Upon examination of the electronics manufacturing industry, a clear trend is apparent as many are shifting away from cleaning with pure DI-water to chemistry assisted cleaning processes. A number of reasons can be cited supporting therecent trend toward cleaning with chemistry. First, there is the increased use of lead free solder which requires higher soldering temperatures. This results in more burnt-in fluxes that are much more difficult to remove as they begin to produce water-insoluble contamination. DI-water alone has a very limited to no ability to solubilize non-ionic residues on the board’s surface. Second, the cleaning of leaded and lead free watersoluble fluxes (especially under low standoff components) has also become a lot more difficult since water with its high surface tension of over 70 dynes/cm cannot effectively penetrate under low standoff components. As standoff heights decrease and component densities increase, companies will have to improve their existing cleaning process. Chemistry-assisted cleaning can reduce the surface tension to 30 dynes/cm and below. Interestingly, the industry so far has mostly reverted to adjusting the cleaning process to its respective limits. This entails, for example, an increase in operating temperature to above 150°F (in some cases up to 180°F), an increase in spray pressures and a lowering of belt speeds to improve and prolong the exposure time. With pure water-soluble fluxes in an eutectic environment, such measures can provide sufficient cleaning results. Given the introduction of lead free solder pastes, however, the solubility of residues in DI-water becomes limited. If non-ionic contamination is produced, water alone cannot chemically dissolve such contamination. Another commonly overlooked consequence is that higher pressures might allow the water to penetrate low standoff components by forcing water underneath or into the capillary spaces. Unfortunately, the cleaning equipment will be challenged to remove dissolved contamination during the drying process, what leads to entrapment. It is of utmost importance to verify a dry and clean environment under components after cleaning, to limit the formation of electrochemical migration or leakage currents. Cleaning agents, on the other hand, can be easily rinsed and dried as the lower surface tension allows for quickremoval. Research and methodology
During a recent, comprehensive in-house study the authors were able to validate a number of research hypotheses. The main objective was to determine the differences of cleaning water-soluble flux residues with DI-water versus using a chemistry-assisted process. Test boards with 0603 chip capacitors and more than 20 watersoluble, lead free solder pastes were used. The research design compared three different cleaning media within identical cleaning equipment. Cleanliness was determined underneath four 68-LCC (leadless chip components) components placed on an IPC-B-36 coupon. A commonly used water-soluble, eutectic solder paste was used for this study. All test assemblies were reflowed in a 10 stage oven to simulate production conditions as closely as possible. A special arrangement of components (4 quads) on the test boards was found to be optimal based on prior experience gained through cleaning under low standoff components and customers’ feedback. Six of the test boards were cleaned at the customer’s site with the existing cleaning process as a benchmark. The remaining 12 boards were tested at ZESTRON’s Technical Center. Table 1 shows the test parameters as they were used during this case study at both sites. The paste was screened onto the test substrate. The components were applied and reflowed according to the guidelines supplied by the solder paste manufacturer. A standard IPC B-36 circuit board was used as test vehicle. Each sample was populated with four 68-LCC components as shown infigure 1. 
Results
All of the components were removed for visual analysis. Any residue detected under or around any of the four leadless components on the board constituted failure of the entire board – see figures 2a, 2b and 3a. Figure 4summarizes the results visually. 
In summary, the following conclusions were reached: • At lower wash temperatures, the tested cleaning agents demonstrated superiority over the pure DI-water cleaning process when cleaning water-soluble flux residues.
• At three percent concentration versus five percent concentration level, the cleaning results were comparable.
• Out of 12 pastes, five were more responsive to an increase in wash temperature in terms of cleanability.
• The use of a cleaning agent with a concentration level as low as three percent provided up to 111 percent better cleaning results underneath the low standoffcomponents when compared to pure DI-water. Conclusion
Current DI-water users are encouraged to take the time and closely investigate their current cleanliness levels, especially under low standoff components. One previously highlighted advantage of using a chemistry- assisted process is that users can operate at lower temperatures and with a wider process window and clean not only OA but also RMA and no-clean fluxes. Despite all the valid arguments encouraging the use of chemistry assisted processes, most machines currently dedicated strictly to DI-water are not properly equipped to use a closed looped chemistry. This means that they do not have the necessary chemical isolation section. The latter is an essential part not only to conserve chemistry but also to minimize cross contamination in the rinse tank. DI-water machines take advantage of cascading DI-water tanks from front to back. Employing a chemical product in the wash tank would lead to a continuous dilution of the recommended application concentration by DI-water. Companies that are strategically planning their capital purchases are therefore well advised to incorporate the mechanical option to run aqueous chemistries. A slightly higher investment will provide significantly more process flexibility in years to come,and might lead to additional savings. About the authors
Harald Wack, PhD Org Chem, is the President of ZESTRON Worldwide. He can be reached at h.wack@zestronusa.com. Umut Tosun, MS Chem Eng, is the Application Technology Manager at ZESTRON America. He can be reached at u.tosun@zestronusa. com. Jigar Patel, MS Chem Eng, is an Application Engineer at ZESTRON America. He can be reached at j.patel@zestronusa.com. Ravi Parthasarathy, MS Chem Eng, is the Senior Process Engineer at ZESTRON America. He can be reached at r.parthasarathy@ zestronusa.com. References:
[1] "Why Switch from Pure DI-Water to Chemistry?”- SMTAI Paper, October 2009
[2] "IPA-Water (75/25) – Why are we putting our customers at risk?” - Dr. Harald Wack,SMT Week Column, June 2009
[3] "DI-water vs. Chemistry” - Dr. Harald Wack, SMT Advisory Column, June 2008
[4] "Fluid Flow Mechanics – New Advances in Low Standoff Cleaning” – ZESTRON,presented at the SMTAI 2008
[5] "Fluid Flow Mechanics – Key to Low Standoff Cleaning” – ZESTRON, presented atthe IPC/APEX 2008
[6] "New Definition of Low Standoff Cleaning” – ZESTRON/Speedline, presented atSMTAI 2007
[7] "What is Innovation in Chemistry?” – Dr. Harald Wack, SMT Advisory Board,September 2009
[8] "Precision Cleaning under Flip Chips” – ZESTRON in Wafer & Device Packaging andInterconnect, July/August 2009 | 
