How to control the quality of lead-free reflow soldering points

Due to the characteristics of "reflow" and "self positioning effect" in the lead-free reflow soldering process, the requirements for mounting accuracy are relatively loose, making it easier to achieve high automation and speed in welding. At the same time, due to the characteristics of reflow and self positioning effect, the reflow soldering process has stricter requirements for pad design, component standardization, component end and printed board quality, solder quality, and process parameter settings. JEENOCE will share with you how to control the quality of lead-free reflow soldering points.

Quality judgment of lead-free reflow solder joints

At present, most of the methods used in the industry to control welding results are MVI (visual inspection) or AOI (automatic optical inspection), combined with ICT (online electrical testing) and FT (functional testing). The former belongs to appearance inspection, although it can detect some process problems, it cannot cover all appearance failure modes. The method of manual visual inspection using a microscope is more capable. However, due to the relationship between speed and cost, it was not adopted.

Although the AOI speed efficiency is good, the detection rate is not yet ideal. The latter two types of testing belong to electrical testing rather than process testing. That is to say, the process problem must be so severe that it has already caused electrical problems or differences during testing, in order for this process problem to be detected by these two methods. For example, process issues with small solder joints often fail to cause electrical issues or differences. Such process problems cannot be identified or detected.

Both the appearance inspection of the former and the electrical inspection of the latter cannot have a high detection rate for the lifespan of solder joints. Previously, we discussed the external and internal structural factors of quality. In terms of internal structural factors, these commonly used practices lack the ability to verify. So strictly speaking, our current inspection technology cannot provide sufficient quality assurance.

Quality assurance of lead-free reflow soldering joints

1. Adequate and good wetting; Adequate and good wetting is an important indicator for us to know the solderability and condition. It is difficult for an unwetted solder joint to have sufficient IMC formation, which indirectly tells us that the welding quality is poor. It should be noted that although there are signs of wetting indicating solderability, it cannot fully indicate the qualification of IMC. The degree or condition of IMC formation is the key to determining the reliability of solder joints. This is an important limitation of visual inspection capability.

2. Appropriate solder joint size; The size of the solder joint directly determines its mechanical strength, as well as its ability to withstand fatigue fracture and creep. In lead-free reflow soldering technology, the material of the solder joint generally comes from the printing amount of solder paste. In situations where the matching of the solder joint material with the device is not ideal, large solder joints can sometimes serve as a buffer for quality issues. From the above perspective, we hope that larger solder joints are preferable. However, too large solder joints can also cause problems. For example, it affects the wettability of the inspection, and can easily cause process problems such as tin absorption and bridging, and may even shorten the lifespan of electromigration faults.

3. Good external contour. The contour of the solder joint is also important. Due to the different stresses borne by various parts inside the solder joint structure during use, the size and factors of the solder joint mentioned above must also be considered together with the external contour and factors. For example, the problem of insufficient tin appearing on wing pins and toes is not as serious in terms of reliability as it appears on heels and other areas.

Internal structural factors affecting the quality of lead-free reflow soldering joints

1. Appropriate intermetallic alloy layer; The formation status of intermetallic alloy (IMC) is the key to determining the mechanical strength of solder joints. Different metals can form IMCs with different composition combinations, and their strength also varies. So, the selection of components, matching of PCB pad coating metal and solder paste metal is an important task to ensure quality. After selecting the appropriate materials, the next step is to control the welding process to achieve a good thickness of IMC. When IMC is not formed, we refer to the solder joint as a virtual weld, and its structure is not sturdy. However, due to the fact that IMC itself is a fragile metal, once it forms too thick, the solder joints are also prone to fracture in the IMC structure. So controlling the thickness of IMC has become a key focus in the welding process.

2. Rich internal structure of solder joints; The interior of the solder joint must be solid. Due to the phenomenon of gas emission from solder paste and PCB materials in lead-free reflow soldering process, if the appearance of the solder joint appears appropriately qualified, there may be some large and small pores inside due to the emission of these gases. If the performance of the solder joint is actually similar and the solder joint is small, the reliability is threatened.

3. The microcrystalline structure inside the solder joint. The microcrystalline structure of the solder joint is affected by heating temperature, time, and cooling rate. Structures of different thicknesses also exhibit different fatigue resistance abilities. The impact of this issue on traditional tin lead is not significant. However, after entering lead-free technology, reports indicate that it is sensitive to certain alloy materials. When users choose lead-free materials, they give necessary evaluations based on their own situation, considering the quality of "non solder joints". We are concerned about the heat resistance of materials (devices and PCBs). As users, we usually request technical information from suppliers in the DFM (Design for Manufacturability) process when selecting these materials. Currently, the popular practice among suppliers is to provide users with a standard similar to the reflux curve, which indicates temperature and time limits, for users to follow. In fact, this approach needs improvement. Because devices are not a single material, but products made of different materials, structural designs, and processes.