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Ususally, electronics solder comes with a built-in flux core. Among the solders I have laying around, I have a silver/tin solder with an "F-SW-21" flux core (ISO 9454-1: 3.1.1, this is zinc chloride and/or ammonium chloride), and a lead/tin solder with a rosin flux core.

As far as I know, these fluxes are there to "break up" the oxide layers on the metal surfaces. But how does this work, chemically? What are the products of this reaction, and where do they go? I am especially wondering about the voids that can form inside the solder: Are these mainly due to gaseous flux that simply boiled due to high temperature, or are these gaseous products of a chemical reaction?

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Flux consists of four major components.

  1. Activators - chemicals dissolving the metal oxides.
  2. Vehicles - high-temperature tolerant chemicals in the form of liquids or solids with a suitable melting point. They act as an oxygen barrier to protect the hot metal surface against oxidation, to dissolve the reaction products of activators and oxides and carry them away from the metal surface, and to aid in heat transfer. A common "vehicle" in electronics soldering is rosin.
  3. Solvents - added to aid in the processing and deposition of the solder joint. Incomplete solvent removal leads to boiling off and spattering of solder particles or molten solder.
  4. Additives - Additives can be corrosion inhibitors, stabilizers, antioxidants, thickeners, and dyes.

Short answer: Flux removes oxidation, aids in heat transfer, cleans and prepares the joint to accept the solder, and promotes even solder flow.

http://en.wikipedia.org/wiki/Flux_(metallurgy)

Many various fluxes contain metal halides which are metals combined with halogens. Halogens are a group in the periodic table consisting of five chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These halides are the activators. Because flux has a low melting point it will liquefy before solder solidifies. Metal halides often promote corrosion which will help aid in the dissolution of the oxide allowing the contaminate to flow away from the joint. Then the solder will flow into the joint forming a strong bond that actually fuses with the metals involved. Which is why metals like lead and tin are used to solder metals like copper because they form a bond with the metal that creates a thin layer of alloy metals. I do not believe there are any "products" from this reaction. There was a saying in Chemistry I learned once "like dissolves like". It takes corrosion to remove it. However, heavy corrosion will not remove from the metal by just using solder flux which is very mild and not acidic as flux used in copper tube welding.

I wasn't able to research "voids" in the solder. In my experience that is due to soldering with extremely high temperatures. Lead melting point is about 621 degrees Fahrenheit. If your iron is too hot it can super heat the lead and cause it to "explode" or pop off the joint. Perhaps this is a cause of voids. Also, if the material being soldered is very dirty it could cause contaminates to be trapped under the solder that the solvents in the flux are unable to clean away. Which, as mentioned above, can cause spattering and boiling off of solder particles which could cause the "voids".

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    \$\begingroup\$ It may be worth holding back on the value of metal astatides as a flux: I don't reckon they'd by RoHS compliant! ;-) \$\endgroup\$ – Dan Sheppard Aug 17 '15 at 18:41
  • \$\begingroup\$ agreed. any holes or voids in the solder are usually from overheating solder to boiling point. if working in a quiet area, you can hear a crackling sound when the solder reaches boiling temp... that sound is the little bubbles boiling and popping in the liquilted metal. even if your solder is successful, it is weaker and unreliable under these conditions. \$\endgroup\$ – user119979 Aug 6 '16 at 22:03
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The "flux" is very wide term. In the case of soldering process, the flux works in two ways:

  1. Cleaning: Using some chemical reaction they removes the oxides from the metal surface and this way improves the wetting of the metal with the melted solder. Usually, some acids are used for cleaning the oxidation. In the case of rosin, these are high molecule resin acids, that become active only in liquid form and high temperatures.

  2. Protecting: The liquid flux protects the surface from reaction with the oxygen during the soldering process. Notice, that the liquid solder sinks in the flux and wet the surface without contact with the air.

Note 1: Zinc chloride and/or ammonium chloride fluxes, mentioned in the question, are too active to be used for soldering electronic devices.

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This is a VERY good question! ALL fluxes throughout welding, brazing, soldering and even the production of metals, work all manner of magic at different stages of their given processes.

  1. Oxides are in the way of your metals that you want to join. Flux cuts through (cleans) this layer. If the metals are eutectic: you can bond them - but NOT to or through their oxides! Flux is a solvent. Activated (caustic/alkyline/acidic) - or not.

  2. A catalyst is required for the two metals to exchange electrons and form a bond. At temperature - flux provides for this.

  3. A protective envelope is needed to keep air / oxygen out of the metal(s) when in the molten / bonding temperature and state. Flux acts like a shield for the process - a layer between air and the metal while the metal is molten - at its most vulnerable state.

  4. Flux is also like soap suds. A surfactant. Lifting and floating out any impurities. Without flux - impurities have no reason or means to move away from the work area.

Appreciating what flux does; I made my own. I enjoy tinkering with my own formulas for different needs. Here's my latest batch:

  • Pine Rosin (it's tree sap, distilled, turpentine removed from it.)
  • 99% isopropyl alcohol - to make a liquid of the rosin.
  • Oxalic Acid - white powder form - equal parts with rosin. Woodworkers bleach wood with it. It's Formic acid with CO2. Formic acid is what ants make their scent trails and mean bites with. It's in Rhubarb leaves. It's the simplest carboxylic acid - what Hughes Aircraft co-researched and found to be optimal. If I use this to clean and tin heavily corroded pads / traces on an iPhone or MacBook circuit board - I have to clean it away very quickly! It ate away a pad first use because I let it sit for 20 seconds. It works! I need to add less.
  • Glycerin - I got it at Walgreens. AfriKare was the name brand. It's 100% pure. Simple stuff. It helps the rosin last longer before it turns dark and tar/sappy under time at heat. It also helps the ingredients keep in solution.
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    \$\begingroup\$ Good first answer, welcome! Just a quick point, oxalic acid isn't the simplest carboxylic acid (it's an oxidised dimer of formic acid), but it forms strong complexes with metal ions, which will help with the cleaning process. \$\endgroup\$ – awjlogan Oct 24 '18 at 20:18

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