Factors Affecting The Explosion Board In PCBA Reflow Welding

    A source of volatile matter is a necessary condition for the production of explosive boards.

       1.Moisture absorption problem

Through the existence form of water in PCB, the way of water vapor diffusion and the variation of water vapor pressure with temperature, it is revealed that the existence of water vapor is the primary reason leading to the explosion of PCB board.

       The moisture in PCB mainly exists in resin molecules, and the macroscopic physical defects (such as voids, microcracks) in PCB. The water absorption rate and equilibrium water absorption rate of epoxy resin, The larger the free volume, the faster the water absorption rate in the initial stage and the affinity of the polar group to the water, which is mainly determined by the free volume and the concentration of the polar group. This is also the main reason that epoxy resin has high water absorption. The larger the polar group content, the greater the equilibrium water absorption. In conclusion, the initial water absorption rate of epoxy resin is determined by free volume. The equilibrium water absorption is determined by the content of polar groups.

        On the one hand, the temperature of PCB increases during lead-free reflow welding, resulting in the formation of hydrogen bonding water in free volume and polar groups, which can obtain sufficient energy to diffuse in the resin. Water diffuses outward and aggregates at the void or microcracks. The mole volume fraction of water in the void increases.

        On the other hand, with the increase of welding temperature, the saturated vapor pressure of water increases at the same time, when the welding temperature rises to 260 ℃, The saturated vapor pressure of water vapor is even up to 5000kPa.When the bonding strength between layers is lower than the saturated vapor pressure produced by water vapor, the material will explode. Therefore, the moisture absorption before welding is one of the main reasons for the stratification of PCB and the explosion of the board.

        2.Influence of Wet Vapor in Storage and production process. The existence of water in the multi-layer PCB is a kind of humidity sensitive component, which has an extremely important effect on its performance.

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