Hand Soldering Techniques

[-mjk-]

This is going to be an ongoing series of posts, and I hope that you won't have to wait too long between them. Feel free to directly ask questions or prompt me to expand on any particular topic. Here are the basics:

1. Fluxes are used to remove oxidation not contamination. Always clean the parts being joined by soldering.
2. Use the lowest activity flux that you can. In order from low to high, they are R, RMA, RA and OA. Never use OA flux for hand soldering electronic components on a PCB. Avoid RA unless you have a very good reason to do so (you won't have a good reason most likely).
3. Use eutectic solder formulated at 63/37. Do not use 60/40 solder. I will explain why in a later post called "Heat Control".
4. Pre-solder all components. That means "tinning" wires or leads to be soldered, and also solder cups on connectors. For large projects such as building patchbays that require hundreds of wire tips to be pre-soldered, consider a solder pot.
5. Never solder crimped connectors. Either solder, or crimp, but do not solder a crimped connector under any circumstances. If a crimped connection is failing, replace it by cutting the union off and re-crimping, or soldering.
6. Keep the cleaning sponge on your soldering station slightly damp, and not soaked. The recommendation is to use distilled water to prevent calcium and other chemical contaminants from getting on the soldering iron tip and interfering with the soldering operation.
7. Use a well-regulated soldering iron. Once you touch the iron to the joint, if the iron cannot hold the temperature reasonably well (in other words, it's underpowered and can't supply enough heat) that will interfere with the creation of a proper intermetallic bond.
8. Properly performed hand soldering is a controlled operation. Desoldering however, is an uncontrolled operation, and all desoldered components should be discarded and replaced using heat control techniques. Suspect that all desoldered components may be heat damaged. In practice, this may not be possible, so use extreme caution when desoldering any component that must be reused.

 

[-mjk-]

Heat Control

Heat control is a crucial aspect of successful soldering by hand. By the term "heat control" we are not referring to the ability of the soldering station to regulate temperature. The term refers to a technique when using a hand soldering iron for electrical soldering.

The goal of soldering is to create a permanent, reliable electrical connection. That requires the formation of an intermetallic bond between the materials to be joined with the intermediate material we call solder.

60/40 solder melts at 188 °C / 370 °F. However 63/37 solder melts at 183 °C or 361 °F, and has the lowest melting point of all solder formulations used in electrical soldering. The reason that this is important is that any other tin/lead solder formulation has a melting range and not a melting point. Let me explain:

We call 63/37 solder, eutectic solder. The word eutectic comes from the Greek meaning "easy to melt" and the formulation of 63/37 tin/lead has a precise melting point as described above. 60/40 solder melts at a slightly higher point, but, at the point where the eutectic formula is melted, the non-eutectic formula is approaching the melting point, but enters it's plastic state. This is why we say that 60/40 has a melting range and not a melting point. Any metal alloy with the lowest melting point can be referred to as eutectic.

The reason that this is important is because eutectic solder has no plastic state.

If you solder with 60/40 and while the solder is melting it's not actually flowing until it reaches 188 °C / 370 °F. Extreme care must be used to ensure that the solder has actually fully melted and flowed, and due to the plastic state of the solder, the joint is not disturbed until the solder freezes. So-called "cold solder joints" are in reality disturbed joints where there was so much heat used that the thermal inertia kept the joint hot enough to hold the solder in the plastic state just long enough for the solder to not freeze before the components moved, even microscopically. Any movement immediately destroys the intermetallic bond.

If we insist on using 63/37 eutectic solder, then 3 important points come into play:

1. If the solder is melting, it is flowing and not in it's plastic state (because there is no plastic state).
2. The solder freezes immediately after the heat is removed because there is no melting range/plastic state.
3. We can employ heat control to protect components being soldered!

What is heat control?

Heat control is a technique that takes advantage of eutectic solder's precise melting point. Just like a pot of boiling water, as long as there is water in the pan, the temperature of the pan cannot exceed the boiling point of the water. So, if we touch the solder to the joint before we apply the heat with the soldering iron, the melting action of the solder will not allow the temperature to rise above the melting point of the solder.

Some of you are probably shaking your head after reading that. When I was teaching a class on hand soldering back in 1988 to a group of repair technicians, one of them raised an objection, saying "But you have to let it flow" and he insisted that one must keep the iron on the joint after applying and removing the solder. This is because his experience is based on using non-eutectic solder. He had issues over the course of his experience with the plastic state of the solder he was using.

Keep this in mind: "If eutectic solder is melting, it is flowing. There is no need to remove the solder and "flow it" by keeping the iron on the joint. Your soldering station has a lot of power to overcome the thermal loading caused by touching a cool joint and heating it. If you remove that solder from the joint, the temperature will immediately climb up and out of control.

This is why desoldering is not a controlled process. There is no solder applied to the joint for heat control.

In that class I taught in 1988, it took about a week for the technicians to actually trust that heat control works and that they could get reliable joints without holding the iron on the joint for extra time after they removed the solder. They were instructed to apply the solder to the joint first, then the iron. Instantly upon completion of the joint, they were to remove the iron just before the solder. But, the data spoke for itself. After that company implemented the techniques of proper hand soldering the reliability rate of the products increased significantly.

So, check your solder. If it says "60/40" you can use it on other stuff, but please do not use it for electrical soldering. While of course it is possible to create solder joints with 60/40 formula solder, you will be subjecting your components to a higher than necessary temperature, and most likely subjecting them to heat damage when you try to "flow" the solder. Not only that, but the higher the temperature used in the process, the larger the intermetallic joint will become. This is not desirable. A joint with a large intermetallic bond has a shorter reliability life span, as the intermetallic bond continues to grow over time until it overtakes the entire joint and must be resoldered.

In order to ensure success, eutectic solder should always be used.

 

[-mjk-]

Pre-Solder Everything

Strangely, what we call soldering in electronics is not actually soldering, technically speaking. The definition of soldering includes the requirement that the surfaces being joined by the intermediate substance (solder) do not melt. But, since all electronic components and PBD assemblies are in fact, pre-soldered, the surfaces of those components certainly do melt. Joining metals where the surfaces do melt is welding.

By why care about that? Simply because solder bonds and flows very easily together from the surfaces of pre-soldered components to form a reliable, permanent, electrical connection - which is the goal. So, a necessary step in the process is to make sure that our components are properly pre-soldered and this will avoid bonding issues and ensure success, as well as saving us time in the end. Solder bonds to solder readily and easily. However, sometimes getting the solder to bond to a base metal presents challenges.

In the first post, I made reference to fluxes and their activity levels. We want to use the lowest activity flux that we possibly can in order to avoid issues with high activity fluxes, such as dendritic growth (which is like a plague on electrical assemblies!). But, some components can be finicky and difficult to bond well with solder (called "wetting"). So, how do we solve this problem?

First off, we have to understand the function of flux. The only purpose of a flux in soldering is to remove oxidation and prevent it from reforming until the intermetallic bond has fully formed. That's it. Flux is not a cleaner which explains why adding more is unlikely to improve wetting on difficult components. So, there are 2 main barriers to wetting, and those are:

1. Contamination: This is any foreign substance other than oxidation. If you strip a wire of insulation and twist the strands with your fingers, you have added skin oil to the surface. Most times you can get away with this. But, flux will not clean skin oil so this explains why adding more and more solder (for the embedded flux in the core) just doesn't seem to help the solder wet the wire. Giving the wire a quick clean with an alcohol wipe usually solves the problem.
2. Oxidation: This is basically, rust. This is what fluxes are designed to remove, and keep from reforming until the soldering operation is complete. Since a flux can be any substance or state that accomplishes the above, even an inert gas can be used. Indeed some critical aerospace components are soldered in a non-oxygen environment. When silver-soldering ground straps and radial wires in a broadcast antenna grounding system, putting a torch on the strap cleans the oxidation off the copper very well. Keeping the torch on the metals being bonded until the solder is completely flowed works perfectly and eliminates the need for liquid or paste fluxes and the subsequent cleaning issues. The heated gas jet is the flux in that application. But you can't solder a PCB with a torch, so let's get into practical application.

Some components are made from a base material that just doesn't want to wet very well. PL-259 coaxial cable connectors, patchbay jack terminals and phone plugs come to mind. These components may not have been properly pre-soldered from the factory and you are left with a component that you cannot easily solder in the field.

Since we want to avoid the use of high activity fluxes, we don't just want to prepare our coaxial cable (in the PL-259 example), shove it into the connector and douse it with liquid OA flux to solder it. We must pre-solder the PL-259 and the cable.

The goal is to use high activity fluxes away from the main assemblies. So, we would apply our aggressive flux to the connector only, and pre-solder away from other components and only assemble them together after cleaning the active flux off the component. This also goes for wires too. You could apply flux and pre-solder terminals on a finicky patchbay and then clean the flux off. After that the terminals will be properly prepared but the highly active flux will not be present at assembly.

XLR Connectors

We are always dealing with XLR connectors in the audio realm. Nearly every professional technician and amateur builder encounters problems with solder-cup terminal XLR connectors at some point. Usually, the solder doesn't fill the cup entirely, and the solder took so long to flow that the insulation on the wire melted back resulting in a large insulation gap. Some people actually put off repairs because they don't want to deal with XLR connectors! I can understand that, so, let's review an XLR action plan. This plan can be applied to most solder type connectors:

Preparation

1. Thoroughly clean your XLR connector's solder cups (alcohol should do nicely, but there are some good electrical cleaners - just don't use a cleaner/lubricant or that defeats the purpose).
2. Put your XLR connector in a small work vise, solder cups facing up, with the open side of the cups facing so you can apply heat on the outside rear wall of the cup while feeding solder into the cup opposite the soldering iron.
3. Using eutectic solder, fill the solder cups by first touching the solder to the cup on the inside back wall, and then touching your soldering iron tip to the outside back wall. Note: always melt a small drop of solder on the tip to facilitate heat transfer.
4. Finish all of your connectors with the above steps and put them aside. They are ready for soldering.
5. Prepare your wires by stripping the insulation back for soldering. The general rule of thumb is that you strip the insulation so that when the wire is inserted into the bottom of the cup, the insulation gap to the top of the solder cup is equal to one wire diameter. Then pre-solder the wire tips using the heat control technique. This will only take 1 second thereabouts, so be prepared to take the iron off quickly. We will preserve the neat appearance of the insulation by not melting it. Prepare all of your wires and put them aside. If you have any trouble wetting the wires, give the wire strands a quick cleaning wipe. In extreme cases where some high-tech alloy is being used, you may have to apply liquid OA flux and use a solder pot. I've only had to do this a few times though. Most audio wires are standard copper alloys.

Procedure

1. Make sure that you have placed any back shells, insulators or strain reliefs on the cable prior to soldering (if applicable).
2. Place your first XLR connector in your work vise. Rotate the connector to whatever position is comfortable for you to apply heat to each cup while feeding the wire with the opposite hand (similar to the pre-soldering operation).
3. Pick up a cable and isolate the first wire to be soldered.
4. Wet the tip if your soldering iron with a drop of solder.
5. Place the wire at the top of the solder fillet in the solder cup before applying heat.
6. Touch the soldering iron to the outside rear wall of the soldering cup while you apply light pressure on the wire.
7. The wire will quickly enter the solder cup just as the solder reaches the melting point. Immediately remove the heat. The solder should freeze virtually instantly. The fillet should demonstrate what is called an "upward meniscus" which means that the solder fillet has raised up to meet the wire, and the fillet does not look like the wire is pushing the solder down into the cup.

If you meticulously follow the guidelines of pre-soldering, you will save yourself a world of soldering headaches.

On a personal note, after I attended soldering school and applied these principles to soldering, I actually began to look forward to large scale jobs, like patchbays and sub-D audio connectors. I have no reservations about taking on new projects that require soldering because knowing the underlying principles has eliminated the mystery of why sometimes things don't solder well, but more importantly how to fix it when that happens.

After soldering school, I started getting more jobs requiring specialized soldering techniques. If you learn these principles you can make money utilizing them. I've been involved in the build-out of 13 radio stations and I always got all the soldering jobs. I built both the studio and transmitter ground systems, and wired the entire studio with every soldered connector. Those all required diverse soldering techniques, but the principles were identical.

I'll bring you more articles on soldering soon.

 

[Phil Tipping]

Great posts MJ thanks.
Just one tip to add: When soldering connectors like XLRs, 5-pin DINs, RCA/phonos etc., plug them into a spare plug/socket so that the pins are held in the correct position while soldering. Some plugs have plastic mouldings which hold the pins in place, so if this softens when soldering, the pins can move out of alignment.

 

[-mjk-]

Great tip Phil! Heat control combined with that tip will keep things lined up properly.

 

[Arjan P]

Now there's some in-depth soldering information! Great posts @-mjk-

One thing I'd like to add regarding pre-soldering: It can help a great deal to use some fine grain sandpaper to prepare the surface of the item to be pre-soldered. Another thing in this regard is to use connectors from a reputable brand AND source. I once bough a batch of Neutrik TRS plugs online that turned out to be fakes: they were impossible to pre-solder and therefor for the waste bin! (actually I could send them back, fortunately..)

Also, when I was creating a lot of studio cabling - and therefor soldering a lot of XLR connectors - I created a heat sink by having a socket or pin connected to the contact to be soldered. In that socket or pin the thickest possible wire and connected to that a terminal to a relatively big metal item (a bolt with a nut should do). It really makes a difference, certainly with some types of wire insulation - sometimes you don't have a choice here.

 

[-mjk-]

@Arjan P thank you. You make some great points as always. Thanks for your input.

Reputable sources! Even a genuine lower grade connector is better than a fake higher grade connector. Glad you could return them!

I also occasionally use some kind of abrasive on those "dog" connectors. The reason I didn't mention that is because you can also create another issue by taking the plating off and instead of trying to solder to nickel plate, you're trying to solder to brass or (even worse) some waste alloy like "key metal". One has to weigh the risks. Use with caution. Connectors that are that oxidized might not be a good choice.

The idea of a heat sink is useful, but I would like to reiterate that if you use heat control - the practice of only using eutectic solder and continuous contact, there really shouldn't be any issues with the insulation as all insulation melts at a higher temperature than eutectic solder (as was taught in soldering school - this may or may not be the case in recent years). That being said, there is always more than one way to accomplish any given task, and it's the results that matter, not so much the method. Methods are simply known starting points and known subsequent pathways.

I'm looking forward to getting some real-world feedback as Forum users as they try out these techniques.