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Reliable Nickel-Free Surface Finish for High-Frequency-HDI PCB
In this paper, assemblies involving this nickel-less novel surface finish have been subjected to extended thermal exposure, then intermetallics analyses.
Technical Paper
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Authored By:
Kunal Shah, Ph.D.
LiloTree
Redmond, WA, USA
Summary
The evolution of internet-enabled mobile devices has driven innovation in the manufacturing and design of technology capable of high-frequency electronic signal transfer. Among the primary factors affecting the integrity of high-frequency signals is the surface finish applied on PCB copper pads – a need commonly met through the electroless nickel immersion gold process, ENIG. However, there are well-documented limitations of ENIG due to the presence of nickel, the properties of which result in an overall reduced performance in high-frequency data transfer rate for ENIG-applied electronics, compared to bare copper.
An innovation over traditional ENIG is a nickel-less approach involving a special nano-engineered barrier designed to coat copper contacts, finished with an outermost gold layer. In this paper, assemblies involving this nickel-less novel surface finish have been subjected to extended thermal exposure, then intermetallics analyses, contact/sheet resistance comparison after every reflow cycle (up to 6 reflow cycles) to assess the prevention of copper atom diffusion into the gold layer, solder ball pull and shear tests to evaluate the aging and long-term reliability of solder joints, and insertion loss testing to gauge whether this surface finish can be used for high-frequency, high density interconnect (HDI) applications.
Conclusions
The Ni-less surface finish of cyanide-free immersion gold plated onto a nano-engineered barrier layer on top of copper is a viable solution for high-frequency-HDI applications. The surface finish was tested for change in contact/sheet resistance after each reflow cycles (up to 6 cycles), intermetallic growth, solder ball brittle failures, and insertion loss. The results showed that this surface finish performs better than other currently available Ni-free surface finishes, such as DIG and EPIG/EPAG, due to its smaller intermetallics, lack of brittle solder joint failures, and extremely low insertion loss compared to bare copper. Also, change in contact/sheet resistance after 6 reflow cycles is insignificant suggesting nano-engineered barrier layer prevent copper atoms diffusion into gold layer. This Ni-less surface finish with nano-engineered barrier layer is a good solution to the current need for a reliable surface finish for high-frequency, HDI PCB applications.
Initially Published in the SMTA Proceedings
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