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I have a board that has fairly high working voltages up to 100V with small clearances down to 12 mil between the high voltage and ground or low voltage nets. This is unavoidable because the whole board is tiny and in some cases the HV and low voltage are adjacent pads on the same component.

This is currently assembled using a water wash cleaning step.

Is it possible to assemble this using no-clean? What are the possible problems with having flux residue on a high voltage board like this? Is it better to clean (with the possible ionic contamination from the detergents) or not clean (with flux residue)? Could I do conformal coating over flux residue?

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    \$\begingroup\$ To the best of my knowledge conformal coating requires very clean surfaces for proper adhesion. At least in our lab all boards are cleaned using a complex cleaning procedure which has a rinsing with deionized water including conductivity monitoring as the final step. For 100V a track clearance of at least 0,16 mm is recommended according to my information so 12 mil = 0,3048 mm should be fine. From my experience even the "no clean" can leave some serious amount of residue which can impact electrical signal quality. At least it happend to me in HF applications with 0.4mm BGA ICs. \$\endgroup\$ – Christian B. Jun 2 at 6:36
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    \$\begingroup\$ @ChristianB. - that looks like most of an answer. Could you add some details and make an answer of it? \$\endgroup\$ – JRE Jun 2 at 8:40
  • \$\begingroup\$ @JRE done. I am never sure if something "qualifies" as an answer and when to post it as comment... \$\endgroup\$ – Christian B. Jun 2 at 10:18
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    \$\begingroup\$ @ChristianB.: If it answers the question, it is an answer. If it asks for more information, or explains how to get needed information it is a comment. \$\endgroup\$ – JRE Jun 2 at 10:20
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To the best of my knowledge conformal coating requires very clean surfaces for proper adhesion (see doi: 10.1088/1741-2560/10/3/031002 ). At least in our lab all boards are cleaned using a complex cleaning procedure which has a final "rinsing with deionized water including conductivity monitoring" step (doi: 10.1109/EMBC.2016.7591102 ). For 100V a track clearance of at least 0.2 mm is recommended according to my information so 12 mil = 0.3048 mm should be fine ( http://www.creepage.com/ ). From my experience even the "no clean" (I typically use SMD291 and SMD291AX) can leave some serious amount of residue depending on the used amount which can impact electrical signal quality. At least I observed signal degeneration in HF applications with 0.4 mm BGA ICs. After additional cleaning steps with deflux etc the signals typically normalize.

residuals after using SMD291 and only sloppy mechanical cleaning: residuals after using SMD291 and only fast mechanical cleaning

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    \$\begingroup\$ Flux residues (and they exist for all types as you note, including 'no-clean' are a major issue in high speed and very dense circuits as well. See kester.com/Portals/0/Documents/Knowledge%20Base/Publications/… for an excellent summary of issues. \$\endgroup\$ – Peter Smith Jun 2 at 13:36
  • \$\begingroup\$ Very interesting publication! I especially like the general overview but the presentation of data is suboptimal. e.g. I can just guess that the y axis in the plots are something like resistance in ohm(?). Many information regarding the test structure dimensions etc are lacking, which renders reproducting and understanding almost impossible and almost all pictures are without a proper scale. This is really a pitty considering the relevance of this topic. \$\endgroup\$ – Christian B. Jun 2 at 18:34

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