View Full Version : Soldering 101

02-05-2010, 02:42 AM
I thought maybe an article about soldering would be something I could contribute here that would be of interest - and the purpose of this post is to gauge the relative interest of all here in such an article. If any are interested - I can follow up wit more details/pics and take some more effort in it.

What is soldering?

Soldering might be thought of as a "gluing" of two metals together with "metal glue" - and is actually much like brazing or welding metals together - only the type of metal (copper, brass, aluminum, or steel) is different in the various processes. Of course the strength of the joint is different as well.

In all of the above processes - the metals are actually not simply "glued" together with some inert substance - they are actually joined such that they form a metallurgical bond and become "one piece" - both structurally (of importance when brazing or welding) - and electrically (of importance most of the time when soldering).

While this article is focused on soldering copper and aluminum electrical connections, it should be mentioned here that there is also soldering used to join plumbing connections of copper pipe, and "silver soldering" used by jewelers for fastening gold and silver bracelets and chains.

What is solder?

The solder itself is generally a compound of tin and lead - usually "60-40" - 60% lead and 40% tin - but in modern times might have NO lead at all in industrial processes (due to "green" considerations). The solder used for the structural connection of copper plumbing pipe is generally 100% lead wire.

Solder generally comes as a reel of wire - which is used in the soldering process similar to the way a welding rod is used in the welding process.The solder itself cannot be considered alone - as will be seen solder always must come along with some "flux" for the soldering process to be successful.

The importance of flux.

As mentioned, soldering, like brazing or welding, is not simply "gluing" together two metals together with some "foreign" substance - it IS the metallurgical and chemical bonding of the two metals to be joined. Such a bond is required for either maximum strength or electrical conductivity.

In order for this process to happen under standard atmospheric conditions, it is necessary for both of the metals to be joined be ABSOLUTELY CLEAN - free of all clinging films or impurities - and also desirable that the entire process be conducted in the absence of an oxygen atmosphere. This is the function of "flux".

Now we finally diverge from the analogy of soldering to welding - as in welding there is no "flux" required for this function - the metals to be joined are ground "bright" - and the oxygen is removed either by the "arc" - or the inert gas employed (e.g. MIG and TIG).

In soldering - we COULD actually "grind" both surfaces to be joined "clean" - but that is quite inconvenient to do in most cases as the parts are quite small - and still for the best connection we still need the absence of oxygen to achieve it.

What is flux? It is simply a chemical that melts and cleans the heated metal surfaces to remove impurities - and in that very brief process also emits gases that displace any oxygen present in the vicinity of the connection to be made. Thus here comes the "flux" to our rescue to make everything easier - we can get everything clean as an operating room even if we have greasy hands and can perform the operation without oxygen messing things up and still breath in the process.

The flux can come in two ways - most solder you will buy is not only "solder" - it also has a "core" of flux. One might also buy a "tub" of flux. Either way - there is either acid or rosin flux.

For the case of joining plumbing pipes - acid flux is used - yet this flux is never used for electrical connections - as it will cause "galvanic corrosion" of the electrical connections.

Rosin flux is always used for electrical soldering.

Why would I solder an electrical connection?

The simple answer here is not really "simple". One must take into account the expansion and contraction properties of metals as well as electrical parameters.

If I were to crimp some terminal on a wire - then I have a mechanical connection which also acts as an electrical connection. This connection is normally between a copper wire and an aluminum terminal. These two metals have a different coefficient of expansion and contraction - thus when the temperature changes - this mechanical connection between the two metal surfaces night have more or less surface area.

What this means in "real terms" is that this crimp connection might have some small resistance when it is cold - but have a larger resistance when it heats up and the metal of the connector expands at a greater rate than that of the wire. This is an situation that accelerates itself - i.e. the connection has a higher resistance as temperature increases - and the higher resistance causes the connection to heat up even more - which causes the resistance to increase - which causes the temperature to increase ....

With a properly soldered connection - we have a point of equilibrium that would be reached - without such dire circumstances resulting from the heating effect of the resistance and the increased resistance due to the heating effect.

For some low current connection like a sensor to the ECU - probably no problems to crimp it so long as the crimp does not allow it to go "open" - but for a high-current connection like an alt to a battery or lights - this "temperature effect" might be a problem without a well soldered connection.

02-05-2010, 02:49 AM
Dude, I can't even tell you how funny this is.....

I published a paper on proper soldering techniques years ago.

It is to date the only how-to on the subject on my car audio site 3 years running. lol

I am really glad you posted this here.

Make you a deal:

You solder up a few simple connections and such to illustrate and post a step by step with good photos and I will sticky this and add it to the running for the contest.

Sound good? :)

- Mario

02-05-2010, 03:05 AM
You've got a deal dude - I'll do it - and glad to have some niche to contribute to.

Always sorry to only "take" - need to give something.

02-05-2010, 03:08 AM
Glad to hear it, and I look forward to the photos and write-up. :)

02-05-2010, 04:12 AM
I would love to see some cold solder joints thrown in there.

joe's gt
02-05-2010, 07:47 AM

Awesome info man. Thanks

02-18-2010, 11:50 PM

Where are we with this? :)