Electrochemical characterization of plasma coatings on printed circuit boards

Progress in Organic Coatings 137 (2019) 105256

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Electrochemical characterization of plasma coatings on printed circuit boards

T

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Aliakbar Khangholia, , Reynier I. Revillaa, Alexander Lutza, Samir Loulidib, Eva Roggeb, Guy Van Asschec, Iris De Graevea a

Research Group of Electrochemical and Surface Engineering, Department of Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Europlasma NV, De Bruwaan 15, 9700 Oudenaarde, Belgium c Research Group of Physical Chemistry and Polymers, Department of Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium b

A R T I C LE I N FO

A B S T R A C T

Keywords: PCB Corrosion Plasma coatings ORP-E

Printed Circuit Boards (PCB) are the backbone of electronic devices and communication technology. The corrosion of copper tracks in PCBs is the main reliability issue in case of exposure to humidity. To overcome this, several surface finishing methods have been introduced in the industry such as plasma coatings. In this study, several electrochemical characterisation methods are explored on blank and plasma-coated PCBs to identify reliable measurement techniques and to get insight in the protective properties of the coatings on an overall macroscopic visual scale, a dedicated surface-average scale, as well as on a localised microscopic scale. The macroscopic visual corrosion behaviour was evaluated by standard salt-spray exposure tests. Odd Random Phase multisine Electrochemical Impedance Spectroscopy (ORP-EIS) provided insight into the average barrier properties of the coatings. Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM) were used to evaluate the samples’ reactivity on a local scale. Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX) analysis was used as a complementary surface analysis method to characterise the coatings. Blank PCBs were studied as reference substrates, and their behaviour was compared to three plasma-coated PCB variants, based on fluorinated organic precursor chemistry. The salt-spray tests illustrated the protective behaviour of the coatings but did not allow to differentiate between the different coatings. With ORP-EIS, it was observed that the coatings act as a barrier against the electrolyte. This is confirmed on the local scale, where no activities were seen on the surface of the coated samples using SVET, except after 10 days of immersion when single layer plasma coatings showed higher activities. SKPFM revealed much higher surface potentials for the coated PCBs compared to the blank PCB allowing to differentiate between the various coated samples.

1. Introduction Printed Circuit Boards (PCBs) are the heart of the communication and electronics industry [1]. The on-going miniaturisation, increasing complexity, and use of PCBs in all kinds of environments are making them much more sensitive to corrosion. The copper tracks used in PCBs are highly susceptible to humidity and contamination: exposure to humidity can lead to corrosion of printed circuit boards and subsequent failure of the electronic devices, and even a small amount of contamination on the PCB can result in the physical breakdown of a device [2–4]. Contamination, continuous exposure to humidity and condensation result in the formation of a thin electrolyte layer on the surface of PCBs that connects the adversely biased tracks on PCBs.

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Presence of an electric field in the surrounding causes the migration of dissolved ions on the anode toward the negatively biased electrode (cathode) and this phenomenon is called electrochemical migration [3–5]. The electrochemical migration can result in the connecting of these two electrodes by the formation and growth of (dendritic) corrosion products. As a result, a short circuit occurs in the system, causing device failure [6]. Finding solutions for protecting printed circuit boards is crucial. Several surface-finishing methods have been introduced on an industrial scale, such as covering the tracks using Electroless Nickel immersion gold (ENIG), Organic Solderability Preservatives (OSP), and immersion silver (ImAg). However, most of these methods have some complications, such as the difficulty of the process due to the daily increase of complexity and sensitivity of the integrated

Corresponding author. E-mail address: [email protected] (A. Khangholi).

https://doi.org/10.1016/j.porgcoat.2019.105256 Received 26 April 2018; Received in revised form 1 July 2019; Accepted 29 July 2019 0300-9440/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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Microscopy (SKPFM), provide insight into the localised processes occurring at the coated metals. Using a movable vibrating electrode, SVET detects potential gradients above the surface of a sample in a solution. The vibration of the electrode allows measuring local potential variations that are then converted into local current densities. These current density variations are associated with ionic fluxes originating from the reactions occurring on the active surface. In combination with a microscope image, the localised current density map allows one to detect local defects in organic coatings and their electrochemical activity [26,27]. In-situ SVET measurements provide information about the protective behaviour of the coating on the microscopic scale [10,28]. SKPFM delivers nano-scale topography maps and the corresponding potential distribution of the sample’s surface. The Volta potential maps obtained by SKPFM represent the surface potential with respect to a metal probe (the AFM tip), and it is generally referred to as contact potential difference (CPD). The measured Volta potential or CPD is proportional to the work function of the sample, and therefore proportional to the chemical potential of the sample’s surface [29]. According to Rohwerder et al., SKPFM is more prone to artefacts compared to SKP [30]. However, if care is taken and different conditions such as reproducibility of data and previous calibration of the cantilever are satisfied, then the measured surface potential could be correlated to the corrosion potential [30–35]. SKPFM has been extensively used to study the local corrosion behaviour of a wide variety of materials [31–36]. In this work, we study the corrosion behaviour of coated PCBs using salt spray test, ORP-EIS, SVET and SKPFM. With this combination, we aim at gaining information on the global, surface average and local scale. While ORP-EIS shows the general surface averaged behaviour of different plasma coatings, used for a qualitative comparison, SVET and SKPFM give us both quantitative and qualitative information on a more local scale.

circuits, the presence of surface defects, or a short lifetime [7–9]. Protective polymer films, in general, have been receiving a great deal of interest due to their capacity to confer unique properties such as protection against corrosive media, self-healing, metallic-like conductivity and biocompatibility depending on synthesis parameters [10–13]. Despite a large number of studies in this area, only a few works have been reported regarding the application of protective polymer films in electronics [14,15]. Various plasma deposition methods are adapted to apply a conformal plasma-polymer layer on different materials. Plasma treatment or plasma polymerisation is a very versatile surface treatment technique. It allows modification of a substrate’s surface without affecting the bulk properties. Plasma treatment is gaining a great deal of interest due to the miniaturisation of items (e.g. electronics) in industries which requires surface modification without affecting the functionality of these items. Plasma polymerisation process allows deposition of thin organic coatings from several angstroms up to several microns. The possibility of using a wide range of gasses or their combinations enables full customisation resulting in unique surface properties needed for different applications. Additionally, the use of monomers for plasma polymerisation results in chemical and physical properties which can be different from the ones of conventional polymer films; for instance, the plasma polymerisation process can produce a highly cross-linked coating. Thus, plasma polymer coatings are chemically more stable than conventional polymers. Other benefits compared to competitive techniques (e.g. wet chemical) are the absence of solvents and water which makes it a more environmental-friendly technology. Treatment of temperature-sensitive materials due to medium vacuum operations is also another benefit of this method [16–19]. Plasma coatings are not typical polymer layers, as their formation depends on the degree of fragmentation of the organic precursor molecule in the plasma, followed by its recombination and layer formation upon deposition on the destined substrate, in this case, a PCB with or without electronic components already soldered on top. Although there is a clear interest in plasma coatings for corrosion protection [13,20], little is known about the electrochemical behaviour of plasma coatings on printed circuit boards [21]. In this research, various electrochemical methods are explored to gain a deeper insight into their potential to study the electrochemical behaviour of coated PCBs. Electrochemical Impedance Spectroscopy (EIS) and local electrochemical techniques, such as Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM), were used to study the protective properties of various plasma polymer coatings on design PCBs. Electrochemical impedance spectroscopy is an effective technique for analysing and studying organic coatings on metallic substrates [22–24]. However, there can be challenges regarding the accuracy and complexity of the data, and as a result misinterpretation. To avoid these problems, Odd random Phase multisine Electrochemical Impedance Spectroscopy (ORP-EIS) can be used. ORP-EIS is an improved EIS technique developed in our group, which notably reduces the possibility of data misinterpretation by integrated measuring and modelling of EIS based on odd random phase excitation signals. Thereby, the disturbing noise, non-linear and non-stationary behaviours of the system can be measured and quantified. Hence, the optimal measurement conditions can be obtained using this method [25]. To be more clear the interpretation of the impedance data is based on modelling of the experimental results. This modelling is only valid when the system behaves linear, and the stationarity behaviour of the system is satisfied. However, these conditions are rarely confirmed in the literature even though it is known that non-linear behaviour is very common in electrochemistry. Moreover, the behaviour of an electrochemical system can change quickly during the measurement which can cause non-stationarity in the system. Therefore, it is of great importance to detect the non-linearities and non-stationarities in the system. Local electrochemical techniques such as Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force

2. Experimental procedures 2.1. Materials and sample preparation Blank PCB samples were purchased from Elprinta NV (Belgium). The copper tracks are coated with an Electroless Nickel/Immersion Gold (ENIG) surface-finishing method. A schematic of a cross-section of a PCB and the design of the PCB samples are shown in Fig. 1a–b. The information and parameters related to the blank samples are listed in Table 1. To study the samples without the influence of the design a simpler design consisting of a circular surface with a diameter of 10 mm was produced specifically for the electrochemical Impedance measurement (Fig. 1c). To characterise the cross-section of the blank PCB sample, the circular area (see Fig. 1c) was cut in half with a diamond saw and embedded. Then the cross sections were polished with diamond paste. An Industrial plasma polymerisation reactor (Nanofics CD600 model, Europlasma NV, Belgium) with a capacitively coupled plasma source was used for the deposition of the plasma polymer coatings. The plasma polymerisation reactor is a rectangular reactor made of aluminium, having a volume of 0,282 m³. The reactor contains four radiofrequency powered electrodes which are water-cooled, and each RFelectrode has a dedicated grounded electrode with a 3 cm gap between the electrodes. The reactor walls are also grounded (see Fig. 2). Medium vacuum conditions are used for the deposition process resulting in the expansion of the plasma zone beyond the plasma generation zone; the medium-vacuum conditions are obtained by using a Roots compressor pump. A 13.56 MHz continuous RF is used for generation of the plasma with powers varying from 10 to 1000 W, work pressures for deposition processes ranging from 3 to 20 Pa, gas-flows from 5 to 200 sccm and at a reactor temperature of 40 °C. The studied coatings in this work are based on fluorinated acrylates monomers (see Fig. 3) which are plasma polymerised with the aim of obtaining hydrophobic barrier coatings 2

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Fig. 1. (a) Schematic and (b) Blank PCB with interdigitized tracks (Comb design), (c) simple circular design of the Blank PCB.

Several coatings were studied: single layer plasma coatings with a thickness of 1 ± 0.15 μm and 9 ± 1.35 μm, and a stacked plasma coating with the thickness of 3 ± 0.45 μm. The plasma coatings were deposited after an optimal plasma pre-treatment of the substrate to increase the adhesion of the coatings to the substrate. The process of applying a stacked coating requires a post-treatment on the single coatings, to be able to deposit another layer on top. The process of coating deposition followed by the post-treatment was repeated several times to obtain a multi-layer or stacked coating on the substrate (full process conditions are proprietary information of Europlasma).

Table 1 Thickness values for the elements of the blank PCB (producer information).

Thickness (μm)

Copper track

Nickel-Phosphorous layer

Gold coating

35

3-5

0.05

2.2. Electrochemical measurements The instrument used for ORP-EIS was a combination of an EG&G Princeton Applied Research potentiostat (PAR model 2273) and a National Instruments PCI-4452 DAQ-card with a built-in anti-aliasing filter. The applied signal was digitally built by Matlab software 2011. Matlab was also used for collecting and treating the data and controlling the DAQ-card. The applied amplitude or root mean square (RMS) was 10 mV. The frequency range of the measurement was between 0.1 Hz and 68 kHz. The measurements were conducted with the samples immersed in 0.4 M Na2SO4 (99.0% purity, Merck Co.) electrolyte at room temperature. A non-aggressive electrolyte was used in this work (for a sufficient conductivity) for the impedance measurements since the main intention was to study the barrier properties of the coatings. The starting point of the measurement is precisely after verifying that the system is linear (When noise + non-linearities coincide with the noise level) and that the noise level of the system is two or more orders of magnitude lower than the signal.

Fig. 2. Schematic of the plasma polymerisation reactor (purple is the plasma zone). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

with varying thicknesses. Substrates are placed outside of the plasma generation zone (zone between RF-electrode and grounded electrode) on dedicated substrate fixtures. The system is pumped down to a base pressure (1.3 Pa). An inert gas or monomer vapour is introduced in the reactor to fill up the reactor to the predefined pressure, Upon reaching the pressure, the plasma is ignited, and then plasma activation and/or plasma polymerisation is initiated. The exposure time is determined by the desired thickness. At the end of the process, the system is flushed removing any remaining gas/vapour, and the system is aerated to atmospheric pressure.

2.2.1. Analysis of ORP-EIS data The obtained data by ORP-EIS consists of the experimental data, the noise level, non-stationarity and non-linearity behaviour of the system [25]. ORP-EIS allows checking the quality of the experimental data by analysing the signal-to-noise ratio, non-linearities and non-stationarities. This is necessary in order to obtain correct experimental data. Fig. 3. Single layer plasma coating's chemistry (1H,1H,2Hperfluorodecyl acrylate).

3

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1.4 mm × 0.68 mm) was used for the measurements. SVET measurements were performed on both the comb and the simple designed PCB samples.

The standard deviations on the excited and non-excited frequencies are used for measuring the noise level. 4 consecutive periods are used to build one ORP-EIS experiment. These are also used for calculation of the standard deviations. The analysis of the noise in the system is explained as follows: 1) The noise level is the measure of the standard deviation on the non-excited even frequencies, 2) The non-stationarities are the difference between the noise level and the standard deviations on the excited frequencies. Therefore, for a stationary system there must be no difference between the two, 3) The non-linearities are the difference between the noise level and the standard deviation on the non-excited odd frequencies, for a linear system noise level + non-linearities should overlap with the noise level.

2.4. FE-SEM/EDX characterisation Field Emission Scanning Electron Microscopy was used for imaging, morphology and microstructural characterisation. A FE-SEM JEOL JSM7100F was used for this purpose. The acceleration voltage and probe current used during the measurements were 15 kV and 30 pA, respectively. The working distance was 10 mm. Energy Dispersive X-ray Spectroscopy (EDX) connected to the FE-SEM was used for the elemental characterisation of the samples.

2.2.2. Fitting of the ORP-EIS data Fitting of the ORP-EIS data can be performed after validating the quality of the experimental data. A cost function as a measure of the distance between the experimental data and the model is considered for the goodness of the fit. This function was lessened to obtain the best fit. Besides, the standard deviation of the parameters is measured as an extra confirmation for fitting reliability. The fitting was done with the in-house developed Matlab program, using a Gauss-Newton followed by Levenberg-Marquardt minimisation scheme [25]. The complex residual of the modelling (the difference between the experimental data and the model) is also considered for the goodness of the fit. In brief, the fitting is assessed based on the physical validity of the model, the low complex residuals with respect to the noise level, the physical explanation of the parameters and the low standard deviation of these fitted parameters.

2.5. Salt spray test Salt spray tests were performed on comb-designed PCB samples according to ASTM B117 standard for an exposure time of 168 h to salt fog. The ASTM B117 standard describes a static test implemented at a constant temperature of 35 °C. 3. Results and discussion 3.1. Samples characterization A SEM image of a cross-section of a blank PCB is shown in Fig. 4a. Fig. 4b–f shows the elemental mapping obtained with SEM-EDX. The different regions/elements of the sample can be clearly observed. The copper track is covered by a nickel-phosphorous layer and a thin gold layer. This multilayer structure is the result of the ENIG surface finishing process. The EDX mapping also shows the uniform distribution of the nickel-phosphorus layer over the copper track. The thickness of this layer was measured around 5 μm, which agrees with the values given by the producer (see Table 1). The thickness of the gold layer obtained from the SEM-EDX images ranged from 0.015 to 0.05 μm. The slight differences obtained between these values and the values given by the company (Elprinta NV) are possibly due to the mechanical polishing process for cross-section sample preparation. A SEM image of the surface of the as-received blank sample is shown in Fig. 5. The typical amorphous nodular structure [37] obtained by the electroless deposition of nickel can be clearly seen. To visualise the plasma coating on the PCB, cross-sectioning methods were not all successful. This is attributed to the intrinsic properties of the coated PCB, i.e. a polymeric carrier substrate with soft copper tracks, covered by an ENIG coating with a different hardness than the copper, and a soft plasma coating on top. Various approaches were tested. It was possible to remove some parts of the coating by tape thus creating a step. Then the cross-section of the remaining coating or the created step on the PCB could be visualised by tilting the sample in the SEM chamber (Fig. 6). This was a successful method for acquiring higher-resolution images of the coating’s cross-section. Furthermore, by analysing several images, we were able to estimate the thickness of the coatings (2.4 μm), which are in good agreements with the producer’s information obtained by reflectometry on Si wafer coupons that were coated together with the PCBs in the same process batch.

2.3. Local electrochemical characterization 2.3.1. SKPFM instrumentation and experimental procedure To analyse the Volta potential, a Park System XE-100 Scanning Kelvin Probe Force Microscopy (SKPFM) was used. All the measurements were performed in air at room temperature. Cr/Au-coated tips on rectangular conductive cantilevers (NSC36/Cr-Au) with a nominal resonant frequency of 65 kHz and a nominal spring constant of 0.6 N/m were used. Topography and contact potential difference between the probe and the sample were simultaneously obtained using a single-pass methodology. The recorded potential signals were inverted to reflect the real relation between the Volta potential values of the surface under investigation. For Volta potential measurements on the cross-section, the sample was prepared in the same way as described in Section 3 (Sample characterisation). However, the bottom of the embedded samples was cut and polished to have enough electrical contact to the instrument for this measurement. To ensure that the measured potential values could be compared to each other, all the samples were tested on the same day. Besides, after completing the first set of measurements on all different samples, they were repeated 3 times (exchanging the samples each time). This was done for several reasons; 1) to ensure that the tip has not faced any geometrical changes or plastic deformation, 2) to exclude variations from environmental factor, 3) to eliminate the influence of possible contamination of the sample/tip; which will change the absolute value of the potential. 2.3.2. SVET instrumentation and experimental procedure SVET measurements were performed using an Applicable Electronics Instrument coupled with ASET 2.10 software. The vibrating probe was calibrated before the measurements. Size of the used tip in the experiments is around 30 μm. For every 40 μm, one measurement point of the vertical component of the current density was recorded to build up the entire current density map. For the measurement, the probe is placed 100 μm above the surface. The measurements were conducted in 0.05 M NaCl (99.7% purity, VWR Co.) electrolyte at room temperature. For SVET, except the measurement area (4 mm2) the rest of the sample was masked with adhesive tape (3 M polyester tape No. 8402). However, only a part of this exposed area (35 × 17 points/

3.2. Macroscopic visual corrosion test Salt spray testing was performed on all types of coated samples (comb design). The test was also done on blank PCB samples, which were used as a reference to evaluate the protective behaviour of the coatings in harsh conditions. After 168 h exposure to the salt fog, blank PCB sample shows severe corrosion of the copper tracks (see Fig. 7). From the optical analysis, it can be seen that the borders of the copper tracks are more prone to corrosion. Moreover, the delamination of the gold layer was observed after 24 h immersion in an aqueous solution 4

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Fig. 4. Cross-sectional EDX mapping of blank PCB: (a) SEM image, (b) EDX mapping, (c) gold layer, (d) Nickel present in Ni-P layer, (e) Phosphorous present in Ni-P layer, (f) Copper track.

(figure not shown). Hence, this delamination could be one of the reasons for corrosion initiation. Most of these corrosion products have a green/blue colour, which is characteristic of some copper compounds or copper corrosion products. This corrosion products as confirmed by Raman spectroscopy technique are the polymorphs of Cu2(OH)3Cl (Atacamite, botallackite and Clinoatacamite), Cu2O and NiO. It can also be seen that corrosion has mainly occurred close to the edges of the copper tracks. All different types of coated samples were tested, but due to the similarity of their behaviour only the 1 μm coated sample is shown here (Fig. 7c and d). It can be noticed from the optical images obtained before and after the test that these coatings act as a strong barrier against corrosion.

3.3. Electrochemical impedance spectroscopy Fig. 5. Surface morphology of ENIG surface (blank sample) obtained with SEM.

3.3.1. Collecting the ORP-EIS data ORP-EIS measurements were performed in order to estimate and compare the protective properties of the different types of plasma

Fig. 6. SEM image of plasma coating’s cross-section. 5

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Fig. 7. Optical image of the (a) blank PCB before the salt spray test and (b) blank PCB after the salt spray test (c) 1 μm plasma coated sample before and d) 1 μm plasma coated sample after the salt spray test.

Fig. 8. Bode amplitude (a) and phase (b) for the blank and plasma coated PCBs just after immersion and checking the reliability of the measurements.

the different noise levels in the system. ORP-EIS allows determining the noise level, non-stationarities and non-linearities at each frequency. This is necessary in order to analyse the measurement errors for a correct model and the correspondent values of the obtained parameters. The experimental data and their noise levels are presented in Fig. 9. The analysis is based on the explanation in Section 2.2.1. For a linear behaviour, the noise level + non-linearities should overlap with the noise level. The same should happen for the noise level + non-stationarities in order to have a stationary behaviour. It can be noticed that for the coated samples the signal-to-noise ratio is worse at low frequencies and that the noise and the impedance signal cross each other (around 0.3 Hz). This indicates that the impedance signal is interrupted by the noise present in the system. When the impedance is high (low current), the signal-to-noise ratio at lower frequencies will be worse. However, ORP-EIS allow to quantify and analyse this behaviour. Hence, care must be taken for the data analysis of the plasma coated samples. By using the in-house developed MATLAB

coating at the initial stage of immersion (after stabilisation of the sample in the electrolyte). Fig. 8 presents the Bode plots of the plasma coated PCB samples. The interpretation of the Bode plots is more straightforward since the observed changes between the different samples are more clear. The impedance magnitude of the plasma coated PCB samples is at least 3 orders of magnitude higher than that of the blank PCB. The 9 μm single layer plasma coated PCB has the highest impedance magnitude while the 1 μm single layer plasma coated PCB has the lowest. It can also be seen from the phase angle that the response at high frequencies (after 100 Hz) altered after application of the coatings on the blank PCB substrate. The 9 μm single layer coating has a phase angle of – 90° in most of the frequency range, while the behaviour of the 3 μm stacked coating deviates from that of the 9 μm single layer at frequencies below 100 Hz. On the other hand, the 1 μm single layer plasma coating shows a very different behaviour compared to the other coatings. The reliability and quality of the measurements were evaluated by 6

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Fig. 9. Comparison between modelled and experimental data of (a) Blank PCB, (b) 1 μm single layer plasma coatings, (c) 3 μm stacked coating and (d) 9 μm plasma coating.

program as explained in Section 2.2.2, different weighting factors can be chosen such as amplitude or noise level. Noise level weighting factor allows for a fitting with more weight assessment to high-quality data points (linear behaviour or lowest noise level) and less weight to lowquality data points [38]. Non-stationarities are observed in most of the samples except the 9 μm single layer plasma coated PCB. Non-stationarities are typically related to the activity of the surface such as corrosion on the blank PCB or the water uptake in the organic coatings [38,39]. Furthermore, high levels of non-stationarities will influence the response at non-excited frequencies. This implies that non-linearities are observed even if the system is linear. This behaviour is distinguished when the non-linearities and non-stationarities are parallel to each other as observed for the blank sample [40]. However, in most of the frequency range, the nonstationarities are 3 orders of magnitude lower than the impedance magnitude. Therefore, their influence can be neglected in the modelling of the obtained experimental data.

ZCPE =

1 Q (jω)α

(1)

When the value of α reaches 1, then CPE represents a pure capacitive behaviour. In this work for the gold layer and some of the coatings, the α value does not approach 1, and this eliminates the use of a pure capacitor. It can be seen in Fig. 9a, that the model matches the experimental data sufficiently. To assess the goodness of the fit and the model, the complex residuals is compared to the noise level. For the best-fit, the complex residuals should overlap noise + non-stationaries. The complex residuals in this case slightly differ to the noise level + non-stationarities. However, the complex residual error is always less than 10%. Moreover, the relative error of the parameters are less than 6%. Therefore, it can be concluded that this model is acceptable. The estimated values and their relative errors are presented in Table 2. Based on this analysis, it can be concluded that the PCB starts corroding as soon as it is exposed to an electrolyte. Plasma coated samples showed different behaviour than the blank PCB. The 9 μm plasma coated PCB sample exhibited the highest impedance magnitude compared to the other coatings. The magnitude of the impedance of the plasma coating with the thickness of 9 μm is an order of magnitude higher than the 3 μm stacked coating and two orders of magnitude higher than 1 μm plasma coating (Fig. 8a). The increase of impedance magnitude is attributed to the larger thickness of the coatings [46]. All the plasma coated PCB samples show relatively much higher impedance magnitude compared to the blank PCB. The higher magnitude indicates the barrier properties of the plasma coatings. Fitting of the plasma coated PCBs were performed in order to gain an insight into the protective behaviour of plasma coated samples and to be able to compare them. The equivalent electrical circuits used for these samples are illustrated in Fig. 10. The equivalent circuit presented in Fig. 10b was used for modelling the data of the 9 μm single layer coating and the 3 μm stacked coating. Similar models have been used in literature to describe the behaviour of a blocking coating exposed to an aqueous solution [47]. In the demonstrated circuits, R0 is a measure of the resistance of the electrolyte plus the electrical contribution from the

3.3.2. Modelling and fitting of the ORP-EIS data Modelling of the data was performed after validation of the quality of the measurements. The blank sample is modelled with the Equivalent Electrical Circuit (EEC) presented in Fig. 10a. A similar model was used in other works [41,42]. The EEC for the blank PCB is shown with the scheme of the blank PCB to illustrate the correspondence of each element to the different layers. In the suggested equivalent electrical circuit, Rs is the resistance of the electrolyte which is observed as a plateau in the bode magnitude plot present in the very high-frequency regions. CPEAu/dl is due to the capacitive behaviour of the gold double layer. However, a constant phase element is used, representative of the heterogeneity on the surface of the gold layer [43]. Rp represents the resistance of the pore in the gold layer. CNiP/dl-Rct expresses the corrosion reaction at the interface of the electrolyte and Ni-P layer as reported previously [44,45]. A CPE is usually used instead of a pure capacitor to compensate the heterogeneities of the substrate. The impedance of a constant phase element is calculated using the following equation:

7

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Fig. 10. Equivalent electrical circuit of schematically represents (a) blank PCB, (b) the 9 μm single layer and 3 μm stacked plasma coatings, (c) expected EEC for the 1 μm single layer plasma coated PCB and (d) used EEC for the 1 μm single layer plasma coated PCB immersed in 0.4 M Na2SO4.

straightforward. By considering that the presented EEC in Fig. 10d is the correct model, then all the models match the experimental data adequately (Fig. 9). Similar to before, complex residuals were compared to the noise level to assess the goodness of the fit. The complex residuals are less than 10% (except for the very low frequency where the signalto-noise ratio is very low). The relative errors of the parameters for the models of 9 μm single layer and 3 μm stacked coatings are mostly less than 4% except for R0 and Rct. These values are more difficult to determine since the frequency was not broad enough to be able to precisely measure these parameters; R0 appears at very high frequencies while Rct is observed at very low frequencies. Table 3 shows the circuit parameters, and their relative error for the 9 μm single layer and 3 μm stacked plasma coatings. The circuit parameters and their relative errors for the 1 μm single layer plasma coated PCB samples are presented in Table 4. The relative error for all the circuit parameters is below 5% except for the R0 and Rct. As mentioned before, these parameters are more difficult to be determined since the frequency range is not broad enough.

Table 2 Best-fit parameters and their relative error obtained from fitting the experimental data for the blank PCB after immersion. Parameter

Value

Rel. error (%)

Rs (Ω) Q-CPEAu/dl (Ω−1 sn) αAu/dl Rpore (kΩ) CNiP/dl (F) Rct (kΩ)

20.56 1.33E-5 0.91 2.51 8.40E-7 46.06

0.01 0.01 0.08 5.98 5.82 5.23

coating (due to stray/parasitic capacitances at the interface of the coating and the electrolyte) [11,48,49]. CPEc is the constant phase element of the coating which describes the behaviour of the coating while Rc is the resistance of the coating. Nevertheless, this model cannot be used for modelling of the 1 μm single layer plasma coated PCB due to large fitting errors. Therefore, a new model based on the physical explanation as shown in Fig. 10c is proposed which considers the ingress of electrolyte through the coating and eventually accessing the substrate. Although the model can be used to fit the data, the relative errors of parameters are very high. On the other hand, it is observed that a simpler model as shown in Fig. 10d can be used to fit the data. If the time constants of the gold and Ni-P layer are close to each other, then they would be convoluted in each other and could appear as a single time constant similar to the one shown in Fig. 10d. The CPE is possibly linked to the double layer behaviour of the gold and Ni-P layer. Moreover, R could contain partially the charge transfer reactions at the interface of the electrolyte and the PCB substrate. However, the interpretation of these parameters is not

Table 3 Best-fit parameters and their relative errors obtained from fitting the experimental data of the 9 μm single layer and 3 μm stacked plasma coated PCBs. Parameter

R0 (kΩ) Q-CPEc (Ω−1 sn) α Rc (GΩ)

8

3 μm stacked coating

9 μm single layer

Value

Rel. error (%)

Value

Rel. error (%)

1.31 1.34E-9 0.96 1.17

6.53 0.01 0.01 3.53

4.51 3.97E-10 0.96 9.39

12.37 0.12 0.03 12.52

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Table 4 Best-fit parameters and their relative errors obtained from fitting the experimental data of the 1 μm single layer plasma coated PCBs. Parameter

Value

Rel. error (%)

R0 (kΩ) Cc (F) Rc (kΩ) Q-CPE (Ω−1 sn) α R (kΩ)

0.29 2.59E-8 2.23E3 1.40E-8 0.86 8.91E4

7.41 4.70 4.94 0.24 0.09 12.31

The electrochemical behaviour of the plasma coatings was analysed using the shown equivalent electrical circuits. The model for the 9 μm single layer and the 3 μm stacked coatings confirmed the initial protective properties of these coatings. The capacitance values of these samples were estimated using the approach introduced by Brug et al. [50]. The capacitance values for all three types of samples are presented in Table 5. The 9 μm single layer plasma coating has the lowest, and 1 μm single layer plasma coating has the highest effective capacitance value. The measured capacitance value (F) is dependent on the thickness of a coating and its dielectric constant by Eq. (2):

Capacitance =

εε 0 A d

Fig. 11. SVET measurement on blank PCB (a) optical image (b) current mapping. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

density maps show negative current density indicating the cathodic activities on the samples. Anodic activities were not detected. The possible explanation could be that the anodic processes are happening under the coating or it could be that the cathodic reaction occurs outside the mapped area [53]. According to the results, the single layer 9 μm plasma coated sample showed much higher activity in comparison to the other samples. On the contrary, 3 μm stacked plasma coated PCB sample shows almost no activity after these 10 days. Moreover, the optical images after 10 days of immersion are shown in Fig. 13. The most apparent changes in the coating are observed for the 9 μm single layer coating. It seems that the electrolyte diffused through the coating and it is trapped there after removing the sample from the electrolyte. This feature is barely evident when it is immersed in the electrolyte. In order to check the validity of the experiment, some scratches were intentionally made in the 1 μm coated sample using a scalpel (Simple design PCB). Fig. 14 shows the SVET current map of this sample. Immediately after immersion, a high anodic current density was observed. This anodic current is possibly related to the dissolution of nickel in the electrolyte. On the other hand, cathodic reactivity was more difficult to detect as this is less localised or an alternative explanation could be that the cathodic reactions are taking place under the coating. Another reason could be that the current lines do not reach the height of the probe; hence, they might not be detectable [53,54]. Bastos et al. inspected the same behaviour on the transparent coated steel with a scribe exposing the substrate, and they correlated this to the large cathodic region [53]. SVET was also performed deliberately at the edges of a track covered by the 1 μm plasma coating (on combed PCB sample) immediately after immersion. The result in Fig. 15 indicates that the coating is also blocking the electrolyte diffusion around these edges.

(2)

Where ε is the dielectric constant of the layer, ε0 is the permittivity constant of the vacuum (8.84 × 10−12 F m−1), A is surface area (m2), and d is the thickness of the layer (m). Therefore, the smallest capacitance value was predicted for the 9 μm single layer coating as this coating is thicker than the rest of the samples (capacitance˜1/thickness). However, it was observed from the modelling of the impedance data that the 1 μm single layer plasma coatings do not entirely block the ingress of the electrolyte through the coating to the substrate. Nevertheless, the much higher impedance magnitude and the charge transfer resistance of the 1 μm plasma coated sample indicates that the corrosion rate decreases drastically. 3.4. SVET Fig. 11 shows the microscopy image and the current density map obtained by SVET on the blank PCB sample (track design) immediately after immersion in 0.05 M NaCl electrolyte. Analysis on the blank sample shows the presence of anodic and cathodic activity on the surface (Red and blue zone respectively). The anodic activity in the current density map is related to the dissolution of nickel in the electrolyte [51,52]. This indicates the poor protection behaviour of the gold layer on the blank sample due to heterogeneities. Furthermore, as it can be observed in the current density map, the anodic reaction is mostly localised at the edge of the copper track. The result implies that the ENIG surface finish offers only minimal protection against corrosion. Conversely, the current density in the electrolyte close to the surface of the plasma coated samples (simple design) remains zero (see Fig. 12). These current density maps were recorded for 10 days. The first and last measurements are presented in Fig. 12. The protection performance for all three different types of coating is very similar for the first measurements. However, after 10 days a noticeable difference is observed between the different types of plasma coatings. The related current

3.5. Volta potential analysis by SKPFM In order to gain more insight regarding the protective behaviour of these coatings, SKPFM was used to map the potential distribution of the samples’ surface. Similar works also reported the application of this technique for studying the behaviour of non-conductive materials such as aluminium oxide (Insulator) and samples with an organic coating

Table 5 Capacitance values of the plasma coatings. Parameter

Effective capacitance (F)

1 μm single layer

9 μm single layer

3 μm stacked coating

Value

Rel. error (%)

Value

Rel. error (%)

Value

Rel. error (%)

2.59E-8

4.70

2.37E-10

2.41

7.56E-10

0.08

9

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Fig. 12. SVET measurements on the surface of (a), (d), (g) 1 μm single layer, 3 μm stacked and the 9 μm single layer plasma coatings respectively (b), (c) ionic current maps of 1 μm coated PCB immediately after immersion and after 10 days (e), (f) ionic current maps of 3 μm stacked coated PCB immediately after immersion and after 10 days (h), (i) ionic current maps of 9 μm coated PCB immediately after immersion and after 10 days respectively.

which means higher energy needed to extract electrons from the surface. This correlates with previously reported works [59]. Earlier studies have shown that gold has a higher protective behaviour than Ni-P layer against corrosion in ENIG surface finish [42]. Ni-P has a lower Volta potential, which means that this layer will be degraded first after exposure to the electrolyte. This is in agreement with the literature that the sacrificial corrosion of nickel happens in the Ni-P layer and a P-rich layer will be formed in order to protect the PCB against water ingress. As described in other works, This barrier mechanism is related to the surface accumulation of dissolution products [45,60]. Due to the small thickness of the gold layer, there might be an influence from the embedding material and the nickel layer itself on the measured Volta potential values. A number of researches report the influence of the relating size of phases on Volta potential measurements [61–65]. The potential of the thin layer of gold will be influenced by the surrounding matrix. This may be a possible explanation why the gold, in this case, has a seemingly lower potential than the copper track. Volta potential and topography maps were also performed on the outer surface of all samples (Fig. 17). The topography of the surface of the blank PCB is in good correlation with the SEM image obtained on the blank sample (Fig. 5), in which the nodular structure obtained by the ENIG surface finishing method can be observed. The values of the nano-scale roughness measured by SKPFM are shown in Table 6.

[55–57]. Nazarov et al. investigated the influence of organic coatings on the measured surface Volta potential of the metallic substrates [56]. Other works described the correlation between the Volta potential shift and the inhibition of possible electrochemical reactions at the interface of the coating and the metallic substrate [58]. In the present work, SKPFM was performed to study and compare the Volta potential shift after applying the different types of plasma coating on the PCBs. A topography map, Volta potential map and a line scan of the crosssection of a blank PCB are shown in Fig. 16. In both maps, the different layers of the sample in the cross section can be distinguished. The dark space between the gold and the nickel layer in the topography image (Fig. 16a) is probably the result of damages caused by the polishing process. It is important to mention that the measured potential values are the potential difference between the Volta potential of the material of the probe and the analysed surface. Therefore, the magnitude of these variations in the CPD map is the real potential difference between the different surface locations. In the potential map shown in Fig. 16b, it can be seen that the copper track and gold layer have higher potential than the Ni-P layer. A line scan across the different layers is shown in Fig. 16c. It can be seen that the potential of the Ni-P layer is about 0.5 V more negative than the potential of the copper track. The line scan shows that the potential of the gold layer is clearly higher than that of the Ni-P layer. Higher Volta potential implies a higher work function,

Fig. 13. Optical images (obtained with SVET) of the plasma coated samples after 10 days of immersion in 0.05 M NaCl solution (a) 1 μm single layer plasma coating, (b) 3 μm stacked coating plasma coating (c) 9 μm single layer plasma coating. 10

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Fig. 14. (a) Scratched sample with a coating of 1 μm on top, (b) ionic current density map of the sample immediately after immersion.

According to the results, the values for the roughness of the 1 μm plasma coating and the blank sample are very similar. Conversely, the 9 μm coating showed a much lower value of the surface roughness (about 4 times lower than the surface roughness of the other samples). This is clearly associated with the thickness of the coating. For thinner coatings, there is a higher influence of the substrate on the final topography of the sample. It is important to notice that the colour bar of 9 μm plasma coated PCB in Fig. 12 is 4 times lower than the others. Latter confirms the smooth surface topography of this type of coating compared to the other ones. The 3 μm stacked coating presented a nanoscale roughness similar to that of the blank PCB sample. The process of applying the 1 μm single layer plasma coating is a continuous one-step procedure, while the process for the stacked coatings includes several steps (Section 2.1). This means that the stacking process is performed by applying a much thinner layer on the surface that follows a similar topography of the blank PCB. After each post-treatment, the newly deposited thin layer will follow the same topography as the previous layer. Therefore, the topography of the stacked coating is more similar to the blank PCB than the 1 μm single layer coating. From Fig. 17 can be seen that the potential maps have a relatively homogenous distribution. Consequently, it can be said that topographical features did not influence the potential measurements. This homogeneity allows us to perform a quantitative comparison of the outer surface Volta potential of the different samples. For this, a statistical analysis was carried out. After imaging about 10 areas on the surface of each specimen, the average of all the potentials measured was calculated. The obtained data are summarised in Fig. 18. The blank PCB sample shows clearly a very low potential compared to the coated PCB samples, in average 800 mV lower than the sample with 1 μm plasma coating. All the plasma coated PCB samples show relatively higher potential. Moreover, a slight increase of the Volta potential value of the plasma coated samples is observed with the increase of the coating thickness. The plasma coated PCB sample with the thickness of 9 μm shows a higher Volta potential than the plasma coated PCB with the thickness of 1 μm which is due to the influence of the increase in the thickness [55,66]. On the other hand, the measured Volta potential for 3 μm stacked coatings is very close to the Volta potential values of 9 μm plasma coating.

4. Discussion In the present work, the protective properties of plasma coatings on PCBs were studied. Salt spray experiments as a macroscopic corrosion testing showed that the plasma coated samples did not corrode after a week of exposure. The results confirmed the protective properties of the different type of presented plasma coatings against corrosion. ORP-EIS measurements were performed to observe and compare the behaviour of different types of plasma coated PCBs under full immersion conditions at the initial stage of immersion. Modelling the ORP-EIS data revealed more information on both qualitative and quantitative scales regarding these coatings. It was observed that the 9 μm single layer and 3 μm stacked coatings act as a protective barrier, while the 1 μm single layer plasma coated PCB allowed the electrolyte to reach the substrate through the coating. Nonetheless, the impedance magnitude of the 1 μm single layer coating was much higher than the blank PCB samples indicating that the coating does show some initial barrier properties. The SVET measurements showed that initially no activities were observed above the plasma coated samples. The results from SVET measurements are consistent with the obtained results with EIS for the 9 μm single layer and 3 μm stacked plasma coated PCBS. However, it was observed from the ORP-EIS data that the 1 μm single layer plasma coating does not act as a perfect barrier which is in contrast with the obtained results from SVET for this sample. As the results of the ORPEIS measurements were reproducible, it can be supposed that SVET is not able to detect the activities under the coating. Moreover, as discussed before the charge transfer resistance of the 1 μm single layer is significantly higher than the blank sample. This indicates the current density of the electrochemical reaction is probably too small to be detected by SVET. It can also be noted that the 9 μm single layer coating showed the highest activities compared to the other samples. As-produced 9 μm single layer plasma coatings have a sticky-like structure. Antonella et al. studied the influence of the plasma polymerisation process parameters on the final structure and behaviour of the plasma coating. This work showed that the medium power and long process duration results in a coating with a sticky character [67]. This is probably the possible explanation for the behaviour of 9 μm single layer plasma coatings. On the other hand, it was observed that the 3 μm plasma coated PCBs show lower activities compared to the single layer

Fig. 15. SVET measurements on the edges of the copper track on the 1 μm coated PCB immediately after immersion (a) optical image (b) ionic current density map. 11

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Fig. 16. a) Topography and b) Potential map of blank sample’s cross-section; c) potential profile on the red line shown in b) scan size: 20 μm × 20 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

plasma coated samples after 10 days of immersion in the solution. Application of plasma coating on the PCBs resulted in a significant Volta potential shift of almost 800 mV as measured by SKPFM. Studies showed that the shift of Volta potential on the coated samples is attributed to the chemistry and the interfacial bonds between the coating and the surface of the substrate. Titz et al. associated the Volta potential shift to the insulating properties of the films against possible electrochemical reactions. This was also confirmed previously with the SVET results as plasma coatings inhibited the electrochemical reactions as observed on the Blank PCB (Fig. 11). Moreover, it appears that the differences in the measured Volta potential values for the coated samples should be correlated to the thickness [55,66].

Table 6 Nano-scale roughness measured by SKPFM for the tested samples. Sample

Blank PCB

1 μm

9 μm

3 μm Stacked coating

Roughness (nm)

69.7

63.8

17.3

74.1

5. Conclusion The challenge in this study concerns the evaluation of the electrochemical behaviour of coated PCB samples in view of the corrosion protective properties of the thin plasma coatings in a reliable way. We were able to characterise the electrochemical behaviour of plasma coated PCBs by using various electrochemical exposure and analysis techniques, going from an overall macroscopic evaluation given by standard salt spray testing, to a surface-average evaluation of the coating barrier properties by ORP-EIS and to a local reactivity analysis using SVET and SKPFM. A number of conclusions can be drawn from this study:

Fig. 18. Measured Volta potential difference by SKPFM for all the samples shown in a box chart.

- Salt-spray testing gave a visual impression of the corrosion behaviour of the blank and coated PCBs in extreme salt-spray environment. This test is a standard test but very severe in nature. The

coated PCBs all performed much better than the blank PCB confirming their corrosion protective ability, but this test did not allow us to differentiate between the various thin coating systems.

Fig. 17. Topography and potential map on the surface of (a), (b) blank PCB (c), (d) 1 μm plasma coated (e), (f) 9 μm plasma coated (g), (h) 3 μm stacked plasma coated sample. 12

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- ORP-EIS is a method that enables monitoring the surface-average electrochemical behaviour of blank versus coated PCBs. Using this technique, we were able to detect the noise present in the system in order to obtain accurate data. It allowed us to qualitatively and quantitatively compare the different type of coatings by their protective behaviour. To go towards quantification of the protective properties as a function of immersion time in an electrolyte solution, complex dedicated modelling of the EIS data is required. This is the subject of an ongoing study. - SVET and SKPFM allow us to monitor the behaviour of plasma coatings at the local scale on PCBs. By using SVET, local anodic and cathodic activities on the surface of the plasma coated PCBs were detected. SKPFM gives the potential distribution across the sample surface. Both local methods confirmed the initial protective properties of the plasma coatings; additionally, SKPFM could differentiate between coatings, showing a higher potential for thicker coatings. - Even though the single layer 9 μm plasma coated samples showed higher initial protection, they do not perform well for a longer period of time. On the contrary, PCB samples with stacked coatings on top, have shown higher stability and protection against corrosion in comparison to the single layer coatings.

[13]

[14] [15]

[16] [17]

[18] [19] [20]

[21]

[22]

[23]

Data availability

[24]

The raw/processes data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

[25]

Declaration of Competing Interest

[26] [27]

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

[28] [29]

Acknowledgements

[30]

The research is funded by Vlaams Agentschap Innoveren en Ondernemen (VLAIO). The Company Europlasma NV is acknowledged for fabrication of the samples and funding. The authors would like to thank Bart Lippens for assistance with sample preparation.

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