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The transition to lead-free has focused increased attention on selecting an effective surface finish for printed wiring board (PWB) and IC substrates. Surface finishes must withstand more aggressive assembly conditions and meet the need for continuously decreasing feature geometries. Assemblers and OEMs could benefit from a cost-effective surface finish that is suitable for both soldering and wire-bonding applications.
For fine-feature solder ball array applications, several lead-free surface finishes are currently in use, including electroless nickel immersion gold (ENIG), OSP, immersion silver (Ag), and immersion tin (Sn). However, each poses assembly challenges with accompanying solder joint reliability issues. For example, surface finishes that form a Cu/Sn intermetallic compound (IMC) - OSP, immersion Ag, and immersion Sn - may allow uncontrolled copper dissolution during lead-free soldering.
ENIG has been accepted as a viable surface finish for soldering. Nickel is a suitable barrier to prevent copper dissolution during the soldering operation. However, excessive nickel corrosion by the immersion gold step has been a source of reliability concerns, such as black pad. At the higher tin content and reflow temperatures required for lead-free soldering, a greater potential exists for the formation of brittle nickel-tin IMCs.
For gold wire bonding, the pad finish has historically been electrolytic gold over electrolytic nickel. Although proven to be a reliable substrate bonding surface, variations in plating thickness pose problems as wiring densities increase. An alternative surface finish is electroless gold over ENIG, which is more suitable for finer pad-pitch applications. Unfortunately, an electroless gold bath can be aggressive to solder masks, and there are concerns regarding gold-to-gold adhesion. Associated precious metal consumption makes the cost-effectiveness of both processes an issue.
The electroless nickel/electroless palladium/immersion gold process (Ni/Pd/Au) was introduced more than 10 years ago to offer a finish for both soldering and wire-bonding applications. The finish consists of a nickel layer (4-6 µm) deposited directly on copper, followed by a thin palladium deposit (0.1-0.3 µm). This is then followed by a thin immersion gold deposit (0.01-0.02 µm). Both electroless nickel and electroless palladium are co-deposited with phosphorus (P).
The suitability of Ni/Pd/Au for both gold and aluminum wire bonding has been reported. Based on wire-pull evaluations, this finish equaled the performance of electrolytic nickel/electrolytic gold. Ni/Pd/Au is receiving renewed attention because of its suitability for lead-free soldering applications.
As part of a design of experiment (DoE), more than 4,000 solder-ball shear and cold-ball pull tests were performed, comparing the performance of ENIG and Ni/Pd/Au finishes. Results included low- and high-speed pull and shear testing. In comparison to ENIG, the Ni/Pd/Au surface provided superior performance in terms of pull and shear strength. The Ni/Pd/Au finish also yielded lower incidence of brittle fracture.
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Figure 1. Comparison of IMC formation after five lead-free solder reflow cycles for ENIG (top) and Ni/Pd/Au (bottom) with palladium thickness of 0.3 µm.
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Microscopic examinations of bond failures showed interfacial fractures occurring along the plane where the bulk solder meets the surface of the Cu/Ni6Sn5 IMC. Figure 1 shows the difference in IMC structure between solder joints formed after five reflows with traditional ENIG and Ni/Pd/Au. In the ENIG sample, a portion of the nickel deposit was converted to create a visible Cu/Ni6Sn5 IMC. A NiSnP IMC and a thick phosphorus-rich Ni3P layer also are located directly above the originally plated Ni/P deposit, much of which has been consumed to create these IMCs. The sample with the Ni/Pd/Au finish, however, shows little or no visible Cu/Ni6Sn5 IMC. A palladium thickness of 0.30 µm was plated over the nickel. The original Ni/P layer has been reduced in thickness, but to create the comparatively thinner Ni3P and thicker NiSnP layers.
The presence of a palladium layer appears to protect the electroless nickel deposit from corrosion by the immersion gold step. Furthermore, this layer acts as a barrier to specifically limit formation of the undesirable Cu/Ni6Sn5 IMC and minimize the growth of the phosphorus-rich Ni3P layer. It is theorized that the presence of the Cu/Ni6Sn5 IMC in measurable thickness is a leading cause of interfacial fractures with lead-free solder joints on traditional ENIG.
Unlike surface finishes that create a Cu/Sn IMC, Ni/Pd/Au provides a barrier to prevent uncontrolled copper dissolution during lead-free soldering. It also demonstrates improved response in terms of ball shear and pull test results, compared to traditional ENIG. Ni/Pd/Au offers the maximum flexibility for lead-free soldering, gold and aluminum wire bonding, and contact-switching applications. Considering the alternatives, Ni/Pd/Au may be a universal finish.
For a list of references, contact the author.
Hugh Roberts, global manager, OEM/EMS Technology Exchange, Atotech, may be contacted at hugh.roberts@atotech.com.
SMT September, 2007
Author(s) :
Hugh Roberts
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