Soldering Techniques

Soldering is the process of joining two or more metal parts using a filler metal, known as solder, which has a melting point lower than that of the metals being joined. During this process, the base metals do not melt; only the solder is melted to create a bond between them. In electrical applications, solder is typically a conductive alloy composed of varying proportions of tin (Sn) and lead (Pb). However, specialized solder alloys containing metals such as silver (Ag), zinc (Zn), and antimony (Sb) are also available. The soldering process is based on the principle that the solder melts, flows between the surfaces to be joined, solidifies upon cooling, and thereby bonds the metals together.

The primary purpose of soldering is to provide not only a mechanical connection but also electrical continuity, especially in electronic circuits. The ability of solder to melt and solidify quickly, along with its resistance to corrosion, has made it an indispensable part of electronic manufacturing and repair processes. Soldering is divided into two main categories: soft soldering and hard soldering. This classification is based on the melting temperature of the filler metal; processes below 450°C are classified as soft soldering, while those above 450°C are classified as hard soldering.

Soldering Methods and Heating Technologies

Soldering is performed using various methods and technologies that differ according to the application area, material type, and production volume. These methods cover a wide range from manual applications to fully automated systems.

Manual Soldering Methods

Iron Soldering

This is the most basic and widely used soldering technique. In this method, the tip of a tool called a soldering iron is heated using electrical energy. The heated tip is brought into contact with the metal surfaces to be joined, raising their temperature to the melting point of the solder. The solder wire is then applied to the area, and the molten solder fills the gap between the metals. When the iron is removed, the solder cools and solidifies, completing the joint. For effective soldering, it is recommended to position the iron tip at approximately a 45-degree angle between the component lead and the pad, and to apply a small amount of solder initially to enhance heat transfer.

Hot Air Soldering

In this technique, a gun or station that blows hot air is used as the heat source. Similar to iron soldering, the hot air stream is directed directly at the area to be soldered to provide heating. This method is particularly useful in situations where direct contact with the soldering iron tip is difficult or could damage the component, such as when soldering or desoldering surface mount devices (SMDs). It is also commonly used to repair faulty solder joints on circuit boards.

Automatic and Semi-Automatic Methods

Reflow Soldering

This is a semi-automatic method commonly used in mass production and assembly of complex circuit boards. In this technique, solder paste is first applied to the areas to be soldered. Electronic components are then placed on the board, and the board is inserted into a furnace calibrated to a specific temperature profile. Inside the furnace, the solder paste melts and simultaneously forms all connections. This method offers high efficiency, especially for soldering small and densely packed components.

Flame and Pressure Soldering

These techniques are typically used in hard soldering applications. In flame soldering, heating is performed manually using a flame source such as a gas torch, and it is preferred for joining small parts. In pressure soldering, the filler metal is placed between the parts and then heated to achieve the joint. This method can be used in automated assembly lines.

Heating Technologies

The key factor determining the performance of soldering equipment is the heating technology used.

Resistive Heating Technology

This is the oldest and most classical heating method. It is based on the principle of heating a resistance wire wound around a core or ceramic bobbin using electric current. Over time, this technology may experience performance degradation; the resistance wire wears out and must be replaced periodically. Thermal stability is low and frequent calibration is required. Additionally, it can lead to high electricity consumption and is prone to human error, as the operator must manually adjust the temperature for each material.

Induction Frequency Heating Technology

This is a modern and reliable technology. High-frequency current (typically 450 kHz or 13.56 MHz) generated by a power supply is transmitted to a specially designed soldering tip, directly heating it. Unlike resistive systems, this method does not use consumable materials. Its greatest advantage is extremely high thermal stability (±1°C). The system automatically adjusts its power according to the metal density of the point being soldered, eliminating the need for manual temperature adjustment by the operator. This eliminates the risk of cold solder joints and prevents sensitive components from overheating. It offers high efficiency with lower energy consumption and minimizes operator errors.

Hard Soldering

Hard soldering is a metal joining process performed at temperatures above 450°C. The filler metals used in this technique are typically high-melting-point alloys such as silver, copper, and gold. Joints created by hard soldering exhibit significantly higher mechanical strength compared to soft soldered joints. Therefore, they are preferred in critical applications requiring durability under harsh conditions, such as automotive, aerospace, space, and mechanical manufacturing. Hard soldered joints also provide good electrical conductivity. The application steps include surface preparation, component assembly, heating at the appropriate temperature, soldering, and controlled cooling. Due to the high temperatures involved and the potential generation of harmful fumes during the process, proper use of personal protective equipment and adequate ventilation are of great importance.

Solder Quality and Application Tips

The quality of a solder joint is critical for both the reliability of the connection and the performance of the circuit. A good solder joint should be shiny, smooth, and have a conical fillet shape that fully surrounds the component lead. Poor or defective solder joints are referred to as cold solder joints. Cold solder joints have a dull, matte appearance and may exhibit surface irregularities and cracks. Such connections are mechanically weak and can lead to intermittent electrical failures over time.

Some key tips for achieving good soldering results include:

• Cleanliness: The surfaces to be soldered and the soldering iron tip must be clean. Oxide, oil, and dirt prevent the solder from adhering properly to the surface. The iron tip should be regularly cleaned with a damp sponge or brass wool. After soldering, any residual flux on the circuit board must be removed using a solvent such as cellulose thinner and a brush, as it can cause corrosion.
• Temperature Control: The soldering iron temperature should be set according to the thermal mass of the component and the board. Too low a temperature can cause cold solder joints, while too high a temperature can damage the component or circuit board and compromise the solder structure.
• Proper Heating: The iron tip should simultaneously contact both the component lead and the pad on the circuit board to ensure even heating of both surfaces. Solder should not be applied until the surfaces are sufficiently heated.
• Use of Solder Paste (Flux): Solder wires typically contain flux internally. However, additional flux application helps remove oxide layers from the surface and reduces surface tension, allowing the solder to flow more easily and form a higher quality joint.

Solder Materials and Desoldering

Various materials are used in the soldering process. The most common are standard tin-lead solders sold in wire form with different diameters. Additionally, solder paste, used for soldering surface mount devices (SMDs), contains alloys such as 62% tin, 36% lead, and 2% silver. For specialized applications such as jewelry, silver solder is available in forms such as pellets, sheets, wire, or powder.

The process of removing an incorrect or outdated solder joint is called desoldering. Two primary methods are used for this. The first involves heating the soldered area with a soldering iron and removing the component while the solder is in liquid form. The second, and cleaner, method involves removing the molten solder from the area using either a desoldering pump (a vacuum tool) or a desoldering braid (copper mesh wire). In particular, for components with many leads that are difficult to desolder, an effective technique is to add a small amount of new solder to the area to facilitate melting of the old solder, followed by complete removal of all solder.