Electronic – Where does the flux go when desoldering

flux

I have a piece of equipment with a right angle miniUSB port that went bad. After pulling the old port I have a big blob of solder shorting out the traces.

So obviously I must get rid of the blob before I solder in a new port. The main question is this: using a soldering iron, wick flux, where should I apply the flux before desoldering, to the blob to the wick?

Best Answer

Flux's job is to clean copper by removing impurities so that solder can properly wet (think "adhere"). When desoldering, you should apply flux to whatever the solder is supposed to wet to. If you are using a copper desoldering braid, then apply a bit of flux to it.

It can't necessarily hurt to apply a little bit of flux to the solder blob as well, if just to help clean it, but the main idea would be to ensure that the solder does not have a coating of dust or other residue on it, such that the iron will make good thermal contact and heat it up properly.

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Electronic – Solder Flux Classifications

For thru-hole work (which it sounds like you are doing) I would go with an active flux that is water soluble. You need to wash the residue off since it is corrosive and can cause shorts. I have had a number of boards mysteriously come back to life after a good cleaning.

For thru-hole parts that may get damaged by cleaning (LED lenses) or SMD parts with heatsink pads I would use the no clean flux.

I have been using the flux pens from Kester. 2331-ZX for water soluble flux and the 951 for no-clean. To clean the water soluble flux I use an "acid" brush with the bristles clipped to around 0.5". I run the board under the hottest tap water I get (probably around 140-160degF) and scrub with the brush. If I do a small repair job I will clean with the isopropyl alcohol I have on my bench.

Here some of the whys --

Why you need flux -- to get a quality joint at a lower soldering temperature. Using a liquid flux lets you spread the flux better over the two surfaces to be soldered.

Why choose an aggressive flux -- an aggresive flux is more tolerant of soldering temperatures. As Leon mentioned this would be a good candidate for a hobbyist. I like the 2331-ZX for this. Since these leave ionic compounds you need to clean.

Why choose no-clean -- eliminates a process step and the issues associated with the extra process step. For example for soldering XLamp LEDs I use a no-clean flux rather than risk scratching the lens.

For more details take a look at http://www.finishing.com/Library/flux.html and the Kester website.

Electronic – How does flux work, chemically, and what are the products

Flux consists of four major components.

Activators - chemicals dissolving the metal oxides.
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.
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.
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".

Best Answer

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Electronic – How does flux work, chemically, and what are the products

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