SMT007 Magazine

SMT-May2018

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14 SMT007 MAGAZINE I MAY 2018 The ability to deliver a higher performance level in thermal fatigue resistance is particularly critical to connecting those high- power, large-size IC components onto the circuit board, as these components impose a larger amount of thermal stress on solder joints during power-on/power-off and/or elevated temperature excursion. In mobile electronics, these thermal excursion-related stresses may be compounded with machinal-shock related stresses that could be occurring during the product's useful life. SnAgCuBi is one of the quaternary systems studied [1] . Again, it is important to accentu- ate that the scientific base to design the SnAg- CuBi system was not to add an element (in this case, Bi) to an SAC alloy. Rather, it was a mate- rial innovation holistically using the underly- ing science and engineering of metallurgical principles by taking the commonly-occurring solder joint failure mechanisms into consid- eration. In other words, the objective was to mitigate those likely failure mechanisms so that solder joints can reliably connect the ever- powerful semiconductor chips to the outside world by serving as electrical, thermal and physical conduits at chip level, package level and on circuit boards. How do the proper compositions of the SnAgCuBi alloy system (containing 2.5–3.5 % Ag, 0.2–2.5% Cu, 0.5–4.0% Bi, balance Sn) perform in comparison with the stan- dard alloys? (Note: All compositions expressed herein are in weight percent.) Comparison with SnPb Eutectic— 63Sn37Pb As an example, take Sn3.0Ag0.5Cu2.0Bi as the composition. It offers higher strength as well as more than 200% higher fatigue life than 63Sn37Pb in accordance to ASTM Stan- dard E606-92 (Standard Practice for Strain- Controlled Fatigue Testing). Comparison with SnAg Eutectic— 96.5Sn3.5Ag The composition of Sn3.0Ag0.5Cu3.0Bi has a melting temperature 209-212°C that is 9°C lower than the eutectic 96.5Sn3.5Ag (221°C). When comparing the basic mechanical properties with 96.5Sn3.5Ag, Sn3.0Ag0.5Cu3.0Bi composition performs better in strength and fatigue life— more than 150% higher in fatigue life. Comparison with SnCu Eutectic— 99.3Sn0.7Cu Sn3.0Ag0.5Cu3.0Bi demonstrates signif- icantly better performance in strength and fatigue, but lower plasticity than 99.3Sn0.7Cu. Its melting temperature is 15°C lower than 99.3Sn0.7Cu. Comparison with SnAgCu Near-eutectic— Sn3.0Ag0.5Cu (SAC305) Sn3.0Ag0.5Cu2.0Bi exhibits high strength (both yield and tensile strengths and higher thermal fatigue life). Another important advan- tage of SnAgCuBi over SnAgCu is the ability to offer lower liquidus temperature. The compo- sition of Sn3.0Ag0.5Cu2.0Bi offers 7°C lower than SAC305. Further, the intrinsic wetting ability of SAC system does not measure up to that of SnPb or SnCu. With SAC305's high liqiudus temperature, the tendency to use a process peak temperature below the optimal temperature often leads to a marginal process, which further aggravates the SAC305's lower wetting ability, thus increasing potential production defects. Focusing on the integrity of a printed circuit board assembly, the liquidus temperature of the interconnecting solder alloy plays an impor- tant role in alleviating any potential defects or thermal damages to components or PCB, which can be detectable or undetectable on the production floor or during quality control verification. Concentrating on solder joint reli- ability, the thermal fatigue resistance sits front and center to the performance and reliability of a circuit board. Overall, the SnAgCuBi system offers more robust performance than any of practical binary alloys, such as 63Sn37Pb, 96.5Sn3.5Ag, or 99.3Sn0.7Cu, and ternary alloys, such as SnAgBi and SnAgCu. In comparison with SAC305, Sn3.0Ag0.5Cu2.0Bi exhibits higher

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