With automotive electronics booming, especially those in electrical vehicles (EV), more sensors and power moderators are increasingly required for electrical vehicles and self-driving cars. Lead-free tin-silver-copper (SnAgCu), also known as SAC, has been a popular solder alloy choice for surface mount technology (SMT) assembly in the electronics industry. While SAC has served the electronics industry adequately well, its adoption for automotive applications has proved to be challenging for several reasons.
Key amongst them is uncertainty in service temperature range capability. There is no question that automotive applications demand high reliability; however, that high reliability is required not only under moderate temperatures, but also under high service temperature conditions. Only limited success has been achieved up to now.
In this research, a novel SnAgCu-based solder alloyed with Sb was developed and characterised for its reliability performance in chip resistors and CABGA192 under thermal cycling testing (TCT) of -40 to 125°C.
Optimisation of Sb content in Sn/3,2Ag/0,7Cu alloys
In the recent development of high-performance Pb-free solder alloys, Sb plays a key role in improving the thermal fatigue resistance of solder joints in harsh thermal cycling or thermal shock conditions. According to the binary Sn-Sb phase diagrams, the solubility of Sb in Sn is approximately 0,5wt% at room temperature, and about 1,5wt% at 125°C. Due to the dissolution of Sb in Sn-based Pb-free solders, solid solution strengthening is expected in these alloys.
Apart from solid solution strengthening, alloying with Sb also has the potential to form various intermetallic phases (IMCs) with Sn, resulting in the precipitation hardening. In literature, 1,5 to 9,0wt% of Sb has been reported. Those alloys showed different thermal fatigue resistance, depending on the concentration of the alloyed Sb. The fine SnSb IMC particles nucleate and grow (cluster of different atoms in certain stoichiometric ratio) after solder solidification during reflow. These SnSb particles are reversely dissolved back into Sn matrix to form a solid solution with increasing temperature, and then precipitate out with the drop in temperature.
A sufficient quantity of Sb is important to harden the solder alloy by providing both solid-solution and precipitation strengthening to the alloy. When the amount of Sb is reduced below 3,0wt%, fine SnSb particles are completely dissolved back into the Sn matrix to form an SnSb solid solution when serving at 150°C and above; no SnSb fine particles remain to strengthen the alloy.
Strengthening in alloys is associated with interrupting the dislocation movement. Both fine particles embedded in the alloy matrix and solute atoms in the solid solution act as obstacles to block the dislocation slide along the favourable lattice direction. At high temperatures (homologous temperature > 0,6), atomic diffusion plays an important role in assisting the dislocation movement. For small obstacles like solute atoms, atomic diffusion can easily assist the dislocation to bypass or ‘climb over’ the obstacles.
For large obstacles like precipitates, more atomic diffusion steps are needed to allow the dislocations to bypass or ‘climb over’. Thus, precipitates are more valuable to maintain high-temperature strength through interrupting the dislocation movement.
Therefore, 4,5wt% and above of Sb is expected to keep the alloys maintaining enough precipitate strengthening, even at 150°C and above. However, if the Sb addition exceeds 10wt%, the solder alloys will have a liquidus temperature above 266°C, making it impossible to be reflowed by the conventional SAC305 process (the peak reflow temperature is usually below 245°C).
Findings
In this research, the thermal performance of five Sn/3,2Ag/0,7Cu/xSb (x in range of 4,5 to 6,5wt%) alloys were compared to select the optimised Sb content. On conclusion of the research, it was noted that based on shear testing at various temperatures, and at different intervals of TCT -40 to 125°C, 90,6Sn/3,2Ag/0,7Cu/5,5Sb (Indalloy276) showed the best performance in those Sb-containing alloys.
Thus, this composition with the addition of 5,5wt% of Sb was identified and developed for testing in targeting high reliability with a wide service temperature capability. Indalloy276 has a melting temperature range from 223 to 232°C and could be processed with traditional SAC305 reflow profiles. The crack resistance of Indalloy276 in the components of CABGA192 and chip resistors are better than SAC305 under thermal cycling of -40 to 125°C. Alloying with 5,5wt% of Sb dramatically improved the thermal fatigue resistance compared to SAC305.
To read the full research paper visit www.dataweek.co.za/*jul24tech
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