1. Characteristics of selective wave soldering

Since the welding parameters of each welding point can be "customized" when using selective welding, we do not have to "make do with it". Engineers have enough process adjustment space to optimize the welding parameters of each welding point (spraying amount of flux, welding time, welding peak height, etc.), thus reducing the defect rate. We may even achieve zero defect welding of through-hole components Selective welding is only for selective spraying of flux at the required welding points, so the cleanness of circuit board is greatly improved, and the amount of ion pollution is greatly reduced. If the Na + ion and Cl - ion in the flux remain on the circuit board, they will combine with the water molecules in the air to form salt, which will corrode the circuit board and the solder joint for a long time, and finally lead to the open circuit of the solder joint. Therefore, the traditional production mode often needs to clean the circuit board after welding, and the choice of welding fundamentally solves this problem The heating and cooling process in welding will bring thermal shock to the circuit board, and its strength is particularly prominent in lead-free welding. The peak temperature of lead-free wave soldering is about 260 ℃, which is 10-15 ℃ higher than that of lead wave soldering. During welding, the temperature of the whole circuit board experiences the process from room temperature to 260 ℃ and then cooling to room temperature. The thermal shock brought by the two temperature changes of rise and fall will cause the shear stress of different materials on the circuit board due to the different coefficient of thermal expansion and contraction. For example, BGA device will form the shear stress at the top and bottom of the solder ball when it bears the thermal shock When the shear stress is large enough, BGA will be stratified and microcracked. Such defects are difficult to detect (even with the help of X-ray machine and AOI), and the solder joints are still conductive on the physical connection (also unable to pass the function test), but when the product is affected by external factors such as vibration in actual use, it is easy to form an open circuit. Selective welding is only for the welding of specific points. No matter in spot welding or drag welding, it will not cause thermal shock to the whole circuit board, so it will not form obvious shear stress on surface mount devices such as BGA, so as to avoid all kinds of defects caused by thermal shock. The temperature required for lead-free welding is high, the solderability and fluidity of solder are poor, and the melting copper of solder is strong

2. Insufficient selective wave soldering

A. high purchase cost
One reason is that the function of selective wave soldering is more complex than general wave soldering, so the machine structure will be more complex and the manufacturing cost will be higher Another reason is that the mainstream selective wave soldering is still an imported product. At the beginning of localization, there are only a few in China. The market demand is not much, and the market competition is not strong. Our company is one of the first batch of domestic enterprises to develop and produce selective wave soldering. We have been striving to make high-quality selective wave soldering for more than ten years. It's a good choice.

B. low efficiency

The advantages of selective wave soldering in the quality control of solder joints are very obvious, but at the same time, compared with the traditional wave soldering, its disadvantages in the output are also very obvious, because one nozzle can only weld one solder joint at the same time, although there are machines now, which increase the output by increasing the number of nozzles, it is still an important deficiency in the output of selective wave soldering.

II. Characteristics and disadvantages of wave soldering

1. Wave soldering features

Wave soldering is suitable for the welding of large area and large batch PCB, and it is widely used in the industrial production of electronic products welding. Wave soldering is to make the welding surface of the plug-in plate directly contact with the high-temperature liquid tin to achieve the purpose of welding. The high-temperature liquid tin keeps a slope, and the special device makes the liquid tin form a wave like phenomenon, so it is called "wave soldering", and its main material is solder strip Wave soldering machine is mainly composed of transport belt, flux adding area, preheating area and wave soldering furnace. The main purpose of the transport belt is to send the circuit board to the wave soldering machine, along the way through the flux adding area, preheating area, wave soldering furnace, etc. Flux adding area is mainly composed of infrared sensor and nozzle. The function of the infrared sensor is to sense whether there is a circuit board entering. If there is a sensor, the width of the circuit board will be measured. The function of flux is to form a protective film on the welding surface of circuit board. The preheating zone provides sufficient temperature to form good solder joints. The infrared heating can make the circuit board evenly heated. In the double wave peak welding system, the shock wave part of the wave prevents missing welding, which ensures the proper distribution of solder passing through the circuit board. The solder penetrates through the slit at a relatively high speed, so as to penetrate the narrow gap. The spray direction is the same as that of the circuit board. Shock wave alone can not properly weld components, it leaves uneven and excess solder on the solder joints, so a second wave is needed. The second smooth wave eliminates the burr and welding bridge caused by the first shock wave. The smooth wave is actually the same as the wave used in traditional through-hole plug-in components. Therefore, when the traditional components are welded on a machine, the shock wave can be turned off and the traditional components can be welded with smooth wave.

2. Insufficient wave soldering

A. high operation cost.
In the practical application of wave soldering, the whole plate spraying of flux and the production of tin slag have brought high operation cost; especially in lead-free soldering, because the price of lead-free solder is more than three times of that of lead solder, the increase of operation cost brought by the production of tin slag is amazing. In addition, the lead-free solder continuously melts the copper on the pad, and the composition of the solder in the tin cylinder will change over a long period of time, which needs to be solved by adding pure tin and expensive silver on a regular basis;
B. trouble in maintenance.
The residual flux in the production will stay in the wave soldering transmission system, and the generated tin slag needs to be removed regularly, which brings more complicated equipment maintenance work to users;

C. poor circuit board design brings some difficulties to production.
When some circuit boards are welded, due to the fact that the designer does not consider the actual production situation, no matter what kind of wave soldering parameters we set and various clamps are used, the welding effect is always difficult to fully satisfy people (for example, some key parts always have defects such as poor tin penetration or bridge connection). After wave soldering, repair welding has to be carried out, which reduces the long-term reliability of the product.

III. comparison between selective wave soldering and traditional wave soldering

1. To sum up, the advantages and disadvantages of selective wave soldering and traditional wave soldering are sorted out.

Selective wave soldering has high welding quality and low welding operation cost. It is suitable for small batch and multi model high-quality products. Because of its single point welding and low efficiency, it is not suitable for mass production. The equipment is relatively expensive.

The welding quality of wave soldering is relatively low, but its efficiency is high, it can be mass produced, but its operation cost is high, and some high quality requirements cannot be achieved. The equipment is relatively cheap.