Chip bonding: Connect the chip, connect the future

Overview of wire bonding technology

In the field of connecting chips to external packages, wire bonding technology occupies an important position due to its efficiency and ubiquity. Among them, the hot press bonding technology is unique, which cleverly integrates the principles of low-temperature diffusion and plastic flow, and plays an important role in gold wire bonding. Ultrasonic bonding technology takes a different approach, achieving reliable connections between gold or aluminum wires through the synergy of plastic flow and friction.

The complete process of wire bonding includes several key links, first of all, the cleaning of the pad and the shell, which is the basis for ensuring the bonding quality; Then there is the precise debugging of the wire bonding machine to ensure that the equipment is in the best working condition; Then perform the wire bonding operation; Finally, there is strict quality inspection to troubleshoot possible problems.

There are two main types of shell cleaning technology, plasma cleaning technology uses a high-power radio frequency source to convert gas into plasma, and uses the characteristics of plasma to remove contaminants attached to the surface; UV ozone cleaning technology uses specific wavelengths of ultraviolet light to decompose substances to achieve the purpose of cleaning.

In terms of wire bonding technology, ball bonding process and wedge bonding process are the two main methods. The ball bonding process is suitable for fine wires, especially when the pad spacing is greater than 100 microns; The wedge bonding process is suitable for the combination of gold wire and aluminum wire, where the aluminum wire is ultrasonically bonded at room temperature, and the gold wire is hot-ultrasonically bonded at 150 degrees Celsius, which is widely used in precision-sized connections.

The bonding method is divided into forward bonding and reverse welding bonding, with the first bond point located on the chip surface and the first bond point on the surface of the shell.

Detailed explanation of hot press bonding and ultrasonic welding technology

The core principle of thermocompression bonding is to combine low-temperature diffusion with plastic flow to facilitate contact between atoms, resulting in a robust solid diffusion bond. During the bonding process, the stressed parts need to undergo a series of carefully designed temperature and pressure cycle treatments, during which the contact surface will undergo plastic deformation and diffusion. Plastic deformation plays a crucial role in destroying the oxide layer on the contact surface and achieving the fusion of metal surfaces. In hot press bonding, the deformation of the welding wire is mainly manifested as plastic flow, and this technology is mainly used in the field of gold wire bonding.

Ultrasonic welding technology combines plastic flow and friction to achieve effective welding of welding wires. The mechanism of operation is to use quartz crystals or magnetic drives to transmit frictional action to a metal sensor known as a "HORN". When the quartz crystal is energized, the metal sensor stretches; After a power outage, the sensor shrinks. These actions are produced by an ultrasonic generator and are generally maintained at an amplitude of 4 to 5 microns. The end of the sensor is equipped with a welding tool, and with the telescopic vibration of the sensor, the welding wire causes friction at the bonding position and plastic deformation under the action of top-down pressure. Most plastic deformation occurs after the bonding point absorbs ultrasonic energy, while pressure-induced plastic deformation is relatively rare. This is because the action of ultrasonic waves on the bond point reduces its hardness, allowing the wire to produce greater plastic deformation at the same pressure. This bonding method is suitable for welding gold or aluminum wires.

In-depth analysis of the wire bonding process

The wire bonding process mainly includes the cleaning of the pad and shell, the commissioning of the wire bonding equipment, the wire bonding operation and the subsequent inspection work. At present, molecular-level cleaning techniques are commonly used for shell cleaning, including plasma cleaning and ultraviolet ozone cleaning.

Plasma cleaning technology excites the gas into a plasma state through a high-power RF source, and then the high-speed moving gas ions hit the surface of the bonding area, and the pollutant is sputtered removed by combining with pollutant molecules or physically splitting. In this process, commonly used gases include oxygen (O₂), argon (Ar), nitrogen (N₂), and a gas mixture consisting of 80% argon and 20% oxygen, or a gas mixture consisting of 80% oxygen and 20% argon. In addition, the plasma combination of oxygen and nitrogen is widely used, especially for the degassing treatment of epoxy resins.

UV ozone cleaning technology uses specific wavelengths of radiation to clean surfaces. The specific process is as follows: UV light at a wavelength of 184.9 nanometers can break the chain of oxygen molecules to form atomic oxygen (O + O), which then combines with other oxygen molecules to form ozone (O₃). Under the action of ultraviolet light at a wavelength of 253.7 nanometers, ozone is further broken down into atomic and molecular oxygen. At the same time, water molecules are broken down into free hydroxide ions (OH⁻). These substances can react with hydrocarbons to form carbon dioxide (CO₂) and water (H₂O), which eventually detach from the attached surface as gases. UV light at 253.7 nanometers also breaks the molecular bonds of hydrocarbons, thereby accelerating the oxidation process.

Although these two cleaning methods are effective in removing organic contaminants from pad surfaces, their effectiveness is largely influenced by specific contaminant species. For example, oxygen plasma cleaning technology does not improve the weldability of gold thick films, while O₂+Ar plasma or solution cleaning methods are better options. In addition, certain contaminants, such as chloride ions (Cl⁻) and fluoride ions (F⁻), cannot be removed by the above methods due to the formation of chemical bonds. In this case, solution cleaning methods may be required, such as using vapor phase fluorocarbons or deionized water.

The wire bonding process is mainly divided into two types: ball bonding and wedge bonding. Ball bonding usually uses fine gold wire with a diameter of less than 75 microns, because fine gold wire is easily deformed under high temperature and pressure conditions, has good oxidation resistance and excellent ball formation. Ball bonding is suitable for pad spacing greater than 100 microns, and has been used in cases with a 50 micron pitch.

The wedge bonding process is suitable for both gold and aluminum wires, with the main difference being that aluminum wires are ultrasonically bonded at room temperature, while gold wires are thermally sonicly bonded at 150°C. The main advantage of wedge bonding is its suitability for fine size soldering, such as pad spacing below 50 microns. However, due to the rotational motion of the bonding tool, its overall speed is usually lower than that of thermal ultrasonic ball bonding. The most common wedge bonding process is aluminum wire ultrasonic bonding, which has a relatively low cost and bonding temperature. The main advantage of gold wire wedge bonding is that since the solder joints formed are smaller than ball bonds, there is no need for hermetic packaging, which is especially suitable for microwave devices.

There are two main methods of bonding technology. Forward bonding, that is, the bonding is completed on the chip first, followed by a second bonding on the packaging case; Back-soldering bonding is first bonded on the shell and then completed on the chip. When implementing forward bonding, the chip bond point is usually equipped with a tailstock; In the process of reverse bonding, the chip is usually not equipped with a tailstock. Which bonding method to choose for circuit connection needs to be judged according to the specific situation.

Failure problems and analysis in the bonding process

The phenomenon of pad pitting

Pad pitting is one of the most common failure problems in the bonding process, and it can occur for a variety of reasons. When the intensity of the ultrasonic energy exceeds the threshold, it can cause misalignment of the Si lattice layer, which can lead to pad pitting. In the wedge bonding process, if the bonding force is too large or too small, this problem is also prone to occur. Normally, a bonding tool that hits the substrate too quickly will not cause potholes in Si devices, but they can cause potholes in GaAs devices. During ball bonding, if the ball size is too small, it may cause the hard bonding tool to come into direct contact with the metallized layer of the pad, resulting in pitfalls.

The thickness of the pad is also related to the degree of damage to the pit, generally speaking, pads with a thickness of 1 to 3 microns have less damage, however, there may be potential problems when the pad thickness drops below 0.6 microns. When the pad metal matches the hardness of the lead metal, the bond quality is significantly improved and the pitting phenomenon is effectively reduced. In the process of ultrasonic bonding with Al wire, if the hardness of the wire is too high, it may cause depression on the surface of the Si sheet.

Other common failure problems

In addition to pad pits, there are also a variety of failure problems in the bonding process. Lead contamination can affect the reliability of the bond, the wrong wire angle can lead to poor connections, clogged wedge through-holes can hinder normal bonding operations, dirty tools can affect the bonding accuracy, and improper fixture clearance and improper fixture pressure can adversely affect the bonding effect.

If the length of the wire is too short, the force applied to the first bonding point will be concentrated in a small area, resulting in excessive deformation. Conversely, if the tail wire is too long, it may lead to a short circuit between the pads.

Bond stripping phenomenon

When pulling off, the root of the bond point may partially or completely separate from the bond surface, forming a smooth fracture, a phenomenon known as bond stripping. This type of peeling phenomenon usually stems from improper selection of process parameters or a decline in the quality of the bonding tool, which is an early warning sign of failure in the bonding process and needs to be paid enough attention.

Lead bending fatigue

Lead bending fatigue is mainly caused by cracks at the root of the lead bonding point, which may be due to mechanical fatigue during the bonding process or thermal stress fatigue caused by temperature cycling. The existing experimental data show that the ultrasonic bonding of Al filament shows higher reliability than that of Al filament in the environment of temperature cycling. In the 0.1% mg content of aluminum wire, its performance is significantly better than that of aluminum wire containing 1% silicon; To ensure effective mitigation of lead bending, the height of the lead closed loop should be at least 25% of the bond point spacing.

Corrosion of bond joints and pads

Corrosion of the bond point to the pad can cause one or both ends of the lead to break completely, causing the lead to lose constraint inside the package, move freely, and cause a short circuit. Moisture and dirt are the two main causes of corrosion. For example, when the bond location contains chlorine (Cl) or bromine (Br), the corresponding chloride or bromide is formed, which in turn corrodes the bond point. Corrosion causes the resistance at the bond point to gradually increase until it eventually leads to device failure. In most cases, the packaging material exerts a certain amount of pressure on the chip surface and its adjacent bond points, and only when the corrosion is severe enough to cause electrical connection problems.

Corrosion of lead frames

Corrosion in lead frames is primarily caused by excessive residual stress or excessive surface contamination introduced during surface coating processes (e.g., nickel plating to protect 42 alloys or copper matrix metals). The most vulnerable area is often the interface between the sealing compound material and the lead frame, which requires attention.

Metal migration

Metal migration refers to the growth process of metal dendrites starting from the position of the bonding pad, which involves the electrolytic behavior of metal ions migration from the anode region to the cathode region, and its occurrence is closely related to the availability of metals, ion types, potential differences and other factors. Metal migration can lead to an increase in leakage current in the bridge area, and when the bridge area is fully formed, it is more likely to cause short circuit problems. Among the various metal migration phenomena, silver (Ag) migration is the most frequently reported, and such migration phenomena also exist in metals such as lead (Pb), tin (Sn), nickel (Ni), gold (Au) and copper (Cu). Given that metal migration is closely related to device failure, it is a failure mechanism that gradually emerges.

Vibration fatigue

Vibration fatigue can cause structural resonance, which in turn can cause damage to the bond point. Specifically, for gold (Au) wire, this critical frequency is between 3 and 5kHz; The critical frequency of aluminum (Al) wire is about 10kHz. Normally, the vibration fatigue failure of wire bonding mostly occurs in the ultrasonic cleaning process. Therefore, to ensure the reliability of the cleaning equipment, its resonant frequency should be controlled in the range of 20 to 100kHz.

Inner lead breakage and disconnection

There are three main situations of internal lead fracture and debonding: First, the lead breaks in the middle part, which is closely related to the damage degree and triggering mechanism of the inner lead, and does not only appear in the early failure stage. Damage to the bonding wire can cause the area of the damaged lead to shrink, which in turn increases the current density, which is more susceptible to burnout, and its ability to resist mechanical stress is weakened, eventually causing the inner lead to break at the damaged area. The main causes of damage include mechanical damage and chemical corrosion of the bonding wire.

The second is that the lead breaks at the root near the bonding point, which is mainly due to process defects, of which thallium (Tl) contamination is a key factor. Thallium can combine with gold (Au) to form eutectic phases with low melting points, which penetrate from the gold-plated lead frame into the gold wire. During the bond point formation stage, thallium can diffuse rapidly and accumulate in the grain boundary region above the ball neck, forming a eutectic phase. When tested by plastic seals or temperature cycling, the ball neck is prone to breakage, leading to device performance failure.

Third, the lead is debonded, and its risk is affected by a variety of factors. The formation of the interfacial insulation layer is an important reason, if the photoresist or window passivation film in the chip bonding area is not thoroughly cleaned, it will lead to the formation of the insulating layer; In addition, the poor quality of the gold plating layer of the tube shell often causes problems such as looseness, redness, bubbling and peeling on the surface; In the process of intermetallic bonding, if it comes into contact with an environment containing oxygen, chlorine, sulfur or water vapor, the metal material will often react with these gases, forming insulating layers such as oxides and sulfides, or suffering from the corrosion of chlorine, which in turn increases the contact resistance and reduces the reliability of the bonding.

Metallization layer defects also increase the risk of debonding, which is mainly manifested by the thin metalization layer of the chip, resulting in a lack of sufficient buffering during the bonding process, resulting in defects. In addition, alloy dots appear in the metallized layer, which are prone to defects at the bonding; There is also an unstable adhesion of the metallized layer, which leads to the risk of shedding pressure points.

Surface contamination can hinder the diffusion between atoms, which can arise from various production processes such as chips, tube shells, splitting knives, gold wires, tweezers, tungsten needles, etc. If the purification of the external environment is insufficient, it may lead to dust pollution; poor purification of the human body may cause organic matter and sodium pollution; If the chip, tube shell, etc. are not thoroughly cleaned in time, the remaining gold plating solution may cause potassium and carbon pollution. This type of contamination is batch in nature and can lead to the scrapping of entire batches of tube shells or lead to corrosion of the bond points and thus failure. In addition, if gold wire and tube shells are stored for a long time, they are not only easy to contaminate, but also easy to age, and the hardness and ductility of gold wire will also change.

The uneven stress distribution between the contact materials is also one of the factors leading to debonding, and the stresses generated during the bonding process are divided into thermal stress, mechanical stress and ultrasonic stress. If the bonding stress is too low, the bond will not be stable enough; However, excessive bonding stress can also adversely affect the mechanical properties of the bond point. Excessive stress may cause damage to the root of the bonding point, which in turn will cause the root of the bonding point to break and lose its function. In addition, excessive stress can damage the chip material below the bond point and even cause cracks.

Intermetallic compounds cause failure and response to Au-Al systems

Interdiffusion and the formation of intermetallic compounds in Au-Al systems

In the early stages of bonding, a very thin diffusion layer is gradually formed between gold and aluminum, which is mainly composed of AuAl₂ (purpura). Further heating will promote the continuation of Au-Al diffusion, and as the gold atoms continue to penetrate into the aluminum film, the originally pure aluminum layer will gradually melt. At the same time, on one side of the golden ball, a compound film composed of Au₅Al₂ will form.

The thickness of the diffusion layer does not increase indefinitely due to the upper limit of the supply of aluminum elements and the significant difference in the rate of interdiffusion between aluminum and gold. We note the diffusion rate as D, where the diffusion rate of aluminum to gold (D Au→Al) is greater than the diffusion rate of gold to aluminum (D Al→Au). If 1 micron is used as the initial aluminum film thickness, the total thickness of the entire diffusion layer is roughly between 4 and 5 microns. During further heating, the gold will diffuse into the diffusion layer and form an Au₄Al compound on the surface side of the gold ball, while continuing to grow towards the semiconductor chip side.

As the temperature rises further, the diffusion of gold (Au) in the diffusion layer continues until only Au₅Al₂ and Au₄Al remain in the diffusion layer. In addition, due to the Kirkendall effect, cavities will appear around the diffusion layer. If the heating continues, the gold (Au) diffusion in the cavity-free region will be enhanced, which will promote the formation of the Au₄Al layer in the central region  .

In molded integrated circuits, these substances act as  catalysts for the oxidation of aluminum in the Au₄Al layer, given the flame retardants contained in the resin material. Bromide penetrates into the bonding point through micropores, which in turn oxidizes the aluminum element in the Au₄Al layer. Therefore, at the interface between the center of the gold ball and the compound layer, a high-resistance barrier will be formed, which can lead to disconnection failure.

Effects of impurities on the Au-Al system

In the early stages of lead development, the focus was on improving mechanical strength, which included fine control of the lead structure and size, so metal-to-metal fracture was not a major consideration. However, as pad spacing shrinks and control windows narrow, advances in wire bonding technology are increasingly constrained by intermetallic phase issues. So far, the research on the doping effect of lead is insufficient.

At present, the introduction of doped impurities and the slowing down of the diffusion rate of the intermetallic phase are considered to be an effective way to reduce intermetallic failure. In fact, doping impurities does not effectively inhibit the growth of intermetallic phases when doping concentrations reach 100ppm. As a result, the doped impurities in some commonly used lead products have been increased to 1%, where doped impurities are effective in preventing the spread of Au and Al. However, the effect is not as expected, and the presence of adulterated impurities can also lead to a decrease in lead conductivity. Therefore, there is an urgent need to explore new solutions to address these issues more efficiently while ensuring that conductivity performance is not adversely affected.

Improve your strategy

There are many reasons why intermetallic compounds fail, so it is difficult to minimize them simply by adjusting for a single factor. We were able to do this by selecting the most suitable epoxy molding compound to reduce encapsulation stress, selecting the appropriate capillary splitter to create a tighter intermetallic phase structure, and optimizing process parameters to minimize irregular grain growth and improve initial intermetallic phase coverage.

The results show that the most critical influencing factor is the type of lead. The type of capillary splitter also has a significant impact on the formation of intermetallic phases. However, once the intermetallic phase coverage exceeds 70%, coverage itself is no longer a decisive factor. When we use 70μm soldering