Metal Turnover Research Articles

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.

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.

Effect of Bath Life of Ni(P) on the Brittle-Fracture Behavior of Sn-3.0Ag-0.5Cu/ENIG

The effect of bath life of Ni(P) on the brittle-fracture behavior of Sn-3.0 wt.%Ag-0.5 wt.%Cu (SAC)/electroless nickel immersion gold (ENIG) was evaluated in this study. The bath lives of Ni(P) for the ENIG surface finish in this study were varied from 0 to 3 metal turnover (MTO), which were indirectly indicative of Ni(P) bath life, with “0 MTO” denoting the as-make-up state and “3 MTO” denoting almost waste plating solution. The SAC/ENIG sample when Ni(P) was plated in the 3 MTO bath (3 MTO sample) had thicker (Cu,Ni)6Sn5 and P-rich layers than when Ni(P) was plated in the 0 MTO bath (0 MTO sample). The brittle-fracture behavior of the 0 and 3 MTO samples was evaluated by use of a igh-speed shear (HSS) test with a strain rate of 0.1–2.0 m/s. The shear strength of the 0 MTO sample was higher than that of the 3 MTO sample. The incidence of brittle fracture increased as the bath life of Ni(P) of ENIG (= MTO of Ni(P)) increased. Observation by transmission electron microscopy (TEM) revealed nano-sized voids (or particles) in the Ni-Sn-P layer. As the MTO of the Ni(P) increased, the number of nano-sized voids in the Ni-Sn-P layer of the SAC/ENIG interface increased. The poor brittle-fracture behavior of the 3 MTO sample originated from the weak interface at the thick P-rich layer and from the large nano-sized voids.

Investigation into the Roles of Sulfur-Containing Amino Acids in Electroless Nickel Plating Bath

Typical sulfur-containing amino acids, cysteine and methionine, were employed as stabilizers in place of lead(II) acetate in the electroless nickel plating (ENP) system. Each of these compounds presents a critical stabilizing concentration of ca. 10−5 mol/L, below which the ENP rate is slightly promoted with increasing concentration but above which it is significantly suppressed. Their role in stabilizing the ENP bath was found to involve inhibition of the anodic reaction of hypophosphite. The chemical adsorption of these two amino acids on active nickel sites at the Ni plating surface was studied via their adsorption on an in situ generated Ni powder. In addition, the stabilization mechanism was investigated by using potentiometry, X-ray photoelectron spectroscopy (XPS), and UV−vis/infrared spectroscopy. Finally, the bath stabilizing capabilities of these two amino acids and the change in ENP properties with time were assessed through a continuous operation comprising four metal-turnover (MTO) runs.

The effects of shrimp trawling on sediment oxygen consumption and the fluxes of trace metals and nutrients from estuarine sediments

The effects of shrimp trawling on sediment oxygen consumption and the sediment–water exchange fluxes of nutrients (ammonium, phosphate and silicate) and trace metals (Mn, Ni, Cu, Cd and Pb) were determined for two separate experiments conducted during July 1998 and May 1999. Oxygen microelectrode measurements showed that during the 1998 experiment, the surface sediments were oxic down to depths of approximately 15 mm suggesting that the study area had not been recently trawled. However, after passage of the trawl gear oxygen was present to depths of only 5 mm. This suggests resuspension of the upper 1 cm of sediment and resulted in ammonium and manganese fluxes increasing by a factor of 2–3, which was attributed to a similar mechanism, i.e. a strengthening of the diffusive gradient. The measured fluxes of oxygen, phosphate and the trace metals Ni, Cu and Pb were not affected by trawling, while the flux of Cd was affected. During the 1999 shrimp trawl experiment, pre- and post-trawl oxygen penetration was limited to the upper 1 mm of sediment suggesting the area had been recently trawled. The diffusive fluxes of oxygen were in agreement with directly measured oxygen fluxes, both prior to and after trawling; further indication that trawling had little to no effect on the exchange of oxygen across the sediment–water interface during this time. Macrofaunal abundance was significantly reduced during 1999 and consistent with sediment oxygen consumption rates, which were also lower during this time. This resulted in the fluxes of oxygen, which were enhanced by biological processes during 1998, becoming diffusion controlled during 1999. This resulted in pre- and post-trawl fluxes of oxygen, ammonium, silicate, Mn, Ni, Cu and Pb, when considering the measurement error, which did not differ significantly. Cd again seemed affected by shrimp trawling activities. An increase or decrease in a trace metal's turnover time (the time in days required to replenish the water column concentration) can have important implications towards the overall health of an estuary. In Galveston Bay, trace metal turnover times averaged 0.4±0.5, 52±32, 27±15, 22±23 and 5.3±3.0 d for Mn, Ni, Cu, Cd and Pb, respectively. These relatively short turnover times indicate that the sediments can be an important source of trace metals to the overlying water column, especially during low-flow periods of the Trinity River, which can sometimes last more than 1 yr. This, in turn, could possibly lead to the non-conservative mixing behaviors, for both nutrients and trace metals, previously reported for the Trinity Bay region of Galveston Bay, as conservative mixing is observed in the lower estuary. Our study shows that the effects of shrimp trawling are largely dependent on the prevailing sediment redox conditions due to the shallow penetration depth of both oxygen and the trawl gear. Therefore, it is unlikely that trawling activities adversely affect the overall health of Galveston Bay. However, repeated trawling with removal of the upper oxic sediment layers could trend surface sediments towards anoxia and ultimately lead to changes in benthic–pelagic coupling.