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Evaluating Cleaning System Performance
December 31, 1969 |Estimated reading time: 5 minutes
IN SUMMARYThough quantitative tools to assess the performance of electronics assembly equipment and processes abound, the industry lacks a reasonable means for evaluating cleaning systems’ performance. A test system has been designed by two industry suppliers in cooperation with Lockheed Martin to study impingement and fluid dynamics. What combination of equipment and materials variables most efficiently removes flux residues under low-standoff components?
BY Steve Stach, Austin American Technology, and Mike Bixenman, Kyzen Corporation
Though quantitative tools to assess the performance of electronics assembly equipment and processes abound, the industry lacks means for evaluating cleaning systems’ performance. Post-process assessments have been around for years, but little is offered to quantify cleaning performance at the equipment level. This test system helps study impingement and fluid dynamics of penetration and flux residue removal from tight spaces.
The objective of spray-in-air batch and in-line cleaning systems is to provide maximum cleaning performance in minimal process time. Fluid dynamics within the system should maximize physical energy delivered at the surface to be cleaned as part of an overall process that meets throughput requirements, uses minimal chemical energy, and is energy- and space-efficient. Equipment designs have different nozzle types, configurations, wash/rinse times, pumps, and pressure differentials. What nozzle configuration best flushes residues from under low-standoff components? Does a combination of flow and high pressure perform strongest? What about temperature and time variables? The study suggests a configurable, programmable test bed to evaluate these factors “on the fly.”
Surface mount components are difficult to clean due to size, spacing, and standoff. To remove residues under these components, static and dynamic cleaning action break through the flux dam left around the perimeter, create flow under the part, and flush detritus. Factors include time in the wash section, nozzle type, pressure, cleaning fluid type, and temperature. The gating factor is time.
Studies assert that cleaning can involve 5-15 minutes in the wash section. In-line at desired conveyor speed, this creates an unreasonably long machine. This study determines incremental improvements that could reduce machine length and/or improve cycle time.
The timing and sequence of events in the cleaning process are critical. Prewash should thoroughly wet parts with the wash solution and provide sufficient flow and contact time to bring the assembly up to wash temperature. Static cleaning occurs during the “soak” period between prewash and wash. During wash, several high-impingement sprays should occur, separated by brief soak periods. Questions arise as to the optimal number of passes under spray, impingement pressure, manifold pressure, nozzle design, flow rate, temperature, and other variables.
Nozzle Design and Pressure
This study concentrates on a test bed and experimental design to evaluate the impact of nozzle design and pressure variations on cleaning performance. Testing was performed on glass substrates bumped with anisotropic epoxy. Glass dies (25 × 25 mm) were placed on the substrate at 75-mm pitch, 900 I/O and high-solids flux, reflowed using a typical lead-free process profile. A CCD camera and video recorder provided documentation and images for analysis.
For the test, a front-loading system with a wash chamber on the left and rinse on the right (Figure 1) was used. This vehicle differs from a large, in-line system because the standard wire-mesh conveyor belt is replaced with a programmable lead screw drive, allowing bi-directional motion and programmable speed control at each stage. The system’s footprint is shortened significantly. A middle/load zone serves as a chemical isolation section, housing a jet manifold dryer. Wash chemistry was stripped off the substrate to limit carryover into rinse sump. Manifolds, nozzles, flow rates, pressures, and speed/number of passes were configured to set test conditions.
Figure 1. Programmable bi-directional cleaning.
Prior research indicated that higher-pressure nozzles do not necessarily produce better results. Video analysis shows that a 5-psi impingement jet bounced much less off the substrate than a 15-psi jet. Therefore, the 5-psi nozzle spread cleaning fluid more evenly. Fluid mechanics suggests that energy delivered to the surface = the mass × velocity2. However, manifold pressure doesn’t necessarily ensure high-impact force. Impingement pressure is dependent on nozzle type and distance from the substrate. Typical pressure drops are 50% for fan, 75% for conical, and 25% for coherent nozzles for each inch traveled.
Optimal machine design involves nozzle type, spacing and arrangement, and process. Aqueous-engineered cleaning fluids vary based on solvency, reactivity, wetting, and compatibility. Concentration affects static cleaning and wash temperature affects the dissolution rate. Impingement energy determines the force and velocity applied to the substrate. Movement varies exposure time to impingement force and soaking interactions. For this study, the test bed included six nozzle designs for which there were two test simulations. Nozzles 1-3 were standard fan-style nozzles; nozzles 4-6 were developmental. One test held the nozzle at a fixed position at the leading edge of the die using low pressure. A second used high pressure (Table 1). A standard cleaning fluid chemistry was used against the assembler’s existing cleaning process.
Figure 2 shows flux removal under nozzle 1, pressure 1; nozzle 1, pressure 2; and nozzle 6, pressure 2 test conditions. Videography confirms that three variables influence dynamic cleaning: nozzle type, flow, and pressure. The images illustrate differences in cleaning performance, with pressure 2, nozzle 6 achieving best results. This, along with fluid mechanics testing, verifies that the nozzle producing high pressure and flow at the edge of the die, not necessarily highest pressure at the manifold, would yield best results.
Figure 2. Flux removal over 1 min. on nozzle 1, pressure 1; 1, 2; and 6, 2.
The cleaning fluid and 150ºC wash proved effective at dissolving rosin flux residue. The focal point (center) of the nozzle jet provides the highest pressure at point of contact. Diameters closely aligned to the focal point clean faster. As distance increases from the focal point, cleanliness diminishes, even with greater flow. Data show that higher flow with pressure decreases cleaning time.
Conclusion
Dynamic cleaning rates reduce time spent and improve the overall process cleaning rate. Researchers find that coherent nozzle jets providing the greatest force and flow at the leading edge of the component will achieve the best dynamic cleaning. With the correct nozzle selection identified, position manifolds for overlapping coverage at a specific distance from the focal point. The rate of dissolution and static cleaning ability is a function of each particular cleaning fluid. Selecting a fluid that exhibits a high static cleaning rate for contaminants involved and combining this with appropriate mechanical design improves the process cleaning rate.
REFERENCES
Contact the authors for a complete list of references.
Mike Bixenman, CTO, Kyzen Corportation, may be contacted at mike_bix@kyzen.com. Steve Stach, president and CEO, Austin American Technology, may be contacted at sstach@aat-corp.com.