| dc.description.abstract | A high reliability solder joints for passive devices must meet the following conditions: (1) Good wettability on the surface of the metal plating of terminations (2) Appropriate interfacial reaction (3) No Sn whisker formation. So, the focuses on this study are wettability, Sn whisker formation mechanism, and interfacial reaction. Our results indicate that we can increase wettability by prolonging the reflowing time and increasing the thickness of Sn finish layer on the substrate.
Sn whiskers formation on Sn(Cu) finishes has been studied. (1) Thickness effect: Sn whisker density for pure Sn and Sn0.7Cu finishes has a linear relationship with the finish thickness. The safety thickness, i.e., with small Sn whisker number, for Sn and Sn0.7Cu finishes are about 10 and 20 µm, respectively. (2) Alloying effect: Sn whiskers formation was found to be retarded by increasing Cu content in Sn(Cu) finishes. We conclude that the Cu additives could reduce the two major driving forces of the Sn whisker formation, i.e., metal under-layer dissolution and thermal stress. The Cu additives formed a self-formed Cu-Sn compound barrier layer, which effectively prevented the reaction and dissolution with the metal under-layer. On the other hands, the Cu additives precipitated out as Cu-Sn compound in the Sn(Cu) finish layer, which is believe to be the reason for smaller CTE values of Sn(Cu) alloys. The smaller CTE values results in a lower thermal stress level in the Sn(Cu) finishes.
Electroless Ni(P) substrate was extensively used as the multi-layer metallization pad for Flip-Chip and ball-grid array (BGA) solder bumps. The correlation exists between the interfacial reaction and mechanical strengths of Sn(Cu)/Ni(P) solder bumps. For pure Sn, Ni3P layer and Ni-Sn compound formed more rapidly than Sn0.7Cu and Sn3.0Cu. Upon Sn/Ni(P) solid-state aging, a diffusion controlled process was observed and the activation energy is 42 KJ/mol. For Sn0.7Cu, the morphology of the interfacial Ni-Sn compound is needle-like at the initial aging, and then transformed to the mixture of Ni-Sn and Cu-Sn compounds which is layer-like shape afterwards. For Sn3.0Cu case, the Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted in a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. In contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu bumps is relatively weak and exhibits poor resistance against the aging process. So the Cu additives in Sn(Cu) solder (0.7 wt%) can increase the interfacial strength of solder bumps in the solid-state reaction. | en_US |