Flux under the wire insulation
It often happens that during the wire tinning stage, the flux will get between the wires and the cable insulation. Then, as a result of the increased temperature of the process, it escapes from under the insulation, which can permanently damage the system.
Almost every wire must be stripped and tinned before assembly. It very often happens that during the tin-plating stage, the flux gets between the cable insulation and the wires themselves. Braided cables, which are more prone to flux leakage, are particularly prone to this problem. What's more, as a result of the increased process temperature, the flux may leak from under the insulation at subsequent assembly stages and even cause very serious problems.
The problem of flux getting between the insulation and cable conductors is most often associated with the use of too much flux and/or the lack of proper cleaning after soldering, which we write about later in the article. However, as Rick Perkins, president of Chem Logic, says in one of the blogs, the root cause may also be poor-quality cable cutting: it may happen that, as a result of poor-quality equipment, insulation is pulled from the wires when cutting the cable, and a clean-cut is not made. If the insulation is pulled off, it facilitates the seepage of flux and solvents into the gap between the wire and the insulation.
In turn, what is the flux leakage mechanism itself, explains Terry Jeglum, president of Electronic Technology Corporation: 'The answer is the law of physics. Flux is applied to the wires - it often happens that there is simply too much of it - and when immersed in the solder bath, the flux rises and penetrates under the insulation. The heat from the solder bath also heats the insulation and allows the flux to migrate between the insulation and the conductors themselves. Over time, the insulation returns to its previous form and 'squeezes out' the excess flux, which manifests as flux oozing from under the insulation, which follows the path of the lowest physical resistance. '
As a way to prevent this unfavorable phenomenon, Terry Jeglum indicates process control at the flux application stage and immersion of the cable only to a certain, safe depth. Leo Lambert, Technical Director at EPTAC also says more about the fluxing and tinning stage: 'Since there is no way to properly clean the flux under the insulation, the key to solving the problem is an overview of the tinning and fluxing operations. At these stages, the flux should only be applied to the end of the wire: it is enough to literally touch the flux to apply the correct amount of flux. In the second stage of tinning the wire in a soldering crucible, the cable should be lowered in the crucible, keeping a safe distance from the insulation. This allows the solder to act capillary and wet the cable wires. On the other hand, if the tinning is done with a soldering iron and a wire with a core containing flux, this flux is sufficient to moisten and prepare the wire for soldering. Touch the soldering iron to the stripped wire without touching it, and then apply solder moving the tip towards the end of the wire. This should prevent flux from getting under the insulation. [...] Medium or high activity fluxes should not be used for tinning the wire, because these types of flux negatively affect the reliability of the wire and the solder joint '.
Mike Bixenman, technical director of Kyzen Corp., disagrees with the previous participant's claim that it is impossible to clean flux from under the insulation: 'Both fluxes and cleaners are designed for wetting and penetration. […] Many people choose the cleaning process using the steam degreasing process, and when cleaning the wiring harnesses, it is often preferable to use a cleaning agent capable of achieving high steam pressure. Provided that the cleaning agent is properly selected for the application, the cleaning process can effectively remove flux residues that penetrate under the insulation. Due to the volatile properties of the cleaning agent, its small amounts may remain under the insulation, but they evaporate shortly after the cleaning process.'
As an alternative, Mike Bixenman also points to water-based cleaning fluids, which also wet and remove flux residues under the wire insulation very well. However, a water-based washing process can present two problems:
1. Flushing out any cleaning agent that has penetrated under the insulation is quite a challenge.
2. Water-based cleaning fluids are less volatile. As a result, upon completion of the cleaning step, the cable may not be completely dry, and if the aqueous cleaning fluid is not rinsed, flux and flux residues may leak from under the insulation in later stages of assembly.
Summarizing, Mike Bixenman suggests washing with detergents based on easily evaporating solvents and using a steam degreasing process.