| dc.description.abstract | ABSTRACT
In the first part of this thesis, the interaction between Cu-Sn and Ni-Sn interfacial reactions in a soldering system has been studied by using a Ni-Sn3.5Ag-Cu sandwich structure. A layer of Cu-Sn intermetallic compound was observed at the interface of the Ni foil after 30 seconds reflowing. Two stages of the Cu-Sn compound growth on the Ni side were observed: (1) in the first minute of reflow, the fast Cu-Sn compound formation was rate limited by Cu diffusivity in the Cu-Sn compound layer of the opposite Cu side. (2) after one minute of reflow, the Cu-Sn compound growth was very sluggish and depended on the Ni diffusion in the Cu-Sn compound of the Ni side. Very little Ni can be detected in the Cu side. This implies that Cu diffused and dissolved in the molten Sn3.5Ag solder much faster than Ni. When the dissolved Cu arrived at the interface of the Ni foil, a Cu-Sn compound layer formed on the Ni interface to prevent the Ni foil from reacting with solder.
The driving force of the dissolved Cu atoms toward the Ni side attributed to the Cu solubility difference across the molten solder was established due to the reduction of the Cu solubility near the Ni interface. The reduction of Cu solubility was caused by the presence of dissolved Ni near the Ni interface. Knowing the experimental value of the Cu flux toward the Ni side and assuming the diffusion of Cu atoms in the molten solder following Fick’s first Law, the diffusivity of Cu is found to be 10-5 cm2/s.
Spalling phenomenon on UBM (Under Bump Metallization) is one of the current urgent reliability issues for the Pb-free solder implementation in flip chip technology. In the second part, we report that spalling of Ni thin UBM can be prevented during the soldering reaction, if a Cu reservoir is introduced into the structure of C4 (Controlled Collapse Chip Connections) solder joints. Once molten Sn3.5Ag solder was saturated with Cu atoms, Cu precipitated out as a layer of Cu-Sn compound on Ni thin UBM. Cu-Sn compound layer served as a reaction barrier to retard the consumption of Ni thin UBM. So, spalling is retarded. After prolonged reflowing, Ni thin UBM was converted to ternary Cu-Sn-Ni compounds. Unlike interfaces of Ni-Sn compound/Cr, interface of Cu-Sn-Ni compound/Cr is very stable and no spalling was found. Furthermore, the effect of Cu content in Sn(Cu) alloys on the interfacial reaction between Ni thin film and Sn(Cu) alloys has investigated. We have found that the variation of Cu content has a strong influence on the spalling of the Ni thin film. With small Cu additives in the Sn, spalling was deferred to longer reflowing time. When the Cu content increased to about 1.0wt.%, a layer of Cu-Sn compound formed on the Ni thin film and no spalling was observed after 20 minutes reflowing. The possible mechanism of spalling deferring is proposed. A Cu flux from the solder to the interface compensated the ripening flux of the semi-spherical compound grains, therefore, spalling was retarded. The driving force of the Cu flux was attributed to the reduction of Cu solubility due to the presence of Ni at the interface of the Ni thin film. The Cu flux from solder to the interface is calculated to be in the same order with the ripening flux of Cu6Sn5 compound grains, which confirms the proposed mechanism of spalling deferring. For the Sn(Cu) alloys having Cu content over 1.0wt.%, the Cu-Sn compound layer grew so fast that the surface of the interfacial compound layer was free of Ni. There was no Cu flux to compensate the ripening flux, therefore, the ripening flux dominated and spalling occurred after short reflowing time.
In the third part, We have studied interfacial reactions between Sn(Cu) alloys and Ni(P) substrates. Comparing to Sn/Ni(P), the formation of Ni3P layers in Sn(Cu)/Ni(P) reaction couples were very limited. The sluggish growth of Ni3P layer attributed to a layer of Cu-Sn compound layer formed on the Ni(P) substrate, which effectively isolated the Ni(P) substrate from reacting with solder. The eventual formation of Ni3P compound layer depended on the Ni diffusion in the Cu-Sn compound. Also, we found that the higher Cu-content Sn(Cu) alloys had less Ni3P formation.
We have investigated interfacial reactions between Sn(Cu) alloys with Au and Ni-coated Au foils in the fourth part. As Au foils reacted with Sn-based alloys, Au foil quickly dissolved into the molten solder and form large amount of Au-Sn compound in the solder and compound layer at the interface. As the Au coated with a thin Ni layer reacted with the Cu content over 1.8 wt.%, a layer of ternary (Cu,Ni)6Sn5 compound was observed at the interface between Sn(Cu) alloys and Au foils. This ternary compound layer effectively isolated the Au foil from reacting with the molten solder. Also, as Au foil reacted with Sn(Cu) alloys. The Au consumption rate depended on the Cu content. In the middle Cu content region, we found that it has the least Au consumption rate. | en_US |