The interfacial structure and strength of solder joints between Sn-9 mass%Zn solder and plated Au/Ni-P alloy film on a Cu substrate have been investigated. Three reaction layers with 0.2 to 0.5 µm thickness were formed along the interface between the plated Ni-P alloy films and Sn-9 mass%Zn solder. The outermost layer contains a Ni-Sn intermetallic compound. The middle layer contains approximately 40 mass%Au, 35 mass%Zn, 20 mass%Ni and 5 mass%Sn. The thickness of the Au layer is 0.1 µm, so the Au layer does not dissolve. The innermost layer contains about 63 mass%Zn, 25 mass%Ni, 10 mass%Au and 2 mass%Sn. The strength of the Sn-9 mass%Zn solder joints take almost the same values independent of P concentration. The strength of Sn-Zn solder joints with Sn-Zn/Ni-2 mass%P, Sn-Zn/Ni-4 mass%P and Sn-Zn/Ni-8 mass%P joints were found to be almost constantly independent of reflow cycles. Therefore, Sn-9 mass%Zn solder is considered to be an excellent solder material for plated Ni-P alloy films.

One of the critical issues which needs to be solved in the packaging technology of high speed and high density semiconductor devices is the enhancement of micro-solder joint reliability and strength. The reliability and strength of the solder joints depend on the interfacial structures between metallization and lead free solder. Both the interfacial structures and the strengths of the solder joints between plated Ni-P alloy films with various P concentrations and various solder materials have been investigated. The places where intermetallic compounds crystallized were found to vary according to the P concentration in plated Ni-P alloy films and the composition of the solder. Pyramidal intermetallic compounds that formed on plated Ni-P alloy films had the following compositions: Sn-3.5 mass%Ag/Ni-2 mass%P, Sn-3.5Ag-0.7 mass%Cu/Ni-P(2, 8 mass%) and Sn-50 mass%Pb/Ni-P(2, 8 mass%). Whereas intermetallic compounds were crystallized in the solder of the Sn-3.5 mass%Ag/Ni-8 mass%P sample. A P-enriched layer was formed between the plated Ni-P alloy films and the intermetallic compounds. The thickness of the P-enriched layers of each sample increased with the reaction time. In experiments using the same solder material, the P-enriched layer of the solder/Ni-8 mass%P sample was much thicker than that of the solder/Ni-2 mass%P sample. In experiments with plated Ni-8 mass%P alloy films, the P-enriched layers became thicker in this order: Sn-50 mass%Pb/Ni-8 mass%P; Sn-3.5Ag-0.7Cu/Ni-8 mass%P; Sn-3.5 mass%Ag/Ni-8 mass%P. The strengths of the solder joints decreased with the P concentration in plated Ni-P alloy films for all solder materials. However, it was found that the strength degradation ratio varied with the solder materials and they increased in the following order: Sn-50 mass%Pb; Sn-3.5Ag-0.7 mass%Cu; Sn-3.5 mass%Ag. Therefore, it was found that the solder joint strength is very sensitive to the thickness of the P-enriched layer at the solder joint and the solder joint strength decreased with the thickness of the P-enriched layer independent of the solder materials. Therefore, research into the interfacial structures between electroless plated Ni-P alloy film and solder is very important. It has been reported that reliability degradation occurs at the interface between plated electroless Ni-P alloy film and solder, when a P-enriched layer is formed at the interface during the soldering process.8) However, no-one has yet confirmed the relationship between the interfacial structure, including intermetallic compound and the P-enriched layer, and the mechanical behavior of the solder joints.The first purpose of the present paper is to investigate the interfacial structures formed during the soldering of plated Ni-P alloy films having various P concentrations with solder materials. Next, solder joint strength was investigated as a function of the solder materials and P concentrations in plated Ni-P alloy films. Finally, the relationship between the interfacial structure and the solder joint strength was investigated.

A new method for transmission electron microscope (TEM) specimen preparation using a focused ion beam (FIB) system that results in a lower rate of gallium (Ga) implantation has been developed. The method was applied to structural and analytical studies of composite materials such as silicon (Si)-devices and magneto-optical disk. To protect the specimens against Ga ion irradiation, amorphous tungsten (W) was deposited on the surface of the specimen prior to FIB milling. The deposition was quite effective in reducing the Ga implantation rate, and energy-dispersive X-ray (EDX) analysis of these specimens detected 0.3Ð1.5% Ga incorporated in the thinned area. FIB milling times for these specimens were 1.5Ð2 hr. Although the milling rate was high, all the materials were properly prepared for TEM study, and clear crystal lattice images were observed on all specimens.

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A technique to cut out small pieces of samples directly from chips or wafer samples in a focused ion beam (FIB) system has been developed. A deep trench is FIB milled to cut out a small, wedge-shaped portion of the sample from the area of interest A micromanipulator with tungsten (W) probe is employed for lifting the micro-sample. The lifted micro-sample is then mounted on a carrier to prepare electron transparent thin foil specimens for transmission electron microscope (TEM) observation. We have also developed a method for site-specific TEM specimen preparation. In this method, FIB system and TEM/scanning transmission electron microscope (STEM) equipped with secondary electron (SE) detector are employed. An FIB–TEM/STEM compatible specimen holder has also been developed so that a specimen can be milled in the FIB system and observed in a TEM/STEM without remounting the specimen. STEM and scanning electron microscopy (SEM) images are used for locating a specific site on a specimen. SEM image observation at an accelerating voltage of 200kV enabled us to observe not only surface structures but also inner structures near the surface of a cross section with depth of field of around 1 micrometer. The STEM image allows the observation of inner structures of 3-5 micrometer thick specimens. Milling of a specimen by FIB and observation of the milled sample by SEM and STEM are alternately carried out until an electron transparent thin foil specimen is obtained. The position accuracy of the method in TEM specimen preparation is approximately 100nm.

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