DIE ATTACH ADHESIVE COMPOSITIONS

Abstract: A novel thermally reworkable die attach adhesive composition for attaching a semiconductor device to a substrate is provided. The composition comprises (a) a thermally reworkable cross-linked resin produced by reacting at least one dienophile having a functionality greater than one and at least one 2,5-dialkyl substituted furan-containing polymer, and (b) at least one thermally and/or electrically conductive material present in an effective amount up to 90 % by weight of the die attach composition to provide a conducting medium.

Description: DIE ATTACH ADHESIVE COMPOSITIONS Field of Invention This invention relates to an adhesive composition. In one aspect, the invention relates to die attach adhesive compositions suitable for attaching semi- conductor devices to carrier substrates. Background of the Invention Die attach adhesives are used to bond the semiconductor device such as a silicon die or chip to a substrate such as a lead frame or a printed circuit board. During assembly of the semiconductor package, the die attach adhesive holds the device firmly in place during wire bonding and encapsulation. They can provide electrical and/or thermal contact between the device and the substrate by means of incorporating electrically and/or thermally conductive fillers in the adhesive formulation. Examples of commonly used die attach adhesives are eutectic solders, conductive epoxies, and conductive polyimides. Eutectic solders are metal alloys typically made with gold. A "preform", which is a metal foil cut to the shape and size of the semiconductor chip, is deposited on the desired substrate of the package and is heated to a temperature near the melting point of the preform. The chip can then be placed onto the preform with a scrubbing motion. The eutectic solders are expensive and difficult to process. Conductive epoxies are typically low viscosity pastes containing electrically conductive fillers. The epoxy is applied to the substrate by conventional means and the device is then placed in contact with the coated substrate. The epoxy can then be cured in one step. Conductive polyimides are similar to conductive epoxies. Thermosetting die attach adhesives, primarily epoxy-based formulations are used to bond semiconductor devices to substrates. Rework processes to replace defective chips usually involve the use of heat both beneath the substrate and on top of the device accompanied by a shearing force to remove the die from the substrate. Historically, rework has been relatively simple since the devices are small and widely spaced from each other. However, emerging packaging technologies such as multichip modules use both larger devices and smaller spacing between them and therefore raise the possibility of damage to the substrate during rework. Thus, it is desirable to provide a die attach adhesive composition which allows the reworking to be more readily processable. Summary of the Invention According to the invention, a thermally reworkable die attach composition is provided comprising: (a) a thermally reworkable crosslinked resin produced by reacting at least one dienophile having a functionality greater than one and at least one 2,5-dialkyl sub- stituted furan-containing polymer, and (b) at least one thermally and/or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium. Such a die attach composition provides a readily reworkable semiconductor board and/or semiconductor package . Detailed Description of the Invention There may be several ways by which the polymer chains of the thermally reworkable crosslinked resin can be produced. The thermally reworkable crosslinked resin can be produced by reacting at least one dienophile having a functionality greater than one and at least one 2,5-dialkyl substituted furan-containing polymer connecting to one another via Diels-Alder addition. In one embodiment the 2,5-dialkyl substituted furan groups are attached to or form part of the polymer chains. The reversible furan to dienophile reaction to form the Diels-Alder adduct can be represented by: 2,5-DIALKYL DIENOPHILE DIELS-ALDER SUBSTITUTED ADDUCT FURAN where Y is either C< or N- . For a thermally reworkable crosslinked resin, all or a portion of the Diels-Alder adduct can revert to the furan and dienophile upon heating such that the resin is a liquid (flowable material) . A crosslinking agent which contains in its molecular structure two or more dienophiles can also be used in this embodiment. These dienophiles are connected to each other by chemical bonds or by bridging groups. Accordingly, the present invention also contemplates a die attach adhesive composition containing a polymer which comprises moieties of a 2,5-dialkyl substituted furan and a crosslinking agent which comprises two or more dienophiles in its molecular structure. The dienophiles may also be attached to or form part of the polymer chains. Crosslinking agent which comprises in its molecular structure two or more 2,5-dialkyl substituted furan groups can also be used. In yet another embodiment the dienophile is attached to the polymer chains to which the 2,5-dialkyl substituted furan groups are also attached or which contain the 2,5-dialkyl substituted furan groups as a part of their polymer chains. Accordingly, the 2,5-dialkyl substituted furan-containing polymer can also contain moieties of a 2,5-dialkyl substituted furan and moieties of a dienophile. The 2,5-dialkyl substituted furans may or may not be substituted at their 3- and 4-positions. Preferred substituents are inert substituents such as for example alkyl or alkyloxy groups, typically having up to 10 carbon atoms, such as methyl, ethyl, 1-propyl, methoxy and 1-hexyloxy groups. Resins containing furans whose 2 and 5 positions are not substituted are susceptible to side reactions which can cause irreversible gelling and interfere with its reversibility. The 2,5-dialkyl substituted furan groups may be attached to the polymer chains of the polymer (s) on which the crosslinked resin is based. They may be attached thereto directly via a chemical bond or via a divalent organic bridging group for which any of the substituents of the furans or the 3- or 4-positions of the furans may function as the point of attachment. The alkyl substituents at the 2- and 5-positions of the furans may be the same or different and will typically have up to 10 carbon atoms. Examples of suitable alkyl groups are methyl, ethyl, 2-propyl and 1-hexyl groups. Examples of suitable furyl groups which can be attached to a polymer chain are 2, 5-dimethylfur-3-yl, 2,5-di- ethyl-3-methylfur-4-yl, 5-ethylfurfuryl or 5- (1-butyl) furfuryl groups. The type of polymer chains to which the 2,5-dialkyl substituted furan groups may be attached is not critical. Suitably the polymer chains are chains of a polyolefin, such as polyethene, polypropene, polystyrene, poly (acrylic acid) or a copolymer of ethene and acrylic acid or ester, chains of random or alternating copolymers of carbon monoxide and olefinically un- saturated compounds (for a further elaboration on such copolymers cf. hereinafter), or chains which contain heteroatoms, such as chains of polyamide or polyester. It is preferred that the 2,5-dialkyl substituted furans form a structural element of the polymer backbone itself. In such a case it is particularly preferred that each of the 2,5-dialkyl substituents of the furans are alkylene groups which also form part of the polymer chain and which may or may not be substituted. Such a structure can be produced by furanizing copolymers of carbon monoxide and olefinically unsaturated compounds which contain 1, 4-dicarbonyl entities in their polymer chains, i.e. by converting such 1, 4-dicarbonyl entities into furan moieties. Alternatively, a 2,5-dialkyl substituted furan-con- taining polymer can be directly produced by reacting carbon monoxide and olefinically unsaturated compounds in the presence of a strong acid. Perfectly alternating copolymers of carbon monoxide and olefinically unsaturated compounds which contain 1, 4-dicarbonyl entities in their polymer chains are known. They can be prepared by palladium catalyzed polymerization using the methods known from, for example, EP-A-121965, EP-A-181014 and EP-A-516238. The polymers so prepared are alternating copolymers of carbon monoxide and the olefinically unsaturated compounds, i.e. copolymers of which the polymer chains contain the monomer units originating in carbon monoxide (i.e. carbonyl groups) and the monomer units originating in the olefinically unsaturated compounds in an alternating arrangement so that every fourth carbon atom of the polymer chain belongs to a carbonyl group. Alter- native copolymers of carbon monoxide and olefinically unsaturated compounds which contain 1, 4-dicarbonyl entities may be random copolymers, i.e. copolymers of which the polymer chains contain monomer units in a random order. The latter copolymers can be prepared by radical initiated polymerization using the methods known from, e.g., US-A-2495286 and US-A-4024326. The furanization of the copolymer of carbon monoxide and olefinically unsaturated compounds may be effected by methods known in the art, for example, by applying phosphorus pentoxide as dehydrating agent, as disclosed by A. Sen et al. (J. Polym. Science, Part A. Polym. Chem. 32_ (1994) p. 841), or by heating in the presence of a strong acid, such as p-toluenesulphonic acid, as disclosed in US-A-3979373. These methods allow the conversion of 1, 4-dicarbonyl moieties in the polymer chains into furan moieties at a variable conversion level, depending upon the reaction conditions selected. It is preferred to employ in the furanization an alternating copolymer of carbon monoxide and olefinically unsaturated compounds because these have a higher content of 1, 4-dicarbonyl groups in the polymer back- bone so that the furanization can be accomplished efficiently at a high level of incorporation of furan groups. If, nevertheless, a low degree of furanization is desired, the conversion of carbonyl groups into furan groups may be kept low. The copolymers of carbon monoxide and olefinically unsaturated compounds may be based on hydrocarbons as the olefinically unsaturated compounds. It is preferred that the copolymer is based on an olefinically unsaturated hydrocarbon, suitably an α-olefin, in particular an α-olefin having up to 10 carbon atoms. Very suitable are aliphatic α-olefins, in particular those having from 3 to 6 carbon atoms and more in particular those having a straight carbon chain, such as propene, 1-butene, 1-pentene and 1-hexene. Propene is most preferred. The copolymer can be regioregular or irregular, stereoregular or atactic. A 2,5-dialkyl substituted furan-containing polymer where a polymer based on propene and carbon monoxide are furanized can be represented by the formula: The precise nature of the dienophile from which the Diels-Alder adduct is obtained is not critical, as long as the Diels-Alder adduct has such a thermal stability that the crosslinked resin is reworkable. Usually the minimum temperature above which the reworkable cross- linked resin will be reworked depends on the maximum temperature requirements for the semiconductor device used. The reworking is suitably carried out at a temperature from 100 °C, preferably from 130 °C, to 250 °C, preferably to 200 °C. Suitable dienophile functionality can be represented by Y=Y where Y is either C< or N-, or -C≡C- . Preferably the dienophiles are, for example, alkynes having electron withdrawing groups attached to both sides of the ethyne moiety, such as ester and keto groups. Examples are mono- and diesters of butynedioic acid (i.e. acetylenedicarboxylic acid) and substituted but-2- yne-1, 4-diones . Other suitable dienophiles are compounds which contain a but-2-ene-l, 4-dione moiety included in a 5- or 6-membered ring, in particular compounds of the general formula: wherein X denotes 0, S, N-, P- or -R- where R is alkylene, wherein at least one of the free valencies is occupied by a bridging group which connects the dienophile with one of the polymer chains or with another dienophile, and wherein the remaining valencies, if any, are occupied by lower alkyl or acyl substituents or, preferably, hydrogen. The lower alkyl substituents suitably contain up to 4 carbon atoms and are, for example, methyl or ethyl groups. Dienophiles of this general formula are preferably cyclic derivatives of maleic anhydride and, in particular, maleimide (i.e. X denotes 0 or, in particular, N-) . Examples of other suitable dienophiles include, bis (triazolinediones) , bis (phthalazinediones) , quinones, bis (tricyanoethylenes) , bis (azodicarboxylates) ; diacrylates, maleate or fumarate polyesters, acetylene- dicarboxylate polyesters. As indicated hereinbefore, in one embodiment use is made of a crosslinking agent which comprises in its molecular structure two or more dienophiles from which the Diels-Alder adducts are obtainable. The dienophiles may be connected to each other by one or more bridging groups. For example, three dienophiles may be connected to one another by a trivalent bridging group. However, it is sufficient that a crosslinking agent is used in which two dienophiles are connected to one another by a bivalent bridging group. The dienophiles may also be connected to one another by chemical bonds. Both the molecular weight and the chemical nature of the bridging group of the crosslinking agent may be varied to a large extent. It has been found that such variations of the crosslinking agent lead to re-moldable crosslinked resins covering a wide range of mechanical properties. The bridging group may contain only carbon atoms in the bridge but it is also possible that it contains heteroatoms in the bridge, such as oxygen, silicon or nitrogen atoms. The bridging group may be flexible or rigid. For example, polymeric bridging groups having flexible polymer chains, such as poly (alkylene oxide) or polysiloxanes, having a number average molecular weight of, say, more than 300, provide rubbery reworkable crosslinked resins. When the polymeric flexible chain has a number average molecular weight in the order of 1500 - 5000 or more reworkable crosslinked resins may be obtained which could replace thermoplastic rubbers. Accordingly, suitable crosslinking agents of this kind are the bis-maleimido-capped poly (alkylene oxide) s, such as poly (ethylene oxide) s or poly (propylene oxide) s, and bismaleimido-capped polysiloxanes, for example the bismaleimides of polysiloxanes of the general formula H2N-CH2 [-0-SiR2] n _0~CH 2 ~ " NH 2' wherein n is an integer, on average, of more than 10 and in particular in the range of 20 - 70, and each R is independently an alkyl group, in particular having up to 5 carbon atoms, preferably a methyl group. Very good results can be obtained with the bismaleimide of bisamino capped poly (propene oxide), in particular having a number average molecular weight of at least 300, more in particular in the range of 1500 - 5000. Low-molecular weight bridging groups, i.e. bridging groups which typically have up to 20 carbon atoms in the bridge, may also be used. Cycloaliphatic and aromatic bridging groups render the bridging groups rigid. Low- molecular weight cycloaliphatic and aromatic bridging groups tend to provide re-moldable crosslinked resins which are hard and brittle, and have a relatively high glass transition temperature. Examples of cycloaliphatic and aromatic low-molecular weight bridging groups are groups containing a norbornane skeleton in the bridge, 1, 3-phenylene groups and groups of the following formulae: -f-CH 2 -f-, -f-O-f-O-f-, -f-0-f-S0 2 - f-O-f- and -f-C (CH3) 2 - _ , wherein -f- denotes a 1, 4-phenylene group. Other suitable bridging groups are alkylene and oxycarbonyl (ester) groups and combinations thereof. Suitable low-molecular weight crosslinking agents are, for example, the bismaleimides of hydrazine, 2 , 4-diaminotoluene, hexamethylenediamine, dodeca- methylenediamine, diamines of the general formula: and bisamino-capped (poly) siloxanes of low molecular weight, such as polysiloxanes of the general formula H2N-CH2 [-0-SiR2l n -0-CH2-NH2, wherein n ranges, on average, from 1 to 10, preferably from 1 to 5 and the groups R are preferably methyl groups. An isomer mixture of the diamines of above formula is commercially available from HOECHST. Very good results can be obtained with bis ( 4-maleimidophenyl) methane and dimethylbis [ (N-maleimidomethyl) oxy] silane . Other suitable crosslinking agents on the basis of maleic anhydride are compounds of the general formula: wherein A denotes a bridging group as described hereinbefore, in particular bridging group having up to 20 carbon atoms in the bridge. More in particular the bridging group A is an alkylene group, such as a hexamethylene group, or groups -D-O-CO- or -CO-O-D-O-CO- wherein D denotes a bivalent hydrocarbyl group, for example an alkylene group, such as a hexamethylene group. Again other suitable crosslinking agents are polyesters based on butynedioic acid and a diol, such as ethylene glycol, a poly (ethylene glycol), propylene glycol or a poly (propylene glycol). These polyesters may be low molecular weight crosslinking agents, such as described hereinbefore, or they may have a number average molecular weight of, for example, more than 400, such as in the range of 2000 - 6000. The present invention also relates to crosslinking agents such as bis-maleimido-capped poly (alkylene oxide) s, in particular bismaleimido-capped poly (propene oxide) s. Such agents have a number average molecular weight of at least 300, preferably in the range of 1500 - 5000. The bismaleimides of polysiloxanes have the general formula H2N-CH2 [-0-SiR2l n -0_CH 2" NH 2' wherein n is an integer of at least 1 and each R is independently an alkyl group, in particular having up to 5 carbon atoms, preferably a methyl group. The bismaleimido- capped polysiloxanes can be prepared by N-hydroxy- methylation of maleimide with formaldehyde and subsequent reaction with the appropriate dichloro- dialkylsilane in the presence of base and water using generally known methods. As noted above, certain embodiments relate to a crosslinking agent which comprises in its molecular structure 2, 5-dialkylfuran moieties. In this cross- linking agent the 2,5-dialkyl substituted furan groups can be connected to one another via a chemical bond or via a bridging group. The nature of this bridging group is generally the same as the bridging group of the crosslinking agents which comprise two or more dienophiles, as described hereinbefore. Examples of suitable crosslinking agents are bis (5-ethylfurfuryl) adipate and the bis-amides of (5-ethylfurfuryl) acetic acid and the diamines mentioned in the preceding paragraphs . The 2,5-dialkyl substituted furan moieties and/or moieties of a dienophile may be connected to the polymer chains by means of a chemical bond or by means of a bridging group. This bridging group may be of the same type as the bridging groups of the crosslinking agents. Examples may be given as follows. When the polymer is a polystyrene, maleimide, as the dienophile, may be attached thereto by tin (IV) chloride catalyzed alkylation of the polystyrene with N-chloromethylmaleimide, and when the polymer is a (styrene/maleic anhydride) copolymer a 5-ethylfurfuryl group may be attached thereto by esterifying the (styrene/maleic anhydride) copolymer with 5-ethylfurfuryl alcohol in pyridine. When the polymer is a copolymer of carbon monoxide and olefinically unsaturated compounds which comprises 1, 4-dicarbonyl entities in their polymer chains, 2,5-di- alkylfurans and dienophiles may be attached thereto by reacting the copolymer with an appropriately substituted primary hydrocarbylamine, e.g., using the methods known from US-A-3979374. In this reaction 1, 4-dicarbonyl entities are converted into pyrrole entities which form part of the polymer chain and which are N-substituted with the substituted hydrocarbyl group. For example, a copolymer of carbon monoxide and olefinically unsaturated compounds which comprise 1, 4-dicarbonyl entities may be reacted with the mono-amide of maleic acid and hexamethylenediamine or with the mono-amide of maleic acid and bis (4-aminophenyl) methane, followed by ring closure of the acid-amido moieties to maleimide moieties. This will yield a polymer having N- ( 6-maleimidohexyl) pyrrole or N- { 4- [ (4 ' -maleimido- phenyl) methyl] phenyl }pyrrole entities in the polymer chain. When it is desired to use a polymer which contains 2,5-dialkyl substituted furan moieties and moieties of a dienophile a portion of the 1, 4-dicarbonyl entities of a copolymer of carbon monoxide and olefinically unsaturated compounds may be converted into furan moieties and another portion of the 1, 4-dicarbonyl entities may be converted into N-substituted pyrrole entities, of which the N-substituent comprise a dienophile . The molecular weight of the polymer (s) on which the reworkable crosslinked resin is/are based may vary between wide limits. Suitably the polymer (s) have a number average molecular weight within the range of at least 500, preferably 700, to 30,000, preferably to 20,000. The quantity of Diels-Alder adducts present in the thermally reworkable crosslinked resin depends on the quantity of 2,5-dialkyl furan groups and the quantity of the dienophile present in the composition from which the Diels-Alder adducts are formed. One skilled in the art will appreciate that a certain minimum quantity of Diels-Alder adducts is needed to be present to effect that the crosslinked resin is a solid material below the temperature at which the Diels-Alder adducts reverse to the 2,5-dialkyl substituted furan and the dienophile. It will also be appreciated that this minimum quantity depends on the molecular weight and the type of the polymer on which the resin is based and, if any cross- linking agent is used, on the number of dienophiles or 2,5-dialkyl furan groups per molecule (i.e. functionality) of the crosslinking agent. Lower molecular weights of the polymer will require a higher quantity of Diels-Alder adducts. The number of a Diels- Alder adducts may be lower when a crosslinking agent is used which has a higher functionality. Generally good results can be achieved by using the 2,5-dialkyl furan-containing polymer having a furan groups to ketone groups ratio of from 1:16 to 4:1. The molar ratio of the 2,5-dialkyl substituted furan groups to dienophiles amounts typically from 10:1 to 1:5, preferably from 5:1 to 1:3. CONDUCTIVE MATERIAL In order to impart thermal or electrical conductivity to the adhesive, an effective amount of a thermally or electrically conducting material is incorporated into the die attach adhesive composition. The preferred thermally conducting material are such as for example, beryllia, boron nitride, aluminum oxide (single crystal) , aluminum nitride coated with copper (see U.S. Patent No. 5,288,769) and the like. Electrical contact is provided by means of incorporating electrically conductive materials (fillers) such as silver, nickel, copper and aluminum particles as well as alloys of such metals in the die attach adhesive formulation. Both powder and flake forms of conductive material may be used in the die attach paste compositions. A preferred amount of thermally and/or electrically conductive material ranges from 60, preferably from 70, to 90, preferably to 80 percent by weight of the total die attach adhesive composition, although lesser and greater amounts can be used. DIE ATTACH ADHESIVES Thermally reworkable die attach means that the device can be removed from a substrate by liquefying the die attach composition (or adhesive) by heating. The die attach composition is liquefied when the composition is flowable. The device can be removed without shearing or use of excessive force. The thermally reworkable die attach composition of the invention contains (a) a thermally reworkable crosslinked resin produced by reacting a dienophile having a functionality greater than one and a 2,5-dialkyl substituted furan-containing polymer, and (b) at least one thermally and/or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium. The thermally reworkable die attach composition can be worked and/or reworked at a temperature where the die attach composition melts. Typically, the thermally reworkable die attach adhesive can be worked and/or reworked at a temperature within the range from 100 °C, preferably from 130 °C, to 250 °C, preferably to 200 °C . If the composition is heated for an extended period of time at high temperature, for instance, for 12 hours at 200 °C, the composition undergoes irreversible cross- linking and it is no longer thermally reworkable. The thermally reworkable die attach composition or adhesive can also contain other additives such as ion scavengers (e.g., tricalcium phosphate), free radical inhibitors (e.g., hydroquinone, phenothiazine) , elastomeric modifiers (e.g., silicones), and other conventional additives used in die attach adhesives. For a longer reworking time, it is preferable to use ion scavengers and/or free radical inhibitors. A process for attaching a semiconductor device to a substrate is also provided comprising: (a) providing the substrate; (b) dispensing on the substrate a thermally reworkable die attach adhesive composition comprising (i) a thermally reworkable crosslinked resin produced by reacting at least one dienophile having a functionality of greater than one and at least one 2,5-dialkyl substituted furan-containing polymer, and (ii) at least one thermally and/or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium, at a temperature which is sufficiently high to convert the die attach adhesive composition into a liquid thereby producing an adhesive-attached substrate; (c) attaching the semiconductor device on the surface of the thermally reworkable die attach adhesive on the adhesive-attached substrate while the adhesive is a liquid, thereby bonding the semiconductor device to the substrate; and (d) cooling the thermally reworkable die attach adhesive to providing an assembly. The semiconductor device can be placed on the adhesive prior to substantial cooling by time or by heating to maintain or to reliquefy the adhesive to be a flowable material. The die attach composition can be dispensed on the substrate by using standard equipment such as a syringe or a motionless mixer that mixes the components of the die attach composition and precisely dispenses the composition on the substrate. Semiconductor devices can be bonded to various substrates such as for example, various metal, ceramic or laminate substrates that include printed circuit boards (PCB) e.g., chip-on-board; to leadframes that are then resin transfer molded into packages such as dual inline package (DIP) and plastic quad flat package (PQFP) ; and other packaging configurations such as ball grid array (BGA) . Repair and rework of defective devices are becoming increasingly important for die attach adhesives. Multichip modules (MCM) use existing package technology with the only difference being that a number of bare chips are adhesively attached to a substrate and packaged together. For instance, a BGA configuration might have a number of dies adhesively attached to a substrate, tape automated bonding to make the connection from the die to the substrate, an encapsulant coating all the dies as one package, and solder balls that make the connection to the printed circuit substrate from the BGA substrate. As spacing between the semiconductor device and nearby components such as other semiconductor devices gets closer, repair and rework without damage to adjacent devices becomes extremely difficult. A thermally reworkable die attach adhesive of this invention enables the user to perform repair and rework with ease on closely spaced devices and high density substrates . The thermally reworkable die attach adhesive composition bonded to a semiconductor device on one side and a substrate on another side can be reworked by steps comprising (a) heating the die attach adhesive composition, said composition comprising (i) a thermally reworkable crosslinked resin produced by reacting a dienophile having a functionality greater than one and a 2,5-dialkyl substituted furan- containing polymer, and (ii) at least one thermally or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium, at a temperature which is sufficiently high to convert the die attach adhesive composition into a liquid thereby providing a liquid die attach adhesive composition; (b) removing the semiconductor device from the liquid die attach adhesive composition; (c) optionally, dispensing a fresh thermally reworkable die attach adhesive composition, said composition comprising (i) a thermally reworkable crosslinked resin produced by reacting a dienophile having a functionality greater than one and a 2,5-dialkyl substituted furan- containing polymer, and (ii) at least one thermally or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium; (d) optionally, providing another semiconductor device on the surface of the liquid die attach adhesive, thereby bonding another semiconductor device on the die attach adhesive; and (e) cooling the liquid die attach adhesive to a temperature which is sufficiently low to solidify the resin. The die attach adhesive composition can be postbaked to enhance the die attach thermal and mechanical properties (e.g., glass transition temperature and mechanical strength) . In order to preserve the thermal reworkability of the crosslinked resin, the die attach adhesive can be heated to a temperature within the range from 70 °C, preferably from 90 °C, to 200 °C, preferably to 160 °C for a period of time up to 4 hours. If thermal reworkability is not required, the die attach adhesive composition can be postbaked at a temperature within the range of from 150 °C, preferably from 180 °C, to 300 °C, preferably to 250 °C for a period of time up to 4 hours to improve the thermal properties. Illustrative Embodiment The following illustrative embodiments describe the novel epoxy resin composition of the invention and are provided for illustrative purposes and are not meant as limiting the invention. Example 1 An autoclave was charged with methanol and propene (approximately 2:1 weight ratio), heated to 90 °C, and then charged with carbon monoxide to a pressure of 72 bar. A catalyst solution of palladium acetate, 1, 3-bis (diethylphosphino) propane, trifluoromethane sulfonic acid, in a weight ratio of 0.6 : 0.62 : 1 and 0.3 pyridine, in a tetrahydrofuran and methanol solution (15 : 1 volume ratio) were injected and the reactor pressure was maintained constant at 72 bar during the reaction by means of a continuous supply of carbon monoxide. Removal of solvent yielded an alternating propene/CO copolymer with a number average molecular weight of 733. Example 2 The alternating propene-CO copolymer with a number average molecular weight of 733 in Example 1 was dissolved in toluene and cyclized in the presence of a catalytic amount of p-toluene sulfonic acid by heating at reflux. The resulting polymer was analyzed by C-13 NMR which showed that 82% of the ketones in the starting polyketone were cyclized to furans ( furan : ketone ratio 2.28:1) by the appearance of C-13 NMR signals (furan resonances) centered at around 107, 114, 147 and 153 ppm. Example 3 A system was made by blending the furanized polyketone made in Example 2 and a stoichiometric amount of toluene diamine bismaleimide (Compimide Resin TDAB, Technochemie Gmbh) at 171 °C (340 °F) . The blend was removed from the gel plate and stored at room temperature. A solder-masked 8-ply (epoxy-glass) printed circuit board was placed on the gel plate at 171 °C (340 °F) and allowed to heat up to temperature. The blended system was dispensed on the board and a silicon chip was placed on top of the system and allowed to adhere to the board. The board was removed from the gel plate and allowed to cool to room tempera- ture . The die stayed adhesively attached to the board as the system formed a crosslinked solid at room temperature. The board was re-introduced back to the hot gel plate and allowed to heat for one minute. The chip was removed from the board easily as the adhesive system reverted back to its uncrosslinked liquid state. The chip was reattached back on the board at its original location by means of the adhesive film that was still present. Example 4 An alternating olefin-CO copolymer (27% ethylene, 73% propylene) with a number average molecular weight of 1472 was prepared in a similar manner to Example 1 from propene and ethylene. The copolymer was dissolved in toluene and cyclized in the presence of a catalytic amount of p-toluene sulfonic acid by heating at reflux. The resulting polymer was analyzed by C-13 NMR which showed that 56% of the ketones in the starting poly- ketone were cyclized to furans (furan: etone ratio 0.64:1) . Example 5 A gel plate was set to 171 °C (340 °F) and the furanized polyketone made in Example 4 was dispensed onto the plate. A stoichiometric amount of TDAB was blended with the furanized polyketone until a homogeneous blend was obtained. The blend was removed from the gel plate and stored at room temperature. Example 6 An ICI cone and plate viscometer was set to a temperature of 175 °C and allowed to equilibrate to the set point. A small amount of blend from Example 5 was placed on the plate and allowed to come up to temperature. The cone was brought down and spun to obtain a good film between the cone and plate. This was verified by lifting the cone up to check for good film formation. Subsequently the blend was allowed to equilibrate for 90 seconds and two viscosity readings were taken while the cone was rotating at a fixed speed. The cone was lifted up and the blend retrieved from both the cone and plate. The blend was allowed to cool at room temperature to a crosslinked solid. The above sequence of events i.e. load on ICI cone and plate, measure viscosity at 175 °C, remove blend, cool to room temperature, was repeated three times with the same blend. The three consecutive readings for viscosity were 3-5 poise, 3-5 poise and 3-5 poise. This experiment shows that the blend can alternate rever- sibly between a crosslinked state at room temperature and a low viscosity uncrosslinked liquid at 175 °C. Example 7 An alternating propene-CO copolymer with a number average molecular weight of 1616 prepared in a similar manner to Example 1, except that 1, 3-bis (di-o-methoxy- phenylphosphino) propane was used in the catalyst solution instead of 1, 3-bis (diethylphosphino) propane . The copolymer was dissolved in toluene and cyclized in the presence of a catalytic amount of p-toluene sulfonic acid by heating at reflux. The resulting polymer was analyzed by C-13 NMR which showed that 57% of the ketones in the starting polyketone were cyclized to furans (furan: ketone ratio 0.66:1). Example 8 A gel plate was set to 171 °C (340 °F) and the furanized polyketone made in Example 7 was dispensed onto the plate. A stoichiometric amount of methylene dianiline bismaleimide (Compimide Resin MDAB, Technochemie Gmbh) and 0.2 moles of phenothazine (Phenothiazine, Aldrich Chemical) per mole of MDAB was blended with the furanized polyketone until a homogeneous blend was obtained. The blend was then removed and stored at room temperature. A small portion of the blend was placed on the gel plate at 171 °C (340 °F) and mixed with 60% by weight of silver powder (Silver powder, -325 mesh, Johnson Matthey) . The compo- nents were mixed and the blend was then removed and stored at room temperature. Example 9 An ICI cone and plate viscometer was set to a temperature of 175 °C and allowed to equilibrate to the set point. A small amount of blend from Example 8 was placed on the plate and allowed to come up to temperature. The cone was brought down and spun to obtain a good film between the cone and plate. This was verified by lifting the cone up to check for good film formation. Subsequently the blend was allowed to equilibrate for 90 seconds and two viscosity readings were taken while the cone was rotating at a fixed speed. The cone was lifted up and the blend retrieved from both the cone and plate. The blend was allowed to cool at room temperature to a crosslinked solid. The above sequence of events i.e. load on ICI cone and plate, measure viscosity at 175 °C, remove blend, cool to room temperature, was repeated three times with the same blend. The three consecutive readings for viscosity were 70-75 poise, 75-80 poise and 75-80 poise. This experiment shows that the blend can alternate between a crosslinked state at room temperature and an uncross- linked liquid at 175 °C. Example 10 A solder-masked 8-ply (epoxy-glass) printed circuit board was placed on the gel plate at 171 °C (340 °F) and allowed to heat up to temperature. The silver- filled system of Example 8 was dispensed on the board and a silicon chip was placed on top of the system and allowed to adhere to the board. The board was removed from the gel plate and allowed to cool to room temperature. The die stayed adhesively attached to the board as the system formed a crosslinked solid at room temperature. The board was re-introduced back to the hot gel plate and allowed to heat for one minute. The chip was removed from the board easily as the adhesive system reverted back to its uncrosslinked liquid state. The chip was reattached back on the board at its original location by means of the adhesive film that was still present and the board was removed from the gel plate and cooled to room temperature. This sequence was repeated two more times i.e. place board back on hot surface, remove chip, reattach chip to board, cool to room temperature. Example 11 Furanized polyketone made in Example 7 and a stoichiometric amount of TDAB along with 6.5% by weight of phenothiazine were heated to 180 °C, mixed and poured in a 3.2 mm (1/8 inch) thick metal mold. The mold was cooled quickly and the resulting casting was tested for properties. The flexural modulus of the sample was found to be 43 bar (628 psi) , a value similar to that of a crosslinked epoxy made with bisphenol-A epoxy cured with an anhydride hardener. The dielectric constant and dissipation factor were 3.17 and 0.013 respectively. Example 12 Furanized polyketone made in Example 7 was reacted with a 2:1 stoichiometric ratio of MDAB, 0.1 mole of phenothiazine/mole of MDAB and 0.015 gm of 2-ethyl hexanoic acid/gm of furanized polyketone. A differential scanning calorimetry scan was performed on the sample at a ramp rate of 20 °C/min. The onset of the glass transition temperature occurred at 105 °C. Example 13 Furanized polyketone made in Example 4 was reacted with a stoichiometric amount of TDAB and 0.1 moles of phenothiazine/mole of TDAB on a gel plate at 171 °C (340 °F) . This sample was ground and placed in a Parr bomb with water in a 10:1 ratio (water : sample) . The Parr bomb was kept at 60 °C for 20 hours and the water extract was analyzed for ions by ion chromatography. The extract contained 14 ppm acetate, < 3 ppm glycolate, formate, propionate, < 0.25 ppm chlorine, < 1 ppm nitrate, 1.7 ppm sulfate, 4.8 ppm sodium, 0.8 ppm magnesium, 2.5 ppm calcium and 0.2 ppm ammonium ion.

Claims: C L A I M S 1. A thermally reworkable die attach adhesive composition comprising: (a) a thermally reworkable crosslinked resin produced by reacting at least one dienophile having a functionality greater than one and at least one 2,5-dialkyl substituted furan-containing polymer, and (b) at least one thermally and/or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium. 2. The thermally reworkable die attach adhesive composition of claim 1 wherein the crosslinked resin is reworkable at a temperature within the range of from 100 °C to 250 °C. 3. The thermally reworkable die attach adhesive composition of claim 1 or 2 wherein the dienophile is an alkyne having electron withdrawing groups attached to both sides of an ethyne moiety, or a cyclic derivative of maleic anhydride. 4. The thermally reworkable die attach adhesive composition of claim 3 wherein the dienophile is selected from the group consisting of compounds containing but-2-ene-l, 4-dione moieties in 5- membered rings, and compounds containing but-2-ene-l, 4-diene moieties in 6-membered rings. 5. The thermally reworkable die attach adhesive composition of claim 3 wherein the thermally reworkable resin further comprises a residue of a cross-linking agent selected from the group consisting of, bismaleimido- capped poly (alkylene oxide) s, bis-maleimido-capped polysiloxanes, bismaleimides of hydrazine, 2,4-diamino- toluene, hexamethylenediamine, dodecamethylenediamine, and substituted and unsubstituted diamines of the formula : 6. The thermally reworkable die attach adhesive composition of claim 1 or 2 wherein the 2,5-dialkyl substituted furan-containing polymer is produced by reacting carbon monoxide with at least one olefinically unsaturated compound. 7. The thermally reworkable die attach adhesive composition of claim 1 or 2 wherein the 2,5-dialkyl substituted furan groups in the 2,5-dialkyl substituted furan-containing polymer and the dienophiles are combined in a molar ratio of from 10:1 to 1:5. 8. The thermally reworkable die attach adhesive composition of claim 1 or 2 further comprising (c) a free radical inhibitor and/or an ion scavenger. 9. A process for attaching a semiconductor device to a substrate comprising: (a) providing a substrate; (b) dispensing, on at least a portion of said substrate, a thermally reworkable die attach adhesive composition, said composition comprising (i) a thermally reworkable crosslinked resin produced by reacting at least one dienophile having a functionality greater than one and at least one 2,5-dialkyl substituted furan-containing polymer, and (ii) at least one thermally and/or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium, at a temperature which is sufficiently high to convert the die attach adhesive composition into a liquid thereby producing an adhesive-attached substrate; (c) attaching the semiconductor device on the surface of the thermally reworkable die attach adhesive on the adhesive-attached substrate while the adhesive is in a liquid, thereby bonding the semiconductor device to the substrate; and (d) cooling the die attach adhesive to a temperature which is sufficiently low to solidify the resin thereby providing an assembly. 10. The process of claim 9 further comprising the steps of: (e) heating the thermally reworkable die attach adhesive composition of the assembly at a temperature which is sufficiently high to convert the die attach adhesive composition into a liquid thereby providing a liquid die attach adhesive composition; (f) removing the semiconductor device from the liquid die attach adhesive composition; (g) optionally, dispensing a fresh thermally reworkable die attach adhesive composition comprising (i) a thermally reworkable crosslinked resin produced by reacting a dienophile having a functionality greater than one and a 2,5-dialkyl substituted furan- containing polymer, and (ii) at least one thermally or electrically conductive material present in an effective amount up to 90% by weight of the die attach composition to provide a conducting medium; (h) optionally, providing another semiconductor device on the surface of the liquid die attach adhesive, thereby bonding another semiconductor device on the die attach adhesive; and (i) cooling the liquid die attach adhesive to a temperature which is sufficiently low to solidify the resin thereby providing a reworked assembly.

Inventors: IYER SHRIDHAR RATNASWAMY, WONG PUI KWAN, IYER, SHRIDHAR, RATNASWAMY, WONG, PUI, KWAN
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1997
Main classification: C09J9
IPC codes: C08G67, C09J9, H01L21, H01L23