PCB Technical - Basic training for manual soldering of PCB circuit boards

This article introduces a basic training, PCB manual soldering.

A strong solder joint requires the use of a well-soldered, well-maintained soldering iron tip, at a temperature of approximately 100°F above the liquid temperature of the solder. The soldering tin on the soldering iron tip improves the rapid heat transfer from the soldering iron to preheat the workpiece. Preheating is required to establish good flow and wetting. Pads, holes and component leads with good solderability characteristics will help to form good solder joints in the shortest time. At elevated temperatures, a short time is the key to avoid damage to the PCB substrate, damage to the bonding of the pad and the PCB substrate, and excessive metal-to-metal growth. Solder spots exposed to repeated temperature cycles above the liquefaction temperature of the solder and/or PCB substrate Tg may suffer cumulative degradation of reliability. The best method is to complete the solder joint in less than 5 seconds, preferably about 3 seconds. This time includes all necessary operations required to create a connection.

Manual welding process

A recommended manual soldering procedure is to quickly touch the heated and tinned soldering iron tip to the cored wire, and then touch the solder joint area, and use the molten solder to help the initial heat transfer from the soldering iron to the workpiece. Then move the tin wire away from the tip of the soldering iron that will touch the soldering surface. Some people recommend to first touch the soldering iron tip to the pin/pad; place the solder wire between the soldering iron tip and the pin to form a thermal bridge; then quickly move the solder wire to the opposite side of the solder joint area. Either method, if done correctly, will give satisfactory results.

The purpose of these two technologies is to ensure that the temperature of the pins and pads is sufficient to melt the tin wire and form the required metal-to-metal bond. If the soldering iron directly contacts and melts the tin wire during the formation of the solder joint, the surface to be soldered may not be hot enough to improve the solder flow, and the formed solder joint may not be really wet to the pad or solder hole (barrel) and pin (lead). When the process is implemented correctly, the flux will melt and flow on the surface to be soldered before the solder, pre-treating the surface, so the solder will wet and flow on the surface, enter the gap, and form a joint. Once the wetting is established and there is sufficient solder flow to form the desired solder joint, the tin wire and subsequent soldering iron are removed from the solder joint area.

After training, practice and relatively formal application, these procedures are not too difficult for motivated and experienced personnel to implement. Some people are faster than others and like it better, and even the most experienced and smartest operators will have to master the process for a few days. This difference comes from the operation considered to be controlled. For this reason, good initial training and regular updates should be provided to the operator. These aspects should include the art and construction of manual welding, the factors that control the formation of welded joints, and the company's standards for accepting and rejecting welded joints.

Possible problems caused by manual welding

The problems in producing strong and acceptable manual solder joints are usually caused by the use of inappropriate temperature, too much pressure, extended retention time, or a combination of the three. However, the root causes of these problems are often related to the tools they use, rather than the skills and motivation of the operators.

Use skilled, trained, responsible and motivated operators to see if the manual welding operation needs improvement in other parts of the process. Some thick PCB designs may require different methods and/or assistance, such as auxiliary heating with hotplates.

Another reason may be poor solderability components, which can usually be dealt with by changes in component specifications or long-distance transportation. Approval and replacement of a different cored wire flux may be appropriate. The use of externally applied liquid flux is another short-term alternative. In this case, the minimum required amount should be used in a controlled manner. Before using any liquid flux to aid manual soldering, tests should be conducted to confirm the compatibility of the flux and residues. If the material comes from the same PCB manufacturer, then you can get the data. If the material comes from different manufacturers, it usually brings the burden of experimentation to the user, because there are too many possible combinations for any supplier.