How to improve reflow soldering when founding PCB burst-2

08-08-2019

1.2 Factors affecting the explosion

        (1) The source of the formation of volatiles is a necessary condition for the explosion

        1 moisture absorption problem

        In the following, the existence of water in the PCB, the way of water vapor diffusion and the change of water vapor pressure with temperature, reveal that the existence of water vapor is the primary cause of PCB explosion.

        The moisture in the PCB is mainly present in the resin molecules, as well as macroscopic physical defects (such as voids, microcracks) present inside the PCB. The water absorption rate and equilibrium water absorption of the epoxy resin are mainly determined by the free volume and the concentration of polar groups. The larger the free volume, the faster the initial water absorption rate, and the polar group has affinity for water, which is also the main reason for the higher water absorption of the epoxy resin. The greater the content of polar groups, the greater the equilibrium water absorption. In summary, the initial water absorption rate of the epoxy resin is determined by the free volume, and the equilibrium water absorption is determined by the content of the polar group.

        On the one hand, the temperature of the PCB during lead-free reflow soldering increases, causing water in the free volume and water to form hydrogen bonds in the polar group, and sufficient energy can be obtained to diffuse in the resin. The water diffuses outward and accumulates at voids or microcracks, and the molar volume fraction of water at the voids increases.

        On the other hand, as the soldering temperature increases, the saturated vapor pressure of the water also increases.

multi-layer PCB assembly

The saturated vapor pressure of water vapor at 2400 kPa at 224 ° C; the saturated vapor pressure of water vapor at 250 ° C is 4000 kPa; and when the welding temperature is raised to 260 ° C, the saturated vapor pressure of water vapor even reaches 5000 kPa. When the bond strength between the material layers is lower than the saturated vapor pressure generated by the water vapor, the material explodes. Therefore, the moisture absorption before welding is one of the main reasons for the delamination and explosion of the PCB.

         2 Influence of moisture during storage and production HDI laminated multi-layer PCB is a moisture sensitive component, and the presence of water in the PCB has an extremely important impact on its performance. E.g:

         (a) The moisture stored in the environment causes a significant change in the characteristics of the PP (prepreg);

         (b) In the absence of protection, PP is highly hygroscopic, and Figure 1.3 shows the hygroscopicity of PP when stored under relative humidity of 30%, 50%, 90%;


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