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STEP 8: Fluid Flow Mechanics
December 31, 1969 |Estimated reading time: 6 minutes
This study was designed to investigate the impact of mechanical versus chemical energy contributions during the removal of contamination under 1- to 2-mil-standoff components. To validate the results obtained, extensive experiments were conducted with prepared test assemblies, iterative experimentation, as well as new mechanical innovations that will help users in the future.
By Harald Wack, Ph.D.; Joachim Becht, Ph.D.; Umut Tosun, M.S.Chem.Eng.; ZESTRON America; and Steve Stach, M.S. Chem.Eng., Austin American Technologies
Mechanical innovations in the cleaning step include, but are not limited to, various flow pattern designs and industry-leading cleaning agents. As a result, the authors also have included experimental data to address fluid flow mechanics, temperature, and solvent-concentration-related effects.
In recent years, various studies have been issued on cleaning under low-standoff components; most, however, provide incomplete information. It is essential to revisit and describe the latest challenges in the market, identifying obvious gaps in available information. Such information is crucial for potential and existing users to fully address the cleanliness levels under their respective components. With the emergence of lead-free soldering and even smaller components, new challenges have arisen including cleaning in gaps of less than 1 mil.
Initial results obtained indicate that cleanability of residues under low-standoff components has become a non-trivial issue.1 Not only are residues becoming harder to remove, the penetration of the cleaning agent seems to be in direct relationship with the geometry and height of the components in question.
Figures 1A and 1B. Flux residues around (A) and under (B) 0204 components.
Looking at the evolutionary path, the industry has transitioned from flux being around the component (thru-hole and SMT outline packs); to flux located under the part (SMT arrays); to completely filling flux under tightly spaced components, such as 0204s and flip chips (Figure 1).
Fluid Flow Theory
The basic tenets are straightforward. To get reasonable rates of cleaning in tight spaces, a suitable cleaning agent technology must be presented with sufficient force, flow, and agility to create fluid flow in these tight spaces. How much force depends on the application and the chemical ease of cleaning. In the easier cases, where open air gaps remain, capillary forces must be overcome to create flow.
Adding surface-tension-reducing agents, commonly called “wetting agents,” lowers surface tension and reduces the resistance to flow. The same effect can be achieved by using organic solvents with lower surface tension. It is essential to stress that gap size, cleaning agent, and the fluxed surfaces determine total flow in the gap. If flux residue fills or partially blocks the fluid path, the residue outer shell must be softened to allow fluid flow channels to be forced into the flux matrix. Recent experiments conducted with glass slides used to simulate flip chip configurations show that a three-step process is required to remove a fully blocked gap; the timeframe to soften the residue is very limited and varies from seconds to minutes depending on the flux and the solder reflow profile. For rapid removal of flux there are three mechanical steps required: soften the outer solvent-depleted shell and flux matrix; jet fluids with sufficient energy to create flow channels in matrix; completely erode away bulk flux residue by flow channels.
In-line Progressive Energy Dynamics Approach
This research focuses on a new approach to designing the wash section sprays of an in-line cleaner. Dubbed “progressive energy dynamics,” this involves a manifold design that is optimized to distribute the wash energy needed at each step of the cleaning process. This is contrasted with the current approach of using bigger pumps and adding more manifolds, which adds length to the cleaner and requires more power.
Progressive energy design is a fluid delivery system that recognizes the three-step process required to clean flux-filled spaces, delivering only what is needed at each step. This approach does two things — it guarantees that the appropriate amount of energy is available at each step and, secondarily, it helps to avoid a waste of energy by applying less energy in the beginning and more at the final spray.
Experiments
Test boards were populated with 0603 chip capacitors having an average standoff height of 1 mil. Each board was populated to its maximum component density (30 components per board). Tests were conducted in three different phases. The phases differ in the configuration of the spray bars as well as the spray nozzle type. This subsequently gave different spray manifold pressures for each phase. Soldering was performed in a 10-stage reflow oven under an air atmosphere. Reflow under nitrogen had previously been demonstrated to provide significantly better cleaning results. The authors therefore opted for reflow with air to produce worst-case scenarios. During all experiments, only one parameter was changed at a time and the results recorded before the next experiment was conducted.
Test Results
In the first phase, a standard cleaning manifold design was tested to yield a base line. The results showed that even belt speeds of 0.4 ft./min. and lower still had minor residues under components for leaded and lead-free formulations. These results correspond with results from other in-line machines processing no-clean lead-free fluxes.
Figure 2. Cleaning under low-standoff components. Phase 1=blue. Phase 2=green.
For the second phase, the same machine was modified by removing the wash spray manifolds and replacing them with manifolds designed to provide increasing flow as the board progresses through them. The results from this phase showed a major improvement in cleaning performance by changing the spray configuration to the progressive energy dynamics approach. Chemistry A was able to clean under the low-standoff components effectively at belt speed of 1 ft./min. (employing a 3-ft.-long wash section), which corresponded to a 3-min. exposure time. A further increase in belt speed yielded only partially cleaned residues. The results were significantly better than the authors were able to achieve in a previous study (employing a wash length section of 5 ft.) with the same chemistry and test substrates. These findings allow the conclusion that the second spray configuration, which was using progressive energy dynamics, enhanced and expedited cleaning under low-standoff components.
Conclusion
It is safe to affirm that components will continue to get smaller, board densities will increase and assemblies will get even more difficult to clean. With all these challenging parameters, the old approach to cleaner design — adding bigger pumps and lengthening the machine while using surfactant-based cleaning agents — is not the most efficient cleaning method.
With this approach, marginal cleaning was achieved at belt speeds not commensurate with the demands of a production environment. After thorough analysis of the interaction between chemical and mechanical energy in the cleaning process, a new approach was evaluated that optimizes pressure and flow by increasing impingement force of the cleaning agent as the board is conveyed through the system.
Progressive energy manifold design, in conjunction with the latest cleaning agent innovation, improves overall performance. Cleaning performance achieved with this new design and product was the best seen to-date in similar types of tests conducted over a period of years.
REFERENCES:
1.“Cleaning under Low-standoff Components,” Umut Tosun, M.S. Chem. Eng., and Dirk Ellis, SMT, February 2008.
Harald Wack, Ph.D., Joachim Becht, Ph.D., and Umut Tosun, M.S.Chem.Eng., ZESTRON America, may be contacted at h.wack@zestronusa.com. Steve Stach, M.S. Chem.Eng., president, Austin American Technologies, may be contacted at sstach@aat-corp.com.