Wedge bonding crater test method and reliability study

Research on wire bonding crater test method and reliability

Wire bonding technology, as the core process of realizing the electrical connection between the chip and the packaging pin in the integrated circuit packaging process, is becoming more and more important in today's rapid development of the semiconductor industry. In recent years, with the innovation of integrated circuit design concepts and the rapid advancement of advanced packaging technology, chips have rapidly evolved in the direction of miniaturization and multi-functionality, which has led to the emergence of complex structures such as multi-layer wiring, integrated devices and circuits under the chip pad in the internal design of chips. At the same time, 2D/3D packaging technology, copper wire bonding process, and chip interconnect bonding process have become the mainstream trends in the current packaging field.

Different chip processes and chip pad structures have specific requirements for bonding equipment, bonding processes, bonding wires, fixture design, and process parameters. The mismatch of any one element may directly lead to the appearance of craters after the product is bonded, and eventually lead to the electrical failure of the product (see Figure 1). It should be noted that the bullet pit is the damage caused by the solder ball to the inside of the chip pad during the bonding and welding process, and this defect is extremely insidious, which requires special analysis and detection methods to be carefully observed under a microscope [1]. Therefore, how to prevent the generation of crater after chip bonding during the integrated circuit packaging process, and to more accurately detect the bonding reliability and monitor product quality through crater testing, so as to ensure the reliability of packaged products, has become an important topic of increasing concern in the industry [2].

1 Hazards and test principles of craters

In the quality inspection of the bonding process, abnormal problems such as ball de-balling and virtual welding can be intuitively found and measured through visual inspection, welding wire tension measurement, welding ball thrust measurement, etc., while the crater is hidden under the bonded weld ball and cannot be detected by conventional visual inspection, and must be identified by complex analysis and inspection methods in professional laboratories.

(1) Crater phenomenon

Crater is a phenomenon that causes damage to the aluminum layer of the chip pad and the underlying silicon compound due to the combination of factors during the bonding process of integrated circuit packaging [2]. This is mainly caused by improper settings of contact, bond, and bond power when the solder ball presses on the surface of the chip pad, as shown in Figure 2. When the crater damage is relatively minor, its shape is generally crescent-shaped; When the damage is severe, the crater is ring-shaped, and bright traces of silicon loss can be clearly seen on the surface of the silicon layer of the chip.

(2) Bullet crater hazards

Once the crater defect causes damage to the silicon layer of the chip, it will directly cause the electrical defect of the product function, mainly manifested as abnormal leakage and low reverse breakdown voltage. When the product is powered on for a long time, the leakage phenomenon will gradually intensify, which will lead to a continuous drop in the reverse breakdown voltage, and may even cause a breakdown short circuit, which will have a serious impact on the electrical function of the product [3]. Crater damage failure is usually latent, leading to problems such as decreased bond strength and dielectric insulation, and accelerating the occurrence of failure under the action of electrical or temperature stress during product application [4]. Serious craters will directly cause product function failure, affect product reliability, and this defect is not easy to detect in time.

(3) Crater test principle

The crater test is a destructive test based on chemical corrosion, which corrodes the surface metal and solder ball of the chip pad with a chemical solution, and then observes and confirms whether there are cracks and silicon layer damage in the structure and circuit under and inside the metal layer of the chip pad under a high-power microscope [5].

Because aluminum is easily soluble in potassium hydroxide, sodium hydroxide, phosphoric acid, hydrochloric acid, dilute sulfuric acid and other solutions, but difficult to dissolve in water, potassium hydroxide (KOH) and phosphoric acid (H3PO4) are usually used as chemical solutions for crater tests. The chemical reaction of aluminum with these two solutions is as follows:

Potassium hydroxide is highly alkaline and corrosive, and it is very easy to absorb moisture in the air and deliquescence, and potassium carbonate will be formed after absorbing carbon dioxide. The chemical equation for its reaction with aluminum is:

2Al + 2KOH + 2H2O = 2KAlO2 + 3H2↑

where KAlO2 is potassium metaaluminate.

Phosphoric acid is a common inorganic acid, which is a medium-strong acid and has the characteristics of non-volatility, decomposition, and almost no oxidation. The chemical equation for its reaction with aluminum is:

2H3PO4 + 2Al = 2AlPO4 + 3H2↑

where AlPO4 is aluminum phosphate.

(4) Crater test can prevent the occurrence of batch quality problems

The bonding wires used in the bonding process mainly include gold, copper and aluminum wires. Among them, the gold wire process is the most mature and stable, and is generally used in products with high reliability requirements; After solving the reliability problem, copper wire has become the most widely used bonding material; Aluminum wire is often used for high-power products.

Due to the physical characteristics of the bonding wire, copper wire and aluminum wire have relatively high hardness, which is more likely to cause damage to the chip pad when bonding products, resulting in crater defects. Crater testing can effectively monitor chip crater hazards caused by improper bonding during first-article inspection and in-process sampling inspection, thereby preventing batch quality problems.

As shown in Figure 3, (a) the picture is a picture of the chip after the wire is bonded, and the quality check at this time can only confirm the appearance quality of the bond, and cannot judge whether there is a crater under the solder ball; (b) The picture shows the qualified chip picture after the crater test, the aluminum layer of the chip has been corroded, the welding ball has fallen off, and the absence of crater can be clearly observed; (c) The picture shows a picture of an unqualified chip after the crater test, with an aluminum pad pit-shaped anomaly, and the silicon layer can be seen.

2 Crater test process

Since crater testing requires the use of chemical agents, it is crucial to ensure the personal safety of test personnel. In the crater test room, the tester must wear work clothes, protective masks and gloves, and the whole operation process needs to be carried out in a fume hood.

After receiving the test sample, it is first necessary to check and confirm that the surface of the chip must be exposed, and for the encapsulated product, it needs to be opened before the test can be carried out. Secondly, use a pick pin to cut off all the solder wires of the test sample from the second solder point (fishtail) to prevent the pad from being damaged before the test due to the force of the wire arc when the chip is removed later. After removing the chip from the test sample with the tool that has picked the second solder joint, it is ready for subsequent tests.

(1) Crater test method

oneThe chemical solution used in the test is poured into the beaker to achieve the scale requirement that the test sample can be completely submerged. According to the process regulations, the beaker is placed on the heating plate to the set test temperature, and then the test sample is placed into the beaker and heated according to the corrosion changes of the chip pad and the potion until the solder ball falls off. If you can't easily brush off the ball, it may be that the aluminum layer of the chip pad has not been completely corroded, and the sample can be put back into the chemical solution to continue heating and soaking. Remember not to forcibly peel off the solder ball with external force, otherwise it will cause pits on the surface of the pad, which will not truly reflect the impact of the solder ball on the pad [6].

twoThe corrosion time required for different potions and chip pads varies, which requires careful observation and strict control. After the solder ball is removed, the tested sample is cleaned in an ultrasonic cleaner with deionized water, and then the chip pad is observed under a high-power microscope by absorbing the moisture from the chip surface with filter paper.

(2) The specific process of crater testing

3 Various methods and comparisons of crater tests

3.1 Key factors in the selection of crater test method

Due to the significant differences in the Pad structure and surface metal layer composition of different chip processes, the chemical solution, formulation, and test time used in the crater test also vary. When conducting crater testing, it is necessary to clearly understand the chip process and Pad structure of the sample under test in order to select the correct test method and obtain accurate test results. If the crater test method is not properly selected, it will lead to incorrect test results, resulting in losses in cost, production efficiency and product quality.

In addition, the chip pad process is usually divided into three categories, namely nickel-palladium pads (NiPdAu Pad), aluminum pads (Al Pad), and copper-aluminum pads (CuAl Pad); The composition and thickness of the metal layer of each process will vary depending on the chip process, see Figure 5 for details. Generally speaking, the thickness of the aluminum or copper layer of the Pad is required to be greater than 0.8~1 μm, and if the thickness is too thin, if a cavity defect is formed in some positions during the deposition of the aluminum layer, it will be impacted by the bonding force during subsequent bonding and packaging, resulting in NSOP (solder ball non-stick) and Pad crater at this position [7].

3.2 Comparison of Pad process methods of three types of chips

Nickel-palladium gold pad (NiPdAu Pad): Since the surface of the pad is a gold layer, water regia (also known as royal acid, nitrohydrochloric acid, which is a highly corrosive and yellow misty liquid composed of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) in a volume ratio of 3:1) must be used to corrode this type of pad. The advantage of this method is that the corrosion rate of gold is fast, but the disadvantage is that the residual substances after corrosion are difficult to remove cleanly, and aqua regia is extremely oxidizing, which has a corrosive or negative impact on many materials.

Aluminum pad (Al Pad) and copper-aluminum pad (CuAl Pad): Since the surface of the pad is an aluminum layer, using the amphoteric nature of aluminum (that is, aluminum can react with both acid and strong alkali), there are many types of chemical agents that can be selected during the test, usually sodium hydroxide/potassium hydroxide solution, phosphoric acid/hydrochloric acid solution, etc.

The use of sodium hydroxide/potassium hydroxide solution dissolves the aluminum layer between the sand ball and the pad, ultimately separating the pad and ball for inspection and analysis. The solution concentration can be adjusted according to the characteristics of the product, but it is not suitable to use high concentrations of alkali for corrosion, and the time needs to be strictly controlled, otherwise it will cause damage to the chip.

In addition, the use of phosphoric acid/hydrochloric acid solution directly dissolves the solder ball, which is preserved due to the passivation effect of the aluminum, so it is possible to check the aluminum layer topography of the pad and to observe the chip pad after continuing to remove the aluminum layer.

In the crater test, if there is a deviation in the concentration of the chemical solution, the test temperature, the test time, the test method, etc., the test result may be "a millimeter difference, a thousand miles of error", so the correct test method is the key to the success of the crater test.

In summary, according to the methods and experience of actual crater testing in long-term production, the advantages and disadvantages of different crater test conditions and methods are compared, and summarized in Table 1.

As can be seen from Table 1, the chemical solutions used in the crater test are different, and the corresponding test conditions and methods are also different, and each has its own advantages and disadvantages, so it is necessary to accurately identify the pad process of the test sample before the test. Due to the characteristics of sodium hydroxide/potassium hydroxide solution in the test, such as strong stability, short test time and clean surface treatment, it is widely used in aluminum pad and copper and aluminum pad testing. In order to accurately determine the results of the crater test, you can also use different chemical solutions of acid and alkaline to test separately, and compare the effects after the test to determine which solution is more suitable for corrosion testing.

4 Reliability verification and research of craters

Crater damage is latent and will affect the reliability and service life of the product. In order to study the degree of risk caused by craters in different chip pad structures, the impact of integrated circuit products is verified and determined by conducting routine reliability project assessments and then functional tests.

Randomly select 3 groups of CUP (CUP: Circuit Under Pad) chips and NO CUP chips, deliberately increase the bonding parameters to make the chip pad crater, and perform slicing analysis to confirm the depth of the chip pad structure damaged by the crater. After the packaging is completed, functional tests and reliability assessments are carried out (3 times of 260°C reflow soldering, 500 times of temperature cycling, and 96 hours of high-pressure cooking), and then functional tests are carried out again to confirm whether the product is qualified, and the verification results are summarized in Table 2.

It can be seen from Table 2 that through the assessment and verification of reliability test items such as reflow soldering, temperature cycling, and high-pressure cooking, it is concluded that the circuit products (CUP) under the chip pad will have functional test failure problems after the reliability test. The circuitless product under the chip pad (NO CUP) passed the functional test after the reliability test.

The reason for this analysis is that the structural design of the chip pad is different, and the first layer (Top1) and the second layer (Top2) of such pad structure are the same potential, as shown in Fig. 6, so the crater after the corrosion of the first layer will not lead to the failure of the electrical performance parameters. The second layer (Top2) and the third layer (Top3) of the structure are not the same potential, so the crater phenomenon cannot occur after the corrosion of the second layer.

According to the above verification results, after long-term tracking of crater test results and functional test yield of production line products, it is found that the impact of crater is directly determined by the design of chip PAD structure.

5 Conclusion

As an important means to evaluate bonding parameters and bonding reliability, crater test can better monitor product quality and has become a key detection method in the bonding process. This paper summarizes the technical methods of crater testing, compares the basic conditions, advantages and disadvantages of various test methods, verifies the impact of craters on product reliability, and provides technical reference for industry technicians.

When correctly identifying and mastering the chip pad process, understanding the characteristics and reliability impact of different chip processes, it is necessary to be familiar with the characteristics of chemical solutions, select the most suitable chemical solutions, test conditions and methods according to different chip pad processes, and operate correctly according to the standard test process to ensure the safety of the test process, obtain accurate test results, and improve the efficiency of quality inspection.

With the continuous development of integrated circuit chip design and manufacturing technology, more complex chip processes and chip pad structures have emerged, which puts forward higher requirements for the technical methods of crater testing. Therefore, packaging technicians need to continue to explore, summarize, and compare the advantages, disadvantages, safety, and effectiveness of various test methods to adapt to the development of integrated circuit chip technology, packaging technology, and testing and analysis technology.