Pastes for thermal, electrical and mechanical bonding

Abstract

A class of paste materials for thermal, and mechanical bonding, and in some cases electrical interconnection, of two solid surfaces includes particles and an organic vehicle which is partially or completely removed during processing. The paste includes hybrids of inorganic materials for meeting the thermal, electrical and mechanical bonding functionality requirements and organic materials for meeting the process, application and protection requirements. The inorganic materials include high thermal and optionally electrical conductivity materials in forms from nanoparticles to micro-powders. The organic materials may include small molecules, surfactant, oligomers, and polymers.

Claims

1. A hybrid paste composition, comprising: a nanostructured inorganic material having particles having a largest dimension of about 1-30,000 nanometers, and a bulk thermal transfer coefficient of at least about of about 10 W/mK, and being sinterable at a temperature below about 450 C. to form an interconnected network; and at least one organic material effective for dispersing and stabilizing the particles within a flowable paste for application between surfaces prior to heating to a temperature above at least 70 C., the at least one organic material separable from the nanostructured inorganic material under heat at temperatures below about 450 C. and compression between the surfaces; the hybrid paste composition being configured to form a sintered thermal interface material comprising an interconnected network of sintered inorganic material configured to withstand a cyclic shear stress of at least 0.2 MPa, and having at least 50% by weight inorganic material and a bulk thermal conductivity higher than about 10 W/mK, after heating of the hybrid paste composition to a temperature between about 70 C. and 450 C. and compression.

2. The composition according to claim 1, wherein the particles comprise: a first particle type having a size in each dimension of about 100-30,000 nanometers, having a bulk thermal transfer coefficient in excess of about 10 W/mK; and a second particle type having a size in each dimension of about 1-100 nanometers, having a bulk thermal transfer coefficient in excess of about 10 W/mK, the first particle type being sinterable in a presence of the second particle type, at a temperature below about 450 C.

3. The composition according to claim 2, wherein the nanostructured inorganic material comprises at least 25% by mass of the first particle type and the at least 25% by mass of the second particle type.

4. The composition according to claim 2, wherein the nanostructured inorganic material comprises at least 25% by volume of the first particle type and the at least 25% by volume of the second particle type.

5. The composition according to claim 1, wherein the hybrid paste composition is stable at room temperature in a ready-to-use form for at least 2 weeks.

6. The composition according to claim 1, wherein the particles comprise a material selected from the group consisting of copper, tin, indium, silver, gold, gallium, aluminum, silicon, boron, lithium, magnesium, palladium, and carbon.

7. The composition according to claim 1, wherein the nanostructured inorganic material comprises an alloy having a sintering temperature at least 2 C. below that of at least one component of the alloy.

8. The composition according to claim 1, wherein at least a portion of the particles are heterogeneous, having a plurality of discrete particle regions of a respective particle, each having different respective composition.

9. The composition according to claim 1, wherein the particles have an at least bi-modal size distribution, having first size nanoparticles having a size in each dimension of 1-100 nm and a second size particles having a size in each dimension of 1-30 m.

10. The composition according to claim 9, wherein the first size nanoparticles and the second size particles have different chemical composition.

11. The composition according to claim 1, wherein the particles comprise nanowires having a cross section size of 1-30 nm and a length of 0.1-30 m.

12. The composition according to claim 1, wherein the particles comprise nano-platelets having a thickness of 1-100 nm and a size along an axis perpendicular to a thinnest dimension of the platelet of 0.1-20 m.

13. The composition according to claim 1, wherein the at least one organic material comprises at least one material selected from the group consisting of a surfactant, an oligomer, a polymer, a cross-linking agent, an alcohol, a sulfate, a sulfoxide, an acrylic, a ketone, acetone, acetonitrile, an alkane, a cycloalkane, an alkene, an ether, benzene, and an aromatic solvent.

14. The composition according to claim 1, wherein the at least one organic material comprises a solvent with a polarity intermediate between polar solvents and non-polar solvents, selected from the group consisting of an acetate, furan, an amine, and a monomer with one or more unsaturated bonds.

15. The composition according to claim 1, wherein the at least one organic material comprises at least one surfactant having an ionic end-group selected from the group consisting of a sulphonate, a carboxylate, a sulphate, an amine, octoxynol, a polyethylene glycol ester, a carboxylic ester, and a carboxylic amide.

16. The composition according to claim 1, comprising 20-95% by mass of the nanostructured inorganic material and 15-80% by mass of the organic material.

17. The composition according to claim 1, wherein the particles have a respective particle surface, and further comprise at least one of an oligomer and a polymer absorbed to the particle surface.

18. The composition according to claim 1, wherein the at least one organic material comprises discrete stable domains of aggregated organic material suspended within an organic liquid.

19. The composition according to claim 1, wherein the particles comprise nanoparticles having a first chemical composition and particles which are larger than nanoparticles having a second chemical composition, difference from the first chemical composition, the nanoparticles being fusible with the non-nanoparticles at temperatures between about 70 C. and 450 C.

20. A hybrid paste composition, comprising: a nanostructured inorganic material comprising particles having a largest dimension of about 1-30,000 nanometers, and a bulk thermal transfer coefficient of at least about of about 10 W/mK, having a composition which is sinterable at a temperature below about 450 C. to form an interconnected network of the inorganic material; and at least one organic material effective for dispersing and stabilizing the particles within a flowable paste for application between surfaces prior to heating to a temperature above at least 70 C., the at least one organic material being separable from the nanostructured inorganic material under heat at temperatures below about 450 C. and compression between the surfaces; the hybrid paste composition being configured, after being heated to a temperature between about 70 C. and 450 C. and compressed between surfaces, to form a sintered interconnected network of sintered inorganic material between the surfaces, configured to withstand a cyclic shear stress applied between the surfaces greater than about 5 MPa without persistent change in thermal conductivity, and having at least 50% by weight inorganic material and a bulk thermal conductivity higher than about 10 W/mK.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B show schematic diagrams of the paste and formed TIM according to the present technology;

(2) FIGS. 2A and 2B show the temperature and relative luminescence of LED/heat-sink package placed on paper box using thermal grease and Ag nanopaste TIMs, respectively.

(3) FIGS. 3A and 3B show the temperature and relative luminescence of LED/heat-sink package placed on an optical table using thermal grease and Ag nanopaste TIMs, respectively.

(4) FIG. 4. Steady state thermal gradient measurements on nanopaste TIM showing interfacial thermal resistance of 2.40.210.sup.6 (m.sup.2K/W) and bulk thermal conductivity of 201 (W/mK).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) FIG. 4 shows the steady state thermal gradient measurements on a microstructured nanopaste TIM showing interfacial thermal resistance of 2.40.210.sup.6 (m.sup.2K/W) and bulk thermal conductivity of 201 (W/mK), for silver nanoparticles having a flake size of about 100 nm-1 m, and a thickness of 10's of nanometers.

(6) Tests prove the concept of Ag flake-particle combination nanopaste design and applications. For example, an LED's junction temperature has an impact on its luminescent efficiency. The test setup is shown in FIG. 1, which shows an LED array package on a heat sink, a photo detector to measure the luminescence, and a thermocouple to monitor junction temperatures. The power supply controls both the voltage and current to the 9 LED arrays, and maintains a constant power output at 7.4 W. Photo detector is fixed at a certain distance right above the LED lamp. Relative luminescence is calculated by normalizing the measured intensity to that at the initial time.

(7) To maintain a well-defined surface and improve the adhesion with TIM, the back side of the as-received LED chip as well as the heat sink surface were coated with copper. The LED chip substrate was coated with a 100 nm Cu film by sputtering; while the heat sink was electroplated with 2.5 m Cu. The performance of Ag nanocomposite TIMs was compared to that of a commercial thermal grease G-751. Tests on the LED package without any TIM were also carried out for comparison. The heat sink was placed on a paper box supported on a stainless steel optical table. The thickness of the TIM layer was 100 m. Silver paste was applied between the LED board and the heat sink, and sintered at 200 C. for 1 hour under a 1.3 lb metal block. FIGS. 2A and 2B show the temperature and relative luminescence of LED/heat-sink package placed on paper box using G-751 thermal grease and Ag nanopaste TIMs, respectively.

(8) The TIM Ag paste includes silver nanoparticles which are about 4-6 nm in diameter and silver nanoflakes which are about 0.3-2 m in lateral dimension. The LED board temperature goes up as the LED is turned on, and reaches a stable temperature in about 30 min; meanwhile relative luminescence drops to reach a relatively constant value. The total luminescence drops about 2.8% for thermal grease and about 1.5% for Ag paste, while in both cases the board reaches the final temperature at about 48 C., which is much lower than the 82 C. reached when no TIM was used. For LED packages placed directly on optical table, as shown in FIGS. 3A and 3B, however, the final temperature was about 35 C. for the G-751 thermal grease and about 33 C. for the Ag paste. Accordingly, the lighting efficiency dropped about 1% for the thermal grease and about 0.4% for the Ag paste.

(9) The overall performance of composite silver TIMs according to the present technology is comparable to or slightly better than that of best thermal grease the inventors could find on the market, which has thermal conductivity of about 5 W/mK. The bulk thermal conductivity of sintered Ag paste according to the present technology was measured to be about 20 W/mK. The thermal resistance between the Ag TIM and Cu plates is 610.sup.6 m.sup.2K/W, and the bond yield strength is 2 MPa.

(10) So in principle, Ag-paste should be seen to perform much better. However, aforementioned values were obtained from Cu/Ag-paste/Cu sandwiches prepared by sintering at high pressure, at about 1000 psi. The LED sample described with respect to FIG. 2B was prepared with very low pressure, so the contacts may not be as tight and the bulk density not as high. Consequently, the TIM in LED pack has higher interfacial contact resistance and lower bulk thermal conductivity than possible through application of increased pressures during TIM formation.

(11) More generally, a thermal resistance of less than 10.sup.5 m.sup.2K/W will be achieved according to the present technology, along with mechanical properties, such as an ability to withstand a cyclic shear stress, i.e., to handle shear without permanent deformation or flow. Traditional powders or pastes and liquids, cannot withstand shear forces. Solid interfaces (such as fused solder) may fail under temperature cycling or at extreme temperatures due an inability to accommodate forces resulting from thermal temperature coefficient mismatch, for example.

(12) In another example, a low viscosity paste containing 20% silver nanoparticles in organic solvent is applied to a heated substrate. The paste is compatible with inkjet printing. Solvent is dried during the printing. The paste is sintered at 180 C. under pressure at the joint surfaces, during which the remaining organic content is evaporated. The specific thermal resistance is 1.410.sup.5 K/Wm.sup.2, electric resistivity is 10 .Math.cm, and yield strength of 2 MPa.

(13) The base formulation of the Ag nanocomposite paste contains Ag-NFs and Ag-NPs as functional inorganic components and at least three organic materials for stabilization and dispersion of nanomaterials and the application of pastes in assembly. The relative composition of NFs and NPs, or the NF/NP ratio, may be used to optimize the thermal properties, process window, formulation recipe, and materials cost. By tuning the NF/NP ratio, the optimal parameters for specific applications can be achieved.

(14) For example, in LED applications, the performance requirement bar may be lower, however, demands on cost and easy assembly is very high. The overall cost structure needs to consider not only the materials and manufacturing costs, but also the gains in operating luminescent efficiency and longer lifetime. In addition to NF/NP ratio, the wettability of the paste to solid substrates can be addressed by tailoring organic components in the paste. Although a general design principle according to the present technology is to remove all organic components after assembly, a certain level of organic residuals in the final product is tolerable, and these residuals may be functional.

(15) Three parameters are generally important for process optimization: TIM bondline thickness, sintering temperature, and sintering time.

(16) The thickness of the paste layer is adjusted according to the roughness of the solid surfaces (e.g., those of chip board and heat sink for LED packing, and those of lid, die or substrate in the case of high performance electronics packaging) so that upon being pressed between the solid surfaces, Ag pastes would fill all interfacial voids.

(17) Because the paste formulation involves multiple organic components with different vapor pressure and boiling temperature, control of thermal annealing temperature and time profiles is important in the resulting TIM performance. Currently, pastes were annealed in two steps, a low temperature step (room temperature to 70 C.) to dry the low boiling temperature solvents, and a high temperature (130 C. to 200 C.) to remove remaining organic molecules. Optimal processing conditions vary with formulations. Because removal of the organic components reduces non-void volume, the TIM is applied in excess as compared to the final module.

(18) Further process optimization also involves proper treatment of solid surfaces. In previous examples, Cu coating was used to improve the bond strength between the TIM and substrate. As direct bonding of Ag TIMs with Al is weak, reasonably strong bonding was achieved with an aluminum heat sink after acid treatment. In another approach, thin layers of Ag-NPs are deposited on both surfaces from a solution and sintered at 130 C. for 5 min to form a metallic Ag film. Subsequent application of Ag TIM paste therefore make good contact to the Ag pre-layer and conforms to the roughness features of the solid surfaces, intimately joining the two solid surfaces with high thermal conductivity passages.

(19) With seamless metallic contacts and strong bonding, the assembly can sustain high operation temperatures without the need for the external pressure. One advantage of the TIMs is for high temperature application, especially since organics that might outgas and leave voids are removed during original TIM formation.

(20) If localization of TIM application is needed, such as to address distributed singular hotspots over large areas, paste formulations may be provided to allow for inkjet printing of TIM layers. The inkjet may be of the piezoelectric kind, in which droplets are ejected by piezoelectric elements in narrow passages which generated pressure pulses, or of the bubblejet kind, in which a component of the fluid is boiled by a localized heating to eject TIM from a nozzle. The suspension for inkjet printing may include water or a low boiling point organic liquid, either of which will evaporate immediately after deposition in the pattern, leaving the organic matrix and inorganic silver nanoparticles and nanoflakes in the desired pattern.

(21) An alternate scheme for depositing the TIM may involve electrographic deposition. See, U.S. Pat. Nos. 8,304,150, 8,138,075, 8,066,967, 7,585,549, 6,815,130, 6,579,652, and 6,524,758, expressly incorporated herein by reference. Note that the nanoparticles and/or nanoflakes may be formed as particles having an organic shell, which is degraded during heating and which facilitates sintering of the particle core.

(22) Using any of a variety of printing techniques, TIM layers with carefully designed patterns (such as lines, grids, or pads) are deposited. The design criterion is to deliver an optimal amount of material to fulfill the thermal management requirement at minimal material usage with best thermo-mechanical stress tolerance. Excess material leads to larger interface distances and higher material cost and perhaps process cost, while insufficient material leads to voids and gaps. Thermo-mechanical stress tolerance is a distinct factor, which does not necessarily lead to a maximization or minimization of TIM thickness, and indeed, may lead to different TIM optimal thickness in various spatial regions of a complex TIM interface.

(23) The morphology of TIM layer after assembly is a key that links formulation and processing parameters to the thermal, electrical, mechanical properties and performance.

(24) In many instances, the TIM also serves a role as an electrical conductor. This has two impacts; the efficiency of the electrical device will be impacted by the electrical conductivity of the TIM, and electrical resistance of the TIM under load will lead to intrinsic heat dissipation. In both these instances, use of silver is advantageous, because it is highly conductive for heat and highly electrically conductive. Thus, for example, LED packages may use the lower surface as both an electrode and a heatsink.

(25) The optimal formulation of the paste may encompass characterization of the individual components, as well as their interaction during processing to form the final TIM. That is, the silver nanoparticles, silver nanoflakes, and organic components of the paste are generally not defined independent of each other. Further, the paste may also include additional components. For example, the paste may include carbon nanotubes, particles of other metals, or the like. In most instances, precipitation of tin from an alloy and formation of tin whiskers is considered a liability; however, in a TIM, a carefully controlled and limited formation of crystalline structures may improve thermal, electrical, and/or mechanical performance. In some cases, the formation of crystalline precipitates from alloys may improve thermal, electrical, and/or mechanical properties of the interconnected network.

(26) The structure, processing and property relationships may be further optimized by computer modeling to optimize the TIM formulation and properties for various uses.

(27) A TIM design is provided in the form of solid films that integrate silver nanoflakes and silver nanoparticles. The paste formulation and process are optimized to minimize both the bulk and interfacial thermal resistances.

(28) It is to be understood that the various embodiments described and taught herein are to be considered as if described in each possible combination, subcombination and permutation.