Advanced Technology Symposium Conference Proceedings


Authors: Donald Abbott, Douglas Romm and Bernhard Lange
Company: Texas Instruments
Date Published: 6/11/2002   Conference: Advanced Technology Symposium

Abstract: Nickel/palladium (Ni/Pd) has been used for >12 years as an integrated circuit (IC) lead finish. This paper looks at an enhanced version of this lead finish that is nickel/palladium/gold (NiPdAu). Any time gold (Au) is in a solder joint there is a potential for Au embrittlement.

This study compares NiPd and NiPdAu lead finishes. The goal is to understand the potential for Au solder joint embrittlement caused by Au from the component finish or the printed wiring board (PWB) pad finish. Components were prepared at 3 different Au thickness for the NiPdAu finish. PWB’s were made with organic solderability preservative (OSP) and electroless nickel-gold (NiAu), at 2 different Au thickness. Samples were built from the components described above and exposed to 1000 temperature cycles. Lead pull testing and metallurgical analysis of the solder joints were performed to determine if the variations in Au content, either from the component lead or the board or both, led to Au embrittlement of the solder joints.

Theoretical calculations show that 3 wt.% Au would not be exceeded in the solder joint for these samples. The Au embrittlement literature predicts no solder joint embrittlement would occur below this Au concentration. Lead pull data show no catastrophic drop in solder joint strength that would be expected with Au embrittlement. The cross sections show no SnAu intermetallics in the bulk of the solder for joints made with "normal" Au thickness components and boards. Thin layers of SnAu intermetallics are seen at the solder/component and solder/PWB interfaces. This is expected. Only in solder joints made with high Au thickness components and PWB’s are there the acicular, SnAu intermetallics that can cause Au embrittlement. The NiPdAu IC lead finish investigated here poses an extremely small risk of solder joint embrittlement when compared to the risk posed by NiAu finish PWB’s.

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