Use of preceramic polymers as electronic adhesives

Abstract: A method of adhering an electronic component to a substrate in which a layer of a preceramic polymer is applied between the electronic component and the substrate to which it is to be adhered followed by heating to convert the preceramic polymer into a ceramic. The method forms a strong bond which is not affected by the environment. The method is valuable for adhering integrated circuit chips to carriers or circuit boards.

Description: The present invention relates to the use of preceramic polymers as adhesives in the electronics industry. These adhesives form secure bonds between electronic components which are not subject to degradation. Conventionally, electronic components have been interconnected with substrates, such as carriers or circuit boards, by metallic solders. Such solders are advantageous in that they form high strength bonds with excellent electrical and thermal conductivity. Moreover, such bonds are formed quickly at a low cost. The use of solder, however, has a number of drawbacks. For instance, solder is generally applied at high temperatures which can damage either the electronic component or the substrate. Moreover, the processes used for applying these solders are very sensitive such that temperature control, cleaning, fluxing, etc. are critical to prevent problems such as corrosion, weak joints, stress cracking and the like. To overcome these problems, a number of polymer based alternatives have been suggested. They include materials such as epoxies, acrylic resins and silicones which are filled with conductive particles and used to adhere electronic components to substrates. These polymers have the advantage of low temperature curing, simplified processing and formation of strong, electrically conductive bonds which have fatigue and shock resistance. The polymer alternatives have additional advantages and utilities in the electronic industry. For instance, unfilled polymers can be used as potting compounds to merely adhere large electrical components to the circuit board. Similarly, the polymers can be filled with materials that are excellent thermal conductors to distribute heat away from temperature sensitive devices. The polymer alternatives, however, also have disadvantages. For instance, polymer application techniques are sensitive to factors such as polymer concentration, solvent type, polymer temperature and the like. Moreover, the cure process with is necessary to harden polymers is sensitive to factors such as temperature, humidity, air concentrations and contaminants. Finally, the hardened polymers are often sensitive to the elevated temperatures found in subsequent processing of the electronic component. The use of preceramic polymers as adhesives is also known in the art. For instance, such polymers have been used to adhere a variety of materials such as plastics, ceramics and the like. It was not, however, known to use such materials to adhere electrical components to substrates. We have now determined that preceramic polymers can be used to form secure ceramic bonds between electronic components. The present invention introduces a method of adhering an electronic component to a substrate comprising: providing an integrated circuit chip and a substrate selected from a chip carrier and a circuit board, wherein the integrated circuit chip and substrate each have a surface to be adhered to the other; applying a layer of a composition comprising a hydrogen silsesquioxane resin between the surfaces to be adhered; joining the surfaces to be adhered to form an assembly comprising the integrated circuit chip and the substrate with the hydrogen silsesquioxane resin therebetween; and heating the assembly to a temperature sufficient to convert the hydrogen silsesquioxane resin to a ceramic. The present invention also introduces a method of adhering an electronic component to a substrate comprising: providing an integrated circuit chip carrier and a circuit board, wherein the integrated circuit chip carrier and circuit board each have a surface to be adhered to the other; applying a layer of a composition comprising a hydrogen silsesquioxane resin between the surfaces to be adhered; joining the surfaces to be adhered to form an assembly comprising the integrated circuit chip carrier and the circuit board with the hydrogen silsesquioxane resin therebetween; and heating the assembly to a temperature sufficient to convert the hydrogen silsesquioxane resin to a ceramic. The present invention is based on our unexpected discovery that a hydrogen silsesquioxane resin can be used to form a ceramic bond between an electronic component and a substrate. This bond is easy to form, secure and not subject to thermal or environmental degradation. Essentially, the process of the present invention comprises positioning a layer of a hydrogen silsesquioxane resin between the electronic component and the substrate to which it is to be adhered followed by heating to convert the hydrogen silsesquioxane resin into a ceramic. As used in the present invention, "electronic components" are integrated circuit chips and integrated circuit chip carriers. The "substrates" employed herein are circuit boards and chip carriers. Hydrogen silsesquioxane resins ("H-resin") include hydridosiloxane resins of the formula HSi(OH) x (OR) y O z/2 . in which each R is independently an organic group or a substituted organic group which, when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent, x = 0-2, y = 0-2, z = 1-3, x + y + z = 3. Examples of R include alkyls such as methyl, ethyl, propyl, butyl, etc.; aryls such as phenyl; and alkenyls such as allyl or vinyl. As such, these resins may be fully condensed (HSiO 3/2 ) n or they may be only partially hydrolyzed (i.e., containing some Si-OR) and/or partially condensed (i.e., containing some Si-OH). Although not represented by this structure, these resins may contain a small number (e.g., less than 10%) of silicon atoms which have either 0 or 2 hydrogen atoms attached thereto due to various factors involved in their formation or handling. The above H-resins are methods for their production are known in the art. For example, U.S. Patent 3,615,272, teaches the production of a nearly fully condensed H-resin (which may contain up to 100-300 ppm silanol) by a process comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate hydrolysis medium and then washing the resultant resin with water of aqueous sulfuric acid. Similarly, U.S. Patent 5,010,159 discloses an alternative method comprising hydrolyzing hydridosilanes in an arysulfonic acid hydrate hydrolysis medium to form a resin which is then contacted with a neutralizing agent. Other hydridosiloxane resins, such as those described in U.S. Patent 4,999,397, those produced by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic hydrolysis medium and those described in JP-As 59/178749, 60/86017 and 63/107122, or any other equivalent hydridosiloxane, will also function herein. If desired, particulate fillers can also be added to the hydrogen silsesquioxane resin to modify the characteristics of the resultant ceramic. For instance, metals such as gold, silver, platinum, aluminum, copper and the like can be added to render the ceramic electrically conductive. Similarly, fillers such as the oxides, nitrides, borides and carbides or various metals and non-metals (e.g., glass, alumina, silica, titanium dioxide, SiC, SiN, AlN, B 4 C, ZrB, ZrC) can be added to modify properties and characteristics such as thermal conductivity, viscosity of the polymer, color, etc. Such fillers and their uses are known in the art. Generally, if fillers are used, they are added in amounts of 1-90 wt%. Still other materials may be present in the coating compositions of the invention. For instance, adhesion promoters such as glycidoxypropyltrimethoxysilane, mercaptopropyltrimethoxysilane and vinyltriacetoxysilane or suspending agents known to those skilled in the art can be included herein in amounts of 1-20 wt%. A layer of the hydrogen silsesquioxane resin and any fillers is applied between the surface of the electronic component to be bonded and the surface of the substrate to be bonded. This is accomplished in any desirable manner, but a preferred method involves dissolving or dispersing the hydrogen silsesquioxane resin in a solvent and applying the mixture to the surface of either the electronic component or the substrate by a means such as drop dispensing, dip coating, spray coating or flow coating and following by evaporating the solvent. Other equivalent means such as melt coating are also deemed to be operable. Generally the two surfaces to be bonded are then brought together with the layer of hydrogen silsesquioxane resin therebetween. If desired, pressure can be applied and maintained to more firmly bond the component to the substrate. The hydrogen silsesquioxane resin is then typically converted to a ceramic by heating it to a sufficient temperature. Generally, the temperature is in the range of 50 to 1000°C depending on the pyrolysis atmosphere and the resin. Preferred temperatures are in the range of 50 to 800°C and, more preferably, 100-500°C. Heating is generally conducted for a time sufficient to ceramify, generally up to 6 hours, with less than 3 hours being preferred (e.g., 5 minutes - 3hrs). The above heating may be conducted at any effective atmospheric pressure form vacuum to superatmospheric and under any effective oxidizing or non-oxidizing gaseous environment such as those comprising air, O 2 , ozone, an inert gas (N 2 , Ar, etc.), ammonia, amines, moisture, N 2 O, H 2 , etc. Any method of heating such as the use of a convection oven, rapid thermal processing, hot plate or radiant or microwave energy is generally functional herein. The rate of heating, moreover, is also not critical, but it is most practical and preferred to heat as rapidly as possible. When the heating is complete, the ceramic forms a strong bond between the electronic component and the substrate. This bond can have a variety of physical and electrical properties (conductive to dielectric) depending on any fillers or additives which may have been used. Moreover, the bond is resistant to damage by the environment, temperature, chemicals and physical manipulation. The following non-limiting examples are included so that one skilled in the art may more readily understand the invention. EXAMPLE 1 Hydrogen silsesquioxane resin, 12 g, produced by the method of U.S. Patent 3,615,272, 36 g of plasma alumina microballoons (6 micrometer), 3 g of glycidoxypropyltrimethoxysilane and 15 g of dimethylcyclosiloxanes were mixed to form a thick paste adhesive. Aluminum panels, 2.5 cm x 7.5 cm x 0.16 cm, were used to test adhesive strength. The panels were cleaned with toluene and then placed in an ultraviolet ozone cleaning system for 15 minutes. Next, 5 mL of the adhesive paste was placed in the center of one of the panels and a second panel was placed cross-wise on the first panel to form a cross with a 2.5 sq. cm center area that bonded the 2 panels together. A 50 g weight was placed on the center area and the bonded panels were pyrolyzed at 400°C. for either 1 or 4 hours. An Instron ™ Testing Machine with a 4.5 kg load cell and a pull rate of 0.625 cm/min was used to determine the adhesive bond strength. The results are as follows: 400°C./1 hour 400°C./4 hours Adhesive Strength 322 g/cm 2 (2 sample average) 420 g/cm 2 (4 sample average) (1 sample > 700 g/sq cm) EXAMPLE 2 Hydrogen silsesquixane resin produced by the method of U.S. Patent 3,615,272 was diluted to 10 wt% in toluene. 50 ppm of platinum, based on the weight of H-resin, was added to the solution. The solution was applied to the unpolished side of a silicon wafer and the unpolished side of a second wafer was applied on top of it. The bonded wafers were pyrolyzed at 400°C for 1.5 hours. The 2 wafers stuck together very well and could not be broken apart by applying lateral force. EXAMPLE 3 Hydrogen silsesquioxane resin produced by the method of U.S. Patent 3,615,272 was diluted to 10 wt% in toluene. 50 ppm of platinum, based on the weight of H-resin, was added to the solution. This solution was applied to the polished side of 4 silicon wafers and the polished side of 4 other wafers were applied on top of them. The bonded wafers were pyrolyzed at 400°C for 1.5 hours. 3 of the 4 wafer sets stuck together and 1 wafer set cracked and did not adhere.

Claims: A method of adhering an electronic component to a substrate comprising: providing an integrated circuit chip and a substrate selected from a chip carrier and a circuit board, wherein the integrated circuit chip and substrate each have a surface to be adhered to the other; applying a layer of a composition comprising a hydrogen silsesquioxane resin between the surfaces to be adhered; joining the surfaces to be adhered to form an assembly comprising the integrated circuit chip and the substrate with the hydrogen silsesquioxane resin therebetween; and heating the assembly to a temperature sufficient to convert the hydrogen silsesquioxane resin to a ceramic. A method of adhering an electronic component to a substrate comprising: providing an integrated circuit chip carrier and a circuit board, wherein the integrated circuit chip carrier and circuit board each have a surface to be adhered to the other; applying a layer of a composition comprising a hydrogen silsesquioxane resin between the surfaces to be adhered; joining the surfaces to be adhered to form an assembly comprising the integrated circuit chip carrier and the circuit board with the hydrogen silsesquioxane resin therebetween; and heating the assembly to a temperature sufficient to convert the hydrogen silsesquioxane resin to a ceramic. The method of claims 1 or 2 wherein the hydrogen silsesquioxane resin further comprises a filler. The method of claims 1 or 2 wherein the assembly is converted to a ceramic by heating at a temperature in the range of between 150 and 600°C for a period of time between 5 minutes and 6 hours.

Inventors: BEARINGER CLAYTON R, CAMILLETTI ROBERT CHARLES, CHANDRA GRISH, GENTLE THERESA EILEEN, HALUSKA LOREN ANDREW, BEARINGER, CLAYTON R., CAMILLETTI, ROBERT CHARLES, CHANDRA, GRISH, GENTLE, THERESA EILEEN, HALUSKA, LOREN ANDREW
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1996
Main classification: C09J5
IPC codes: C09J5, H05K7, H01L21