Common gold-aluminum bonding problems in wire bonding

I will enrich the details and elaborate in depth on the basis of not changing the original intention from the aspects of the phenomenon, principle, differences of different systems, influencing factors, manifestations and countermeasures of the gold-aluminum bonding problem, etc., to present the pseudo-original effect.

# In-depth analysis and countermeasures of common gold-aluminum bonding problems in wire bonding

In the reliability test area of the integrated circuit packaging workshop, a batch of samples stored at high temperature for 300 hours is undergoing strict inspection - under the microscope, the connection interface between some gold wire ball welds and aluminum pads has obvious debonding, and the edges of the originally bright bond points show irregular gray areas. This seemingly small defect can cause the chip to suddenly fail during service, paralyzing the entire electronic system. Gold-aluminum bonding is a classic interconnect technology in semiconductor packaging, and its reliability has always been a research hotspot in the field of electronics manufacturing. This paper systematically analyzes the failure mechanism of gold-aluminum bonding, from the growth law of intermetallic compounds to the evolution process of Kirkendall cavities, comprehensively presents the essence of this complex interface phenomenon, and provides targeted solutions.

1. The phenomenon and characteristics of gold-aluminum bonding failure

The failure of the gold-aluminum bonding system does not occur suddenly, but undergoes a gradual process from microstructural changes to macroscopic performance degradation, showing characteristic performance at different stages.

1.1 Visual and microscopic morphological characteristics

The appearance and microstructure of the gold-aluminum bond points after high-temperature storage will change significantly: the intact bond points show a uniform metallic luster, while the failed samples will have the gold wire separated from the pad (Fig. 1a, b), and the black oxide ring will appear on the edge of the aluminum wire bondingpoint, and obvious debonding gaps will be visible in severe cases. An uneven reaction layer is visible on the pad side after debonding, and some residual intermetallic compound (IMC) fragments are left on the wire side (Figure 4), and the distribution of these fragments is closely related to the initial bond strength. By high-power SEM observation, the IMC coverage of the underside of the normally bonded gold sphere can reach 84% (Figure 5a), while the unbonded area of the failed sample is significantly enlarged (Figure 5b), and cracks and voids appear in the IMC layer; Statistics from a laboratory show that under the high-temperature storage conditions of 175°C, the failure probability of gold-aluminum bonding points increases exponentially with time, and the proportion of failed samples can reach 23% after 300 hours, which is much higher than the 3% of copper-copper bonding.

1.2 Quantitative performance of performance degradation

The degradation of bond quality is directly reflected in the changes in electrical and mechanical properties: after 1000 hours of storage at 150°C for a 25μm diameter wire bond point, the average tensile force decreases from the initial 7.5g to 4.2g, a decrease of 44%, and the fracture position is transferred from the wire body to the bonding interface. As the IMC layer thickens and cavities form, the contact resistance increases from the initial 20mΩ to more than 150mΩ, and even jumps by orders of magnitude in high temperature and humidity environments. In the temperature cycling test of -55°C~125°C, after 1000 cycles, the bond strength of the failed sample decreased by 58% of the initial value, far exceeding the 20% limit allowed by the industry standard. These performance changes are not linear, but there are obvious inflection points - when the IMC layer thickness exceeds 1 μm or the voiding rate reaches 30%, the performance will fall off a cliff, which is also a key threshold for gold-aluminum bonding reliability management.

2. Mechanism analysis of gold-aluminum bonding failure

The failure essence of gold-aluminum bonding is the result of metallurgical reactions and diffusion behavior at the interface, involving the formation and growth of intermetallic compounds and the evolution of cavities induced by the Kirkendall effect.

2.1 Complex evolution of intermetallic compounds

Gold and aluminum form five different intermetallic compounds (IMCs) at the interface, each with significant differences in physical and chemical properties:

Au₄Al: Low formation temperature (about 150°C), relatively stable structure, conductivity about 60% of pure gold

Au₅Al₂: Grows rapidly in the range of 175-200°C, is the dominant phase in the middle stage, and is prone to internal stress

Au₂Al: Commonly known as "white spot", it has a brittle and hard texture, with an electrical conductivity of only 30% of pure gold, and is prone to cracks at the interface

AuAl₂: One of the main forming phases in the early stage, which gradually transforms into other phases with increasing temperature

AuAl: purple ("purpura"), poor stability at high temperature, and easy to produce interfacial stress with the surrounding phase

The formation of these compounds has a significant temperature dependence: at 175°C annealing, the initial stage (0-2 hours) is dominated by Au₄Al and AuAl₂; In the middle (2-24 hours) Au₅Al₂ becomes the dominant phase; The long-term (>100 hours) is dominated  by the continuous growth of Au₄Al (Figure 7). This phase transition process, accompanied by significant volume changes (total expansion rate of about 3-5%), is a major source of internal stress.

2.2 Kirkendall effect and cavity formation

The difference in diffusion rates between gold and aluminum is another key factor in interface failure: at 200°C, the diffusion coefficient of gold in aluminum (1.2×10⁻¹⁴cm²/s) is 4.8 times that of aluminum in gold (2.5×10⁻¹⁵cm²/s), and this asymmetric diffusion leads to vacancies at the interface. The initially formed tiny vacancies (< 50 nm in diameter) gradually accumulate and grow into Kirkendall cavities (up to 1-5 μm in diameter) at high temperatures, and the bonding fails when the cavities connect to each other to form through cracks.  The internal stress generated by IMC growth accelerates the growth and expansion of cavities, and experiments show that the growth rate of cavities in stress concentration areas is 2.3 times higher than that in uniform areas. Through the dynamic observation of FIB-SEM, it was found that the initial hole with a diameter of 100nm will grow to 2μm after 100 hours at 175°C, and it is mainly distributed at the interface between the IMC and the aluminum pad, which is related to the accumulation of vacancies left by the rapid diffusion of aluminum atoms.

3. Failure differences of different gold-aluminum bonding systems

According to the different combinations of bonding materials, gold-aluminum bonding can be divided into two major systems: Au/Al and Al/Au, and their failure processes and mechanisms are significantly different.

3.1 Au/Al System (Gold Wire - Aluminum Pad)

The failure of this system is characterized by the depletion of aluminum pads, presenting a three-stage degradation process:

Stable growth stage (0-100 hours): The thickness of the IMC layer increases linearly with time (growth rate is about 0.01μm/h), mainly consuming aluminum pad material, and the bond strength remains stable

Lateral phase change stage (100-500 hours): When the aluminum pad (typically 1-2μm thick) is completely consumed, the IMC begins to expand laterally towards the aluminum material at the edge of the pad, the vertical growth slows down, and the bond strength begins to decline

Crack propagation stage (>500 hours): The IMC layer in the center region of the solder joint becomes thinner, the edges thicken, and the Kirkendall cavity accumulates to form cracks that propagate from the interface inward, eventually leading to debonding

A chip packaging facility case showed that after 500 hours of storage at 175°C with an Au/Al bond point with a 1μm thick aluminum pad, the IMC layer was 2.3μm, the voiding rate exceeded 40%, and the bond tension dropped to 52% of the initial value.

3.2 Al/Au system (aluminum wire - gold plating); The failure process of this system is more complicated due to the thicker gold plating (usually 5-10 μm): the aluminum atoms continue to diffuse towards the gold plating, and cavities form inside the aluminum wire (especially thick aluminum wires with a diameter of > 50 μm), resulting in a reduction in the aluminum wire cross-section; The five IMCs existed at the same time and were unevenly distributed, with AuAl₂ dominated  on the side near the aluminum wire and Au₄Al on the side near the gold layer. For thin gold plating (<3μm), the gold may be completely consumed, forming a direct contact between the aluminum and the IMC, accelerating failure; In a 200°C high-temperature test, the bond point between a 250 μm diameter aluminum wire and a 5 μm gold plating was completely depleted after 1942 hours, resulting in a crack up to 5 μm wide at the interface (Figure 10) and an increase in contact resistance by more than 10 times the initial value.

4. Key factors affecting the reliability of gold-aluminum bonding

The failure rate of gold-aluminum bonding is affected by a variety of environmental and process factors, among which temperature, humidity and current play the most significant roles.

4.1 Acceleration of temperature

Temperature is the most critical factor affecting diffusion and phase transitions, with exponential effects: according to the Arrhenius equation, the IMC growth rate increases by 1.5-2 times for every 10°C increase in temperature, and at 250°C it is 8 times higher than at 150°C. High temperature promotes the conversion of unstable phases (e.g., AuAl) to stable phases (e.g., Au₄Al), and at the same time intensifies volume changes and stress accumulation. GJB 2438B-2017 specifies that aluminum wire bonding samples must be baked at 300°C ×1 hour before being tested for bond strength to evaluate high-temperature stability

Comparative data from a study show that the median failure time of gold-aluminum bonding is 2500 hours, 800 hours and 300 hours respectively at three temperatures of 100°C, 125°C and 150°C, which fully reflects the strong acceleration effect of temperature.

4.2 Humidity and galvanic corrosion

Humidity alone has less effect on IMC growth, but when combined with temperature, it can cause galvanic corrosion:

Electrolyte formation: The high temperature and high humidity environment makes the interface residues absorb moisture to form an electrolyte, and gold (electrode potential + 1.5V) and aluminum (-1.66V) form an electric pair to accelerate the anode dissolution of aluminum. The corrosion products of aluminum (Al (OH)₃) can clog the interface gaps, exacerbate stress concentration, and disrupt the continuity of the IMC layer. At 85°C/85% RH, the corrosion rate of gold-aluminum bonding was 3.5 times higher than that of dry environment, and interface peeling was the main failure mode. Reliability tests in automotive electronics show that after 1000 hours of temperature and humidity cycling (40°C/95% RH~85°C/30% RH), the failure rate of gold-aluminum bonds reaches 18%, which is much higher than 5% in dry environments.

4.3 Joule thermal effect of electric current

The current itself has little effect on the growth of the IMC, but the Joule heat generated will significantly accelerate the failure: when a large current (>1A) passes through the bonding point, the Joule heat generated by the contact resistance increases the local temperature by 20-50°C, which is equivalent to the increase in ambient temperature; After the formation of the cavity, the current density increases sharply in the remaining conductive channels (up to 10⁴A/cm²), further exacerbating local heating. When the void rate exceeds 50%, instantaneous fusing may occur, especially in power devices. Test data from Power Semiconductor shows that the local temperature at the 250μm aluminum wire bond point is 42°C higher than the ambient temperature when carrying 3A, increasing the probability of 500-hour failure from 12% to 35%.

5. Systematic response strategies for gold-aluminum bonding problems

In view of the inherent defects of gold-aluminum bonding, measures should be taken from various aspects such as material selection, process optimization and structural design to delay the failure process.

5.1 Alternatives to material systems

Avoiding gold-aluminum heterogeneous bonding is the most fundamental solution: using gold-gold or aluminum-aluminum bonding systems to eliminate IMC growth problems at the source, such as copper-copper wire bonding, which is becoming increasingly popular in semiconductor packaging; Introducing a barrier layer (such as nickel and palladium) between gold and aluminum can effectively prevent cross-diffusion with a thickness of 50-100nm, reducing the growth rate of IMC by 70%. R&D of palladium-coated gold wire, copper-clad aluminum wire and other composite wires, using surface materials to inhibit diffusion, a certain type of palladium-coated gold wire extends the life of the bond point by 3 times

Gold-aluminum bonding (GJB 2438B-2017 Appendix D) has been strictly prohibited for aerospace-grade circuits, all with gold-gold or copper-copper bonding, reducing the probability of bond failure for satellite payloads to less than 0.1%.

5.2 Parameter optimization of the bonding process

Reduce initial defects and improve bond quality through process control: Thick spherical bonding (spherical diameter/wire diameter ratio > 2.5) results in a more uniform IMC distribution and a 40% reduction in stress concentration (Fig. 13b). Optimize ultrasonic power (typically 30-50mW) and time (15-30ms) to ensure a continuous, but not too thick, initial IMC layer (ideal thickness 0.2-0.5μm);  The bonding temperature is reduced from the traditional 250°C to 180-200°C, reducing thermal damage and initial diffusion; A packaging plant reduced the initial voidage rate of the gold-aluminum bond point from 8% to 2% and the failure rate from 23% to 7% after 500 hours of storage at 175°C through process optimization.

5.3 Diffusion suppression technology

Inhibition of gold-aluminum cross-diffusion through material modification: 0.1-0.5% copper or palladium is doped in the gold wire to form a diffusion barrier layer, which reduces the Au diffusion coefficient by 50%;  The thickness of the gold plating for aluminum wire bonding should be controlled at 0.5-1μm (Figure 15) to ensure the bond strength and avoid excessive diffusion. Plasma treatment was used to form a dense oxide layer (thickness 5-10nm) on the aluminum surface to slow down the initial reaction rate. Experimental data showed that 0.3% copper-doped gold wire reduced the growth rate of IMC by 45%, and the cavitation rate was only 50% of pure gold wire after 1000 hours of storage at 200°C.

5.4 Quality control system

Establish a quality control mechanism for the whole process: the impurity content (nickel, iron, etc.) needs to be < 50ppm, and the plating solution should be changed regularly and the ion exchange method should be used to remove metal impurities. X-ray was used to detect the internal cavity of the bonding point, and ultrasound scanning imaging was used to evaluate the uniformity of IMC distribution. Critical products are screened for 175°C×100 hours of high-temperature storage to remove early failure samples

After an automotive electronics company introduced strict quality control, the on-site failure related to gold-aluminum bonding was reduced from 200ppm to 15ppm, reducing after-sales costs by about 8 million yuan per year.

epilogue

The reliability problem of gold-aluminum bonding is essentially an embodiment of diffusion and phase transition laws in materials science in microscale interconnections, and its complexity stems from the competitive growth of five IMCs and the dynamic evolution of Kirkendall holes. Although the metallurgical reaction between gold and aluminum cannot be completely eliminated, the effective life of the bond point can be significantly extended through a combination of material substitution, process optimization, and diffusion inhibition.

With the development of semiconductor technology towards higher integration and greater power, gold-aluminum bonding is gradually being replaced by new technologies such as copper-copper bonding and silver paste interconnect, but it will still exist for a long time in the low-end field. For engineers, a deep understanding of the failure mechanism of gold-aluminum bonding and mastering its control methods is not only a need to solve the current problem, but also an important way to understand the nature of interconnect technology - after all, the connection of any heterogeneous material faces similar interface challenges.

In the future, with the development of nano-coating, new alloys and other technologies, the reliability of gold-aluminum bonding may be breakthrough-improved, but before that, scientific control based on existing cognition is still the key to ensuring the reliability of electronic devices.