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.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
With over 50 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.
Jim Hall
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.
Transcript
Phil
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.
Jim
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.
Phil
We would suspect that DI water probably is not quite hacking it.
Jim
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.
Phil
He may be talking about a bare board that they've stored too long and they're tarnished.
Jim
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.
Phil
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.
Jim
You want to get a product that's designed to be compatible with all your electronic materials.
Phil
Happy scrubbing and happy cleaning. And when you go to solder those boards, whatever you do -
Jim
Don't solder like my brother.
Phil
Don't solder like my brother.
Comments
I cant speak to rigid PCBs, but we have been sandblasting Flex Circuits and FFC for that fact with this stuff for 25 years for post (C02) laser cleaning (carbon removal). We then use a panel wash with high pressure to rinse these. We have never... NEVER had an issue with this process. My $0.02.
Jason Michaud, Miraco Inc.
10-15 years ago we recovery cleaned hundreds of PCBAs with immersion silver plating that was exposed to a high sulfur environment that exhibited heavy creep corrosion using a sodium bicarb solution. It was very effective at cleaning off the creep corrosion. We processed those assemblies through an inline rinse process at 1.0 FPM to be sure to remove all the sodium bicarb. Those units went back into the field for years with no known issues. So in my opinion sodium bicarb is a good option for some specific recovery cleaning projects.
I don't think it would be a good option for removing flux or other process residues. That requires being sure you can flush under components and as we all know that is very difficult when cleaning bottom terminated components like QFNs. The surface tension of the media/solution would be so high it would stuggle to flow under the parts.
Eric Camden, Foresite
Carbonates are used as builders in cleaners. A 5% solution of sodium bicarbonate will have a pH of ~ 8 - 9. Upon evaporation of water, some powder particles will be left on the surface. Therefore, it is important to thoroughly and completely rinse the carbonate from the surface. Ultrasonic cleaning and ultrasonic rinsing in a 2-stage counter-flow configuration works very well...as long as the components on the board can withstand the impact of the ultrasonic energy.
There are alternate, far more effective cleaners versus basic carbonate solutions. These cleaners most often use different buffering agents/builders to carbonates. For example, amines such as TEA can be used. It serves as a pH adjuster and emulsifier, aiding in the removal of oily and greasy soils. It is free rinsing. However, TEA can leach many metals including copper, aluminum, and cobalt. TEA does not readily attack silver or tin on its own. In most cleaning formulations containing TEA, azole chemistries are added to the formulation to protect against such attack/leaching anyway.
Our company has worked with companies such as Caterpillar to successfully clean populated circuit boards for their GPS and laser positioning systems for example. The cleaner used is a mildly alkaline cleaner, and is free-rinsing. Cleaning and rinsing was performed in ultrasonic tanks. I would NOT recommend the use of a 3% - 5% solution of hydrochloric acid to neutralize carbonate residues. HCl is a strong acid and will most certainly attack any silver and tin coatings.
If carbonates are used, use a weak organic acid such as citric, lactic or glycolic acid at a 0.5% - 1% concentration followed by thorough rinsing in high quality D.I. water. Blow dry the circuit board using convection hot air or bake the board in a convection lab oven at 50oC - 60oC for 1 hour to ensure that it is 100% dry.
Wade Rohland, Northern Technologies International Corporation
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.
Norman Berger
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.
