Doug Pauls Explains Ion Chromatography
Residues from manufacturing operations may adversely impact the reliability of printed circuit assemblies in the field. But not all residues are harmful or detrimental: What is critical is to understand what kinds of residues are present and in what amounts. ROSE (Resistivity of Solvent Extract) techniques have been used as a measure of ionic contamination by bare board manufacturers and assemblers since the 1970s, and there is a huge installed base of ROSE test equipment. But does ROSE testing give sufficient information about the nature and origin of the residue in the context of present-day electronics manufacturing? Ion chromatography offers a more precise and selective analytical technique, but how does it work and how can it be used to determine electronic cleanliness? What ion-chromatography-based specifications exist and how they were derived, and how you would use this tool for process troubleshooting and optimization?
Principal Materials and Process Engineer at Rockwell Collins and Chair of the IPC Cleaning and Coating Committee, Doug Pauls drew upon his 25 years' experience in the use of ion chromatography for electronics assembly improvement to answer these questions, in a webinar presented on behalf of SMART Group and moderated by Bob Willis.
Pauls began by reviewing the status and limitations of ROSE testing, and discussed the reasons why the industry should not continue to use ROSE for acceptance testing against a background of almost everyone having the equipment for a test that was called up in the majority of purchasing specifications, that was easy to carry out, and easy to pass. But the methods and pass-fail criteria came from an era where high-solids rosin fluxes and CFC solvent cleaning were standard, and these were no longer relevant. Present-day assemblies were vastly different in component density, complexity, materials of construction, and manufacturing methods.
He drew attention to IPC-TR-583 ‘An In-Depth Look at Ionic Cleanliness Testing’, published as long ago as 1993 but still applicable, which had concluded that ROSE tests tended not to be repeatable or reproducible, the "equivalency factors" were not valid and that the equipment should not be used for product acceptance but only for the process control purposes for which it was originally intended. An example he gave from his own production facility was when a sudden increase in the result of routine ROSE testing gave early warning of a failure in the works DI water plant. But in general, his comment was: "You may know something is making the extract solution conductive, but you don’t know what. Many modern contaminants won’t even make an ionic cleanliness tester twitch! How comfortable are you using reliability criteria developed in the 1970s? You need a tool that is much more specific on the soils on your product. ROSE is a broad bladed axe – ion chromatography is a surgeon's scalpel!"
To explain how ion chromatography worked, he began with one of his renowned graphic analogies: this time vegetable beef soup, consisting of macaroni letters, beef, carrots, tomatoes, potatoes, broth, salt and spices, and a magical strainer with which he could separate all the individual constituents and examine them. In this way, he could determine the main differences between Doug Pauls’ soup and Bob Willis’ soup. But even if they contained the same proportions of macaroni letters, beef, carrots, tomatoes and potatoes, Bob's soup tasted different to Doug’s soup, and Doug’s primary magical strainer was unable to separate the components of the broth, so he needed an even finer magical strainer to resolve the salt, pepper, other spices and Bob’s secret ingredients. (Edible flowers, Willis later admitted…)
Back to the reality of ionic residues on printed circuit assembles: the extract solution was essentially a chemical soup of residues that needed to be separated out into individual species for identification and quantification, and this was achieved in the ion chromatograph by a specialised fractionating column and conductivity detector. Different columns and eluents were used for anions and cations, and the equipment worked at very high pressure although the pipework was of capillary diameter so actual flow-rates were very small, typically 0.25 ml/minute. Using his own system as the example, Pauls explained the function and contribution of each major component. "It might look complicated, but if you appreciate how a domestic plumbing system works, you will have no difficulty in understanding the basic principles." The sample, typically 10 microlitres, was introduced, either manually or with an autosampler, into a sample loop, and a multi-position valve admitted it to the separation column via a guard column designed to protect the separation column from any accidental gross contamination - it could cost $2000 to replace - where it was absorbed on the ion-exchange medium.
Individual ionic species were more or less strongly absorbed so that when the eluent was pumped through the column at increasing concentration, they were progressively displaced at different rates so that they arrived at the detector at different time intervals and were displayed as individual peaks on the chromatogram. Pauls used potassium hydroxide as eluent for anions and methyl sulphonic acid for cations.
Although his equipment at Rockwell Collins was to research laboratory standards, and represented an investment of over $100,000, perfectly functional systems were available at much lower cost, as demonstrated by the small unit that Bob Willis had used in his IPC Apex workshop. Neither did the system require a qualified chemist to operate it: a technician could be straightforwardly trained by the equipment supplier, and IPC-WP-008: "Setting Up Ion Chromatography Capability" was a useful reference guide - "All the things the salesman never told you!"
Pauls described how the equipment was calibrated using traceable standards, typically for nine anions: bromide, chloride, fluoride, nitrate, nitrite, phosphate and sulphate, and six cations: ammonium, calcium, lithium, magnesium, potassium and sodium. He then showed examples of actual chromatograms and explained to interpret them - how to identify and quantify the species for which the system had been calibrated, and how to go about characterising any additional peaks which might appear - from particular flux activators for example. The chromatogram was actually a trace of conductivity versus time, and the integrated area under each peak could be calculated and converted into micrograms per unit area of surface extracted.
He went on to explain how to carry out the actual extraction, globally or locally, and what solvents to use to give the best analytical opportunity whilst minimising any risk of damage to the assembly? A typical global extraction involved sealing the sample assembly in a disposable plastic bag with a measured volume of solvent, generally 75/25 IPA/water, and immersing it in a water bath at 80ºC for 60 minutes. Several ingenious techniques had been devised for sampling specific local areas.
"So you've got some results - what do they mean?" he asked, "what's 'good' and what’s 'bad'?"
Predictably, his answer was "It depends... The days of the 'one size fits all' metric are gone. Present-day electronics are too varied in construction, materials and manufacturing methods for a single metric to apply, although that’s what everyone would like to have, so that no one has to think!"
For bare boards, IPC-5704 'Cleanliness Requirements for Unpopulated Printed Boards' had been in effect since 2009, and was being called out in OEM purchase orders even though many PCB fabricators were reluctant to be bound by it. But for assemblies there were not yet any universal standards, although equipment manufacturers and independent test laboratories had issued guidelines and recommended cleanliness limits, generally qualified by a statement along the lines of "Please note that the various residue levels shown in the table are only recommended starting points, and should not be construed as industry limits."
Pauls gave examples of typical recommended cleanliness limits and Rockwell’s own limits for bare boards, and post-assembly cleaned and no-clean assemblies, and explained how the values had been derived - generally from test assemblies exhibiting electrochemical failure mechanisms. There was no universal cleanliness value and every assembly configuration would have its own sensitivity, considering factors like component density, materials of construction, manufacturing processes, end use environment, design life of the product and the possible consequences of failure. "But if you don’t know what your cleanliness is, or what your cleanliness level should be, then these values are a good starting place until you can determine those metrics for your product.” He suggested IPC-5704 as a good starting point for bare board cleanliness, and the recommendations of Precision Analytical Laboratory and Foresite Inc for assemblies. And he cited IPC-9202 'Material and Process Characterization/Qualification Test Protocol for Assessing Electrochemical Performance', and IPC-9203 'Users Guide to IPC-9202 and the IPC-B-52 Standard Test Vehicle' as good guides to help in examining manufacturing materials and processing.
Ion chromatography was useful for many purposes, including cleaning material selection and qualification and process optimisation, no-clean process qualification, bare board cleanliness evaluation and vendor qualification, component cleanliness assessment and forensic trouble shooting, and was particularly valuable as a tool for what Pauls termed WITCH analysis (What Is This Crap Here?).
The most informative webinar I have attended for a long time! Doug Pauls commented: "Any one of the subtitles in the realm of 'cleanliness' could occupy a whole day's discussion" but in the space of an hour he managed to give his audience a clear and fluent explanation of the principles, techniques and applications of ion chromatography, illustrated where appropriate with meaningful analogy and interspersed with dry Iowa humour. And his presentation provoked a flood of questions seeking his opinions on specific scenarios and case histories, which he answered expertly and constructively until time ran out.