Electrostatic bonding of engineering materials

TWI Bulletin, October 1985

by Dick Sharples

Dick Sharples, BA, is a Research Metallurgist in the Materials Department.

The advantages and disadvantages of the electrostatic bonding (ESB) process are discussed in relation to existing glass-to-metal joining technology, and the results are presented of some preliminary bonding trials aimed at evaluating the use of ESB for applications involving commercially available engineering materials. These indicate that the technique could be used for non load-bearing applications.

Electrostatic bonding was first reported in 1969, ^[1] and offered the possibility of joining glasses to metals and semi-conductors at temperatures well below the softening point of the glass*. The components to be joined must be polished to a smooth flat surface finish, placed in contact at a temperature high enough to ensure adequate ionic conductivity and subjected to an applied DC voltage such that the glass is at a negative potential with respect to the metal.

* The change of glass viscosity with temperature is gradual, and the softening point is arbitrarily defined as the temperature at which the viscosity equals 10 ⁷ Nsecm- ² .

Present theory ^[2] suggests that, as for the glass-to-metal bond in enamelling, the presence of a layer of oxide is necessary for bonding to occur. Upon application of a voltage, intimate contact is facilitated by electro-chemical etching of the high spots between glass and metal. As a result of the applied voltage, the mobile cations of glass ( e.g. Na ⁺ and K ⁺ ) migrate away from the interface under the influence of the electric field ( Fig.1). The layer of glass adjacent to the anodic substrate becomes depleted in mobile charge carriers, the corresponding negative charge of the silicon-oxygen network being effectively immobile, and an increasingly large part of the voltage drop occurs across this layer ( Fig.1a). A high electric field strength is developed at the interface, and it is this that provides the force to close the gap between glass and metal surfaces and allows a chemical bond to develop.

Fig.1. Changes occurring during electrostatic bonding on application of voltage (left) and during electrostatic bonding (right)

a) voltage distribution;

b) electric field strength distribution;

c) joint configuration

The process has found a limited number of applications, almost exclusively within the field of electronics, and these are concerned with specific combinations of rather specialist materials ^[3] (e.g. Pyrex-to-silicon for solar cell encapsulation or borosilicate glass-to-Kovar for encapsulation of opto-electronic devices). However, in principle the process should have a broader potential for other joints involving glassto metal ( e.g. sealing of radioactive waste, laser gyros and flange windows).

Joining of glass to metal

Conventional techniques for the joining of glasses to metals may be divided into direct bonding (in which the glass is usually heated to a temperature sufficient to allow it to deform into contact with the metal and to bond with itchemically), mechanical joining, soldering and brazing, and adhesive bonding.

Demands made of a glass-to-metal joining technique include:

1. The use of low temperatures in joining - direct bonding and brazing techniques involve relatively high temperatures which, because of expansion mismatch, result in the development of high stresses in the region of the joint on cooling. In addition, the properties of some components, particularly in the microelectronics industry, may be impaired by high temperatures experienced during fabrication. Both of these problems are reduced by the lower temperatures involved during electrostatic bonding.
   
   
2. Hermeticity - most joints between glass and metal are made for applications requiring hermeticity ( e.g. for the protection of microelectronic packages from the environment, or for components of vacuum systems). However, those techniques such as adhesives and mechanical seals, which allow the joint to be made at relatively low temperatures and thereby reduce the problems discussed above, do not, in general, ensure hermeticity. The ESB process is capable of producing a hermetic seal, ^[4] despite the lower temperatures required for bonding.
   
   
3. Limited distortion - the direct bonding technique will commonly involve deformation of the glass, as a result of the use of temperatures in excess of its softening point, and this may be unacceptable. The use of temperatures below the softening point of the glass means that ESB is capable of joining glass to metal without distorting the glass component.
   
   
4. High temperature service - adhesively-bonded joints suffer the most serious limitations in terms of allowable service temperature, although soldered joints and those mechanical joints made by a 'shrink-fit' technique are also affected. Joints made by ESB consist of a direct chemical bond and are, in this respect, similar to conventional direct bonds which, along with brazed bonds, generally exhibit the best high temperature service capability attainable with conventional techniques. At the same time, the fact that the joint is made directly between glass and metal means that there is no mechanism for accommodation of strains developed by expansion mismatch, as there would be in a joint brazed with a ductile filler metal for example.
   
   

It is apparent that ESB may have a place most especially where hermetic seals are required to be made at joining temperatures which must be low enough to avoid damage to the components ( e.g. distortion of the glass) or to minimise stress developed in the region of the joint as a result of thermal expansion mismatch.

The limitations of the ESB process arise from the stringent demands placed upon surface finish and surface cleanliness, together with the necessities for the glass and metal to have compatible expansion coefficients and for theglass to possess sufficient ionic conductivity at the bonding temperature. However, other glass-to-metal joining techniques such as adhesives, brazing and soldering also require careful preparation of bonding surfaces (e.g.metallisation in the latter two cases) and all techniques, to some degree, require matching of glass and metal expansion characteristics.

Research at the welding institute

Experimental details

Materials

Bonding trials were performed between common engineering steels (BS 970: Part 1 080A40 and 030Mo7) and several glasses, and between the structural Al-Mg-Si 6082 alloy and glass. The glasses were commercially available borosilicatetypes intended for enamelling steel and aluminium, and were chosen using available data so as to have, as far as possible, thermal expansion characteristics compatible with those of the metals. The softening point temperatures of theglasses bonded to steel were ~520°C, whilst that of the glass bonded to aluminium was 460°C.

Equipment and procedure

Trials were carried out with The Welding Institute's ESB equipment (described in ref. ^[3] ) which enables bonding pressure, temperature and applied voltage to be preset and automatically activated at, and for, set times. Bonds were made under different conditions, defined by the main bonding parameters oftemperature, pressure, voltage and time, to examine the influence of such procedural variables upon bond strength. It was found that the applied voltage was limited by breakdown of the air between the platens and hence could beincreased as temperature was decreased. In addition to the four main procedural variables, the current passing through the glass was recorded during bonding.

Preparation and assessment of specimens

Metal specimens of 15mm diameter were bonded to glass discs of similar diameter and ~ 1 mm thickness ( Fig.2). The metal specimens were bonded, after ultrasonic cleaning, in a lapped condition thereby ensuring a macroscopically flat surface although one with a slightly higher roughness average (RA < 0.l µm) than that ofa polished surface. The glass discs were supplied by UQG Ltd, Cambridge, in an optically flat and polished condition, and were cleaned prior to bonding. Bond strength was assessed using a simple shear test ( Fig.3); when a figure for the shear stress at fracture is given, it is an average figure derived by assuming that bonding had occurred over the whole glass-to-metal interface (although this was often not the case).

Fig.2. Glass disc electrostatically bonded to steel specimen (steel specimen has had a step machined into it, to reduce the likelihood of arcing between it and the platens)

Fig.3. Shear testing rig:

a) Side view where P is shear force and F is clamping force;

b) Plan view

Such figures will therefore always be underestimates of the actual bond strength.

Results

Steel-to-glass

Bonds of sufficient strength to warrant testing were achieved under the following range of conditions.

The results of shear testing are shown in Fig.4 and 5 and, whilst the strengths are low when compared with those of the steel and the glass, they are significantly higher than previously achieved in shear tests of electrostatic bonds made between silicon and Pyrex, and aborosilicate glass and Kovar. ^[3] Cracking of the glass commonly occurred whilst the bond was cooling down to room temperature, particularly from the higher bonding temperatures. All bonds failed at the glass-to-metal interface.

Fig.4. Shear strengths of bonds between steel (030M07) and enamel glass 2 using a bonding time of 300sec and applied voltages, 500V at 400°C, 800V at 350°C, 1000V at 300°C

Aluminium-to-glass

Bonds of sufficient strength to warrant testing were achieved under the following range of conditions:

- Current

Particularly on cooling from 250°C, the glass tended to crack to such an extent as to render the bond unsuitable for shear testing. The results of the shear tests performed are shown in Fig.6. The stronger bonds fractured within the glass and not at the glass-to-metal interface ( Fig.7), and thus the bond strengths measured are minimum values.

Discussion

The clearest trend to emerge from the results ( Fig.4 and 6) is that bond strengths increase with bonding temperature. Whilst Fig.4 and 6 demonstrate that the significance of applied voltage is secondary to that of bonding temperature, Fig.5 indicates that, at a constant temperature, bond strength does tend to increase with applied voltage, Indeed, the general procedure adopted was to apply the maximum voltage possible, at a given temperature, consistent with the avoidance of electrical breakdown of air or glass. There appear to be no drawbacks associated with this procedure and it would seem that it can be generally recommended for electrostatic bonding.

Fig.5. Shear strengths of bonds between steel (030M07) and enamel glass 2 using a bonding temperature of 400°C, and bonding pressure of 12Nmm ^-2

Although Fig.4 and 5 do suggest that bond strength increases with pressure and time respectively, no consistent trends emerged and all that can be concluded with confidence is that the strongest bonds were achieved at the highest pressure (12Nmm^-2 ) and for bonding times > 100sec.

Fig.6. Shear strength of bonds between aluminium (6082) and enamel glass with a bonding time of 100sec and bonding pressure of 12Nmm ^-2

Fig.7. Concoidal fracture within glass of shear tested bond between aluminium alloy and glass

The current flowing through the glass during bonding was found to decrease with time, indicating an increase in resistance in accord with theory. ^[1] No correlation between bond strength and either the initial magnitude of the current or the total charge passed was found. This however contrasts with work by Arata et al ^[5] who found a non-bonding/bonding transition as the total charge passed was increased. All that can be concluded from the reported work is that the passage of some current is necessary for bond formation.

Whilst the relatively low bonding temperatures involved are advantageous in the reduction of stresses developed on cooling because of expansion mismatch, nevertheless cracking commonly occurred, particularly on cooling from the higher bonding temperatures. Choice of metal and glass with more closely matching expansion characteristics than those used in the reported work would be possible for a particular application, since expansion characteristics of glass may be 'tailormade' to some degree. Further attention to joint design could be beneficial in alleviating thermal strains.

Summary

Electrostatic bonding was shown to be capable of producing bonds between engineering grades of aluminium and steel, and commercially available glasses, at temperatures significantly below the softening points of the glasses. The practical advantages of such a low temperature joining process include a reduction in thermal stresses resulting from mismatch of expansion coefficients, the absence of distortion of the glass component and the avoidance of damage to temperature sensitive components.

Although recorded bond strengths were considerably less than those of the corresponding parent aluminium and steel alloys, such strengths would be adequate for non load-bearing engineering applications. In the case of the joints between aluminium and glass, bond strength commonly exceeded that of the glass in the test. Of the four bonding parameters investigated, temperature appeared to be the most critical and, to a limited extent, reductions in bonding temperature could be compensated for by increasing the applied voltage.

References