Ask the Experts
October 5, 2018
White Discoloration After Ultrasonic Cleaning
We are using halogen free solder paste SAC305type "no clean" for our circuit board assemblies. We are cleaning using an ultrasonic cleaning system with an aqueous cleaning medium specifically designed for ultrasonic cleaners. The problem is that after cleaning there is a white discoloration on the soldered areas. What could be the cause?
Expert Panel Responses
Without actually seeing the discoloration, it' difficult totell if it's an oxidation problem possibly caused by a component of thecleaning agent or a true white residue issue. If it is a white residueproblem, that may be even more difficult to pinpoint the cause as these may berooted in the following:
A few questions:
- Reaction with the laminate and cleaning agent
- Reaction with the laminate and paste
- Reaction with the paste and cleaning agent
- Reaction involving all three
If anything has been recently changed, this may give you anindication of where to look. I would also suggest contacting yourcleaning agent vendor for an analysis of your problem and a recommendation.
- Is the material being rinsed post-cleaning?
- Is this either a new process for cleaning, a new paste in theprocess, or a new cleaning agent?
- What if anything has changed in either the solder or cleaningprocess (settings, cycle times, temps, etc.)?
- Was the current entire process evaluated prior toproduction?
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.
Wedo not use the soldering materials/process that you do, so I can't add value toyour question. However, I want to comment on the cleaning process you describe.J-STD-001, 8.2.b prohibits ultrasonic cleaning of assemblies unless you havedocumentation (available for review) that" ...the use of ultrasonics does notdamage the mechanical or electrical performance of the product or componentsbeing cleaned." Obviously I don't know your design or the type of parts on theassembly, but I thought it should be pointed out that ultrasonic cleaning candamage quite a wide range of components.
NASA/Marshall Space Flight Center
Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.
Your residue could be one of two things:
If spot cleaning with an aggressive solvent such as denaturedethanol does not remove any of the residue, then it is most likely metal salts.Most if not all no-clean residues can be removed with such a solvent. You maywant to consult with the paste manufacturer to be certain. Cleaning as soon aspossible after reflow will ease removal of the flux residues.
If you find that theresidues are residual flux, then you will certainly want to consult with boththe paste manufacturer and the cleaning chemistry manufacturer tooptimize the process.
- Un-dissolved flux that has been attacked by the chemistry.Partially dissolved flux residues, after drying, can have this whiteappearance.
- Metal salts. The flux forms metal salts as it cleans oxides.After removal of the flux, these salts, which are not water soluble, can beleft behind and can be quite difficult to remove. They are not normallyharmful.
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.
Some no clean solderpastes are very difficult to clean. Typically this is due to making apaste pin probable so if this is what you are using, pin probable paste thenyou can either make a change in the paste or look at a different cleaner. I know Kyzen has developed a cleaner that works for these types of solderpastes. Feel free to contact me if you need specifics.
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.
Thewhite residue is uncleaned no clean flux residue. Ultrasonic cleaningdoes not clean well for most water soluble fluxes let alone for no clean fluxresidues. You should call me and we can discuss the risk of residues youare leaving in that no clean flux residue.
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
Discoloration can be caused by several things. The first one Iwould check, is if the discoloration occurs across all solder joints.Is it a film or crystalline by-products of the partially removedflux? If it is a film only, it could be a surface reaction between theflux system and the SAC alloy.
Obviously if you notice crystalline residues usually white tooff white in color it is flux residue that have not been removed. Doing ionic tests at this point is important to insure theseforms of residues or reactions do not leave conductive residues behind. If ionic molecules are present, cleaning has not been effective.
It is also important to avoid extended times above liquidusduring reflow, higher peaks and times above 217C will at times render the residues harder to remove. So lower temperatures, TAL ortotal area under this curve will make residues after soldering more easily cleaned.
The other concern would be effectiveness of the cleaning agentto remove completely the flux residues and the by-products of soldering. Cycle time, temperature and the concentration of the cleaningagent are important parameters to look at.
In reference to the thin whitish film, this is usually anoxidation reaction between the flux in the solder paste and the solder alloy,it is usually not an issue butmostly cosmetic. Ionic tests should be done to confirm this.
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.
White residue left on the bumpsurface may be an indication of over-heat exposure during reflow. As circuitassemblies increase in density, more and smaller components are assembled ontoa circuit card. Often times, this requires a longer soak time during the reflowsoldering process. In combination with higher lead-free reflow temperatures,the soils can thermally degrade with the potential to cross link andpolymerize. These chemical reactions can render a cleanable solder paste moredifficult to clean.
There are a number of other potential factors that may also be the rootcause. One is that lead-free flux residues are more difficult to clean. Theycontain higher molecular weight materials to improve wetting and guard againstthermal burn out. If you have cleaned these residues with the existing processin the past, the issue is most like due to thermal heat exposure at reflow. Ifnot, the issue may be a poorly match cleaning agent for the residue inquestion. Other factors to investigate are wash time, wash temperature, andbath maintenance.
Mr. Bixenman is the CTO for Kyzen Corp. Kyzen Corp. is a leading provider of engineered cleaning fluids for high technology manufacturing environments.
I would check thecompatibility of the flux to the cleaning solvent. The fluxes can be eitherrosin or resin based or water soluble based and this has to be defined prior toselecting the cleaning solvents to be used. Using solvents which don't removeall the residues will leave visual traces of the remaining flux films.
Many of the low-solidcontent fluxes are not made to be cleaned off and when subjected to cleaningsolutions some of the residual films are partially removed and hence the lightis reflected differently and appear as a whitish residues on the surfaces ofthe solder joints. Since this is a paste application the solder joints and theperiphery of the solder joints is the only place the flux residues are going tobe visible and these most likely will be the areas that turn whitish inappearance.
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.
After many years of involvement in the PCB and precisioncleaning industry the visible white residue issue by far has to be the mostcommon and unpredictable anomaly associated in the fabrication and cleaningsteps. Surface residues, such as flux residues, i.e. OA, Rosin or No-Clean andanother typically defined as tin-salts which are insoluble in normal solvents,are generally considered a failure condition.
The chemistry and/or compositionof the visible white residues are complex; the deposits themselves are of anumber of types and origins, and their incidence is associated with bothmaterials and process parameters attributed to before and after cleaning.Fluxing agents react with the tin and lead oxides on thesolders' surface. Doing what flux is designed to do it breaks down the oxidesand allows the solder to flow and be joined. The oxides now are defined asmetal salts thus effectively inert and water insoluble. Despite their waterinsolubility, under controlled processing conditions they will be washed awayduring the cleaning cycle as sufficient quantities of the fluxes solubleingredients, in many cases the rosin component remains.
Enter the 21'st century and new paste formulas especially those termed"no clean" or low residue is the lower rosin % is now replaced by other activeingredients and thixotropic materials. Plasticizers and other "secretingredients" sometimes listed on the MSDS. Halogen free solder pasteformulas with SAC305 alloys are the most difficult of the new formulas toremove.
It is very important to match the cleaning agent to the flux residue.This is likely a prime factor in why you are seeing the white residue or it canbe as simple as not being at the recommended in-use ratio of your cleaningagent. If it cleaned successfully before and now you are seeing white residueverify that the concentration percentage is correct. Also not defined isyour wash temperature or time of wash. These details are also important toconfirm. If you wish to discuss this more do not hesitate to contact me.
Technical Expert Sales Support
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.
Ican suggest two areas of investigation, it is common for lead-free materials to react with alcohol. Perhapsthere is a de minimus long-chain alcohol content in the cleaning materialthat is causing the chemical reaction with the flux that subsequently cannot beremoved by the cleaning fluid.
Two tests suggest themselves:
Overall, thesubmitter should consider doing a wider variety of tests, with differentcleaning systems and chemistries, to try to isolate the failure determine theroot cause. This can be VERY time-consuming, but until you do it you're reallyjust shooting in the dark.
- try cleaning with an alcohol-based cleaner,and see if MORE white residues occur. Alternatively, try cleaning with amaterial that is completely alcohol-free, and see if the residues occur or canbe removed. One such product for testing is MicroCare PowerClean defluxer;available from distributors everywhere.
- Anotherpossibility is insufficient rinsing; it may be that due to thetemperatures/duration of the soldering process, the cleaning is taking longerand requires more agitation than the current cleaning machine can deliver. Acleaning test using manual cleaning, with a vigorous aerosol rinse, should besufficient to test this hypothesis.
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.
White discolorationmeans a lot of possibilities... Could be partially cleaned flux residues, or someinteraction with joint etc. A photo would help us a lot to identify the issue.I am suspecting the concentration could be low, or the temperature may not beat appropriate level. Additionally we need to know the wash exposure time andthe orientation of the boards inside the ultrasonic tank with respect tolocation of transducers. If you would like, contact us directly so that we candiscuss the details and offer specific recommendations.
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".
A cleaning solvent that is chemically matched to the fluxbeing used is required for satisfactory cleaning. It is also recommended that acleanliness test of some type is performed as a process control to validatethat the cleaning system is meeting the requirements of the contract. If thesoldering process for each CCA requires more than an hour to complete, thenin-process cleaning should be used pending completion of the soldering and thefinal cleaning. In your case this is theultrasonic cleaner (US).
There is nothing wrong with using an ultrasoniccleaning system, as long as you understand its capabilities, and moreimportantly, its limitations. Be sure you are using a system designed for cleaning ofelectronics, not mechanical parts. The correct systems for electronics use arandom sweeping oscillation to prevent cavitation in the cleaning fluid.Cavitation can totally destroy electronic component wirebonds and die bonds. Irecommend Crest http://www.crest-ultrasonics.com/.I have no monetary interest, just a lot of really good experience with them.
Spray-in-air wash systems typically use some type of airknife after the final clean rinse to remove at least most of the rinse waterfrom the CCA. A standard bench-top US system does not provide either a finalrinse or a blow-off function. What this means is that the suspended flux solids(and other contaminants) are simply suspended in the wash solvent, be it water,IPA, or some other semi-aqueous solution. If the wash or rinse solution isallowed to simply evaporate, then the reaction products and any suspendedsolids are simply left behind on the surface of the product, and that is typicallythe white residue you see.
My recommendation would be to:
- Verify the flux-cored wire solder and the solder pasteflux match any liquid fluxes that you are using. Never mix flux chemistries.
- Talk to the vendor(s) of the flux(s) you are using andget their recommendation on what to use in the US cleaner. CAUTION! Never useflammable solvents or IPA in an ultrasonic cleaner. The vapor produced can bevery explosive.
- When using a no-clean flux with an ultrasonic cleaner,I have had excellent results using hot DI water (140 deg. F) with a 4 to 6%inert saponifier mix.
- Upon completion of the US wash, follow that with aclean hot DI water rinse, and a thorough blow-off, and perhaps a 5-minute bakeat 105 deg. C after that.
- Perform some type of periodic cleanliness test. 6. Perform some type of magnified inspection to make surethe residues don't come back.
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.
Is it residue? Take a solder sample from the same lot (do not print)and run through oven and then place in ultrasonic cleaner for similar time as your problem board(s). "Residue" present? If yes, look at condition of LPI mask cure. No? Print with paste, reflow and place in cleaner. Residue? Review what appears to be a solvent for the residue, do ionic contaminant testing, and investigate compatibility of cleaner chemistry with paste as recommended by others.
Robb Spoerri, USA
More than likely the Waxes as the Rosins will be normallyvery soluble portion of flux. The Paste manufacturers use Waxes for tack lifeand they are extremely difficult to remove if left on any longer than 48 hoursafter reflow. The Wax is insoluble in most normal Machine chemistries once agedany longer than 48 hours but can be cleaned using hand aerosolcleaners/defluxers due to the more active solvents used in aerosols. For bestresults get them cleaned in the tank at around 60C the hotter the better, witha higher Ph solution or the US or FA+ works well from Zestron in Ultrasonics ifdone quickly.
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.