Summary of Circuit Board Welding
Welding is one of the important processes in manufacturing electronic products. Without corresponding process quality assurance, any well-designed electronic product is difficult 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 produce one or two circuit boards, and manual soldering is often required. In mass production, everything from component screening and testing to circuit board assembly and soldering is completed by automated machinery, such as automatic testing machines, component cleaning machines, tinning machines, shaping machines, insertion machines, wave soldering machines, leg cutting machines, printed board cleaning machines, etc. These computer-controlled production equipment have played an important role in modern large-scale electronic product production, ensuring consistency in process conditions and soldering operations, and improving product quality.

                                                                                                                              
The classification of welding and the conditions for soldering. The classification of welding technology is widely used in the electronics industry, and almost all welding methods are used in the manufacturing process of electronic products. However, the most commonly used and representative method is soldering. Soldering is a type of welding that involves heating a solder joint and a solder material with a lower melting point than the solder joint to the soldering temperature. Without melting the solder joint, the solder melts and infiltrates the soldering surface, forming a connection between the two through atomic diffusion. Its main features include the following three points:
⑴ The melting point of the solder is lower than that of the welded component; ⑵ When welding, heat the solder and the workpiece together to the soldering temperature, causing the solder to melt while the workpiece remains unmelted The formation of welding relies on the infiltration of melted solder into the welding surface, and capillary action causes the solder to enter the gap of the welded part, forming an alloy layer, thereby achieving the bonding of the welded part.
Except for some alloy materials containing a large amount of chromium, aluminum and other elements that are not suitable for soldering, most other metal materials can be soldered. The soldering method is simple and only requires the use of simple tools (such as soldering iron) to complete the process of soldering, solder joint repair, component replacement, and re soldering. In addition, soldering also has the advantages of low cost and easy automation. In electronic engineering technology, it is the earliest, most widely used, and largest proportion of soldering methods.
The physical basis of soldering that must be met is "wetting", also known as "wetting". To explain infiltration, let's start with the water droplets on lotus leaves: there is a layer of impermeable waxy material on the surface of lotus leaves, and the surface tension of water keeps it in a bead shape. It rolls on the lotus leaf but cannot spread out, and this state is called inability to infiltrate; On the contrary, if a liquid spreads out on the contact surface with a solid and fully spreads and contacts, it is called infiltration. The process of soldering is to melt, flow, and infiltrate the lead tin solder on the welding surface through heating, allowing lead tin atoms to penetrate into the surface of the copper base material (wire, solder pad) and form a brittle alloy layer of Cu6-Sn5 on the contact surface between the two.
The angle formed by the contact between the solder and the base metal during the welding process is called the wetting angle, as shown in Figure 5.13. (a) In the picture, at that time, the solder did not wet the base material and could not form a good solder joint; (b) In the picture, at that time, the solder infiltrated with the base material, forming a good solder joint. By carefully observing the wetting angle of the solder joint, the quality of the solder joint can be determined.
Figure 5.9 shows the infiltration and infiltration angle. Obviously, if there is dirt or oxide layer on the welding surface that blocks infiltration, it cannot generate an alloy layer of two metal materials, or if the temperature is not high enough to fully melt the solder, it cannot cause the solder to infiltrate. The following conditions must be met for soldering:
⑴ Weldable components must have good weldability, which refers to the performance of the alloy that can form a good bond between the welded metal material and the solder at an appropriate temperature. Not all metals have good weldability, some metals such as chromium, molybdenum, tungsten, etc. have very poor weldability; Some metals have good weldability, such as copper, brass, etc. During welding, the high temperature causes an oxide film to form on the metal surface, which affects the weldability of the material. To improve weldability, measures such as surface tin plating and silver plating can be taken to prevent oxidation of the material surface.
⑵ The surface of the welded component must be kept clean in order to achieve good bonding between the solder and the welded component. Even weldments with good weldability may produce harmful oxide films and oil stains on the surface of the weldment due to storage or contamination. Before welding, it is necessary to remove the dirty film completely, otherwise the welding quality cannot be guaranteed. Mild oxide layers on metal surfaces can be removed by the action of solder, while metal surfaces with severe oxidation should be removed by mechanical or chemical methods, such as scraping or acid washing.
The purpose of using appropriate soldering flux is to remove the oxide film on the surface of the solder joint. Different soldering processes require the use of different fluxes, such as nickel chromium alloys, stainless steel, aluminum, and other materials. Without specialized special fluxes, it is difficult to implement soldering. When soldering precision electronic products such as printed circuit boards, rosin based soldering flux is usually used to ensure reliable and stable soldering. It is generally used to dissolve rosin into rosin perfume with alcohol.
When the welded component is heated to an appropriate temperature for welding, the function of thermal energy is to melt the solder and heat the welded object, allowing tin and lead atoms to obtain sufficient energy to penetrate into the lattice of the welded metal surface and form an alloy. The welding temperature is too low, which is not conducive to the penetration of solder atoms and cannot form alloys, making it easy to form virtual welding; Excessive welding temperature can cause the solder to be in a non eutectic state, accelerating the decomposition and volatilization rate of the flux, resulting in a decrease in the quality of the solder, and in severe cases, it can also cause the solder pads on the printed circuit board to detach.
It should be emphasized that not only should the solder be heated to melt, but also the solder should be heated to a temperature that can melt the solder at the same time.
Suitable welding time refers to the time required for physical and chemical changes throughout the entire welding process. It includes several parts: the time for the metal to reach the welding temperature, the melting time of the solder, the time for the flux to function, and the time for the formation of the metal alloy. After the welding temperature is determined, the appropriate welding time should be determined based on the shape, properties, characteristics, etc. of the welded part. Welding time is too long, which can easily damage components or welding parts; If it is too short, it will not meet the welding requirements. Generally, the maximum welding time for each solder joint is no more than 5 seconds.
Preparation before welding - Tin plating In order to improve the quality and speed of welding and avoid defects such as virtual welding, the welding surface should be subjected to solderability treatment - Tin plating - before assembly. Printed circuit boards that have not been cleaned and coated with flux should be processed according to the methods described in Chapter 5. Plating solder on the solder surface (leads or other areas that require soldering) of electronic components is a very important process before soldering, especially for some components with poor solderability, tin plating is crucial. Professional electronic manufacturers have specialized equipment for solderability treatment.
Tin plating, also known as "tinning", is actually the infiltration of liquid solder onto the surface of the metal being soldered, forming a bonding layer that is different from both the metal being soldered and the solder. This bonding layer firmly connects the solder and the metal to be welded, two materials with different properties and compositions.
The basic skills of manual soldering with an electric soldering iron are not difficult to master, but there are certain technical essentials. People who have been engaged in electronic product production for a long time improve the quality of welding from four aspects: materials, tools, methods, and operators.
The most important of course is still human skills. Without a considerable amount of welding practice and careful experience and understanding, one cannot master the technical essentials of welding; Even technicians who have been engaged in welding work for a long time cannot guarantee that the quality of each welding point is completely consistent. Only by fully understanding the principles of welding and practicing diligently can one learn the basic skills of welding in a relatively short period of time. The specific methods and key points introduced below are all summaries of practical experience and are shortcuts for beginners to quickly master welding skills.
Beginners should practice diligently and continuously improve their operational skills, and should not leave welding quality issues to be resolved during the overall circuit debugging.
Mastering the correct welding posture can ensure the physical and mental health of the operator and reduce labor injuries. To reduce the harm of chemical substances emitted during the heating of solder to humans and minimize the inhalation of harmful gases, the distance from the soldering iron to the nose should generally be no less than 20cm, with a recommended distance of 30cm.
There are three ways to grip an electric soldering iron, as shown in Figure 5.15.
Figure 5.10 Schematic diagram of holding a soldering iron Figure 5.11 Holding method of soldering wire The reverse holding method has stable movement and is not easy to fatigue after long-term operation, suitable for the operation of high-power soldering iron; The positive grip method is suitable for operating medium power soldering irons or soldering irons with bends; When soldering printed boards and other components on the control panel, the pen holding method is commonly used.
There are generally two ways to handle solder wire, as shown in Figure 5.16. Due to the presence of a certain proportion of lead in solder wire, which is a harmful heavy metal to the human body, gloves should be worn or hands should be washed after operation to avoid ingestion of lead dust.
After using the soldering iron, be sure to securely insert it into the iron holder and be careful not to touch the soldering iron tip with wires or other debris to avoid scalding the wires and causing accidents such as electric leakage.
The basic steps of manual welding operation are to master the temperature and welding time of the soldering iron, select the appropriate contact position between the soldering iron tip and the welding point, in order to obtain good welding points. The correct manual welding process can be divided into five steps, as shown in Figure 5.17.
Step 1: Prepare for welding (Figure (a)) Hold the welding wire in your left hand and the soldering iron in your right hand to enter the backup welding state. Require the soldering iron tip to be clean, free of solder slag and other oxides, and coated with a layer of solder on the surface.
Step 2: Heat the solder joint (Figure (b)). Place the soldering iron tip against the connection between the two solder joints and heat the entire solder joint for approximately 1-2 seconds. For soldering components on a printed circuit board, it is important to ensure that the soldering iron tip contacts both objects being soldered simultaneously. For example, the wires and terminals, component leads and solder pads in Figure (b) should be uniformly heated simultaneously.
Step 3: When the welding surface of the workpiece is heated to a certain temperature by feeding the welding wire (Figure (c)), the solder wire contacts the workpiece from the opposite side of the soldering iron. Caution: Do not place solder wire on the soldering iron tip!
Step 4: Remove the welding wire (as shown in Figure (d)). Once the welding wire has melted to a certain extent, immediately move it 45 degrees to the left.
Step 5: Remove the soldering iron (as shown in Figure (e)) from the soldering area where the solder has infiltrated the solder pad and component. Then, move the soldering iron to the upper right at a 45 ° angle to end the soldering process. The time from the third step to the fifth step is approximately 1-2 seconds.
The five step soldering method shown in Figure 5.12 can be simplified into three steps for soldering components with small heat capacity, such as connecting thinner wires on printed boards.
① Preparation: Same as steps 1 and 2 above Heating and wire feeding: After placing the soldering iron tip on the workpiece, insert the welding wire.
③ Wire removal soldering iron: After the solder has infiltrated and diffused on the welding surface to the expected range, immediately remove the welding wire and move the soldering iron away, and be careful not to delay the time of removing the welding wire from the soldering iron.
For welding components that absorb low heat, the entire process takes only 2-4 seconds. The rhythm control of each step, accurate mastery of the sequence, and proficient coordination of movements all require extensive practice and careful experience to solve. Someone has summarized the method of controlling time in a few seconds in the five step operation method: count one or two after the soldering iron contacts the solder joint (about 2 seconds), count three or four after feeding the welding wire, remove the soldering iron, and the amount of melting of the welding wire depends on observation. This method can be used as a reference, but due to factors such as differences in soldering iron power and solder joint heat capacity, there is no fixed rule to follow in actual mastery of welding temperature, and specific conditions must be treated accordingly. Imagine, for a solder joint with a large heat capacity, if a low-power soldering iron is used for soldering, during the above time, the heating temperature may not be enough to melt the solder, making soldering impossible.
The welding temperature and heating time have been mentioned more than once when introducing the mechanism and conditions of soldering, and appropriate temperature is essential for forming good solder joints. How to control this temperature? Of course, based on relevant data, the optimal temperature required for different welding materials can be clearly identified, and relevant curves can be obtained. However, in the general welding process, it is not possible to use instruments such as thermometers to detect the temperature at any time, but rather to use more intuitive and clear methods to understand the temperature of the welded parts.
Through experiments, it has been found that the time the soldering iron tip stays on the solder joint is directly proportional to the increase in solder joint temperature. When using the same soldering iron to heat workpieces with different heat capacities, achieving the same welding temperature can be achieved by controlling the heating time. But in practice, the heating time cannot be determined solely based on this relationship. For example, when using a low-power soldering iron to heat larger workpieces, regardless of how long the soldering iron stays, the temperature of the workpiece cannot rise because the heating capacity of the soldering iron is less than the heat dissipated by the workpiece and the soldering iron in the air. In addition, to prevent internal overheating and damage, some components are also not allowed to be heated for a long time.
What are the effects and external characteristics of heating time on welded parts and joints? If the heating time is insufficient, the solder cannot fully infiltrate the workpiece, resulting in resin slag inclusion and virtual welding. On the contrary, excessive heating not only may cause damage to components, but also has the following hazards and external characteristics:
⑴ The appearance of the solder joint deteriorates. If excessive heating is continued after the solder has already infiltrated the workpiece, it will cause the flux to evaporate completely, resulting in overheating of the molten solder and reducing its wetting performance; When the soldering iron leaves, it is easy to pull out the solder tip, and at the same time, the surface of the solder joint turns white, rough particles appear, and loses its luster.
⑵ High temperature causes the decomposition and carbonization of the added rosin flux. Rosin generally begins to decompose at 210 ℃, not only losing its function as a flux, but also forming carbon slag inside the solder joint, becoming a slag inclusion defect. If rosin turns black during the welding process, it is definitely caused by excessive heating time.
Excessive heating can damage the adhesive layer of copper foil on the printed circuit board, leading to the peeling of copper foil pads. Therefore, accurately mastering the heating temperature at the appropriate heating time is the key to high-quality welding.