Solder materials Lead-free solders

A strain-energy based reliability model similar to a previously developed model for Sn-Pb assemblies (Ref 16, 44-46) is under development. In lieu of finite element analysis, the model uses strength of materials techniques to capture the deformations of substrates and components. One main item presently under investigation is the selection of an adequate constitutive model for solder in lead-free solder joints (Ref 47, 49). This is a critical part of the model development effort, given significant differences in the mechanical behavior of bulk solder and small size solder joints. 

Use in wave soldering machines—Lead-free solders, such as Sn-Pb solders, oxidize very quickly in the liquid state. In both cases, the oxides form at the surface immediately after the oxide skin has been removed. Dross, the material formed at the molten wave in a wave soldering machine, consists of solder metal surrounded by an oxide skin. The tendency to form dross in the case of lead-free solders is greater than that in eutectic Sn-... 

Typical environmentally regulated substances are arsenic, cadmium, mercury, and lead. Presently, there are few products including mercury. Some electronic devices include arsenic or cadmium, and are used in a computer and so on. Lead is used in the battery of a car, and its alloys are used as solders. In addition, it is included in polyvinylchloride (PVC) cables as a stabilizer. The battery has been treated appropriately. The solder was replaced by a lead-free solder after the RoHS directive. However, a lead-free solder includes lead below 1000 (mass) ppm. The quantity of lead in PVC cables has been decreased. The consumption of the environmentally regulated material seems to decrease, but the discarded products may be the products produced before the enactment of the regulations. 

Luttrull, D., and Hickman, E, New Halogen-Free PCB Materials for High-Speed Applications and Lead-Free Solder Processes, Fwtore Circuits, March 2001. 

Increased expansion causing premature PTH failures Base materials expand when exposed to soldering temperature.This expansion becomes pronounced above theTg of the resin system in the base material. The increased temperature of lead-free soldering can cause premature failure of the PTHs due to this increased expansion. See Fig. 51.3 for PTH expansion characteristics with temperature. 

A tremendous number of researchers have explored the reliability of Pb/Sn solder alloys. A wealth of data, material properties, and proposed constitutive relationships have been captured in the literature. However, there is relatively little information in the literature on the material properties of lead-free solders. Moreover, the material property information available is on slightly different compositions of lead-free solders, with tests performed in different ways. 

TABLE 59.2 Material Properties Anand s Constants for Lead-Free Solder... 

A more comprehensive list of mechanical material properties of lead-free solders and other materials used in electronic packages can be found in Reference 22. 

Two issues have arisen regarding RoHS requirements for the flexible circuit materials flame-retardant molecules with bromine in adhesive resins, and heat resistance for high-temperature processing with lead-free soldering. Although the issue of bromine is not actually a part of the RoHS requirement, it has been linked to the general environmental issues of printed circuit materials and processes. 

If adhesiveless laminates are used as the major materials, high temperatures can be used with lead-free soldering. In the case of SMT assembling, the flexible circuit should be fixed on a carrier frame. This allows it to be processed by the same mounting and soldering equipment as used for rigid boards. 

Amalu, E.H., Ekere, N.N. Mallik, S. (2011). Evaluation of Rheological Properties of Lead-Free Solder Pastes and their Relationship with Transfer Efficiency During Stencil Printing Process. Materials and Design, Vol. 32, (February 2011), pp. 3189-3197. 

The example of glass fiber-reinforced PA 6 shows that the thermal-mechanical behavior of polyamide 6 and polybutylene terephthalate can be improved by electron irradiation so that the material can withstand an industrial soldering process at a temperature of 255 °C (such as used in lead-free soldering) [807], [720]. Figures 5.250 and 5.251 illustrate this by two examples. 

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