This can include: the use of solder paste with a thermally stable flux system that provides and maintains stickiness and a paste with metal particles with two different eutectic points - 50 percent melts at 422.6☏ (217☌) for Sn95, 5Ag3, 8Cu0, 7, and the remainder at 429.8☏ (221☌) for Sn96Ag4. The elimination of tombstones, or at least a significant minimization, can be achieved in part by using the correct SnAgCu lead-free solder paste. The greater the pad geometry and the area of the molten solder, the stronger the effect of surface tension, which is why attention must be paid to the thermal mass of the component connection surfaces. Secondly are defects related to the temperature (ΔT) and heat dissipation. First, there are defects related to the surface of the PCB and components, such as solderability, oxidation and physical damage to the plating. There are basically two causes of tombstoning. Therefore, in addition to the obvious benefits of vapor phase soldering, other precautions must be taken to minimize tombstones. However, an additional delay in initial wetting to reduce ΔTs is exactly what is needed to minimize tombstones. The fast, initial wetting does not leave time to reduce ΔTs. Vapor phase soldering ensures a clean surface for optimal wetting. This implies a significantly lower oxidation of the metallic surfaces during transit into reflow, compared with reflow processes under normal process environment conditions. The oxygen-free vapor blank in vapor phase soldering prevents the re-oxidation during soldering. High surface tensions lead to uneven masses, lifting two-pole SMD components. In general, the better (the more oxygen-depleted) the soldering process, the lower the interface tension. Possible cause of tombstoning include inaccurate layout, incorrect stencil geometry, incorrect placement, and poor surface quality. This effect is mainly observed in small and light two-pole SMD components, such as 008004, 01005, 0201, and 0805 (0201, 0402, 0603, and 2012 metric) resistors and capacitors. Generally, one end of the component is soldered to the PCB, while the other lifts off, resembling a cemetery tombstone. Tombstoning is a defect that occurs when an SMT component is partially or completely lifted from the PCB. By Marc van Stralen, IMDES Creative Solutions
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