Effects of Reflow Cooling Rate on the Growth of Ag3Sn Platelets and Deformation of Solder Balls

Abstract

In the electronic packaging, 1 to 4 wt. % Ag is commonly added in the solders to improve the electrical property, melting point, and reliability of materials in the transition from Pb-containing solders to Pb-free solders. Generally, solders undergo multiple reflow during bumping and assembly. After reflow, the growth of leaf-shaped Ag3Sn is observed in the solidified Sn-Ag solders but the solder appearance may deform as bigger Ag3Sn platelets protrude severely. Severe protrusion of Ag3Sn may cause short circuit as it touches the adjacent bumps and solder balls. This reliability concern becomes more severe as electronic products move towards lighter, thinner, fine pitch packages and the spacing between bumps or solder balls becomes smaller. The morphology of precipitated Ag3Sn phase is affected by the content of Ag and the cooling rate during the solidification process. In this study, metallic solders such as SAC305, SAC405, SB05 and electroplated bumps with various Ag compositions were selected to investigate the effects of cooling rate and multi-reflow on the growth of the Ag3Sn platelets. For the solder balls, the SAC405 solder ball appearance deformed by protrusion of huge Ag3Sn platelets more severely than SAC305 at a slow cooling rate of 0.91 °C/sec, which is due to higher Ag content and slow cooling rate enabling the Ag3Sn phase to have more time to grow bigger. From the morphology of intermetallic compound (IMC) observed in the SB05 solder joint, a small amount of Bi (bismuth) retarded the diffusion between Sn and Cu and also reduced the undercooling of molten Sn. Therefore, the Sn matrix solidified quickly which inhibited the formation of huge Ag3Sn platelets and prevented the solder ball deformation even at a slow cooling rate. On the other hand, the deformed solder balls with abnormal Ag3Sn protrusion caused by slow cooling rate could recover its normal ball appearance after an additional reflow with a faster cooling rate of 8.16 °C/sec. The huge Ag3Sn platelets re-melted in the molten solder during the additional reflow and precipitated in a smaller size at a faster cooling rate. For the electroplated bumps, two commercial Sn/Ag co-deposition chemical solutions were studied under the reflow process condition of non-flux coating with a constant cooling rate of 2.44 °C/sec and formic acid gas atmosphere, while the Ag composition was controlled at 1.91~4.41 wt. %. For the Ni UBM, deformed bumps were observed as Ag composition exceeded 2 wt. %, regardless of the chemical solutions. Study showed that plated bumps suffered a more severe appearance deformation causing them to be more sensitive to Ag composition compared to solder balls, which was due to a larger volume change from mushroom-like structure transforming into spheres during reflow process. For the Cu UBM, the Ag3Sn phase formed mainly at the UBM interface. When most of the Ag3Sn phase grew at the interface, it reduced the amount of Ag3Sn precipitated on the bump surface and thus avoided the solder deformation. Similarly, the deformed bumps could also be recovered by an additional reflow with a faster cooling rate of 8.16 °C/sec. With the increase of stacking number in the electronic packaging, the cooling rate becomes slower considering the total package structure. For the bumping process, the volume of solder is less but pitch is small. When the content of Ag exceeds 2 wt. %, the solder appearance deforms seriously and the risk of short circuit is high in view of slower cooling rate. For the assembly, the risk of short circuit caused by protruded Ag3Sn is low because of the wider pitch and larger solder volume. Therefore, short circuit problem may be likely to happen in the bumping process as compared with the assembly especially at a slower cooling rate because of a shorter pitch. In this study, it was found that solder deformation was not caused by the number of reflows but by the rate of cooling process. The slower the cooling rate the more severe the deformation it caused. Therefore, it is important that cooling rate is being controlled without affecting overall structure of the package as spacing between solders becomes smaller and smaller in the future.