Doug Pauls Explains Ion Chromatography

Estimated reading time: 6 minutes

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.

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