Basics of quality soldering.

Basics of Quality Soldering. How to Learn to Solder

• Basics of Soldering
• Various Soldering Methods
• Soldering Printed Circuit Boards
• Enameled Wire Connection Technique

Basics of Soldering

The first thing you need to do is prepare everything necessary for soldering electronic components: a soldering iron, a soldering iron stand, a wooden block, solder, flux, pliers or tweezers, and wire cutters.

Before soldering, the soldering iron must be prepared. To do this, use a file to shape the tip of the soldering iron at a 45-degree angle (this is especially true for a new soldering iron, as the anti-burn coating on a new tip prevents tinning and, consequently, picking up solder). After cleaning the tip, plug the soldering iron into the mains. When it heats up to the melting point of the solder, slightly process the tip with the file once more until the working part shines, and immediately touch the flux and solder with the tip. A portion of the solder should remain on the tip; then you just need to rub the working surface of the tip against the prepared wooden block. After this, the soldering iron can be considered ready for further work. During operation, to keep the tip clean, occasionally wipe it with a multi-layered cotton cloth.

Before soldering a component, it must be prepared. Using narrow pliers or tweezers, bend the component leads so they fit into the holes on the board (this is called forming the component leads). It is useful to have a special tool for forming component leads to specific distances between mounting holes. Insert the component into the holes on the board. Make sure to observe the correct placement (polarity — if applicable) of the component, for example, diodes or electrolytic capacitors. After that, slightly spread the leads on the opposite side of the board so the component does not fall out of its mounting place. Do not spread the leads too much.

Let's Start Soldering!

Place the soldering iron tip between the lead and the board pad, as shown in the picture, and heat the soldering joint. The heating time should be no more than 3-5 seconds to avoid damaging the component or the board.

After 1-2 seconds, bring the solder wire to the joint. When the solder touches the soldering iron tip, the flux may splatter. Once the necessary amount of solder has melted, pull the solder wire away from the joint. Hold the soldering iron tip at the joint for another second so the solder distributes evenly. After that, without moving the component, remove the soldering iron. Still without moving the component, wait a few moments until the solder joint cools down completely.

Now you can cut off the excess lead lengths using wire cutters. Be careful not to damage the solder joint while doing this.

Criteria for Quality Soldering

• A high-quality solder joint securely connects the contact pad and the component lead, and has a smooth, shiny surface.
• If the solder joint is spherical or connects with adjacent contact pads, reheat the joint until the solder melts and remove the excess solder. A small amount of solder always remains on the soldering iron tip.
• If the solder joint has a dull, matte surface and looks scratched, it is called a "cold solder joint". Reheat the joint until the solder melts and let it cool without moving the components. Add a little solder if necessary. After this, you can remove the flux residue from the board using a suitable solvent. This step is not mandatory — the flux can remain on the board. It does not interfere with or affect the circuit's functionality in any way (though for aesthetic reasons, it is better to remove the flux residue).

Various Soldering Methods

How to solder correctly? The paragraphs below should answer this question. They are intended for beginner radio amateurs looking for something more than just theoretical knowledge.

Soldering Free Wires

Let's get straight to practice with the very first example. We need to connect an LED with a current-limiting resistor and solder a power cable to them. No mounting pins, boards, or other auxiliary elements are used here. The following operations must be performed:

1. Strip the insulation from the ends of the wire. The thin copper conductors are absolutely clean, as they were protected from oxygen and moisture by the insulation.
2. Twist the individual strands of the wire core. This prevents them from fraying later.

3. Tin the ends of the wires. During tinning, the heated soldering iron tip must be applied to the wire simultaneously with the solder. The wire must be heated well so that the solder is distributed evenly over the surface of the twisted strands. Gently rubbing with the tip helps distribute the solder along the entire length of the tinned area.

4. Shorten the leads of the LED and resistor, and tin them as well. Although the leads were tinned during the manufacturing of the components, a thin layer of oxidation may have formed on them during storage. After tinning, the surface will be clean again. If you are using very old components salvaged from old boards, they are usually heavily oxidized. The leads of such components must be cleaned of oxides before tinning, for example, by scraping them with a knife.
5. Holding the leads to be connected parallel to each other, apply a small amount of molten solder to them. The soldering spot should heat up quickly, using about 2-3 mm of solder (assuming a 1.5 mm diameter). As soon as the solder evenly fills the gaps between the connected leads, quickly remove the soldering iron. The solder joint must remain undisturbed until the solder hardens completely. If the components move prematurely, micro-cracks will form in the joint, degrading the mechanical and electrical properties of the connection.

A Bit of Theory

Soldering is the process of joining metals using another, more easily melted metal. In electronics, solder containing 60% tin and 40% lead (60/40 rosin core solder) is generally used. As you might guess, the numbers in the solder designation indicate the percentage of tin in the alloy. This alloy melts at just 180 degrees Celsius. Modern solders used for electronic circuits are produced in the form of thin tubes filled with special rosin, which acts as a flux. The heated solder creates an internal bond with metals such as copper, brass, silver, etc., provided the following conditions are met:

• The surfaces of the parts to be soldered must be cleaned, meaning the oxide films formed over time must be removed.
• The part at the soldering joint must be heated to a temperature exceeding the melting point of the solder. Certain difficulties arise with large surfaces that have good thermal conductivity, as the power of the soldering iron may not be sufficient to heat them.
• During the soldering process, the joint must be protected from exposure to atmospheric oxygen. This task is performed by the flux (rosin), which forms a protective film over the soldering area. The flux is contained within the solder as a thin core. When the solder melts, it spreads over the surface of the liquid metal.

Typical Beginner Mistakes and How to Fix Them

• Beginners often touch the soldering joint with just the very tip of the soldering iron. This does not transfer enough heat to the joint. An experienced assembler has a feel for optimal heat transfer. They apply the soldering iron tip in a way that creates the largest possible contact area between it and the soldering joint. Moreover, they very quickly introduce a little solder between the tip and the part to act as a thermal conductor.
• Beginners melt a bit of solder on the iron and, with some delay, bring it to the joint. During this time, part of the flux evaporates, leaving the solder without a protective layer, causing an oxide film to form. A professional, on the other hand, always touches the joint with the soldering iron and the solder simultaneously. This ensures the joint is enveloped by a drop of clean melt before the flux has time to evaporate.
• Beginners are often unsure whether the solder joint is overheating. They pull the soldering iron away too early, then have to bring it back to reheat, pull it away again, etc. The result is a gray solder joint with uneven edges because the joined parts weren't heated sufficiently, and the process took too long, causing the rosin to evaporate. A master, however, heats the joint quickly and intensely, and finishes the process sharply and definitively. They are rewarded with a smooth, silvery solder surface.

Soldering Printed Circuit Boards (PCBs)

Soldering components onto a printed circuit board requires less effort than joining free wires, as the holes in the board serve as good anchors for the component being soldered. However, here too, the result depends on experience and luck. The first circuit or project assembled on a perfboard will most likely fail at the very first solder points, which might end up looking like one solid blob of wire... But after some practice, each connection will look better and better.
In the example below, an integrated circuit (IC) is being mounted on a board. The goal is to make consistently good connections. Let's describe the individual steps:

1. The solder and the soldering iron tip are applied to the mounting point simultaneously. The soldering iron tip must touch both the component lead and the PCB pad.

2. The position of the soldering iron tip is held steady until the solder covers the entire contact area with an even layer. Depending on the temperature of the soldering iron, this takes half a second to a full second. This time provides sufficient heating for the solder joint.

3. Now the soldering iron tip should be moved in a semicircle around the contact being soldered, simultaneously moving the solder in the opposite direction. This applies about 1 mm more solder to the joint. The joint is heated enough that the melted solder evenly distributes across the entire contact pad due to surface tension.

4. Once the required amount of solder has been applied to the joint, the solder wire can be pulled away.

5. The final step is quickly pulling the soldering iron tip away from the joint. The still-liquid solder, covered with a thin layer of flux, takes its final shape and solidifies.

If the soldering iron tip is at the optimal temperature, the entire process takes no more than one second. After a little practice, all your solder joints will start looking exactly the same, like two drops of water.

Enameled Wire Connection Technique

There are two main options for assembling (experimental) electronic circuits at home: stripboards and raster perfboards using point-to-point wiring with enameled wire (magnet wire). The enameled wire connection technique is also suitable for larger projects. With this technique, thin enameled wires are routed between the solder points. The enamel coating is burned off (melted) at the exact spots where the soldering takes place. A little practice won't hurt here, so we will do our first experiments on an old, unneeded board. So, the enameled wire must be routed between two existing solder points.

First, one end of the enameled wire is soldered. Depending on the temperature of the soldering iron, it takes one to three seconds for the enamel to melt. The remnants of melted and burned enamel stick to the soldering iron tip, which must be cleaned regularly and re-tinned with fresh solder.

After that, the end of the wire is soldered into the required place. Then the wire is routed to the second solder point and bent around it to form an acute angle, marking the spot on the wire that will be soldered.

The bend is soldered next. This operation takes longer than the one described above because you now have to process an enamel-coated section, which has poorer thermal conductivity compared to the clean end of the wire. However, even here, with a bit of patience and solder, you can melt the enamel and tin a few millimeters of the wire.

The bend is soldered next. This operation takes longer than the one described above because you now have to process an enamel-coated section, which has poorer thermal conductivity compared to the clean end of the wire. However, even here, with a bit of patience and solder, you can melt the enamel and tin a few millimeters of the wire.

By the way, you might have noticed that the second solder joint wasn't made exactly at the planned point. During soldering, the soldering iron tip slipped, and the joint was made at the adjacent point. That's totally fine since this exercise was done on a scrap board. However, this proves that before tackling a serious project, you should thoroughly master the technique of performing basic operations.

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