Asao Nishimura

Chip scale packaging for memory devices

A low cost, high reliability chip scale package has been developed for memory devices. The developed CSP can be applied to center pad type devices such as DRAM and to peripheral pad type devices such as SRAM and Flash. Reliability and high volume productivity are the main technological challenges that have to be overcome for chip scale packaging. This paper unveils Hitachis original CSP concept and shows how our CSP overcomes these challenges.

Whisker Initiation Behavior from Electrodeposited Sn/Cu Coating on Cu Leadframe

Tin/copper (Sn/Cu) coatings on Cu leadframes (CUFE), in which Fe atoms are doped as a minor element, showed whisker initiation at room temperature over a long period of 47 months. By means of the planar slice method and electron back scattering pattern (EBSP) measurement, the whisker roots were consistently found to be located at the intersections of grain boundaries in the coating. Whisker roots were also located above the peaks and ridge lines of intermetallic compound (IMC) Cu6Sn5, which was formed with a pyramid-shaped configuration between the Sn/Cu coating and Cu leadframe. Using finite element analysis (FEA), we calculated stress distribution in the coating. The results indicated that compressive stress normal to the grain boundary was induced with a gradient toward the surface in the coating. Therefore, the compressive stress gradient induced by the pyramid-shaped IMC is thought to be the root cause of whisker initiation in Sn/Cu coatings on Cu leadframes. When the Cu leadframe with a minor doped element of Cr atoms (CUCR) was used as the substrate with the same Sn/Cu coating, no whisker initiation was observed even after a longer storage time of 65 months. Through field-emission scanning transmission electron microscopy (FE-STEM) and field-emission transmission electron microscopy (FE-TEM) microstructural observations of vertical sections of each sample, the shape of the IMCs formed between the coating and the leadframe in the Sn/Cu-CUFE sample was found to be different from that in the Sn/Cu-CUCR sample. The difference in whisker initiation tendency can therefore be explained by the difference in compressive stress depending on the shape of the Cu-Sn IMCs, because stress distribution in the coating of the Sn/Cu-CUCR sample calculated using FEA revealed a smaller stress gradient than that in the Sn/Cu-CUFE sample.

Stress measurement by X-Ray diffraction method for electrodeposited SnCu coating on alloy 42 substrate

A stress measurement method using SnCu coating on an Alloy 42 substrate sample, which indicated non-linear sin2 Psi diagram for the coating under ordinary X-ray diffraction conditions was investigated. Diffraction profile versus 2thetas diagrams for peak indexes of (312), (501), (213), (600), (323), (541), and (631) obtained using an iso inclination method and a fixed Psi method using CuKalpha radiation showed clear peaks for some Psi angles. However, in other cases, the diagrams showed flat forms (no peaks). It was found that removing the Psi data with the flat forms from the diffraction profile versus 2thetas diagram gave a linear sin2Psi diagram for each index. Indexes with higher 2thetas degrees had a smaller confidence limit. The (631) peak with the highest 2thetas degree, which had no flat forms in the diffraction profile versus the 2thetas diagram, even with seventeen Psi data points, indicated a linear sin2thetas diagram and a confidence limit of less than plusmn2 MPa.