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Clean “no-clean,” or use a water wash solder paste?


diameter was measured. After cross sectioning the joint, solder height and solder angle could be defined (the smaller the wetting angle the better the wetting of the solder). For tin-lead alloys, the best spreading


was achieved using a fast ramp up in the preheat part with a peak temperature of 215˚C in a nitrogen atmosphere. The water wash paste has stronger


activators, which result in better wetting. The average wetting angle for water wash was 1˚ smaller than the “no-clean” score.


Lead vs. lead-free Lead-free soldering introduces numerous additional challenges to the cleaning process. Water wash is an attractive option for lead-free soldering because higher activated flux systems can be used. However, due to the higher soldering temperatures, the residues become harder, making cleaning more difficult. The flux residue is more difficult


to clean because of, a higher molecular weight, more complicated ingredients structure, and more side reaction products in flux residue.4 The surface tension of lead-free


alloys is approximately 20 percent higher than the SnPb. This affects the wetting properties. The result can be found when measuring the wetting angle of the solder joint. For each solder paste, the optimal


settings were defined with the Taguchi approach. Afterwards, confirmation runs with the best settings showed the data found in Table 5. The TGA defines, the remaining


flux amount after soldering. For lead- free solders, there are less flux residues because the thermal profile has higher temperatures than a SnPb reflow profile. The water wash flux system is


completely different from the “no-clean.” By weight, there are more residues left on the printed circuit board (PCB). These residues have a completely different composition. They are hydroscopic, and active, but can be easily removed even with DI-water.


Conclusion Cleaning agents have been improved and cleaning has become a value-added production step in soldering processes where reliability in the field is critical of corrosion and current leakage. DI-water alone may not be sufficient


to remove flux residues under small SMD components. It is limited to removing non-


Figure 11. What if all residues are not cleaned? This graph shows the reliability risk for different formulations.


ionic residues from the surface of a PCB. Due to its high surface tension, DI-water is not able to penetrate underneath low stand-off devices. “No-clean” solder paste can be cleaned,


but DI-water alone is not able remove the hard residues that emit water instead of dissolve in water. A saponifier is required to remove all the resins. When washed off completely a water


wash solder paste is preferred, because it is easy to clean, has stronger activators and is safe after cleaning. There is the risk that if not completely washed off (miniaturization increases the risk due to low stand- off height, high population, and finer conductor widths and spaces), reliability becomes critical.


References 1. C. Lea, “After CFCs? Options for Cleaning Electronic Assemblies,” Electrochemical Publications Ltd., 1992


2. W. Rubin, “The Progress of “no-clean” Technology,” Electronic Production, November 1998, p27


3. IPC J-STD-001E “Requirements for Soldered Electrical and Electronic Assemblies,” p43


4. Dr. Ning-Cheng Lee, “Lead-Free Flux Technology and Influence of Cleaning.”


5. Dr. Mike Bixenman, “OA Flux Cleaning Studies on Highly Dense Advanced Packages Parameters.”


Gerjan Diepstraten is process support manager at Cobar Europe B.V. Gerjan has a mechanical engineering background and began his career in 1989 as a mechanical designer at Soltec B.V.


Over the years Gerjan has done workshops and presentations at different seminars including APEX, EIPC, IPC conferences, FAPS, FED, Brasage Soldering and CEMEX.


Tim Lawrence is a chemist with Cobar/ Balver Zinn. Dr. Lawrence received his PhD


in physical chemistry from Leicester University, UK.


16 – Global SMT & Packaging – Celebrating 10 Years – December 2010


www.globalsmt.net


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