Grain refinement of solder materials by minor element addition

Owing to environmental concern, SnAg and SnAgCu alloys have been widely applied to electronic packaging industry since the RoHS announcement. However, there are still some reliability issues waiting for systematic research to discover promising ways to alleviate them. For instance, electromigration-resistance issue of advance packaging becomes more critical due to increased current density of smaller solder joint. It has been characterized that different dominating diffusion flux would cause distinct failure model of electromigration (EM). When the interstitial diffusion flux of Cu/Ni in Sn crystals dominates, intermetallic compounds (IMCs) and metallization layer dissolutions will be the main model of EM failure. On the other hand, solder joints tend to degrade by pancake void formation-and-propagation model when the Sn self-diffusion flux dominates under current stressing. Recently, tin´s anisotropic properties have been widely reported. Especially, some researches proved that the interstitial diffusion of Cu/Ni along the c-axis of tin grains is much faster than along the a-axis and b-axis. This phenomenon will lead to serious EM degradation via IMCs and metallization layer dissolutions if the electron flow parallels to the c-axis of Sn grain. Unfortunately, there are only few Sn grains or even one grain existing in a single joint of Sn-rich solder alloys. Consequently, the probability of occurring c-axis effect causing EM-failure mentioned above would be increased much greater. The objective of this work is to refine the microstructure of solder joints. With finer tin grains and more crystal-orientations, EM-resistance might be enhanced theoretically. Several researches about reducing undercooling or improving mechanical reliabilities of Sn rich solders with minor additions of Ti, Mn, Zn, respectively, have been published. Nevertheless, the effect of grain refinement via minor addition has not been clearly understood yet. Hence, the microstructures of the Sn2.4- g solder alloys on Cu pads without/with minor additions of Ti, Mn, and Zn, respectively, were investigated by Optical Microscopy (OM) cross-polarized images meticulously including as reflowed and thermal aging under 150°C conditions in our work. Interfacial reactions between these solder alloys and Cu pads were also observed by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) to characterize the IMC growths of solder joints with minor additions systematically. According to our experimental results, Ti addition is able to refine the microstructure effectively. We assume that Ti addition might provide extra sites which enhance the nucleation upon solidification. After the critical nuclei form, the pre-existing Ti-Sn IMC may pin the grain boundaries. As a result, grain growth after nucleation could be suppressed. However, Zn and Mn additions seem not to influence the microstructures of Tin. After 1000 hours prolong aging, the growths of the Cu6Sn5 are similar with/without Ti and Mn additions but the Cu3Sn could be suppressed slightly by the minor additions of Ti and Mn. On the other hand, Zn addition can suppress the interfacial IMCs including Cu6Sn5 and Cu3Sn drastically.