Analysis of Chip Bonding Technology

System analysis and reliability improvement strategies for failure modes of semiconductor bonding systems

In the complex process chain of semiconductor packaging, the bonding system is like a "neural hub" connecting the chip to the outside world, and its reliability directly determines the interconnect quality of electronic components. Statistics show that among all kinds of failures of microelectronic devices, bonding system-related failures account for as high as 35%-40%, which not only affects product yield, but may also lead to sudden failures of terminal equipment during use. The failure mechanism of bonding systems is highly complex, involving the intersection of materials science, thermodynamics, electrochemistry and other disciplines, and the same failure phenomenon may originate from completely different physicochemical processes. This paper focuses on three typical bonding systems, Au-Al, Cu-Al, and Al-Al, reveals the essence of various failure modes through physical and chemical analysis, and establishes a full-chain solution of "failure mode, mechanism analysis, detection method, and improvement strategy" to provide systematic guidance for improving the reliability of bonding systems.

1. Bonding process foundation and system composition

1.1 The core process of the bonding process

The bonding process is essentially a precision manufacturing process that establishes the electrical connection between the chip electrode and the substrate through metal leads, and although the bonding processes of different metal wires have commonalities, there are key differences due to the difference in material properties:

As the most mature technical solution, the standardization process of the gold wire bonding process includes four key steps:

Melt ball formation: The lighter emits a high-voltage arc (usually 3-5kV), melting the end of the gold wire into a spherical shape with a diameter of 1.5-2 times the diameter of the wire (3mil gold wire forms about 4.5mil melt ball); The first bond point is formed: the splitting knife carries the molten ball down to the chip pad, and applies 10-30g pressure and 20-40kHz ultrasonic vibration to make the melt ball and the aluminum pad form a metallurgical combination to form a spherical bonding point; Lead arc control: The splitting knife moves up and laterally, pulling out a gold wire with a specific arc (arc height is usually 2-3 times the thickness of the chip) to ensure that the lead is neither affected by excessive tension nor interferes with other components; Second bond point formation: The splitting knife applies pressure and ultrasonic vibration to the corresponding position of the lead frame, and after forming a wedge-shaped bonding point, the gold wire is cut off, and 0.5-1mm tail wire is retained for the next melt ball.

The process steps of copper wire bonding are basically the same as those of gold wire, but they need to be carried out in an inert atmosphere:

Protective gas environment: The bonding area is continuously injected with 95% N₂+5% H₂ gas mixture (flow rate 5-10L/min), and the oxygen content is controlled below 50ppm to prevent oxidation of copper wires; Parameter adjustment: Because the hardness of copper is 2-3 times that of gold , the bonding pressure needs to be increased by 30%-50% (from 15g to 20-25g), and the ultrasonic power is increased by 20% to ensure the rupture of the interface oxide layer. Equipment adaptability transformation: The material of the splitting knife has been changed from ceramic to tungsten carbide (WC) to improve wear resistance and extend the service life (from 500,000 times for gold wire bonding to 300,000 times for copper wire). Aluminum wire bonding adopts room temperature ultrasonic bonding technology, and the process characteristics are significantly different: no melting ball step: directly use a splitting knife to apply pressure (50-100g) and ultrasonic vibration (60kHz) to the aluminum wire, and destroy the surface oxide layer through plastic deformation;

Double wedge bonding: The first and second bond points are wedge-shaped, and the bond strength mainly depends on the atomic diffusion of the aluminum-aluminum interface. High-Power Adaptation: Aluminum wires with a diameter of more than 300μm are used for power devices, and 150-200g pressure is applied simultaneously during bonding to ensure large-section contact. Industry standards clearly quantify the requirements for bond strength: AEC-Q100 specifies a shear force of 3 mil gold ball ≥ 30.8 g and a 3 mil gold wire tensile force ≥ 15 g; MIL-STD 883 is more stringent, with a shear force of ≥ 35g and a tensile force of ≥ 18g for the same specification. These indicators directly reflect the stability of the bonding process.

1.2 Comparison of the characteristics of bonding materials

Ideal bonded leads need to balance conductivity, chemical stability, and mechanical properties, with significant differences between the three mainstream materials: gold, copper, and aluminum:

The core advantages of gold wire (99.99% purity) are chemical inertness and process maturity: Performance parameters: Resistivity 2.4μΩ・cm, tensile strength 140MPa, elongation 30%, stable mechanical properties can still be maintained at 150°C; Process advantages: The bonding yield can reach more than 99.9%, which is suitable for automated large-scale production; Main limitations: The cost is 8-10 times that of copper, and the intermetallic compound (IMC) formed with aluminum pads grows quickly, and the thickness of the IMC can reach 3.5μm after 1000 hours of storage at 150°C, resulting in embrittlement at the bond point. Copper wire (99.95% purity) is the mainstream choice for consumer electronics due to its cost and electrical performance advantages: Performance parameters: resistivity 1.7μΩ・cm (better than gold), tensile strength 300MPa (2 times that of gold), outstanding fatigue resistance; Application advantages: 30% higher download flow rate than gold for the same diameter, 50% longer power cycle life; Process challenges: Easy oxidation (oxidation rate is 10 times higher than gold), high sensitivity of the bonding process to parameters, and increased risk of chip damage by 20%. Aluminum wire (99.5% purity) is irreplaceable in the field of power devices: Specification features: The wire diameter is usually 300-500μm, the ampacity of the single filament can reach 18-30A, and the multi-filament parallel can meet the demand of hundreds of amps; Cost advantage: The price is only 1/20 of that of gold wire, suitable for high-power and low-cost applications; Limitations: The resistivity is 2.8μΩ・cm, which is 17% higher than gold, and it is easy to form an oxide layer (Al₂O₃), which affects the bond quality.

1.3 Structural composition of the bonding system

A complete bonding system consists of a complex multi-layer structure of leads and pads, and pad design is critical to bond reliability:

A typical profile structure for a chip pad includes, from top to bottom: Passivation layer: typically SiO₂ or Si₃N₄, the thickness is 1-2μm, the internal structure of the chip is protected, and the bonding window accuracy is controlled at ±5μm; Top layer aluminum metal: thickness 0.8-2μm, purity above 99.5%, addition of 1% Si to prevent electromigration, is the direct contact surface of bonding; Through-hole array: tungsten (W) material, diameter 1-2μm, density 50-100/μm², to achieve electrical connection between the top layer and the lower layer of metal; Dielectric layer: SiO₂ or polymer material, thickness 2-5μm, isolating different metal layers; Lower Metal: Typically Cu or Al, 1-3μm thick, serves as the main channel for current conduction.

Key design parameters for pad construction directly impact bonding performance:

Top layer aluminum thickness: Increasing the thickness can improve the stress buffer capacity, and experiments show that the bond cracking rate is reduced by 40% from 1μm to 2μm;

Via density: High-density arrays (>80 per μm²) reduce the risk of dielectric layer cracking, reducing dielectric cracks from 12% to 3% after density increase in one case; Window Size: 10%-20% larger than the diameter of the bond point, ensuring complete coverage of the bonding area and avoiding edge effects.

2. Analysis of typical failure modes and mechanisms of bonding systems

2.1 Cracking in the bonding process (crater effect)

Cracking in the bonding process is the most common failure mode of Cu-Al systems, and its essence is the interface damage caused by the mismatch between the process parameters and the material properties: Microscopic characteristics of the failure phenomenon: Appearance Manifestations: Pits or cracks can be seen on the surface of the pad after chemical opening, and in severe cases, the cross-sectional characteristics of the underlying dielectric layer are exposed: Observed by focused ion beam (FIB) cutting, the cracks radiate from the center of the bonding point to the surrounding area, and the depth can reach 1-3μm; Distribution law: It mostly occurs at the first bonding point of copper wire bonding, especially thin copper wires with a diameter of ≤2mil, which is 3-5 times more common than gold wire.

The underlying mechanism stems from the improper transmission of ultrasonic energy:

When the copper ball is not fully deformed, the ultrasonic vibration acts directly on the Cu-Al interface, generating a local stress concentration (up to 500MPa).

The plastic deformation of the aluminum pad cannot absorb all the energy, resulting in stress transmission to the lower dielectric layer, causing brittle fracture.

The fracture toughness of dielectric materials (such as SiO₂) is only 2-3MPa·m¹/², which is much lower than that of aluminum's 10MPa·m¹/², which became a weak link. DOE experiments at a packaging plant revealed key influencing factors: Ultrasonic power: When the power exceeded 300mW, the cracking rate jumped from 5% to 25%; Bonding pressure: insufficient (<15g) leads to poor contact, and excessive (>30g) increases the risk of cracking;

Copper ball hardness: When the Vickers hardness is > 80HV, the probability of cracking increases significantly.

2.2 Degradation of intermetallic bonds

Overgrowth of intermetallic compounds (IMCs) is a common failure risk for both Au-Al and Cu-Al systems, but there are differences in manifestations and mechanisms: The degradation of the Au-Al system is characterized by the following characteristics: IMC composition: formation of Au₂Al, AuAl, AuAl₂, Au₄Al at high temperatures and Au₅Al₂, of which Au₅Al₂ accounts for 60%-70%; Growth rate: It takes only 0.3 seconds to form a 100Å IMC at 200°C, and the thickness can reach 3.5μm after 1000 hours of storage at 150°C (as shown in Figure 4).

Failure Manifestations: High brittleness of IMC layer (fracture toughness < 1MPa·m¹/²), resulting in a 40%-50% decrease in bond strength, and the appearance of "purple spots" (AuAl₂) or "white spots" (Au₂Al). ) phenomenon. The Kirkendall effect is a unique degradation mechanism of the Au-Al system: diffusion difference: the diffusion rate of Au to Al is 5-10 times that of Al to Au, resulting in vacancies at the interface; Hole formation: vacancies aggregate to form micron-sized cavities that expand under thermal cycling conditions; Failure Evolution: Cracks form after the cavity joins, eventually causing the bond point to fall off, a process that begins to manifest at about 1000 hours at 125°C. The IMC growth of the Cu-Al system showed different characteristics: the main phase composition: CuAl₂ (θ phase), CuAl (η phase) and Cu₉Al₄ (ε phase), in which CuAl₂ was dominant. Growth kinetics: It takes 20 seconds to form 100Å IMC at 200°C, which is more than 60 times that of Au-Al.

Degradation Manifestation: IMC thickness increases linearly with time (approximately 0.5μm per year at 150°C), resulting in a 5%-10% increase in contact resistance per year.

2.3 Contact corrosion (electrochemical corrosion)

Contact corrosion is particularly prominent in Cu-Al systems and is a progressive failure dominated by electrochemical principles:

Electrochemical mechanism of failure: Potentiometric difference drive: The potential difference between Cu (+0.337V) and Al (-1.662V) reaches 2.0V, forming a galvanic battery in the presence of the electrolyte. Reaction process: Al is oxidized as the anode (Al→Al³⁺+3e⁻), Cu is reduced as the cathode (O₂+2H₂O+4e⁻→4OH⁻); Product Effects: The resulting Al (OH)₃ is non-conductive and expands in volume, resulting in increased contact resistance and eventually an open circuit. The acceleration effect of environmental factors is significant: humidity: when the relative humidity is > 60%, the corrosion rate increases by 3 times, and the water vapor condenses to form an electrolyte layer; Contaminants: Halogen ions such as Cl⁻ and Br⁻ (concentration > 100ppm) in the encapsulation material accelerate the reaction and are catalysts for corrosion; Temperature: For every 10°C increase, the corrosion rate increases by 20%-30%, and the corrosion rate at 125°C is 5 times faster than at room temperature. Typical characteristics of failure: Appearance: Copper bond points can be seen to fall off after chemical opening, and there are corrosion marks on the edges of residual aluminum pads. Cross-section: SEM observations show that the Al layer is uniformly corroded, forming a honeycomb structure; Composition: EDX detected high concentrations of Cl⁻ (>0.5wt%) in areas where corrosion was visible. Notably, while the potential difference between Au and Al is greater (3.16V), the Au-Al system has a lower risk of corrosion, as the rapidly forming thick IMC layer blocks the ongoing electrochemical reaction, acting as a natural barrier.

2.4 Degradation of bonding wires of power devices

The degradation of the bonding wire of power devices is a complex process under the action of multiphysics, which directly affects the product life: Driving mechanism of degradation:

Thermal cycle stress: The change of junction temperature (ΔTj=100-150°C) in the power cycle leads to thermal expansion differences between the bond wire and the chip (CTE mismatch). Current effect: The Joule heat generated by a large current (>10A) increases the temperature of the bonding wire by 50-100°C, intensifying the softening of the material; Mechanical fatigue: The root of the bonding wire is subjected to alternating stress, and the strain amplitude can reach 0.5%-1%, far exceeding the fatigue limit of the material. Typical manifestations of degradation: Displacement and disengagement: The aluminum bonding wire undergoes plastic deformation under cyclic stress, and the bonding point gradually detaches from the pad (as shown in Figure 7). Root cracking: High-stress zones are concentrated at the root where the bonding wire connects to the pad, forming microcracks and propagating (as shown in Figure 8). Parameter changes: A 10% increase in thermal resistance (Rth) and a 5% increase in saturation pressure drop (VCE) are clear signs of degradation.

Power cycle test is an effective method to evaluate degradation: test conditions: ΔTj=150°C, cycle 10 seconds (heating 6 seconds, cooling 4 seconds); Failure criteria: 20% increase in VCE or bond wire breakage;

Lifetime characteristics: According to the Weibull distribution, the B10 lifetime (10% failure probability) requirement of a typical power device ≥ 100,000 cycles.

3. Failure detection and analysis methods

3.1 Physical opening technology

Physical opening is the basic means to observe bonding failure, and the appropriate method should be selected according to the packaging type: Chemical opening process of plastic packaging: corrosion liquid formula: 95% sulfuric acid + 5% fumed nitric acid (volume ratio), temperature 80-100°C; Procedure: Immerse the device in corrosion liquid and stir magnetically (300rpm) until the molding compound is completely removed (usually 5-10 minutes); Post-treatment: Ultrasonic cleaning with deionized water (5 minutes), alcohol dehydration, to avoid residual acid corrosion of the bonding point. Mechanical opening method for metal casing: Tool selection: diamond cutting disc (thickness 0.1mm) with an accuracy of ±0.01mm; Process control: first cut along the edge of the package, and then gradually remove the shell to avoid damage to the internal bonding structure; Applications: Metal case packages for power devices (such as TO-247) that preserve the original state of the bonding system.

3.2 Micro analysis techniques

A variety of microscopic characterization methods can be used together to accurately identify failure modes: Scanning electron microscopy (SEM) analysis: observation content: bond point morphology, crack distribution, corrosion product morphology; Operating conditions: acceleration voltage 10-20kV, working distance 5-10mm, resolution up to 10nm; Advantages: It can directly observe three-dimensional topography and distinguish different phases in combination with backscatter electron imaging. X-ray spectroscopy (EDX): Application scenarios: determining the composition of corrosion products (e.g., detecting Cl⁻ to confirm contact corrosion), analyzing IMC elemental composition; Quantitative accuracy: the error of the main elements (>1wt%) is < 5%, and the element distribution can be analyzed semi-quantitatively; Limitations: Light elements (e.g., H, He) are undetectable with a spatial resolution of about 1mm. Section preparation technology: Ion grinding (CP): Bombardment of samples with an Ar ion beam (3-5keV) to prepare stress-free sections; Focused Ion Beam (FIB): Accuracy up to 5nm, suitable for preparing ultra-thin sections of specific areas; Observation advantage: Clearly display IMC thickness, cavity distribution, and interface bonding status.

3.3 Reliability test method

Accelerated test simulates failure under long-term use conditions: High Temperature Storage Test (HTSL): Conditions: 150°C or 175°C for 1000-2000 hours; Objective: To accelerate IMC growth and assess the risk of intermetallic degeneration; Monitoring indicators: bond strength retention, contact resistance change rate. Power Cycle Test: Equipment: Power Cycle Tester (such as T3Ster), which can control ΔTj accuracy ±2°C; Monitoring parameters: real-time recording of VCE, junction temperature, thermal resistance; Data analysis: Extrapolation of lifetime under real-world usage conditions through the Arrhenius model. Temperature cycling test: Conditions: -55°C~125°C, 1000-2000 cycles;

Stress source: CTE differences between different materials (e.g., Al wire CTE=23ppm/°C, Si chip CTE=2.6ppm/°C);

Applicable scenarios: Evaluate bond failures caused by mechanical stress (e.g., broken leads, solder joints falling off).

4. Systematic improvement strategies and implementation cases

4.1 Solutions for cracking in the bonding process

Multi-dimensional optimization significantly reduces the risk of cracking in Cu-Al systems:

Process parameter optimization:

Segmented ultrasonic power: 200mW (0.1 seconds) initially, then increased to 300mW (0.2 seconds) to reduce impact stress;

Pressure-Power Matching: For 2mil copper wire, recommended

System analysis and reliability improvement strategies for failure modes of semiconductor bonding systems

In the complex process chain of semiconductor packaging, the bonding system is like a "neural hub" connecting the chip to the outside world, and its reliability directly determines the interconnect quality of electronic components. Statistics show that among all kinds of failures of microelectronic devices, bonding system-related failures account for as high as 35%-40%, which not only affects product yield, but may also lead to sudden failures of terminal equipment during use. The failure mechanism of bonding systems is highly complex, involving the intersection of materials science, thermodynamics, electrochemistry and other disciplines, and the same failure phenomenon may originate from completely different physicochemical processes. This paper focuses on three typical bonding systems, Au-Al, Cu-Al, and Al-Al, reveals the essence of various failure modes through physical and chemical analysis, and establishes a full-chain solution of "failure mode, mechanism analysis, detection method, and improvement strategy" to provide systematic guidance for improving the reliability of bonding systems.

1. Bonding process foundation and system composition

1.1 The core process of the bonding process

Although the bonding process of different metal wires has commonalities, there are key differences due to the difference in material properties: as the most mature technical solution, the standardized process of the gold wire bonding process includes four key steps: melt ball formation: the lighter releases a high-voltage arc (usually 3-5kV), and melts the end of the gold wire into a spherical shape with a diameter of 1.5-2 times the wire diameter (3mil gold wire forms a melt ball of about 4.5mil); The first bond point is formed: the splitting knife carries the molten ball down to the chip pad, and applies 10-30g pressure and 20-40kHz ultrasonic vibration to make the melt ball and the aluminum pad form a metallurgical combination to form a spherical bonding point;

Lead arc control: The splitting knife moves up and laterally, pulling out a gold wire with a specific arc (arc height is usually 2-3 times the thickness of the chip) to ensure that the lead is neither affected by excessive tension nor interferes with other components; Second bond point formation: The splitting knife applies pressure and ultrasonic vibration to the corresponding position of the lead frame, and after forming a wedge-shaped bonding point, the gold wire is cut off, and 0.5-1mm tail wire is retained for the next melt ball. The process steps of copper wire bonding are basically the same as those of gold wire, but they need to be carried out in an inert atmosphere: Protective gas environment: 95% N₂+5% H₂ mixed gas (flow rate 5-10L/min) is continuously passed through the bonding area, and the oxygen content is controlled below 50ppm to prevent oxidation of copper wire; Parameter adjustment: Because the hardness of copper is 2-3 times that of gold , the bonding pressure needs to be increased by 30%-50% (from 15g to 20-25g), and the ultrasonic power is increased by 20% to ensure the rupture of the interface oxide layer. Equipment adaptability transformation: The material of the splitting knife has been changed from ceramic to tungsten carbide (WC) to improve wear resistance and extend the service life (from 500,000 times for gold wire bonding to 300,000 times for copper wire).

Aluminum wire bonding adopts room temperature ultrasonic bonding technology, and the process characteristics are significantly different: no melting ball step: directly use a splitting knife to apply pressure (50-100g) and ultrasonic vibration (60kHz) to the aluminum wire, and destroy the surface oxide layer through plastic deformation; Double wedge bonding: The first and second bond points are wedge-shaped, and the bond strength mainly depends on the atomic diffusion of the aluminum-aluminum interface. High-Power Adaptation: Aluminum wires with a diameter of more than 300μm are used for power devices, and 150-200g pressure is applied simultaneously during bonding to ensure large-section contact. Industry standards clearly quantify the requirements for bond strength: AEC-Q100 specifies a shear force of 3 mil gold ball ≥ 30.8 g and a 3 mil gold wire tensile force ≥ 15 g; MIL-STD 883 is more stringent, with a shear force of ≥ 35g and a tensile force of ≥ 18g for the same specification. These indicators directly reflect the stability of the bonding process.

1.2 Comparison of the characteristics of bonding materials

Ideal bonded leads need to balance conductivity, chemical stability, and mechanical properties, with significant differences between the three mainstream materials: gold, copper, and aluminum:

The core advantages of gold wire (99.99% purity) are chemical inertness and process maturity: Performance parameters: Resistivity 2.4μΩ・cm, tensile strength 140MPa, elongation 30%, stable mechanical properties can still be maintained at 150°C; Process advantages: The bonding yield can reach more than 99.9%, which is suitable for automated large-scale production; Main limitations: The cost is 8-10 times that of copper, and the intermetallic compound (IMC) formed with aluminum pads grows quickly, and the thickness of the IMC can reach 3.5μm after 1000 hours of storage at 150°C, resulting in embrittlement at the bond point. Copper wire (99.95% purity) has become the mainstream choice for consumer electronics due to its cost and electrical performance advantages: Performance parameters: resistivity 1.7μΩ・cm (better than gold), tensile strength 300MPa (twice that of gold), outstanding fatigue resistance;

Application advantages: 30% higher download flow rate than gold for the same diameter, 50% longer power cycle life; Process challenges: Easy oxidation (oxidation rate is 10 times higher than gold), high sensitivity of the bonding process to parameters, and increased risk of chip damage by 20%. Aluminum wire (99.5% purity) is irreplaceable in the field of power devices: Specification features: The wire diameter is usually 300-500μm, the ampacity of the single filament can reach 18-30A, and the multi-filament parallel can meet the demand of hundreds of amps; Cost advantage: The price is only 1/20 of that of gold wire, suitable for high-power and low-cost applications;

Limitations: The resistivity is 2.8μΩ・cm, which is 17% higher than gold, and it is easy to form an oxide layer (Al₂O₃), which affects the bond quality.

1.3 Structural composition of the bonding system

A complete bonding system consists of a complex multi-layer structure of leads and pads, and pad design is critical to bond reliability:

A typical profile structure of a chip pad from top to bottom includes:

Passivation layer: usually SiO₂ or Si₃N₄, thickness 1-2μm, protect the internal structure of the chip, and the bonding window accuracy is controlled at ±5μm;

Top layer aluminum metal: thickness 0.8-2μm, purity above 99.5%, addition of 1% Si to prevent electromigration, is the direct contact surface of bonding;

Through-hole array: tungsten (W) material, diameter 1-2μm, density 50-100/μm², to achieve electrical connection between the top layer and the lower layer of metal; Dielectric layer: SiO₂ or polymer material, thickness 2-5μm, isolating different metal layers; Lower Metal: Typically Cu or Al, 1-3μm thick, serves as the main channel for current conduction. Key design parameters of pad construction directly affect bonding performance:

Top layer aluminum thickness: Increasing the thickness can improve the stress buffer capacity, and experiments show that the bond cracking rate is reduced by 40% from 1μm to 2μm;

Via density: High-density arrays (>80 per μm²) reduce the risk of dielectric layer cracking, reducing dielectric cracks from 12% to 3% after density increase in one case; Window Size: 10%-20% larger than the diameter of the bond point, ensuring complete coverage of the bonding area and avoiding edge effects.

2. Analysis of typical failure modes and mechanisms of bonding systems

2.1 Cracking of the bonding process (crater effect) Cracking of the bonding process is the most common failure mode of the Cu-Al system, which is essentially the interface damage caused by the mismatch between the process parameters and the material properties: Microscopic characteristics of the failure phenomenon: Appearance Manifestation: After chemical opening, pits or cracks can be seen on the surface of the pad, and in severe cases, the underlying dielectric layer is exposed (as shown in Figure 2). Cross-sectional characteristics: Cracks radiate from the center of the bond point to the surrounding area by focusing ion beam (FIB) cutting, up to a depth of 1-3μm; Distribution law: It mostly occurs at the first bonding point of copper wire bonding, especially thin copper wires with a diameter of ≤2mil, which is 3-5 times more common than gold wire. The underlying mechanism stems from the improper transfer of ultrasonic energy: when the copper ball is not sufficiently deformed, the ultrasonic vibration acts directly on the Cu-Al interface, resulting in a local stress concentration (up to 500MPa). The plastic deformation of the aluminum pad cannot absorb all the energy, resulting in stress transmission to the lower dielectric layer, causing brittle fracture. The fracture toughness of dielectric materials (such as SiO₂) is only 2-3MPa·m¹/², which is much lower than that of aluminum's 10MPa·m¹/², which became a weak link. DOE experiments at a packaging plant revealed key influencing factors: Ultrasonic power: When the power exceeded 300mW, the cracking rate jumped from 5% to 25%; Bonding pressure: insufficient (<15g) leads to poor contact, and excessive (>30g) increases the risk of cracking; Copper ball hardness: When the Vickers hardness is > 80HV, the probability of cracking increases significantly.

2.2 Degradation of intermetallic bonds

Overgrowth of intermetallic compounds (IMCs) is a common failure risk for both Au-Al and Cu-Al systems, but there are differences in manifestations and mechanisms: The degradation of the Au-Al system is characterized by the following characteristics: IMC composition: formation of Au₂Al, AuAl, AuAl₂, Au₄Al at high temperatures and Au₅Al₂, of which Au₅Al₂ accounts for 60%-70%; Growth rate: It only takes 0.3 seconds to form 100Å IMC at 200°C, and the thickness can reach 3.5μm after 1000 hours of storage at 150°C (failure performance: IMC layer is brittle (fracture toughness < 1MPa·m¹/²), resulting in a 40%-50% decrease in bond strength, and the appearance of "purple spots" (AuAl₂) or "white spots" (Au₂). Al) phenomenon.

The Kirkendall effect is a unique degradation mechanism of the Au-Al system: diffusion difference: the diffusion rate of Au to Al is 5-10 times that of Al to Au, resulting in vacancies at the interface; Hole formation: vacancies aggregate to form micron-sized cavities that expand under thermal cycling conditions; Failure Evolution: Cracks form after the cavity joins, eventually causing the bond point to fall off, a process that begins to manifest at about 1000 hours at 125°C. The IMC growth of the Cu-Al system showed different characteristics: the main phase composition: CuAl₂ (θ phase), CuAl (η phase) and Cu₉Al₄ (ε phase), in which CuAl₂ was dominant; Growth kinetics: It takes 20 seconds to form 100Å IMC at 200°C, which is more than 60 times that of Au-Al. Degradation Manifestation: IMC thickness increases linearly with time (approximately 0.5μm per year at 150°C), resulting in a 5%-10% increase in contact resistance per year.

2.3 Contact corrosion (electrochemical corrosion)

Contact corrosion is particularly prominent in Cu-Al systems and is a progressive failure dominated by electrochemical principles: Electrochemical mechanism of failure: Potentiometric differential drive: The potential difference between Cu (+0.337V) and Al (-1.662V) reaches 2.0V, forming a galvanic cell in the presence of the electrolyte; Reaction process: Al is oxidized as the anode (Al→Al³⁺+3e⁻), Cu is reduced as the cathode (O₂+2H₂O+4e⁻→4OH⁻); Product Effects: The resulting Al (OH)₃ is non-conductive and expands in volume, resulting in increased contact resistance and eventually an open circuit. The acceleration effect of environmental factors is significant: humidity: when the relative humidity is > 60%, the corrosion rate increases by 3 times, and the water vapor condenses to form an electrolyte layer; Contaminants: Halogen ions such as Cl⁻ and Br⁻ (concentration > 100ppm) in the encapsulation material accelerate the reaction and are catalysts for corrosion; Temperature: For every 10°C increase, the corrosion rate increases by 20%-30%, and the corrosion rate at 125°C is 5 times faster than at room temperature. Typical characteristics of failure: Appearance: After chemical opening, the copper bond point can be seen to fall off, and there are corrosion marks on the edge of the remaining aluminum pad; Cross-section: SEM observations show that the Al layer is uniformly corroded, forming a honeycomb structure; Composition: EDX detects high concentrations of Cl⁻ (>0.5wt%) in areas where corrosion is visible. Notably, while the potential difference between Au and Al is greater (3.16V), the Au-Al system has a lower risk of corrosion, as the rapidly forming thick IMC layer blocks the ongoing electrochemical reaction, acting as a natural barrier.

2.4 Degradation of bonding wires of power devices

The degradation of the bonding wire of power devices is a complex process under the action of multiphysics, which directly affects the product life: Driving mechanism of degradation:

Thermal cycle stress: The change of junction temperature (ΔTj=100-150°C) in the power cycle leads to thermal expansion differences between the bond wire and the chip (CTE mismatch). Current effect: The Joule heat generated by a large current (>10A) increases the temperature of the bonding wire by 50-100°C, intensifying the softening of the material; Mechanical fatigue: The root of the bonding wire is subjected to alternating stress, and the strain amplitude can reach 0.5%-1%, far exceeding the fatigue limit of the material. Typical manifestations of degradation: Displacement and disengagement: The aluminum bonding wire undergoes plastic deformation under cyclic stress, and the bonding point gradually detaches from the pad. Root cracking: high-stress zones are concentrated in the roots where the bonding wire connects with the pad, forming microcracks and spreading; Parameter changes: A 10% increase in thermal resistance (Rth) and a 5% increase in saturation pressure drop (VCE) are clear signs of degradation. Power cycle test is an effective method to evaluate degradation: test conditions: ΔTj=150°C, cycle 10 seconds (heating 6 seconds, cooling 4 seconds); Failure criteria: 20% increase in VCE or bond wire breakage; Lifetime characteristics: According to the Weibull distribution, the B10 lifetime (10% failure probability) requirement of a typical power device ≥ 100,000 cycles.

3. Failure detection and analysis methods

3.1 Physical opening technology

Physical opening is the basic means to observe bonding failure, and the appropriate method should be selected according to the packaging type: Chemical opening process of plastic packaging: corrosion liquid formula: 95% sulfuric acid + 5% fumed nitric acid (volume ratio), temperature 80-100°C; Procedure: Immerse the device in corrosion liquid and stir magnetically (300rpm) until the molding compound is completely removed (usually 5-10 minutes); Post-treatment: Ultrasonic cleaning with deionized water (5 minutes), alcohol dehydration, to avoid residual acid corrosion of the bonding point. Mechanical opening method for metal casing: Tool selection: diamond cutting disc (thickness 0.1mm) with an accuracy of ±0.01mm; Process control: first cut along the edge of the package, and then gradually remove the shell to avoid damage to the internal bonding structure; Applications: Metal case packages for power devices (such as TO-247) that preserve the original state of the bonding system.

3.2 Micro analysis techniques

A variety of microscopic characterization methods can be used together to accurately identify failure modes: Scanning electron microscopy (SEM) analysis: observation content: bond point morphology, crack distribution, corrosion product morphology; Operating conditions: acceleration voltage 10-20kV, working distance 5-10mm, resolution up to 10nm; Advantages: It can directly observe three-dimensional topography and distinguish different phases in combination with backscatter electron imaging. X-ray spectroscopy (EDX): Application scenarios: determining the composition of corrosion products (e.g., detecting Cl⁻ to confirm contact corrosion), analyzing IMC elemental composition; Quantitative accuracy: the error of the main elements (>1wt%) is < 5%, and the element distribution can be analyzed semi-quantitatively; Limitations: Light elements (e.g., H, He) are undetectable with a spatial resolution of about 1mm. Section preparation technology: Ion grinding (CP): Bombardment of samples with an Ar ion beam (3-5keV) to prepare stress-free sections; Focused Ion Beam (FIB): Accuracy up to 5nm, suitable for preparing ultra-thin sections of specific areas; Observation advantage: Clearly display IMC thickness, cavity distribution, and interface bonding status.

3.3 Reliability test method

Accelerated testing simulates failure under long-term service conditions: High Temperature Storage Test (HTSL) conditions: 150°C or 175°C for 1000-2000 hours; Objective: To accelerate IMC growth and assess the risk of intermetallic degeneration; Monitoring indicators: bond strength retention, contact resistance change rate. Power Cycle Test: Equipment: Power Cycle Tester (such as T3Ster), which can control ΔTj accuracy ±2°C; Monitoring parameters: real-time recording of VCE, junction temperature, thermal resistance; Data analysis: Extrapolation of lifetime under real-world usage conditions through the Arrhenius model. Temperature cycling test: Conditions: -55°C~125°C, 1000-2000 cycles; Stress source: CTE differences between different materials (e.g., Al wire CTE=23ppm/°C, Si chip CTE=2.6ppm/°C); Applicable scenarios: Evaluate bond failures caused by mechanical stress (e.g., broken leads, solder joints falling off).