Ask the Experts
October 9, 2018
Cleaning with Sodium Bicarbonate
Is there an affect on PCBA long termreliability 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 veryimportant to remove all ionic cleaning by-products after the wash cycle iscompleted.It is important to test after the final rinse the circuit cardin 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 adequateremoval of sodium and carbonates has occurred.If residual ionic contaminants remain, electrical reliability isthe risk associated with this cleaning method since sodium bicarbonate is veryconductive.
Senior Market Development Engineer
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
Alot depends upon what will be done to the silver after it is scrubbed. Ifthe silver plated pads will be soldered AND all residue from the scrubbingagent have been completely removed, there should not be any reliabilityissues. If the silver finish is used for something other than soldering(e.g. ZIF contacts, ACF connection), the scrubbing would most likely removeany anti-tarnish agents and could cause the silver to tarnish much morequickly.
Senior Applications Engineer
Flexible Circuit Technologies
Mark Finstad has over 30 years in the flex circuit industry in both design and manufacturing. He is a regular speaker at IPC APEX (Professional development courses) and PCB West (flex circuit design courses). He is also vice chair of IPC-2223 and active member of IPC-6013. Finstad has extensive experience with both domestic and off-shore manufacturing.
That gets an "ItDepends" kind of answer. Whether or not you have long term reliabilityissues depends on the thoroughness of your DI water rinse. High levels ofresidual sodium could result in leakage currents in humid conditions, but wouldprobably not result in corrosion. What soil are you trying to remove ordissolve with sodium bicarb and why did you choose bicarb as your cleaningmedia?
Principal Materials and Process Engineer
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
I am assuming you are relying on the abrasiveness of thesodium bicarbonate as fine scrub compound to remove the oxide and return theshine. Are these bare PCB's? Once again your question is vague so I am assumingthey are bare, without mounted components. As far as using it on a PCB, well this is an unusual request butI foresee no issues provided it is properly rinsed. Sodium bicarbonate orcommonly called baking soda is a very safe substance, used in antacids andcooking. About a decade or more back one company found a way to incorporate itinto an aqueous flux cleaner in lieu of a traditional saponifier.
Personally I would focus on preventing the tarnish. Why are yougetting tarnish to begin with? Silver tarnishes quickly when exposed withhydrogen sulfide in the air. How old are these boards? After building the boards are theyshipped and stored in airtight packaging? It is also common to have acid freetissue paper placed between each board to prevent tarnishing.
Another method instead of manual scrubbing and perhaps causingdamage 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 postedon your efforts.
Technical Expert Sales Support
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.