An Overview on Global Change in Ionic Cleanliness Requirements
|Publication Date:||1 September 2018|
For many years, military (e.g., MIL-STD-2000) and commercial (e.g., IPC-J-STD-001) manufacturing standards have required manufactured circuit card assemblies (CCAs) to meet an ionic cleanliness requirement of 1.56 microgram (μg) of sodium chloride (NaCl) equivalence per square centimeter (cm2) of extracted surface, using Resistivity of Solvent Extract (ROSE) testing per IPC-TM-650, method 2.3.25.
It is the position of the IPC committees that the value of 1.56 μg NaCl equivalence per square centimeter should be considered as obsolete for the following reasons:
● This test methodology was originally developed in the 1970s; it was never intended to be used as a cleanliness test, nor as a test for product acceptability, it was only intended to be used as a process control method.
● The use of the ionic contamination value as a measure of product acceptance was the result of a US Department of Defense desire to implement a pass/fail criteria.
● This ionic contamination value, and those derived from them, were originally developed for high solids (35% solids) rosin fluxes and ozone depleting chemical (ODC) cleaning. The flux chemistries and cleaning solutions used today are completely different from those used when the ROSE limits were established.
● Modern assemblies are simply too complex in terms of residues to have a single "one size fits all" cleanliness criterion.
● There is mounting evidence that as CCA component density increases, so does the sensitivity of the circuit to ionic contamination. Modern circuit assemblies have far greater component densities than found in the 1970s. This also means that residues that had minimal impact on 1970s component technologies can now have a significant impact on component reliability.
● For many assemblies, ROSE testing is no a longer a sufficient test regimen to adequately predict acceptable levels of ionic residues. IPC has compiled a list of technical presentations showing the inadequacy of ROSE to predict ionic residues for high performance electronics (see last page of the document).
● It is recognized that ionic residue testing is critical for reliable circuit function and so the ROSE test has continued in use until a more suitable alternative can be identified and implemented.
The IPC Technical Report 583 (IPC-TR-583, 1995) documents a review of the ionic contamination testers on the market at that time. The study reported an apparent lack of repeatability and reproducibility between the different systems available at the time and that the use of "equivalency factors", correlating the results between different kinds of testers, should be considered invalid.
It should also be noted that the TR-583 conclusions refer to the ionic testers of that era (1990s). Many of the more modern configurations of the ionic testers have been shown to be repeatable and reproducible. See Reference  as an example of testing modern instruments for repeatability and reproducibility.
The Executive Summary of IPC-TR-583 is included with this document as Appendix A. The full technical report is available from the IPC.
Many of the other references in the Bibliography section can assist the reader in understanding the history of the ROSE test, as well as some of the shortcomings of using ROSE as a measure of product acceptance