The same PCB board generallyhave to go through SMT SMT patch processing, and then through flow welding,wave soldering, rework and other procedures. It's likely to form differentresidues. In humid environment and under a certain voltage, the electrochemicalreaction can occur with the electric conductor. , causing the surfaceinsulation resistance (SIR) to drop. If electromigration and dendrite growthoccur, there is a short circuit between the wires, creating a risk ofelectromigration (commonly known as "leakage").

In order to ensure electricalreliability, the performance evaluation of different clean - free flux isrequired. Use the same solder on the same PCB, or clean after welding.

According to the mechanicalstrength of solder joints reliability analysis, tin whiskers, porosity, crack,intercellular compounds, mechanical vibration fault, thermal cycle failure,electrical reliability, any failure is more likely to happen in the solderjoints under the existence of defects: the thickness of the intermetalliccompound is too thin and too thick after welding, pore and micro cracks in thesolder joints or interface; Small solder joint wetting area (small bonding sizebetween the welding end and the welding plate): the microstructure of thesolder joint is not compact, with large crystal particles and large internalstress. Some defects can be detected by visual inspection, AOI and X-ray. Forexample, the overlap size of welding spot is small, there are pores on thesurface of welding spot, and cracks are more obvious.

However, the microstructure,internal stresses, internal voids and cracks in the solder joints, especiallythe thickness of intermetallic compounds, are hidden defects that cannot bedetected by manual or automatic SMT processing. Various reliability tests andanalyses are required, such as temperature cycle test, vibration test, droptest, high-temperature storage test, humid heat test, electromigration test,high accelerated life test and high accelerated stress screening. Thenelectrical and mechanical properties (such as solder joint shear strength,tensile strength) are tested. Finally, the judgment was made through visualexamination, X-ray fluoroscopy, metallographic section, scanning electronmicroscope and other tests and analysis.