Surface Mount International Conference Proceedings


Author: Alan Gickler
Company: Johnson Manufacturing
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: Since the mid 1980’s a search has been on to replace lead bearing solders with “lead free” solders. The reasons are well known and do not bear extensive reanalysis here. The initial group of lead free solders were pointed toward the plumbing industry and attempted, sometimes successfully, to mirror 50Sn/50Pb in liquidus and the range between melting and liquidus. Many are patented and all are more expensive than the leaded counterpart. Since 1992 a search has been on for the electronics and electrical industry to replace 63Sn/37Pb and other leaded solders. Although the reasons to switch are not as compelling as was the case in plumbing and no legislation has been enacted in the US banning the use of leaded solders in electronics/electrical to date, most reasonable people accept the need to eliminate lead in these industries. Whether due to corporate philosophies, unhappiness with some of the characteristics of leaded solders, or the assumption that lead will be banned sometime in the near future (if not in the US then in Europe, Canada, Australia, or whatever which will, in turn force the multinationals to switch worldwide), extensive research is underway by both the users and producers. Most observers expect a “family” of solders to emerge which will replace 63/37, but at a higher cost, higher melting temperature, less tolerant processing window and requiring some equipment modification or total replacement. Offsets may include better fatigue resistance (thereby giving longer product life) and elimination of the expenses due to lead as both an effluent, airborne hazard or landfill concern. contamination a concern). One of the premiere studies just published is one by a consortium (NCMS) including EMPF, Ford, GM Delco, GM Hughes, Lucent, NET, Rockwell, RPI, Sandiz TI and United Technologies. Generally called the “NCMS LEAD FREE STUDY’, it identifies three solders that show promise in electronics. These are all eutectic solders in the SN/BI, SN/AG and SN/AG/BI families. The authors of this paper have added the eutectic SN/AG/CU alloy and the “lean” version of SN/AG/CU, which was developed by Ames Lab at Iowa State University and Sandia National Laboratory. Copper pickup in a wave soldering machine (or the older drag machines) is of concern since one of the problems relating to 63SnJ37Pb is copper pickup from the board finish, Joint quality or appearance problems once the level is above 0.5 weight percent is usually the trigger to “dump the pot” and recharge with new solder. If the new lead free solders pick up copper faster, then a major problem would be encountered and if they pick up slower, a hidden cost component of the 63Sn/37Pb would be reduced or eliminated. Lead contamination concerns relate to any bismuth containing lead free solder and the low melting eutectic phase of the SN/PB/BI system at 96 degrees C. Non

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