EIPC 2017 Winter Conference Review of Day 1

Estimated reading time: 13 minutes

There had been renewed interest in vapour phase reflow technology, particularly for lead-free soldering, where benefits such as enhanced wettability and temperature homogeneity with no risk of overheating could be realised. Emmanuelle Guéné from Inventec Performance Chemicals in France reported the results of a reliability study of no-clean chemistries for lead-free solder paste in vapour phase reflow.

The study set out to compare firstly the cleanability and secondly the chemical reliability of several solder paste residues after soldering in convection or vapour phase reflow.

For the cleanability experiments, four pastes were chosen, of which three were nominally no-clean. Test assemblies were subjected to aqueous detergent spray at different concentrations, co-solvent cleaning and mono-solvent cleaning. Although it had been expected that the vapour phase-reflowed samples would be more easily cleaned, no significant differences were observed, and it was concluded that the nature of the solder paste and the cleaning process have more influence on the results than the reflow method itself.

In the second part of the study, four no-clean pastes were subjected to SIR testing to IPC TM-650 method 2.6.3.3 Rev. A, and to the harsher BONO residue corrosivity assessment test. It was observed that SIR values were always slightly lower after vapour phase reflow, and that pastes that were close to the SIR limit when reflowed in convection might fail if reflowed in vapour phase. It was concluded that if high chemical reliability was desired, it was advisable not to use “aggressive” activators such as halogen, especially with vapor phase reflow. Regarding test methods, it was commented that because the BONO test is more drastic and more sensitive than standard SIR test, it is being used as a tool for solder paste development and capable of predicting the risk of solder pastes generating corrosion in harsh environments. 

A second presentation from Inventec Performance Chemicals was given by Patrick Duchi, on optimisation of chemistry for a vapour phase de-fluxing process for no-clean lead-free materials.

Reviewing trends in electronics assembly, he made it clear that miniaturization, together with lead-free soldering and the use of no-clean fluxes was having a direct impact on the cleanability of PCB assemblies. And one of critical consequences of miniaturisation, so far as it affected cleaning, was the smaller standoff under components. Legislation had restricted the choice of available cleaning products, but there was still a range of products available, including those based on detergents and surfactants, petroleum distillates, formulated hydrocarbons, terpenes, glycols, hydrofluorocarbons and hydrofluoroethers.

He discussed the significance of contact angle, surface tension and viscosity, and the concept of solvency power as measured by the KB (Kauri-Butanol) index, and listed typical compositions of flux residues, then presented the results of a series of case studies designed to compare the performance of a range of cleaning formulations across a range of equipment types, measured by visual inspection and ionic contamination testing.

The results indicated that the cleaning process should include mechanical agitation, and that ultrasonics had not been observed to cause damage to quartz components. In terms of performance, a segregated co-solvent process using a hydrofluoroether azeotrope had shown particularly good results, especially with immersed jet agitation, in removing all types of flux residue, and in terms of comparative cleaning costs, the segregated co-solvent process was also the most economical. 

The final presentation of the first day was given by Professor Karl Ryder from the University of Leicester in the UK, presenting the findings of the MACFEST project. It was interesting to compare the content of this presentation with the earlier one from MacDermid, since electroless nickel corrosion effects in the ENIG process were of relevance in both. Whereas the MacDermid development had centred on the optimisation of traditional aqueous chemistry, the MACFEST project had approached the problem from a different angle, using ionic liquid chemistry as the basis of the immersion deposition process. Professor Ryder explained the characteristics of ionic liquids and described a specific class known as deep eutectic solvents, which were readily and cost-effectively available. The example used in the MACFEST project was a mixture of choline chloride and ethylene glycol, which had unusual solvation properties for metal salts and had been demonstrated in several metal finishing applications to modify the electrochemical characteristics of metal ions in solution.

The MACFEST project had set out to develop a “universal surface finish” for PCBs, wire-bondable as well as solderable, using a conventional proprietary electroless nickel as a base on which to deposit immersion palladium followed by immersion gold, both from ionic liquid chemistry. The “ENIPIG” finish had been independently tested for solderability and had shown excellent results, comparable with existing finishes. Moreover, focused ion beam cross-sectioning had shown no evidence of the grain boundary corrosion on electroless nickel characteristically associated with aqueous deposition. 

A long conference day was rounded off with a visit to the spectacular Red Bull Hangar-7, owned by Red Bull founder Dietrich Mateschitz, with its collection of historical aeroplanes, helicopters and Formula One racing cars, followed by a conference dinner at the M32 restaurant, high above the Mönchsberg museum of modern art, with panoramic views over the city of Salzburg.

I am grateful to Alun Morgan for allowing me to use his photographs.

To read "EIPC 2017 Winter Conference Review of Day 2", click here.

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