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Knocking Down the Bone Pile: Tin Whisker Mitigation in Aerospace Applications, Part 3
The Effectiveness of 75% IPA/25% DI Extraction Solution on No-clean Flux Residues
June 13, 2019 | David Lober and Mike Bixenman, DBA, KYZEN; and Marietta Lemieux and Mark McMeen, STIEstimated reading time: 12 minutes
It is relatively easy to find the solubility of most WOAs in water. It is somewhat more difficult, but still possible, to find the solubility of WOAs in common solvents, such as IPA. What is virtually impossible to find in literature is the solubility of WOAs in a mixed solvent system. Many complex factors and interactions govern solubility. These factors are not additive nor linear. A literature review found only a few cases where the solubility of abietic acid or any dicarboxylic acids of interest were measured [12 & 13]. Generally, this was under conditions that were not similar to those in industry. It was with this stumbling block the solubility was to be investigated.
Methods
1. Gravimetric
To assess the solubility of abietic acid, glutaric acid, malic acid, succinic acid, and sodium succinate, saturated solutions of each were prepared in the solvent systems indicated in Table 2. To do this, each acid was obtained from Sigma-Aldrich and used without any further purification. Next, the acids were placed in 250-mL jars with a magnetic stir bar and a quantity of the chosen solvent. The jars were sealed and vigorously stirred at room temperature (20–25°C) for at least one week.
A substantial excess of acid was used to ensure that solubility was reached as indicated by a large amount of undissolved acid. Then, each solution was transferred into four pre-weighed vials using a syringe and a 5-μm membrane filter to remove any suspended solids. The weight of all of the vials was recorded. The vials were then placed in a dry bath at 110°C until they reached a constant weight, indicating dryness. For all weights, an analytical balance with an accuracy of ±0.0001 g was used.
Table 2: Solvent systems.
2. Ion Chromatography
A parallel study was undertaken to approach the issue for the other direction, namely looking at changes in ion concentration as a function of extraction solvent as measured by IC. A total of 25 boards were printed with an industry standard NC solder paste. The boards were reflowed without populating the QFN components in a standard reflow profile. After reflow, boards were randomly assigned an extraction solution from Table 2. A total of five boards were used for each solvent tested. Each board was extracted individually per IPC-TM-650 2.3.28 (80°C for one hour). The extraction volume was held constant for all boards. Twelve of the samples were analyzed immediately by Laboratory A who produced the boards. Thirteen of the samples were sent to Laboratory B for analysis.
Results
Gravimetric
The results of the gravimetric method are presented in Figure 2. The plots show all four replicates plotted for each IPA concentration as well as a LOESS fit with the standard error shown.
Figure 2: Gravimetric results.
Ion Chromatography
Due to instrument errors, the full data set is not available at the time of writing. Preliminary data is available and discussed along with future work. IC results for the total WOA extracted from each board is presented only to protect proprietary information (Figure 3).
Inferences
The results of these findings would seem to have practical impacts on measuring the reliability of PCBAs. The gravimetric results indicate that more “modern” WOAs are soluble in pure water about as much as they are in IPA. However, if the WOA forms a salt with a base, the trend is the opposite. These salts would still be capable of electrochemical migration and to serve as a leakage path for current. The IC data shows a different picture, however. The maximum extraction appears to be around 50%/50% IPA/DI water. This indicates that it is possible that ROSE and IC under report contamination as there could be more ions present than are soluble under a different concentration of IPA. It is suspected that a non-extraction based technique, such as SIR, may have an advantage over ROSE or IC.
Figure 3: Preliminary IC results.
Conclusions
Gravimetric
At the present time, there is not enough data in the literature to confirm the gravimetric findings, so additional work will be needed. The WOA with the most available solubility data from reputable sources is succinic acid. The solubility in pure DI water ranges from 77 mg/mL [14] to 83 mg/mL [15]. Our reported value was 84.8 ± 0.069 mg/mL, which is in good agreement. However, the solubility in IPA and/or IPA+ water is not readily available. The solubility of succinic acid in similar alcohols is 57 mg/mL for ethanol and 158 mg/mL for methanol [14]. Our value for pure IPA was 60.0 ± 0.024 mg/mL, which seems plausible. The data for malic acid and glutaric acid is suspect as our values are substantially less than those found in the literature.
While there is a lack of solid data in the literature for the solubility of the WOAs in solvent mixtures, the trends do match what might be expected. Abietic acid needs a critical amount of organic solvent to begin to be solubilized. While not discussed in this article, sodium succinate was expected to follow an opposite trend than the other materials, as it is a salt, because salts tend to have less solubility in organic solvents than their acid counterparts. The flux reactions should generate different salts of WOAs, which would be less detectable, but equally as harmful as their parent acid compounds.
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