Figure 1: Solder wire
Solder is an alloy with a low melting point; it permanently bonds two or more metal surfaces. There are various types of solders available, such as lead-based, lead-free, and silver-based, each with its unique properties and benefits. Choosing the appropriate solder type and flux is essential to ensure that the joint is strong, durable, and long-lasting. This article explores the different types of solder, their features, and their applications.
Understanding the various types of solder and their properties is important to ensure that the solder joint can withstand the environmental and mechanical stresses to which it will be exposed to.
By selecting the right type of solder, you can ensure a strong, reliable, and long-lasting connection that will meet the specific requirements of your application.
Lead-based solder contains lead as one of its primary components. Tin-lead solder, the most common lead solder, contains approximately 60% tin and 40% lead. Tin is added for its lower melting point. Additionally, lead prevents tin whisker growth (small, hair-like protrusions called whiskers grow from the surface of tin, leading to short circuits). Lead solder has been widely used in various applications, including electronics, plumbing, and metalworking.
The EPA (Environmental Protection Agency) has established regulations for using lead-free pipes, fittings, fixtures, solder, and flux for drinking water, requiring no more than 0.2% lead when used with respect to solder and flux. Lead-free solder typically contains a combination of metals such as tin, copper, silver, and sometimes small amounts of other metals.
Flux core solder contains a flux core inside the solder wire. Flux is a material that helps to clean and prepare the metal surfaces to be joined by removing oxides, contaminants, and other impurities. It also helps to facilitate the flow of molten solder by reducing surface tension and preventing oxidation during the soldering process.
The flux core inside the solder wire comprises a mixture of organic and inorganic compounds designed to react with the metal surfaces as they are heated during the soldering process. There are two forms of flux core solders.
As the solder wire melts, the flux core is released and cleans the surfaces to be joined. This makes the solder's flow easier and creates a stronger, more reliable bond.
Figure 2: Soldering a circuit board using a wire
Solder comes in several forms, including wire, preform solder, paste, and powder. Choosing between these solder types demands a thorough examination of the features and application of each type, as seen in Table 1.
Features Solder wire Preform soldering Solder paste Appearance A thin, metal wire coated with a flux material Precut pieces of solder A mixture of finely powdered metal solder and flux. How to use Cut the solder wire to the desired length and then melt it with a soldering iron or another heat source. Place the preform on the metal surfaces to be joined and then heat the components to a temperature above the melting point of the solder. Solder paste is typically applied to the pads on a PCB assembly using a stencil, which allows for precise placement and control of the amount of solder paste used. Once the solder paste has been applied, the electronic components are placed onto the pads. The entire assembly is heated in a reflow oven to melt the solder and create a permanent electrical connection. Advantages Easy to use, suitable for a variety of applications, can be used with different types of flux Preforms provide precise control over the amount of solder used in each joint, which helps to reduce waste and improve consistency. They eliminate the need for messy flux materials and can help to reduce solder voids and other defects in the finished product. Allows for the simultaneous soldering of multiple components, saving time and improving production efficiency. Also, solder paste is ideal for high-density PCBs, where traditional wire soldering may be impractical. Limitations Limited control over the amount of solder applied can be messy and may require additional flux The preforms should be properly placed and aligned on the metal surfaces before heating. This requires careful attention to detail and specialized equipment, such as pick-and-place machines or automated assembly equipment. Solder paste requires careful handling and storage to prevent drying out or contamination of the flux material, which can affect the quality of the solder joints. Application General-purpose soldering and repair work Automated assembly and precise joint configurations Fine-pitch components and surface mount technologyTable 1: Comparison of various solder types based on their form factor (physical features)
A cold solder occurs when the solder melts partially due to insufficient heating. Cold joints are harsh and irregular in appearance, resulting in an unstable joint. If this happens, reheat the junction until the solder flows again. Also, ensure that the soldering iron is appropriately preheated and is running at the correct power.
A solder wick, also known as desoldering braid or soldering wick, is a braided copper wire used to remove excess solder from a circuit board or electronic component. The braided copper wire of the solder wick is coated with flux, which helps to remove the solder from the joint or component being worked on.
To use a solder wick, place the braided wire over the joint or component and heat it with a soldering iron. As the solder melts, it is drawn into the braided wire through capillary action, effectively removing it from the joint or component.
Solder wick is particularly useful for removing excess or unwanted solder, for example, when correcting a solder bridge (when two or more connections are accidentally joined together with excess solder) or when replacing a component. It is also commonly used when salvaging electronic components from old or damaged devices, as it allows the removal of the old solder to make way for new connections.
Figure 3: Solder wick
When selecting a suitable solder for a particular application, there are several critical criteria to consider.
Read our article on soldering for a step-by-step guide on how to solder a component to an electronic board.
The composition of solder can vary depending on its intended use, but typically it is made of a mixture of metals such as tin, lead, silver, or copper.
Yes, stainless steel can be soldered, although it can be difficult due to its high oxidation resistance and low thermal conductivity. Use a solder with a melting point that is lower than the melting point of steel. Also, choose a soldering iron that works at higher temperatures.
Solder flux is used to clean the metal surface, reduce surface tension, and protect the joint from oxidation, while solder paste is used to join components to a circuit board by melting the solder in the paste.
A soldering wire is a thin wire made of a metal alloy that combines two metal surfaces by melting the wire with a soldering iron. Solder paste is a mixture of metal alloy powder and flux suspended in a paste-like substance.
Common soldering wire materials are tin, lead, silver, or copper. Flux is often added to the solder wire to remove oxidation, impurities, and promote adhesion to metal surfaces being joined.
Solder powder is a fine metallic powder of one or more metals like tin, lead, and silver. Solder powders are often mixed with flux.
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