Ask the Experts
January 12, 2023 - Updated
April 3, 2012 - Originally Posted

Flux Oozing from Insulated Wires

We have a problem with flux and wiring. Our wires are stripped and tinned. It appears that throughout the build cycle flux oozes out from under the insulation. We clean and find later in the process that flux seeps out again.

What could cause this?


Expert Panel Responses

I would bet you are using stranded wire. When you flux the wire, capillary forces pull the flux far up under the insulation. Heat causes the solvent to expand and so it bubbles back out.

If you try to clean the flux out, the cleaning solution also gets pulled up by capillary forces under the insulation.

Doug Pauls
Principal Materials and Process Engineer
Collins Aerospace
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.

Are your wires stranded or solid? Stranded wires have a higher propensity for wicking flux, which can later reflow at elevated assembly temperatures.

Robert "Bob" Lazzara
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.

When I have seen flux oozing from between the wire insulation and the wire, the reason has been attributed to the use of too much flux and the lack of adequate cleaning after soldering.

This may also be attributed to poor quality wire trimming where the insulation is pulled away from the wire rather than a clean cut being made. If the insulation is pulled away, it will cause wicking of flux and solvents into the gap between the wire and insulation. This solvent / flux will emerge later in the process, usually when heat is applied to the system. So
  1. make sure the wire trimming process does not pull the insulation from the wire,
  2. check to make sure too much flux is not being used,
  3. clean the soldered connection with a low surface tension solvent (such as one of the brominated or fluorinated or terpene-based solvents); alcohol doesn't qualify, and
  4. if this is a high-reliability application, consider baking out the boards / assemblies with a slight vacuum and heat.
Sounds like overkill, but "fugitive flux" could cause grave problems when the unit is placed in service.

Rick Perkins
Chem Logic
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 law of physics is the answer.

Wires are fluxed (in your case to much) and when dipped into the solder bath the flux is driven up and under the insulation. With the heat from the solder bath also gets the insulation warm (from the heat from dipping the wires) allows the flux to migrate under the insulation.

Over time the insulation return to its former form (retentively)and expels the excess flux out of the insulation and it shows up as oozing out from under the insulation. Flux will follow the path of least physical resistance.

To control the depth of wire fluxing a weir valve in the flux pot will control the height of flux deposited on the wire and with that control of flux height on the wire and you will be able to control the flux migration.

Terry Jeglum
Electronic Technology Corporation
Mr. Jeglum has 35+ years experience and is the founder of Electronic Technology Corporation. He is responsible for 22 years of program management for the Company.

Both flux and cleaning agents are designed to wet and penetrate. When soldering wire harnesses, one of the risks is flux wicking up the wire and under the insulation. When cleaning the wire harness, use of a cleaning agent with a high vapor pressure is often preferred. Many select a cleaning process using a vapor degreasing process.

Assuming that the cleaning agent is matched to the soil, the cleaning process effectively removes flux residues that wick up into the insulation. Due to the volatility of the cleaning agent, little residual cleaning agent is left behind. Any cleaning agent left behind volatilizes soon after the cleaning process.

Aqueous cleaning fluids also wet very well and will remove flux residues under a wire harness. The issue with aqueous processes is two-fold:
1. Rinsing the cleaning agent that wicks up under the insulation may be a challenge and
2. Aqueous cleaning fluids are less volatile. As such, total drying may not be complete once cleaning is done. If the aqueous cleaning fluid is not rinsed, the flux and residual cleaning agent may ooze out after the build and cleaning process is complete.

Due to the nature of wire harnesses, I suggest cleaning with a solvent that readily evaporates. Best practice is to clean the harness using a vapor degreasing process.

Mike Bixenman
Kyzen Corp.
Mr. Bixenman is the CTO for Kyzen Corp. Kyzen Corp. is a leading provider of engineered cleaning fluids for high technology manufacturing environments.

Capillary action during first tinning.

Mahendra Gandhi
SME - PWB Technologies
Northrop Grumman
Mahendra Gandhi has been working in interconnect industry since 1972.

Flux oozing! Great question! Someone had to put the flux there to start with, so reviewing the tinning operation and the application of flux prior to tinning the wires is the key to the answer as there is no way to properly clean any flux that is beneath the insulation.

The tinning process applies flux to the end of the wire only and if the wire is dipped in a flux pot, just touching the flux is enough to apply the correct amount of flux. The second step of tinning the wire, which if done in a solder pot, is to lower the wire into the solder pot to a depth that is just shy of the insulation. This allows the wetting action of the solder or the capillary action of the solder to wet the strands of the wire. Do not let the flux and solder rise up beneath the insulation as this is where the problem began.

If the tinning is done with a soldering iron and cored solder, the flux in the cored wire solder is enough to wet and prepare the wire for soldering. Apply the solder iron to the stripped wire no touching the insulation and then apply the solder moving the iron and solder towards the end of the stripped wire. This again will prevent getting excess flux beneath the insulation.

In any case one last word, do not use medium or high activity fluxes to tin the wire as this type of flux will impact the reliability of the wire and the solder joint.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
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.

K.C. the root cause of this is the flux itself. Flux must first be prevented from wicking up there. A wire heat sink needs to be applied during soldering. This should help minimize or even eliminate the flux (and solder) traveling up the wire. Sadly in many cases this is almost impossible due to production requirements, the complexity of the wire harness, etc.

You make no mention of what your cleaning agent is. I am going to assume it is IPA.During the cleaning cycle the cleaner meets the flux under the wire insulation. Think of the flux in there now as a drain pipe choked with mud. The cleaner is trying to dissolve this and only partially successful. Mostly because of the location in the small area between the insulation and wire surface.

Usually overnight the residual cleaner still present up inside the wire helps soften the flux enough to cause it to flow back down the wire.(It will follow the path of least resistance.)

Honestly, there is no way to remove it with great success once inside the insulation. I know of several accounts that have moved away from the traditional dip/soak in liquid flux followed by molten bath solder dip tinning to now just tin the wires using a low solids flux core wire solder.

Kyzen Cybersolv 141R due to it very low surface tension has had some mild success at one of my military customers cleaning up this problem. If you would like further assistance do not hesitate to contact me.

Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
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.

All comments above are noted, I would thin back your existing flux by 50% and check soldering. IF OK reduce again and check to see if acceptable wetting wise. You also need to control the Sg of the flux and not let it thicken up while in use.

Again cleaning maybe forcing more of the flux into the strands and into insulation so thin back as much as possible and rely on the heat of the dipping to burn off the excess flux. You should really choose an ORL0 flux for this application.

Greg York
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.
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