The causes and solutions of poor wetting in reflow soldering

Poor wetting during reflow soldering is also known as non wetting or semi wetting. It refers to the situation where during the welding process, the solder and substrate welding areas do not form a reaction layer with each other after being wetted, resulting in missed or insufficient welding faults. Its harm is that the solder joint has low strength and poor conductivity. JEENOCE will explain the reasons and solutions for poor wetting during reflow soldering.

1、 Reasons for poor wetting of solder joints on circuit boards after reflow soldering

Poor wetting is caused by the high heat absorption of the device, resulting in a lower temperature on the solder joints compared to the temperature on the PCB pads. The cause of this malfunction involves various factors such as material type or characteristics, packaging, inventory, logistics handling, and technology. From the perspective of supply chain or industrialization, it involves the work of departments such as design, suppliers, warehouse logistics, and production processes. In terms of technical integration management requirements, these departments must cooperate and define their respective responsibilities, and ensure that they do their respective work well. This can prevent the occurrence of problems. The so-called cooperation between them refers to defining work indicators based on technical principles and cost-benefit considerations. So before integrating technology management, we must have a sufficient understanding of the entire assembly technology in order to make correct and appropriate decisions.

2、 A method to solve poor wetting of solder joints on circuit boards after reflow soldering

1. However, to ensure wetting, the primary condition is the characteristics of the welded metal. Wetting is a relative property, so matching between materials is crucial. On a typical solder joint using reflow soldering, three materials are included. That is, the surface coating of the solder joints, solder paste, and PCB pads of the device. From the user's perspective, unfortunately, suppliers have invented not just one, but a considerable number of matching combinations. In lead containing technology, although there are not many types of solder paste alloys, there are also many choices for PCB pad coatings, especially for device solder end coatings. The relative wettability of these materials is not consistent.

Even worse, when these materials are combined with other considerations, such as having a wide variety of device types on the same PCBA, the coating thickness of each device's solder joint, the inner layer material of the solder joint, the electroplating process of the solder joint, inventory time and conditions, etc., a complex and variable characteristic difference situation can be formed. The selection of materials is the responsibility of the user design department, so ensuring that the selected materials are suitable for their own or external manufacturing capabilities is a top priority. This is the DFM element in technology integration management. If this part of the work is done properly, users can ensure that the required materials have a suitable degree of wettability.

2. Another necessary task is to establish process capabilities. That is to choose the process and establish process characteristic parameters. The example we are giving here is reflow soldering, so the process selection is also reflow soldering. As for the process characteristic parameters, for the fault mode of "poor or insufficient wetting", it lies in the second to fourth steps of the reflux process. That is to say, there are three processes: volatilization, soldering assistance, and reflow. Especially the latter two processes are the main control points. If the volatilization process is not done well, the residual volatiles will affect the effectiveness of the welding flux, which will also affect the wetting ability. If the welding time is not properly controlled, the welding efficiency may not be fully utilized if it is too short, and re oxidation may occur if it is too long, which also affects or reduces wettability. So this process must also have good temperature and time control. The temperature of the reflux will determine the degree of wetting. The higher the temperature, the lower the surface tension of the molten metal, which is beneficial for wetting. So raising the temperature is a way to enhance wetting. However, this certainly has its limitations, such as the need to simultaneously take into account the potential thermal damage, deformation, and other issues that high temperatures may cause. It should be noted that in practical work, we are not facing a single solder joint with uniform temperature. But rather thousands of solder joints with varying wettability, temperature, and even uneven temperature of a single solder joint itself. And we must set a common temperature and time to handle their different characteristic requirements. This requires process engineers to flexibly use various modulation methods to formulate the optimal settings.