As of july 1, 2006, the weee directive will eliminate the use of lead in solder paste. The industry is intensively developing solder-paste systems that serve as an alternative to the tin/lead (SnPb) solder paste used. But switching to lead-free paste will increase the need for chemically supported cleaning.
As early as the 1950s, electronic circuit failures were documented due mainly to electrochemical migration of solder connections containing silver. Because of the low silver concentration, the dendrites caused by electrochemical migration (EM) cannot be determined via discontinues in resistance measurements (Figure 1). Temporary failure, however, may occur during the measurement period. The high affinity of silver used to form silver hydroxides and sulfides, respectively, is the reason for the increase in EM (Figure 2). The larger quantities of activators, which tend to be highly hygroscopic in character, are required due to increased reflow temperatures. If they are not partially or fully removed, they can induce moisture films on the assembly. This, in turn, leads to electrochemical migration and the observed formation of dendrites. It seems difficult to encapsulate the high activator concentrations reliably in current lead-free paste formulations, which indicates that No Clean technology has reached its process limit. Many companies have reported that the reliability of the components soldered with lead-free solder pastes can no longer be guaranteed using “only No Clean” methods. Boards are continuously subjected to complex and difficult climatic tests and conditions. Due to the larger quantity of hygroscopic activators, lead-free solder pastes have no sufficient safety margins to pass these tests. Actual assembly cleaning, particularly the removal of increasingly present residues, can contribute to an improvement in reliability under environmental stress, and prevent susceptibility to migration.