Elimination of Au-embrittlement in solder joints on Au/Ni metallization
- PDF / 294,979 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 133 Views
RAPID COMMUNICATIONS The purpose of this Rapid Communications section is to provide accelerated publication of important new results in the fields regularly covered by Journal of Materials Research. Rapid Communications cannot exceed four printed pages in length, including space allowed for title, figures, tables, references, and an abstract limited to about 100 words.
Elimination of Au-embrittlement in solder joints on Au/Ni metallization M.O. Alam and Y.C. Chana) Department of Electronic Engineering, City University of Hong Kong, Kowloon Tong, Hong Kong
K.N. Tu Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, California 90095-1595 (Received 14 July 2003; accepted 8 September 2003)
Systematic experimental work was carried out to understand the mechanism of Au diffusion to the solder interface, and a novel method was proposed to eliminate Au embrittlement by circumventing the continuous layer of (Au,Ni)Sn4 at the solder interface. Contrary to the usual expectation, it was determined that utilization of a very thin Ni metallization in the Au/Ni/Cu under bump metallization (UBM) was an effective means of maintaining mechanical integrity of the solder joint. It was found that the out-diffusion of Cu during the aging period changes the chemistry and morphology of the intermetallic compounds at the interface.
The metallization of a tri-layer Au/Ni/Cu creates a popular under bump metallization (UBM)/bond pad for chip to substrate and/or substrate to printed circuit board interconnection through solder joint. The outermost Aulayer serves to protect the UBM/pad from corrosion and oxidation as well as to increase solderability.1–3 The Ni layer serves the purpose of a diffusion barrier to inhibit the out-diffusion of Cu to the Au-protective layer during storage time and to prevent the rapid soldering reaction with the Cu layer.3–7 The Au coating (usually 0.1–1 m) dissolves very quickly into the molten solder during reflow and precipitates as AuSn4 in the matrix of the solder upon cooling while the Ni reacts with Sn to form a thin Ni3Sn4 intermetallic layer at the interface.8–11 However, it is surprising that after the package is subjected to several hundred hours of solid-state aging at 100–150 °C, most of these AuSn4 needlelike precipitates migrate to the Ni interface and form a continuous layer of (Au,Ni)Sn4 over the Ni3Sn4 layer.9–11 The aged joints were found to be significantly weaker than the same joints tested before aging. The redeposition of the (Au,Ni)Sn4 as a continuous layer degrades the strength
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0170 J. Mater. Res., Vol. 19, No. 5, May 2004
and ductility of the solder/pad interface, eventually resulting in Au-embrittlement of the solder interconnection. This (Au,Ni)Sn4 ternary intermetallic (TIMC) was reported to grow at a rather faster rate than the binary intermetallic (BIMC) during the aging treatment.9–13 It is now confirmed that the utilization of a Ni
Data Loading...
