Ask the Experts
February 12, 2018
Un-cleaned PCB Assemblies Potted
We received assembled PCBs that were soldered with a washable flux vs. a no-clean flux. The flux was not removed and the boards were potted with an epoxy based resin and then mounted inside an enclosure.
What difficulties do you think we will have?
Expert Panel Responses
Aqueous-cleansolder chemistries must be cleaned to avoid corrosion even in the case ofpotted PCAs. The presence of flux residue will impede proper adhesion ofthe potting compound to the PCA thus creating a miniscule gap between the boardsurface and the potting compound; a potential conduit for corrosion. Corrosion will occur with time especially between any twooppositely charged conductors on the board surface.
Corrosion productswill result in electrical shorting eventually. The encapsulant may retardingress of water but eventually corrosion will occur. Aqueous solderchemistries are notorious for adsorbing water from the atmosphere.Adsorption will begin prior to encapsulation. Note that corrosion fromsolder chemistry can occur beneath solder mask on occasion too. Corrosiongeneration can be confirmed with an abbreviated heat and humidity test andcircuit verification or direct observation for corrosive products (may bedifficult with presence of potting compound).
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
There are many variablesassociated with your question. For example, was this "washableflux" a water-based OA type of flux or was it a rosin-based R type offlux? Also, what is the expected reliability level of thecomponents? Does failure of the board risk human life? Or, doesfailure of the board mean that someone's toaster or hair-dryer will no longerfunction.
There is a high likelihood that these boards will fail, sooner than later, andthe decision to scrap the boards needs to be seriously considered. The OAflux is highly acidic and the rosin-based products could lead to much fasterthan normal dendritic growth and shorts... depends on humidity and temperatureand current.
If the application is a low reliability application with a product that wouldpose no risk if it fails, then so be it. All that is at sake is yourcompany's reputation. If you want to ensure the product will meetthe designed reliability life and maintain your company's reputation I wouldeither scrap or rework the boards. If it is a system that couldrisk human-life if it fails, there is no doubt that the boards must bescrapped.
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.
The key to answering this question would be to know exactly whatflux was used. The ionic activity of the flux is what you want tounderstand. A highly ionic flux may have long term effects ofpotential corrosion. Most rosin based fluxes are typically inert aftersoldering, and should not cause a problem. The other concern would be ifthe epoxy potting adhesion was affected by the flux residue. Are theboards truly encapsulated or not?
I would recommend amoisture and insulation resistance test of a sample unit to gain confidence ofthe assembly.
Minco Products Inc
Nick has been in the circuit industry since 1985, in technical and management roles. Nick is the Vice Chair of the IPC Flexible Circuits Committee, and Chair of IPC 6013 Flexible Circuits Perfomance Subcommittee. Nick holds a Bachelors Degree in Mechanical Engineering and a Masters Degree in the Management of Technology from the University of Minnesota.
There are two very different possible scenarios here:
In the first case, you have a rather disastrous reliabilityoutlook. The flux is highly ionic and will inhibit any bond between the pottingcompound and the board. When power is applied, it is almost certain that a conductivepath will be formed, with the result being corrosion and raid failure of theassembly. If a water-soluble flux was used, the assemblies are not useable inany scenario I can imagine.In the second case, if the flux is relativelybenign, like an RMA with "zero halides", and if it has beenwell-oxidized so that the residue is a hard resin, the outlook may be muchbetter. In fact, some RMA residues can, under good conditions, be as benign asa no-clean.
There is another problem, and that is knowing whether thespecific flux used will inhibit cure and/or adhesion of the potting compound.This could be tested directly compared to your specified flux.Ask the assembler which flux specifically was used; if it was awater-soluble, do not use the assemblies. If it was rosin-based and a no-halideRMA (not an RA) then:
- The flux used was a water-soluble flux
- The flux used was a rosin-based flux such as an RMA
Ultimately, yourdecision as to whether to use or scrap the assemblies if an RMA was used willrest on the result of a risk assessment based on the technical data you able toobtain, and the reliability you require.
- Contact the flux manufacturer and ask if they have test data onSIR when the residues are left on
- Contact the potting compound manufacturer and ask aboutcompatibility with the specific flux chemistry; do they expect problems withadhesion or inhibition of cure?
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
All of the water-soluble fluxes I am familiar with are highlyactivated. These residues must be removed, and as soon aspossible, after the soldering process to prevent the activating acids and/orsalt forms from corroding the assembly. By coating over the residues,this essentially has trapped the culprits with no viable way of removingthem. However, the corrosion will continue until a probable board failureoccurs during service.
I'm familiar withthese corrosive elements under "conventional" conformal coating, but not theepoxy variety. I assume many of the same failures would occur,though. You can either perform a life cycle test on the board in the sameservice environment and make a risk assessment at that time. Epoxy isextremely difficult to remove, so that may not be a feasible option. Youmay end up removing these particular boards from service and clean the residuesin the future prior to coating.
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.
In all likelihood, this assembly will suffermultiple failures. First, the coating over OA flux will most likely fail due todelimitation of the coating. Second, OA (water soluble) fluxes among the mostaggressive flux types, both before reflow and after. OA flux retains much ofits activity after reflow. The flux will remain corrosive and conductive. Thereare few viable options available once the assemblies have been coated. Ifcoating removal is possible (potting compounds are not normally removed) thenthe assemblies may be cleaned however the damage has most likely been done.
The best solution is to ensure that this doesnot happen again. Water soluble fluxes MUST be soon after reflow. This is notan option. Other flux types (RMA, No-Clean), should be removed prior tocoating.
Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.
Itis possible that you would see corrosion and/or electromigration.
Renee has been with Trace and an IPC member for 16 years. She has managed all military and commercial PB qualification and conformance testing and training, as well as product qualification and testing in the areas of solder pastes, fluxes, solder masks, and conformal coat. She is the chairman of the IPC Testing and the IPC-J-STD-004 Flux Specification Committees and the Vice Chairman of the Assembly and Joining Committee. She has published more than a dozen papers and presented at numerous electronics conferences.
If a water washable flux was used, it is important to wash offcompletely all ionic residues and normally assess cleanliness prior to potting.Water washable flux residues are designed to be removedcompletely after soldering. Ionic constituents vary from one formula to anotherbut all having good activity in common.
If the water washable residues were completely removed in awater cleaning system and board cleaniness assessed under parts and in anypart, it should be fine.It is not recommended to leave uncleaned water washable fluxresidues in contact with the assembly for any length of time.
No-clean fluxes arehowever designed from the start to be left on without cleaning.
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.
Interesting condition. I would think it depends upon the initialmoisture in the environment prior to potting the product. If the flux was awater soluble flux and was not cleaned off, it could still be active and ifmoisture was available it could create corrosion products on the surface of theboard. You also could generate dendritic growth between conductors if theproper conditions were met.
If the board was completely coated with epoxy resin the moistureabsorption would be minimal, so the long term risk may be minimal, butcorrosion is corrosion and once started who know how it will progress.
I would suggestevaluating units from the field to determine if any damage or deteriorationoccurred on the surface of the board due to material left behind prior to epoxycoating or potting.
Vice President, Technical Director
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Ifthe flux was intended to be removed by a wash and was not then the consequencescould severe. Many washable fluxes are highly corrosive and MUST be removed.You will have corrosion and failures in the field.
SME Production Technical Excellence Staff
Subject matter expert in the field of electronics assembly and soldering.
Isee a number of potential difficulties that may arise in this instance, but thebig unknown will be the behavior of the activators in the flux. If this fluxhas a significant activation, the chance for corrosion of the metal leads oncomponents is high. This will be compounded by the heat on the boards, entrappedby the potting compound, as heat increases the corrosive effects of the fluxresidues. In addition, the presence of the flux probably will impair thebonding of the potting compounds on to surface of the boards; at MicroCare wealways recommend rigorous cleaning before the application of conformal coatingsor potting compounds to ensure quality bonding.
On the other hand, if thebonding holds, the potted boards will be protected from mechanical vibrationand from other environmental problems, such as moisture, which will mitigatesome of the corrosive effects of the flux residues.
If these were my boards, I'd base my use/scrapdecision upon the application of the product. If they are going into a hi-relelectronics system, I'd scrap the boards and fire the subcontractor for sloppyworkmanship. If they're going into a simple, non-critical application with ashort product life, I'd probably use them and set aside a bit extra as awarranty reserve.
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.
If the residues that were left onthe PCB are potentially corrosive then they will essentially shorten theworking life of the device.
Ioniccontaminants are typically flux residues or harmful materials left behind aftersoldering. They can degrade the reliability of the electronic components andassemblies by contributing to current leakage between the circuitry, causingcorrosion and promoting dendrite growth.
The epoxy resin in this case has essentiallyencapsulated the residues onto the PCB. The extent of damage will dependon the type and quantity of residues left on the PCB. It is advisable to reworkhowever epoxy resin is never easy to remove. Electrolube have a couple of resinremoval products to offer and I would be happy to discuss this with you furtherif required.
European Technical Support Specialist
Jade Bridges is the European Technical Support Specialist for Electrolube. She is responsible for technical support within Europe, offering assistance to customers with product selection, implementation and after sales support across the range of Electrolube products. Her expertise is carried over from her position as R&D Manager for Electrolube, where she was responsible for the new product development and technical support across an array of chemical products for the electronics industry, including conformal coatings, encapsulation resins, thermal management products, contact lubrication and electronic cleaning solutions.
To give an exact answerI would need to know exactly what the flux is.Generally speakingfluxes that are designed to be washed off are higher activity fluxes and arenot designated as no clean.They need to be removedfor a reason and that reason is that they are potentially still active orconductive after reflow.Corrosion and potential dendrite growth underthe potting compound is a potential problem.
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
If the equipment is battery powered then an additional risk is a greatly reduced battery life resulting from any leakage currents that may occur due to the flux residues. I have heard of equipment designed with a battery life of several years that were failing within months or even weeks due to a similar situation.
David Whalley, Loughborough University, UK
All of these comments are good ones, but to add to David Whalley's comment about batteries on the assembly, don't forget that every capacitor represents some form of energy storage. So even without any battery, you may have some power applied, and it certainly does not take much to promote dendritic growth.
Richard D. Stadem
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.