Development of lead-free reflow process technology -influence of heat history on solder joint quality

摘要

Concerning environmental problems, the movement of world regulations has been advancing recently. Now lead-free soldering technology is being developed all over the world in which conventional solder that contains lead - an environmentally burdening material - is being substituted in electrical and electronic equipment. We developed a novel Sn-Ag-Bi solder system, which lowers the melting point of Sn-Ag solder by adding Bi. In addition, this solder system maintains total balance of joint strength. In 1998, we introduced this Sn-Ag-Bi lead-free solder in one of our products, the portable MD player, and were the first in the world to do so. This was a major breakthrough in the practical use of lead-free solder. In the solder joint, which includes Bi the phenomenon in which joint quality deteriorates was determinted under heating conditions. That is in the mounting condition of two-side reflow soldering and one-side flow/one-side reflow soldering the joint part on the first side may be re-heated when the second side is soldered resulting in joint quality deterioration. In the present study, we determined what kind of influence the heat history of the joint part of the Sn-Ag-Bi lead-free solder had on joint quality by analyzing joint interface characteristics. We found that the compound layer between Cu and Sn increased on re-heating the solder joint, and segregation of the Bi, Pb and Sn alloy occurred We concluded that this phenomenon caused joint quality deterioration.