With the increasing complexity of electronic functions, there’s a demand for more intricate printed circuit boards. Manufacturing processes require solder pads to be smaller, with finer lines, flat surfaces, and higher solderability. In response, as SMT Technology advances, the chemical nickel-gold (ENIG) surface finish has been developed to meet assembly requirements. However, a downside emerges—ENIG plating exhibits black pad defects after soldering, posing a challenge to the PCBA industry. In this article, TechSparks delves into the origins of black pad issues following nickel-gold plating and presents potential solutions.
What Is ENIG Black Pad in PCB and How Does It Occur?
ENIG black pad is a common PCB problem, manifested by irregular dark-colored corrosion or oxidation on the metal surface of solder pads. It often appears as a “black” appearance, hence the name “ENIG black pad.” It should be noted that ENIG cannot be reworked, the only solution is prevention!
During the processes of gold deposition and nickel dissolution, a displacement reaction occurs on the circuit board, sealing the gold layer and inhibiting nickel dissolution while also halting gold deposition. However, due to the presence of voids and an imperfect structure in the gold layer, gold can still react slowly under certain conditions. Under the influence of specific factors, the reactivity of the gold solution becomes excessive, leading to irregular and excessive oxidation of the local nickel surface. Even with the deposition of the gold layer on top, undesirable oxides may form between the gold layer and the underlying nickel. This is the underlying principle of ENIG black pad, but the actual manifestation of this issue usually occurs during the PCB soldering process. Improper soldering temperature and duration, as well as poor solder quality, can trigger reactions between the metal surface and the components in the solder, resulting in the formation of undesirable intermetallic compounds and accelerating the occurrence of black pad.
Experimental Analysis of ENIG Black Pad
Relationship with Nickel Thickness
Observing the nickel surface using a scanning electron microscope reveals a certain relationship between the thickness of the nickel layer and the size of its crystal lattice. A looser crystal structure leads to less corrosion of nickel grains during the gold displacement process. When the deposition time for nickel is longer, the crystal lattice might undergo a certain degree of enlargement. This enlargement could potentially aid in reducing the corrosion of nickel grains during the gold displacement process. It’s important to note that the crystal lattice size is not solely determined by deposition time but is influenced by multiple factors, including deposition conditions and the material’s crystal structure.
Relationship with Gold Thickness
Under varying gold deposition thicknesses, for the same nickel surface, as the deposition time for gold increases, the gold layer gradually thickens, leading to intensified corrosion on the nickel surface. This phenomenon might occur due to the thicker gold layer promoting more frequent reactions between gold and the nickel layer, thereby influencing the corrosion condition of the nickel surface.
Relationship with Gold Concentration
During the processing, the gold concentration in the solution affects the deposition rate and reactivity of gold. When the gold concentration in the bath is too low, the deposition rate of gold may slow down, leading to insufficient gold deposition. On the other hand, if the gold concentration is too high, the deposition rate of gold increases, intensifying the degree of reaction between gold and nickel, thereby exacerbating the corrosion on the nickel surface.
Mitigating Potential Impacts of Black Pad in ENIG Process
(1) When the nickel layer is sufficiently thick, the lattice of nickel expands, resulting in a smoother nickel surface. Consequently, during the gold-nickel displacement process, the likelihood of reactions between gold and nickel grains diminishes, reducing the corrosiveness of the nickel surface. To address this, TechSparks suggests increasing the nickel layer thickness to over 4 μm in the production process, mitigating potential black pad risks and preventing gold permeation through the nickel layer that could impact soldering.
(2) Longer deposition of gold leads to a thicker gold layer and increased potential for nickel surface corrosion. To manage this, it is recommended to maintain gold layer thickness within the range of 0.025 μm to 0.0625 μm. To reduce gold deposition time, adjustments can be made to the pH value and temperature of the gold bath.
(3) Accelerating the gold deposition process through pH and temperature adjustments in the previous step may enhance deposition rates. Therefore, it is advisable to choose immersion gold baths with mild semi-reductive and semi-displacement characteristics. Ensuring proper compatibility between gold baths and nickel tanks helps mitigate the risk of PCB black pad.
(4) The concentration of gold in the gold bath has limited impact on nickel surface corrosion. Insufficient gold concentration in the gold bath intensifies the trend of reactions, thereby increasing the degree of nickel surface corrosion, which affects PCB solderability. Therefore, maintaining the gold concentration in the gold bath at around 0.8 g/l is recommended.
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