Cause: The root cause of the tombstone phenomenon is that the wetting force on both sides of the element is unbalanced, so the moment at both ends of the element is also unbalanced, resulting in the tombstone phenomenon.

The following conditions can lead to an unbalanced wetting force on both sides of the component during reflow:

1.1 The pad design and layout are unreasonable. If the pad design and layout have the following defects, the wetting force on both sides of the component will be unbalanced.

1.1.1 One of the pads on both sides of the component is connected to the ground wire or one side of the pad area is too large, and the heat capacity at both ends of the pad is uneven;

1.1.2 The temperature difference across the surface of the PCB is too large to cause uneven heat absorption on both sides of the component pads;

1.1.3 Large device QFP, BGA, small chip component pads around heat sinks will experience temperature unevenness across both ends。

Solution: Change the pad design and layout。

1.2There is a problem with solder paste and solder paste printing. The activity of the solder paste is not high or the solderability of the components is poor. After the solder paste is melted, the surface tension is different, which will cause the pad wetting force to be unbalanced. The amount of solder paste printed on the two pads is uneven, and the more side will absorb more heat from the solder paste, and the melting time will lag, resulting in an unbalanced wetting force.。

Solution: Use a solder paste with higher activity to improve the printing parameters of the solder paste, especially the window size of the stencil.

1.3 The uneven force in the Z-axis direction of the patch displacement will lead to uneven depth of the components immersed in the solder paste, and the wetting force on both sides will be unbalanced due to the time difference during melting. If the component patch is shifted, it will directly lead to tombstone.

Solution: Adjust the process parameters of the placement machine.

1.4 The furnace temperature curve is incorrect. If the furnace body of the reflow soldering furnace is too short and the temperature zone is too small, the working curve of the PCB heating will be incorrect, so that the humidity difference on the board surface is too large, resulting in an unbalanced wetting force.

Solution: Adjust the appropriate temperature curve according to each different product.

1.5 Oxygen concentration in nitrogen reflow soldering. Reflow soldering with nitrogen protection will increase the wetting force of the solder, but more and more examples show that the phenomenon of tombstones will increase when the oxygen content is too low; it is generally believed that the oxygen content is controlled at (100 ~ 500) × 10 to the negative 6th power is the most suitable.