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Reassessing Surface Finish Performance for Next-gen Technology, Part 2
March 4, 2024 | Frank Xu, PhD and Martin Bunce of MacDermid Alpha, and John Coonrod of Rogers Corp.Estimated reading time: 1 minute
The introduction of 5G/6G has created a growing demand for faster rates of data transfer and operation at higher frequencies, pushing signals to travel toward the outer edges of conductors. As a result, the surface finish applied over the copper circuitry is now gaining more attention.
It has previously been shown that the low conductivity and magnetic properties of the electroless nickel (EN) layer negatively affect electrical signals as they travel along the conductor’s outer surfaces, leading to insertion losses. Subsequent studies show that reducing the EN thickness can offset some insertion losses observed. In more recent times, nickel-free finishes, such as EPIG (electroless palladium, immersion gold, no EN), thin EN ENEPIG, silver-gold (immersion silver followed by immersion gold), have been promoted as solutions for improving signal integrity at higher frequencies.
In Part 1 of this article, we reported the signal loss properties of the new nickel-free surface finishes, along with immersion silver, organic solderability preservative (OSP), and ENIG/ENEPIG surface finishes. As expected, ENIG and ENEPIG are challenged by signal losses at higher frequency, with OSP and immersion silver demonstrating no contribution to the signal losses. The new generation surface finishes (silver-gold, thin EN ENEPIG, and EPIG) perform similarly—outperforming standard ENIG/ENEPIG finishes but still contributing slight signal loss compared to immersion silver and OSP.
One of the concerns for nickel-free finishes is the solder joint reliability with removed or greatly reduced nickel diffusion barrier layer thickness. Part 2 of this article addresses this concern and focuses on various solder joint reliability testing, including high-speed ball shear, drop shock, and solder joint reliability electromigration tests.
This is an excerpt from the February 2024 issue of Design007 Magazine, continue reading.
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