|Ask the Experts|
October 13, 2017
Cleaning No-clean Flux with Tap Water
What happens when cleaning no-clean flux with normal tap water? We are currently using a no-clean process for our SMT board assembly. However, on occasion, our thru-hole assembler will use a solder with a water soluble flux to assemble the thru-hole portion of the board. The assembler will then attempt to clean the board using ordinary tap water using a brush, allowing the water to run over the board. The assembler then dries the board with compressed air.
|Expert Panel Responses|
This approach is not a good practice for one obvious reason and one less obvious reason. The more clear-cut bad practice is the use of tap water to clean a circuit board. Tap water contains many ionic contaminants that would be left on the board after cleaning, which can lead to long-term reliability issues. De-ionized (DI) water is really the only option for the cleaning of any type of electronic assembly and should be used in this situation. The less obvious potential issue is the unknown interaction between the no-clean residue and the water. Most no-clean residues are not removed in water, but many of them can chemically react with water in such a manner that can impact board cosmetics and reliability. In some circumstances, the partial wash of no-clean residues can leave residues that are not completely chemically reliable. It is therefore advised to verify that the no-clean residues are reliable when subjected to a partial wash in any kind of water - even DI water. The best way around all of this may be to introduce no-clean materials at the through-hole soldering operation, thereby forsaking the improved activity of the liquid flux but gaining long-term reliability. Although no-clean liquid fluxes are not as robust as water-soluble varieties, they can be sufficiently optimized to produce excellent results and eliminate cleaning completely.
General Manager - Electronic Assembly Americas
You have to ask the question of How Clean is Clean? It is probably not a good practice, but will the world stop because of it NO. Tap water has all kinds of mineral in it and in some cases it has added halogen, yup, fluorides. If it is dried off with high pressure air, the residues left behind would be miniscule. If the customer has a problem then use Deinonized water and the minerals and other additives would not be an issue.
Vice President, Technical Director
Residues of no clean fluxes are meant to be left on the board. The reason is that paste residues have a resin in them that is not water soluble thus making it very difficult/impossible to clean with water alone. One of the attributes of resin in the flux is that they also encapsulate the activators making the residue non-corrosive and non-conductive. By using water to clean you might be making your non-corrosive/non-conductive no clean flux corrosive and conductive.
Depends on the no clean paste being used. Most likely it will turn the no clean solder paste or flux residue to a white or hazed color. Mainly visual. I'd be more concerned with the active water soluble flux being manually cleaned off in a sink to be honest.
What happens is; the no clean flux is coatedwith a surface layer of what ever ions were present in the tap water they usedto clean the water soluble flux. Theamount will vary depending on the water quality and how much was blown offverses evaporated. The bulk flux is unlikely to be affected otherwise as aclass the no cleans are designed to be hard and water resistant. The bigger problem here is what happens to the rest ofthe board where these ions will also remain behind under components, inconnectors, and between exposed conductors where is has been shown tosignificantly increase the chance of electrical failure over time. High reliability manufacturing practices require a washimpingement pressure sufficient to establish flow under all components and apurified water final rinse when cleaning water soluble fluxes. The normal practice is to cleaning in anautomated cleaner where time, temp, pressure and rinse purity can be set andautomatically monitored.As a minimum, I would recommend your vendor use a DIwater final rinse. It's a small price topay to sleep well and cheaper than raising the product liability insurance.
Austin American Technology
It is rarely a good idea to do any cleaning of flux/process residues with tap only. As previously mentioned there is at a minimum the opportunity to leave behind ions from the tap water and that composition can vary wildly by municipality. The next thing that jumps to my mind is the possibility that you are partially removing the no-clean residues and that is akin to removing a bandage over an open wound. The resin that is binding any still active part of the flux will be removed and it is now a lot easier to set up an electrical leakage event or even electrochemical migration. Partially cleaning a no clean can be as detrimental as not cleaning a water soluble flux in some cases but certainly not all. IF it's an option it is a much, much better practice to ensure that the residues left behind have been exposed to a proper thermal excursion and are as close to benign as possible, then leave them alone.
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