Particles and other residues that accumulate on precision assemblies and support equipment during manufacturing processes must be removed to prevent yield loss. Precision cleaning may be needed at various stages to control particle and other residue burden.

There are many factors to consider for cleaning and inspection operations within the cleanroom-based manufacturing process flow. For example, automated part transfer systems (vacuum-picks) that are not cleaned regularly can be a source of contamination. Particles and residues generated during assembly and test processes accumulate on the support equipment and can transfer to subassemblies during handling (Figure 1).

Off-line precision cleaning using immersion (wet) cleaning processes is a common approach, but can be inconvenient, labor intensive, and cause line stoppages. Cleaning complex support equipment can require multiple operations including disassembly, removal from the cleanroom for precision cleaning, return to the cleanroom, reassembly, and calibration. Likewise, precision assemblies requiring cleaning during build are often batched, precision cleaned, dried, and returned to the manufacturing line. All of these procedures are disruptive to product flow and create additional contamination burden.

Carbon dioxide cleaning is a proven strategy for improving particle and residue cleanliness in precision manufacturing processes for support equipment as well as for assemblies during build to reduce particle and residue burden.^1-3

Conventional carbon dioxide “snow” cleaning has been employed for precision cleaning applications over the past 20 years. Properly designed carbon dioxide snow cleaning systems with inert atmospheres in clean cabinets can be very effective for cleaning precision parts that cannot or should not be cleaned by liquid immersion.

Clean, high volume

In high-volume manufacturing, cost is usually a prime concern. For an automated dry cleaning system to be acceptable, it must demonstrate high reliability, high throughput, and excellent cleaning performance at a reasonable cost. Most importantly, it must be able to operate within the cleanroom atmosphere. This has proven to be a challenge for conventional carbon dioxide snow cleaning processes. Moreover, portability and tool integration have been constraints common to conventional carbon dioxide snow cleaning processes. It is difficult and expensive to completely isolate the substrate to be cleaned from the cleanroom atmosphere, particularly for on-the-fly utilization within a cleanroom-based manufacturing operation. Cleanroom atmospheres contain water vapor, organics, salts, and particles. These airborne contaminants are easily condensed, concentrated, coalesced, and entrained into very cold and electrostatically charged jet sprays, common to conventional carbon dioxide spray cleaning systems. Such contamination can be deposited onto the surfaces of critical support fixtures and assemblies being cleaned.

Advanced carbon dioxide composite spray cleaning technology addresses the drawbacks and limitations of conventional carbon dioxide snow spray cleaning, offering several advantages. These include carbon dioxide conservation and control; spray impact energy control; elimination of local surface condensation (and surface re-deposition phenomenon); and easier adaptability to automation equipment, assembly tools, production lines, and processes within controlled environments. The spray technology can be packaged as stationary manual cleaning cells, robotic spray cleaning tools, integrated assembly tools, and mobile cleaning tools.

The cleaning energy required to remove microscopic particles and thin films rises exponentially as the diameter of the particle or thickness of the film decreases, increasing to several million g-forces for sub-micron particles.⁴ Fluid velocities rapidly decrease from turbulent flow (high energy) to laminar flow (low energy) at microscopic distances from the surface. This is where the small particles and thin films hide out. To overcome this energy barrier commonly known as “the wall” or “boundary layer”, high fluid velocities must be achieved to increase fluid flow characteristics from laminar to turbulent, which increases viscous drag (also known as shear stress) upon the particle. Carbon dioxide composite sprays (Figure 3) are superior to conventional high-pressure spray cleaning using liquid solvents or blow-off gases in two ways. First, the chemistry provides a physical scouring action plus chemical solvency. Second, the spray directly impacts the substrate surface, overcoming the energy barrier.

The physicochemical cleaning principles involved in advanced composite carbon dioxide spray cleaning are analogous to micro abrasive cleaning, with a few significant distinctions. One difference is the low hardness of carbon dioxide particles (<1 Mohs). The microscopic carbon dioxide particles are nonabrasive. In addition, the carbon dioxide spray cleaning processes involve physical momentum transfer, phase change phenomenon, and solvent energy mechanisms to remove particulate and thin-film contamination from a surface.

For example, dense phase carbon dioxide exhibits a solvent chemistry similar to halogenated cleaning solvents,⁵ which enhances the surface cleaning effect. Performance testing using various composite spray nozzles, pressures, temperatures, flow rates, and particle sizes has demonstrated surface impact stresses (cleaning energy) to be precisely controlled from less than 1 MPa to as high as 60 MPa (8,700 psi). This  is more than sufficient shear stress to cause an impinging solid spray particle to change phase to liquid at the substrate surface.⁶ These tests (Figure 4) also demonstrated that carbon dioxide composite spray cleaning energy can be sustained for relatively long distances (12 in or more) using coarse carbon dioxide particle streams.

Carbon dioxide composite spray cleaning technology developed by CleanLogix has been demonstrated in high-volume hard disk drive (HDD) manufacturing.7 Ultra clean robotic spray cleaning tools, designed for Class 10 environments, can simultaneously clean different types and volumes of complex HDD assemblies.

A major HDD manufacturer documented (Figure 5) the effectiveness of robotic carbon dioxide composite spray cleaning for motor base assemblies (MBA). The cleaning specification for a new, prime MBA is difficult to achieve. A typical liquid particle count analysis shows an average of 12% more 0.5-µm sized particles than the cleaning standard (target) using a conventional ultrasonic wet process (New MBA). MBAs measured during rework operations (Rework MBA) showed particle counts exceeding 20% of specification. An initial robotic carbon dioxide spray cleaning process (CleanFlex, CleanLogix LLC) removed 33% of total particles, exceeding the new parts specification by 18% (CleanFlex A). Continued optimization of the initial cleaning recipe yielded additional reductions, shown under CleanFlex B and CleanFlex C. This was achieved by adjusting spray pressure, particle concentration, particle size, and robot scan speed and distance.

Summary

Carbon dioxide composite spray cleaning technology provides flexible and adaptable precision cleaning options for producing products in controlled environments. The systems and methods adapt to cleanroom manufacturing and assembly lines, production equipment, and processes. The technology can decrease maintenance burden without damage to support hardware. Carbon dioxide composite spray cleaning can be used to produce cleaner small form-factor and complex parts to improve cleanliness for assembly processes such as clean assembly, plating, bonding, and coating.

References
1. Jackson D, et al. Using Solid-state CO2 in Critical Cleaning, Precision Cleaning. 1999;8(5).
2. Mee P, et al. Management of Disk Drive Component Microcontamination, IDEMA Insight.1997; 10(2).
3. Jackson D, et al. Effective Alternatives to Traditional Spray and Immersion Cleaning Processes. Process Cleaning. 2007;March/April.
4. Musselman RP, et al. Shear Stress Cleaning for Surface Departiculation. J. Env. Sci., 1987;Jan/Feb:51.
5. Barton AF, Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press; 1985.
6. Technology Brief, Cleaning Action and Chemistry of a CO2 Composite Spray, CleanLogix LLC.
7. Knoth G, et al. Automated CO2 Composite Spray Cleaning System for HDD Rework Parts. Journal of the IEST. 2009;52(1).

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David Jackson has developed more than a dozen products using recycled carbon dioxide and has been issued more than 30 patents worldwide. He is a member of IDEMA, ASM, SME, SAMPE, and IMAPS organizations. Contact: david.jackson@cleanlogix.com.

This article appeared in the September 2012 issue of Controlled Environments.