CO2 Surface Preparation for Wire Bonding

Summary

The Clean Imagineering Application Note AN1001 thoroughly examines the critical role of surface preparation in wire bonding processes, emphasizing the necessity of contaminant-free bond pads to ensure strong and reliable wire bonds. Wire bonding is a widely used method to electrically connect components on electronic substrates, and surface contamination significantly undermines bond quality and reliability. The report identifies typical contaminants—including halogens, hydrocarbons, metals from plating, sulfur, organic residues, and human-derived particles—and explains their negative impacts on bondpad cleanliness and bonding performance. Traditional cleaning methods, such as solvent wipes, immersion cleaning, and vacuum plasma treatments, face challenges including residue re-deposition, inconsistent cleaning efficiency, process complexity, and cross-contamination.

This note highlights an alternative cleaning technology: selective CO₂ spray cleaning, combining CO₂ snow spray and plasma treatment to provide a simpler, dry, and efficient cleaning method. A detailed experimental evaluation involving 106 ceramic substrates with TiW/Ni/Au metallized bond pads compared the existing Solvent/Plasma cleaning process with the CO₂ cleaning method under rigorous contaminant challenge conditions. Contaminants applied included tape adhesive, finger oils, flux, silicone oil, and mixed contaminants, simulating worst-case scenarios. The CO₂ cleaning was evaluated via bond pull strength tests, defect rates (DPM), and a novel non-contact surface contamination evaluation technique called Optically Stimulated Electron Emission (OSEE).

Results showed that CO₂ cleaning performed equally or better than the traditional solvent/plasma process across most contaminants, delivering higher bond pull strength, significantly lower defect rates, and more consistent cleaning outcomes (higher CpK values). The CO₂ process also eliminated waste generation associated with solvents and wipes, and it enabled automated, repeatable, selective cleaning via a Cartesian robotic platform (SnoBotTM) integrated with CO₂ plasma technology (BlueFireTM) for enhanced cleaning energy and electrostatic control. The OSEE technique proved effective for real-time, on-line contamination detection, facilitating improved process control.

In conclusion, the CO₂ spray cleaning system replaced existing cleaning methods in the evaluated defense application, enabling a more efficient, robust, and environmentally friendly surface preparation process suitable for high-reliability wire bonding in aerospace, medical, microelectronics, and other industries.

Highlights

• Surface cleanliness is critical for reliable wire bonding and is achieved by removing various organic and inorganic contaminants.
• Selective CO₂ spray cleaning offers a dry, waste-free, and highly effective alternative to traditional solvent and plasma cleaning methods.
• Rigorous comparative tests demonstrated CO₂ cleaning matches or exceeds the bond strength and defect reduction of solvent/plasma processes.
• Human-derived contamination is a significant contamination source, requiring stringent cleaning and handling protocols.
• Automated robotic CO₂ cleaning (SnoBotTM) improves cleaning consistency and labor efficiency with integrated plasma and ionized air features.
• Optically Stimulated Electron Emission (OSEE) is validated as a reliable, non-contact method for on-line contamination and cleanliness monitoring.
• CO₂ cleaning reduces environmental impact by eliminating solvent waste and is applicable across aerospace, defense, medical, and microelectronics sectors.

Key Insights

• Comprehensive Contaminant Identification Essential for Effective Cleaning: The diversity of contaminants—from halogens and hydrocarbons to metals and human particulates—necessitates a robust cleaning approach that addresses multiple contamination types simultaneously. CO₂ spray cleaning’s physical removal mechanism effectively counters this complexity, outperforming solvent/plasma methods especially for mixed contaminant scenarios where chemical cleaning alone struggles. This highlights the advantage of combining physical and plasma cleaning energies in modern surface preparation.
• Process Simplification Enhances Reliability and Throughput: The prior process involved multiple manual steps—wiping, immersing, and plasma treatment—each adding variability and opportunity for recontamination or error. The automated CO₂ cleaning method provides a single-step, programmable, dry system reducing handling and operator dependency. This process simplification minimizes variability and leads to improved consistency as shown by tighter bond strength distributions and higher CpK values, which are critical in defense and aerospace applications where failure is not an option.
• Integration of Robotics and Real-Time Monitoring Optimizes Cleaning Control: The combination of a Cartesian robotic system and OSEE probe offers precise, repeatable cleaning with immediate feedback on surface cleanliness. This integration supports inline quality assurance and rapid adjustment of cleaning parameters, providing a high level of process control uncommon in conventional cleaning systems. OSEE’s sensitivity to molecular contamination enables detection of residues invisible to visual inspection and permits proactive management before bonding, reducing waste and failures.
• Statistical Analysis Validates Superior Cleaning Performance: The evaluation’s use of pull strength data, defect-per-million (DPM) rates, and CpK analysis provides a rigorous proof framework. CO₂ cleaning achieved a defect rate just 2% of that found with solvent/plasma methods and surpassed minimum military standard requirements for bond strength by a large margin. CpK above 1 indicates excellent process capability and control, translating to higher confidence in manufacturing reliability and lower risk of field failures.
• Environmental and Operational Benefits of CO₂ Cleaning: Unlike solvent-based processes that create hazardous waste streams needing disposal, the CO₂ method is environmentally benign, producing no liquid waste or chemical residues. Its dry and selective nature eliminates precision drying challenges seen with immersion cleaning, reducing cycle time and energy consumption. These benefits align with industry trends favoring green manufacturing and lean process methodologies, making CO₂ cleaning appealing beyond technical performance.
• Human Contamination Control Remains a Crucial Factor: Despite advanced cleaning, human-sourced contaminants remain a significant risk due to natural shedding of skin, hair, cosmetics, and fibers. The report underscores the high particle counts generated even by stationary personnel and the exponential increase with movement, emphasizing the importance of cleanroom protocols and automation to minimize human contact and contamination sources. This factor affects overall bond reliability and stresses that cleaning systems must be robust enough to address continuous contamination influx.
• Broad Industry Applicability and Scalability of the CO₂ Cleaning Approach: Although the study was conducted in a defense missile system context, the technology is applicable to a wide range of industries including aerospace, medical devices, electro-optics, microelectronics, and hard disk drives. These fields share stringent cleanliness and reliability demands where wire bonding is critical. The CO₂ cleaning system’s modular robotic design and tunable parameters make it scalable and adaptable to various substrate sizes, contaminant profiles, and throughput requirements, offering a flexible surface preparation platform for evolving industry needs.