2 meters to simulate the user's actual working environment.

Vibration, bending and torsion testing

Vibration testing is used to evaluate the resistance of products to vibrations that may occur during transportation. The Sine Vibration 10 experimental method that complies with JEDEC standards is usually used.

*^10^*Sine Vibration: Vibration whose frequency changes with time.

Other tests include bending tests and torsion tests. The bend test is used to evaluate solder joint defects caused by warping or bending of the printed circuit board (PBC); the torsion test, also known as the twist or torque test, is used to evaluate the solder joint problems and warpage resistance of the product when subjected to torsional stress. force.

Ensure reliable semiconductor products

The reliability tests and standards introduced in this article are the foundation for ensuring that these important components meet the stringent standards of today's technological world. Various evaluation methods, including environmental condition testing, mechanical factor testing, and product life testing, all reflect the semiconductor industry's determination to produce reliable and durable products.






Review Editor: Liu Qing


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