Insulating coating bonded copper wire SSB

Insulating Coating Bonded Copper Wire SSB Interconnect Technology: Process Challenges and System Solutions

Introduction: Copper Wire Bonding Revolution in the Era of High-Density Packaging

In the process of evolving integrated circuit packaging technology to multi-chip integration and 3D stacking, the wire bonding process is undergoing a historic transformation from gold wire to copper wire. Insulating coating bonded copper wire has become a new favorite in the industry with three core advantages: the material cost is only 1/8-1/5 of that of gold wire, the conductivity (58MS/m) is 29% higher than that of gold (45MS/m), and the mechanical strength (tensile strength > 300MPa) is more suitable for fine-pitch bonding needs. Among them, the SSB (Stand-off Stitch Bond) process, as a key technology to achieve multi-chip interconnection, completes two bonding operations on the same pad through the two-step method of "ball planting-soldering", increasing the chip interconnection density in the package by more than 40%, providing a feasible integration solution for complex devices such as 5G RF modules and automotive-grade MCUs.

However, the inherent properties of insulating coating-bonded copper wires pose significant challenges to the SSB process: the CuO oxide layer (hardness > 3GPa) formed on the surface of the copper wire is much higher than that of pure copper (hardness 1.5GPa), resulting in a 3-fold increase in the damage rate of aluminum pads during the bonding process. The impact of the two bonds causes the occurrence of "aluminum extrusion" phenomenon up to 15%; Improper control of the shielding gas can result in a FAB (airless ball) dimensional deviation of > 5 μm, which directly affects bond consistency. Xi'an Taifengrui Electronics' production data shows that the yield of the unoptimized SSB process is only 68%, but with systematic process improvements, the yield can be increased to more than 97%. Based on actual production experience, this paper comprehensively analyzes the difficulties and solutions of insulating coating bonded copper wire SSB technology from three dimensions: material characteristics, equipment parameters, and process control.

1. SSB process principle and technical bottleneck

1.1 The unique implementation path of multi-chip interconnection

The SSB process uses an innovative two-step operation of "ball planting + bonding" to build a flexible inter-chip connection network, and its technical characteristics are reflected in three typical application scenarios: to realize the signal transmission of chips with different functions, such as connecting the control chip and the power chip through φ18μm copper wire in the power management module, and the bonding spacing can be reduced to 50μm, which reduces the space occupation by 30% compared with traditional wire bondingSolving the connection needs of non-adjacent pads on a single chip, it can achieve signal winding in the range of 1mm in the RF chip, reduce signal delay by more than 20%, and reverse connection from the pin in the lead frame to the chip ball planting point, shortening the heat dissipation path of the power device by 50%, and reducing the thermal resistance to less than 0.8°C/W

A stacked image sensor packaging case showed that the SSB process increased the number of chip stacks from 2 to 4 layers while maintaining a bond yield of 85%, which is difficult to achieve with traditional bonding techniques.

1.2 Process challenges caused by inherent contradictions

There are three core sets of contradictions between the physicochemical properties of insulating coating-bonded copper wires and the demands of the SSB process: 0.5 seconds of exposure to air can form a 2-3 nm thick CuO layer, resulting in irregular bulges (height difference of > 2 μm) during FAB molding, extending the bond strength fluctuation range to ±25%. Experimental data show that for every 1% increase in CuO content, the bonding shear strength decreases by 3-5%, and when the oxide layer thickness exceeds 5nm, a completely non-stick failure mode occurs. The Vickers hardness of insulating coated copper wire (HV 120-150) is 1.5 times higher than that of gold wire (HV 80-100), and the aluminum pad (HV 30-40) is prone to plastic deformation under the two impacts of the SSB process. Microscopic observations have shown that pad depressions of unoptimized processes can reach 1-2μm, more than 30% of the thickness of the aluminum layer, directly threatening the electrical integrity of the underlying circuitry. SSB process parameters involve 12 key variables (including pressure, power, time, etc. of two bonds), and a 10% deviation from the parameter matching can lead to bond failure. A production statistic shows that defects caused by parameter drift account for 62% of total SSB defects, which is much higher than the 35% of traditional bonding processes.

These contradictions are particularly pronounced in high-density packages, where bond yields cliff-drop when pad sizes shrink to less than 50×50 μm².

2. Systematic analysis of key influencing factors

2.1 The decisive role of chip surface quality

The surface state of the chip directly determines the initial conditions of bonding, and three typical defects can lead to significant process problems:

Organic residues (> 5 nm thick) can cause microbubbles (> 1 μm in diameter) to form at the bonding interface, triggering interfacial delamination during temperature cycling. XPS analysis showed that bond strength decreased by more than 40% when the C content exceeded 15% in the contaminated area.

Excessive probe prints (> 30μm in diameter) can disrupt the continuity of the aluminum layer, resulting in stress concentration during bonding, and the risk of failure of such defects is 8 times higher than that of normal chips in -55°C~125°C thermal cycling tests. There is a strict window for matching aluminum layer thickness to copper wire diameter (Table 1), and when the aluminum layer thickness matched by 18μm copper wire is reduced from 1.2μm to 0.8μm, the "lost aluminum" defect rate rises from 2% to 18%. The verification data of an automotive-grade chip shows that the first-time pass rate of SSB bonding can be increased to more than 95% by strictly controlling the surface roughness of the chip (Ra<5nm) and the purity of the aluminum layer (>99.5%).

2.2 Optimization space for the protective gas system

Gas protection is the core means to control the oxidation of copper wires, and its effect is reflected in the dimensional stability and shape consistency of FAB: N₂/H₂ mixture (5% H₂+95% N₂) effectively removes CuO through the reducing properties of H₂ (standard electrode potential - 0.828V), and the experiment shows that the CPK value of its FAB size (1.34-1.57) is significantly higher than that of pure N₂ protection (1.13-1.19). More importantly, the gas mixture can control the roundness error of the FAB to less than 3%, which is often more than 8% with pure N₂ protection. Low flow rate (<0.4L/min) will lead to inadequate protection and oxidation spots on the surface of the FAB (> 5% of the area); Excessive flow rates (>0.8L/min) can cause airflow disturbances, increasing the standard deviation of FAB size from 0.3μm to 0.8μm. The optimized flow control accuracy should reach ±0.05L/min, and the response time should be < 100ms. The annular blowing structure provides uniform air supply at 360°, so that the shielding gas forms a stable positive pressure environment (pressure > 5Pa) in the bonding zone, and the FAB pass rate (96.63-99.90%) is much higher than that of the unilateral blowing structure (68.68-82.38%). Simulation analysis shows that the airflow uniformity index (deviation <5%) of the annular structure is more than 5 times higher than that of the unilateral structure (deviation > 25%). The transformation case of a packaging plant proves that after changing from single-sided blowing to annular blowing, the FAB defect rate of the SSB process is reduced from 12% to 0.5%, saving more than 2 million yuan per year.

2.3 Synergy of bonding parameters

The two bonding operations of the SSB process require precise parameter matching, and the optimization directions determined by DOE experiments include:

The 65mA/320μs ignition condition results in a copper ball with a lower hardness (HV 80-90) and a 30% increase in exposed copper area compared to 45mA/380μs, effectively reducing aluminum pad damage. Lower ultrasonic power (80-100mW) and longer bonding time (25-30ms) were used to control the deformation of the aluminum layer at the ball implantation point within 0.5μm. The optimized combination of pressure (15-20gf) and power (100-120mW) allows the amount of "aluminum extrusion" to drop from 5μm to less than 2μm without compromising the bond strength (>15g). In actual production, these parameters need to be dynamically adjusted according to the thickness of the chip aluminum layer to form multiple sets of parameter matrices to ensure that different products can obtain the best bonding effect.

3. Construction of process control system

3.1 Customized design of special fixtures and splitting knives

The SSB process puts forward special requirements for auxiliary tools and forms a series of customized solutions: the zonal temperature control design is adopted, so that the temperature difference at each point of the heating block is <±3°C, and the difference in bond strength caused by uneven temperature (<5%) is avoided. The vacuum adsorption force needs to be stabilized above -80kPa to ensure that the lead frame is not loose (displacement < 1μm). Head roughening (Ra 0.5-1μm) increases the copper wire grip and increases the tensile strength of the second solder joint by 15-20%. The match between the guide corner diameter (CD) and the pad size should be met: CD = 0.8× Minimum pad side length - 4μm to prevent both eccentric balls and crushing of the aluminum pad.

One test data showed that the shedding rate of the SSB second solder joint decreased from 3.2% to 0.6% after using a custom splitter, while the splitting life was extended from 500,000 to 800,000 cycles.

3.2 Material selection and management specifications

The characteristics of the wire have a direct impact on the SSB process, and the proven preferred solution includes: the surface gold plating (0.1-0.3μm thickness) provides additional oxidation protection, and the FAB maintains good bondability after 3 seconds of exposure to air, which is much better than pure copper wire (<1 second). The temperature in the nitrogen cabinet is maintained at 20-25°C (fluctuating <±2°C) and the relative humidity is < 50%, which can stabilize the shelf life of the wire at 6 months and change the bonding performance rate of < 3% within 9 days after opening.

After implementing strict material control processes, the wire-related defect rate of a company decreased from 8% to 1.5%, significantly improving process stability.

3.3 Whole-process quality monitoring strategy

Establish a three-level control system covering pre-production, production, and post-production: use automatic optical inspection (AOI) to inspect the chip surface to identify defects such as contamination and scratches, with a resolution of 2μm, to ensure that defective chips do not flow into the bonding process. Real-time acquisition of bonding parameters (pressure, power, temperature, etc.), setting a 3σ control limit, automatic shutdown alarm when the range is exceeded, and a response time of < 1 second.

Each batch of samples was sampled: tensile test (n=50): minimum tensile force ≥ 10g (18μm line), shear test (n=30): minimum shear force ≥8g, profiling (n=5): IMC thickness 1-3μm, no obvious voids

Through this system, a factory increased the Process Capability Index (CPK) of the SSB process from 1.0 to 1.6, meeting the stringent requirements of automotive electronics.

4. Technological innovation and future prospects

Insulating coating-bonded copper wire SSB technology is evolving towards finer and more reliable technology, with three major innovation trends:

Introducing machine learning algorithms, the model trained based on 5000+ sets of process data can predict bond quality (accuracy > 95%), and automatically adjust parameters to compensate for material differences, so that the standard deviation of bond strength of different batches of wires is controlled within 5%. Developed graphene composite coated copper wire, which uses graphene's high conductivity (10⁶S/m) and oxidation resistance to extend the stabilization time of FAB in air to 10 seconds, while at the same timeReduces bond resistance by 10-15%. Effective bonding below 150°C is achieved through ultrasonic energy focusing (power density increased to 5W/mm²), reducing thermal damage to the aluminum layer by 60%, which is especially suitable for the integration of heat-sensitive devices. These technological innovations will drive the SSB process from the current 50μm pitch to 30μm or even 20μm, providing critical interconnection support for advanced packaging forms such as chiplets.

conclusion

The proven application of insulating coating bonded copper wire SSB technology is the result of synergistic optimization of material properties, equipment performance and process control. By systematically solving the three core problems of oxidation control, pad protection, and parameter matching, this technology has achieved a stable yield of more than 97%, showing significant advantages in the field of multi-chip integration. Practice shows that the successful implementation of the SSB process requires the establishment of a full-chain control system of "material-equipment-process-testing": strictly control the quality of wires and the storage environment on the material side; Customized fixtures and splitting knives are used on the equipment side; Optimize gas protection and bonding parameters at the process end; Implement whole-process quality monitoring at the testing end. This systematic approach transforms the SSB process from laboratory technology to large-scale production capabilities, meeting the demands of mass production for high-density packaging. As integrated circuits develop towards higher integration and higher reliability, insulating coating bonded copper wire SSB technology will continue to innovate, achieve new breakthroughs in bonding density, reliability and cost control, and become a key supporting technology in the field of advanced packaging.