Chip bonding: connect chips, connect the future

<!--[if !supportLists]-->1.<!--[endif]-->Wire Bonding Wire bonding is the most common and effective joining process for interconnecting chips to external packages.

1.1 Commonly used wire bonding methods

1.1.1 Hot pressing bonding: The principle of hot pressing bonding is the combination of low-temperature diffusion and plastic flow, which promotes the contact of atoms with each other, and then produces solid diffusion bonding. Plastic deformation and diffusion occur on the contact surface after a certain period of time, temperature, and pressure at the part that is subjected to pressure during bonding. Plastic deformation is necessary to destroy any contact surfaces so that the metal surface can be fused. During the bonding process, the deformation of the wire is a plastic flow. This method is mainly applied to gold wire bonding. 1.1.2 Ultrasonic bonding: Ultrasonic bonding of welding wire is a combination of plastic flow and friction. The friction action is transmitted to a metal sensor (Metal "HORN") by means of a quartz crystal or magnetic control. When the quartz crystal is energized, the metal sensor stretches; When the power is lost, the sensor shrinks accordingly. These actions are generated by an ultrasound generator and the amplitude is usually 4 - 5 microns. When a welding tool is installed at the end of the sensor, when the welding tool vibrates before and after the sensor expands and contracts, the welding wire creates friction at the bonding point and forms plastic deformation under top-down pressure. Most of the plastic deformation occurs after the bonding point is subjected to ultrasonic energy, and the plasticization caused by pressure accounts for only a very small part, because when ultrasonic waves act on the bonding point, the hardness of the bonding point will decrease, so that the same pressure can produce a large plastic transformation. This bonding method can be used for gold or aluminum wire bonding.

1.2 Wire bonding process The wire bonding process includes: pad and housing cleaning, wire bonding machine adjustment, wire bonding and inspection. Molecular cleaning methods are currently commonly used in the enclosure cleaning method, i.e., plasma cleaning or ultraviolet ozone cleaning.

(1) Plasma cleaning - this method uses a high-power RF source to convert the gas into plasma, and high-speed gas ions bombard the surface of the bonding zone, and sputter to remove the pollutants by combining with the pollutant molecules or making them physically split. The gases used are generally O2, Ar, N2, 80%Ar + 20%O2, or 80%O2 + 20%Ar. In addition, O2/N2 plasma has an application, which is an effective degassing material for the removal of epoxy resins.

(2) Ultraviolet ozone cleaning is performed by emitting radiation at wavelengths of 184.9 mm and 253.7 mm. The process is as follows: ultraviolet rays with a wavelength of 184.9nm can break the O2 molecular chain to form an atomic state (O + O), and the atomic oxygen combines with other oxygen molecules to form ozone O3. Under the action of ultraviolet light at a wavelength of 253.7 nm, ozone can be decomposed into atomic oxygen and molecular oxygen again. Water molecules are able to be broken to form free OH-roots. All of these substances are capable of reacting with hydrocarbons to form CO2 + H2O, which eventually leaves the bonding surface as a gas. The 253.7nm wavelength UV also breaks the molecular bonds of hydrocarbons to accelerate the oxidation process. Although these two methods are able to remove organic contamination from the pad surface, their effectiveness is highly dependent on the specific contaminant. For example, oxygen plasma cleaning does not improve the solderability of Au thick films, and the best cleaning method is O2 + Ar plasma or solution cleaning. In addition, some contaminants, such as Cl ions and F ions, cannot be removed by the above methods because of the formation of chemical binding. Therefore, in some cases, solution cleaning is also required, such as vapor fluorocarbons, deionized water, etc. (3) There are two types of wire bonding process: ball bonding process and wedge bonding process. Ball bonding generally uses fine Au wires with a diameter of 75 μm or less. This is mainly because the fine Au filament is easy to deform under high temperature and pressure, has good oxidation resistance and good spheroidization. Ball bonding is typically used when the pad spacing is greater than 100 μm, but there are also examples where the pad spacing is 50 μm. The wedge bonding process is suitable for both Au and Al wires. The difference between the two is that Al wire is ultrasonically bonded at room temperature, while Au wire is thermally ultrasonic bonded at 150°C. A major advantage of wedge bonding is that it is suitable for fine sizes, such as pad spacing below 50 μm. However, due to the rotational motion of the bonding tool, its overall speed is lower than that of thermoultrasonic ball bonding. The most common wedge bonding process is Al wire ultrasonic bonding, which has a lower cost and bonding temperature. The main advantage of Au wire wedge bonding is that there is no need for a sealed package after bonding, because the solder joint formed by wedge bonding is smaller than that of ball bonding, which is especially suitable for microwave devices.

(4) There are two ways to bond. Positive bonding: the first point is bonded to the chip, and the second point is bonded to the package shell; Reverse solder bonding: The first point is bonded to the shell and the second point is bonded to the chip. When using positive soldering bonding, the bonding point on the chip generally has a tail wire; With reverse bonding, there is usually no trailing wire on the chip. Exactly which bonding method to use for circuit bonding needs to be determined on a case-by-case basis.

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<!--[if !supportLists]-->1.<!--[endif]-->Failure caused by bonding process errors 2.1 Pad pit The pit phenomenon usually occurs in ultrasonic bonding, which refers to the damage to the semiconductor material layer below the pad metallization layer. This damage is sometimes a visible dent and more often is an invisible damage to the structure of the material. This damage can degrade device performance and cause electrical damage. The reasons for this are as follows:

(1) The ultrasonic energy is too high, resulting in Si lattice lamination fault;

(2) The bonding force is too high or too low when the wedge is bonded;

 (3) The impact velocity of the bonding tool on the substrate is too large, which generally will not make the Si device out of the pit, but will cause the GaAs device out of the pit; (4) The solder ball is too small during ball bonding, causing the hard bonding tool to contact the pad metallization layer;

(5) The thickness of the pad is too thin. Pad thickness of 1 - 3 μm is relatively small, but pads with a thickness of less than 0.6 μm can be problematic;

(6) When the hardness of the pad metal and the lead metal is matched, the bonding quality is the best, and the pit phenomenon can also be minimized;

 (7) When the Al wire is ultrasonic bonded, the hard metal wire may cause the Si sheet to go out of the pit. 2.2 Tail filament inconsistency This is the most common and difficult problem to solve when wedge bonding. Possible causes are as follows:

 (1) The surface of the lead wire is dirty;

(2) The wire transmission angle is incorrect;

(3) The wedge through hole is partially blocked;

 (4) The tool used to clamp the lead is dirty;

 (5) The fixture clearance is incorrect;

(6) The pressure exerted by the fixture is incorrect;

 (7) Wire stretching error. A tail filament that is too short means that the forces acting on the first bonding point are distributed over a smaller area, which will result in excessive deformation. A tail wire that is too long may cause a short circuit between the pads. 2.3 Bonding peeling Peeling refers to the partial or complete detachment of the root of the bonding point from the bonding surface when pulling, and the fracture is smooth. Stripping is mainly caused by the wrong selection of process parameters or a decrease in the quality of the bonding tool. It is an early signal of bond-related failure. 2.4 Lead bending fatigue The cause of this failure is the crack at the root of the wire bonding point. It can be mechanical fatigue during bonding operations or thermal stress fatigue due to temperature cycling. The results of the existing experiments show that:

(1) Under the condition of temperature cycling, the ultrasonic bonding of Al wire is more reliable than that of hot pressing bonding of Al wire;

 (2) 0.1% Mg的Al丝性能优于含1%Si的Al丝;

(3) The height of the lead closed loop should be at least 25% of the bonding point spacing to reduce bending. 2.5 Bond and Pad Corrosion Corrosion can cause one or both ends of the lead to be completely disconnected, allowing the lead to move freely within the package and causing a short circuit. Moisture and dirt are the main causes of corrosion. For example, the presence of Cl or Br at the bond site can lead to the formation of chloride or bromide, which corrodes the bond point. Corrosion increases the resistance of the bond point until the device fails. In most cases, the encapsulation material exerts pressure on the chip surface and adjacent bonding points, and electrical connection problems only occur when corrosion is very severe. 2.6 Lead frame corrosion is caused by excessive residual stress or excessive surface contamination introduced in the surface plating (e.g., Ni) process that prevents corrosion of the lead frame base metal (Alloy 42 or Cu). The most sensitive area is at the interface between the sealing compound material and the lead frame. 2.7 Metal migration refers to the growth of metal dendrites starting at the bonding pad. This is an electrolysis process in which metal ions migrate from the anode region to the cathode region, which is related to the availability of metals, ion types, potential differences, etc. Metal migration can lead to an increase in leakage currents in the bridging area, which can cause a short circuit if the bridging is fully formed. Ag migration was the most commonly reported, but other metals such as Pb, Sn, Ni, Au and Cu also migrated. It is a phenomenon of gradual failure due to its failure association. 2.8 Vibration fatigue The minimum frequency at which resonance can be generated and therefore damage to the bonding point, 3 - 5 kHz for Au filaments and 10 kHz for Al filaments. In general, the vibration fatigue failure of wire bonding occurs during the ultrasonic cleaning process, so the resonant frequency of the ultrasonic cleaning equipment should be within 20 - 100kHz.

<!--[if !supportLists]-->2.<!--[endif]-->Inner Lead Fracture and Debonding The methods of inner lead break are generally divided into three categories: middle lead breakage, lead break at the root near the bonding point, and debonding.

(1) Intermediate fracture of the lead The intermediate fracture of the lead does not necessarily occur in early failure, because it is related to the degree of damage of the inner lead and the mechanism induced by the damage. Bond wire damage will make the area of the damaged part of the lead wire smaller, resulting in: increased current density, so that the damaged part is easy to be burned; The ability to resist mechanical stress is reduced, resulting in fracture of the inner lead where it is damaged. The causes of damage are: first, the bonding wire is subject to mechanical damage, and second, the bonding wire is eroded by chemical corrosion.

(2) Bond wire breaks at the root near the bonding point This phenomenon is mainly caused by the process. In the presence of a thallium (Tl) contamination source, Tl can form a eutectic phase with a low melting point with Au and transport from the Au-plated lead frame into the Au filament. During bond point formation, Tl can be rapidly diffused and enriched at grain boundaries above the ball neck to form a eutectic phase. On plastic sealing or temperature cycling, the ball neck breaks and the device fails.

(3) Bonding point detachment In automatic wire bonding technology, bond point detachment of semiconductor devices is the most common failure mode. This failure mode is difficult to remove by conventional screening and testing, and can only be exposed under strong vibrations, so it is extremely harmful to the reliability of semiconductor devices. The main factors that can affect the reliability of an internal wire bond are:

<1> Formation of the insulation layer on the interface: The photoresist or window passivation film in the bonding zone on the chip is not cleaned, and the insulation layer will be formed. The poor quality of the gold plating layer of the tube shell will lead to problems such as surface porosity, redness, bubbling, and peeling. In the case of metal-to-metal bonding contact, in the presence of oxygen, chlorine, sulfur, and water vapor, the metal often reacts with these gases to form an insulating interlayer such as oxide and sulfide, or is corroded by chlorine, resulting in an increase in contact resistance and thus reducing the bonding reliability.

 <2> Metallization layer defects: Metallization layer defects mainly include the chip metallization layer is too thin, so that there is no buffering effect during bonding, and alloy points appear in the chip metallization layer, forming defects at the bonding place; The metallization layer of the chip is not firmly adhered and is most likely to drop the pressure point.

 <3> The surface is contaminated, and the atoms cannot diffuse each other: including chips, tube shells, capillary knives, gold wires, tweezers, tungsten needles and other links may cause contamination. The external environment is not purified enough, which will cause dust pollution; Poor purification of the human body will cause organic matter contamination and sodium contamination; If the chips and shells are not cleaned in time, the residual gold plating solution will cause potassium contamination and carbon contamination, which is a batch problem, which may lead to the scrapping of a batch of shells, or cause bond point corrosion, resulting in failure; If the gold wire and tube shell are stored for too long, it is not only easy to stain, but also easy to age, and the hardness and elongation of the gold wire will also change.

<4> Improper contact stress between materials: Bonding stress includes thermal, mechanical, and ultrasonic stress. Too little bond stress can lead to poor bonding, but too much bond stress can also affect the mechanical properties of the bond point. Excessive stress can cause damage to the root of the bond joint, cause the root of the bond to break and fail, and also damage the chip material under the bond point, and even crack the joint.

<!--[if !supportLists]-->3.<!--[endif]-->Intermetallic compounds fail the Au-Al system 4.1 Interdiffusion and intermetallic formation in the Au-Al system The interdiffusion and intermetallic formation process in the Au-Al system is as follows:

(1) In the early stage of bonding, a thin diffusion layer is formed between Au-Al, the composition of which is AuAl2 (purple spot);

 (2) Further heating will cause the Au-Al diffusion to continue, and the pure Al layer will disappear as Au continues to diffuse into the Al film. At the same time, an Au5Al2 compound layer was formed on the side of the Au silk ball.

(3) The thickness of the diffusion layer does not increase indefinitely, because the source of Al is limited, and there is a difference in the mutual diffusion velocity between the two. Define D as diffusion velocity, DAu→Al> DAl→Au. Assuming that the initial Al film thickness is 1 μm, the total diffusion layer thickness is about 4 - 5 μm. When further heated, Au diffuses into the diffusion layer and forms Au4Al on the side of the Au filament ball, and grows to the side of the semiconductor chip.

(4) When further heated, the diffusion of Au into the diffusion layer continues, and finally the composition of the diffusion layer is only Au5Al2 and Au4Al. At the same time, due to the Kirkendal effect, cavities will be created around the diffusion layer;

(5) If the heat continues, the Au diffusion in the non-void position will be further strengthened, resulting in the formation of the Au4Al layer in the central part.

(6) For plastic ICs, it will be a catalyst for Al oxidation in the Au4Al layer due to the bromide content of the flame retardant in the resin material. The bromide passes through the void into the bonding point and oxidizes Al in the Au4Al layer, resulting in the formation of a highly resistive layer at the interface between the center of the Au sphere and the compound layer, which will result in a kind of disconnection failure.

4.2 Effect of impurities on Au-Al systems In the early days of lead development, the main purpose was to enhance the mechanical strength, such as the control of the lead structure and length, so that the problem of metal-to-metal fracture was not given too much consideration. However, as the pad spacing continues to decrease and the control window continues to narrow, the development of wire bonding technology begins to be limited by the intermetallic phase problem. So far, the lead doping effect has not been studied in depth. The addition of doping impurities and slowing down the diffusion rate of the intermetallic phase are considered to be the only means to reduce intermetallic failure. In fact, at a doping concentration of 100 ppm, doping impurities cannot effectively prevent the growth of intermetallic phases. For this reason, the content of doping impurities in some commonly used leads is increased to 1%, and the doping impurities can prevent the diffusion of Au and Al. However, it is not as effective as expected, and it also reduces the conductivity of the leads. As a result, new approaches are needed that can address these issues more effectively without compromising conductivity performance. 4.3 Improvement Methods There are many causes of intermetallic failure, so it is difficult to minimize them by controlling one factor. What we can do is select the best EMC (Epoxy Molding Compound) to reduce encapsulation stress, select the best capillary capillary type to form a denser intermetallic phase, and optimize process parameters to minimize irregular growth and increase initial intermetallic coverage. The results of the study show that the most effective influencing factor is the type of lead. The capillary capillary type also affects the formation of intermetallic phases. However, when the intermetallic phase coverage is greater than 70%, the intermetallic phase coverage is no longer the main factor. When we use capillary capillary and lead types with a pad spacing of 70 μm for 40 μm, we experience high-temperature superconductivity (HTS) and temperature cycling failures. However, by selecting the best capillary capillary type, lead type, and EMC, we can achieve good results in reliability performance.

<!--[if !supportLists]-->4.<!--[endif]-->5.1 Effect of Peak Temperature of Thermal Cycling on Metallographic Structure The metallographic structure of thermal cycling under different peak temperature conditions is shown in Figure 1. As can be seen from Figure 1, when the peak temperature of the thermal cycle is 1350°C, it transforms into coarse low-carbon martensitic + a small amount of side slat bainite tissue after cooling. When the peak temperature of the thermal cycle is 950°C, the cooled tissue is significantly refined. When the peak temperature of the thermal cycle is 750°C, the part of the quenching zone corresponding to the heat-affected zone, due to the short residence time of the high temperature, the homogenization of the austenite composition is very insufficient, so that the structure of the zone is ferrite + granular bainite structure. When the peak temperature is 600°C, it does not exceed the high-temperature tempering temperature during quenching and tempering treatment, and the structure is dominated by tempered sostenite. 5.2 Effect of Peak Temperature of Thermal Cycle on Impact Energy Under the action of thermal cycling at different peak temperatures, the impact energy decreases with the increase of the peak temperature of thermal cycle. When the peak temperature of the thermal cycle exceeds 1100°C, the impact energy has been reduced to a low level. It can be seen that with the increase of the peak temperature of the thermal cycle, the grain growth tends to increase, and when the peak temperature of the thermal cycle is 1350°C, the austenite grain grows severely, resulting in the lowest impact energy in this region.

<!--[if !supportLists]-->5.<!--[endif]--> 5.3 Lead fatigue In the thermal fatigue test of Au nanoleads, a sinusoidal alternating voltage (Vpp = 10V) is fed into the lead, resulting in an alternating thermal stress in the lead. The frequency of the alternating voltage signal in the experiment is 50Hz - 100Hz. Let the temperature change generated by the alternating electrical signal in the lead be ΔT, then the thermal strain generated in the lead is Δε = (αAu - αSi)ΔT, where (αAu - αSi) is the difference between the thermal expansion coefficients of Au and Si (αAu = 1.42×10 - 5/°C; αSi = 3×10 - 6/℃）。 This strain will cause the Au lead to undergo a compression-fatigue cycle. In the experiment, the fatigue failure life of each specimen (the number of fatigue cycles when the guide wire is open) was recorded, and the topography of the lead surface was observed by scanning electron microscopy (SEM). The fatigue failure results of three different widths of Au leads with a length of 20 μm and different widths are shown under the same alternating voltage signal (Vpp = 10V) and different voltage frequencies. For the same input voltage, the number of failure cycles decreases significantly as the lead linewidth decreases.

**I. Content Summary**

1. **Wire Bonding Overview**

   - Wire bonding is a common and effective joining process between a chip and an external package.

   - Commonly used wire bonding methods include hot pressing bonding (the principle is the combination of low-temperature diffusion and plastic flow, mainly used for gold wire bonding) and ultrasonic bonding (which is a combination of plastic flow and friction, which can be used for gold wire or aluminum wire bonding).

2. **Wire bonding process**

   - Includes pad and housing cleaning, wire bonder adjustment, wire bonding, and inspection.

   - Enclosure cleaning methods include plasma cleaning (which uses a high-power RF source to convert gas into plasma to remove contaminants) and UV ozone cleaning (which removes contaminants by substances produced by specific wavelengths of ultraviolet light).

   - Wire bonding processes include ball bonding (for fine Au wires, often used when the pad spacing is greater than 100 μm) and wedge bonding (for Au wires and Al wires, Al wires are ultrasonically bonded at room temperature, and Au wires are thermally ultrasonic bonded at 150°C, suitable for fine sizes).

   - Bonding methods are positive bonding (the first point is bonded to the chip) and reverse bonding (the first point is bonded to the housing).

3. **Failure due to bonding process errors**

   - Pad pits: Mostly in ultrasonic bonding, the reasons include excessive ultrasonic energy, improper bonding force, etc.

   - Tail wire inconsistency: Wedge bonding tends to occur, which is related to a variety of factors such as dirty lead surfaces.

   - Bond stripping: Caused by incorrect process parameters or deterioration in the quality of the bonding tool.

   - Lead bending fatigue: Caused by mechanical fatigue or thermal stress fatigue due to temperature cycling.

   - Corrosion of bond joints and pads: Moisture and dirt are the main causes.

   - Lead frame corrosion: excessive residual stress or contamination of the surface plating.

   - Metal migration: is a metal ion migration process that causes short circuits.

   - Vibration fatigue: The minimum resonant frequency varies depending on the lead material.

   - Inner lead breakage and debonding: including intermediate lead breakage (related to bond wire damage), root break of the bond wire near the bond point (caused by the process), bond point debonding hazard (affected by a variety of factors, such as the formation of interfacial insulation, etc.).

4. **Intermetallic compounds disable Au-Al systems**

   - The interdiffusion and intermetallic formation processes in the Au-Al system are complex, and the composition and structure of the compound layer will change due to heat, and bromide will lead to disconnection failure.

   - In terms of the influence of impurities on the Au-Al system, increasing the content of doped impurities can prevent diffusion but reduce the conductivity.

   - Improvement methods include selecting the best EMC, capillary capillary type and optimizing process parameters.

5. **Thermal cycling makes the lead fatigue and fails**

   - The peak temperature of thermal cycling has different effects on the metallographic structure, and the metallographic structure is different at different peak temperatures.

   - The effect of the peak temperature of thermal cycling on the impact energy decreases with the increase of temperature.

   - In the Au nanometer lead thermal fatigue test, the thermal strain generated in the lead wire causes fatigue cycling, and the number of failure cycles is reduced by reducing the lead width at the same input voltage.