|Ask the Experts|
January 11, 2021
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.
Principal Materials and Process Engineer
Are your wires stranded or solid? Stranded wires have a higher propensity for wicking flux, which can later reflow at elevated assembly temperatures.
Circuit Connect, Inc.
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
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.
Electronic Technology Corporation
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.
Capillary action during first tinning.
SME - PWB Technologies
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.
Vice President, Technical Director
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.
Technical Expert Sales Support
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.
Technical Sales Manager
BLT Circuit Services Ltd
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