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Compatibility and Aging for Flux and Cleaner Combinations



Compatibility and Aging for Flux and Cleaner Combinations
A materials study for high reliability electronics cleaning is presented in this paper.
Analysis Lab

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Authored By:


Kim M. Archuleta and Rochelle Piatt
Sandia National Laboratories
Albuquerque, New Mexico

Summary


A materials study of high reliability electronics cleaning is presented here. In Phase 1, mixed type substrates underwent a condensed contaminants application to view a worst-case scenario for unremoved flux with cleaning agent residue for parts in a silicone oil filled environment. In Phase 2, fluxes applied to copper coupons and to printed wiring boards underwent gentle cleaning then accelerated aging in air at 65% humidity and 30 OC. Both sets were aged for 4 weeks. Contaminants were no-clean (ORL0), water soluble (ORH1 liquid and ORH0 paste), and rosin (RMA; ROL0) fluxes.

Defluxing agents were water, solvents, and engineered aqueous defluxers. In the first phase, coupons had flux applied and heated, then were placed in vials of oil with a small amount of cleaning agent and additional coupons. In the second phase, pairs of copper coupons and PWB were hand soldered by application of each flux, using tin-lead solder in a strip across the coupon or a set of test components on the PWB. One of each pair was cleaned in each cleaning agent, the first with a typical clean, and the second with a brief clean. Ionic contamination residue was measured before accelerated aging.

After aging, substrates were removed and a visual record of coupon damage made, from which a subjective rank was applied for comparison between the various flux and defluxer combinations; more corrosion equated to higher rank. The ORH1 water soluble flux resulted in the highest ranking in both phases, the RMA flux the least. For the first phase, in which flux and defluxer remained on coupons, the aqueous defluxers led to worse corrosion. The vapor phase cleaning agents resulted in the highest ranking in the second phase, in which there was no physical cleaning. Further study of cleaning and rinsing parameters will be required.

Conclusions


Conclusions of this initial study resulted in the recommendation of no application of the ORH1 flux when possible, and if the higher activity is required for function, additional testing in cleaning process development will determine the extra rigor required to ensure a good clean of flux residue. The use of aqueous cleaning agents looks promising for assemblies that can be wetted, but further investigation into proper rinsing will be needed. For assemblies that require a solvent or vapor clean, supplementary testing is recommended. And lastly, the vapor defluxer V3 is recommended to no longer be considered for program and will be removed from further testing.

The path forward for continuation of assessment of fluxes and defluxers to use in high reliability applications includes testing for cleaning efficacy with physical cleaning on test vehicles with BGA, QFN and miniature components. An important aspect of this will be evaluation of rinse capability for various defluxers. Another vital factor is the ability to determine cleanliness level validation methods for small amounts of residue well concealed under components.

New test methods are desired, and validation of cleanliness testing methods must be corroborated with destructive testing for direct evidence of results. Possibilities include power spray and/or extended time, or a series of solvent solutions to facilitate removal of more residues, for residue extraction used in resistivity of solvent extract or ion chromatography test methods.

Initially Published in the IPC Proceedings

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