Crack evolution and rapid life assessment for lead free solder joints

Abstract

This paper reports on the crack behavior and microstructure evolution of lead free solder joints during thermal and isothermal fatigue testing. Daisy chained Ball Grid Array (BGA) assemblies were subjected to either accelerated thermal cycling or cyclic bending at room temperature. In the case of thermal cycling effects of strain range were assessed by comparing assemblies with different distances to neutral point. Crack areas were measured by dye and pry and recrystallization detected by optical microscopy with crossed polarizers. The stochastic nature of the fatigue crack growth associated with variations in Sn grain orientation was addressed by using pattern recognition with Artificial Neural Network (ANN). Different statistics were calculated for the different crack curves at specific life percentages to address the change in crack growth distributions at these different live intervals.

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

“…64,65 The same mechanism is also active in thermal cycling, leading to an almost immediate initiation of a transgranular crack. 11 However, the dominant mechanism is the formation and ongoing rotation of new grains leading to ever greater misorientation and eventually crack growth. 24,26 The transgranular crack growth mechanism may involve local recrystallization near the crack tip, but from a modeling perspective, at least it is completely different.…”

“…[5][6][7][8][9][10] A modification of the conventional dye-and-pry technique allowed the measurement of very small fatigue cracks and, contrary to popular belief, the crack initiation stage was found to be negligible in thermal cycling. 11 Crack growth did, however, remain quite limited until a continuous network of high-angle grain boundaries had been established across the high-strain region in the joint. 11,13 At that point, fatigue cracks started to propagate much more rapidly through the region (Fig.…”

“…11 Crack growth did, however, remain quite limited until a continuous network of high-angle grain boundaries had been established across the high-strain region in the joint. 11,13 At that point, fatigue cracks started to propagate much more rapidly through the region (Fig. 6).…”

“…Instead, failure in thermal cycling almost always occurs by recrystallization of the Sn to form a network of high-angle grain boundaries across the joint [5][6][7][8][9][10] , followed by cracking along these boundaries. [11][12][13] It is our belief that the general picture below can be extended to explain the superior performance of a so-called interlaced twinning structure, 14 but that may require more work. The most credible approaches to quantitative modeling rely on the finite element modeling (FEM)-based calculation of stresses and strains, and often other factors derived from these (work, entropy, etc.…”

Recrystallization behavior of lead free and lead containing solder in cycling

“…Different brightness regions in this figure indicate three dominant crystal orientations, divided into six separated twin regions. Figure 1: Cross polarizer image of cross section of 30 mil SAC305 BGA joint showing "beach ball" structure Accelerated thermal cycling of SnAgCu based assemblies usually leads to recrystallization across the high strain region of a joint, and failure then happens by cracking along the new grain boundaries [2][3]. Figure 2 shows a recrystallized SAC305 joint in thermal cycling, where different Sn grain orientations again have different brightness.…”