Is there an effect on PCBA long-term reliability when cleaned with a sodium bicarbonate scrub followed by DI water rinse? Jim Hall and Phil Zarrow, The Assembly Brothers, discuss this secnario and share their own experiences. Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
He has vast experience in SMT equipment, materials and processes.
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics.
He is a pioneer in the science of reflow.
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, coming to you from high atop Mount Rialto here in New Hampshire.
Our question today is about cleaning. It comes from L.P. "Is there an effect 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?
I assume that you have checked the compatibility of all the materials on your circuit board, your components, compatibility with the sodium bicarbonate and your cleaning operation. But most important is that sodium bicarbonate to the best of my knowledge is a caustic solution, as many traditional cleaners have been, and you absolutely need to get it all off the board when you're done.
We would suspect that DI water probably is not quite hacking it.
It depends upon the components on your board, the clearances and so forth and the mechanical action during this "scrub," whether it's forcing sodium bicarbonate under low clearance parts such as a QFNs.
If you leave sodium bicarbonate on your board, you are definitely taking a risk.
He may be talking about a bare board that they've stored too long and they're tarnished.
He's talking about an assembly so I interpret that as having soldered components on the printed circuit board, or a printed wiring board if we want to be more descriptive.
Immersion silver finish has been around for a while now. And it's becoming really popular. This is a common problem. I would check with some of the industry cleaning companies.
You want to get a product that's designed to be compatible with all your electronic materials.
Happy scrubbing and happy cleaning. And when you go to solder those boards, whatever you do -
Don't solder like my brother.
Don't solder like my brother.
Imm Ag PCB have other problems. Ag is highly reactive. In environments where there is high amounts of sulfur in the air due to pollution...think Mexico City or some places in China or India, the sulfur will react with the PCB finish. The results can be ugly.
I notice that the picture associated with the article is of a box of washing soda, which is sodium carbonate, not sodium bicarbonate. We use sodium bicarbonate (baking soda) routinely in our cooking, but not washing soda so much.
Wayne Miller, Wayne Miller Associates
Sodium bicarbonate is frowned upon as a cleaning method for flux removal or for other soils requiring a neutral pH solvent. Potassium bicarbonate, on the other hand, is the main ingredient in many saponifiers such as Armakleen 2000, for example. They clean extremely well and render a pH-neutral solution when mixed with clean DI water at about a 6 to 10% ratio. The downside however, is that ANY bicarbonate-bearing product is not completely soluble, so you will deal with bicarbonate build-up on all of the surfaces of the cleaning equipment, including pump seals, bearings, etc., that are almost impossible to clean off.
Richard Stadem, General Dynamics Mission Systems
Sodium bicarbonate can act as a mild abrasive but it it may enter the surface of whatever you are scrubbing (the same as any abrasive), leaving a possibility of problems. I would say it is imperative to give a mild acid rinse (say 3-5% hydrochloric acid) at room temperature for 5 minutes BEFORE at least 2 good DI rinses. This will decompose any poorly-soluble weak ionic residues into very soluble ones, which will dissolve in the rinses.