The reliability of lead-free solder wire
depends not only on intrinsic characteristics, but also on the design and components and boards to be mounted, the process used to form the solder joints, and the long-term use environment. Also, the properties of the solder joints are different from those of the bulk lead-free solder wire.
Consequently, some established bulk solder wires and solder joints may not have identical mechanical and temperature characteristics. The main reasons for this are the high ratio of the board layer surface to the amount of solder wire, the formation of many heterogeneous nuclei during curing, and the variation in elemental or metallurgical concentrations as lead-free solder dots are formed. Either condition can result in a structure that lacks uniformity in the reaction. This interface decay will be more pronounced as the thickness of the solder line is reduced. As a result, the characteristics of the solder joint may change and the failure mechanism may not be the same as that obtained with bulk lead-free solder wire.
The three fundamental mechanical properties of lead-free solder wire include stress-to-stress properties, creep resistance and fatigue resistance. Strength is important because most welds experience shear stress in service.
Creeping is a general plastic deformation when both temperature and stress (load) are kept constant. This time-dependent deformation can occur at any temperature above absolute zero. However, cowardice only becomes important at "lively" temperatures.
Fatigue is the failure of an alloy under alternating stress. The alloys can withstand less stress under cyclic loading than under static loading. Electronic packaging and installations using moderate lead-free tin wire typically experience low frequency fatigue (fatigue life less than 10,000 cycles) and high stress.
Thermal mechanical fatigue is another form of test used to characterize solder wire. The material is subject to the temperature extremes of the cycle, a form of temperature fatigue testing. Each method has its own characteristics and advantages, both of which affect the strain cycling on lead-free tin wires. Performance and design of external components and circuit boards also have a significant impact on lead-free solder contact characteristics. For example, the design of the solder mask associated with the pad (eg, restricted or unrestricted solder mask) will affect the performance and failure mechanism of lead-free solder joints.