With the emergence of pH-neutral defluxing technologies in early 2009, pH-neutral formulations promised to set a new standard for material compatibility, while proving valuable to those who worked toward environmentally sound processes. As a result, potential users are very interested in assessing the differences between alkaline cleaning agents and the newer pH-neutral products, with regard to both, cleaning performance and material compatibility. One area of particular interest is the cleaning agent impact on sensitive metals. Preventing corrosion
Material compatibility issues between sensitive metals and cleaning solution arise when corrosion, i.e. the electrochemical deterioration of a metal due to the reaction with its environment, takes place. To prevent corrosion caused by the very cleaning solution that is meant to safeguard the assembly from corroding infield and potentially fail, inhibitors come into play. In general, corrosion inhibitors are chemicals that form coordinative chemical bonds with metallic surfaces (adsorption), thereby developing a thin protective layer. They are normally distributed through a solution or by dispersion. Inhibitors slow corrosion processes by either increasing the anodic or cathodic polarization behavior, by reducing the movement or diffusion of ions to the metallic surface or by increasing the electrical resistance of the metal's surface. Corrosion inhibitors can beclassified as either inorganic or organic, with the latter being more prevalent due to solubility advantages, performance,and fewer environmental concerns. Examplesof typical corrosion inhibitors are silicates, borates,alkanolamines, naphthalenesulfonic acid, triazoles, carboxylicacids, molybdates, polyols, and phosphate. If the respective cleaning media do not work as intended, several types of corrosion can commonly occur on electronic assemblies, such as gas phase, uniform, pitting, electrolytic metal migration, and galvanic. Fortunately, this has been an area of much research and electronics manufacturers today have a variety of cleaning choices to prevent such issues with the newer and more effective aqueous alkaline chemistries strongly preferred over solvents or traditional surfactants. Recently, however, the choices of aqueous products available for defluxing have expanded significantly with the introduction of pH neutral formulations. Collaborative study
A collaborative study was conducted to compare the material compatibility and cleaning effectiveness of pHneutral and alkaline technologies at low operating concentrations. In the first part of the assessment, material compatibility effects were examined. Particularly sensitive materials were chosen for exposure under worst case conditions. They included but were not limited to anodized aluminum, copper and certain nickel alloy substrates. For the cleaning performance testing, the study employed the use of IPC approved B52 test boards (figure 1). While the most challenging component geometries were chosen, the authors also determined the need to quantify cleanliness to provide data that was not included in previous publications related to this topic. Extensive SIR, ion chromatography and analytical test data were accumulated to validate the visual residue analysis. Methodology
The research design compared the material compatibility of two alkaline cleaning agents with varying degrees of inhibition and a pH-neutral cleaning agent with sensitive metals. For this part of the study, various types of sensitive metals were exposed to the same alkaline and pH-neutral cleaning agents as well as DI-water. A visual inspection was performed after 15 minutes and 24 hours exposure as well as after three weeks of storage under normal environmental conditions. These data were subsequently used to determine the methodology in the second part of the study. While conducting several extensive preliminary cleaning trials to define the process settings for further and more detailed analyses in the second phase of the cleaning performance testing, ten most commonly used leaded and lead-free no-clean and water-soluble solder pastes were applied to the boards and reflowed in a 10-stage oven. Tables 1 and 2 show the reflow profiles. Subsequently, the boards were cleaned applying the process settings outlined in Table 3 and visually inspected. In the second phase of the cleaning trials, a fewer number of test boards and only two solder pastes were chosen. After subjecting the boards to the same reflow parameters, they were cleaned with both agents and inspected via SIR analysis and ion chromatography. All results were recorded and analyzed before drawing final conclusions. Material compatibility
Material compatibility tests clearly demonstrate that while short-term exposure to aqueous cleaning solutions may not have any influence on material compatibility, the long term effects also need to be examined. First, the authors are not surprised that slight hazing developed on the copper substrate after exposure to DI-water. De-ionized water over time becomes inherently acidic via CO2 absorption and copper, dust, etc. rapidly supply ions, thus re-ionizing de-ionized water. Therefore, the addition of inhibitors in all media is important to prevent corrosion especially if users are concerned about the long term reliability of their product. Second, the fact that some material changes (discoloration) were observed on the lead frame that had been exposed to cleaning agent B is noteworthy. More importantly, though, the fact that cleaning agent A caused a signifi cant amount of post rinse degradation is quite worrisome. The authors hypothesize that any deterioration of the substrate's surface may have been caused by a possible lack of rinsability of the corrosion inhibitor in this particular cleaning medium. Oftentimes the amount and type of inhibitor added to the cleaning solution can impact its rinsability. Additionally, any environmental effects on the inhibitor residue may have also caused the degradation over time. It is important for users to realize that even if substrates are deemed perfectly clean and compatible with their cleaning solution soon after exposure, the long term study results (3 weeks after cleaning and environmental exposure) indicate that material changes may happen later in the fi eld, which can cause reliability problems in the long run. Cleaning performance
Results from preliminary cleaning trials indicate that since obvious differences in surface and under-component cleanliness levels do exist, the pH-neutral agent clearly outperformed alkaline cleaning agent A. A board that is perfectly clean on the surface can fail when the spaces underneath the components are examined. The authors conclude that there are several potential reasons for these results. First, any lack of performance could be related to concentration, i.e. 10 percent may be too low of an effective concentration to clean these challenging boards properly, especially for the alkaline product. Second, the observed cleanliness issues may be due to the inhibition packages (type and amount) that the cleaning agents contain. Third, cleaning product formulations and mechanisms also play an important role. Finally, the residues found may be a result of inhibitors bonding with the very contamination the cleaning agent was intended to remove thereby preventing the dissolution of the residue. In order to further quantify and qualify these findings, several additional test vehicles were chosen, refl owed, and cleaned with both solutions. Subsequently, the boards were subjected to SIR and ion chromatography analyses. In summary, after conducting extensive testing and analyses, results indicate that when it comes to cleaning performance, the pH-neutral product and the alkaline cleaning agent A are both very viable solutions. Both chemistries were able to provide a clean surface. In the undercomponent cleanliness visual examination, the pH-neutral cleaning agent performed signifi cantly better overall. Both solutions left some residues behind. After examining the SIR test results, the pH-neutral agent finished slightly ahead of its counterpart. The ion chromatography values also confirmed these findings, as again, the pH-neutral chemistry did a slightly better job of removing ionicresidues. ------------------------------------------------------------------- Choosing the right inhibitor
For all aqueous solutions to do a superior job without affecting sensitive metal substrates, i.e. corrosion control, manufacturers have to add inhibitors. Studies have shown that choosing the correct type and amount of inhibition chemistry is critically important. Otherwise, the inhibitors themselves can present several problems in the SMT production process. First, the solubility of certain inhibitors in concentrate chemistry is sometimes low and only a small percentage of the inhibitor can be added to the cleaning product formulation. Therefore, to achieve proper protection of sensitive metals using such problematic inhibitors, a higher recommended operating concentration is often required in the wash tank, which leads to unnecessary chemistry consumption. On the other hand, lowering the concentration leads to a lower amount of inhibitor available to protect sensitive metals. Second, these organic additives can have detrimental effects on the cleaning process as they also interact with any residue as well as the environment and inhibit the dissolution of such residue into the cleaning fluid. Finally and most importantly, certain inappropriate inhibitors are tightly bound to the metal surface and are more difficult or impossible to rinse from the substrate's surface and under components, where they linger insidiously, causing a host of problems over time. This contamination can adversely increase the electrical resistance of the contaminated areas, lead to conformal coating issues and cause unpredictable failures, thereby threatening the long-term reliability of the assembly. The type and amount of inhibitors selected is also a function of the pH conditions in the process. Some inhibitors that work well at a certain pH will not function as well or at all if the pH is outside of this range. Therefore, pH-neutral cleaning agents offer distinct advantages. They require very small amounts of inhibitors because at this pH range (7 +/- 0.5), a unique and customized set of corrosion inhibitors is very effective, thereby solving the problems mentioned above. Due to their lower surface tension (less than 30 mN/m vs. 72 mN/m for water), pH-neutral solutions can penetrate the tiny spaces in and around components, do their job of removing contamination even at low concentrations and can be easily rinsed and dried. Furthermore, pH-neutral cleaners are more environmentally friendly and eliminate waste water neutralization processes. Most importantly, however, using pH-neutral agents has been shown to eliminate material compatibility concerns in cases where alkaline agents have failed, thereby offering users asolution that previously did not exist. ------------------------------------------------------------------- ------------------------------------------------------------------- Impact on copper
To further analyze and exemplify the long-term environmental exposure effects after cleaning with the above agents, the authors chose to examine the impact on copper. With its wide range of applications in the electronics manufacturing industry, copper is usually deemed resistant toward environmental and chemical influences. It is, however, known that in aggressive media, this metal is particularly susceptible to corrosion. Close-up views of the copper lead frames under 40x magnification demonstrate the changes observed after three weeks of environmental exposure. Figure 2 shows the untreated control substrate with no changes. The copper is still shinywithout any discoloration or corrosion. The lead frame cleaned with the pH-neutral cleaning agent experienced no change after having been exposed to the environment for three weeks. In other words, the substrate looked just like it did after having been exposed to the pH-neutral solution for 24 hours. Minor discoloration specs and lines hadalso been noted at that time (figure 3). The lead frame cleaned with alkaline cleaning product A, on the other hand, became significantly discolored and corroded after 3 weeks of environmental exposure, which constitutes a major transformation as the substrate was perfect after 24 hours of exposure. The copper lead frame showed a tremendousamount of corrosion in the difficult to rinse areas as well as on the flat surfaces. Major discoloration, i.e. significant lines, wasnoted throughout the substrate's surface (figure 4). Figure 5 demonstrates that there was some material change observed after 3 weeks of environmental exposure. This lead frame had been subjected to alkaline cleaning agent B. After 24 hours of exposure, the copper substrate was still perfect. After 3 weeks of environmental exposure, however, the substrate's surface appeared hazy and showed some discolored specs and lines (figure 5). Lastly, the authors inspected the copper lead frame that had been exposed to DI-water only. The image shows that after 3 weeks of storage in the environment no change was observed(figure 6). ------------------------------------------------------------------- ------------------------------------------------------------------- Summary
Overall, the study results are quite encouraging with regard to cleanliness, proving that pH-neutral cleaning agents can compete with alkaline products. In fact, in this particular case, the pH-neutral product outperformed the alkaline product. Both can definitely do a superior job of removing all residues from the board's surface, whereas, in this case, the pH-neutral chemistry took the lead in cleaning residues from the tight spaces underneath the components. There are, however, some distinct differences with regard to material compatibility. As noted in the first part of this study, it is important for users to realize that even if substrates are deemed perfectly clean and compatible with their cleaning solution soon after exposure, long term product related effects can compromise the reliability of the assembly. This study proved that the pH-neutral cleaning agent was far superior in the area of material compatibility as both alkaline products experienced significant problems during the different test phases. Whether choosing a pH-neutral or an alkaline product, all aqueous precision cleaners in the electronics manufacturing industry must contain some sort of inhibiting additive in order to avoid corrosion affecting sensitive metal substrates. The question of how much and what kind of inhibitor to use depends on the cleaning agent, as this study clearly suggests that differences do exist. Unfortunately, there is no universal solution for protecting sensitive metals as finding the proper combination of specific inhibitors is a complex function of numerous significant parameters. It is important to note, that there are some distinct differences between alkaline and pH-neutral cleaning agents that may influence the user's choice. pH-neutral chemistries offer an unprecedented level of environmental friendliness, thereby reducing worker safety and government regulation compliance concerns as well as the need for costly waste water neutralization processes. They are also easy to rinse and gentle on precision cleaning equipment. ------------------------------------------------------------------- | 
