Brief description of welding tools

Hand soldering and rework are process steps that require excellent operator skills and good tooling; an inexperienced operator can create reliability nightmares. When equipped with adequate tools and training, the operator should be able to create reliable solder joints. Surface mount hand soldering is sometimes more challenging than through-hole soldering because of the smaller pin pitch and higher pin count. During the rework process, care must be taken not to overheat the printed circuit board; otherwise, both the plated through-holes and the pads can be easily damaged. This article will review contact soldering and heated gas soldering, the two most common types of hand soldering. Contact Soldering

　　Contact soldering is accomplished when a heated tip or collar is in direct contact with the solder joint. The tip or collar is mounted on the soldering tool. Tips are used to heat a single solder joint, while collars are used to heat multiple solder joints simultaneously. There are a variety of designs for single tip soldering tools and tips. There are also a variety of designs for tips in the form of collars. There are two or four discrete collars that are primarily used for component removal. The collars are designed primarily for multi-pin components, such as integrated circuits (ICs); however, they can also be used to remove rectangular and cylindrical components. Collars are useful for removing components that have been glued together. After the solder melts, the collar can be twisted around the component to break the glue connection.

　　Four-sided components, such as plastic lead chip carriers (PLCCs), create a problem because it is difficult for the soldering iron ring to contact all the pins at the same time. If the soldering iron ring does not contact all the pins, heat conduction will not occur, which means that some solder joints will not melt. Especially on J-type lead components, all the pins may not be on the same reference plane, which makes it impossible for the soldering iron ring to contact all the pins at the same time. This situation can be catastrophic because the pads that are still soldered to the pins will be pulled out of the PCB when the operator removes the component.

　　Soldering tips and rings require frequent preventive maintenance. They need to be cleaned and sometimes tinned. Frequent replacement may be required, especially when using small soldering iron tips. Contact Soldering Systems

　　Contact soldering systems can be categorized from low price to high price and usually limit or control temperature. The choice depends on the application. For example, surface mount applications usually require less heat than through-hole applications.

　　Constant temperature systems provide continuous, constant output, delivering heat continuously. For surface mount applications, these systems should operate within a temperature range of 335 to 365°C. Limited temperature systems have a temperature limiting capability that helps keep the system temperature within an optimal range. These systems deliver heat discontinuously, which prevents overheating, but heat recovery can be slow. This can cause the operator to set a higher temperature than desired, speeding up soldering. The operating temperature range for surface mount applications is 285 to 315°C. Controlled temperature systems provide high output capabilities. These systems, like temperature limited systems, deliver heat discontinuously. Response timing and temperature control are superior to limited temperature systems. The operating temperature range for surface mount applications is 285 to 315°C. These systems also provide better deviation capabilities, typically 10°C. Features associated with contact soldering systems include: In most cases, contact soldering is the easiest and lowest cost method for touch-up and component removal and replacement. Components attached with glue can be easily removed with solder rings. Contact soldering equipment is relatively low cost and readily available. Problems associated with contact soldering systems include: Systems that do not contain the tip or ring are susceptible to temperature shock, which raises the tip or ring temperature above the desired range. The soldering iron ring must be in direct contact with the solder joint and the lead to be effective. Temperature shock can damage ceramic components, especially multilayer capacitors. Heated gas (hot air) soldering

　　Hot air soldering is accomplished by directing heated air or an inert gas, such as nitrogen, through a nozzle toward the solder joint and pins. Hot air equipment options range from simple handheld units that heat a single location to complex automated unit designs that heat multiple locations. Handheld systems remove and replace rectangular, cylindrical, and other small components. Automated systems remove and replace complex components, such as fine-pitch and area array components.

　　Hot air systems avoid localized thermal stresses that can occur with contact soldering systems, making them the first choice in applications where uniform heating is critical. Hot air temperatures typically range from 300 to 400°C. The time required to melt the solder depends on the amount of hot air. Larger components may require more than 60 seconds of heating before they can be removed or replaced.

　　Nozzle design is important; the nozzle must direct the hot air toward the solder joint, sometimes away from the component body. Nozzles can be complex and expensive. Adequate preventive maintenance is necessary; nozzles must be cleaned regularly and properly stored to prevent damage. Characteristics associated with hot air systems include: The low efficiency of hot air as a heat transfer medium reduces thermal shock due to slow heating rates. This is an advantage for certain components, such as ceramic capacitors. Using hot air as a heat transfer medium eliminates the need for direct soldering iron tip contact. Temperatures and heating rates are controllable, repeatable, and predictable. Issues associated with hot air systems include: Hot air soldering equipment ranges in price from medium to high. Automatic systems are quite complex and require a high skill level to operate. Flux and solder flux can be dripped from small bottles and can be used with sealed or refillable flux pens. Often, operators use too much flux. I prefer to use flux pens because they limit the amount of flux used. I also prefer to use flux cored solder, which contains flux and solder alloy. When using cored solder and liquid flux, make sure the fluxes are compatible with each other.

　　Surface mount soldering usually requires smaller diameter wire, typically in the 0.50-0.75mm range. Through-hole soldering usually requires larger diameter wire, in the 1.20-1.50mm range. Solder paste can also be applied with a syringe, although many manual soldering methods heat the paste too quickly, causing spatter and solder balls. Flux glue, rather than solder paste, is very useful for replacing area array components.