Welding is one of the important links in manufacturing electronic products. Without the corresponding process quality assurance, it is difficult for any well-designed electronic product to meet the design requirements. In the process of scientific research and development, design and trial production, and technological innovation, it is impossible and unnecessary to use automatic equipment to make one or two circuit boards, and it is often necessary to assemble and weld them manually. In mass production, from the screening and testing of components to the assembly and welding of circuit boards, it is all done by automated machinery, such as automatic testing machine, component cleaning machine, tin plating machine, setting machine, plug-in machine, wave soldering machine, leg breaking machine, PCB cleaning machine and so on. These computer-controlled production equipment have played an important role in the production of modern large-scale electronic products, which is conducive to ensuring the consistency of process conditions and assembly and welding operations and improving product quality.

Welding classification and welding conditions

Classification of welding

Welding technology is widely used in electronic industry. In the process of manufacturing electronic products, almost all kinds of welding methods are used, but the most common and representative method is soldering. Brazing is a kind of welding, which is to heat the weldment and solder with a melting point lower than that of the weldment to the brazing temperature. Without melting the weldment, the solder melts and soaks into the welding surface, and the connection of the weldment is formed by the diffusion of two atoms. Its main features are as follows:

⑴? The melting point of solder is lower than that of weldment;

⑵? When welding, the solder and the weldment are heated to the brazing temperature at the same time, and the solder melts but the weldment does not melt;

⑶? The formation of welding depends on the molten solder infiltrating the welding surface, and the solder enters the gap of weldments through capillary action to form an alloy layer, thus realizing the combination of weldments.

Except for some alloy materials containing a lot of elements such as chromium and aluminum, which are not suitable for soldering, most other metal materials can be soldered. The welding method is simple, and welding, solder joint renovation, component replacement and re-welding can be completed only by using simple tools (such as electric soldering iron). In addition, welding has the advantages of low cost and easy automation. In electronic engineering, it is the earliest, widest and largest welding method.

Basic skills of manual soldering iron welding

Manual welding with electric soldering iron is not difficult to master, but it has certain technical essentials. People who have been engaged in the production of electronic products for a long time can improve the welding quality from four aspects: materials, tools, methods and operators.

Of course, the most important thing is human skills. Without considerable welding practice and detailed experience, it is impossible to master the technical essentials of welding; Even a skilled worker who has been engaged in welding for a long time cannot guarantee that the quality of each solder joint is completely consistent. Only by fully understanding the welding principle and practicing attentively can we learn basic welding skills in a short time. The following specific methods and precautions are a summary of practical experience and a shortcut for beginners to quickly master welding skills.

Novices should practice frequently and constantly improve their operation skills, and can't leave the welding quality problem until the whole circuit is debugged.

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 of chemicals emitted by the flux during heating and the inhalation of harmful gases, under normal circumstances, the distance from the soldering iron to the nose is not less than 20cm, generally 30cm.

Causes and harm of virtual welding

Virtual welding is mainly due to the oxide and dirt on the surface of the metal to be welded, which makes the solder joint in a connection state with contact resistance, which leads to abnormal circuit operation, unstable connection and irregular increase of noise, which brings great hidden dangers to circuit debugging, use and maintenance. In addition, some virtual solder joints keep good contact for a long time after the circuit starts to work and are not easy to be found. However, under the action of environmental conditions such as temperature, humidity and vibration, the contact surface gradually oxidizes and the contact gradually becomes incomplete. The contact resistance of virtual solder joint will cause local heating, and the increase of local temperature will further worsen the incomplete contact of solder joint, and finally even make the solder joint fall off, and the circuit will not work normally at all. This process can last for a year or two, and its principle can be explained by the concept of "primary battery": when the solder joint is wet, water molecules dissolve metal oxides and dirt to form electrolyte, and the copper and lead-tin solders on both sides of the virtual solder joint are equivalent to the two electrodes of the primary battery. Lead-tin solder loses electrons and is oxidized, while copper material gains electrons and is reduced. In such a galvanic cell structure, metal corrosion occurs at the virtual solder joint, local temperature rise intensifies the chemical reaction, and mechanical vibration makes the gap between them expand continuously until a vicious circle finally forms an open circuit at the virtual solder joint.

According to statistics, nearly half of the failures of electronic products are caused by poor welding. However, it is not easy to find out the virtual solder joint that causes the fault from the electronic devices with thousands of solder joints. Therefore, virtual welding is a hidden danger of circuit reliability and must be strictly avoided. Pay special attention to manual welding operation.

Generally speaking, the main reasons for virtual soldering are: poor solder quality; Poor reducibility or insufficient flux consumption; The surface of the welding part is not cleaned in advance, and the tin plating is not firm; The temperature in the welding head is too high or too low, and there is an oxide layer on the surface; Welding time is not well mastered, too long or too short; When the solder does not solidify during the welding process, the welding element is loose.

Power-on inspection

After the appearance inspection, it is considered that the connection is correct before the power-on inspection, which is the key to test the circuit performance. Without strict appearance inspection, power-on inspection is not only more difficult, but also may damage equipment and instruments and cause safety accidents. For example, if the power supply cable is soldered in white, then the equipment will be found unable to be electrified after being electrified, and of course it can't be checked.

Power-on inspection can find many tiny defects, such as invisible circuit bridging, but it is not easy to find hidden dangers of internal virtual welding. Therefore, the fundamental problem is to improve the technical level of welding operation, and the welding problem cannot be left to the inspection process.

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