BGA IC soldering process principle

（一）The scientific process of chip surface mounting and soldering technology improves the success rate of BGA disassembly and soldering.

1. Firstly, it is necessary to preheat the BGA IC, especially for large-area plastic packaging BGA IC, to avoid sudden high temperatures that can cause damage due to thermal expansion and deformation of the PCB board. The general method of preheating is to raise the hot air nozzle, pull it away from the IC, and blow the IC evenly. The time is generally controlled within 10 seconds.

If a mobile phone that has been exposed to water needs to have its BGA IC soldered, the preheating area can sometimes be slightly increased to facilitate a more thorough removal of the IC and PCB board. When purchasing BGA ICs and soldering them onto the board, it is also important to use hot air to remove moisture. Many people overlook the preheating step, which can be fatal for BGA ICs.

2. The melting point of rosin is 100 ° C. Before this temperature, attention should be paid to controlling the maximum heating rate, and experts recommend 40 ° C/second. In fact, during the entire welding process after bringing the air nozzle closer to the IC, attention should be paid to controlling the heating rate.

3. The melting point of solder is 1830 ° C, but both in the production line and during rework, the "natural loss" and "heat loss" of heat must be considered. The actual soldering temperature used will increase. Experts suggest that the reflow soldering temperature for the production line should be between 2150 ° C and 2200 ° C.

When we disassemble and solder BGA in mobile phone repairs, we use a value above this recommended temperature. I think manual disassembly and soldering may result in more heat loss compared to wave soldering on the production line. Additionally, the scenery and heat of different manufacturers' hot air guns vary. For example, we have used different brands of hot air guns, and the temperature adjustment always varies. Sometimes, the disassembly and soldering IC temperature can reach as high as 300 ° C, 400 ° C, or even higher. Thirdly, when using solder pastes with different melting points, it is often necessary to use different temperatures, which everyone has experienced deeply. So, sometimes when we listen to others introduce their welding experience, we must not simply copy how much temperature and air volume we use. We can only refer to them because everyone uses different hot air guns, materials, and welding parameters, and each person has their own set of methods to 'tame' the hot air gun. Therefore, we need to determine the accurate temperature based on our own actual situation.

Of course, everyone may use different temperatures and times on the surface, but in reality, when disassembling and soldering the same BGA and C, the temperature and time should be basically the same, and it is absolutely impossible for the differences to be too outrageous. This is determined by the fundamental characteristics of materials such as solder, chips, PCBs, etc.

4. Another key point here is the time element of the reflux zone. From the curve graph, it can be seen that the time it takes for the chip and PCB board to fuse together is 40-90 seconds. At this time, in addition to the previous temperature factor, controlling the time is another major factor for successful desoldering. If the heating time is too short, it is easy to cause breakpoints when removing the chip, and soldering the core characteristics can also cause virtual soldering. Heating for too long can easily cause the chip to rise and the PCB board to break. Once the operating temperature and time are not organically coordinated, the above-mentioned 'tragedy' will occur.

Although we cannot use computers to set the time and temperature standards for heating areas, driving areas, and other areas like surface mount technology on production lines, we can at least summarize the patterns in our own successful disassembly and welding practices.

5. Hot air reflow soldering is the best heating method for soldering BGA ICs, especially plastic encapsulated ICs. There are currently two commonly used heating methods for welding: infrared heating and hot air heating. The former achieves welding through infrared radiation heating. Due to the different abilities of PCB boards and chip components to absorb infrared wavelengths (black components have the strongest infrared absorption ability), there is a significant temperature difference across the entire PCB board, sometimes up to 20%. However, the hot air reflow soldering method can make the temperature more uniform, with a general temperature difference of only a few degrees.

In summary, whether disassembling or soldering BGA ICs, it is simply the best grasp of the two main factors of hot air temperature and soldering time, which are the fundamental guarantees for improving the quality and success rate of disassembly and soldering.

（二） The physical and chemical properties of soldering improve the quality of BGA disassembly and soldering

1. The viscosity of screen printing and solder paste

Before installing electronic components onto the PCB board of the entire machine, screen printed steel plates need to be made. The BGA solder plates we use now are derived from screen printed steel plates on the production line, which are "extracted" from the entire steel plate. Due to the viscosity of solder paste, when it is brushed onto the corresponding solder joints on the PCB board through online holes, this physical property determines that the slower the brushing speed, the less liquid leakage will occur, while the faster the speed, the more liquid leakage will occur. That is, the brushing speed is inversely proportional to the amount of solder paste. If this speed is not well controlled, it is either due to insufficient solder paste causing solder leakage or due to excessive solder paste causing solder joints to connect. These phenomena are more pronounced in chip soldering such as QFP (Four sided Flat Package External Lead Out Pin), especially for pin spacing below 0.5mm, QFP is so difficult to "serve" because BGA pin spacing mostly reaches 1mm or more. We pay attention to even scraping when replanting BGA ICs, and do not squeeze the BGA IC during the heating process. Generally, there will be no virtual soldering or short circuiting phenomenon.

2. The chemical properties of solder to prevent oxidation of solder

We know that the chemical composition of solder is not just tin, it is a mixture of elements such as tin and lead. In welding, a solder paste with a composition ratio of 63n/37Pb that requires no cleaning is often used. Under high temperature, it undergoes a chemical reaction with the copper dots on the PCB board, fuses together, and forms an alloy, which "eats" into the PCB board.

Therefore, we should pay attention to avoiding two situations when welding.

(1) The amount of solder at the welding point should not be too small, otherwise it may cause virtual soldering.

(2) The temperature during soldering should not be too high, and the time should not be too long, especially when using non cleaning solder paste, otherwise it is easy to form solder joints.

When we solder the solder pins of QFP chips during maintenance, if we repeatedly solder one point without adding some solder, it is easy for the pins to detach due to this "aging" reason. When BGA IC is virtual soldered, in many cases, we have to remove it and re screen it with solder paste or hot air soldering, which is also because of this reason. Often, some people only add a little rosin to the BGA IC and blow it with a hot air gun to finish. Sometimes soldering can be successful, but for solder joints with severe virtual soldering or oxidation, it is temporary or completely useless.

3. The tension and pulling force of soldering

After the solder melts and has tension and tension, this physical property brings us great convenience in soldering. In cases where there is a slight deviation between the solder joint and the PCB solder joint position, the melted solder has the ability to pull and align the IC pins. When soldering BGA ICs, although we cannot see the bottom pins with the naked eye, we can visually align the IC with the PCB solder joints. The diameter of BGA solder pins is wider than that of QFP chip pins, resulting in higher alignment accuracy. In addition, the tension and pulling force of the solder itself make it possible to align the solder joints manually without the need for specialized equipment.

In fact, BGA packaging technology has unparalleled advantages over QFP chip technology in terms of production and repair yield, and is the current and future trend of chip packaging. At the beginning, we had a fear of difficulty when manually disassembling and soldering BGA ICs, mainly because we didn't understand its characteristics and soldering process. Once we mastered its "temperament", wouldn't we just manipulate it to be "obedient"? The manual disassembly and soldering process of BGA varies from person to person, but scientific soldering standards must be followed, which can be described as "different paths leading to the same goal". The purpose of writing this article can be inspired by Chairman Mao's sense of pride: to disdain BGA strategically and to value BGA tactically.