The rapid growth of the printed circuit board industry of the 1950s had a significant effect on the development of hard-gold processes. The objective was to find alternatives to the alkaline pure gold processes, which at high temperatures attacked resist materials and bonding agents and provided deposits with limited abrasion resistance.13 Rinker and Johns were the first to develop a cyanide-based gold electrolyte utilizing nickel, cobalt or indium as alloying metals.14 At that time, iron (belonging to the same subgroup as nickel and cobalt), was not taken into consideration because of possible concerns that co-deposits would result in brittleness issues.15 However, later intensive studies of gold-iron deposit characteristics have allayed these concerns. It is only appropriate therefore, that iron together with cobalt and nickel can fulfill industry MIL and ASTM standards.16,17 The following investigation involves a mildly acidic gold-iron high speed electrolyte.** Table 1 summarizes typical parameter ranges.
Table 1 - Working parameters of the gold-iron process.
4.2 - 4.7
60°C (Range = 40-65°C)
2.0 - 20 g/L
2.5 - 100 A/dm2
Morphology studies with focused ion beam (FIB)/ scanning electron microscope (SEM)
The deposit morphology was studied using a focused ion beam (FIB) in conjunction with a scanning electron microscope (SEM). All FIB/SEM samples consisted of a stainless steel substrate coated with an initial 5 µm nickel (from a sulfamate electrolyte) followed by 5 µm of gold. This FIB/SEM technique requires a high thickness due to gallium ions bombarding the test sample, creating the necessary fine micro-cuts perpendicular to the surface (Fig. 2).
The following FIB/SEM images (Fig. 3) illustrate sections measuring 10, 4 and 1 µm. A current density of 30 A/dm2 was used during gold-iron deposition. These images highlight the very low nano-porosity of the gold-iron deposits.
Quartz crystal microbalance study of metal deposition characteristics Deposition rate and coating information of gold-iron electrodeposits at varying current density may be obtained by means of a quartz crystal microbalance (QCM).18-20 The QCM is a very sensitive analytical method that permits the in situ detection of interfacial processes, e.g., weight gain by surface electrodeposition. The measurement principle is based on the change in the natural frequency of a quartz resonator that can be excited to a resonant oscillation by AC voltage (piezoelectricity). In 1959, Sauerbrey was the first, to develop a method for correlating changes in the oscillation frequency Δf and the mass deposited on it Δm:18
∆f = (2fo2 ∙ ∆m) / ((ρq μq )1/2 ∙ A) (1)
The equation defines fo as the frequency of the quartz resonator, ρq and μq are the density and shear modulus of the quartz, respectively, A is the area and Δm the adsorbed mass of substance under investigation.
Inserting the Sauerbrey constant Sf allows Equation 1 to be simplified to:
∆f = - (Sf ∙ ∆m) / A (2)
The proportionality factor Sf is a material-specific parameter and is also referred to as the “integral coating weighing sensitivity.” The QCM is a very surface sensitive technique and so the detection limit for mass deposition lies on the order of a few nanograms.
In this study the dimensions of the quartz crystal used were 0.3 mm thickness, 14 mm diameter and a surface area of 0.5 cm2. Figure 4 shows the details of the electrode set-up. The electrolyte is pumped at constant flow onto the oscillating quartz crystal (WE) via an injection tube.
From the frequency / time diagrams measured using the quartz crystal microbalance, it is possible to determine deposition rates and current efficiencies. Figure 5 provides typical experimental data with respect to change in current density and frequency as function of time. The time resolution is 1 sec and the frequency drift < 1 Hz to 300 kHz.
Table 2 shows the parameters used for sample preparation.
Table 2 - Experimental parameters for the gold-iron process study.
|pH ||4.6 |
|Temperature ||60°C |
|Gold concentration ||8.0 g/L |
|Iron concentration ||0.05 g/L |
|CD Range ||Variable (1.0 - 70 A/dm2) |
Figure 6 shows the current efficiency and deposition rate data as function of the current density from the gold-iron QCM experiments. At 5.0 A/dm2, the current efficiency is 71% and progressively falls at increasing current density to 23% at CD = 70 A/dm2. At 1, 5 and 10 A/dm2, the deposition rate values were 0.4, 2.4 and 4.3 µm/min, respectively. At 70 A/dm2 and 8.0 g/L gold, a deposition rate of 10.7 µm/min was achieved.
Investigations of deposition rates at high flow rate
Deposition rate data was measured using a high-speed plating cell at a fixed flow rate of 2,200 L/hr (Fig. 7).
The effect of temperature on the deposition rate was investigated and resulted in an expected speed increase at higher values (Fig. 8). The highest achievable rate of 17.2 µm/min was observed at CD = 70A/dm2, T = 65°C for 8.0 g/L Au content.
A further investigation shows that at increased gold concentration (from 8 to 16 g/L), greater deposition rates are achievable (Fig. 9). Above 30 A/dm2, the differences become very noticeable. For example, at CD = 50 A/dm2 and T = 60°C, the deposition rate at 8.0 and 16 g/L are 13.6 (Fig. 8) and 19.6 (Fig. 9) μm/min, respectively.
Contact resistance measurement
A detailed study on the effect of thermal aging on measured contact resistance was carried out. Testing was carried out with the contact resistance measuring device in accordance with the EN IEC 512 Standard21 (Measuring parameters: I = 10 mA, U = 20 mV, F = 5 cN). For each test sample, the mean from 30 individual measurements was taken. Gold thicknesses of 0.3 and 0.8 µm were used over a 1.5-µm nickel underlayer. Contact resistance measurements of “as-plated” deposits produced constant values of 2.5 – 3.0 mΩ over a current density range of 10 – 50 A/dm2 (Fig. 10). Little influence noted at varying thickness. Even after artificial aging (16 hr at 260°C), constant contact resistance values below 5.0 mΩ were observed (Fig. 11). At increased temperature (300°C for 5 min) little change to contact resistance was observed (Fig. 12). Hence, over a wide 10 to 50A/dm2 current density range, the Au/ Fe deposits provide consistently low contact resistance values, irrespective of aging conditions and thickness.
Iron codeposition investigations
The weight percentage of alloying metal was measured using atomic absorption spectroscopy (AAS). Analyses show that the wt% of co-deposited iron decreases at increasing current density. Under optimum conditions, 0.28 wt% Fe was found to occur at CD = 10 A/dm2 decreasing to 0.14 wt% at CD = 70 A/dm2 (Fig. 13).
Hardness evaluation of gold deposits
To measure hardness by the Martens’ procedure, test samples were prepared comprised of 5 µm each of bright nickel followed by the gold electrodeposit. Marten’s hardness (HM) is defined as maximum bearing stress Fmax divided by the contact area As:22
HM = Fmax / As (3)
Martens’ hardness values incorporate both plastic and elastic deformation forces and are applicable for all metals. This procedure is universally accepted and defined in both Berkovich and Vickers methods.23 The Fisherscope H100C (Fig. 14) was used for the measurements. Hardness measurements at varying current densities were carried out using optimum process conditions to determine the effect of iron incorporation on micro-hardness. Figure 15 confirms a similar trend to wt% Fe co-deposition at varying current densities, peaking at low current density.
The investigations carried out outline the performance of the gold-iron process over a wide range of parameters (including gold content and flow rate) using both quartz microbalance technique and high-speed plating cells. The Au/ Fe process provides an effective and efficient alternative to the traditional gold-cobalt or gold-nickel systems and electrodeposits satisfy the internationally accepted MIL Spec G45-204, type II/ grade C specification.
FIB/SEM deposit examinations highlight low porosity and contact resistance measurements suggest excellent stability even after accelerated aging / heat treatment (to 300°C / 5 min).
Deposition rate studies using both quartz microbalance at varying electrolyte flow rate and also the MiniLab high-speed test cell demonstrate exceptionally high values over a wide current density range. Wt% iron co-deposition trends were found to be comparable to the Au/ Co system, i.e., decreasing with increasing current density. The highest Marten’s hardness value of 1844 was measured at 10A/dm2 for a 0.28 wt% Fe co-deposit.
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