David Lober; Mike Bixenman, DBA
Marietta Lemieux, Mark McMeen
Two of the three primary cleanliness test methods for PCBs, Resistance Of Solvent Extract (ROSE) and Ion Chromatography (IC), rely on the assumption that all ionic contamination on a PCB will be soluble in a solution of 75% isopropyl alcohol (IPA) 25% De-ionized water (DI). This assumption is made in the most critical step of an analytical procedure, the extraction step. If this extraction step is not functional or even optimal, then no instruments further down the logical path can correct for poor extraction and you will not know the actual contamination levels on the PCBA. This is why, in many analytical textbooks, extraction studies are covered in the first few chapters, as poor extraction efficiency is the death of an analytical method.
Considering the importance of the extraction step, and that two-thirds of the primary cleanliness assessment methods rely upon the same extraction solution, one would assume that there would be studies that demonstrate the appropriateness of the 75% IPC/25% DI extract solution. Studies showing the relevance of this extract for rosin-based fluxes that were popular in the 1970s have currently existed, but no such studies on newer materials exist on No-Clean fluxes. This is a troublesome fact for the industry as a whole, as subject matter experts agree that the current extraction solution and procedure is unlikely to be efficient with common materials now. However, no studies as to the relevance of the 75% IPA/25% DI extraction studies currently exist. In fact, the solubility of the specific ions tested for in the IC analysis in 75% IPA/25% DI is not only unknown in the industry, but literature searches for this data have yielded nothing. This paper attempts to begin to rectify this situation and promote open discussions on the future of ionic cleanliness test methods.
This study is broken into two parts, first a review of the solubility of some critical ions in different ratios of IPA/DI and at different temperatures. As noted above, despite using this extraction solvent for decades, this essential piece of information is missing. The second is to look at the influence of extraction time on several fluxes under the current standard. Prior work conducted by this team, indicated that there was a statistically significant increase in the concentration of extracted ions with a 3-hour extraction vs. the standard 1-hour extraction. Due to sample and time constraints, this relationship was not explored further. In this study, we will repeat that study but with more samples, and additional extraction times to determine if this trend is essential or not.
At the present time, there is not enough data in 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/mL14 to 83 mg/mL15. 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 methanol14. 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.
Initially Published in the SMTA Proceedings