SMT007 Magazine

SMT-July2018

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14 SMT007 MAGAZINE I JULY 2018 almost linearly with Ag up to around 4.0 wt %; and its plasticity increases with decreasing Ag content [1]. Overall, the lower strength is associated with lower Ag content, in congruence with the metal- lurgical 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 liqui- dus 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 composi- tion, 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 resound- ingly 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 inter- nal structure. By any practical measures, all materials and components used in the assem- bly 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 tempera- ture 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 estab- lished in the industry. With the goal of meet- ing the relentless demands of enhanced perfor- mance 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 require- ments 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 perfor- mance 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 publica- tions. 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 plat- 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.

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