SMTA China Conference Proceedings


Authors: Hugh Roberts and Sven Lamprecht, et al.
Company: Atotech Inc.
Date Published: 4/8/2008   Conference: SMTA China

Abstract: Following the adoption of recent Pb-free requirements, PWB and IC substrate fabricators, assemblers and OEMs continue to search for a surface finish that will minimize solder joint reliability concerns. Historically, electroless nickel / immersion gold (ENIG) has provided a reliable finish for soldering applications. The nickel layer provides the added benefit of serving as a barrier to minimize copper dissolution during the soldering operation. Preservation of the underlying copper will become increasingly important as circuit features continue to shrink and signal integrity becomes a more critical issue. However, ENIG is not trouble-free; as the immersion gold reaction can often excessively corrode the nickel layer, leading to the occurrence of “black pad” and tendencies toward brittle failure in solder joints. As an alternative to ENIG, an electroless palladium layer over the nickel (NiPd) should provide a solderable surface while minimizing impact to the nickel deposit. By eliminating the corrosive immersion gold reaction, the resultant intermetallics should contribute to improved solder joint integrity, even after multiple Pb-free reflow operations.

This paper summarizes the results of recent investigations to examine the effect of electroless nickel process variations with respect to Pb-free (Sn-3.0Ag-0.5Cu) solder connections. These investigations included both ENIG and NiPd as surface finishes intended for second level interconnects in BGA applications. Process variations that are suspected to weaken solder joint reliability, including treatment time and pH, were used to achieve differences in nickel layer composition. Immersion gold deposits were also varied, but were directly dependent upon the plated nickel characteristics. In contrast to gold, different electroless palladium thicknesses were independently achieved by treatment time adjustments.

Results of investigations include: (1) Cold ball pull testing to evaluate solder joint integrity, (2) SEM examinations of the underlying nickel surface and (3) IMC examinations to quantify nickel thickness degradation after multiple solder reflow cycles. The paper discusses the relatively simple approach that, if proven effective in large-scale fabrication, may significantly reduce or eliminate many of the well-known reliability concerns associated with nickel-based intermetallics in a Pb-free assembly environment.

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