NDLI: Reaction of solder with Ni/Au metallization for electronic packages during reflow soldering

Content Provider	IEEE Xplore Digital Library
Author	Ho, C.E. Tsai, S.Y. Kao, C.R.
Copyright Year	1999
Abstract	Gold over Ni is one of the most common surface finishes for Cu soldering pads in ball-grid-array (BGA) and other electronic packages. The Au layer is for oxidation protection, and the Ni layer serves as a solderable diffusion barrier. In this study, eutectic Pb-Sn solder-balls were reflowed on the Au/Ni/Cu pads, and the chemical interactions between the solder and the surface finish were studied. Quenched-in microstructures at different stages of the reflow were carefully examined using the scanning electron microscopy. It was found that the solder melted locally along the solder/pad interface at the very early stages of the reflow before the whole solder ball had reached the Pb-Sn eutectic temperature. This was because a ternary eutectic reaction L=(Pb)+(Sn)+AuSn/sub 4/ occurred at 177/spl deg/C, six degrees below the Pb-Sn eutectic temperature. Four distinct stages were identified for the reflow process. The four stages are: (1) partial melting of solder balls and the initial reaction of Au with Sn; (2) complete reaction of An with Sn; (3) separation of (Au/sub x/Ni/sub 1-x/)Sn/sub 4/ from the pad; (4) complete melting of solder balls and the reaction of Ni with Sn. After a typical reflow, with a 225/spl deg/C peak reflow temperature and 115 s reflow time, all the An and Au-bearing intermetallic compounds left the interface and the only intermetallic compound at the interface was Ni/sub 3/Sn/sub 4/ with a thickness of about 2 /spl mu/m.
Sponsorship	IEEE Components, Packaging, and Manufacturing Technology Society IEEE Components, Packaging and Manufacturing Technology Society
Starting Page	493
Ending Page	498
Page Count	6
File Size	170354
File Format	PDF
ISSN	15213323
Volume Number	24
Issue Number	4
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2001-11-01
Publisher Place	U.S.A.
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Gold Metallization Electronics packaging Tin Temperature Surface finishing Intermetallic Soldering Oxidation Protection
Content Type	Text
Resource Type	Article
Subject	Electrical and Electronic Engineering

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