PCB Tech - Causes and solutions of PCB board welding defects

Process characteristics of selective welding

The process characteristics of selective welding can be understood by comparing with wave soldering. The most obvious difference between the two is that in wave soldering, the lower part of the PCB board is completely immersed in liquid solder, whereas in selective welding, only a certain area is in contact with the solder wave. Since the PCB itself is a poor heat conduction medium, it does not heat and melt solder joints in adjacent components and PCB areas during welding. Flux must also be precoated before welding. In contrast to wave soldering, flux is only applied to the lower part of the PCB to be welded, not the whole PCB. In addition, selective welding is only applicable to the welding of plug-in components. Selective welding is a new method, a thorough understanding of selective welding process and equipment is necessary for successful welding.

Selective welding process

Typical selective welding processes include flux spraying, PCB preheating, dip welding and drag welding.

Flux coating process

Flux coating process plays an important role in selective welding. During welding heating and at the end of welding, the flux should be sufficiently active to prevent bridging and oxidation of the PCB. The X/Y manipulator carries the PCB through the top of the flux nozzle, and the flux is sprayed to the position of the PCB to be welded. Flux has single nozzle spray, micro - hole spray, synchronous multi - point/graphic spray. The most important thing for microwave peak selective welding after reflow welding process is that flux is sprayed correctly. The microporous jet does not stain the area outside the solder joint. The minimum flux spot graph diameter of micro-spot spraying is greater than 2mm, so the flux position accuracy of the spray deposited on the PCB is ±0.5mm, so as to ensure that the flux always covers the welding part. The tolerance of the flux amount is provided by the supplier. The technical manual should specify the flux amount, and the safe tolerance range of 100% is usually recommended.

Preheating process

The main purpose of preheating in selective welding process is not to reduce thermal stress, but to pre-dry flux to remove the solvent before entering the solder wave, so that the flux has the correct viscosity. During welding, the influence of heat preheating on welding quality is not the key factor. PCB material thickness, device package specification and flux type determine the setting of preheating temperature. In selective welding, there are different theoretical explanations for preheating: some process engineers believe that PCB should be preheated before flux spraying; Another view is to weld directly without preheating. The user can arrange the selective welding process according to the specific situation.

Welding process

There are two different selective welding processes: drag welding process and dip welding process.

Selective drag welding is done on a single solder wave. The drag welding process is suitable for welding in very tight Spaces on PCB. For example: individual solder joints or pins, a single row of pins can carry out the drag welding process. The PCB moves over the solder wave at different speeds and angles to achieve the best welding quality. In order to ensure the stability of welding process, the inner diameter of welding nozzle is less than 6mm. After the flow direction of the solder solution is determined, the nozzle is installed and optimized in different directions for different welding needs. Manipulator can be from different directions, that is, 0° ~ 12° between different angles close to the soldering wave, so the user can weld various devices on the electronic components, for most of the devices, the recommended tilt Angle is 10°.

Compared with dip welding process, drag welding process has better thermal conversion efficiency than dip welding process due to the solder solution and PCB movement. However, the heat required to form the weld connection is transferred by solder wave, but the solder wave of single nozzle is small in quality, only the temperature of solder wave is relatively high, can meet the requirements of drag welding process. Example: solder temperature is 275 degree Celsius ~ 300 degree Celsius, dragging speed of 10mm/s ~ 25mm/s is usually acceptable. Nitrogen is supplied in the welding area to prevent soldering wave oxidation, soldering wave eliminates oxidation, making the drag welding process to avoid the generation of bridge defects, this advantage increases the stability and reliability of the drag welding process.

The machine has the characteristics of high precision and high flexibility, module structure design system can be completely customized according to the special production requirements of customers, and can be upgraded to meet the needs of future production development. The motion radius of manipulator can cover flux nozzle, preheating and solder nozzle, so the same equipment can complete different welding process. Machine - specific synchronous process can greatly shorten the single - board process cycle. The manipulator has the capability to enable this selective welding with high precision and high quality welding characteristics. First of all, the manipulator is highly stable and accurate positioning ability (±0.05mm), which ensures that the parameters of each plate production are highly repeated and consistent; Secondly, the 5-dimensional motion of the manipulator enables the PCB to contact the tin surface at any optimized Angle and orientation to obtain the best welding quality. The tin wave height measuring needle installed on the splint device of the manipulator is made of titanium alloy. It can measure the tin wave height regularly under the control of the program, and control the tin wave height by adjusting the speed of the tin pump to ensure the stability of the process.

Despite these advantages, single-tip wave tow welding process also has disadvantages: the welding time is the longest in flux spraying, preheating and welding. And because the solder joints are drag welding one by one, with the increase of solder joints, the welding time will greatly increase, the welding efficiency is not compared with the traditional wave soldering process. However, this is changing. Multiple nozzle designs can maximize yield. For example, double welding nozzles can double the yield, and the same can be done for flux.

Immersion selective welding system has a plurality of solder nozzle, and PCB welding spot is one to one design, although the flexibility is not as manipulator type, but the output is equivalent to the traditional wave welding equipment, equipment cost is relatively low manipulator type. Depending on the PCB size, single-board or multi-board transfer can be carried out in parallel. All pending solder joints will be sprayed, preheated and welded in parallel at the same time. However, due to the different distribution of solder spots on different PCBS, special solder nozzles should be made for different PCBS. The size of the nozzle should be as large as possible to ensure the stability of the welding process and not affect the surrounding components on the PCB, which is important and difficult for the design engineer, because the stability of the process may depend on it.

The use of immersion selection welding process, can weld 0.7mm ~ 10mm solder spot, short pin and small size pad welding process is more stable, the possibility of bridging is also small, the adjacent solder spot edge, device and nozzle spacing should be greater than 5mm.