Ask the Experts
January 11, 2019
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
Unlessyou have a check valve behind the dispenser brush, chances are high forpotentially drawing a contaminant back into the container. It's simpleenough to verify and replace if necessary. Aquick rinse & dump with a little fresh solvent at each refill never hurtseither. Not being familiarwith your specific process, I'd suggest contacting the manufacturer of the soilto be removed (flux etc.) for tips.
Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.
The first point you must understand is that you are not"actually" cleaning the boards when you use an acid brush and a smallsolvent bottle.
You are, in fact, only spreading the contamination around on theboard. This is because while the solvent evaporates, thecontamination does not. So, say you have 1 gram of flux residue ona 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 aroundthe soldered areas.
The solvent dilutes the flux on the CCA, andspreads it around on the surface and some of it on your brush. Thenthe solvent evaporates leaving behind the flux residue both on the board and onyour brush. Its a common fallacy to believe that thecontamination evaporates with the solvent; it simply does not.
Depending on the level of reliability you require for your CCAs, will steer youto different types of cleaning processes. Vapor Defluxing (hotvapor condensing on the surface actually carries away the flux), or water-basedcleaning (wash, rinse, dry), or immersion into one, two, or three solvent tanks(brush, dunk, dunk).
So, the fact is that you are NOTcleaning your CCAs, it is only an intermediate step, and if cleaning is yourgoal, you need to be talking to one of us "experts" to help youfigure that out. You know where to find me!
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
Spotcleaning is always a reliability issue if not done correctly. It is recommendedto avoid smearing flux residues across the assembly since this causesadditional contamination. A complete rinse after cleaning and testing for ionicresidues is always the safest. The cleaning solution should be kept clean atall times. Using the correct solvent is also important, some solvents are bestfor one flux type and another works best with other types.
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.
In response to the question about "small bench-topsolvent dispensers with acid brushes" for touch-up cleaning, I'd liketo 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 thesolvent when you did a dirty brush back into the pump dispenser. You probablyhave seen this already as the fluid in the "clean" fluid in the pumpbottle gradually darkens in use. This is because the solvent dissolves the fluxresidues on the brush and re-deposits them on the top of the pump bottle.
Thepump bottle valve gradually clogs with flux residues and fairlyquickly fails to re-seal. This allows the dirty solvent on the top of thepump bottle to slip back into the bottle itself. This means you are trying toclean your PCBs with dirty solvent, which is never a good idea.
B. You can test this your self quite easily. Take a bottlethat 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 ofthe pump bottle. Quite quickly you will see a thin "thread" ofcolored ink spread across the top of pump bottle dispenser, and thengradually filter down into the cleaning fluid, coloring it.
Clearly, if ink canget into your "clean" solvent, then flux residues can, too. Oneremedial strategy would be to frequently empty the solvent dispensers of dirtysolvent, reclean the bottles thoroughly, plus re-clean the interior of thepump-bottle valve. Unhappily, this is very hard to do. Quite quickly you'reback to cleaning with dirty solvent.
C. Another aspect of this style of cleaning is that it isextremely hard to rinse the contamination from PCBs. There are four steps tocleaning anything: wet, scrub, rinse and dry. With pump bottles and acidbrushes, you can wet the PCB, and you can scrub, but it's almost impossible torinse. This means you move the flux residues around on the board,but you usually do not remove them. They linger behind, ready tocause corrosion and dendrites and other headaches. Quality rinsing is essentialto quality cleaning.
In December 2007, there was a very interestingtechnical note in Circuits Assembly Magazine by Mr. Terry Munson,"The Process Doctor." Using SIR testing, the most advanced formof board cleanliness testing, Mr. Munson measured the contamination lefton 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 wasteof your time. Scrubbing without rinsing is another exercise infutility. I would suggest migrating to aerosol benchtop cleaning.Aerosol cleaners are pure, fresh, uncontaminated cleaners; there are a widevariety of chemical choices - better choices than alcohol - and they are frommany different vendors so you can get good, competitive prices.
The three mainproviders of aerosol cleaners in the USA are Chemtronics, TechSpray, andMicroCare. All are available from most of the better distributors, such asStanley Suppy, Techni-Tool, EIS, HISCO, and regional distributors as well. The downside of aerosol cleaners is that they waste a lot of solvent whenrinsing, which costs money.
E. The best answer of all (and forgive me fortooting my company's horn a bit) would be to switch to theTriggerGrip dispensing system from MicroCare. Only MicroCare has anything likethe TriggerGrip tool. This tool fits on top of the aerosol can and amplifiesthe cleaning power of whichever cleaning fluid you select with the mechanicalscrubbing action of the brush on the dispenser. The TriggerGrip also minimizesliquid waste and makes it easy to rinse the PCB.
Typically, when comparedto normal aerosol cans, the TriggerGrip system makes aerosol cans last twiceas long, which can be quite a savings. In addition, MicroCare has documentedtest results showing the TriggerGrip dispenser gets PCBs as clean as long,expensive, high-powered aqueous cleaning systems, which is quite a success foran inexpensive benchtop cleaning tool.
For more detailsabout dip-and-brush cleaning, see:
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.
What type of solventdispenser are you using? There are products on the market that prevent thesolvent 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 considerthat the application of the solvent spreads the dissolved flux to allneighboring components. My concern would be spreading the surface contaminateon the PCB while using the acid brush.
I also think that aconcern would be that the acid brush itself is a source of"spreading" contamination.
This is a bigger concernthan the "possible" contamination of the dispensing bottle.Best to clean the entire board via your cleaningsystem to provide a complete removal of contamination.
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
Most likely you aredragging contamination back into the solvent dispenser, and I would highlyrecommend cleaning the dispenser prior to refilling it. Alternative way is toutilize rework / repair defluxers that come in aerosol can and its own brushtip. Please contact me for product recommendation and sample to try it out. Iwould gladly assist you.
Application Technology Manager
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
Brush cleaning caused me to use the term"Contamination Relocation" because that is what happens more timesthan not. I have written and presented several papers on this topic at both IPCand SMTA conferences in the past. You can seach for "Effective SpotCleaning Regardless of Material Selection" and find some usefulinformation on this exact topic. The main takeaway from that paper is tounderstand rinsing is as important as cleaning.
You can effectively clean thearea of concern but if you aren't careful you will simply relocate thecontamination to neighboring components. This is especially easy when using IPAas it will yield a very low surface tension and the residue will flow under SMTcomponents and eventually can lead to electrical leakage and/or electrochemicalmigration related failures in the field.
To answer your original question you are most definitelytransferring contaminants to your dispensing bottle, and your acid brush will alsohold high levels of flux activators as the carrier is evaporated.
Eric has been in the electronics industry for over 14 years and manages the C3 technical user group, Failure Analysis project management, Rescue Cleaning Division and is one of three Lead Investigators at Foresite.
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. 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.
Jerry Wiatrowski, General Dynamics
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
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.