Wire bonding

Full analysis of wire bonding technology: from principle to industrial application

Introduction: Cornerstone technologies for microelectronic interconnects

In the "last mile" connection of integrated circuit packaging, wire bonding technology has always occupied an irreplaceable position with its unique flexibility and cost advantages. This process of synergizing heat, pressure and ultrasonic energy to form an atomic bond between the metal leads and the substrate pad is like a "bridge project" in the microscopic world, building a stable electrical path between the chip and the external circuit. When metal leads (only 18-50μm in diameter) reach atomic spacing from the pad surface, electron sharing or atomic diffusion triggers an interfacial metallurgical reaction that results in a bond point with ohmic contact characteristics with contact resistance as low as 10⁻⁴Ω which is sufficient for high-frequency signal transmission.

Despite the rapid development of advanced interconnect technologies such as flip soldering and automatic carrier belt soldering, wire bonding is still the mainstream choice in the packaging industry with a market share of more than 90%. This technological dominance stems from a triple advantage: equipment investment is only 1/5-1/10 of flip soldering, material cost is controllable, and can accommodate diverse packaging needs from 0.3mm×0.3mm microchips to high-power devices. According to data from a consumer electronics foundry, the cost of low- and mid-end chip packaging using wire bonding can be controlled at $0.05-0.5 per piece, while the cost of flip soldering solutions of the same scale is as high as $1-5 per piece. This article will systematically analyze this seemingly simple but precision engineering interconnection technology, from material selection to process optimization, and comprehensively present its technical characteristics and industrial value.

1. Technical principles and process flow: precise control of microscopic connections

At its core, wire bonding lies in enabling reliable metal-to-metal connections through energy input, a process that breaks down into a series of precisely controlled physicochemical changes.

1.1 The Nature of Bonding: From Mechanical Contact to Atomic Bonding

The microscopic mechanism of bond formation consists of three key stages:

Contact stage: The lead is in close contact with the surface of the pad under pressure, and the contact area increases nonlinearly with the increase of pressure, and when the pressure reaches 0.5-1.5N, the actual contact area can reach 30-50% of the apparent area

Oxide layer breaking: The tangential force generated by ultrasonic vibration (usually 60-120kHz) causes plastic deformation of the contact surface, breaking the oxide layer with a thickness of about 2-5nm, exposing the fresh metal surface

Atomic Diffusion: Under the synergistic action of temperature (125-350°C) and pressure, atoms at the interface obtain sufficient activation energy to form a diffusion layer about 10-50nm thick through lattice diffusion, and finally achieve metallurgical bonding

High-resolution transmission electron microscopy (HRTEM) observations show continuous gradient changes in the interface transition zone of high-quality bond points with no visible holes or cracks, a microscopic feature that guarantees bond strength (typically 0.05-0.1N) and long-term reliability.

1.2 Complete process chain: precise operation from ball planting to wire breaking

The bonding process of a single lead is like a miniature precision surgery, involving six key steps:

1. Metal wire leading: A metal wire with a diameter of 18-50μm is led from the reel and passes through the capillary tube of the bonding machine (the inner diameter is usually 1.5-2 times the wire diameter), with the end protruding 1-2mm of the capillary

2. Airless ball (FAB) formation: The wire end is melted into a ball under the protection of inert gas by electron flame (EFO), and the ball diameter is controlled to 1.5-2.5 times the wire diameter (such as a 25μm line forms a 40-60μm ball).

3. First solder joint formation: The gold ball is press-soldered on the chip pad, and through the combination of pressure (0.5-1.5N), temperature (125-350°C) and ultrasonic energy, a solder joint with a diameter of 1.2-1.5 times the diameter of the ball is formed

4. Lead arc control: The bonding machine pulls up the lead wire according to the preset trajectory to form an arc with a height of 10-50μm, and the curvature needs to ensure that the lead wire is not plastically deformed and the stress is uniform

5. Second solder joint formation: Wedge-shaped solder joints are formed at the pins in the lead frame to achieve intermetallic connection through ultrasonic vibration

6. Wire break and next cycle: Apply appropriate tension and ultrasonic energy at the end of the second solder joint to break the lead at the predetermined position, and prepare the gold ball formation of the next solder joint.  The entire process can be completed in 0.1-0.3 seconds on a high-speed bonding machine, and the advanced equipment can achieve bonding efficiency of 50,000-100,000 points per hour, with positional accuracy controlled within ±1μm, ensuring accurate operation on pads with a pitch of 0.1mm.

2. Comparison of material system and performance: the material basis of connecting quality

The reliability of wire bonding is essentially a comprehensive embodiment of material properties, and the differences in conductivity, chemical stability, and process adaptability of different metal wires directly determine their application scenarios and cost structure.

2.1 Characteristic map of core lead materials

At present, the mainstream bonded wires have formed a clear division of application, and their performance parameters and applicable scenarios are as follows: Golden wire:

As the "gold standard" for bonding materials, gold wire (99.99% purity) has unmatched comprehensive properties: conductivity: 45MS/m, ensuring signal transmission loss of < 5%, chemical stability: no oxidation in the range of -55°C~150°C, annual change rate of contact resistance < 10%, bonding yield: more than 99.9% under mature processes However, its high cost (about 500 yuan/gram) has become a constraint and is gradually being replaced in cost-sensitive fields such as consumer electronics. According to a statistic, the cost of gold wire accounts for 15-20% of the total cost of low-end chip packaging.

Aluminum wire: (99.5% purity) occupies a place in the field of power devices with low cost (about 2 yuan/gram); Mechanical strength: Tensile strength 120-150MPa, suitable for bonding thick wires with a diameter of 50-100μm. Heat dissipation performance: thermal conductivity of 237W/m·K, which is better than gold wire (slightly lower than 317W/m·K), but its main defects are easy oxidation (formation of Al₂O₃ layer) and low bonding yield (usually 95-97%). This limits its application in high-density packages.

Copper wire: (99.99% purity) As the main subsylmer of gold wire, it has obvious advantages in balancing performance and cost: the conductivity is 58MS/m, which is 29% higher than that of gold wire, suitable for high-frequency signal transmission, and the cost is only 1/20-1/30 of that of gold wire, which can reduce material costs by more than 80%. However, the process is demanding: it must be bonded in a nitrogen-protected environment (oxygen content < 50ppm) and the high rigidity (elastic modulus 110GPa, gold wire 78GPa) makes it difficult to control the lead arc, which is currently about 10-15% permeability in low-to-mid-range products.

Silver Alloy Wire: Silver alloy wire (containing 92-96% silver, with palladium, gold, and other elements) is an emerging choice in the mid-to-high-end segment: 90% reflectivity, 10-15% higher light efficiency in LED packaging, 10⁴A/cm² current density, 1.5 times that of gold wire, 1/3-1/5 cost of gold wire, and storage period of up to 6-12 months (without sealing) 。 Its technical bottleneck lies in patent barriers and process maturity, and the current application rate in domestic packaging factories is about 5%, but it is growing rapidly.

Lead frame and auxiliary materials

The choice of material for the lead frame as the "landing field" for bonding is also crucial: Copper alloy: accounts for more than 70% of the market share, with high conductivity (35-50MS/m) and good solderability, typical grades such as C194 (copper-iron-phosphorus alloy) nickel-iron alloy: widely used in the military industry due to its low coefficient of expansion (CTE 4-6ppm/K) suitable for matching ceramic packaging; Composite metals: such as nickel gold (Ni/Au) plating on copper, taking into account both electrical conductivity and oxidation resistance, the thickness of the coating in the bonding area should be controlled at 0.5-2μm. Another key element in the bonding process is the capillary tube (microtubule), the tip design of which directly affects the energy transfer efficiency: the surface roughness Ra 0.5-1μm, the ultrasonic energy transfer efficiency is increased by 20%, but the contaminant adhesion rate is 3 times that of smooth surfaces, the surface roughness is Ra<0.1μm, and the cleanliness retention time is extended to 48 hours, but the ultrasonic energy loss is increased by 15%

In practical applications, the selection depends on the bonding material: copper wire bonding mostly uses GM type to enhance energy transfer, while gold wire bonding can choose P type to extend service life.

3. Bonding method and process characteristics: optimal combination of energy input

Depending on the energy input method, wire bonding forms three mainstream processes, each with significant differences in temperature, pressure, and energy control.

Hot compression welding: reliable connection at high temperature and pressure

Hot press soldering (T/C) is the most traditional bonding method, which relies on plastic deformation at high temperature to achieve connection: temperature: 200-350°C (chip temperature), pressure: 0.5-1.5N/point (proportional to the diameter of the lead), time: 50-200msThe high temperature softens the metal wire (the yield strength of the gold wire drops to 50% of room temperature at 300°C), and the plastic flow occurs under the action of pressure, and finally forms a mechanical-metallurgical bond with the pad, and the bonding of thick aluminum wire (diameter 50-200μm) of high-power devices can reach 0.05-0.09NThe main limitation is that high temperatures can cause chip damage – experiments have shown that 350°C for 10 seconds drifts the threshold voltage of CMOS devices by 5-10%. As a result, this technology is gradually being replaced by thermoacoustic welding, and its market share has now dropped to less than 15%.

Ultrasonic welding: the miracle of energy at room temperature

Ultrasonic welding relies entirely on mechanical vibration energy to achieve connection, opening up the possibility of low-temperature bonding: temperature: room temperature (25±5°C), pressure: 0.1-0.5N/point, ultrasonic power: 50-200mW (frequency 60-120kHz);  The alternating shear force generated by ultrasonic vibration causes plastic deformation and local temperature rise (up to 150°C) on the contact surface, breaks the oxide layer and promotes atomic diffusion, stabilizes the bond strength at 0.07N, and minimizes the risk of thermal damage to the chip. This technology is particularly suitable for bonding thermal-sensitive devices such as MEMS sensors, but it places extremely high requirements on pad surface quality – bond yields can plummet from 99% to less than 80% when the oxide layer thickness exceeds 3nm. At present, the application in the field of low-temperature packaging accounts for about 25%.

3.3 Thermoacoustic welding: the optimal solution for synergy

Thermal acoustic welding (U/S&T/S) combines the synergistic effects of temperature and ultrasonic energy to become the current mainstream process:

Process window: Chip temperature: 125-300°C, splitting temperature: 125-165°C, pressure: 0.5N/point, ultrasonic power: 100-300mW. Core advantages: The temperature reduction reduces the growth rate of gold-aluminum intermetallic compounds (IMCs) by 50%, controls the annual growth rate of interfacial resistance within 5%, and increases the bonding strength to 0.09-0.1N, which is 10-20% higher than that of hot compression weldingPads that tolerate slight oxidation (oxide layer < 5nm) with yields above 99% Typical Applications: More than 90% of gold wire bonding and 60% of copper wire bonding are used in this process, covering a wide range of fields from consumer electronics to automotive electronics, and a comparative experiment at a packaging factory showed significant differences in bonding results between the three processes on the same chip: IMC thickness of 1.2μm for hot press soldering and ± fluctuations in bond strength for ultrasonic soldering 15%, and thermoacoustic welding strikes a perfect balance between the two, becoming the first choice for mass production.

4. Technology comparison and development trend: market-driven innovation direction

The evolution of wire bonding technology has always revolved around the triangle of "performance improvement, cost reduction, and process simplification", constantly pushing its own limitations in competition with other interconnect technologies.

4.1 Competitive landscape with advanced interconnection technologies

The comparison of the technical parameters of wire bonding, flip soldering, and automatic carrier belt soldering (TAB) clearly shows their respective applicable scenarios:

This difference has led to a clear market differentiation: wire bonding dominates low-end consumer electronics (95%), power devices (80%), and automotive electronics (70%), while flip soldering monopolizes high-end processors and RF chips.

4.2 Cost-benefit analysis of material substitution

The economics of replacing gold wire with copper wire have been well demonstrated: an annual material cost dropped from $50 million to $2.5 million after a packaging plant with an annual output of 1 billion chips switched to copper wire, and the payback period was controlled within one year despite the addition of a nitrogen protection system (about $500,000 per line). Silver alloy wires show potential in the mid-to-high-end segment: in LED packaging, their 90% reflectivity improves light efficiency by 12%, which translates to an 8% reduction in cost per watt, completely offsetting the increase in material costs. According to data from an LED company, flip-chip LEDs using silver alloy wires reduce the combined cost by 5% for the same light output.

The frontier of technological innovation

The future development of wire bonding technology presents three major directions: 1Ultra-fine gold wire bonding: Develop gold wire bonding technology with a diameter of less than 15 μm, with a new capillary design, to achieve high-density interconnects with a 30 μm pitch, targeted for 5G RF modules

2. Copper wire without protective gas process: Through wire surface modification (such as nickel plating 0.1μm), copper wire bonding in an air environment is realized, which is expected to reduce equipment costs by 30%. 3. Intelligent bonding system: Bonding machines with integrated machine learning algorithms can adjust parameters in real time, reducing the standard deviation of bond strength from ±8% to ±3% for different batches of chips. These innovations will enable wire bonding technology to maintain its dominant position in the market for the next 5-10 years, especially in emerging packaging formats such as chiplets, where its flexible interconnect capabilities are expected to open up new application spaces.

epilogue

Wire bonding technology seems to be a simple "two points and one line", but in fact it embodies the wisdom of materials science, precision mechanics and control engineering. From 18μm ultra-fine gold wire to 100μm power aluminum wire, from room temperature ultrasonic to high-temperature hot pressing, this technology adapts to the development needs of the microelectronics industry through continuous innovation. In the foreseeable future, wire bonding will remain the mainstream choice for integrated circuit packaging due to its irreplaceable cost advantages and process flexibility, despite the challenges of advanced interconnect technologies.

For industry practitioners, an in-depth understanding of the characteristics and differences of different bonding materials and mastering the optimization methods of various process parameters are the keys to enhancing product competitiveness. With the increasing reliability requirements of 5G, automotive electronics and other applications, wire bonding technology will continue to evolve in the direction of higher strength, lower loss, and more intelligence, writing a new chapter in the microscopic world of connection engineering.