Method of manufacturing silver flake

Description

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method of manufacturing silver flake, whereby material selected for use as a substrate is that which is easily removed and broken. First, a layer of silver metal is plated on a surface of the substrate, whereafter such test specimen is crushed and the substrate removed, or first remove the substrate material and thereafter crush, and thereby obtain silver powder of extremely uniform thickness. The silver flake thus acquired is provided with superior thermal and electrical conductivity in a specific direction.

(b) Description of the Prior Art

Because silver powder is provided with properties including good conduction of heat and high conduction of electricity, the silver powder. has extensive application in electronic encapsulation, electrode contacts, multilayer integrated electronic components, and so on, and moreover, when heat and electrical conduction of flake-form silver powder is compared to that of spheroidal form of silver powder, a considerable difference in properties of conduction of heat and electricity is found. Due to fact that collection of the latter spheroidal powder only results in point contact between spheroidal bodies of the powder, whereas the former flake-form silver powder amasses by means of directional accumulation, and apart from point contact between the flakes, substantial mass-collection of layer upon layer of surfaces is realized. Furthermore, because points of intersection between the surfaces of silver flake effectuate larger circulation channels, phonons and electrons relatively easily transit such surface contacts therethrough, and thus thermal resistance and electrical resistance is greatly reduced. The silver flake is thus provided with superior thermal and electrical conductivity in a specific direction. For instance, Taiwan patent No. 432085 “Thermal Conductive Paste Component”, utilizes silver flake having longitudinal length of 10˜50 microns and thickness of 1˜5 microns.

With regard to the aforementioned problems in manufacturing silver powder, prior Taiwan and international patents or related publications have disclosed such, including Taiwan patent No. 146827 entitled “Method of Manufacturing Silver Powder”, patent No. 490339 entitled “Preparation Method for Micro-Silver Palladium Alloy Powder”, and patent No. 233315 entitled “Preparation Method for Controlling Surface Area of Silver Powder”, all of which disclose methods for precipitating silver powder out of chemical solution. Patent No. 261554 entitled “Method of Manufacturing Silver Powder by Aerosol Decomposition” adopts thermal decomposition of an unsaturated solution containing silver compounds, and is similar in concept to that seen in patent No. 458829 entitled “Method of Manufacturing Metal Powder”, having a primary characteristic in utilizing thermal decomposition of micro-droplets of a metal salt in solution, and at least one chemical compound can be thermally decomposed to form metallic powder thereof.

However, focus of each of the aforementioned patents is in manufacturing process of manufacturing silver powder, and the patents do not disclose a method of manufacturing the silver flake. More importantly, the aforementioned patents are only able to produce near-spheroidal or irregular-shaped silver powder. An overall view of methods for producing the metal powder include reduction method, reducing chemical compound method, vapor reduction method, chemical vapor deposition method, vapor condensation thermal decomposition method, liquid phase precipitation method, electro-analysis method, electrochemical corrosion method, atomization method and mechanical crushing method, wherein only the mechanical crushing method is able to acquire long flake-form powder. However, length of the flake-form powder is irregular, aspect ratio is excessively large, and shape of the flakes assumes a lengthways form, which disadvantages collective compactness of the powder. An additional shortcoming of excessive bulkiness of the powder reduces utilization value, particularly when applied in high value-added electronic usage.

In conclusion, current art is unable to manufacture silver flake having uniform thickness, whereby uniform thickness of the silver flake realizes improvement in thermal and electrical conductivity of the silver flake for application in electronics industry, and so on.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide a method of manufacturing silver flake that improves upon the aforementioned shortcomings, and which is characterized in utilizing a plating film of uniform thickness and processing that implements technological measures involving pre-plating and post-removal.

Primary steps of the present invention are:

• • a. Select a removable material as a substrate.
  • b. Plate a silver metal layer on the substrate.
  • c. Remove the substrate, and acquire silver flake of uniform thickness.

Apart from the aforementioned steps, the present invention can increase number of processing steps according to requirements of implementation with the following supplementary steps:

• • b1. Crush the substrate containing the silver metal layer or:
  • c2. Crush the silver metal layer having already removed the substrate.

The aforementioned substrate can be fabricated from ceramic (tablet), metal, plastic, and so on.

The aforementioned measures employed in plating manufacturing of silver metal layer can be conventional electroplating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and so on.

Furthermore, the silver metal layer can be silver metal, silver alloy or a metal containing silver as a constituent.

The silver flake thus acquired by means of the aforementioned steps is provided with superior thermal and electrical conductivity in a specific direction.

Another objective of the present invention is to provide a processing method for manufacture silver powder having high thermal conductivity and good electrical conductivity.

To enable a further understanding of the said objectives and the technological methods of the invention herein, the brief description of the drawings below is followed by the detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electron micrograph of silver flake manufactured according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention is described hereinafter:

• • 1. Use a tablet as a substrate (the tablet being fabricated from ceramic material).
  • 2. Silver plate the tablet.
  • 3. Dispose the tablet having a silver metal plated layer into a mortar, and grind the tablet, thereby forming fragments thereof.
  • 4. After retrieving the fragments, steep in water and stir, which thereby effectuates dissolving the tablet.
  • 5. Acquire silver powder of uniform thickness.

Particle size of the aforementioned silver powder can be regulated by grinding process.

Material appropriate for the tablet taken as the substrate in the aforementioned embodiment is that which is easily removed and broken.

During the step when the silver metal is being plated on the tablet, silver is plated on the surface of the tablet by means of an ion beam deposition film method, having primary function to effectuate evaporation deposition of the metal silver, and through assistance of a second ion beam, deposition on the surface of the tablet is realized, thereby forming the metal silver deposited film. Procedural method is as follows: procedure is carried out under a vacuum pressure of 0.00013 Pa with silver of 99.99% purity, and an argon gas having working pressure of 0.01 Pa; the primary ion beam formed has an ion energy and beam flow of 700 eV and 10 mA respectively, moreover, assisting the primary ion beam is an argon ion beam having the ion energy and beam flow density of 500 eV and 0.0001-0.00001 mA/cm² respectively. Growth speed of the silver film is greater than 0.02 nm/s. Prior to deposition of the aforementioned tablet, the tablet is necessarily dried to remove water adsorbed on the surface of the tablet, whereupon silver plating reaction is proceed with.

A second embodiment of the present invention is described hereinafter:

• • 1. Use metal as the substrate.
  • 2. Implement silver electroplating of the metal substrate.
  • 3. Utilize method of chemical corrosion to remove the substrate.
  • 4. Acquire the silver powder of uniform thickness.

Technological conditions for plating and manufacture as employed in the second embodiment are as follows:

Because silver very easily precipitates out of a cyanide solution and onto a metal, which results in irregular density of plating layers, thus, urgent plating processing is necessarily first implemented, resulting in a great deal of nucleation. Composition of the cyanide solution is 3˜6 g/L of silver cyanide and 120˜150 g/L of sodium cyanide, to supplement with high-current density the cyanide solution is thereupon disposed within an electroplating solution, composition of which is 45˜50 g/L of silver cyanide (AgCN), 45˜50 g/L of ionized potassium cyanide (KCN), 10˜14 g/L of potassium hydroxide (KOH), 45˜80 g/L of potassium carbonate (K₂CO₃), and 40˜60 g/L of potassium nitrate (KNO₃). Electroplating current density (stirring) is 8 A/dm², and working temperature is 38˜45° C.

The method of chemical corrosion is utilized to remove the substrate of the second embodiment, and the metal substrate chosen is one provided with corrosiveness, whereas a chemical solvent having non-corrosive properties towards silver metal is utilized to implement corroding effect, which can thus achieve objective of removing the substrate and retrieving the silver flake. In the aforementioned embodiment, the step involving removing the tablet by means of chemical corrosion utilizes aluminum metal as the substrate, and the chemical solvent is 30 wt % sodium hydroxide solution. The aluminum substrate having surface plated with the silver film is disposed into the sodium hydroxide solution, whereupon the aluminum completely dissolves leaving the silver film.

A third embodiment of the present invention is described hereinafter:

• • 1. Use glass (ceramic material) as the substrate.
  • 2. Utilize method of solution deposition to implement silver plating surface of the glass (ceramic).
  • 3. Remove the substrate by means of method of thermal stress.
  • 4. Acquire the silver powder of uniform thickness.

Technological conditions for plating and manufacture as employed in the third embodiment are as follows:

Settle out silver ions from a metal silver compound by means of a reducing agent effect, which reduces the silver ions and deposits the silver on the surface of the glass substrate. Actual method employed is: add 50 g of silver nitrate (AgNO₃) to 2 liters of water, add 50 g of potassium hydroxide (KOH) to 2 liters of water, add 80 g of sugar to 0.8 liters of water, permeate 3.5 ml of nitric acid (HNO₃) in 100 ml of water, and mix all together in a 16:8:1 ratio respectively. Thereafter add ammonium hydroxide (NH₄OH) to the silver nitrate (AgNO₃) and the potassium hydroxide (KOH) solution, thereby forming a non-sediment solution, whereupon nitric acid (HNO₃) is further poured in, whence a silver plating reaction commences.

The method of thermal stress is utilized to remove the substrate of the third embodiment. Regarding heating, because different substances are provided with different coefficients of expansion, thus cold contraction and hot expansion produced by heating and cooling can be utilized to realize removal of the silver powder from the substrate, thereby forming flakes thereof.

A fourth embodiment of the present invention utilizes a method of plastic electroplating to silver plate on ABS plastic (Acrylonitrile Butadiene Styrene plastic), whereby a procedure undertaken to realize such is as follows:

• • 1. Alkaline cleaning of the substrate: Utilize an alkaline to clean the substrate, thereby effectuating removal of contamination from the surface of the substrate, including contamination from being subject to dust in air, grease and fingerprints.
  • 2. Etching: Etching is implemented by means of 90 g/L of strongly oxidized potassium dichromate (K₂Cr₂O₇) and 600 ml/L of sulfuric acid (H₂SO₄), thus enabling the ABS (Acrylonitrile Butadiene Styrene) plastic surface to become hydrophilic therewith. Working temperature is 60-70° C., and time required is 5-10 minutes. However, too short a time will result in local non-plating, whereas too long a time will result in an inferior surface, which reduces adhesive strength of the plating film.
  • 3. Neutralizing: Remove six valence chromium and chromous salts remnants left from etching, which thereupon improves post-manufacturing agent processing of adsorption condition of the surface of the plastic.
  • 4. Catalysis: Subdivide to complete sensitizing and nucleation, wherein sensitizing is undertaken with 20 g/L of tin chloride (SnCl₂), 40 mls of hydrochloric acid (HCl) at 20-25° C., and time required is 1-3 minutes, which thereby enables the reducing agent to adsorb onto the surface. After reducing, a post-nucleating agent reaction is implemented to form a metallic state. Formulation for nucleation is 0.25 g/L of palladium chloride (PdCl₂), 2.5 ml/L of hydrochloric acid (HCl) at 20-40° C., and time required is 0.5-1 minutes. The aforementioned two steps can be executed together. However, the above formulation would thus have to be amended.
  • 5. Acceleration processing: Diluted acid or diluted alkali is utilized to remove residue tin compounds, and palladium is thereby completely exposed, thus enhancing the catalysis function. Working temperature is 20-50° C., and time required is 2-10 minutes.
  • 6. Proceed with electroless silver plating: A plating bath is first prepared having components (A) 60 ml/L of silver nitrate (AgNO₃), 60 ml/L of ammonium hydroxide (NH₄0H) (28% concentration), and (B) 210 ml/L of formaldehyde HCHO (37% concentration). Only upon starting electroplating is one portion of solution (B) added to five portions of solution (A). Upon needing to deploy the plating bath, the silver nitrate is first dissolved in approximately two thirds pure water, whereafter, ammonia is extremely slowly added to solution and mixed, and an object to be plated is thereupon placed into the solution.
  • 7. Remove the substrate: A heating method is utilized to thereby realize melting of the substrate and acquiring of silver flake thereof or a chemical solvent is employed to erode away the substrate, and the silver flake can analogously be procured thereof.

It is of course to be understood that the embodiments described herein is merely illustrative of the principles of the invention and that a wide variety of modifications thereto may be effected by persons skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.