IPA-Water (75/25): Ineffective for Cleanliness Test with Modern Contaminations

Estimated reading time: 4 minutes

Harald Wack, Ph.D., ZESTRON America

As this industry is keenly aware, innovation is what keeps us moving forward, justifying our high standard of living in North America. During my graduate thesis work in asymmetric catalysis at Johns Hopkins University, my professor once told me that he sees "innovation as the ability to see change as an opportunity, not a threat".

To this day, numerous test methods ROSE test IPC TM650 2.3.25.1; IPC TM 650 Method 2.3.28; and MIL-P-28809 section 4.8.3 are used to determine cleanliness. Keep in mind that the electronics industries that remain in North America are primarily focusing on military, medical, and automotive products, which require high cleanliness levels.

These test methods include, but are not limited to, ionic contamination and ion chromatography (IC). IPA-water (75/25) is used with both cleanliness test techniques as an extraction liquid including variations for different temperatures. Higher temperatures increase residues' solubility. One of the drawbacks of this method is that a solvent's solubility is directly proportional to temperature. Increased temperature increases a solvent's ability to dissolve solids. After solvent distillation, once the solvent cools to room temperature, this physical process becomes fully reversible and results in precipitation of the solids that are not soluble at room temperature. In practical terms, it means that, at the moment of injection of a sample (this must be at room temperature) into an analytical instrument such as an ion chromatograph, the precipitation occurs as the liquid cools down. Heating extraction water to increase solubility might not be a solution to the initial problem, which is the poor solvency of IPA-water.

We can generally classify the nature of any residues as ionic and non-ionic. As a result, a "solvent" (in this context a mixture of organic solvents and water) and a highly polar liquid (i.e. water) are required as a viable extraction medium. The rational is that polar water solubilizes all inorganic matter (salts, chlorides, bromides, weak organic acids) and the solvent is used to solubilize non-ionic contamination (resins). So, if the solvent's duty is to solubilize non-ionic contamination, then isn't water really the most important part to make the ionic contamination test work (as you are removing polar, conductive residues)? The limitation of this statement is that water solubility for inorganic residues is only relevant once it gets past the resin layers left on the PCB, which are organic and not soluble in water (RMA, no-cleans). Think of trying to clean under low-standoff components using a process where the cleaning is not optimal. Surface contamination, the rosin-/resin-based residues, will remain under and around components, and flux residues are trapped under them. To quantify how much contamination remains (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.

With the electronics market moving to lead-free materials, higher complexity boards, and lower component standoff heights, the problem is further complicated. Residues are becoming more complex and significantly more difficult to clean as higher reflow temperatures and higher activator content form harder to remove residues. Lead-free is not limited to the low-reliability sectors either. As IPA-water already struggled with cleaning/solvency prior to the proliferation of lead-free products, the problem is now further compounded. A generally observed unit for the "cleaning power" of a solvent is the Kauri Butanol value (Kb value). This international, standardized measure for a hydrocarbon solvent is governed by an ASTM standardized test, ASTM D1133. The result of this test is an index, or the Kb value. A higher Kb value means the cleaner is more active. Mild cleaners have low scores in the 10s and 50s; powerful cleaners like the obsolete chlorinated solvents have ratings in the low 100s. Not surprisingly, the value for IPA is below 50.

Whichever modern, alternative mix will be used instead of IPA-water has to be chemically capable of fully solubilizing all remaining residues on assemblies post cleaning. It could be, for example, a mix of solvents (polar and non-polar) in combination with water; or a solution without water, using instead a highly polar liquid such as Dimethylformamide (DMF). As my first undergraduate chemistry textbook cited: "like dissolves like;" simply put but eternally true. This notion has also been conveyed in great detail by the Hildebrandt and Hanson Solubility parameters research.

The questions I pose are urgent. How is it we are still using IPA-water mix that is proven to be "one of the worst solvents," as recently stated by a member of our industry, mixed with water? Why do we expect it to clean/solubilize modern resin-based residues? And how are customers feeling about assessing perceived cleanliness (for high-end military products, for example) by relying on an inaccurate extraction/analytical method? We have a responsibility to ensure product quality in our industry and we can do better than this. Research is currently underway to provide viable, general replacement solutions that should be evaluated and compared to the present method.

Harald Wack, Ph.D., an SMT Editorial Advisory Board member, is president of ZESTRON America. Wack has authored and published several scientific articles, and has provided technical information for various publications. He received his doctoral degree in organic chemistry from Johns Hopkins University. He may be contacted at (703) 393-9880 or via e-mail at h.wack@zestron.com. Dr. Wack recently wrote Alkalinity, Defluxing, and Materials Compatibility, The Importance of Global Technical Support, and Material Compatibility Worldwide.

For more on ionic contamination and cleanliness test, read:Cleanliness Testing on the Shop FloorEssential Electronics Assembly Cleaning Evaluating Cleaning System Performance