|Ask the Experts|
January 13, 2021
Cleaning with Sodium Bicarbonate
Is there an affect on PCBA long term reliability for assemblies cleaned with a "Sodium Bicarbonate" scrub, followed by DI water rinse, to remove tarnishing of silver immersion plating?
|Expert Panel Responses|
Cleaning with sodium carbonate is an option however it is very important to remove all ionic cleaning by-products after the wash cycle is completed.It is important to test after the final rinse the circuit card in question to see if all ionic residues are completely removed.
Unusually ionic chromatography is the preferred method. If ionics are not present after this test it indicates adequate removal of sodium and carbonates has occurred.If residual ionic contaminants remain, electrical reliability is the risk associated with this cleaning method since sodium bicarbonate is very conductive.
Senior Market Development Engineer
A lot depends upon what will be done to the silver after it is scrubbed. If the silver plated pads will be soldered AND all residue from the scrubbing agent have been completely removed, there should not be any reliability issues.
If the silver finish is used for something other than soldering (e.g. ZIF contacts, ACF connection), the scrubbing would most likely remove any anti-tarnish agents and could cause the silver to tarnish much more quickly.
Senior Applications Engineer
Flexible Circuit Technologies
That gets an "It Depends" kind of answer. Whether or not you have long term reliability issues depends on the thoroughness of your DI water rinse. High levels of residual sodium could result in leakage currents in humid conditions, but would probably not result in corrosion.
What soil are you trying to remove or dissolve with sodium bicarb and why did you choose bicarb as your cleaning media?
Principal Materials and Process Engineer
I am assuming you are relying on the abrasiveness of the sodium bicarbonate as fine scrub compound to remove the oxide and return the shine. Are these bare PCB's? Once again your question is vague so I am assuming they are bare, without mounted components.
As far as using it on a PCB, well this is an unusual request but I foresee no issues provided it is properly rinsed. Sodium bicarbonate or commonly called baking soda is a very safe substance, used in antacids and cooking. About a decade or more back one company found a way to incorporate it into an aqueous flux cleaner in lieu of a traditional saponifier.
Personally I would focus on preventing the tarnish. Why are you getting tarnish to begin with? Silver tarnishes quickly when exposed with hydrogen sulfide in the air. How old are these boards? After building the boards are they shipped and stored in airtight packaging? It is also common to have acid free tissue paper placed between each board to prevent tarnishing.
Another method instead of manual scrubbing and perhaps causing damage to the board surface is a simple experiment in the following link;http://scifun.chem.wisc.edu/homeexpts/TARNISH.htmlI would be curious to see how this works for you. Please keep me posted on your efforts.
Technical Expert Sales Support
Immersion silver is a temporary finish. Its usefulness lasts until the board is soldered. It was never intended to be a durable finish and therefore it should applied only on circuit conductors intended to be soldered.Marwan Rahal, Holaday Circuits, Inc.
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