Usually the same PCB circuit board will go through SMT chip processing re-flow soldering, wave soldering, reworking and other processes, it is likely to form different residues, in a humid environment and under a certain voltage, may be with the conductive body between the electrochemical reaction, causing the decline of surface insulation resistance (SIR). If electromigration and dendritic crystalline growth occur, there is a risk of electromigration (commonly known as “leakage”) due to short circuits between wires. This is shown in the picture below.
To ensure the reliability of electrical, the performance of different no-clean fluxes needs to be evaluated，and the same fluxes should be used as far as possible on the same PCB or for post-solder cleaning.
Through the reliability analysis of 7 aspects of mechanical strength, tin whiskers, voids, cracks, intermetallic compound, mechanical vibration failure, thermal cycling failure and electrical reliability of solder joints, any of the following failures are more likely to occur in solder joints with the following defects: too thin and thick intermetallic compound thickness problems right after soldering: voids and tiny cracks inside the solder joint or at the interface; small wetted area of the solder joint ( small size of the lap between the solder end of the component and the pad): microstructure of the solder joint is not dense, large crystalline particles, high internal stress. Some defects can be detected visually, by AOI or X-ray, such as small lap sizes, pores on the surface of the solder joint, more obvious cracks, etc. However, the microstructure of the solder joint, internal stresses, internal voids and cracks, especially the thickness of intermetallic compounds, which are hidden defects invisible to the naked eye, cannot be detected either by manual or automatic inspection of the SMT process, and require detection using various reliability tests and analyses, such as temperature cycling, vibration tests, drop tests, high temperature storage tests, damp heat tests, electromigration ( ECM) test, high-acceleration life test and high-acceleration stress screening; then electrical and mechanical properties (such as solder joint shear strength and tensile strength) tests; and finally tests and analyses through external inspection, X-ray fluoroscopy, metallographic sectioning, scanning electron microscopy, etc., before judgement can be made.
It is also clear from the above analysis that hidden defects add an element of uncertainty to the long-term reliability of lead-free products. As a result, high reliability products are currently exempt; both visible defects, and hidden defects, are caused by the high tin content of lead-free, high temperatures, small process windows, poor wettability, material compatibility towards the problem, and design, process and management factors.
Therefore, we must consider the compatibility of lead-free materials, the compatibility of lead-free and design, and lead-free and process from the beginning of the design of PCBA lead-free products; fully consider heat dissipation issues; carefully select PCB plates, Pad surface layer, components, solder paste and flux, etc.; SMT process optimization and process control are more detailed than when lead soldering; material management is more rigorous and meticulous.
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