PROCESS, USE AND ARTICLE

Abstract

A process for the production of an article of glass with an applied electrically conductive grid comprising: a) applying a conductive paste to a glass substrate; b) firing the paste to form the electrically conductive grid; and c) soldering an electrical connector to the electrically conductive grid via a lead-free solder; wherein the conductive paste comprises finely divided particles of a conductive metal, particles of glass frit, and an organic medium; and wherein the particles of the glass fit have a particle size D.sub.90 of less than 4 microns.

Claims

1. A process for the production of an article of glass with an applied electrically conductive grid comprising: a) applying a conductive paste to a glass substrate; b) firing the paste to form the electrically conductive grid; c) soldering an electrical connector to the electrically conductive grid via a lead-free solder; wherein the conductive paste comprises finely divided particles of a conductive metal, particles of glass frit, and an organic medium; and wherein the particles of the glass frit have a particle size D.sub.90 of less than 4 microns.

2. The process as claimed in claim 1, wherein the particles of the glass frit have a particle size D.sub.90 of less than 3 microns, preferably less than 2 microns.

3. The process as claimed in claim 1, wherein the glass frit comprises bismuth and/or zinc borosilicate compounds.

4. The process as claimed in claim 1, wherein said glass frit is present in an amount of 0.1-10% by weight of the total weight of said paste.

5. The process as claimed in claim 1, wherein said conductive metal is present in an amount of 50-90% by weight of the total weight of said paste.

6. The process as claimed in claim 1, wherein said conductive metal is silver.

7. The process as claimed in claim 6, wherein said silver has an average particle size of 0.1 to 15 microns.

8. The process as claimed in claim 1, wherein said organic medium comprises of ethyl cellulose, terpineol, and butyl carbitol.

9. The process as claimed in claim 1, wherein said organic medium is present in an amount of 5-50% by weight of the total weight of said paste.

10. The process as claimed in claim 1, wherein the paste further comprises a transition metal oxide in an amount of 3-15% by weight of the total weight of said paste.

11. The process as claimed in claim 1 wherein the lead-free solder comprises tin, silver, and copper.

12. The process as claimed in claim 11, wherein the lead-free solder further comprises nickel, cobalt, zinc or bismuth.

13. The process as claimed in claim 1, wherein step a) is carried out by screen printing.

14. The process as claimed in claim 1, wherein step a) is carried out by ink-jet printing.

15. (canceled)

16. A method for improving the strength of adhesion of an electrical connector to a conductive grid on a glass substrate, comprising: utilizing a conductive paste comprising: finely divided particles of a conductive metal, particles of glass frit having a particle size D.sub.90 of less than 4 microns, and an organic medium.

17. An article of glass with an applied electrically conductive grid, which electrically conductive grid is powered through a connector comprising a lead-free solder and comprises a fired conductive paste comprising finely divided particles of a conductive metal, particles of glass frit having a particle size D.sub.90 of less than 4 microns, and an organic medium.

18. A vehicle comprising an article of glass with an applied electrically conductive grid, wherein the article of glass is obtained or obtainable by the process as claimed in claim 1.

19. (canceled)

Description

DETAILED DESCRIPTION

[0030] Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

[0031] Suitable lead-free solders include solders of the SAC and SACX type referred to above.

[0032] The conductive metal of the conductive paste for use in the present invention may be selected from silver, gold, platinum, palladium and mixtures thereof. Preferably, the conductive metal is silver.

[0033] The conductive paste may contain the conductive metal in an amount of, for example, 50-90% by weight of the total weight of said paste. For example, the content of the conductive metal is in the range of 60% to 88% by weight of the total weight of said paste.

[0034] In a particular embodiment, silver flakes or silver powder may be used. Alternatively, a mixture of silver flakes and silver powder may be used. In relation to technical effect, there are no particular limitations to the particle size of the silver, but in general a particle size D.sub.90 of 0.1 to 15 microns, and especially 0.5 to 5.0 microns, or 1 to 5 microns, is preferred. The term D.sub.90 herein refers to particle size distribution, and a value for D.sub.90 corresponds to the particle size value below which 90%, by volume, of the total particles in a particular sample lie.

[0035] When the silver particles are larger than 15 microns, the coarseness of the particles slows the sintering process and makes it difficult to achieve the desired resistivity. Particles that are too coarse may also lead to screen blockage, negatively impacting the application process, and lead to a poor end result. When the silver particles are smaller than 0.1 micron, the sintering process may proceed too rapidly, resulting in undesirable effects, such as the rising of glass to the surface during sintering.

[0036] In a particular embodiment, the conductive paste of the present invention contains from 50% to 90% by weight, based on the total weight of the paste, of conductive metal particles, for example silver particles, having an average particle size of 1.0 to 5.0 microns. For example, said silver particles may comprise 60% to 88% by weight, based on the total weight of the paste.

[0037] Where silver is used as the conductive metal in the present invention, it is preferably of high purity (99+%). However, depending on the electrical requirements of the pattern, it is also possible to use material of lower purity.

[0038] The chemical composition of the glass frit has little importance on the function of the invention. For example, bismuth and/or zinc borosilicates and leaded frits are widely used in pastes for automotive glass, and can be used in the present invention. Suitable glass frit binders show high acid resistance and low crystallization proneness combined with a low melting point. Mixtures of different types of glass frit may be employed.

[0039] The glass frit of the conductive paste used in the present invention has a particle size D.sub.90 of less than 4 microns, preferably less than 3 microns, more preferably less than 2 microns.

[0040] The particle size may be determined using a laser diffraction method (e.g. using a Malvern Mastersizer 2000).

[0041] The fine milled glass frit is included in the conductive paste employed in the present invention in an amount of 0.1% to 10.0% by weight, and preferably 3% to 5% by weight based on the total weight of the paste.

[0042] A content of more than 10% glass frit may cause sintering of the silver to proceed too far and may also cause glass exudation. A content of less than 0.1% glass frit may result in insufficient sintering and insufficient weather, chemical and mechanical resistance properties.

[0043] The conductive paste employed in the present invention may optionally include one or more transition metal oxides. The transition metal oxide(s) may be used to enhance the wear resistance. Suitable oxides include oxides of the transition metals vanadium, manganese, iron and cobalt. The amount required to achieve the desired effect is 3.0% to 15%, and preferably 5.0% to 10.0%, based on the total weight of the paste. At less than 3.0%, an improvement in the wear resistance cannot be expected. Furthermore, if the amount of the transition metal is greater than 15%, resistivity is increased and sintering is adversely affected. Also soldering is negatively affected when higher amounts of non-conductive materials are added.

[0044] Mixtures of separately added transition metal oxides can likewise be used, so long as the total amount of such oxides is the same as that indicated above. In cases where the transition metal oxides are separately added, the particle sizes of the oxides are not subject to any narrow limitations from the standpoint of technical effects. However, the particle sizes must be suitable to the method of use and the firing method.

[0045] Any suitable inert liquid may be used as the organic medium in the present invention, although a non-aqueous inert liquid is preferred. Use can be made of any one of various organic liquids which may or may not contain a thickener, such as a cellulose derivate, a stabilizer, such as C.sub.12 and higher organic acids, and/or other common additives (for example, staining agents, such as carbon black and/or inorganic pigments, rheology modifiers, such as fumed silica and/or polyamide thixotropes, adhesion enhancers and sintering modifiers.

[0046] Examples of organic liquids that may be used include alcohols, esters of such alcohols (for example, the acetates and the propionates), terpenes (for example, pine oil, terpineol and the like), solutions of resins (for example, polymethacrylates) in organic liquids, solutions of ethyl cellulose in a solvent (for example, pine oil, or terpineol) and the monobutyl ether of ethylene glycol monoacetate.

[0047] A preferred organic medium is composed of ethyl cellulose in terpineol, combined with butyl carbitol acetate.

[0048] The organic medium may account for 5 to 50 wt % of the conductive paste. The amount of thickener used depends on the viscosity of the ultimately desired composition. That is, it depends on the conditions required for printing. Preferably, the viscosity of the conductive paste may be in the range 5 to 100 Pa.Math.s measured at 20 S.sup.1 and 20 C.

[0049] The conductive paste employed in the present invention may be made by mixing all of the components to form a homogeneous mixture. Said mixture may be dispersed by triple-roll milling until a homogeneous, well-dispersed paste is obtained.

[0050] As an example, the paste may be screen-printed, inkjet printed or applied by any other suitable technique on to a desired substrate, and fired. After firing, a connector may be applied by soldering the connector on to the applied paste using a lead-free solder.

[0051] The strength of adhesion of the connector to the fired conductive paste may be determined by testing pullstrength, as follows:

[0052] The substrate with applied connector is placed under a force gauge. The substrate is held in place and the connector is fixed to the force gauge. The force gauge is connected to a device capable of applying a vertical pulling speed of approximately 1.5 mm/s. The force gauge is then zeroed and the pulling is started. The force gauge is set in a way that shows the maximum applied force in the display. At the moment the bond between the connector and substrate breaks, the force is read from the display. This value is registered as being the pullstrength.

[0053] While the composition of the present invention may be utilized on a variety of substrates, the composition has particular utility on glass substrates in the automotive industry. Additionally, the composition of the present invention may have utility on non-automotive glass, such as freezer doors, and display windows.

[0054] The conductive paste of the present invention, when used to form an electrically conductive grid on vehicle windows (windshields) and mirrors for use in a defroster system or any other electrical functional circuit application, provides superior pullstrength when a lead free soldering alloy is used to solder the connector to the fired conductive paste, as is demonstrated in the Examples herein. Improvements in weather, chemical and mechanical resistances are also advantageous for the applied product and are also demonstrated by the Examples herein.

[0055] The present invention is illustrated by the following non-limiting Examples.

EXAMPLES

[0056] Two commercially available bismuth borosilicate frits (glass frit A and glass frit B) were subjected to ball-milling in water to produce particles having a D.sub.90 particle size as set out in Table 1. Particle size distribution was determined via laser diffraction using a Malvern Mastersizer 2000.

[0057] Six conductive pastes were prepared each containing about 80% by weight of a commercial silver powder (AEP-2, available from Ames Goldsmith), about 4.5% by weight of the milled glass frit and standard organic media (containing 90% heavy organic solvents such as terpineol and isotridecanol and 10% polymeric solids such as cellulosics). The silver powder, glass frit and a proportion of the organic medium were mixed for 10 to 15 minutes until a homogeneous mixture was obtained. The mixtures were each dispersed by triple-roll milling until a well dispersed paste was acquired. The paste was then diluted with further organic medium until a viscosity suitable for screen printing was achieved. The exact compositions of the prepared conductive pastes and particle sizes of the glass frits are given in Table 1.

TABLE-US-00001 TABLE 1 Composition of prepared samples Example 1 Example 2 Example 4 (Compar- (Compar- (Compar- ative) ative) Example 3 ative) Example 5 Example 6 Wt % 80.2520 80.2520 80.2520 80.2520 80.2520 80.2520 Silver Powder Wt % 4.5140 4.5140 4.5140 Glass frit A Wt % 4.5140 4.5140 4.5140 Glass frit B Wt % 15.2340 15.2340 15.2340 15.2340 15.2340 15.2340 Organic Medium Milled 8-10 m 4-6 m <2 m 8-10 m 2.6 m 1.3 m D.sub.90 of Glass Frit

[0058] Preparation of Evaluation Samples

[0059] Small-scale defogging circuits were prepared for evaluation of the pastes according to the following procedure. The prepared conductive pastes were applied by screen printing onto a flat glass substrate. The printed pastes were dried at elevated temperatures of 150 C. for 10 to 15 minutes until visibly dry. The applied conductive pastes were then fired in an oven suitable for fast firing glass samples, in air, at temperatures of between 680 and 700 C. for 180 seconds to form a conductive grid on the glass substrate.

[0060] A pre-soldered connector was then applied to each fired sample by soldering the connector onto the fired paste using power controlled soldering. The power controlled soldering was performed by placing two electrodes on the connector above areas where pre-solder was applied. Power was applied until the solder melted and spread evenly over the surface underneath the connector. Power was then removed and the solder allowed to harden and cool.

[0061] The connector used in this case is a commercially available pre-soldered on glass connector. The pre-solder on this connector is a SAC(X) type of solder containing 94.5 to 97.5% Sn, 2 to 4% Ag and 0.5% Cu.

[0062] Pullstrength was tested by the method described above and the results are presented in Table 2.

TABLE-US-00002 TABLE 2 Average Pullstrength values Exam- Exam- Exam- ple 1 ple 2 Exam- ple 4 Exam- Exam- (Compar- (Compar- ple (Compar- ple ple ative) ative) 3 ative) 5 6 Pb- 22 Kg 25 Kg 38 Kg 20 Kg 24 Kg 34 Kg free solder

[0063] As can be seen from the results presented above, the process of the present invention, illustrated by Examples 3, 5 and 6 above, provides articles of glass with applied electrically conductive grids having superior pullstrength when soldered to a connector with a lead-free solder.

[0064] Weather and chemical resistance were measured as follows and the results are presented in Table 3.

[0065] Acid Resistance

[0066] Immersion Test

[0067] Lidded glass jars filled with 0.1N H.sub.2SO.sub.4 test solution to a level of ca. 5 cm above the bottom of the jars were placed into an oven set at 80 C., with the lids tightly secured. After a minimum of 4 hours at 80 C. the jars were removed from the oven and evaluation samples prepared as above were placed into the test solutions such that the circuits were half-immersed. The lids of the jars were closed tightly and the jars placed back into the oven at 80 C.

[0068] After 4, 5, 6, 7, 8, 9, 16, 20 and 24 hours, samples were taken out of the test solution, rinsed with tap water and air dried.

[0069] Release of the conductive grid from the substrate was visually assessed. Visual assessments were performed both before and after treatment.

[0070] Tape Test

[0071] Strips of adhesive tape, e.g., Scotch 2517, were applied over the test area, left for 24 hours, and pulled off. Release of the conductive grid from the substrate was visually assessed. Again, visual assessments were performed both before and after treatment.

[0072] Resistance (R) before and after the immersion test and after the tape test was measured. The absolute and percentage change in resistance is calculated from each strip.

[0073] Visual and calculated judgement before and after the tape test: [0074] OK: No delamination, or minor damage to the circuit with no line breakage and R<10%. [0075] NOK: Delamination or line breakage of the circuit and/or R>10%.

[0076] The results of the acid resistance testing are shown in Table 3 below.

[0077] Weather Resistance

[0078] Evaluation samples prepared as above were visually assessed before testing.

[0079] Plastic bags were filled with moisturized hydrophilic cotton saturated with water, and evaluation samples placed therein with the printed surfaces contacting the cotton. The bags were closed tightly, sealed and placed into a climate chamber set at 80 C. and 96% RH. Samples were removed at 7, 14 and 21 days, rinsed with tap water and air dried.

[0080] Immersion tests and tape tests were carried out as above.

[0081] The results of the weather resistance testing are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Weather and Chemical Resistance Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 Weather <21 <21 n.t. n.t. n.t. >21 Resistance Days Days days NOK NOK OK Acid n.t. <5 n.t. n.t. n.t. >24 resistance Hours Hours NOK OK Acid/ n.t. <4 n.t. n.t. n.t. >9 Tape Hours Hours resistance NOK OK n.t. not tested