Thermally conductive member using graphite film and method for manufacturing the thermally conductive member

Abstract

To provide a thermally conductive member that maintains a high thermal conductivity, is easy to handle, and is highly mass-producible, and a method for manufacturing the thermally conductive member by a novel method for compositing a graphite film.SOLUTION: A thermally conductive member 1 includes a graphite film 2 provided with a plurality of through holes 21, and metal foils 3, 3' covering both sides of the graphite film 2, and the metal foil 3 covering one surface of the graphite film 2 is joined to the metal foil 3' covering the other surface of the graphite film 2 through the through holes 21.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a thermally conductive member using a graphite film and a method for manufacturing the thermally conductive member. More specifically, the present invention relates to a thermally conductive member using a graphite film having a high in-plane thermal conductivity of 1000 W / m K or more, suitable for use as a heat dissipation member for cooling and dissipating heat from heat-generating electronic components, and a method for manufacturing the thermally conductive member. [Background technology]

[0002] As electronic devices become faster, more powerful, and more dense, it is becoming increasingly important to take measures to prevent heat from being released from heat-generating components. One effective way to deal with this is to use materials with as high a thermal conductivity as possible for their components, and the industry is actively adopting and developing materials with excellent thermal conductivity. The metallic materials silver, copper, and aluminum are well known for their excellent thermal conductivity, and are commonly used as heat sink materials for heat dissipation. In addition to the above materials, graphite has also been attracting attention in recent years. Graphite has the second highest thermal conductivity in the in-plane direction of its crystalline grains, second only to diamond, and boasts a thermal conductivity nearly three times that of copper. Conversely, its thermal conductivity in the direction perpendicular to the crystal plane is less than 1 / 200 of that in the in-plane direction, giving it the characteristic of anisotropic thermal conductivity. Active research is being conducted into how to maximize this anisotropic thermal conductivity by using graphite films in which the graphite crystals are oriented. Graphite films include graphite films made from natural graphite, which are produced by expanding the interlayer space between natural graphite particles with sulfuric acid or the like, then crushing them into flakes and then sheeting them in a process similar to that used to make paper. Alternatively, there are graphite films made from synthetic graphite, which are produced by carbonizing polyimide films and then graphitizing them through high-temperature treatment at nearly 3000°C. While synthetic graphite films are expensive to produce, they have significantly better density and crystal orientation than natural graphite films, and their in-plane thermal conductivity is 1200 W / m·K or more, close to the theoretical value of graphite. For example, Patent Document 1 describes a method for producing graphite films with a specific gravity of 1.8 g / cm by baking a polyimide film with a thickness of 10 to 60 μm to form a layered crystal structure in which carbon is covalently bonded, and then rolling the film. 3 or more, and the thermal diffusivity is 10cm 2 A method for producing a heat-dissipating graphite sheet with a viscosity of 1 / s or more and a thermal conductivity of 1260 W / m·K or more is disclosed.

[0003] For example, graphite film is used in various heat-resistance materials for smartphones. To eliminate hot spots caused by heat generated by the semiconductors installed in the smartphone, a graphite film is attached to the back of the smartphone and used as a heat spreader, taking advantage of its high in-plane thermal conductivity. Furthermore, to serve as a heat sink for power semiconductors such as IGBTs, high-power LED elements, or Peltier elements, which are highly desired on the market, vigorous research is being conducted into the use of slices obtained by laminating graphite film and then cutting it perpendicular to the plane (Patent Document 2).

[0004] Various methods have been attempted to manufacture heat dissipation components using graphite. For example, a method in which natural graphite and metal are melted and mixed and then rolled into a sheet (Patent Document 3), a method in which graphite films are laminated and then sintered at high temperature while applying pressure, and a method in which molten copper is pressed between the graphite film layers are also known (Patent Documents 4 to 6). Furthermore, a thermally conductive material in which graphite films are coated with an adhesive resin and then laminated is also known (Patent Document 7). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5098642 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-4733 [Patent Document 3] Patent No. 5640239 [Patent Document 4] Patent No. 4711165 [Patent Document 5] Patent No. 4490723 [Patent Document 6] Patent No. 3345986 [Patent Document 7] Japanese Patent Application Publication No. 2019-10773 [Patent Document 8] Patent No. 3417253 [Patent Document 9] Patent No. 5624684 [Patent Document 10] Japanese Patent Publication No. 2020-188235 Summary of the Invention [Problem to be solved by the invention]

[0006] Graphite laminates obtained by laminating and pressing synthetic graphite films and sintering them at high temperatures have excellent thermal conductivity, but the adhesive strength between the graphite layers is weak, leading to breakage during slicing, making them difficult to fabricate into thin sheets. Furthermore, the method of melting and kneading metals such as copper or aluminum with natural graphite particles, followed by rolling and laminating the resulting sheets, limits the amount of graphite particles that can be packed into the laminate, and the particles are not continuous, resulting in a thermal conductivity of only around 1000 W / m·K. Furthermore, the method of injecting molten metal between graphite film layers requires large-scale equipment and high investment, resulting in a manufacturing cost disadvantage. Furthermore, using the injecting method on dense synthetic graphite film makes manufacturing difficult due to poor wettability with molten copper, making it a method that is only possible with natural graphite film. As a result, the resulting thermal conductivity is significantly lower than that of synthetic graphite film. As such, a manufacturing method for graphite laminates that offers a good balance of performance and cost and satisfies market demands has not yet been established. Furthermore, as the study of its use progressed, it became clear that synthetic graphite film had drawbacks, such as its weak mechanical strength for use as is and the generation of conductive dust. Additionally, because the graphene has an oriented stacked structure, the surface is easily peeled off due to weak bonds formed only by van der Waals forces, and its poor wettability with metals made it difficult to combine with other materials. To address these issues related to combining, various studies are being conducted on composite structures that do not impair heat resistance or thermal conductivity using resins, ceramics, or metals. For example, epoxy resin, silicone resin, or acrylic rubber, which are easily adhesive, can be used to bond synthetic graphite film layers or to form composites with metals or ceramics. However, in this case, the low thermal conductivity and heat resistance of the resin can impair the properties of the graphite, which is undesirable. With regard to composites of synthetic graphite films, the following have been proposed: a graphite composite in which a thin metal film reactive with carbon atoms is provided on at least one surface of a graphite sheet (Patent Document 8); a graphite composite film in which a metal layer is formed on at least one surface of a graphite film having a plurality of through-holes, and a continuous metal layer is formed in the through-holes so as to connect to the metal layer; and a composite heat transfer member in which a carbonaceous material plate made of a composite containing graphene and graphite particles is tightly adhered to a metal casting, enabling efficient heat conduction (Patent Document 10). However, in order to achieve high mass productivity while maintaining the good electrical and thermal conductivity of graphite sheets, there are still many areas that need improvement, which currently prevents widespread application. [Means for solving the problem]

[0007] The present invention relates to a new method for compounding synthetic graphite films that solves the above-mentioned problems, and makes it possible to provide a thermally conductive member and a method for manufacturing a thermally conductive member that maintains high thermal conductivity, is easy to handle, and is suitable for mass production.

[0008] The present invention is summarized as a thermally conductive member as set forth in the following items (1) to (6). (1) a first surface, a second surface opposite to the first surface, and a wire passing through the first surface to the second surface; a graphite film having a plurality of through holes formed therein; and a coating layer covering the first surface of the graphite film. Has a flat surface a first metal foil and a second surface of the graphite film; Has a flat surface A second metal foil is provided, and the first metal foil plane and the second metal foil The plane of the through hole Directly in the metal Joined having a joint A thermally conductive member characterized by: (2) The thermally conductive member according to (1), wherein the graphite film has a thermal conductivity in an in-plane direction of 1000 W / m·K or more and a thickness of 100 μm or less. (3) The thermally conductive member according to (1) or (2) above, wherein the diameter of the through-hole is at least five times the thickness of the graphite film. (4) A thermally conductive member according to any one of (1) to (3) above, wherein the first and second metal foils are made of any of copper, aluminum, tin, zinc, lead, iron, nickel, copper, silver, and gold, or an alloy thereof. (5) The thermally conductive member according to any one of (1) to (4) above, wherein the first and second metal foils have a thickness of 500 μm or less. (6) A thermally conductive member according to any one of (1) to (5) above, characterized in that the first and second surfaces of the graphite film are plated with a metal selected from gold, silver, nickel, titanium, copper, tin, and zinc.

[0009] The present invention also provides a method for producing a thermally conductive member as set forth in (7) below. (7) a first surface, a second surface opposite to the first surface, and a wire passing through the first surface to the second surface; A graphite film having a plurality of through holes is prepared, a first surface of the graphite film is covered with a first metal foil, and the graphite film is No. coating the second surface with a second metal foil; The first metal foil covering the first surface of the graphite film and the second metal foil covering the second surface of the graphite film are joined by ultrasonic bonding. the through hole Within and a bonding step. [Effects of the Invention]

[0010] According to the present invention, a graphite film is processed by pressing or laser processing to form a perforated graphite film with a plurality of through holes, and then both sides of this perforated graphite film are covered with metal foil. After that, ultrasonic waves are applied while pressure is applied, thereby obtaining a thermally conductive member that combines practical strength with high thermal conductivity. Furthermore, by stacking multiple layers of metal foil and perforated graphite film alternately and applying pressure and ultrasonic waves, it is possible to easily manufacture a thermally conductive member with a laminated structure. In addition, by using ultrasonic waves to bond the metal foils together, it is possible to manufacture the product using processes only at room temperature, which also has the effect of preventing warping and discoloration that occur when bonding materials with different thermal expansion coefficients. By incorporating the highly thermally conductive member of the present invention into, for example, a computer CPU, a power semiconductor, or a high-output LED, better heat dissipation characteristics than ever before can be expected. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing a cross section of a thermally conductive member of the present invention, illustrating a state in which metal foils covering both sides of a graphite film are joined to each other via a plurality of through holes provided in the graphite film. [Figure 2] FIG. 2 is a diagram illustrating the state of a graphite film immediately before both sides are covered with metal foil. [Figure 3] 1 is a diagram illustrating an overview of a plurality of through holes provided in a graphite film constituting a thermally conductive member of the present invention. [Figure 4] 1 is a view illustrating another embodiment of a graphite film according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of a thermally conductive member using a graphite film of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing a cross section of a thermally conductive member using a graphite film of the present invention, illustrating a state in which a graphite film 2 having a plurality of through holes 21 and metal foils 3, 3' covering both sides of the graphite film 2 are bonded to each other via the through holes 21. Fig. 2 is a diagram illustrating the state immediately before both sides 22, 23 of the graphite film 2 are covered with the metal foils 3, 3'.

[0013] Graphite film 2 is, for example, a film-like synthetic graphite in which graphite crystals are oriented in the plane direction. The orientation direction of the graphite crystals is not limited. Synthetic graphite films can be produced by carbonizing a polyimide film in an inert atmosphere and then graphitizing it at a high temperature of nearly 3000°C. In particular, synthetic graphite films obtained by carbonizing and graphitizing a polyimide film have high thermal conductivity, and their in-plane thermal conductivity can exhibit performance close to the theoretical value. Graphite film 2 used in the present invention is characterized by an in-plane thermal conductivity of 1000 W / m K or more.

[0014] When using synthetic graphite film, the starting material, polyimide film, is pyrolyzed, allowing the film to be 100 μm thick or less. This makes it significantly thinner and more flexible than natural graphite films made from graphite. To facilitate bonding between metal foils, a synthetic graphite film thickness of approximately 50 μm is preferable. Conversely, a thickness exceeding 200 μm is unsuitable because it increases the spacing between the metal foils, making bonding difficult and increasing the surface irregularities. In the present invention, the graphite film 2 has an in-plane thermal conductivity of 1000 W / m·K or greater and a thickness of 100 μm or less (e.g., 20 to 80 μm). The width of the graphite film 2 is also not particularly limited and can be 100 mm or greater. For example, a thermally conductive member 1 having a width of 400 mm or greater can be produced using a graphite film 2 having a width of 400 mm or greater.

[0015] 3 shows a plurality of through holes 21 formed in graphite film 2 constituting the thermally conductive member of the present invention. Graphite film 2 according to the present invention has a plurality of through holes 21. The size of through holes 21 is preferably large enough to allow metal foils 3, 3′, which sandwich both sides of graphite film 2, to be joined via through holes 21, and the diameter of through holes 21 is preferably at least five times the thickness of graphite film 2.

[0016] The shape of through hole 21 is preferably circular or elliptical, but may also be rectangular or triangular. For example, in the example shown in FIG. 3, through hole 21 in graphite film 2 is circular. FIG. 4 shows graphite film 2a according to another embodiment of the present invention. In the example shown in FIG. 4, graphite film 2a has a combination of cross-shaped through hole 21a and circular through hole 21. Thus, the shape of through hole 21 is not particularly limited. However, in the present invention, metal foils 3, 3′ are bonded within through hole 21 by applying ultrasonic waves while applying pressure to them. Therefore, through hole 21 has a shape and size that allows a sufficient distance within through hole 21 to bond metal foils 3, 3′. From this perspective, through hole 21 preferably has a diameter at least five times the thickness of graphite film 2. For example, when graphite film 2 has a thickness of 50 μm, the diameter of through hole 21 is preferably 250 μm or more, which facilitates bonding of metal foils 3, 3′ via through hole 21.

[0017] Next, a method for providing a plurality of through holes 21 in graphite film 2 will be described. Providing a plurality of through holes in graphite film 2 involves drilling holes in graphite film 2 in the form of a punched metal, and this can be achieved by mechanical processing using a punching machine or laser processing. Note that if the density of through holes 21 (the area of through holes 21 per a given area of graphite film 2) is low, the bonding strength of metal foils 3, 3′ decreases, but the in-plane thermal conductivity can be increased. Conversely, if the density of through holes 21 is high, the bonding strength of metal foils 3, 3′ increases, but the in-plane thermal conductivity decreases. Therefore, it is preferable to appropriately adjust the density of through holes 21, taking into account both the bonding strength of metal foils 3, 3′ and the in-plane thermal conductivity.

[0018] Furthermore, because the graphene layers are only weakly bonded in the thickness direction by van der Waals forces, voids may form between the layers, impairing thermal conductivity. For this reason, methods such as applying pressure to the top and bottom of the graphite film to maintain thermal conductivity have been proposed. However, in the method of the present invention, the graphite film is automatically compressed in the thickness direction by fusing the metal foils together, resulting in stabilized thermal conductivity and improved properties.

[0019] Furthermore, graphite film 2 is preferably a synthetic graphite film, and can be metal-plated, painted, or surface-treated before use. Even in this case, there is no concern about a decrease in the bonding strength between metal foils 3 and 3' via through holes 21 in graphite film 2 of the present invention. The surface of graphite film 2 can be characterized by being plated with a metal made of gold, silver, nickel, titanium, copper, tin, or zinc. For example, a synthetic graphite film can be copper-plated, and then aluminum foils can be bonded thereto as metal foils 3 and 3'. Plating graphite film 2 in this way makes it possible to produce a thermally conductive member 1 with higher strength.

[0020] The metal foils 3, 3' used in the present invention can be made of any of the following metals: copper, aluminum, tin, zinc, lead, iron, nickel, copper, silver, and gold, or alloys thereof. While the metal type is not particularly limited as long as it is a highly thermally conductive metal, such as silver, copper, or aluminum, metal foils with low melting points are easier to bond using frictional heat generated by ultrasonic application, and metals with high rigidity are unsuitable for ultrasonic bonding. Therefore, it is desirable to select metal foils with melting points of around 600°C, such as tin, zinc, aluminum, copper, or silver, or alloys thereof. Furthermore, the metal foils 3, 3' may be made of iron, stainless steel, nickel, or copper as a core metal, with the exterior coated with one of the above low-melting-point metals by plating or other means. Using aluminum, tin, or zinc for the metal foils 3, 3' is preferable not only for improving the strength of the thermally conductive member 1 but also for dust prevention.

[0021] Metal foils 3, 3′ can be characterized by having a thickness of 500 μm or less. The thickness of metal foils 3, 3′ needs to be adjusted appropriately depending on the metal used, but since using a metal foil thicker than 500 μm may impair the high thermal conductivity of graphite film 2, a thickness of 500 μm or less is preferable, and a thickness of 5 to 200 μm is particularly more preferable.

[0022] Thermally conductive member 1 according to the present invention is constructed by ultrasonically bonding metal foil 3 covering first surface 22 of graphite film 2 and metal foil 3' covering second surface 23 of graphite film 2 to each other at through-holes 21. In other words, thermally conductive member 1 is constructed of graphite film 2 and metal foils 3, 3' covering first and second surfaces 22, 23 of graphite film 2, with metal foils 3, 3' bonded to each other at bonding portions 31 within through-holes 21. Ultrasonic bonding can basically bond metals to each other through a reaction at around room temperature, and although ultrasonic waves are generally defined as high-frequency sounds that are inaudible to the human ear, the ultrasonic waves of the present invention can also include all industrially useful sound waves, such as sound waves around 15 kHz, as interpreted in industry.

[0023] In ultrasonic bonding, the sample is sandwiched between an anvil and a vibrator called a probe, and the pressure is 1 to 300 kg / cm 2 By applying ultrasonic waves for several seconds while applying a pressure of 100 kJ / cm2, metal foils 3 and 3' sandwiching graphite film 2 are fused and bonded within through-holes 21. The pressure application time must be adjusted appropriately depending on the combination, but can be completed in a short time of 1 to 20 seconds.

[0024] Furthermore, if the synthetic graphite film is thick or the metal foil is highly rigid and the bonding between the metal foils 3 and 3' is insufficient at the through-hole 21, inserting a flexible resin or rubber plate between the anvil and the probe will ensure uniform pressure on the uneven parts and improve the bonding. When fusing a large area, a long probe is used and the graphite film 2 and the metal foils 3 and 3' are fed in overlapping positions between the anvil and the probe, enabling surface bonding.

[0025] [Manufacturing method] The method for manufacturing the thermally conductive member 1 includes the steps of preparing a graphite film 2 having a plurality of through holes 21 formed therein, coating a first surface 22 of the graphite film 2 with a first metal foil 3, and coating a second surface 23 of the graphite film 2 with a second metal foil 3′, and applying ultrasonic waves while applying pressure to the graphite film 2 to bond the first metal foil 3 and the second metal foil 3′ to each other via the through holes 21.

[0026] [Uses of Thermally Conductive Material 1] The present invention relates to the application of a graphite film having a high in-plane thermal conductivity of 1000 W / m·K or more, suitable for use as a heat dissipation component for cooling and dissipating heat from heat-generating electronic components, etc. For example, the thermally conductive member 1 of the present invention can be used as a heat dissipation heat sink to deal with heat generated by components mounted on electronic devices. Furthermore, because the thermally conductive member 1 of the present invention is produced by combining the graphite film 2 and the metal foils 3, 3' using ultrasound, the thermally conductive member 1 can be manufactured using thinner graphite films 2 and metal foils 3, 3' than conventional ones, and the thickness of the thermally conductive member 1 can be reduced to 500 μm or less (or 300 μm or less). [Example]

[0027] Example 1 A 40 μm thick graphite film 2 (Gravure, manufactured by Kaneka Corporation) obtained by carbonizing and baking a polyimide film was punched with through holes 21, each 3 mm in diameter, at intervals of approximately 5 mm using a press machine used for punching metal plates. This graphite film 2 was used as a core, and both sides 22, 23 were sandwiched between 60 μm thick aluminum foils 3, 3', and 15 KHz ultrasonic vibrations were applied for approximately 3 seconds using an ultrasonic vibrator, so that the aluminum foils 3, 3' on both sides were fused together through the through holes 21 in the graphite film 2, resulting in a self-holding graphite sheet (thermal conductive member 1 according to Example 1).

[0028] Next, the thermal diffusivity of the thermally conductive member 1 according to Example 1 was measured using a thermowave analyzer (manufactured by Bethel Hudson Laboratories, Inc.). As comparative examples, the thermal diffusivities of only copper foil, only aluminum foil, and only synthetic graphite film were also measured. The measurement results of the thermal diffusivities of Example 1 and the comparative examples are shown below. [Table 1] As shown in Table 1 above, it was found that the thermal conductive member 1 of Example 1 exhibited a thermal diffusivity higher than that of metals alone (copper foil alone and aluminum foil), and fully inherited the high thermal conductivity of the graphite film 2.

[0029] Example 2 The perforated graphite film 2 obtained in Example 1 was subjected to electrolytic copper plating in a copper sulfate bath to obtain a copper-plated graphite film having a thickness of 5 μm. This copper-plated graphite film 2 was sandwiched between 60 μm thick tin foils 3 and 3' on both sides of the core, and ultrasonic waves were applied under the same conditions as in Example 1 to obtain an integrated three-layer film (thermal conductive member 1 of Example 2). The thermal diffusivity (heat transfer rate) of the thermal conductive member 1 according to Example 2 was measured and found to be 170 (10 -6 m 2 / s), and it was found to be a useful heat dissipation material.

[0030] As described above, thermally conductive member 1 according to the present invention includes graphite film 2 having a plurality of through holes 21, first metal foil 3 covering first surface 22 of graphite film 2, and second metal foil 3′ covering second surface 23 of graphite film 2. First metal foil 3 covering first surface 22 of graphite film 2 is bonded to metal foil 3′ covering second surface 23 of graphite film 2 via through holes 21. Therefore, thermally conductive member 1 having a thickness of 300 μm or less can be manufactured without special processes such as casting. Prior art has disclosed a method of covering a graphene plate (a plate made of a composite in which graphene aggregates and graphite particles are stacked) with a cast molding such as magnesium or aluminum. However, this requires a casting process, which increases production costs. In addition, the thickness of the graphene plate must be increased to withstand casting, resulting in an overall thickness exceeding 500 μm. Furthermore, it was necessary to include graphite particles to ensure the graphene plate had sufficient strength to withstand casting. In contrast, the thermally conductive member 1 according to the present invention can be manufactured simply by joining the metal foils 3 and 3' using ultrasonic waves, eliminating the need for a casting process. Furthermore, the thickness and strength of the graphene plate required for casting are not necessary for the graphite film of the present invention. Therefore, compared with conventional manufacturing methods, not only can the thermally conductive member 1 be manufactured more easily, but the thickness of the thermally conductive member 1 can also be reduced. Furthermore, the thermally conductive member 1 can be manufactured using a graphite film 2 that does not contain graphite particles. Furthermore, a plurality of thermally conductive members 1 according to the present invention can be stacked to form a block-shaped structure.

[0031] Although the preferred embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the above-described embodiments. Various modifications and improvements can be made to the above-described embodiments, and such modifications and improvements are also included in the technical scope of the present invention. [Explanation of symbols]

[0032] 1...Thermal conductive material 2,2a...Graphite film 21,21a...Through hole 22...First side 23...Second Side 3...First metal foil 3'...Second metal foil 31…Joint part

Claims

1. A graphite film having a first surface, a second surface opposite the first surface, and a plurality of through holes extending from the first surface to the second surface; a first metal foil having a flat surface covering the first side of the graphite film; a second metal foil having a flat surface covering the second surface of the graphite film; A thermally conductive member, characterized in that the plane of the first metal foil and the plane of the second metal foil have a joint portion where they are directly metal-joined within the through-hole.

2. the graphite film has an in-plane thermal conductivity of 1000 W / m K or more; 2. The thermally conductive member according to claim 1, wherein the thickness is 100 μm or less.

3. 3. The thermally conductive member according to claim 1, wherein the diameter of the through-hole is at least five times the thickness of the graphite film.

4. 4. The thermally conductive member according to claim 1, wherein the first and second metal foils are made of any one of copper, aluminum, tin, zinc, lead, iron, nickel, silver, and gold, or an alloy thereof.

5. 5. The thermally conductive member according to claim 1, wherein the first and second metal foils have a thickness of 500 [mu]m or less.

6. 6. The thermally conductive member according to claim 1, wherein the first and second surfaces of the graphite film are plated with a metal selected from gold, silver, nickel, titanium, copper, tin, and zinc.

7. A graphite film is provided having a first surface, a second surface opposite the first surface, and a plurality of through holes extending from the first surface to the second surface; coating a first side of the graphite film with a first metal foil and a second side of the graphite film with a second metal foil; and joining the first metal foil covering the first surface of the graphite film and the second metal foil covering the second surface of the graphite film within the through hole by ultrasonic bonding.

Images