As mentioned by Smetana et al. (2004), one of the critical issues for lead-free products is solder joint reliability. There are many useful reliability results based on, for example, field data, power/temperature cycling tests, mechanical shearing, bending and twisting tests, shock and vibration tests, electrochemical tests and mathematical modelling, for Sn-Pb solder joints (Lau, 1991; Lau and Rice, 1985; Lee, 2002; Wassink, 1989). However, this is not the case for lead-free solder joints because of their immaturity. Thus, the reliability of lead-free solder joints is currently under scrutiny.

A variety of environmental stress factors, e.g. temperature, voltage, humidity, corrosion, current density (electromigration), and mechanical loads, may lead to lead-free solder joint failure. The most common failure modes in practice are overload and fatigue.

Overload failure occurs whenever the stress in the solder joint, brought about by the imposed stress factors, is greater than the capacity of the solder material. An example would be excessive bending and twisting of a printed circuit board (PCB) with solder-bumped flip chip (Lau, 1994, 1996, 2000; Lau and Pao, 1997), chip scale packages (CSP) (Lau and Lee, 1999), or ball grid array (BGA) packages (Lau, 1995) soldered to its surface. In this case, the bending and twisting stresses are larger than the strength or fracture toughness of the lead-free solder alloy.

Fatigue failure takes place via the initiation and propagation of a crack until it becomes unstable. The stress factors that typically cause failure by fatigue are far below the overload...