Part 3 of this series focuses on how Bi plays a role to the answers of these two questions: Why isn’t SAC able to be a universal interconnecting material for electronic circuits, and why does a quaternary alloy system offer a more wholesome approach? (Note: a quaternary system referred herein does not include SAC compositions incorporated with one or more doping elements.)
Overall, SAC305 has performed to expectations—delivered satisfactory solder interconnections for most (but not all) applications under most service conditions. Nonetheless, some performance deficiencies have manifested as anticipated. Specific deficiencies include the undesirable brittleness (loosely defined) relative to SnPb counterpart and the potential occurrence of solder joint surface cracks and other production-related defects and issues (e.g., head-on-pillow, pad-cratering).
One straightforward remedy to alleviate the loosely defined brittleness of SAC305 was to reduce the Ag content, which consequently has led to the introduction of low-Ag SAC alloy compositions (e.g., SAC0308 containing 0.3wt%Ag, 0.8%Cu) to the industry. Apparently, the reduced metal cost of the low-Ag compositions also offers an upside. However, with the reduction of Ag content, the mechanical properties of resulting solder joints (the yield strength, tensile strength and creep resistance) are expected to decrease. In the ranges of Ag and Cu contents of this discussion, the fatigue resistance, which often involves more complex mechanisms, is also expected to decrease with the reduction of Ag content.
Testing measurements coincide well with the expectations. For the effect of Ag content (at a range of 0.5–1.5 wt% Cu), tests showed that yield strength and tensile strength increase almost linearly with Ag up to around 4.0 wt%; and its plasticity increases with decreasing Ag content.
Overall, the lower strength is associated with lower Ag content, in congruence with the metallurgical principles. When an alloy delivers (or fails to deliver) its performance over a period of service time in the fashion largely in line with the expectations before a single test was run, it is immensely comforting and rewarding.
Turning to manufacturing processes, which in turn affect the integrity of the circuit board assembly as a whole, the alloy compositions containing the Ag content lower than 3.0 wt% (SAC305) correspond to increased liquidus temperatures comparing with SAC305. The liquidus temperature increases with the decreasing Ag content, which is nearly in a linear correlation. This was also expected, because SAC305, a near-eutectic composition, essentially is associated with the lowest melting temperature (217–220°C) that can be achieved within the SnAgCu system.
Does that few degrees delta in liquidus temperature matter? The answer is resoundingly a yes.
The increased liquidus temperature requires an increased process temperature to make sound interconnections in the package level or board assembly level. The increased liquidus temperature also demands a higher level of heat resistance of PCB material and PCB internal structure. By any practical measures, all materials and components used in the assembly must have a higher temperature tolerance level in order to be in sync with the process temperature dictated by the lower Ag content.
It should be noted that the liquidus temperature of SAC305 already pushes to the high end of assembly temperature in order to fit a broad spectrum of PCB designs under the current SMT infrastructure. A melting temperature below 213°C is more desirable and forgiving, providing a wider process window. To avoid a “narrow” process window is the prerequisite to minimize production defects.
Specific constraints in the SMT infrastructure including the supply chain have been established in the industry. With the goal of meeting the relentless demands of enhanced performance of electronics, a ternary alloy, such as SAC, is expected to fall short in serving as a reliable interconnecting material to deliver all properties and performance that the advanced electronics requires, particularly the thermal fatigue resistance to withstand the stress/strain that powerful components imposed on the solder joint. The same applies to other ternary systems.
Under the known constraints of the SMT manufacturing infrastructure (e.g., operational flow, process temperature) and the requirements of physical and chemical properties of materials to produce electronic products (e.g., environmental stability), ternary systems unfortunately do not possess the fundamental microstructure and metallurgical foundation to support the higher level of solder joint performance and solder joint reliability.
Advanced electronics designed with higher functionalities and higher power in a smaller form factor imposes a larger amount of “cyclic thermal stresses” on solder joints. This was the genesis of designing quaternary alloys, as addressed in the early 1990s in many of my professional development courses and publications.
It is worth noting that the scientific base to design the SnAgCuBi system was not to add an element (in this case, Bi), to an SAC system. Rather, it was a holistic material design platform using the underlying science and engineering of metallurgical interactions and taking the commonly-occurring solder joint failure mechanisms into consideration. In other words, the design was to mitigate those likely failure mechanisms so that solder joints, by serving as electrical, thermal and physical conduits in chip level, package level and on circuit board, can reliably connect powerful semiconductor chips to the outside world to make reliable advanced electronics that we all enjoy using in all facets of our lives.