Cleanliness Assessment for Class III Lead-Free No-Clean Assemblies
This study qualifies an aqueous cleaning process capable of removing combinations of no-clean flux residues for Class III electronic assemblies.
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Authored By:
Umut Tosun, M.S.Chem.Eng., Ravi Parthasarathy, M.S.Chem.Eng., Michael McCutchen, M.S.Chem.
ZESTRON America, Manassas, VA USA
Summary
For mission critical electronics or Class III products, such as those used within the military, aerospace and medical industries, highest electronic reliability is a requirement as failure is not an option. Within the electronics industry, this means that residues, either ionic or non-ionic, must be fully removed. Partially removed or untouched residues can lead to component and product failures resulting from electrochemical migration, dendrite growth and electrical leakage currents.
The goal of this study was to identify and qualify an aqueous cleaning process capable of removing combinations of no-clean flux residues for Class III electronic assemblies. Teamed with a global electronic manufacturing service (EMS) provider supplying electronics to the aerospace and medical industry, the Design of Experiment (DOE) developed was executed in two phases. Initial testing was completed utilizing EMS boards and final testing was validated using IPC test coupons and standards.
Conclusions
When building Class III products, it is essential to thoroughly understand the cleaning process and be assured that it has been optimized based on board design, cleaning equipment type, cleaning agent selected, operating parameters used, paste and flux types used and residues generated. The OEM for which this DOE was developed understood the importance of the cleaning process and sought empirical data to design and verify the cleaning process.
Using the EMS's boards and paste and flux details, a DOE was developed enabling ZESTRON to select the most suitable cleaning agent and implement a testing program with cleaning equipment similar to that used by the EMS. Cleaning equipment parameters were optimized and excellent cleaning results were obtained as indicated by visual inspection and ionic contamination analyses. The cleaning process efficiency was verified using EMS boards through Ion Chromatography and SEC analyses. Finally, additional verification regarding the effectiveness of the cleaning process as obtained through the use of SIR and ECM analyses employing IPC test coupons.
Once both the OEM and EMC were assured that the recommended cleaning process met their Class III cleanliness requirements, material and component compatibility testing was conducted. Critical board components were found to be completely compatible; however, the label supplier was required to make a top coat change on one of the two labels used.
Based on the results of this DOE, the cleaning process was implemented at the EMS site and cleaning results were subsequently validated as meeting the OEM cleanliness specifications.
Initially Published in the IPC Proceedings
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