PCB Tech - SMT process manual soldering iron welding technology

Hand soldering with an electric soldering iron is not difficult to master, but there are some technical essentials. People who have been engaged in the production of SMT electronic products for a long time have summarized the four elements of welding (also known as 4M): materials, tools, methods, methods and operators. The most important of these is of course human skills. Without a considerable amount of welding practice and careful experience and understanding, one cannot master the technical essentials of welding; even a skilled worker who has been engaged in welding for a long time cannot guarantee the highest quality of each SMT solder joint. Only by fully understanding the principle of welding coupled with careful practice can it be possible to learn the basic skills of welding in a relatively short period of time. Some specific methods and points of attention introduced below are shortcuts for beginners to quickly master welding skills.

Beginners should be diligent in practice, and constantly improve their operating skills, and can't leave the welding quality problem to the whole machine circuit debugging to solve it.

1 Correct posture of welding operation

Mastering the correct operating posture can ensure the physical and mental health of the operator and reduce labor injuries. In order to reduce the harm to people caused by the chemical substances volatilized when the flux is heated, and reduce the inhalation of harmful gases. Under normal circumstances, the distance from the soldering iron to the nose should be no less than 20cm, usually 30cm.

2 Basic steps of SMT welding operation

Only by mastering the temperature and soldering time of the soldering iron, and choosing the proper contact position between the soldering iron tip and the solder joint, a good solder joint can be obtained. The correct welding operation process can be divided into five steps:

2.1 Preparation for welding: Hold the soldering iron in your right hand and enter the state for preparation for welding. The soldering iron tip is required to be kept clean, free of slag and other oxides, and a layer of solder is plated on the surface.

2.2 Heating the weldment: The soldering iron head rests on the joint of the two weldments to heat the entire weldment for about 1-2 seconds. For soldering components on the printed board, pay attention to making the soldering iron head contact the pad and the lead of the component at the same time.

2.3 Feeding the welding wire: When the welding surface of the weldment is heated to a certain temperature, the solder wire touches the weldment from the opposite side of the soldering iron. Note: Do not send the solder wire to the tip of the soldering iron.

2.4 Remove the welding wire: When the welding wire has melted a certain amount, immediately move the welding wire to the upper left 45°C.

3 Welding temperature and heating time

Proper temperature is essential for the formation of good solder joints. How to master this temperature? When according to the relevant data, it is possible to clearly find the optimal temperature required for different weldment materials, and obtain the relevant curve. However, in the general welding process, it is impossible to use thermometers and other instruments to detect at any time, but hope to use a more intuitive and clear method to understand the temperature of the weldment.

Through experience, it is concluded that the time that the soldering iron tip stays on the weldment is directly proportional to the increase in the temperature of the weldment. For the same soldering iron, when heating weldments with different heat, you can achieve the same welding temperature by controlling the heating time. However, in practice, the heating time cannot be determined solely based on this relationship. When a small and medium-power soldering iron is used to heat larger pieces, no matter how long the soldering iron stays, the temperature of the soldering piece will not rise because of the heating capacity of the soldering iron. Less than the heat lost by the weldment and soldering iron in the air. In addition, in order to prevent internal damage due to overheating, some components are not allowed to be heated for a long time. What is the effect of heating time on soldering and solder joints and their external characteristics? If the heating time is not enough, the solder will not be able to fully infiltrate the soldering parts, resulting in rosin inclusions and false soldering.

4 Specific methods of connection operation

Under the goal of ensuring high quality, the specific welding operation methods can vary from person to person, but the following methods summarized by the predecessors can not be ignored for beginners.

(1) Keep the soldering iron tip clean

When SMT soldering, the soldering iron tip is in a high temperature state for a long time, and it is exposed to weak acid substances such as flux, and its surface is easily oxidized and stained with a layer of black impurities. These impurities form an insulating layer. It hinders the heat conduction between the soldering iron tip and the weldment. Therefore, pay attention to rubbing away impurities on the soldering iron stand. Wiping the tip of the soldering iron with a damp cloth or damp sponge at any time is also one of the commonly used methods. For ordinary soldering iron tips, you can use a file to remove the surface oxide layer when the contamination is serious. For long-life soldering iron tips, this method must not be used.

(2) Accelerate heat transfer by increasing the contact area

When heating, all parts of the weldment that need to be infiltrated by solder should be evenly heated, not just a part of the weldment, and do not use a soldering iron to increase the pressure on the weldment, so as to avoid damage or unnoticeable traps. Some beginners try to speed up the soldering by applying pressure to the soldering surface with a soldering iron tip, which is wrong. The correct method is to choose different soldering iron tips according to the shape of the weldment, or trim the soldering iron tip yourself so that the soldering iron tip and the weldment form surface contact instead of point or line contact. In this way, efficiency can be greatly improved.

(3) Heating depends on SMT solder bridge

In non-pipeline operations, the solder joint shapes for SMT soldering are varied, and it is unlikely that the soldering iron tip will be replaced constantly. To improve the heating efficiency, a solder bridge for heat transfer is required. The so-called solder bridge is a bridge that transfers heat between the soldering iron tip and the solder when the soldering tin is retained on the soldering iron tip. Since the thermal conductivity of molten metal is much higher than that of air, the solder joints are heated to the soldering temperature quickly. It should be noted that the amount of tin used as a solder bridge should not be kept too much to avoid misconnection.