Electroless Nickel Immersion Gold Process Research Articles

Morphology and Electrochemistry Drive Corrosion of Electroless Nickel Immersion Gold Films: A Multi-Technique Analysis

Electroless nickel immersion gold (ENIG) is a common surface finish for high-reliability microelectronics circuitry due to its good solderability, corrosion resistance, layer thickness uniformity and applicability for fine pitch applications1. ENIG deposition is a multistep process in which a 3 to 6 micron (118-236 μ-in) layer of electroless nickel (i.e. Ni-P) is deposited onto the substrate material (typically copper), followed by a galvanic displacement reaction replacing the surface layer of Ni with 0.04 to 0.1 microns (1.6 to 4 μ-in) of gold (Au). Despite ENIG’s widespread use, it continues to suffer from a failure mode known as “black pad” or Ni hypercorrosion during immersion gold deposition2. A cohesive mechanistic explanation for the occurrence of Ni hypercorrosion is not yet established despite extensive study. Recent approaches have attempted to characterize in-situ the influence of bath chemistry; however, the competing influence of all variables of the ENIG process (e.g. temperature, pH, solution chemistry, volume mixing, surface morphology, etc.) make global mechanistic conclusions challenging if not impossible experimentally3. Instead, the work here aims to develop tools to assess an ENIG film of unknown origin or deposition conditions for the risk of corrosion. Therefore, this study utilizes a combination of surface characterization via x-ray photoelectron spectroscopy (XPS) depth profiling and transmission electron microscopy (TEM) imaging and electrochemical characterization via cyclic polarization (CP) and scanning vibrating electrode technique (SVET) to build a multi-technique analysis of ENIG failures.ENIG samples with identical geometries and plating requirements from two domestic vendors (A & B) were obtained. XPS with argon ion sputtering allows for comparison of atomic concentration of Ni, Au and P in ENIG films through their depth as a function of the number of sputter cycles. XPS results show that Vendor A films have a high concentration of Ni in the Au layer and a lower concentration of P near the Au-Ni interface, whereas Vendor B films have a low concentration of Ni in the Au layer and a higher concentration of P near the Au-Ni interface. This indicates a more porous morphology in Vendor A versus B films. SVET is an electrochemical scanning probe tool which allows for real-time observation of cathodic and anodic currents (in a 0.35% NaCl solution) representing existing and developing defect sites on ENIG surfaces. SVET experiments show that Vendor A and B samples exhibit preferential anodic currents on ENIG surfaces corresponding to Ni hypercorrosion defects. Focused ion beam (FIB) lift-outs from SVET samples viewed in TEM give ex-situ confirmation that SVET observations align with the real metal layer structure and chemistry as well as confirming the presence and location of corrosion products. CP curves for samples of each film in a 0.35% NaCl solution demonstrate that both Vendor A and Vendor B films exhibit effective corrosion rates above 1.0 mil per year which is on the order of bare Cu wire in the same electrolyte. Overall, a distinct difference in film morphology between Vendor A and B samples exists; however, both films exhibit substantial electrochemical activity in solution indicating corrosion susceptibility. Taken together, these results show that Ni hypercorrosion can be driven by at least two distinct mechanisms: high porosity in the Au film leading to enhanced Ni migration and reaction at the Au surface; or, P enrichment at the Au-Ni interface leading to enhanced P migration and a more electronegative surface composition. Future work will build out characterization of additional ENIG samples to establish a failure matrix for ENIG films, encompassing a wider variety of unique mechanisms. D. Goyal, T. Lane, P. Kinzie, C. Panichas, K. M. Chong, and O. Villalobos, in "52nd Electronic Components and Technology Conference 2002.(Cat. No. 02CH37345)", p. 732-739. IEEE, 2002.R. Ramanauskas, A. Selskis, J. Juodkazyte, and V. Jasulaitiene, Circuit World, (2013).A. Accogli, E. Gibertini, G. Panzeri, A. Lucotti, and L. Magagnin, Journal of The Electrochemical Society, 167 (8), 082507 (2020). SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

Reliable Nickel-Free Surface Finish Solution for High-Frequency, HDI PCB Applications

Abstract 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 printed circuit board 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 the high-frequency data transfer rate for ENIG-applied electronics, compared with 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 study, assemblies involving this nickel-less novel surface finish have been subjected to extended thermal exposure, then intermetallics analyses, contact resistance comparison after every reflow cycle (up to six 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 applications.

Thiourea-Based Extraction and Deposition of Gold for Electroless Nickel Immersion Gold Process

Gold electroless plating for surface finishing of electronic circuits, named electroless nickel immersion gold (ENIG), is widely practiced in the electronics packaging industry. Noncyanide substitu...

Understanding the Failure Mode of Electroless Nickel Immersion Gold Process: In Situ-Raman Spectroscopy and Electrochemical Characterization

Since the early 70s, printed circuit boards (PCBs) are firmly entrenched in all electronic branches, from the consumer electronics to scientific and medical equipment, culminating later in the personal computer industry. Electroless nickel immersion gold (ENIG) process is one of the most used selective finishing in PCBs production. It involves two different electroless deposition mechanisms: (1) NiP autocatalytic deposition and (2) gold galvanic immersion plating in which displacement reactions are involved. Because during ENIG process, NiP is dissolved from the electrode surface into the solution, it can be considered as a controlled corrosion process of the metal substrate: selection of the complexing agents in solution is thus crucial. Since they are the most used complexing agents, EDTA and citric acid have been investigated. In order to understand their effects on the NiP surface during the immersion plating, open circuit potential (Voc) and linear sweep voltammetry (LSV) measurements have been performed. Furthermore, to establish which complexes are adsorbed on the electrode surface during the plating process, in situ Raman spectroscopy was carried out showing EDTA to be the most effective Ni cations chelating agent. To assess the effect of Cu contamination, accountable of reddish gold failure mode, the electrochemical and in situ spectroscopic analyses on both immersion gold solution and polluted revealed that the adsorption of and/or complexes is competitive with To evaluate the effects of copper contamination on gold nucleation and growth FE-SEM and AFM were carried out, while gold layer solderability has been evaluated according to NF-A-89 400 standard.

Effects of Bath Composition and P Contents on the Defects of NiP Layer in Electroless Nickel Immersion Gold Process.

Electroless nickel immersion gold (ENIG) has been widely used for surface finishing in PCB industry, however surface defects are sometimes found during PCB soldering process. These defects cause failures in soldering on PCB and consequently cause poor safety probability of products. The formation mechanism of the defects in the NiP layer was investigated. Three parameters on the black pads were analyzed. First, morphology was analyzed with changing metal turn over (MTO) numbers. Second, pH of Ni solution was changed in the ENIG process. Third, stress of NiP layers was analyzed at different pH. The relationship between each defect and P contents was analyzed. High open circuit voltage (OCV) can cause the poor surface morphologies as well as the defects. High and large difference of P contents in NiP layer can act the important role in the formation of black pads. The high tensile stress can also be another factor of the formation of black pads.

Investigation of surface defects of electroless Ni plating by solder resist dissolution on the ENIG process

Defects, such as a black pads and pinholes, are generated on the Ni/Au interface after the electroless nickel immersion gold (ENIG) process. The contamination of plating solutions, including dissolution of the solder resist (SR), side reaction products, and impurities (Cu, Ni, drag-in), can be a cause of the black pad phenomenon. In this study, electroless Ni plating was investigated to determine the correlation between SR dissolution and black pad generation. Three different SR components, monomer, hardener, and photoinitiator were used to examine the effects of the properties of the coating layer. The surface morphology, plating rate, and phosphorus (P) content of the electroless Ni layer with SR additives were analyzed. The black pad mechanism by SR dissolution was examined by electrochemical analysis including the open circuit voltage (OCV) and cyclic voltammetry (CV) in a three-electrode cell. When a few ppm of SR was added, the OCV increased compared to the pure solution. CV revealed a change in hysteresis loop in the oxidation region upon the addition of SR. The contamination in the solution promotes the oxidation reaction rather than the reduction reaction. Therefore, the content of P in the contaminated solution was higher than that of the pure solution, and defects, such as black pad and pinholes, were observed.

Characterization of the Contamination Factor of Electroless Ni Plating Solutions on the ENIG Process

The electroless nickel immersion gold process often produces defects, such as pinholes and black pads that can cause brittle fractures at the interface between the solder and metal pad. Contamination in electroless Ni plating solutions with increasing metal turn over (MTO) is believed to be one of the causes of the formation of surface defects. MTO means indirectly Ni bath life, with ‘‘0 MTO’’ indicating a freshly plating bath and ‘‘2 MTO’’ an aged plating bath, which is supplemented twice with the initial amount of metallic salts and the reducing agent. In this study, surface defects on the Ni-plated layer were investigated to understand the correlation between the MTO solutions and defect factors by solder resist (SR) dissolution. The characteristics of the contaminated MTO solutions were analyzed by total organic carbon and liquid chromatography mass spectrometry (LC-MS). LC-MS detected the component (melamine) of the hardener of the SR at 2.14 ppm in the 2 MTO, and 2.52 ppm in the 2.5 MTO solutions. Electroless nickel plating was conducted in a 0 MTO solution with hardener addition. Pinholes were observed in solutions containing more than 2 ppm of a hardener. In particular, the content of phosphorus in the defective area was higher than that in the non-defective area. Consequently, as the MTO solution increased, the hardener was dissolved in the Ni solution, which caused defects in the nickel surface layer, such as pinholes and black pads.

In Situ-Raman Spectroscopy and Electrochemical Characterization on Electroless Nickel Immersion Gold Process

Electroless nickel immersion gold (ENIG) process is one of the most used selective finishing in PCB production. It involves two different electroless deposition mechanisms: (1) NiP autocatalytic deposition and (2) gold galvanic plating where coating is obtained by a displacement reaction. Because during the ENIG process, NiP is dissolved from the electrode surface into the solution, it can be considered as a controlled corrosion process. For this reason, selection of the complexing agents in solution is crucial. Since they are the most used complexing agents, in this study EDTA and citric acid have been selected. In order to understand their effects on the NiP surface during the immersion plating, open circuit potential and potentiodynamic detection measurements have been performed. Furthermore, to establish which complexes are adsorbed on the electrode surface during the plating process, in-situ Raman spectroscopy was performed. Electrochemical and spectroscopic analyses reveal that EDTA is the dominant complexing agent, while citric acid is mainly a buffering agent.

In Situ-Raman Spectroscopy and Electrochemical Characterization on the Enig Process

Electroless nickel immersion gold (ENIG) is one of the widely used free-lead final finish in the electronic industry and, even though it is highly expensive due both to the use of precious metal i.e. gold and to process working conditions, its use in PCB production has been growing significantly because it is compatible with a wide range of component assembly methods. ENIG is a very versatile surface finish: it is solderable, aluminium wire bondable and provides an excellent electrical contacting surface. [1,2] Nevertheless, even if this selective finishing is one of the most convenient process for PCB, some problems related to hypercorrosion phenomena during the ENIG process cause an effective loss of solderability. These problems are known as black pad and pinholes: they weaken the adhesion of the gold film to the surface and can lead to severe failures in solder joints and low shear forces after assembly.[3-6] Since the ENIG process can be considered as a controlled corrosion of NiP during a displacement reaction driven by the potential difference between Ni and Au, the purpose of this work is to understand the effects of each complexing agent used for this surface finishing treatment by means of electrochemical and spectroscopic investigations. The effects are studied in terms of Ni corrosion from the electrochemical and from the morphological point of view. To do this the samples are treated in the same way and directly immersed into different electrolytes for the electrochemical characterization. For morphological and microstructural studies, each sample electrochemically tested has been analysed by means of scanning electron microscope (SEM) and x-ray diffraction (XRD). To assess the different species adsorbed on the NiP surface and on the Au layer during the electrochemical process, an innovative study using the in situ-Raman spectroscopy has been done: the spectra are collected in full swing of the Au deposition at the process temperature using different electrolytes. The same procedure will be applied on Cu-contaminated solution in order to understand if Cu presence, accountable of reddish gold issue, can affect the adsorption phenomena on the electrode surface. Moreover, a study on gold deposit nucleation and growth, both for pure and contaminated solution, has been done: samples at different immersion times have been analysed using atomic force microscopy. The Cu effect has been investigated from the applicative point of view performing solderability tests which results will be presented.

Effects of organic additives on the immersion gold depositing from a sulfite–thiosulfate solution in an electroless nickel immersion gold process

An immersion gold-plating process on electroless Ni–P alloy substrate was investigated.

Reliability of LTCC using Electroless Nickel Immersion Gold (ENIG) Plating Technology

In an effort to address the impact of rising gold costs on the fabrication expense for high frequency circuits, and maintain circuit quality and reliability, the DuPont™ GreenTape™ 9K7 low temperature co-fired ceramic materials (LTCC) with top silver conductors have been evaluated using electroless nickel immersion gold (ENIG) plating. Results comparable to the quality of efficiency and reliability of traditional gold conductor systems were achieved at reduced costs. LTCC coupons produced using the ENIG process were tested and compared to standard specifications for coupons with gold printed conductors. The test methods used include wire bond and adhesion pull tests on eight layer coupons, and MIL-STD-883 [1] temperature cycle testing on a layered high frequency test pattern. The reliability of the high frequency characteristics including insertion loss and return loss measurements were evaluated periodically throughout 1090 temperature cycles. Adequate results for most applications were obtained using the ENIG plated system compared to gold printed coupons. In an attempt to better understand the boundaries of the ENIG process on 9K7 LTCC and its use as an alternative for gold conductor systems; future testing will explore the effects of additional environmental testing. The added tests will include thermal shock, and temperature and humidity cycling on 9K7 LTCC test coupons. The testing will be conducted in accordance with MIL-STD-883 as a benchmark, and also evaluate wire bond and adhesion properties. This paper will report the reliability test results on plated GreenTape™ 9K7 LTCC systems after exposure to the stated environmental conditions.

A corrosion couple experiment reproducing the black pad phenomenon found after the electroless nickel immersion gold process

The “black pad” phenomenon, which rarely occurs in electrolessly plated nickel–phosphorous (Ni–P) films, was reproduced. This work reports that nodular variation of the P content induces potential differences large enough to drive galvanic corrosion when the film is exposed to an electrolyte containing gold cyanide. Subsequent corrosion couple experiments using Ni–P films with different P contents demonstrated that preferential corrosion occurred at the low P side, and the propensity of forming black pad increased with the difference in P content (ΔP).

A Study on the Mechanism of Black Pad Formation during Electroless Nickel Immersion Gold process

The “Black Pad” phenomenon referring to blackening of electrolessly plated nickel-phosphorous (Ni(P)) films after the immersion gold (IG) process was reproduced using pure chemicals for the first time. Chemical composition of the electrolyte most suitable for the black pad reproduction was proposed, and it was found that submicron scale nodular variation of the P content of the Ni(P) film induced potential differences large enough to drive galvanic corrosion when exposed to the electrolyte; gold cyanide solution in this case. Instead of conventional potential measurement techniques, corrosion couple experiments using Ni(P) films with different P content were performed to substantiate preferential corrosion, which occurred at the lower P side with the propensity of forming black pad getting more severe with the P content difference (ΔP) between films. Subsequent AFM analysis showed that variations of nodular shapes and curvatures among nodules led to submicron scale variations of the P content in the film, which could promote galvanic corrosion.

Characterization of Black Pad Defect on Electroless Nickel- Immersion Gold (ENIG) Plated Circuits

The advent of the RoHS legislation, which mandated lead-free solders, and the rise in popularity of the BGA package, have resulted in ENIG becoming a popular choice for surface finish due to its excellent long term solderability and surface planarity. One of the pitfalls of this surface finish is the possibility of creating a layer of overly elevated phosphorous content between the electroless nickel and immersion gold, known as black pad defect. Most of the available literature suggests black pad defect is caused by accelerated reduction (corrosion) of the nickel in the nickel- phosphorus (Ni-P) layer during the immersion gold step of the ENIG process. Black pad defect can manifest as corrosion spikes at the nodule boundaries in the Ni-P and can progress to an abnormally thick high phosphorus region at the top of the Ni-P. One failure mechanism associated with black pad defect is the brittle failure of well wetted solder joints from the underlying Ni-P layer due to the presence of the high phosphorus region. In severe cases, black pad defect can cause solderability issues and retard the formation of tin-nickel intermetallic preventing good wetting of the solder joint. We have had the opportunity to examine many different cases of black pad, from severe to mild, and have a large amount of knowledge to share. This paper will give the reader a basic understanding of how to recognize black pad and subsequently determine its severity.

Machine vision in detection of corrosion products on SO 2 exposed ENIG surface and an in situ analysis of the corrosion factors

Corrosion resistivity is an important property in printed circuit boards manufacturing where the electroless nickel immersion gold (ENIG) is a commonly used surface finish. The corrosion resistivity of the surface is tested with a sulphur dioxide (SO 2) test which imitates the influence of atmospheric corrosion. The test result is analyzed manually by vision inspection, which is unsuitable for mass production. The result is also qualitative (failed or passed) and therefore the corrosion severity is impossible to analyze and further mathematical analysis is hard to conduct. In this paper, a new machine vision based corrosion evaluation algorithm is developed for quantitative and objective analyzing of the SO 2 test result. The algorithm produces automatically the same data as the manual method and, in addition, also new continuous corrosion state estimates. The produced data are further used for correlation analysis in which the relations between the parameters of the ENIG process and the corrosion resistivity of the surface are studied and the algorithm is verified.

Origin of Surface Defects in PCB Final Finishes by the Electroless Nickel Immersion Gold Process

We suggest a unique mechanism for surface defect generation causing solder joint or bonding failures in printed circuit boards (PCBs). Surface defects can be defined as corroded holes or spikes of the Ni-P layers on the soldering or wire bonding pads of PCBs. The typical defects are the black pad or pinhole pad defects generated after final finishing by the electroless nickel immersion gold (ENIG) process. Once corroded voids or spikes are plentifully created in nickel/gold interfaces, the bonding strength of solder or wire bonding joints is reduced. Therefore, it is important to characterize the details of these surface defects. In this paper, the defect microstructures and the P content variation with the ENIG processes are investigated. The surface defect selectivity with pad size and pad connectivity is suggested based on the key findings of P content variation. An overall mechanism is proposed based on a mixed mode of concentration cell corrosion and galvanic cell corrosion. Based on these results, more reasonable root causes are suggested.

The Characterization of Al Bond Pad Surface Treatment in Electroless Nickel Immersion Gold (ENIG) Deposition

This study reports a number of experiments that were designed to characterize aluminum bond pad surfaces prior to electroless nickel immersion gold (ENIG). In the ENIG process, aluminum bond pads need special treatment to achieve successful nickel deposition and provide reliable interconnection of under bump metallurgy in advanced packaging. During this treatment process, the aluminum pad was cleaned, activated and then coated with a layer of zinc. Systematic study was carried out to determine the best parameters, through multiple and various exposure times of the zincation process and zincation solution concentration effect on the Ni/Au surface roughness and aluminum dissolution rate on the bond pad during multiple zincation process. The ball shear strength was evaluated between eutectic 37Pb/63Sn solder ball and under bump metallurgy (UBM) interfaces across multiple zincation process. Scanning Electron Microscope (SEM), Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM), Focused Ion Beam (FIB) and ball shear tester were used as analytical tools. The results suggest that the multiple zincation process consistently produces a smoother surface of ENIG UBM and consequently provides a better shear strength.

Characterization of parasitic residual deposition on passivation layer in electroless nickel immersion gold process

The problem of parasitic residual deposition on the passivation layer in electroless deposition process is studied in this paper. The characterization analysis tools involved are focused ion beam (FIB), scanning electron microscopy (SEM), electron dispersive X-ray (EDX) and metallurgical interface analysis. Samples were identified as either good or bad prior to the characterization process via a high power microscope. A good sample is the one without parasitic deposition while a bad sample contains parasitic residual deposition on the passivation surface. Ultrasonic vibration has been able to remove the parasitic deposition and reveal crack presence on the passivation surfaces. Crack was found extended to the aluminum underneath that leads to the formation of the parasitic residual deposition. Additional sacrificial photoresist on top of passivation was able to eliminate this parasitic deposition.

Development of an electroless nickel immersion gold process for PCB final finishes

A new generation of electroless nickel immersion gold process has been developed that is targeted to solve the “Black Nickel issue”. The development involved the understanding of the details of the nanostructure of electroless nickel, the interface between Ni and Au, Ni and solder, and the control of the porosity of the Au surface. A suitable increase in phosphorous content in the nickel deposit reduces the hyperactive corrosion during immersion gold process. In addition, a stabilizer system that gives consistent performance across the entire bath life was used to improve the simplicity of bath control. This combination gives many advantages for the applications in printed wiring boards.

The effect of modifications to the nickel/gold surface finish on assembly quality and attachment reliability of a plastic ball grid array (peer review version)

Electrolytic and electroless Ni/Au are common pad surface finishes on area array (BGA or CSP) packages and printed wiring boards (PWB). The electroless nickel/immersion gold (ENIG) process often is implemented when there is insufficient space to allow bussing for the more common electrolytic Ni/Au plating. The ENIG process continues to be used despite evidence that it may cause catastrophic, brittle, interfacial solder joint fractures. In this investigation a plastic ball grid array (PBGA) test vehicle is used to compare quality and reliability of standard and experimentally modified ENIG surface finishes. The standard electrolytic Ni/Au surface finish is used as the control cell for the experiment. Ball shear tests and optical and scanning electron microscopy are performed on as-received and thermally preconditioned packages to evaluate package quality prior to assembly. Accelerated temperature cycling (0/+100/spl deg/C and -40/+125/spl deg/C) is used to evaluate solder joint attachment reliability. Detailed failure mode analysis is used to compare the fracture modes in the ball shear and thermal cycled samples in the electroless and electrolytic packages. The results are discussed in terms of the failure modes and the characteristics of the different Ni/Au surface finishes.