The main reason why the tombstone phenomenon happen is the imbalance of the wetting force at both ends of the component, which causes the torque at both ends of the component to be imbalanced, resulting in "tombstone".
What will cause the unbalanced wetting force at both ends of the component during reflow soldering, which leading to "tombstone"?
Factor A: Unreasonable pad design and layout
① One of the pads on both sides of the component is connected to the ground or the area of one side of the pad is too large, and the heat capacity at both ends of the pad is uneven;
②The temperature difference across the PCB surface is too large to cause uneven heat absorption on both sides of the component pad;
③The temperature unevenness will appear at both ends of the small chip component pads around the large devices QFP, BGA, and radiator.
★Solution: Engineers adjust the pad design and layout
①The activity of the solder paste is not high or the solderability of the component is poor. After the solder paste is melted, the surface tension is different, which will cause the pad wetting force to be unbalanced.
②The amount of solder paste printed on the two pads is uneven, with thick tin on one side and large tensile force, while the thinning force on the other side is small, causing one end of the component to be pulled to one side to form an empty solder, and one end to be pulled up to form a tombstone. ★Solution: It is necessary for the factory to select a solder paste with higher activity to improve the printing parameters of the solder paste, especially the window size of the stencil.
Factor C: The uneven force in the Z-axis direction of the patch shift when mounting process will cause the uneven depth of the component immersed in the solder paste. When melting, the wetting force on both sides will be unbalanced due to the time difference. If the component is shifted, it will be directly Lead to tombstone. ★Solution: The factory needs to adjust the process parameters of the pick and plcae machine.
Factor D: The reflow oven temperature curve is not correct
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, and the wet difference on the board surface will be too large, which will cause the wetting force to be unbalanced.
★Solution: The factory needs to adjust the appropriate temperature curve according to each different product.
If you need any solution or suggestion on reflow oven, please reach to www.sunzontech.com for support.
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