*Article* **Study on the Wetting Mechanism between Hot-Melt Nano Glass Powder and Different Substrates**

**Yifang Liu 1,2,\*, Junyu Chen 1,2 and Gaofeng Zheng 1,2**

<sup>2</sup> Shenzhen Research Institute of Xiamen University, Shenzhen 518000, China

**\*** Correspondence: yfliu@xmu.edu.cn; Tel.: +86-592-2194957

**Abstract:** The wettability of molten glass powder plays an essential role in the encapsulation of microelectromechanical system (MEMS) devices with glass paste as an intermediate layer. In this study, we first investigated the flow process of nano glass powder melted at a high temperature by simulation in COMSOL. Both the influence of the different viscosity of hot-melt glass on its wettability on SiO<sup>2</sup> and the comparison of the wettability of hot-melt glass on Au metal lead and SiO<sup>2</sup> were investigated by simulation. Then, in the experiment, the hot-melt glass flew and spread along the length of the Au electrode because of a good wettability, resulting in little coverage of the hot-melt glass on the Au electrode, with a height of only 500 nm. In order to reduce the wettability of the glass paste on the Au electrode, a SiO<sup>2</sup> isolation layer was grown on the surface of golden lead by chemical vapor deposition. It successfully reduced the wettability, so the thickness of the hot-melt glass was increased to 1.95 µm. This proved once again that the wettability of hot-melt glass on Au was better.

**Keywords:** wettability; hot-melt glass; flow time; coverage thickness; SiO<sup>2</sup> and Au substrate

## **1. Introduction**

Good vacuum packaging [1], even special packaging in a bad environment [2], is an important means to ensure the reliability of MEMS devices. In the middle layer packaging process with nano glass powder, the MEMS sensor can be electrically interconnected with the outside world through the external lead wire of the metal electrode, and the cap, substrate and lead wire can be tightly sealed together by using nano glass powder through hot press bonding. Nano glass powder or the glass frit inter-layer packaging has the advantages of a high tolerance to the surface roughness of the bonding interface, suitable for various materials in the MEMS, electrical insulation characteristics to simplify the electrode lead extraction process and patterning without an additional lithography process by using screen printing [3–5]. It has been widely used in the packaging of the MEMS pressure switch [6,7], MEMS gyroscope [8] and accelerometer [9]. Many scholars only describe the packaging principle, packaging process and packaging results of nano glass powder, but there is no report on both the mechanism of infiltration and flow process of hot-melt glass on the substrate.

After nano glass powder is made on the glass substrate with metal lead through screen printing, during the process of high temperature melting, the wettability of molten nano glass powder on metal lead and the SiO<sup>2</sup> substrate are different due to a different contact angle, surface tension and adhesion work. After cooling and solidification, the adhesion thickness of the glass powder on the metal lead is different from that on the SiO<sup>2</sup> substrate. If the height of the glass powder inter-layer on the Au metal lead is less than 10 µm [10], the package will fail.

In order to improve the results of the direct packaging of nano glass powder in the MEMS structure with metal leads, the wettability of nano glass powder in a hot-melt state was investigated. Firstly, the whole flow process of hot-melt nano glass liquid on silver substrate from the starting point to the material interface wall was simulated by COMSOL.

**Citation:** Liu, Y.; Chen, J.; Zheng, G. Study on the Wetting Mechanism between Hot-Melt Nano Glass Powder and Different Substrates. *Micromachines* **2022**, *13*, 1683. https://doi.org/10.3390/mi13101683

Academic Editor: Wensheng Zhao

Received: 19 September 2022 Accepted: 4 October 2022 Published: 6 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Then, the wettability of the hot-melt nano glass powder with a different viscosity on the SiO<sup>2</sup> substrate was analyzed and compared by simulation. The wetting effect of hot-melt glass with the same viscosity on SiO<sup>2</sup> and Au substrates were also investigated. Finally, it was verified by experiments that the wettability of hot-melt nano glass on Au metal leads was better than that on SiO2, which leads to a too small adhesion thickness. By depositing a SiO<sup>2</sup> isolation layer on the metal leads, the wettability of hot-melt nano glass on a Au metal lead was successfully reduced, so as to improve its adhesion thickness on the Au.

## **2. Simulation Analysis of Wettability**

Wettability is the degree of difficulty for a liquid to adhere to a solid when it contacts with a solid. It is usually determined by the contact angle between the solid–liquid interface and the liquid–gas interface θ. When the contact angle is less than 90◦ , the liquid can wet the solid. When the contact angle is greater than 90◦ , the liquid is difficult to wet the solid. Zhu Dingyi et al. [11,12] studied the corresponding relationship between liquid surface tension, solid surface tension and the contact angle. Guan C.H. [13] researched the impact of surface roughness on solid–liquid wettability. Li Wei [14] obtained the contact angle between the hot-melt glass and different substrates through experiments, and the better wettability was attained by polishing the surface of the material. In reference [15], the adhesion work was calculated by measuring the contact angle. The viscosity µ of liquid affected the velocity difference of each layer in the flow, which was one of the key factors affecting the fluidity of the liquid. Reference [16] verified that viscosity µ directly affected the fluidity of the hot-melt alloy liquid, and 1/µ was used to characterize the relationship between the wettability and the temperature of the hot-melt alloy. However, the simulations of the wettability of liquids with a different viscosity on the same substrate and liquids with the same viscosity on different substrates have not been reported.

## *2.1. Simulation Model*

The hot-melt glass powder was filled into a silicon pit sputtered with a layer of different substrate materials and heated to reflow to fill the whole pit. Assuming that the bottom radius of the hot-melt glass column was 2 mm and the height was 5 mm, the radius of the sphere equal to its volume was 2.47 mm. Taking the bottom radius of the cylindrical container made of the base material as 3 mm, we got the simulation model as shown in Figure 1. *Micromachines* **2022**, *13*, 1683 3 of 9

The material properties of nano glass powder at room temperature were indicated in

2.221 1000 2003.4

Reference [17], the data in Table 2 were preliminarily sorted out and calculated.

**Density (g/cm<sup>3</sup> )**

tension between solid, liquid and gas can be expressed by "Young's formula".

1 SiO<sup>2</sup> 2.2 1723 457.8 2 Au 19.3 1064 1168

According to the relationship between the surface tension and the temperature in

As shown in Figure 2, the relationship between the contact angle θ and the interfacial

γsg、γsl and γlg represent solid–gas interfacial tension, solid–liquid interfacial ten-

; viscosity, 1000 Pa·s; and surface tension, 2003.4 mN/m.

**Melting Point (**°C**)**

γsg = γsl + γlgcosθ, (1)

**Surface Tension at 950**  °C **(mN/m)**

**Figure 1.** Final simulation model. **Figure 1.** Final simulation model.

Table 1: density, 2.221g/cm<sup>3</sup>

**Density (g/cm<sup>3</sup>**

**Serial Number**

**Table 1.** Material properties of nano glass powder at 950 °C.

**Table 2.** Surface tension of silica and gold substrates.

sion and liquid–gas interfacial tension, respectively.

**Substrate Material**

The material properties of nano glass powder at room temperature were indicated in Table 1: density, 2.221g/cm<sup>3</sup> ; viscosity, 1000 Pa·s; and surface tension, 2003.4 mN/m.

**Table 1.** Material properties of nano glass powder at 950 ◦C.


According to the relationship between the surface tension and the temperature in Reference [17], the data in Table 2 were preliminarily sorted out and calculated.

**Table 2.** Surface tension of silica and gold substrates.


As shown in Figure 2, the relationship between the contact angle θ and the interfacial tension between solid, liquid and gas can be expressed by "Young's formula".

$$
\gamma\_{\rm sg} = \gamma\_{\rm sl} + \gamma\_{\rm lg} \cos \theta,\tag{1}
$$

γsg, γsl and γlg represent solid–gas interfacial tension, solid–liquid interfacial tension and liquid–gas interfacial tension, respectively. *Micromachines* **2022**, *13*, 1683 4 of 9

**Figure 2.** Schematic diagram of surface tension at the junction of contact angle and three phase. **Figure 2.** Schematic diagram of surface tension at the junction of contact angle and three phase.

γlg

The corresponding relationship between the liquid surface tension, solid surface tension and contact angle [18,19] was expressed by Equation (2): The corresponding relationship between the liquid surface tension, solid surface tension and contact angle [18,19] was expressed by Equation (2):

$$\gamma\_{\rm sg} = \frac{\gamma\_{\rm lg}}{2} \times \left(\sqrt{1 + \sin^2 \theta} + \cos \theta\right),\tag{2}$$

the substrate material in Table 2, the contact angle formed when the substrate material and the hot-melted glass were infiltrated and could be calculated by Formula (2). The liquid–gas surface tension γlg = 2003.4 mN/m. At the same time, Equation (2) According to the data of hot-melted glass in Table 1 and the surface tension data of the substrate material in Table 2, the contact angle formed when the substrate material and the hot-melted glass were infiltrated and could be calculated by Formula (2).

was transformed as follows: γsg 2 The liquid–gas surface tension γlg = 2003.4 mN/m. At the same time, Equation (2) was transformed as follows:

> γsg γlg ) 2 ] 2

γsg γlg ) 2 − 1] 2

According to Equation (2), when the contact angle is 90°, the solid surface tension is 1416.6 mN/m. Thus, to consider the positive and negative values of cos θ and convert

According to Equations (4) and (5), the contact angles between each substrate and

Adhesion work is the energy released in the process of adhesion. In the process of adhesion, the surface energy of the solid and liquid is lost, and the surface energy of the solid–liquid interface is generated. The calculation formula of the adhesion work was as

W<sup>a</sup> = γsg + γlg − γsl

According to Formula (8), the adhesion work between hot-melted glass and different substrates could be obtained. The contact angle and the adhesion work which were calcu-

W<sup>a</sup> = γsg + γlg − (γsg − γlg cos θ) = γlg(1 + cos θ), (8)

$$\left(\sqrt{1+\sin^2\theta}+\cos\theta\right)^2 = 4 \times \left(\frac{\chi\_{\rm sg}}{\chi\_{\rm lg}}\right)^2,\tag{3}$$

γlg

(γsg < 1416.6 ∩ > 90°), (5)

(γsg > 1416.6 ∩ < 90°), (6)

, (7)

<sup>4</sup> θ = 1 − [2 × (

<sup>4</sup> θ = 1 − [1 − 2 × (

hot-melted glass could be obtained from the data in Tables 1 and 2.

Combined with "Young's formula" (1), we could obtain:

sin

sin

further:

follows:

lated are shown in Table 3.

$$1 + \cos \theta \cdot \sqrt{1 + \left(\sin \theta\right)^2} = 2 \times \left(\frac{\gamma\_{\rm sg}}{\gamma\_{\rm lg}}\right)^2,\tag{4}$$

According to Equation (2), when the contact angle is 90◦ , the solid surface tension is 1416.6 mN/m. Thus, to consider the positive and negative values of cos θ and convert further:

$$\sin^4 \theta = 1 - \left[1 - 2 \times \left(\frac{\gamma\_{\rm sg}}{\gamma\_{\rm lg}}\right)^2\right] \left(\gamma\_{\rm sg} < 1416.6 \cap \theta > 90^\circ\right),\tag{5}$$

$$\sin^4 \theta = 1 - \left[ 2 \times \left( \frac{\gamma\_{\rm sg}}{\gamma\_{\rm lg}} \right)^2 - 1 \right] \left( \gamma\_{\rm sg} > 1416.6 \cap \theta < 90^{\circ} \right) \tag{6}$$

According to Equations (4) and (5), the contact angles between each substrate and hot-melted glass could be obtained from the data in Tables 1 and 2.

Adhesion work is the energy released in the process of adhesion. In the process of adhesion, the surface energy of the solid and liquid is lost, and the surface energy of the solid–liquid interface is generated. The calculation formula of the adhesion work was as follows:

$$\mathbf{W\_{a}} = \boldsymbol{\gamma}\_{\text{sg}} + \boldsymbol{\gamma}\_{\text{lg}} - \boldsymbol{\gamma}\_{\text{sl}\prime} \tag{7}$$

Combined with "Young's formula" (1), we could obtain:

$$\mathcal{W}\_{\rm a} = \gamma\_{\rm sg} + \gamma\_{\rm lg} - \left(\gamma\_{\rm sg} - \gamma\_{\rm lg} \cos \theta\right) = \gamma\_{\rm lg} (1 + \cos \theta), \tag{8}$$

According to Formula (8), the adhesion work between hot-melted glass and different substrates could be obtained. The contact angle and the adhesion work which were calculated are shown in Table 3.

**Table 3.** Contact angle and adhesion work between hot-melt glass and each substrate [14].


## *2.2. Simulation of Wettability of Hot-Melt Glass with Different Viscosity on SiO<sup>2</sup> Substrate*

By changing the viscosity of hot-melt glass from 500 Pa·s to 1000 Pa·s, the influence of the viscosity of the hot-melt glass on the flow velocity and wettability of the hot-melt glass was studied with the SiO<sup>2</sup> as a substrate. Taking the yellow light band as the reference point, the relationship between the viscosity of hot-melt glass and the time needed to flow to the junction of the material bottom and the material wall was explored in this paper. On the SiO<sup>2</sup> substrate, the steady state of hot-melt glass with a different viscosity flowing to the junction is shown in Figure 3, which corresponds to a different flow time.

Therefore, when the viscosity of the hot-melt glass was 1000, 900, 800, 700, 600 and 500 Pa·s, respectively, the time of the hot-melt glass flowing to the specified distance on the SiO<sup>2</sup> substrate could also be obtained, as shown in Figure 4. It could be seen that the lower the viscosity of the hot-melt glass, the shorter the flow time to the specified distance, the higher the flow speed and the better the wettability. For the same SiO<sup>2</sup> substrate, the solid surface energy of hot-melt glass with a different viscosity was the same, but the lower the viscosity was, the higher the wettability was.

**Serial** 

**Table 3.** Contact angle and adhesion work between hot-melt glass and each substrate [14].

1 SiO<sup>2</sup> 457.8 138.2 509.9 3 Au 1168 103.3 1542.5

*2.2. Simulation of Wettability of Hot-Melt Glass with Different Viscosity on SiO<sup>2</sup> Substrate*

By changing the viscosity of hot-melt glass from 500 Pa·s to 1000 Pa·s, the influence of the viscosity of the hot-melt glass on the flow velocity and wettability of the hot-melt glass was studied with the SiO<sup>2</sup> as a substrate. Taking the yellow light band as the reference point, the relationship between the viscosity of hot-melt glass and the time needed to flow to the junction of the material bottom and the material wall was explored in this paper. On the SiO<sup>2</sup> substrate, the steady state of hot-melt glass with a different viscosity flowing to the junction is shown in Figure 3, which corresponds to a different flow time.

**at 950** °C **(mN/m) Contact Angle (°) Adhesion Work** 

**(mJ/m<sup>2</sup> )**

**Number Substrate Surface Tension** 

**Figure 3.** Flow time of hot-melt glass with different viscosity on SiO<sup>2</sup> substrate: (**a**) the viscosity is 1000 Pa·s; (**b**) the viscosity is 900 Pa·s; (**c**) the viscosity is 800 Pa·s; (**d**) the viscosity is 700 Pa·s; (**e**) the viscosity is 600 Pa·s; (**f**) the viscosity is 500 Pa·s. **Figure 3.** Flow time of hot-melt glass with different viscosity on SiO<sup>2</sup> substrate: (**a**) the viscosity is 1000 Pa·s; (**b**) the viscosity is 900 Pa·s; (**c**) the viscosity is 800 Pa·s; (**d**) the viscosity is 700 Pa·s; (**e**) the viscosity is 600 Pa·s; (**f**) the viscosity is 500 Pa·s. the higher the flow speed and the better the wettability. For the same SiO<sup>2</sup> substrate, the solid surface energy of hot-melt glass with a different viscosity was the same, but the lower the viscosity was, the higher the wettability was.

**Figure 4.** Relationship between flow time and viscosity of hot-melt glass on SiO<sup>2</sup> substrate. **Figure 4.** Relationship between flow time and viscosity of hot-melt glass on SiO<sup>2</sup> substrate.

*2.3. Comparison of Wettability of Hot-Melt Glass Solution between SiO<sup>2</sup> and Au Substrates*

Then, the viscosity of the hot-melt glass was kept at 1000 Pa·s, the simulation was carried out on the SiO<sup>2</sup> and Au substrates and the simulation results, as shown in Figure 5, were obtained. It could be clearly seen from Figure 5 that it took 22 s for the hot-melt

parameters in Table 2, the contact angle between the hot-melt glass and the Au substrate was smaller than that of SiO2, and the adhesion work and surface tension on the Au sub-

**Figure 5.** Simulation results of adhesion of hot-melt glass on SiO<sup>2</sup> and Au substrates: (**a**) SiO2; (**b**)

The micro pressure switch was packaged with nano glass powder. The hot-melt glass was transparent and the surface morphology was compact and smooth, as shown in Figure 6. However, because the wettability between the hot-melt glass and the Au electrode

strate were larger, so the wettability was higher and the flow velocity was higher.

Au.

**3. Experimental**

#### *2.3. Comparison of Wettability of Hot-Melt Glass Solution between SiO<sup>2</sup> and Au Substrates 2.3. Comparison of Wettability of Hot-Melt Glass Solution between SiO<sup>2</sup> and Au Substrates*

**Figure 4.** Relationship between flow time and viscosity of hot-melt glass on SiO<sup>2</sup> substrate.

*Micromachines* **2022**, *13*, 1683 6 of 9

lower the viscosity was, the higher the wettability was.

Then, the viscosity of the hot-melt glass was kept at 1000 Pa·s, the simulation was carried out on the SiO<sup>2</sup> and Au substrates and the simulation results, as shown in Figure 5, were obtained. It could be clearly seen from Figure 5 that it took 22 s for the hot-melt glass to flow to the junction on the SiO<sup>2</sup> substrate and 16.5 s on the Au substrate. With the same viscosity, the surface free energy of the liquid was the same. Yet, combined with the parameters in Table 2, the contact angle between the hot-melt glass and the Au substrate was smaller than that of SiO2, and the adhesion work and surface tension on the Au substrate were larger, so the wettability was higher and the flow velocity was higher. Then, the viscosity of the hot-melt glass was kept at 1000 Pa·s, the simulation was carried out on the SiO<sup>2</sup> and Au substrates and the simulation results, as shown in Figure 5, were obtained. It could be clearly seen from Figure 5 that it took 22 s for the hot-melt glass to flow to the junction on the SiO<sup>2</sup> substrate and 16.5 s on the Au substrate. With the same viscosity, the surface free energy of the liquid was the same. Yet, combined with the parameters in Table 2, the contact angle between the hot-melt glass and the Au substrate was smaller than that of SiO2, and the adhesion work and surface tension on the Au substrate were larger, so the wettability was higher and the flow velocity was higher.

lower the viscosity of the hot-melt glass, the shorter the flow time to the specified distance, the higher the flow speed and the better the wettability. For the same SiO<sup>2</sup> substrate, the solid surface energy of hot-melt glass with a different viscosity was the same, but the

**Figure 5.** Simulation results of adhesion of hot-melt glass on SiO<sup>2</sup> and Au substrates: (**a**) SiO2; (**b**) Au. **Figure 5.** Simulation results of adhesion of hot-melt glass on SiO<sup>2</sup> and Au substrates: (**a**) SiO<sup>2</sup> ; (**b**) Au.

## **3. Experimental**

**3. Experimental** The micro pressure switch was packaged with nano glass powder. The hot-melt glass was transparent and the surface morphology was compact and smooth, as shown in Figure 6. However, because the wettability between the hot-melt glass and the Au electrode The micro pressure switch was packaged with nano glass powder. The hot-melt glass was transparent and the surface morphology was compact and smooth, as shown in Figure 6. However, because the wettability between the hot-melt glass and the Au electrode were stronger than that between the hot-melt glass and the SiO<sup>2</sup> substrate, the hot-melt glass flowed rapidly along the length direction of the Au electrode lead and spread out rapidly, resulting in little coverage of this part of the hot-melt glass. After measurement, the thickness of the hot-melt glass on the Au electrode lead was only 500 nm, as shown in Figure 6b. This thickness was not enough to form a sealed package during bonding. *Micromachines* **2022**, *13*, 1683 7 of 9 were stronger than that between the hot-melt glass and the SiO<sup>2</sup> substrate, the hot-melt glass flowed rapidly along the length direction of the Au electrode lead and spread out rapidly, resulting in little coverage of this part of the hot-melt glass. After measurement, the thickness of the hot-melt glass on the Au electrode lead was only 500 nm, as shown in Figure 6b. This thickness was not enough to form a sealed package during bonding.

**Figure 6.** Sintering effect of hot-melt glass: (**a**) slurry morphology; (**b**) measuring diagram of step meter. **Figure 6.** Sintering effect of hot-melt glass: (**a**) slurry morphology; (**b**) measuring diagram of step meter.

The wettability of hot-melt glass to different materials varies greatly [20,21]. From the above simulation and experimental results, it could be seen that the wettability of hotmelt glass on the Au metal lead was good, so the volume of hot-melt glass passing through the Au metal lead decreased sharply. A silicon wafer sputtered when a large area of Au lines was selected and a thin layer of nano glass powder was manually coated on the The wettability of hot-melt glass to different materials varies greatly [20,21]. From the above simulation and experimental results, it could be seen that the wettability of hot-melt glass on the Au metal lead was good, so the volume of hot-melt glass passing through the Au metal lead decreased sharply. A silicon wafer sputtered when a large area of Au lines

whole surface and melted at a high temperature. Figure 7b showed that the amount of hot-melt glass on the Au metal leads was very small, and a small part shrank to the metal

wire was very strong and the adhesion thickness of the glass paste was not as good as that of the silicon or glass. Based on the verification results, it was proposed that a SiO<sup>2</sup> isolation layer should be formed on the surface of the metal lead by chemical vapor deposition

**Figure 7.** Pre-sintering effect of glass slurry on large-area metal circuit: (**a**) metal circuit; (**b**) mor-

The experimental process and results are shown in Figure 8. The SiO<sup>2</sup> isolation layer successfully reduced the wettability of the hot-melt glass on the Au metal lead, and this part of the hot-melt glass was consistent with that on the glass sheet. The thickness of the hot-melt glass increased from 500 nm to 1.95 µm. It could be seen that there was a significant difference between the thickness of the glass powder on the metal lead covered with

to reduce the wettability of the glass slurry in this area.

phology of hot-melt glass.

meter.

was selected and a thin layer of nano glass powder was manually coated on the whole surface and melted at a high temperature. Figure 7b showed that the amount of hot-melt glass on the Au metal leads was very small, and a small part shrank to the metal free area on the silicon wafer. It was proved that the wettability of glass paste on the Au wire was very strong and the adhesion thickness of the glass paste was not as good as that of the silicon or glass. Based on the verification results, it was proposed that a SiO<sup>2</sup> isolation layer should be formed on the surface of the metal lead by chemical vapor deposition to reduce the wettability of the glass slurry in this area. the Au metal lead decreased sharply. A silicon wafer sputtered when a large area of Au lines was selected and a thin layer of nano glass powder was manually coated on the whole surface and melted at a high temperature. Figure 7b showed that the amount of hot-melt glass on the Au metal leads was very small, and a small part shrank to the metal free area on the silicon wafer. It was proved that the wettability of glass paste on the Au wire was very strong and the adhesion thickness of the glass paste was not as good as that of the silicon or glass. Based on the verification results, it was proposed that a SiO<sup>2</sup> isolation layer should be formed on the surface of the metal lead by chemical vapor deposition to reduce the wettability of the glass slurry in this area.

*Micromachines* **2022**, *13*, 1683 7 of 9

were stronger than that between the hot-melt glass and the SiO<sup>2</sup> substrate, the hot-melt glass flowed rapidly along the length direction of the Au electrode lead and spread out rapidly, resulting in little coverage of this part of the hot-melt glass. After measurement, the thickness of the hot-melt glass on the Au electrode lead was only 500 nm, as shown in Figure 6b. This thickness was not enough to form a sealed package during bonding.

**Figure 6.** Sintering effect of hot-melt glass: (**a**) slurry morphology; (**b**) measuring diagram of step

(a) (b)

The wettability of hot-melt glass to different materials varies greatly [20,21]. From the above simulation and experimental results, it could be seen that the wettability of hotmelt glass on the Au metal lead was good, so the volume of hot-melt glass passing through

**500nm**

**Figure 7.** Pre-sintering effect of glass slurry on large-area metal circuit: (**a**) metal circuit; (**b**) morphology of hot-melt glass. **Figure 7.** Pre-sintering effect of glass slurry on large-area metal circuit: (**a**) metal circuit; (**b**) morphology of hot-melt glass.

The experimental process and results are shown in Figure 8. The SiO<sup>2</sup> isolation layer successfully reduced the wettability of the hot-melt glass on the Au metal lead, and this part of the hot-melt glass was consistent with that on the glass sheet. The thickness of the hot-melt glass increased from 500 nm to 1.95 µm. It could be seen that there was a significant difference between the thickness of the glass powder on the metal lead covered with The experimental process and results are shown in Figure 8. The SiO<sup>2</sup> isolation layer successfully reduced the wettability of the hot-melt glass on the Au metal lead, and this part of the hot-melt glass was consistent with that on the glass sheet. The thickness of the hot-melt glass increased from 500 nm to 1.95 µm. It could be seen that there was a significant difference between the thickness of the glass powder on the metal lead covered with a thin layer of SiO<sup>2</sup> and that on the metal lead not covered with SiO2. This proved once again that the wettability of hot-melt glass on a Au substrate was better. *Micromachines* **2022**, *13*, 1683 8 of 9 a thin layer of SiO<sup>2</sup> and that on the metal lead not covered with SiO2. This proved once again that the wettability of hot-melt glass on a Au substrate was better.

**Figure 8.** Isolation layer pre-sintering: (**a**) deposition of SiO2; (**b**) printing glass paste; (**c**) high temperature hot-melt glass; (**d**) characterization of glass powder thickness. **Figure 8.** Isolation layer pre-sintering: (**a**) deposition of SiO<sup>2</sup> ; (**b**) printing glass paste; (**c**) high temperature hot-melt glass; (**d**) characterization of glass powder thickness.

The wettability of the molten glass powder was studied by simulation and experi-

liquid, the greater the wettability and the higher the flow velocity on SiO2. When the viscosity of the molten glass slurry decreased from 1000 Pa·s to 500 Pa·s, the time for the hot-melt glass to flow to the specified interface on the silica substrate decreased

2. The surface tension of Au metal lead was higher than that of SiO2, the contact angle between the Au metal lead and the hot-melt nano glass was smaller and the wettability of the Au metal lead was stronger. When the molten glass slurry with the same viscosity of 1000 Pa·s flowed on the silica and gold substrates, the time to flow to the

3. Compared with SiO2, Au had a higher adhesion work, a faster spreading speed and

4. By depositing a thin layer of SiO<sup>2</sup> on the Au metal lead, the flattening speed of hotmelt glass could be effectively reduced and the adhesion height of the nano glass

**Author Contributions:** Conceptualization, Y.L. and G.Z.; methodology, Y.L.; validation, Y.L. and J.C.; formal analysis, J.C.; investigation, J.C.; resources, Y.L.; data curation, J.C.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L. and G.Z.; supervision, Y.L.; project administration, Y.L.; funding acquisition, Y.L. All authors have read and agreed to the published ver-

designated interface was 22 s and 16.5 s, respectively.

powder could be increased from 500 nm to 1.95 µm.

a smaller adhesion thickness in a limited time.

**4. Conclusions**

from 22 s to 15.4 s.

sion of the manuscript.

## **4. Conclusions**

The wettability of the molten glass powder was studied by simulation and experimentally. The conclusions obtained in this research are summarized as follows:


**Author Contributions:** Conceptualization, Y.L. and G.Z.; methodology, Y.L.; validation, Y.L. and J.C.; formal analysis, J.C.; investigation, J.C.; resources, Y.L.; data curation, J.C.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L. and G.Z.; supervision, Y.L.; project administration, Y.L.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by Guangdong Basic and Applied Basic Research Foundation (2022A1515010949; 2022A1515010923).

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

