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May 16, 2018

Excess Flux Residue After Hand Soldering

Excess Flux Residue After Hand Soldering
We observe an excessive amount of flux after hand soldering terminals using flux cored wire solder. See the image. Is this amount of excess flux normal? Is there something wrong with our manual soldering operation?


Expert Panel Responses

Theanswer is it depends but this appears to be excessive on the contact, but was adifferent amount of solder wire or larger core used to solder the sample. I would be more concerned is the flux benign or still corrosive (no clean fluxrequires additional heat to complex the flux to create a benign residue andthis looks like it is far from the solder joint heating). The other questionto ask if this is insulative do you want it on the contact that is designed tomake electrical contact.

Terry Munson
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.

Thepicture shows flux residue from soldering using solder wire which is notuncommon.Fluxresidue will depend on the percentage of flux in the wire. Fluxpercent by weight will vary from 0.5% to 3% in solder wires and higherpercentages will leave more visible flux residue. Lessflux in the wire, less flux residues, however you need a good percentage toenable ease of soldering.Usually1, 2 or 3 % flux is best with 0.5% being more difficult to use by operators. Ifthe residue is from flux that are no-clean in nature the residue will not causeissues normally. Mostno-clean solder wires are described as ROL0. No-clean,RA, RMA flux wires have fluxes which are resin based and slightly higher orlower soldering tip temperatures willnot impact the volume of visible flux. Resins tend to have high boiling pointsand do not vaporize with soldering tip temperatures. If temps are too high the resin will darken and burn. This willrender it harder to clean off later.

Peter Biocca
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.

Flux residueis proportional to the percentage of flux in the wire and the amount of wirefed into the joint.Wire solderthat is '2% flux core' is 2% flux core by weight - it's approximately 50% byvolume. Therefore, if the joint accepts a lot of solder, then there isgoing to be more residue. In the image you provided, the terminal looksto hold a lot of solder therefore you could expect more visible residue, but Iwould not consider it excessive. You could experiment with differentmanufacturers wire solder products to see if they leave less residue as fluxformulation can significantly impact location/appearance/volume of residue.

Tim O'Neill
Technical Marketing Manager
Tim O'Neill is the Technical Marketing Manager for AIM Products. AIM is a global supplier of materials for the PCB assembly industry including solders, fluxes and thermal management materials. Tim has a B.A. from Assumption College and post-graduate studies in education. He has 20 years of experience in the electronics soldering industry, beginning his career in 1994 with EFD and was key in business development of their fine pitch solder paste dispensing technology. Tim joined AIM in 1997 and has since assisted many clients with assembly challenges, specializing in Pb-Free process development and material selection.

First, it is assumedthat this is a No-Clean flux-cored solder wire. If not, then the residuewould have to be removed. If this is a No-Clean flux-cored solder wirethere is nothing wrong with your soldering but ways to make it lookbetter. Seeing that the flux-cored solder wire has been heated fully toalloy melting, the flux within has been heated sufficiently to
  1. Remove oxidesas is needed for joining and
  2. Remaining flux residue will be fully activatedand rendered harmless in terms of corrosion.
The amount of residue can becontrolled somewhat by adjusting the diameter of the flux-cored solderwire. Try using a smaller diameter for less flux residue; the smallestdiameter that will do the job. The IPC-610 standard for workmanshipallows flux residue resulting from hand-or machine-soldering as long as theresidue has been heated sufficiently to activate it. Any surface that hasachieved soldering temperature will certainly render the flux residueactivated.

Gary Freedman
Colab Engineering
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.

Theamount of flux does not seem excessive. Soldering this type of terminalrequires a fairly high volume of solder, so there will be more residue than fora smaller joint. It may be possible to reduce the amount of residue bypurchasing solder wire with a lower flux percentage. Speak with your soldermanufacturer about what you are now purchasing, and whether a version withlower flux percentage is offered. If it is, you will need to test it to seewhether the reduced flux activity due to less flux is adequate for theapplication. One other thought; if the flux running down the terminal is of concern,re-orienting the terminal during soldering may avoid this, but of course theresidue will still be present.

Fritz Byle
Process Engineer
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.

From the photo is appears that your operator is adding flux prior tohand soldering. The flux is too far away from the solder joint so I would checkthis first.If this is not the case, from the photo it appears that the flux is notactivated (as shown) which could mean:
  1. Soldering iron temperature is too low.
  2. Soldering time on the joint is too short and not activating the flux.
  3. Flux content in the solder core is too great. Perhaps considering a lessorpercentage.

Gary Goldberg
President and CEO
Mr Goldberg has practical experience in production line layout, process flow and cycle rate analysis. He knows how to avoid bottle necks and most related PCB or pallet handling questions.

Todetermine if the electrical performance will be compromised, a surfaceinsulation resistance test (available in the IPC Test Methods) will show if theresidue will be sufficient enough to compromise the finished PCB.Hand soldering temperatures can vary significantly. Soldering tipin relation to board density (which can act as a heat sink) , not to mentionthe human factor, can add up to inconsistent heating ultimately leading toexcessive residue.

Stephanie Nash
Integrated Ideas & Technologies, Inc.
Stephanie Nash is the Director of Technical Services & Marketing for Integrated Ideas & Technologies, Inc., a premier manufacturer of SMT stencils. She has been instrumental in the stencil design and technical support.

In this case, the soldering appears to be on a crimped lug terminal. No flux should be allowed to wick down inside of the wire insulation, and I am also puzzled as to why a crimped connection requires solder? But if a flux must be used, then an RMA tacky flux is preferred as it is the least problematic if some flux does wick inside the wire insulation, but it should be cleaned if at all possible. Typically the best way to clean it is in an ultrasonic cleaner using a 5-7% mix of inert saponifier in hot deionized water (do NOT put IPA or any other flammable solvent in an ultrasonic cleaner; it has a tendency to go BOOM!) .

Because we are talking about a mechanical subassembly, there is little concern with damage from US cavitation. A tacky flux is much easier to apply and clean in a controlled manner than liquid flux, and will stick around for the whole soldering process, unlike liquid fluxes that typically evaporate or burn off before the soldering is completed but can also wick down into the wire and cause problems later. As to the idea of using a surface insulation resistance test, I am puzzled as to how that is performed on a lug terminal. Would you care to expand on that, Ms. Nash?

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
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.