IPA water is limited as a cleaner and should not be a standard for analytical extraction tests.

A generally well understood unit for the “cleaning power” of a solvent is the Kauri-Butanol value (Kb-value, ASTM D1133). The result of this test is an index, usually referred to as the “Kb-value.” The higher the Kb-value, the more active the cleaning agent. Mild cleaners have low scores in the tens and fifties; powerful cleaners like the old chlorinated solvents have ratings in the low hundreds. Not surprisingly, the value for IPA is below 50.

Whichever modern, alternative mix will be used instead of IPA water, one fact remains: It must match chemically to fully solubilize all remaining residues on assemblies. It could be, for example, a mix of solvents (polar and non-polar) in combination with water, or alternatively a solution without water, and instead a highly polar liquid such as DMF (dimethylformamide). Recent literature reviewed this notion and summarized the Hildebrandt and Hanson Solubility parameters research.^1,2

Based on the situation described here, we examined alternative solvent mixes to establish whether IPA-water mix can be improved. This would provide new avenues to increase analytical test methods in the near future and limit the risk users currently assume. This study is divided into two parts. In Part 1, the authors screened various potentially suitable solvents. In Part 2, a final selection of solvents was applied on commonly available flux residues.

The study was based on two hypotheses. Hypothesis 1 was IPA water is not the best solvent mix to fully solubilize current flux residues. Hypothesis 2 was various solvent mixes across Hansen space should show significant improvement.

The experiment began with a rosin system called “Manila Copal,” which simulates contamination caused by fluxes and/or solder pastes. This natural material is closely related to Kauri Copal (used for Kb-value determination) and was used in all initial solubility experiments. The latter were intended to narrow the solvents tested from the initial count (of more than 20) to five finalists. Table 1 summarizes all solvents used.

During selection, solvents were chosen with a high polarity and varying degrees of hydrogen bonding capabilities. The parameters were calculated according to procedures outlined by Blanks and Prausnitz.³ The dispersion parameters, for example, are based on atomic forces. The size of the atom is important, as corrections are needed for atoms significantly larger than carbon, such as chlorine, sulfur, bromine, etc.

Part 1 experiment. One gram of Manila Copal was stirred on a magnetic stirrer in 100 mL of the investigated solvent at room temperature, and the time measured until the whole copal was fully dissolved. Slight turbidities caused by the dissolved copal were neglected. The maximum time allotted for the dissolving processes was set to 60 min. In case the copal was not completely dissolved, the remaining copal was filtered and the filter paper then gravimetrically dyed for 1 hr. at 100°C (212°F). Table 2 summarizes the results.

Part 1 findings. The authors were able to conclude the performance did vary across the solvents. For example, the solubility obtained for acetonitrile, isopar, and nitrobenzene was found to be less than 5% in solution. Toluene was not able to dissolve anything. Other solvents did solubilize more than 60% of the contamination, such as THF (tetrahydrofuran), Methoxy-Propoxy-Propanol (DMP) and Butyl-acetate. Ethanol, PM, MEK, NMP, and DMF were able to achieve 100% removability, and are summarized below, including their Hansen parameters.

Based on the initial findings and solvent selection, it was important to correlate the results to real-life residues. Very commonly used leaded and Pb-free water-soluble and no-clean pastes were chosen. A sample number of 11 solder pastes was chosen to simulate potential solubility trends. Each board was reflowed according to its recommended profile in a 10-stage state-of-the-art reflow oven. The experimental procedure is described below.

Part 2 experiment. A total of 100 mL of selected organic solvents (75 g of solvent, 25 g of water) and each solder paste layout were immersed into each solvent for 10 min. with mechanical agitation to check the relative cleaning ability. The boards were then dried and examined under 40X magnification to evaluate cleanliness. Conductivity was continuously measured during the whole procedure.

The authors compared 10 commonly used solder pastes based on cleaning performance. The rating was chosen from 1 to 5, with 1 the lowest and 5 the highest. Each solvent was mixed 75%/25% per volume. Much to our surprise, the alternative solvent mixes did not significantly improve the cleaning results. For example DMF, a well-known and powerful solvent, showed lower overall ratings. It outperformed IPA only in the case of solder paste 6. A similar result was observed for MEK. For solder paste 9, it cleaned equally well when compared to IPA. For all other solder pastes, IPA cleaned equally or worse.

Part of our objective in this study was to establish superior solvent/water solutions that would be able to remove remaining flux residues during the analytical cleanliness assessment. Parallel to the cleaning experiments, the authors decided to monitor the conductivity of the solutions. This parameter is essential to ensure that viable alternative solutions can be used to assess cleanliness based on measured conductivity. Table 4 summarizes the values for the selected solvents.

The values demonstrate a direct relationship of solder paste residues between solvents and IPA as the internal standard. The authors were able to confirm our initial assumption that each solvent increased from its starting conductivity, showing the solubilization of conductive contamination. It is noteworthy that due to their ability to better clean water-soluble fluxes (compared to no-clean formulations), their respective conductivity values were found to be higher. This indicates partial solubilization, but does not imply full removability. With the limited cleaning performance observed previously (especially for no-clean pastes), one can conclude that for all selected solvents, the conductivity measurement is a suitable analytical tool.

Conclusions

Based on the research conducted, the authors conclude that none of the selected alternative solvents chosen is a suitable alternative to IPA. Hypotheses 1 and 2 could therefore not be validated. However, IPA failed to show the required cleaning performance to remain a viable extraction fluid of choice for ion chromatography and ionic contamination. It is fair to state that current, modern flux residues cannot compare to traditional RMA formulations, as they provide a significantly more complex structure that impacts its ability for removal. At present, IPA water demonstrates very limited cleaning performance, which in turn confirms that any analytical extraction method using this solvent mix cannot and will not provide an absolute cleanliness assessment, just a relative one. This should not be accepted as a sufficient standard for high-end electronics assemblies. 

Acknowledgments

Special thanks to ERSA North America for the generous support with Hotflow 3/20 reflow oven.

References

1. Steve Stach, “Using Hansen Space to Optimize Solvent Based Cleaning Processes for Manufacturing Electronic Assemblies,” SMTNet, July 9, 2009.
2. Ken Dishart, “Using Hansen Solubility Parameters to Match the Cleaning Material to the Lead-Free Flux Residue,” SMTA Cleaning Conference, September 2008.
3. Charles M. Hansen, Hansen Solubility Parameters: A User’s Handbook, second edition, 2007, pp. 13.

Harald Wack, Ph.D., is president of Zestron Worldwide (zestron.com); h.wack@zestronusa.com. Syed Ahmad is the supervisor at Zestron America’s R&D department. Joachim Becht, Ph.D., is in the R&D department at Zestron America.

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