Chien-Fu Tseng

錫銀銅銲料與無電鍍鎳(磷)/鈀/金底層金屬之微觀結構、可靠度測試及結構方向性

Electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) is widely used as surface finish for metal bond pad in the electronic packaging industries. The widespread adoption of ENEPIG is attributed to its many advantages, and the most important one, it resolves the so-called “black pad” reliability problem in the electroless Ni(P)/immersion Au (ENIG) surface finish. The insertion of Pd layer is believed to relieve the corrosion of underneath Ni(P) layer from immersion Au plating solution. However, the complete interfacial reaction and comparison between these two surface finishes are still lacking in literature. Therefore, this study aims to probe the microstructure variation induced by the Pd addition and to discuss the possible mechanism. The interfacial reactions of Sn-3.0Ag-0.5Cu solder jointed with ENIG and ENEPIG were first to investigated. (Cu,Ni,Pd)6Sn5 grew rather slower in the ENEPIG samples among all aging condition as compared with ENIG. It was demonstrated that ENEPIG could inhibit the formation of Ni3Sn4, which then decreased the growth of columnar Kirkendall voids inside the Ni3P layer. In order to further explore the growth kinetics of (Cu,Ni,Pd)6Sn5, the liquid state reaction was thus investigated. Pd may act as heterogeneous nucleation sites in the initial soldering and lower the activation energy of (Cu,Ni,Pd)6Sn5, as compared to (Cu,Ni)6Sn5. The lower activation energy of (Cu,Ni,Pd)6Sn5 growth ensured that no phase transformation occurred in the SAC305/ENEPIG joints, which may explain why the phase transformation was inhibited in the ENEPIG joints. The detailed impacts of Pd on the growth kinetics of IMC formation was investigated and discussed as well as the mechanism of Ni3Sn4 suppression. To verify that the microstructure variation would affect the interfacial strength, the high speed impact test was utilized. The impact energy of ENEPIG joints declined slower than that without Pd-doped after prolonged reflow. The enhanced impact strength and the transition of failure mode in the ENEPIG joints was attributed to the needle-like morphology of (Cu,Ni,Pd)6Sn5. The detailed mechanism of improved mechanical strength for solder joints with Pd dissolved was deliberately addressed and discussed regarding the distinct microstructural evolution in the ENEPIG joint. Besides, the crystallographic orientation of ENIG/SnAgCu/Cu and ENEPIG/SnAgCu/Cu assembled solder joints was investigated. With the aid of EBSD analysis, various grain structures and preferred growth orientation of IMC on the Cu and Ni(P) substrates were observed. The distinctive growth behaviors of intermetallic compound on the Cu and Ni(P) substrates were associated with the cross-interaction of minor Cu, Ni and Pd elements. Finally, the correlation between microstructure variation and grain orientation was probed and discussed. The possible mechanism was also proposed.

Solid state reaction of Sn3.0Ag0.5Cu solder with Cu(Mn) under bump metallization

In flip chip technology, Cu thin-film is a widely used under bump metallization (UBM). However, the major disadvantages of Cu UBM are fast consumption of copper, rapid growth of IMCs and easy formation of Kirkendall voids [1, 2]. Many efforts have been focused on suppression of Kirkendall voids which are detrimental to solder joints reliability in the microelectronics industry [3, 4]. In this study, a novel Cu(Mn) UBM design was provided by sputtering to improve the conventional Cu metallization. For the higher Mn concentration (10 at.% Mn) in the Cu(Mn) UBM, a new Sn-rich phase formed between Cu6Sn5 and the Cu(Mn) UBM. However, a layer of crack formed after aging. For the lower Mn concentration (2 at.% Mn), the growth of Cu3Sn and Kirkendall voids was significantly suppressed during thermal aging. The interfacial reaction was analyzed by a field emission electron probe microanalyzer (FE-EPMA) to evaluate the composition distribution. Kinetic analysis and X-ray color mapping gave the evidence that Mn diffusion into Cu3Sn slowed down the diffusivity of Cu in Cu3Sn layer. The Mn-enriched Cu3Sn might serve as a diffusion barrier to reduce interfacial reaction and Kirkendall voids formation. These results suggest the Cu-Mn UBM with a low Mn concentration is beneficial to retarding the Cu pad consumption in solder joints.