Study On Printing Performance And Void Formed After Reflow Of Lf2000 Paste

After nearly a decade of development, lead-free solder has been to be the main solder for electronic assembly industry. Compared with lead solder, the lead-free solder is easier to form the defects of bridge, void, false welding, tombstoning and so on because of its high melting point, poor wettability and low density. Paste is the most important materials of SMT. The property of the paste affect the quality of the welding points in directly. The active agent of traditional lead-free contains halogen and a large amount of colophony. After reflowing, the colophony becomes residues with high viscosity. The residues must be washed away. The washing process will prolong the production cycle and increase the cost. These years, more and more people pay attention to the pollution of environment, the use of halogen has been limited. Therefore, the research and development of lead-free paste without halogen and washing has become an urgent task of the paste manufacturer. LF2000 paste is a kind of lead-free paste without halogen and washing explored by Shen Zhen new materials company, it is easy to form bridge and void defects after reflow.The property of LF2000 paste has been tested according to the standard of IPC-TM-650. The results show that all the properties can meet the demand of the standard. Comparing with OM338 paste which is manufactured by Alpha and recognized as an excellent paste, the change of the viscosity in the process of printing has been researched. As the printing, the viscosity of LF2000 paste first reduced and then increased. After 4 hours of printing, the viscosity reached the lowest point and it reduced by 39.4% while the viscosity of OM338 paste hardly changed after printing. The demand of paste property in printing and mechanism of thixotropic agent have been discussed. Three kinds of thixotropic have been added in the paste respectively and the test show that the adding of 4% Y thixotropic could improve the thixotropy greatly.The void defects formed in the reflow has been studied by X-Ray. There are many reasons that might cause void defects, for LF2000 paste the main reason is that the system of solvent, however. The solvent is so volatile that it has volatilized over before the melt of solder. So the residue is hard to discharge from the solder without the help of the solvent. The tests show that the void defects can be improved by adding solvent of G, which has a high boiling point. When the G reaches 4%, the void is the least.