|Ask the Experts|
August 26, 2020
Contamination Using Solvent Dispensers
After solder touch-up we use small bench-top solvent dispensers with acid brushes for local spot cleaning. Are we dragging contamination back into the solvent dispenser each time we replenish solvent on the acid brush?
When we need to refill our solvent dispensers should we first clean them to ensure they are free of contamination? What process do you recommend for local spot cleaning after touch-up?
|Expert Panel Responses|
Unless you have a check valve behind the dispenser brush, chances are high for potentially drawing a contaminant back into the container. It's simple enough to verify and replace if necessary.
A quick rinse & dump with a little fresh solvent at each refill never hurts either. Not being familiar with your specific process, I'd suggest contacting the manufacturer of the soil to be removed (flux etc.) for tips.
The first point you must understand is that you are not "actually" cleaning the boards when you use an acid brush and a small solvent bottle.
You are, in fact, only spreading the contamination around on the board. This is because while the solvent evaporates, the contamination does not. So, say you have 1 gram of flux residue on a section of a CCA that was recently soldered. You take your brush, dip it into the solvent (or the springy top of a Menda bottle) and brush around the soldered areas.
The solvent dilutes the flux on the CCA, and spreads it around on the surface and some of it on your brush. Then the solvent evaporates leaving behind the flux residue both on the board and on your brush. Its a common fallacy to believe that the contamination evaporates with the solvent; it simply does not.
Depending on the level of reliability you require for your CCAs, will steer you to different types of cleaning processes. Vapor Defluxing (hot vapor condensing on the surface actually carries away the flux), or water-based cleaning (wash, rinse, dry), or immersion into one, two, or three solvent tanks (brush, dunk, dunk).
So, the fact is that you are NOT cleaning your CCAs, it is only an intermediate step, and if cleaning is your goal, you need to be talking to one of us "experts" to help you figure that out. You know where to find me!
Spot cleaning is always a reliability issue if not done correctly. It is recommended to avoid smearing flux residues across the assembly since this causes additional contamination.
A complete rinse after cleaning and testing for ionic residues is always the safest. The cleaning solution should be kept clean at all times.
Using the correct solvent is also important, some solvents are best for one flux type and another works best with other types.
Senior Market Development Engineer
In response to the question about "small bench-top solvent dispensers with acid brushes" for touch-up cleaning, I'd like to offer a a couple of suggestions for this process which, as we call it, is "dip-and-brush" cleaning:
A. First and foremost, yes, you are re-contaminating the solvent when you did a dirty brush back into the pump dispenser. You probably have seen this already as the fluid in the "clean" fluid in the pump bottle gradually darkens in use. This is because the solvent dissolves the flux residues on the brush and re-deposits them on the top of the pump bottle.
The pump bottle valve gradually clogs with flux residues and fairly quickly fails to re-seal. This allows the dirty solvent on the top of the pump bottle to slip back into the bottle itself. This means you are trying to clean your PCBs with dirty solvent, which is never a good idea.
B. You can test this your self quite easily. Take a bottle that is filled with alcohol and that has been used to clean a number of PCBs. Using a felt-tipped pen or white board marker, press down on the dispenser of the pump bottle. Quite quickly you will see a thin "thread" of colored ink spread across the top of pump bottle dispenser, and then gradually filter down into the cleaning fluid, coloring it.
Clearly, if ink can get into your "clean" solvent, then flux residues can, too. One remedial strategy would be to frequently empty the solvent dispensers of dirty solvent, reclean the bottles thoroughly, plus re-clean the interior of the pump-bottle valve. Unhappily, this is very hard to do. Quite quickly you're back to cleaning with dirty solvent.
C. Another aspect of this style of cleaning is that it is extremely hard to rinse the contamination from PCBs. There are four steps to cleaning anything: wet, scrub, rinse and dry. With pump bottles and acid brushes, you can wet the PCB, and you can scrub, but it's almost impossible to rinse. This means you move the flux residues around on the board,but you usually do not remove them. They linger behind, ready to cause corrosion and dendrites and other headaches. Quality rinsing is essential to quality cleaning.
In December 2007, there was a very interesting technical note in Circuits Assembly Magazine by Mr. Terry Munson,"The Process Doctor." Using SIR testing, the most advanced form of board cleanliness testing, Mr. Munson measured the contamination left on the boards after dip-and-brush cleaning. He concluded that rarely, if ever, would dip-and-brush cleaning achieve satisfactory results.
D. A better answer would be to stop using pump bottles all together.Alcohol is not a particularly good cleaner, and dirty alcohol is simply a waste of your time. Scrubbing without rinsing is another exercise in futility. I would suggest migrating to aerosol benchtop cleaning.Aerosol cleaners are pure, fresh, uncontaminated cleaners; there are a wide variety of chemical choices - better choices than alcohol - and they are from many different vendors so you can get good, competitive prices.
The three main providers of aerosol cleaners in the USA are Chemtronics, TechSpray, andMicroCare. All are available from most of the better distributors, such as Stanley Suppy, Techni-Tool, EIS, HISCO, and regional distributors as well. The downside of aerosol cleaners is that they waste a lot of solvent when rinsing, which costs money.
E. The best answer of all (and forgive me for tooting my company's horn a bit) would be to switch to the TriggerGrip dispensing system from MicroCare. Only MicroCare has anything like the TriggerGrip tool. This tool fits on top of the aerosol can and amplifies the cleaning power of whichever cleaning fluid you select with the mechanical scrubbing action of the brush on the dispenser. The TriggerGrip also minimizes liquid waste and makes it easy to rinse the PCB.
Typically, when compared to normal aerosol cans, the TriggerGrip system makes aerosol cans last twice as long, which can be quite a savings. In addition, MicroCare has documented test results showing the TriggerGrip dispenser gets PCBs as clean as long,expensive, high-powered aqueous cleaning systems, which is quite a success for an inexpensive bench top cleaning tool.
For more details about dip-and-brush cleaning, see:
What type of solvent dispenser are you using? There are products on the market that prevent the solvent from being returned to the dispensing bottle. The material remains in a"well" that prevents the liquid from returning to the reservoir.
You also need to consider that the application of the solvent spreads the dissolved flux to all neighboring components. My concern would be spreading the surface contaminate on the PCB while using the acid brush.
I also think that a concern would be that the acid brush itself is a source of"spreading" contamination.
This is a bigger concern than the "possible" contamination of the dispensing bottle.Best to clean the entire board via your cleaning system to provide a complete removal of contamination.
Most likely you are dragging contamination back into the solvent dispenser, and I would highly recommend cleaning the dispenser prior to refilling it.
Alternative way is to utilize rework / repair defluxers that come in aerosol can and its own brush tip. Please contact me for product recommendation and sample to try it out. I would gladly assist you.
Application Technology Manager
Brush cleaning caused me to use the term "Contamination Relocation" because that is what happens more times than not. I have written and presented several papers on this topic at both IPC and SMTA conferences in the past. You can search for "Effective Spot Cleaning Regardless of Material Selection" and find some useful information on this exact topic. The main takeaway from that paper is to understand rinsing is as important as cleaning.
You can effectively clean the area of concern but if you aren't careful you will simply relocate the contamination to neighboring components. This is especially easy when using IPA as it will yield a very low surface tension and the residue will flow under SMT components and eventually can lead to electrical leakage and/or electrochemical migration related failures in the field.
To answer your original question you are most definitely transferring contaminants to your dispensing bottle, and your acid brush will also hold high levels of flux activators as the carrier is evaporated.
One thing most everyone neglects to consider when brush cleaning, and it may be the greatest source of harmful contaminants. I'm talking about the acid brush itself.Jerry Wiatrowski, General Dynamics
Many are still made of horse (+ unknown) hair, imagine all the organic debris and fluids that could be in that hair - ewwwww. So, if you don't consider the brush, you could be missing an important source of contamination.
Yes, your solvent is contaminated every time when you put the brush back to the bottle, which will cause additional contamination to the board when the solvent is used again. However, a more critical issue is you are not actually cleaning the board when you use solvent to "clean" the local spot, only relocating the ionic contaminants - unless the board is rinsed with clean solvent afterward.
If no-clean flux is used in the reflow process or rework, using organic solvent to clean the reworked spot will expose more ionic species to the board. These ionic species were originally protected by hydrophobic solid chemicals such as rosin or resin in the flux formulation.
To address the last two questions: yes, you should clean the solvent dispenser before refill. To avoid relocating the contaminants, it would be better to clean the whole board and rinse it with clean solvent.
Director New Product Development
Metallic Resources, Inc
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