Electrochemical migration of lead free solder joints

Abstract

Electrochemical migration (ECM) tests on lead bearing and lead free solder joints on Cu lamination on FR-4 board were conducted by applying constant voltage. This paper first studied the ECM of the soldered joints under distilled water after removing the fluxes. In addition, conditions under high temperature and high humidity were also set up to investigate the changes of the initial surface insulation resistance (SIR) with the residues of no clean fluxes. It is found under distilled water that dendrites of the solder joints take on different morphologies with the different migration elements. For the joints of Sn-37Pb and Sn-36Pb-2Ag solders, the main migration element is Pb. While that of Sn-3.5Ag and Sn-4Ag-0.5Cu solders, it is Cu that usually migrates and forms dendrites due to the poor wettability of the solder paste leads to part exposure of Cu substrate at the wetting brim. For Sn-3Ag-0.5Cu solder joints, Sn leads the migration. While for Sn-Zn-Bi solder joints, it is always Zn to migrate which means Zn is more mobile than Cu. The investigation on SIR shows the fluxes have great effect on the migration behavior. The failure time of the joints with the same solder alloy compositions have different failure time due to the different fluxes. The effect of the wettability and the role of Cu substrate on the ECM behavior of the solder joints are discussed in detail.