Journal of SMT Article

FATIGUE PROPERTIES OF Sn/3.5Ag/0.7Cu SOLDER JOINTS AND EFFECTS OF Pb-CONTAMINATION

Authors: J. Oliver, M. Nylén, O. Rod, C. Markou
Company: Swedish Inst. for Metals Rsrch
Date Published: 10/1/2002   Volume: 15-4

Abstract: Solder containing lead (Pb) has been used in electronics applications for more than 50 years. However, due to environmental concerns it is likely that use of lead in solders will be prohibited in coming years. Thus alternative lead-free solders have to be found and their properties need to be assessed.

The requirements for Pb-free solders are, in general, similar to those of leaded solders. They require similar process windows in terms of soldering time and temperature and should result in equally good or better properties. A great number of lead-free alternatives have been studied in the literature from which alloy Sn/3.5Ag/0.7Cu was chosen to be the studied alternative, as this is probably the one most widely used in Europe [1, 2, 3, 4, 5, 6].

The fatigue properties of Sn/3.5Ag/0.7Cu have been investigated in this work with:

  • Thermal cycling of boards: Printed circuit boards, having different surface finishes and different types of components, were soldered under different atmospheres using the Sn/3.5Ag/0.7Cu lead-free solder. These boards were then temperature cycled between 125 - 15 degrees C. The microstructures of the solder joints were studied after various exposure tests up to 2000 cycles.
  • Mechanical cycling of model joints, at room temperature, using specially designed ring and pin samples: Sn/3.5Ag/0.7Cu lead-free solder alloy has been mechanically cycled to compare with the traditional SnPb solders. Solder alloys containing different low Pb contents have also been investigated to study the effect of Pb contamination (from metallizations) on Pb-free solder.
  • Sn/3.5Ag/0.7Cu has fatigue properties during mechanical cycling which are better than those of the best traditional lead containing samples tested previously. It has, however, been shown that a contamination of 2%-5% Pb can be detrimental to the lifetime of Pb-free solders [7].

    Key words: solder joints, Pb-contamination, fatigue.



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