Generally, the same PCB circuit board needs to undergo SMT patch processing and then flow soldering, wave soldering, rework and other processes. It is likely to form different residues. Under a humid environment and a certain voltage, an electrochemical reaction may occur with the electrical conductor. , Causing a decrease in surface insulation resistance (SIR). If electromigration and dendrite growth occur, there will be a short circuit between the wires, causing the risk of electromigration (commonly known as "leakage").
In order to ensure electrical reliability, the performance of different no-clean fluxes needs to be evaluated. The same PCB should use the same flux as much as possible, or be cleaned after soldering.
According to the reliability analysis of the mechanical strength of solder joints, tin whiskers, voids, cracks, intercellular compounds, mechanical vibration failure, thermal cycle failure, electrical reliability, any failure is more likely to occur in the presence of The solder joints with the following defects: the intermetallic compound thickness is too thin and too thick after welding: there are voids and micro cracks in the solder joints or at the interface; the solder joint wetted area is small (the bonding size of the component welding end and the pad is biased) Small): The microstructure of the solder joint is not dense, the crystal particles are large, and the internal stress is large. Some defects can be detected by visual inspection, AOI, and X-rays, such as small overlap size of solder joints, pores on the surface of solder joints, and more obvious cracks.
However, the microstructure, internal stress, internal voids and cracks of the solder joints, especially the thickness of the intermetallic compounds, these hidden defects are invisible to the naked eye and cannot be detected by manual or automatic inspection by SMT processing. It is necessary to use various reliability tests and analysis for testing, such as temperature cycling, vibration test, drop test, high temperature storage test, damp heat test, electromigration (ECM) test, high accelerated life test and high accelerated stress screening; and then conduct Electrical and mechanical properties (such as solder joint shear strength, tensile strength) test; finally through visual inspection, X-ray fluoroscopy, metallographic section, scanning electron microscope and other tests and analysis, to make a judgment.
It can also be seen from the above analysis that hidden defects increase the long-term reliability of lead-free products by uncertain factors. Therefore, currently high-reliability products are exempted; both visible defects and hidden defects are due to lead-free high tin, high temperature, small process window, poor wettability, material compatibility issues, and design , Process, management and other factors.
Therefore, we must consider the compatibility between lead-free materials, the compatibility of lead-free and design, and the compatibility of lead-free and process from the beginning of the design of PCBA lead-free products; fully consider the heat dissipation problem; carefully select the PCB sheet, Pad surface layer, components, solder paste and flux, etc.; more detailed SMT process optimization and process control than when soldering with lead; more strict and meticulous material management.
