JP4202923B2 - Thermally conductive material, microelectronic device, method of forming thermally conductive material, and method of conducting and removing heat from a microchip - Google Patents

Description

  The present invention relates to the manufacture of circuit boards and integrated circuit packages. More particularly, the invention relates to a low storage modulus, electrically conductive thermal interface for integrated circuit packages.

  Integrated circuits are well-known industrial products and are widely used in industrial and consumer electronics applications. They are particularly useful for large-scale applications such as industrial control devices as well as small-scale devices such as telephones, radios and personal computers.

  As the need for more intensive electronic device applications increases, there is an increasing demand for electronic systems that operate faster and provide more functionality with less occupied space. To meet these requirements, manufacturers design electrical and electronic devices that include a large number of electronic elements that are relatively close together. These elements tend to generate a large amount of heat, which must be dissipated by some means to prevent device breakdown or malfunction.

  Traditionally, electronic devices have been cooled using forced or convective circulation of air within the device housing. The cooling fan is often provided as an integral part of the electronic device or is individually attached thereto to increase the surface area of the integrated circuit package that is exposed to air flow. These fans are used to increase the amount of air circulating inside the device housing. U.S. Pat. No. 5,522,700 teaches the use of a typical fan device to dissipate heat from an electronic device. U.S. Pat. No. 5,166,775 describes an air manifold mounted adjacent to an integrated circuit for directing air flow over an electronic device mounted on the circuit. The air manifold has one air inlet and a plurality of outlet nozzles disposed along a conduit for directing air over the electronic device.

  Unfortunately, as integrated circuits continue to shrink in size while increasing power density, simple air circulation is often insufficient to adequately cool circuit elements. Further heat dissipation achievable by air circulation is achieved by attaching a heat sink or other heat dissipation device to the electronic element. U.S. Pat. No. 4,620,216 describes an integrated heat sink for a semiconductor package having a plurality of cooling fin elements, which heat sink is used to cool a high density integrated circuit module. U.S. Pat. No. 5,535,094 teaches the use of a combination of air blower and heat sink. That patent teaches a module having an integrated blower for cooling an integrated circuit package. The blower is attached to a heat sink mounted on the integrated circuit package. Heat generated by the integrated circuit is conducted to the heat sink. The blower generates a flow of air that flows across the heat sink and removes heat from the package.

  It is known in the art to use a thermal or electrical interface to attach these heat dissipation devices to a heat radiating element. However, it is known that the conventional interface has several drawbacks. Interfaces used in the semiconductor industry typically include a metal interface or a polymer adhesive filled with a conductive filler. Metal interfaces such as solder, silver, and gold provide low resistance but high storage modulus and are not suitable for large IC dies. Furthermore, polymer adhesives can have very low modulus, but their resistance is too high. As the amount of heat radiated from the chip increases, thermal interfaces with low modulus as well as high thermal and electrical conductivity are required for use with integrated circuit packages or semiconductor dies. In addition, the interfaces must be capable of being assembled and processed at a low temperature such as about 200 ° C. or lower.

  The present invention provides a solution to this problem. In accordance with the present invention, a porous, flexible, elastic and thermally conductive material is formed, which material includes a network of metal flakes. These thermally conductive materials are preferably manufactured by first forming a conductive paste comprising a volatile organic solvent and conductive metal flakes. The conductive paste is heated to a temperature below the melting point of the metal flakes, whereby the solvent evaporates and the flakes sinter only at the edges. The ends of the flakes merge with the ends of adjacent flakes such that an open hole is defined between at least some adjacent flakes, thereby forming a network of metal flakes. This network structure allows the thermally conductive material to have a low storage modulus of less than about 10 GPa while having good electrical resistance properties.

  The present invention provides a porous, flexible, elastic, thermally conductive material comprising a network of metal flakes, said flakes having an end, the flake being sintered only at that end And fuse with the ends of the adjacent flakes such that an open hole is defined between at least some of the adjacent flakes.

Furthermore, the present invention provides
a) forming a conductive paste comprising a solvent and conductive metal flakes with edges; and b) heating the conductive paste to a temperature below the melting point of the metal flakes, thereby evaporating the solvent and To fuse the ends of adjacent flakes so that an open hole is defined between at least some adjacent flakes, thereby forming a network of metal flakes. A method of forming a porous, flexible, elastic, thermally conductive material is provided.

The present invention further includes
a) forming a conductive paste comprising a solvent and conductive metal flakes having ends;
b) applying a conductive paste layer between the microchip and the heat spreader, thereby forming a composite material;
c) heating the composite material to a temperature below the melting point of the metal flakes, thereby evaporating the solvent and sintering the flakes only at their ends, thus defining open pores between at least some adjacent flakes Fusing the ends of adjacent flakes to form a thermally conductive material layer between the microchip and the heat spreader, the thermally conductive material comprising a network of metal flakes, and heat from the microchip. Provides a way to conduct and remove.

  The present invention relates to a porous, flexible, elastic and thermally conductive material comprising a network of metal flakes. Initially, a conductive paste containing a mixture of metal flakes and solvent is formed. The paste can be formed using any conventionally known method such as mixing.

  The metal flakes preferably include a metal such as aluminum, copper, zinc, tin, gold, palladium, lead, and alloys and combinations thereof. Most preferably, the flakes contain silver. The flakes preferably have a thickness of about 0.1 μm to about 2 μm, more preferably about 0.1 μm to about 1 μm, and most preferably about 0.1 μm to 0.3 μm. The flakes preferably have a diameter of about 3 μm to 100 μm, more preferably about 20 μm to about 100 μm, and most preferably about 50 μm to about 100 μm. In the most preferred embodiment, each flake has an edge that is thinner than the center of the flake.

  The solvent preferably serves to lower the melting point of the metal flakes. The solvent preferably includes a volatile organic solvent having a boiling point of about 200 ° C. or less. Suitable volatile organic solvents include, but are not limited to, alcohols such as ethanol, propanol, and butanol. Preferred volatile organic solvents include butanol.

  The conductive paste is heated so that the solvent evaporates and the metal flakes sinter only at their edges. The flakes thus fuse at the ends of adjacent flakes so that an open hole is defined between at least some adjacent flakes, thereby making the porous, flexible and elastic substantially A thermally conductive material free of solvent and binder is formed. The conductive paste is preferably heated at a temperature not higher than the melting point of the metal flakes. In preferred embodiments, the heating is in the temperature range of about 100 ° C to about 200 ° C, more preferably about 150 ° C to about 200 ° C, and most preferably about 175 ° C to about 200 ° C.

In a preferred embodiment, the thermally conductive material is manufactured in the form of a thermally conductive material layer. This is preferably done by applying a layer of conductive paste to the surface of a substantially flat substrate and heating the conductive paste layer as described above to form a thermally conductive material layer. The thermally conductive material layer can then optionally be removed from the substrate. Examples of suitable substrates include, but are not limited to, heat spreaders, silicon dies, and heat sinks. A preferred substrate includes a silicon die. The paste can be applied using any known conventional technique, such as dispensing from a syringe. The thermally conductive material layer preferably has a thickness of about 10 μm to about 50 μm, more preferably about 10 μm to 35 μm, and most preferably about 20 μm to about 30 μm. The thermally conductive material layer preferably includes a storage modulus of less than about 10 GPa, more preferably from about 1 GPa to about 5 GPa, and most preferably from about 1 GPa to about 3 GPa. Also, the thermally conductive material layer preferably includes an electrical resistance of about 1 × 10 ⁻⁶ ohm / cm to about 1 × 10 ⁻⁴ ohm / cm, more preferably about 1 × 10 ⁻⁶ ohm / cm to about 1 × 10 ⁻⁶ ohm / cm. 5 × 10 ⁻⁵ ohm / cm, most preferably about 1 × 10 ⁻⁶ ohm / cm to about 2 × 10 ⁻⁵ ohm / cm.

  The heat conductive material of the present invention can be used for various purposes such as a thermal interface between a metal surface and a silicon die or between a heat radiation article and a heat absorption article. In a preferred embodiment, the first surface of the thermally conductive material layer is affixed to the thermal radiation article. Examples of suitable heat emitting articles include, but are not limited to, microchips, multichip modules, laser diodes, and the like. A preferred thermal radiation article comprises a microchip. The second surface of the thermally conductive material layer can then optionally be affixed to the heat absorbing article. Examples of suitable heat absorbing articles include, but are not limited to, heat spreaders, heat sinks, steam chambers, heat pipes, and the like. Preferred heat absorbing articles include a heat spreader. These thermal radiation or heat absorbing articles can be applied to the thermally conductive material layer using any suitable conventional method known in the art. In the most preferred embodiment, the thermally conductive material layer is formed by first forming a composite material comprising a conductive paste layer applied between the heat-radiating article and the heat-absorbing article. The entire composite material is then heated to form a thermally conductive material between the heat-radiating article and the heat-absorbing article. The thermally conductive material of the present invention is particularly useful in the manufacture of microelectronic devices.

  The following non-limiting examples serve to illustrate the invention. It will be appreciated that variations in component proportions and elemental substitutions of the present invention will be apparent to those skilled in the art and are within the scope of the present invention.

  Silver flakes were mixed with an organic solvent to form a homogeneous paste. At least four pastes were mixed. The ratio of the organic solvent to the metal flakes in the paste is shown in Table 1 below.

The paste was then filled into a syringe and the viscosity of the paste was measured using a Haake viscometer with a 1 ° C. cone and plate. Measurements were performed at 22 s ^-1 and 40 s ^-1 . The viscosities of the different materials prepared are shown in Table 2.

表に示したように、ロット＃１とロット＃２は両方とも、約４０００ｃｐｓ程度の類似した粘度であった。これは、それらが両方とも同じ成分の混合比であるので予測できた。しかし、ロット＃３とロット＃４は粘度が大きく異なり、これは、それらのそれぞれの成分の混合比と相関付けることができる。

As shown in the table, both lot # 1 and lot # 2 had similar viscosities on the order of about 4000 cps. This could be predicted because they are both the same component mixing ratio. However, lot # 3 and lot # 4 have vastly different viscosities, which can be correlated with the mixing ratio of their respective components.

  The material was then cured with four different cure profiles as shown in Table 3 below. Curing was performed using the following procedure.

1) At least 3 slides were prepared for each profile as follows.
a) A 1 inch × 3 inch glass slide was washed with isopropyl alcohol and air dried.
b) The glass slide was placed on a glass slide support and fixed firmly.
c) Using a line engraved on the support as a guide, the tape string was placed 100 mils parallel to the length of the slide. The string length used was at least 2.5 inches.
d) There were no wrinkles or bubbles under the tape.
2) A silver paste was placed on one end of the slide. Using a razor blade maintained at an angle of about 30 degrees on the surface of the slide, the silver paste material was drawn toward the other end of the slide and the material was squeezed between the tape strings.
3) The tape was carefully removed.
4) The material was cured in an oven.

  The cured material was then tested for its volume resistance. Lot # 2 was not tested because lots # 1 and 2 had the same component mix ratio. The volume resistance was tested using the following procedure.

1) The cross-sectional area (μm ² ) of the cured material was measured at three different positions along the tape string.
2) Resistance measurement was performed using a Hewlett-Packard 4-point probe, model number 34401A.
3) Resistance measurements were recorded and resistance was determined using the following formula:

P = R (A / 2.54) cm
Where P = specific resistance, ohm-cm
R = measured resistance, ohm A = cross section, cm ²
2.54 = distance between a pair of internal electrodes, cm

  The results are shown in Table 4 below. From the results it is observed that the material did not sinter in profile 3. This is because the 150 ° C. peak temperature was too low for silver sintering.

  The bond strength of the sintered metal flakes to the three metal surfaces was tested using die shearing to determine the bond strength of the silver paste. The test surface was formed by applying a silver paste on a nickel plate surface, a silver spot plated surface, and a bare copper surface. The metal surface was affixed to the lead frame and cured in an oven.

  Cured samples were tested by the following procedure.

1) The substrate was set on a suitable support on a die shear tester.
2) The tip of the die shear tool was aligned with the widest side of the element to be tested. The die shearing tool was set perpendicular to the substrate and as close to the substrate as possible so as not to contact the substrate surface.
3) Press the “TEST” button on the panel of the tester to start the shear test cycle.
4) After the shear test cycle was completed, the level of force displayed on the panel was recorded.
5) Steps 1 to 4 were repeated until all elements were sheared on the substrate.
6) The average shear strength was calculated for the elements on the substrate.

  From the results it was observed that cure profile 1 was too high for industrial use, while profile 3 did not sinter the material well. Profile 3 did not sinter because the peak temperature of 150 ° C. was too low to sinter the silver. Therefore, adhesion measurements were made only for profiles 2 and 4. The paste lot used in this experiment is Lot # 3. The results are shown in Table 5 below.

表５に示すように、材料はニッケルめっき面上では焼結せず、一方裸の銅面上の接着はやや低かった。高温で銅が酸化し、したがって表面の焼結の障壁になった。しかし銀のスポットめっき面は２００℃のピーク温度で良好な接着を与えた。接着は１８０℃では極端に低かった。プロファイル２の破損モードがダイ／ペースト界面であり、一方プロファイル４ではペースト基板界面であったことに留意されたい。これは、プロファイル４の低い硬化プロファイルで焼結がより少なく、したがって材料内部で破損したことを示している。しかしプロファイル２の２００℃での硬化では焼結のレベルが高かった。破損の界面は、裸のシリコンへの接着が低いダイ／ペースト界面に移った。

As shown in Table 5, the material did not sinter on the nickel plated surface, while the adhesion on the bare copper surface was slightly lower. Copper was oxidized at high temperatures and thus became a barrier to surface sintering. However, the silver spot-plated surface gave good adhesion at a peak temperature of 200 ° C. Adhesion was extremely low at 180 ° C. Note that the failure mode of profile 2 was the die / paste interface, while profile 4 was the paste substrate interface. This indicates that the lower cure profile of profile 4 was less sintered and therefore failed inside the material. However, when the profile 2 was cured at 200 ° C., the level of sintering was high. The failure interface moved to a die / paste interface with low adhesion to bare silicon.

  Lot # 4 was then tested for adhesion using profile 2. The results are shown in Table 6 below.

表６に示すように、銀のスポットめっき面の接着が最も高く、裸の銅面が続いた。この実験においてニッケルめっき面が最小の接着を示した。しかし、この特別の材料ロットは、ロット＃３に較べて銀スポットめっき面への接着が極めて低いことを示した。これはロット＃４内の有機溶媒の量がより高いレベルにあったためであろう。

As shown in Table 6, the adhesion of the silver spot plated surface was the highest, followed by the bare copper surface. In this experiment, the nickel plated surface showed minimal adhesion. However, this special material lot showed very low adhesion to the silver spot plated surface compared to Lot # 3. This may be because the amount of organic solvent in Lot # 4 was at a higher level.

Analysis and Conclusion It was found that the three materials were mixed and the viscosity was dependent on the mixing ratio of their organic content. All of these pastes were sinterable. Four different sintering profiles were used. It has been found that silver flakes can be sintered at 180 ° C. or higher. The volume resistivity of the sintered material was found to be lower than that of silver filled epoxy adhesive, but comparable to the conductivity of silver glass and solder. The adhesion of these pastes was mainly temperature dependent. It was also dependent on the surface to which it was glued. In particular, a metallized die surface (eg with silver) would be a better surface. Apart from the surface of the die, the material did not sinter well on the nickel plated surface, but adhered well to the silver spot plated surface.

  Although the invention has been shown and described in detail with reference to preferred embodiments, those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Will be easily recognized. The claims are to be construed to include the disclosed embodiments, the alternatives discussed above and all their equivalents.

It is a top view which shows the heat conductive material layer of this invention. It is an enlarged top view which shows the heat conductive material layer of this invention. It is a side view which shows the heat conductive material layer of this invention. It is a side view which shows the heat conductive material layer of this invention affixed on the microchip. It is a side view which shows the heat conductive material layer of this invention affixed on the microchip and the heat spreader.

Claims (18)

a) forming a conductive paste comprising a volatile organic solvent selected from the group consisting of ethanol, propanol, and butanol and conductive metal flakes with ends; and
b) heating the conductive paste to a temperature below the melting point of the metal flakes, thereby evaporating the volatile organic solvent and sintering the flakes only at its ends, thus at least adjacent to the open holes Porous, flexible, elastic heat transfer manufactured in a method that includes fusing the ends of adjacent flakes as defined between the flakes, thereby forming a network of metal flakes Sex material.   The thermally conductive material according to claim 1, wherein the solvent and the binder are substantially absent.   The thermally conductive material according to claim 1, which has a storage elastic modulus of 10 GPa or less. The heat conductive material of Claim 1 which has an electrical resistance of 1 * 10 < ^-6 > ohm / cm-1 * 10 < ^-4 > ohm / cm.   The thermally conductive material according to claim 1, wherein the flakes have a thickness of 0.1 μm to 2 μm and a diameter of 3 μm to 100 μm.   The thermally conductive material according to claim 1, wherein the flakes include a metal selected from the group consisting of aluminum, copper, lead, zinc, tin, gold, palladium, and alloys and combinations thereof.   A microelectronic device comprising a layer of a thermally conductive material of claim 1 affixed to a thermal radiation article.   A microelectronic device comprising: a microchip; a heat spreader on the microchip; and the thermally conductive material layer of claim 1 affixed between the microchip and the heat spreader. a) forming a conductive paste comprising a volatile organic solvent selected from the group consisting of ethanol, propanol, and butanol and a conductive metal flake having an end; and b) reducing the conductive paste below the melting point of the metal flake. Adjacent flakes such that the volatile organic solvent evaporates and thereby sinters the flakes only at their ends, thus opening holes defined at least between adjacent flakes. A method of forming a porous, flexible, elastic, thermally conductive material comprising fusing the ends of the substrate, thereby forming a network of metal flakes.   The method of claim 9, further comprising the subsequent step of applying a thermally conductive material layer to the microchip.   The method of claim 9, further comprising a subsequent step of applying a thermally conductive material layer between the microchip and the heat spreader.   The method of claim 9, further comprising the subsequent steps of applying a thermally conductive material to the microchip and applying a second surface of the thermally conductive material to the heat spreader.   The method of claim 9, wherein the flakes comprise a metal selected from the group consisting of aluminum, copper, lead, zinc, tin, gold, palladium, and alloys and combinations thereof.   The method of claim 9, wherein the flakes comprise silver.   The method according to claim 9, wherein the flakes have a thickness of 0.1 μm to 2 μm and a diameter of 3 μm to 100 μm. The method of claim 9 , wherein the volatile organic solvent comprises butanol.   The method of claim 9 wherein in step (b) the flakes are heated to a temperature in the range of 100C to 200C. a) forming a conductive paste comprising a solvent and conductive metal flakes having edges;
b) applying a layer of conductive paste between the microchip and the heat spreader, thus forming a composite material;
c) heating the composite material to a temperature below the melting point of the metal flakes, thereby evaporating the solvent and sintering the flakes only at their edges, so that there are open pores between at least some adjacent flakes Fusing the ends of adjacent flakes as defined to form a thermally conductive material layer between the microchip and the heat spreader, the thermally conductive material comprising a network of metal flakes. A method of conducting heat away from a microchip.

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