Ask the Experts
March 26, 2018
Spotting After DI Water Cleaning
We have a close loop 3 Bed DI System fitted with a resistivity monitor. When the beds are new we generally generate water at approximately 18 Mega Ohms. This system was performing well until recently. We had been running 2-3 months between bed changes, now we are changing the beds out every 2 weeks.
We are seeing visible spotting on surfaces yet the puzzling thing is that the water resistivity reads 18 Mega Ohms. This spotting immediately goes away after a bed change. We've double checked the resistivity meters. Do you have any idea what may be causing this?
Expert Panel Responses
This appears tobe some minerals which are still in the water. We had an issue similar whenworking for another company and we found that the town switched its watersupply during the year, from a lake reservoir to wells and it created havoc inthe manufacturing process until we figured this out. So please check with yourtown water department to make sure of the water supply consistency.
I would then have thewater tested for mineral content before and after the DI columns and have thisdone on a regular basis. Many times the town will do this work for you, socheck.
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.
It sounds like there's some organic fouling ofthe IX resin. Is there a carbon pre-filter prior to the IX beds to removeorganics? Have you analyzed the feed and outlet water to/from the IX beds fortotal carbon content? Has there been a process change that matches the declinein bedlife?
Lee Wilmot has 20+ years doing EHS work in the PCB/PCBA industries, including environmental compliance, OSHA compliance, workers compensation, material content declarations, RoHS & REACH compliance. Active on IPC EHS committee and c-chaired committees on IPC-1331, J-STD-609A on labeling & marking, IPC-1758 on packaging and others.
The spotting could conceivably be due to fine, insolublematerial suspended in the water. Such material would not necessarily contributeto reduction in resistivity. I don't consider this to be a likely scenario,however.
I assume that the reason that you are changing beds every twoweeks is to get rid of the spotting. Is the resistivity monitor also showing adrop? If not, check the resistivity meter. It's very possible that it is notfunctioning properly. You should always have a second, hand-held meter toverify the meter on the system.
You don't say what the source of the input water supply is; it ispossible that there has been a change in the quality of the water coming in,namely a rise in the amount of mineral content. This can happen when a utilitychanges sources (different well?), or changes their treatment strategy. Haveyour water treatment supplier test your incoming water. They should be able totell you, based on the test results, how often you would be expected to need tochange beds. If this does not match up with what you are seeing, then furthertroubleshooting of the system is needed.
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.
Have you changed flux?There is something obviously fouling the beds if they are going bad in 2 weeks.Having sold recirc systems for Separation Tech, Trek & Stoelting over theyears, experience would point to either a change in the flux or something hascontaminated the system. I remember a situation many years ago where thestainless piping became contaminated between the , remote, DI recirc system,& the cleaners. The system was completely purged, flushed with peroxide& the problem went away.
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
The client needs to let us know if they startedto use any cleaning agent or additives (such defoamer etc.) in their washprocess. Also, calibration of the built in resistivity sensor needs to be verifiedwith a handheld resistivity-meter just to make sure the sensor is working fine.At a certain frequency level, the original acquired canisters (for containing resin and carbon) need to be disposed of, andreplaced with new canisters (never been used) for the regenerated resinand carbon materials.
The aged canisters could cause problems in the longrun. Also, the regenerated resin beads should also be disposed of,and the client should start with fresh, never been processed, resin beads.They don't have to be concerned with carbon, because each time new,unprocessed carbon is provided to the end user. Please ensure to have a firstparticle filter, then carbon bed placed in front of the mixed beds(anion+cation).
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".
DI resins will only removeionizable material and it looks like you are building a concentration ofnon-ionic compounds in the rinse. Ifthis is the problem it can be solved by adding a activated carbon inline beforethe DI tanks to absorb these compounds.
Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.
You can have very clean, properly de-ionizedwater coming in and cleaning the soils and fluxes from your CCAs, but a key toproper cleaning is not only to have the right solvent or DI water, but a cleanrinse and dry as well. Some drag-out from the cleaning process is alwayspresent in the water still left on the CCA after cleaning.
In fact, the betteryour cleaning process is, the more suspended solids are in the remaining water.If you allow that remaining water to evaporate from your assembly, thosesuspended solids, oils, and other impurities are allowed to remain on the boardas "high-water" marks. If you then bake the board dry, those highwater residues are then baked on and can be very difficult to remove.
So you should always be sure a proper blow-off of theremaining water takes place, whether air knives or simple ionizing blowoff gunsare used. The blowoff will take all of the suspended solids away, and then ifyou bake away any remaining moisture, you will be left with a clean, dry, pristineCCA with very little ionic residues left.
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.
The various scenarios given above are all possible. However, at a different company, a 3 bed DI system (Strong base, cation, mixed bed polisher) was used and a similarly drastic reduction of the tank life observed. It was determined that the strong base and cation tanks had been installed in the wrong order once and the positions were copied during subsequent replacements. Tank life returned to normal after the proper order was restored.
Mark Rasmussen, Itron