In order to address the issue of solder joint lines resulted from inadequate post-welding cooling in vacuum vapor phase welding of multi-layer printed boards,this study identified the underlying cause of solder joint lines in multi-layer boards and enhanced the post-welding cooling zone,enabling disclosure of the effects of different welding processes on surface morphology,internal structure morphology, and mechanical properties of solder joints.Experimental findings demonstrated that fast cooling led to thinner interfacial metal compound (IMC) within solder joints and a more uniform distribution of Pb in Sn. The grain diameter of IMC layer formed under fast cooling was approximately 1 µm, while shear strength reached 17.26 MPa-an increase by 43% compared to slow cooling rate - thus achieving fine crystal strengthening objectives. Furthermore, significant improvements have been observed in surface quality by fast cooling reducing solder joint lines.