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Optimization of Ag Composition in Cu Pillar Bumps with Sn-xAg Solders
Keywords: Cu Pillar Bump, Sn-Ag Lead-free Solders, Intermetallic Compound
In order to minimize the large plate-like growth of Ag3Sn potentially adversely affecting the mechanical behavior and reducing the reliability of solder joints, the Ag composition of less than 3 wt% has been suggested in BGA ball Sn-xAg solder alloys. The suggestion (< 3wt% Ag in the solders) is invalid in small solder bumps, but there is no available reference or guideline of the Ag composition in the small Sn-xAg solders, such as under-100um C4 solder bump and under-50um Cu pillar bumps. In this study, the optimum level of Ag composition in Cu pillar bump with Sn-xAg solder (< 50um in diameter) was investigated to suppress the large plate growth of Ag3Sn IMCs during reflow. In general, Ag atoms in the Sn-Ag solders react to Sn atoms, and form Ag3Sn intermetallic compounds (IMCs) in the solder matrix. Undesirably, pro-eutectic Ag3Sn IMCs grow rapidly within the liquid phase during cooling, and form large plate-like shape. Since the undercooling of Sn-Ag solders increases with a decrease of solder ball size to less than 100um, the Ag3Sn IMCs are more prone to become largely plate-like in small solder bumps. Actually, in Cu pillar bumps with under-50um Sn-Ag solders, the large plate-like growth of Ag3Sn was observed on the top surface of solder bumps even though the Ag composition is less than 3.0 wt% in the solders, such as 2.3 wt% or 2.5 wt%. The large plate-like Ag3Sn causes the noise of bump height inspection and potentially interrupts the wetting of solders on Cu pads during flip chip boning. Through the dynamic scanning calorimeter (DSC) results and the thermodynamic calculations, the optimized Ag composition in under-50um Sn-xAg solder was suggested, and the large plate-like growth of Ag3Sn was effectively reduced in the suggested Ag composition. In addition, another way suppressing the large plate growth of Ag3Sn IMCs, such as cooling rate and reflow profile, was also reported.
Moon Gi Cho, Senior Engineer (Ph.D)
Samsung Electronics Co., Ltd
Hwasung-City, Gyeonggi-Do

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