Tuesday, May 22, 2018
Home/Current Issue >  Industry Articles >  PCB and Assembly > 

Why IPA-Water Extraction Fails!
PC board cleaning.

The performance requirements for mission critical electronic assemblies are constantly increasing as component failures, particularly in the aerospace, automotive, medical and military industries, are simply not an option. Since failures are often due to insufficient cleanliness, cleaning has become an integral and vital part of the production process.

Residues are generally classified as ionic and non-ionic. In order to determine ionic cleanliness levels, an IPA-Water mixture of 75 percent IPA (solvent) and 25 percent water (highly polar liquid) are required as an extraction liquid. The polar liquid is used to solubilize all inorganic particles (salts, chlorides, bromides, weak organic acids) and the solvent is used to solubilize non-ionic contamination (resins). However, if it is the solvent's

responsibility to solubilize non-ionic contamination, is water really the most important part to make the ionic contamination measurement work?

For water to be able to solubilize inorganic residues, it first has to be able to get past the organic and water insoluble resin layers (RMAs, No-cleans). Rosin and resin-based residues remain under and around components and flux residues are trapped. In order to quantify the residues, which is what IPA-water is intended to show, one has to remove ALL residues (ionic and non-ionic) from around and under the components.

IPA-water has been struggling with cleaning/solvency prior to the introduction of lead-free products. As the market is moving towards these materials, more densely populated boards, lower standoff heights and smaller components, achieving perfect cleanliness is becoming more difficult. Residues are more complex and significantly harder to clean as higher temperatures and higher amounts of activators result in even tougher residues.

Whichever modern, alternative mix will be used instead of IPA-water, one fact remains true: it must match chemically in order 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).

Based on the above, the authors have examined alternative solvent mixes to establish whether or not the IPA-water mix can be improved, thereby providing new test methodologies and mitigating the current risks of potential failures of critical components.

Research Part 1:
As an initial hypothesis, the authors' goal was to show that an IPA-water mixture fails to completely solubilize current flux residues when used as an extraction medium.

Secondly, the authors wanted to establish that when applying the Hansen Solubility Parameters, other solvent mixes, compared to IPA-water, are more suitable extraction media.

All initial solubility experiments were conducted with a rosin system "Manila Copal" which simulates contamination caused by fluxes and/or solder pastes. The intention of this initial research was to reduce the solvent selection from 20 to 5. All solvents used are summarized in table 1.
Table1: Standard solvents used to determine initial solubility of Manila Copal applying HSP [4].

During the selection process, solvents with a high polarity and varying degrees of hydrogen bonding capabilities were chosen. The parameters were calculated according to procedures outlined by the Hansen Solubility Parameters. [4]

One gram of Manila Copal was agitated with a magnetic stirrer in 100mL of the investigated solvent at room temperature and the time was measured until the whole copal was fully dissolved. Slight turbidities caused by the dissolved copal were neglected. The maximum time allotted for this process was 60 minutes. If the copal was not completely dissolved, the remaining copal was filtered and the filter paper then gravimetrically died for 1hr at 100xC/212xF. The results are summarized in table 2.
Table 2: Standard solvents used to determine initial solubility of Manila Copal using HSP [4].

Experimental Findings:
The authors were able to conclude, that performance varies among solvents. The solubility obtained for acetonitrile, isopar, and nitrobenzene was found to be less than 5 percent in solution. Toluene was not able to dissolve anything. Other solvents did solubilize more than 60 percent 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 percent removability, and are summarized in table 3 including their Hansen Parameters.
Table 3: Final Five Solvents selected.

Research Part 2:
Based on the initial findings and solvent selection, it was important to correlate the results with real-life residues. Very commonly used leaded and lead-free water-soluble and no-clean pastes were chosen. A sample of 11 solder pastes was selected 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 in detail below.

100mL of organic solvents (75g of solvent, 25g of water) were selected. Each solder paste layout was immersed into each solvent for 10 minutes with mechanical agitation. The boards were dried and examined under 40x magnification to evaluate the cleanliness levels. The conductivity was also continuously measured. The results are summarized in figure 1 and table 4.
Figure 1: Cleaning ability of tested solvents (75/25) with water.

[1] Contamination in all areas untouched/ White residues (WR) [2] Contamination in most areas

[3] Contamination in few areas

[4] Some minute and specs or lines

[5] Clean

Table 4: Conductivity Measurement values for solder pastes 1-5.

Experimental Findings:
The authors compared 11 commonly used solder pastes with regard to cleaning performance. A 1 to 5 cleanliness rating was chosen (5=highest, 1=lowest). Each solvent was mixed 75 percent/25 percent per volume. Much to the authors' 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. Only for solder paste 6 it was able to outperform IPA. 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.
SZ 40 microscope used to examine cleaning effectiveness on PC board.

Part of our objective in this study was to establish superior solvent/water solutions which 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.

The values demonstrate a direct relationship between solder paste residues and solvents. The authors were able to confirm that each solvent's conductivity increased, showing the solubilization of conductive contamination. It is noteworthy to state that due to their ability to better clean water-soluble fluxes (when 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 previously observed, one can only conclude that for all selected solvents the conductivity measurement is indeed a suitable analytical tool.

Based on the above research, the authors conclude that none of the selected alternative solvents chosen are suitable alternatives to IPA. Hypothesis 1 and 2 could therefore not be validated. However, one should point out, that 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 which 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, only a relative one. For high-end electronic assemblies this should not be accepted as a sufficient standard.

Contact: Zestron America, 11285 Assett Loop, Manassas, VA 20109 703-393-9880 fax: 703-393-8618 E-mail: infoUSA@zestron.com Web:

[1] Stach, Steve. "Using Hansen Space to Optimize Solvent Based Cleaning Processes for Manufacturing Electronic Assemblies." www.aat-corp.com June 18, 2009.

[2] Ken Dishart / Bill Kenyon. SMTA Cleaning Conference September 2008.

[3] Wack, Harald, Ph.D. "IPA-Water (75/25): Ineffective for Cleanliness Test with Modern Contaminations." SMT Contributing Article. SMT Week June 2009.

[4] Hansen, Charles, M. "Hansen Solubility Parameters, A User's Handbook." Second Edition, 2007.

Special thanks to ERSA North America for the generous support with its HOTFLOW 3/20 Reflow Oven.

Harald Wack, Ph.D., President ZESTRON Worldwide. Questions may be addressed to h.wack@zestronusa.com.

Syed Ahmad, Ms.Chem., R&D Department ZESTRON America. Questions may be addressed to s.ahmad@zestronusa.com

Joachim Becht, Ph.D., R&D Department ZESTRON Europe. Questions may be addressed to j.becht@zestron.com.

search login