Freeze Drying of Organically Modified Layered Silicates as Key Step to Improve the Barrier Properties of Organic Coatings for High Power Electronics Protection

Organically modified layered silicates can be used as barrier pigments in organic coating systems to reduce the permeation of water vapor and gases, e.g., oxygen as well as harmful gases that promote corrosion. Unmodified layered silicates are not sufficient to reduce the permeation significantly. However, the arrangement and orientation of the particles have an enormous impact on the later performance in the barrier film. In organic solvents or non‐aqueous polymer matrices the non‐modified layered silicates cannot exfoliate adequately. The organic modification influences the layer spacing and the compatibility to organic solvents. Layered silicates are modified with dodecylamine to enhance the spacing layer. The spacing layer is enhanced by 55% caused by the organic modification as it is commonly known. In contrast to previous work freeze drying of the organically modified layered silicates improved the particle distribution and by this barrier properties significantly. The modified particles reduce the diffusion of water and oxygen by ≈99.6% and 97.2%, respectively. The corrosive gas test of electronic circuit boards shows no growth of dendrites and no failure of the electronics in contrast to protective coating without the layered silicates.


Introduction
With increasing electrification of industrial devices, power electronics are used in a wide spectrum of different application conditions and climates.Especially power semiconductor devices, with their non-hermetically sealed packages are directly influenced by the surrounding climate and the ingress of humidity and pollutants can lead to corrosion at the metal interface.Highly reactive contaminants like hydrogen sulfide (H 2 S) are able to diffuse through the encapsulation material (silicone gel) and cause electro-chemical migration (ECM) at the copper-ceramic-substrate in presence of high humidity.As a result, Copper Sulfide (CuS/Cu 2 S) dendrites are formed in the insulation trenches [1] and these conductive paths cause destruction of the device under the influence of high electrical fields or steep transients.Therefore, the development of corrosion resistant packaging materials is mandatory to fulfill the lifetime requirements in modern power electronics applications.A polymeric layer with adapted barrier pigments, e.g.layered silicates can be used to minimize the diffusion of moisture or H 2 S.
Layered silicates in polymeric matrices were used in the last 25 years to enhance various properties of polymers.[12] Nevertheless, the layered silicates are not used exclusively in their natural form for the different applications.[15] This surface modification is achieved by the intercalation of organic molecules in the layered silicates, to exploit the full potential of the inorganic material.
The concept of a barrier to decrease the transport of permeates through a polymeric coating by layered silicates is based on the elongation of the diffusion path.Figure 1 shows the schematic structure of a barrier coating with plate like layered silicate particles.In an ideal case, the particles are arranged in a brick like arrangement with an orientation parallel to the surface.Previous Figure 1.Schematic structure of a barrier coating with plate like particles to extend the diffusion pathway. [23]udies show that thin plates with a high aspect ratio are particularly suitable to form a good barrier.8] Equation (1) shows the ratio of permeation through a film without barrier pigments ( Q0 ) and permeation through a film with barrier pigments ( QN ).This ratio is determined by the filling degree ϕ, the particles aspect ratio , and the free volume .The aspect ratio  includes the distance between the end of two particles in the same plane (2s), the distance of the mid of two particles in the same plane (2d), the thickness of the particle itself (a), and the vertical spacing of particles (b).Layered silicates feature a high aspect ratio of up to 500 for the application as barrier pigments. [19,20]In practice the barrier increase is lower than calculated as the layers are not as perfectly exfoliated and orientated as presumed by the equation.
To achieve a brick like structure in the barrier coating, the layered silicates must be exfoliated in the solvent or polymer, that are used in the formulation of the coating.To build a barrier with a sufficient performance, the particles must be homogeneously distributed and orientated parallel to the surface after the application.In this work, the modification of layered silicates by an ion exchange reaction is carried out.In contrast to many previous articles, [21][22][23] the modified layered silica was freeze dried instead of oven dried.For the improvement of combustion properties of epoxy resins by layered silicates it was shown by Schartel et al. [24] that the freeze-dried layered silicate is superior compared to spray or oven dried ones.We postulate that also the barrier properties of a coating can be improved if the layered silicate is already pre-exfoliated in the dry state.It is the aim of this paper to show this.For barrier coatings for the electronic industry the enhanced barrier properties could be used in coatings to protect semiconductor chips.In previous work this was already examined but was only focused on unmodified layered silicates. [10]

Synthesis of Organically Modified Layered Silicates
To modify the layered silicates, two precursor solutions were prepared.For the first solution 20 g Cloisite Na + were mixed with 400 mL deionized water at 80 °C in a 1000 mL beaker and was stirred for 1 h to exfoliate the particles.The second solution contained 8.82 g (0.048 mol) dodecylamine (DDA) in 100 mL deionized water.5.172 g (0.16 mol) hydrochloric acid was added, and the mixture was stirred at 80 °C for 1 h to protonate the amine groups.After 1 h the amine solution was added to the silicates, and 200 mL deionized water was added additionally.The resulting silicate-DDA dispersion was stirred for 1 h at ambient temperature.
Afterwards, the resulting dispersion was filtrated with Rotilabo round filter type 12A (Carl Roth) with the aid of a vacuum pump.The filtrated silicates were washed three times with 200 mL deionized water and filtered again.The product was transferred into a 500 mL round bottom flask and was directly frozen with liquid nitrogen.The flask was attached to a laboratory freeze dryer Alpha 1-4 from Christ and the product was dried at −18 °C at 0.1 mbar for 72 h.
Oven drying particles were taken after filtration into a crystallizing bowl.The bowl with the organically modified layered silicates was placed in an oven.The drying of the particles was performed at 60 °C for 72 h.
BET surface was measurement with a BET Nova 2000e from Quantachrome.Before the measurement the samples were dried for 24 h at 50 °C and 1 h at 80 °C.

Formulation, Application, and Curing
The formulation of a barrier coating with modified layered silicates CNa + -DDA was carried out as described previously. [10]The polyimide was dissolved in DMF.The modified layered silicate was also dispersed in the solvent, the dispersing agent was added, and the mixture was stirred at 80 °C for 12 h.This particle slurry was combined with the polyimide solution.The dispersion was heated to 140 °C while stirring continuously for 1 h.Table 1 shows the compositions used in this work.
The coatings were applied onto the PET substrates with a TQC Sheen four-sited film applicator.All coatings were applied with a wet layer thickness of 200 μm.Curing of the applied coatings was performed in three steps.After drying the barrier coatings for 10 min physically at room temperature, the samples were exposed to UV radiation from a mercury lamp with an intensity of 0.08 W cm −1 for 5 min.Finally, the barrier coatings were cured in an oven for 20 min at 150 °C.
For lifetime tests, the coatings were applied onto the test boards with a spray gun using a pressure of 2 bar.The curing process did not change.The layer thickness of the dry barrier coatings on the PCB test structure was 15-20 μm.

Characterization
X-ray photoelectron spectroscopy (XPS) measurements were executed with an Escalab Xi+-System from Thermo Fischer Scientific.Excitation was with monochromatic AlK radiation.The acceptance angle of photoelectrons was 0°.A Constant Analyser Energy mode (CAE) with 150 eV pass energy for the overview spectra was chosen.The high-resolution spectra were recorded with 20 eV.The sample area was 0.65 mm 2 , and the neutralisation of electrically non-conductive samples was carried out by a combination of low-energy electrons and low-energy argon ions.
X-ray diffraction (XRD) was used to determine the layer spacing of the silicates.The diffractograms were recorded using the MiniFlex 600 from Rigaku.The excitation was carried out with CuK radiation in a measuring range of 2-80°with a step size of 0.03°and a measuring time of 5 s per angle.
The chemical composition of the silicate particles after modification reaction was determined by FTIR spectroscopy.These measurements were performed with an Alpha II Platinum ATR from Bruker.The resolution was 2 cm −1 , and each spectrum consists of 32 scans in the range of 4000-400 cm −1 .
SEM analyses were performed with a FEI Helios Nano Lab 600 (DualBeam) from Thermo Fisher Scientific.For sample preparation, the coatings including the PET substrates were cut into small specimen.The samples were embedded in Spurr epoxy resin, which was hardened for 12 h at 70 °C.Subsequently, cross-sections were cut from the samples using the Ultra Microtome Leica UltraCut CC.To prevent electrical charging, the crosssections were vapor-coated with a 5 to 10 nm thick carbon layer.
Light microscope images were performed with an Axio Imager M1 from Zeiss.Samples were investigated without further preparation.Twenty times magnification was chosen for investigation the test boards after lifetime test.
The diffusion of water vapor and oxygen was determined by a Labthink C406H tester according to ASTM D3985, ASTM F1249, and ISO 15106-2.Circular samples with an area of 50 cm 2 were cut from the coated PET films and clamped into the tester.Table 2 shows the measurement parameter for WVD and OD.
For corrosive gas tests with H 2 S, two test boards were prepared, one board with a barrier coating with unmodified silicates and one with the barrier coating with DDA modified particles.Cured boards were placed in a sample holder and embedded with a silicone gel (Table S1, Supporting Information), to generate a closed system without external influences (Figure S1, Supporting Information).The samples were placed in the chamber (Figure S2, Supporting Information) and were preconditioned at 45 °C and 93% relative humidity for 3 h.After preheating, the chamber was supplied with 25 ppm H 2 S gas.The test starts 20 min after  gas supply with a voltage ramp from 50 V s −1 to 1 kV.This was equivalent to an electrical field of 1 kV mm −1 on the test boards.
The test was performed with these conditions for 125 h, while the chamber atmosphere was changed three times per hour.After the test, the voltage ramp was settled down, the gas supply was stopped, the sample chamber was flushed with moist air for 1 h, and the samples were dried for 24 h at 50 °C in air.The evaluation of the samples was performed by a functionality test and with the examination with a microscope.To compare the results to uncoated circuits an additional pure board without coating was measured under the same conditions. [1]

Results and Discussion
The layered silicate modified with DDA was obtained as an offwhite very fluffy powder and was characterized by XPS and XRD measurements.XPS was used to control the ion-exchangereaction. Table 3 shows the composition of the particles before and after modification with DDA.The natural pure particles consist mainly of the expected and described silicon oxide lattice.
The content of sodium ions on the surface is 3%.After modification, the carbon and nitrogen amount increase.Simultaneously, the sodium disappears.Furthermore, the amount of protonated amine is 100%, proving that the sodium vacancies were exchanged completely with the protonated DDA.The proportions of silicon and oxygen are decreased due to the changed distribution caused by the additional insertion of the organic modifier.
To verify the ion exchange reaction and the expected layer spacing increase XRD diffractograms were recorded.Figure 2 shows the shift of the first peak to smaller angles.To quantify the variation of the two samples the Bragg equation (Equation 2) was used to calculate the spacing distance between silicate layers.
The unmodified layered silicate has a spacing layer distance of 1.19 nm and the modified one of 1.84 nm, respectively.Hence, the distance between two layers was widened by ≈0.65 nm or 55%.This corresponds to known values [25,26] and shows that the layer distance does depend on the drying technology as we used freeze-drying in contrast to oven-drying to obtain a light fluffy powder instead of a common mineral powder.Widening and hydrophobization of the layered silicates enables penetration of the interspaces with organic compounds and increases the probability of intercalation and exfoliation of the particles.The recorded XRD diffractograms of freeze-dried and ovendried layered silicates are shown in Figure 2. If the two types of drying are compared with each other, it becomes apparent that the freeze dried modified layered silicates have a significantly higher layer spacing of 2.32 nm than the oven-dried silicates with 1.84 nm.The difference in expansion is 26%.This widening of the layer spacing shows the significance of the drying technology on the pre-exfoliation of these modified layered silicates.BET surface measurements underline this postulate.The surface of ovendried silicates is significantly lower in comparison to the surface of the freeze-dried particles (Table 4).
For the application of the particles as barrier pigments in coatings a larger spacing and BET surface improves the possibility to wet the singles particles surface by solvent and polymeric materials which is a prerequisite for proper particle distribution and ideally exfoliation.It is known that the barrier properties of a layered silicate containing coatings are as higher as better the particles are distributed (and aligned).In the follow-  ing strongly improved barrier properties are demonstrated resulting from freeze drying of the organically modified layered silicate.
IR spectroscopy confirmed the organic modification.Figure 3 shows the IR spectra of both particle types.The spectrum of the unmodified particles shows the characteristic signals of a layered silicate.[29] The broad vibration between 3500 and 3000 cm −1 is assigned to water molecules, adsorbed by the material. [27]The strong Si-O band of the crystallite can be observed from the signal at 994 cm −1 [27,29] Bands at 512 and 440 cm −1 are evoked by the Si-O-Al and Si-O-Si vibrations. [29,30]he spectrum of the modified silicate particles shows the same characteristic bands for the hydroxyl groups at 3622 cm −1 , the water uptake at 3500-3000 cm −1 , and the Si-O, Si-O-Al, Si-O-Si bands at 1000, 515, and 442 cm −1 .Slight peak shifts indicate the attachment of hydrogen bonding on the surface, caused by the protonated DDA.Compared to the unmodified montmorillonite, the vibrational band between 3500 and 3000 cm −1 becomes stronger and the right shoulder is more distinct.This effect was induced by the NH 3 + signal of the protonated amine group, which also appears at 3500-3300 cm −1 .After adsorption of dodecyl-ammonium on the surface of the layered silicate the spectrum has two peaks at 2922 and 2851 cm −1 , respectively compared with unmodified montmorillonite.33] For a sufficient barrier performance, the arrangement and orientation of the particles within the coating layer is important.For this aluminum sheet metal was coated and cross-section examined by SEM.In Figure 4 the differences between coatings containing non-modified layered silicates and the modified version are shown.In the coating with unmodified layered silicate areas with agglomerated particles are identified.The light grey to white particles show nearly no delamination and exfoliation in the poly-meric matrix.The original natural stacks of silicates still exist, and a brick like structure by intercalation with the polymer was not formed.Also, the orientation of the pigments is not sufficient to evoke barrier properties of the coating.The majority of the particles are not aligned parallel to the surface.
The coating with modified layered silicates shows a fundamentally different arrangement of the particles.The barrier pigments are agglomerated in a different way and show a homogeneous distribution within the layer.The particles consist of loose aggregates of the single layers which are aspheric and aligned parallel to the surface.The particles show an arrangement, where direct diffusion pathways through the layer are hardly detectable.In accordance with the model in Figure 1 the barrier coating with DDA modified montmorillonite presents a suitable candidate to attain a significant barrier against diffusion of water and gases through the polymeric matrix, and the associated propagation of these corrosives to the surface of the electronic device to be protected.
To investigate the barrier properties of the films, WVD and OD were determined.Figure 5 shows the transmission rates of WVD and OD measurements of the investigated samples.Pure PET foil shows the highest transmission rates with both gases.The sample, which is coated only with the polymer, shows a slightly higher permeation resistance against water and oxygen and the values for the coating with the unmodified silica are not much better.The addition of modified layered silicates leads to a stronger barrier, as expected, and described in previous work. [10]odified layered silicates reduces the permeation of water vapor and oxygen drastically.Compared to the PET foil the coating reduces the permeation of water vapor by 99.6% and oxygen by 97.2%, respectively.Other systems based on polymeric nanocomposites with layered silicates show significantly lower barrier properties of 100 g m −2 d −1 (WVD) for poly (lactic acid)/MMT or 3000 cm 3 m −2 d −1 (OD) for Polyethylene/cycloolefin copolymer/MMT nanocomposites. [34,35]hese results display the enormous effect of the modification compared to unmodified particles and are much higher as known from literature for only oven dried layered silicates. [36,37]This confirms the strongly improved particle distribution and orientation due to freeze drying resulting in very fluffy powder with high BET surface.
The enhanced corrosion protection was examined with the corrosive gas test under influence of hydrogen sulfide.Figure 6 shows the sample after the test with no additional coating on the surface of the board (left), the coating with unmodified particles (middle), and the barrier coating with modified layered silicates.On the uncoated chip the growth of dendrites between the conducting paths of the semiconductor chip is visible.The dendrites are an indication of corrosion and promote the disruptive discharge of the device up to a short circuit.Furthermore, the functionality of the board is no longer provided after the test, the sample failed.Corrosion products of the material cause the dark coloring of the conducting path.In the middle, the chip with the coating containing unmodified layered silicates can be seen.Dendrites can be observed, but the growth is reduced compared to the first sample.Nevertheless, the test board also failed in the conduction test.The dendrites lead to a short circuit and destroy the chip.The last chip with the barrier coating containing the modified layered silicates is shown on the right side.The chip has no discolorations on the conducting path and no growth of den- drites are visible after 125 h of testing.Furthermore, the chip is still intact.The results illustrate that corrosive gases, like H 2 S, are hindered to treat the surface by the barrier coating.A protection against corrosive gases is shown and it could be proven that this investigated barrier coating leads to a longer resistance against corrosion induced by moisture or gases.

Conclusion
Freeze-drying of organically modified layered silicates is a key step to improve the barrier properties of organic coatings containing these silicates as barrier pigments.By testing the effect of dodecylamine as organic modifier, this study ascertained that freeze-dried organically modified silica are pre-exfoliated and can be used to perform a significant increase of barrier in a coating.In contrast to many previous publications modified layered silica were freeze-dried, instead oven dried, to obtain fluffy pre-exfoliated powder.The spacing layer of organically modified particles were widened by 55% by the modification with dodecylamine.This shift in the XRD diffractogram, the higher BET surface and the XPS measurements, indicating a complete ion exchange, show clearly the pre-exfoliated state of the particles after modification and freeze-drying.A coating, which contains the modified silicate, decreases the permeation of water vapor and oxygen by 99.6% and 97.2%, respectively compared to a PET foil.The orientation and arrangement of the particles changes drastically to a brick like structure by modification of the silica.This leads to an improved barrier effect, as molecules are prevented from migrating through the coating by extending the diffusion path.Corrosive gas testing also shows the  improvement of barrier properties.Electronic circuit test boards are still working after the test and no failure by a short circuit can be observed.However, details of the particles and coatings morphology will be examined and presented in near

Figure 2 .
Figure 2. Normalized low angel section of the XRD diffractograms of unmodified and modified layered silicate particles.

Figure 3 .
Figure 3. FTIR spectra of unmodified and modified layered silicate particles.

Figure 4 .
Figure 4. SEM images of barrier coatings with unmodified (left) and modified layered silicates (right).

Figure 5 .
Figure 5. Permeation measurement of PET foil, Polyimide polymer on PET foil, barrier coating with unmodified silicates on PET, and barrier coating with DDA modified silicates on PET.

Figure 6 .
Figure 6.Light microscope images of non-coated chip (left), a sample coated with a coating with unmodified layered silicates (middle), and a coating with DDA modified particles (right) after the corrosive gas test.

Table 2 .
Parameters of permeation tests.

Table 3 .
Composition of unmodified (CNa + ) and modified (CNa + -DDA) layered silicates from XPS measurements with amount of protonated nitrogen.
Composition of the sample [at.%]

Table 4 .
BET surfaces of prepared particles.