US20040170825A1 - Device cover having a gapped adhesive preform thereon for covering a device on an electronic substrate - Google Patents

Abstract

A device cover for covering a device on an electronic substrate according to the invention comprises a cover having a bonding surface defining a closed bonding pattern, and a bonding pattern of adhesive on the bonding surface of the cover. The bonding pattern of adhesive has at least one gap therein, wherein the gap is sufficiently small as to be filled by the adhesive when the adhesive flows when the cover is adhered to the electronic substrate.

Description

• This Application is a division of U.S. patent application Ser. No. 10/170,533 filed Jun. 13, 2002, which is a division of U.S. patent application Ser. No. 09/633,792 filed Aug. 7, 2000, now U.S. Pat. No. 6,432,253, which is a division of U.S. patent application Ser. No. 09/232,936 filed Jan. 19, 1999, now U.S. Pat. No. 6,136,128, which claims the benefit of U.S. Provisional Application Serial No. 60/090,295 filed Jun. 23, 1998 and of U.S. Provisional Application Serial No. 60/092,170 filed Jul. 9, 1998.[0001]
• The present invention relates to a device cover having thereon a flowable adhesive preform having a gap therein for covering a device on an electronic substrate.[0002]
• Many approaches have been tried for packaging electronic devices for protection against external hazards, such as handling and other mechanical damage, environmental factors, chemical attacks, and other potentially adverse elements. Depending on both the functional and aesthetic requirements, these electronic devices are typically packaged in several levels of packaging. The outermost level is most likely a housing or enclosure for the equipment of which such devices are a part.[0003]
• Generally, a useful electronic device, such as electronic circuit or integrated circuit, is packaged within a small package or module providing the first of at least several levels of protection. Electronic devices such as semiconductor devices are often protected by solid organic encapsulation. When several of these packaged electronic devices are put together as a functional unit, such as in an electronic circuit module or on a printed circuit board or other substrate, they are often protected with an exterior lid, cover or other enclosure to form a protective housing. These exterior lids or covers may be attached with adhesive, solder, or by mechanical fasteners, such as screws, bolts and clips.[0004]
• In some applications, an electronic device at the semiconductor device level may not be able to reliably be encased in a solid encapsulant because of the adverse influence of stresses induced in the device owing to direct contact with the encapsulant. In other applications, the use of the encapsulation may be too costly. In still other applications, there may be a need for a lid or cover that is electrically conductive so as to provide shielding against electromagnetic interference (EMI) which may originate in the covered device or which may originate externally and to which the covered device may be susceptible. In this type of EMI-resistant application, the lid must be electrically conductive and must also be connected to the electrical ground of the electronic device. This requirement cannot be easily met with either an insulating organic encapsulant which does not provide shielding or with a conductive encapsulant which is likely to electrically short the electronic device or the conductors connecting thereto. Even the use of an electrically conductive lid that is soldered in place may be inconvenient or impractical because of the adverse effects on the devices that result from the high temperatures required for making soldering attachments. In addition, if one needs to rework the soldered module, the de-soldering operation may also cause overheating or other damage or the inadvertent de-soldering of other electronic elements inside of the package.[0005]
• In fact, most of the electronic devices utilized in aerospace, military and other high reliability applications make use of a hermetically-sealed lid to prevent moisture and other adverse elements from affecting or damaging the electronic components employed therein. However, true hermetically-sealed packages are very expensive to fabricate. Most high-reliability hermetically-sealed packages employ either metal soldering or brazing for lid attachment, especially for applications requiring an electrically conductive housing for EMI protection. In those applications where an insulating lid or cover must be employed, high temperature glass seals are often utilized. In order to prevent damage to the electronic devices from the high-temperature processing necessary to form the glass seals, the packages and lids must be heated up locally only along the rim of the package and lid. As a result, the processing time is long and the work of attaching the protective lids is delicate. In addition, the materials employed in both the glass seal and lid must have respective coefficients of thermal expansion (CTE) that are matched to that of the electronic substrate or package to which they attach. This additional requirement of matching the respective CTEs of the substrate, sealing material, and lid, all increase the difficulty of package design and the cost of the finished device. In general, the cost of both the materials and the processing of matched-CTE packages are prohibitive for commercial electronics products for general use, such as consumer electronic products.[0006]
• Electronic package lids and covers are used, however, to a certain extent in commercial electronics products where required to achieve necessary performance parameters. For example, frequency-determining electronic devices that are susceptible to frequency errors caused by stress-induced mechanical distortion or that must mechanically change to function, such as piezo-electric sound generators and frequency crystals employed in communication equipment, cannot be simply encapsulated and so are protected by a lid. These lids are generally attached with adhesive.[0007]
• Conventionally, adhesive in the form of dispensable paste or die-cut preforms is applied to the device or to the lid immediately before or as part of the lid attachment bonding process. In certain cases, for, example, when the number of lid attachments is high, lids are pre-coated with adhesive or with die-cut adhesive preforms that will flow and cure when applied under heat and pressure conditions during the lid attachment process. However, the cost of adhesive pre-coating and die cut adhesive preform application to lids and covers is still quite high, in part due to the number of steps required and the handling of individual lids and even individual adhesive preforms. Adhesives in liquidous form are typically dispensed with a programmable automatic dispenser or are roller-coated onto the sealing areas of each lid, and are then subsequently dried or B-staged at a temperature and for a time substantially lower than the specified curing temperature and time for the particular adhesive. The liquidous adhesive is thus changed into a solid state either through solvent evaporation or chemical cross-linking of the adhesive during this drying or B-staging.[0008]
• U.S. Pat. No. 5,056,296 issued to Ross et al and entitled “Iso-Thermal Seal Process for Electronic Devices” discloses an apparatus and process wherein the apparatus heats the lid, the package and the surrounding thermosetting adhesive so that they all attain an isothermal condition, i.e. a uniform temperature, before the lid is mated to the package in the bonding process. The Ross et al patent describes the pre-sealing isothermal condition as necessary to prevent differential air pressure between the inside and outside of the package that can cause “blow-out”-induced pinholes along the bond line provided by the sealing thermosetting adhesive if the parts are brought together and then are heated. Because of the time required to raise the temperature of the lid and the package, perhaps several minutes to achieve uniform temperature, the Ross et al process would appear able to achieve significant quantity production only when applied in a batch processing of lids, which often is impracticable. Moreover, because of the long heating time, the Ross et al process would seem to require a slower curing adhesive so as to avoid gelling or partial curing of the pre-heated adhesive before attachment of the lid to the package, thereby also extending the post-attachment curing time of the adhesive and further reducing the ability to achieve quantity production.[0009]
• U.S. Pat. No. 5,427,642 issued to Akiguchi et al entitled “Method for Mounting Electronic Parts on a Printed Circuit Board by Use of an Adhesive Composition” describes an arrangement for mounting components to a substrate so as not to form enclosed areas in which air could be trapped during a soldering operation, and does not relate to a device cover. Specifically, Akiguchi et al relates to an adhesive composition for attaching electronic components to a substrate prior to their being electrically connected by soldering. The adhesive of Akiguchi et al is applied to the substrate and then is partially cured by ultraviolet (UV) light to harden the surface layer thereof. Placing the electronic part then “dents” the adhesive composition. It appears that the hardening of the surface layer before application of the electronic part acts to decrease any flow that might otherwise occur. The gap of Akiguchi et al must remain open for a subsequent soldering operation.[0010]
• U.S. Pat. No. 5,932,875 to R. Chung et al entitled “Single Piece Integrated Package and Optical Lid” relates to an integrated circuit package that has been attached to a PC board and an optical lid that is thereafter placed to cover the previously attached package R. Chung et al describes placing epoxy onto the flange of the lid or onto the PC board (substrate) to which the lid is placed, and does not describe or suggest a bonding pattern or preform of adhesive on the lid wherein the bonding pattern or preform of adhesive has at least one gap therein, and so it cannot describe or suggest that the at least one gap is filled when the adhesive flows.[0011]
• Thus, there is a need for adhesive preform lids and covers that avoid the “blow-out” problem and provide a cost-effective solution for protecting devices such as sensitive electronic components. It is also desirable that such cover lend itself to automated processing and that the adhesive of the preform be melt flowable so as to be removable at a temperature and an applied force that will not damage either the electronic components inside the package and/or on the substrate to which they are attached for covering the electronic components therein and/or thereon.[0012]
• There is also a need for lids and covers that provide shielding against EMI and that can be attached at a temperature substantially below the general soldering temperature of about 220° C., i.e. at a temperature that will not disturb or damage soldered connections, e.g., a melting temperature in a range of about 90° C. to about 180° C. It is also desirable that the adhesive employed therein is electrically conductive and bonds essentially instantly upon reaching the bonding temperature, and that the lids or covers so attached be removable at a temperature below the general soldering temperature so as to eliminate the possibility of thermally-induced damage to or misalignment of components inside the package.[0013]
• To this end, a device cover for covering a device on an electronic substrate according to the invention comprises a cover having a bonding surface defining a closed bonding pattern, and a bonding pattern of adhesive on the bonding surface of the cover. The bonding pattern of adhesive has at least one gap therein, wherein the gap is sufficiently small as to be filled by the adhesive when the adhesive flows when the cover is adhered to the electronic substrate.[0014]
BRIEF DESCRIPTION OF THE DRAWING
• The detailed description of the preferred embodiments of the present invention will be more easily and better understood when read in conjunction with the FIGURES of the Drawing which include:[0015]
• FIG. 1 is a cut-away perspective view of an electronic device including an adhesively attached cover;[0016]
• FIG. 2 is a plan view of a plurality of adhesive preforms on a release substrate;[0017]
• FIG. 3 is a side cross-sectional view of the adhesive preforms and release substrate of FIG. 2 taken along line I-I;[0018]
• FIG. 4 is a side cross-sectional view of the adhesive preforms and release substrate of FIG. 3 with a plurality of lids or covers thereon; and[0019]
• FIG. 5 is a perspective view of a portion of an electronic device having a plurality of lids or covers thereon.[0020]
DESCRIPTION OF THE PREFERRED EMBODIMENT
• FIG. 1 is a cut-away perspective view of an[0021]electronic device10 including anelectronic substrate20 upon which are mounted one or moreelectronic components22, such as semiconductor chips, integrated circuits, transistors, diodes, resistors, capacitors, inductors, and combinations thereof. The electronic devices are connected in circuit by electrical conductors (not visible in FIG. 1) formed on or withinsubstrate20, as is known to those having ordinary skill in the art. Electrical leads24,26 extending outwardly fromsubstrate20 as in a “flat-pack” arrangement, for example, provide conductive connections between the electrical conductors andcomponents22 ofelectronic device10 and the apparatus to whichelectronic device10 is incorporated.
• Because[0022]electronic components22 commonly include very fine features that are delicate and susceptible to damaged by mechanical and electrical means, and/or are susceptible to contamination by moisture or other foreign matter, a protective lid or cover40 is attached over and protectingelectronic components22.Protective cover40 is attached tosubstrate20 by a continuous line of adhesive30 that joins theedges42 ofcover40 to the surface ofsubstrate20 completely around the periphery thereof.Edges42 ofcover40 are a bonding surface that define a bonding pattern, wherein the adhesive30 is deposited in a pattern substantially corresponding in size and shape to that bonding pattern.
• Where[0023]cover40 is a protective cover only, it may be formed of stamped or cast or molded epoxy, liquid-crystal polymer or other suitable plastic, and adhesive30 may be a non-electrically conductive thermoplastic adhesive, such as types TP7150, TP7090, TP7750 and TP7260 or a non-electrically conductive thermosetting adhesive, such as types ESP7675, ESP7670 and ESP7450, all of which are available from AI Technology, Inc. located in Princeton, N.J. Surfaces of plastic covers to which adhesive is to be applied are preferably prepared for improved adhesion, such as by oxidizing the bond surfaces by flame or corona treatment. Covers typically range in size from about 100 mils×100 mils, which could be employed to protect an individual transistor or diode or a small integrated circuit, to about one or two inches by one or two inches, which could be employed to protect a large integrated circuit such as a micro-processor.
• Where[0024]cover40 is for providing electrostatic and/or electromagnetic shielding of the electronic components it encloses, cover40 may be formed of a metal, such as copper, aluminum, steel, stainless steel and alloys thereof, with and without protective plating. Alternatively, cover40 may be formed of a non-conductive material as above and plated with an electrically-conductive coating, such as copper, silver, gold or combinations thereof, or may be filled with conductive particles such as copper, silver, gold, aluminum and/or carbon particles. In the case where such electrostatic and/or electromagnetic shielding is also provided, adhesive30 may be an-electrically conductive thermoplastic adhesive, such as types TP8090 (filled with silver particles), TP8093 (filled with silver-plated copper particles) and TP8150 (filled with silver particles) or an electrically conductive thermosetting adhesive, such as types ESP8680 (filled with silver particles), ESP8450 (filled with silver particles) and ESP8453 (filled with silver-plated copper particles), all also available from AI Technology, Inc. Adhesives of the foregoing types are considered flexible adhesives in that they have a modulus of elasticity that is less than about 200,000 psi over the specified and/or operating temperature range of the electronic devices with which covers40 are intended to be employed, and also will elongate by at least 10% before fracturing. For example, type ESP8450 adhesive has a modulus of elasticity between about 200,000 psi and 20,000 psi over the temperature range of about −55° C. to +150° C. It is noted that covers40 formed with the foregoing materials and employingadhesive preforms30 of the foregoing exemplary materials will be resistant to the passage of moisture and chemical cleaners and solvents commonly employed in the manufacture of electronic devices, such as isopropyl alcohol, volatile methylsiloxane, terpenes and other solvents. The adhesive preforms30 will exhibit volume resistivity in a range of about 100 million ohm-cm to about 0.1 ohm-cm, depending upon the adhesive material and the fillers therein, if any, and so will tend to dissipate electrostatic potential.
• [0025]Covers40 with preformed adhesive30 applied thereto may be made by the following method which is described in relation to FIGS. 2, 3 and4. Arelease substrate32 such as a sheet of steel coated with a layer of poly-tetra-fluoro-ethylene, such as Teflon®, available from E.I. duPont de Nemoirs located in Wilmington, Del., is obtained and a set of at least two relational alignment holes34,36 are made therein, as by punching, die cutting or laser cutting.Release substrate32 may also employ polypropylene plate, and, if a mechanically self supporting release substrate is desired, it may be made of a self-supporting sheet of low surface energy (e.g., surface energy less than 30 dyne/cm) material such as poly-tetra-fluoro-ethylene or may be made of aluminum, stainless steel, steel or other metal and coated with such low surface energy material. The relational alignment holes34,36 are located in known predetermined relationship to each other, as may be seen in the plan view of FIG. 2.
• A flexible adhesive is deposited on[0026]release substrate32 to form a pattern of a plurality ofadhesive preforms30 conforming substantially to the bonding pattern defined byedges42 ofcover40, in positions determined by the relational alignment holes34′,36′ in the screen, stencil or mask employed to deposit the flexible adhesive, which relational alignment holes34′,36′ are in the same known predetermined relationship to the pattern ofadhesive preforms30 as are the corresponding relational alignment holes34,36 inrelease substrate32. Deposition of flexible adhesive may be accomplished by mesh screening, stencil screening, contact screening, mask screening ink-jet printing or other suitable method. Flexible adhesive preforms30 are formed of a deposition of flexible adhesive that may be electrically insulating or electrically conductive, or may be of a thermoplastic or thermosetting adhesive type, as set forth above. Eachadhesive preform30 has a shape that corresponds to the bonding pattern defined by the shape of theedges42 of the cover orlid40 that is to be attached to an electronic substrate. For example, if thecover40 is in the form of a hollow rectangular solid, as is illustrated in FIG. 1,adhesive preform30 is in the shape of a rectangle as is illustrated in FIG. 2, and if thecover40 is in the form of a hollow cylinder (not illustrated),adhesive preform30 is in the shape of a circle.
• FIG. 3 is a side cross-sectional view of the[0027]release substrate32 of FIG. 2 taken along section line I-I with the plurality ofadhesive preforms40 thereon. Eachadhesive preform30 is relatively thin because it need only contain sufficient adhesive to form a bond between acover40 and asubstrate20 when they are pressed together in assembling an electronic device.Release substrate32 with the pattern of wetadhesive preforms30 thereon is ready to receivecovers40 on the respectivewet preforms30.
• A[0028]guide plate50 has a pattern ofreceptacles52 therein corresponding to the pattern ofadhesive preforms30 onrelease substrate32. Eachreceptacle52 is adapted for releasably receiving acover40 therein. Preferably, guideplate50 also has a set of relational alignment holes34′,36′ therethrough located to correspond to the set of relational alignment holes34,36 inrelease substrate32 and in the same known relationship to the pattern ofreceptacles52 as are relational alignment holes34,36 to the pattern ofadhesive preforms30.Guide plate50 is placed overrelease substrate32 so that thereceptacles52 in theguide plate50 are in direct correspondence in shape and size to theadhesive preforms30, preferably passing an alignment pin through each of the corresponding pairs of respective relational alignment holes34,36 and34′,36′. Also preferably,receptacles52 may be several thousandths of an inch larger than the size ofcovers40 to allow easy placement thereof. Thecovers40 are then placed directly on top of the wetadhesive preforms30 throughreceptacles52 inguide plate50. After all of thecovers40 have been placed onadhesive preforms30, theguide plate50 is removed, producing the result shown in FIG. 4.Release substrate32 with thecovers40 on theadhesive preforms30 is dried or B-staged, for example, in a belt oven or a box oven, for a time sufficient to remove solvent from the adhesive and/or for some chemical cross-linking of the adhesive to occur, whereby the wetadhesive preforms30 become solidadhesive preforms30, each one attached to a respective one of thecovers40.
• [0029]Covers40 with driedadhesive preforms30 thereon may then be released from therelease liner32 and are ready to be used, for example, in attachment onto a substrate of an electronic or other functional device. Alternatively, covers40 withadhesive preforms30 thereon may be packaged in either tape-and-reel or waffle packaging for ease of transportation and storage for later use, for example, with conventional “pick-and-place” apparatus.
• Alternatively,[0030]release substrate32 may be employed with conventional “pick-and-place” apparatus in two different ways. Firstly,release substrate32 with wetadhesive preforms30 thereon as shown in FIG. 3 may be transferred to a pick-and-place apparatus, such as a model ECM 93 pick-and-place machine available from Manncorp located in Huntingdon Valley, Pa., which then picks up individual covers40 and places one on each of theadhesive preforms30 onrelease substrate32, thereby also producing the result shown in FIG. 4.Release substrate32 containing the wetadhesive preforms30 is then processed as described above. Secondly,release substrate32 withcovers40 attached thereto by driedadhesive preforms30 as shown in FIG. 4 may be transferred to a pick-and-place apparatus, such as the Manncorp model ECM 93, which apparatus then picks up each cover40 with driedadhesive preform30 attached thereto and places it in the predetermined location on the substrate of an electronic or other functional device. In either of the foregoing ways of utilizingrelease substrate32 with pick-and-place apparatus,release substrate32 may be positioned on such pick-and-place apparatus by employing the relational alignment holes34,36 therein, whereby the location of eachadhesive preform30 and/or of eachcover40, as the case may be, on the pick-and-place apparatus is determined precisely.
• In the perspective view of FIG. 5 is shown a plurality of non-conductive lids or covers[0031]40 and a plurality of electrically conductive lids or covers40′ attached to anelectronic substrate20′ such as a printed circuit wiring board. Eachcover40,40′ covers and protects one or more components that are attached to printedwiring board20′, for example, by adhesives, soldering, wire bonding or other known arrangement. Respective ones ofcovers40,40′ are attached to printed wiring board by an insulatingadhesive preform30 or by an electrically-conductiveadhesive preform30′ that was formed oncovers40 in the manner described herein above.
EXAMPLE 1
• Example 1 involves a[0032]lid40 for protectingsemiconductor devices22 inside asmall module10′ for communication equipment, such as portable electronic pagers and mobile or cellular telephones.Semiconductor devices22 are attached onto afunctional board20′ that is a printedwiring circuit board20′ made of standard FR4 substrate material. Interconnections betweencircuit board20′ anddevices22 may be made, for example, either by conventional wire-bonding or by conventional “flip-chip” bonding. The electronic modules are typically arranged in a panel of multiple repeated circuitry.Lids40 with pre-appliedadhesive preforms30 thereon are placed on top of thecircuit board20′ substrate and are bonded thereto with heat and pressure for a specific period of time determined by the adhesive. In this example, a B-stageable insulating epoxy adhesive type LESP7670 available from AI Technology, Inc. is employed for lid sealing. The LESP7670 adhesive paste is first deposited onto therelease substrate base32 in the form of a pattern of repetitive units ofrectangular preforms30 located in known predetermined relationship with respect to a set of relational alignment holes34,36 as shown in FIG. 2 that have preferably been made outside the area useful for depositingadhesive preforms30. Typically,adhesive preforms30 have a thickness of about 75 to 150 microns. Although deposition methods including screen-printing, stencil-printing, and contact and impact deposition methods have been found useful, stenciling is preferred in this example. Use of the relational alignment holes34,36 is particularly advantageous when adhesive preforms30 are to be deposited onmany release substrates32 that are to be used to facilitate high-volume assembly-line-like deposition of adhesive.Release substrate32 with the wetadhesive preforms30 is then transferred to another station where aguide plate50 is placed overrelease substrate32 and is aligned therewith by a corresponding set of relational alignment holes34′,36′ inguide plate50, as described above.Lids40 are then placed through the receptacle holes52 and directly on the wetadhesive preforms30. After all thelids40 have been so placed,guide plate50 is removed.Release substrate32 withlids40 attached thereon byadhesive preforms30 is then placed in a belt oven or box oven heated to a temperature of about 60-80° C. for a time, such as about 30-60 minutes, sufficient to remove solvent fromadhesive preforms30 and to permit partial chemical cross-linking thereof, so that wetadhesive preforms30 become solid adhesive preforms attached to lids40.Lids40 with dryadhesive preforms30 attached thereto are released fromrelease liner32 and are ready for attachment ontocircuit board20′ by pick-and-place equipment.Lids40 withadhesive preform30 attached thereto are pressed against electroniccircuit board substrate20′ (as shown in FIG. 5) at a temperature of about 150-180° C. for about three to ten minutes with about 10 psi applied pressure, which is sufficient to produce adequate flow ofadhesive preform30, during the bonding process oflids40 tocircuit board20′. Type LESP7670 epoxy adhesive may be used without additional curing.Lids40 may be easily removed without damagingcircuit board20′ by concentrating the stress upon theadhesive preform30, as by pulling the lid, twisting the lid, or prying the lid, and may be facilitated by heating the adhesive preform to a temperature sufficient to reduce its bonding strength.
EXAMPLE 2
• Example 2 is an alternative employing the same adhesive deposition method and adhesive material as in Example 1, however, instead of using[0033]guide plate50 to facilitate precision placement oflids40 on the wetadhesive preforms30, standard pick-and-place equipment conventionally employed for precisely mounting components by surface mounting technology (SMT) is employed. Suitable SMT pick-and-place equipment is commercially available from Mydata Automation located in Peabody, Mass., from Universal Instrument located in Binghamton, N.Y., from Zevatech Inc. located in Morrisville, N.C., and from Manncorp, and can place components (i.e. lids40) onto circuit boards with a positional inaccuracy of one one-thousandth of an inch or less and at a rate greater than one lid per second. In fact, positioninglids40 within two one-thousandths of an inch is more than adequate accuracy for most applications. Oncerelease substrate32 is fully populated withlids40, it is heated for B-stagingadhesive preforms30. The fact that the wetadhesive preform30populated release substrate32 can be handled in much the same way as is a conventional printed circuit board deposited with solder paste and the lids can be handled as components, greatly facilitates automating process of applyingadhesive preforms30 tocovers40, thereby to increase the production rate and uniformity of adhesively preformed covers, while reducing the production cost thereof. Advantageously, the cover of the present invention is compatible with conventional automated assembly equipment that users thereof may already have and so may elect to employ.
EXAMPLE 3
• Example 3 utilizes the same processes for pre-applying[0034]adhesive preforms30 ontoprotective lids40 and forbonding lids40 tocircuit board20′, however, thelid40 in this Example 3 has a wider bonding edge, for example, because the material oflid40 is thicker or the edges thereof are flared to increase the bonding area. As a result,lid40 may be attached with anadhesive preform30 having a lower bonding strength and yet provide the same mechanical protection. To that end, a B-stageable flexible epoxy paste type LESP7450 also available from AI Technology, Inc. is employed. Type LESP7450 has an intrinsic bond strength of approximately 2000 psi at ambient temperature, which is less than about 30% of the bond strength of typical high-strength lid seal adhesives, and is flexible (i.e. has a modulus of elasticity of less than about 200,000 psi) over substantially more than half of its specified operating and storage temperature range, for example, a temperature range of −55° C. and 150° C. The bond strength of type LESP7450 adhesive drops to approximately 300 psi at temperatures at or above about 90° C., i.e. a temperature substantially lower than the melting temperature of solder, thereby to allow easier removal oflid40 by applying torque, prying or other concentration of stress. Ease of removal is a desirable feature, especially for larger lids and lids with larger bonding areas.
EXAMPLE 4
• Example 4 employs an electrically conductive B-stageable flexible thermoplastic adhesive paste, type LTP8090 available from AI Technology, Inc., in conjunction with conductive covers to provide EMI shielding. Specifically, cover[0035]40′ is a metallic shell formed of a magnetic stainless steel sheet having a thickness of approximately 150 microns. Small openings are provided on the top ofcover40′ to allow viewing of the interior thereof, for example, for inspection, and to permit air flow for cooling the electronic components enclosed bycover40′. These openings are small as compared to the wavelength of the electromagnetic radiation of interest and thus prevent EMI from leaking into and out of thecover40′, for example, wherecover40′ is employed in a handset of mobile cellular telephone. Openings smaller than about 5 mm, for example, will not pass electromagnetic signals at frequencies less than about 50 GHz. Type LTP8090 conductive adhesive paste is deposited onto arelease substrate32 in a preform shape to coincide with the bonding area shape ofcover40′ which are placed onto the wet adhesive preforms30′ with aguide plate50 as in Example 1.Covers40′ with the wet adhesive preforms30′ thereon are then B-staged to formdry preforms30′ attached tocovers40′ which are then attached onto theelectronic module20′ at a temperature of about 150-180° C. with about 10 psi pressure. it is noted thatadhesive preform30′ and cover40′ form a Faraday electrostatic shield against EMI leakage. Because type LTP8090 adhesive is a thermoplastic resin having sharp or well-defined melting temperature of about 110° C., covers40′ can be easily removed once the temperature of the bonding areas is raised above that melting temperature. As a result, electronic devices including such covers may be easily reworked at temperatures well below the melting point of solder and the maximum temperature that semiconductor and other electronic components can withstand, thereby avoiding degradation of or damage to such electronic components.
• In Examples 1-4 above, the adhesive preforms are generally preferred to be slightly wider than are the edges of[0036]lids40,40′ that serve as bonding areas, so that the preforms attach to lids40,40′ with sufficient bonding area before they are attached to anelectronic device10,10′. However, wherelids40,40′ have wide bonding edge areas, and particularly whereadhesive preform30,30′ is an electrically conductive adhesive, it may be important to confine the area and volume of adhesive inadhesive preforms30,30′ onlids40,40′ to avoid unwanted electrical connections, bridges and short circuits byadhesive preforms30,30′, such as to electronic components and conductors located close tolids40,40′. It is noted that even insulating adhesives can form a high resistance (e.g., multi-megohm) path that will disturb certain high-impedance circuits. In some cases, it may be advantageous to substantially displaceadhesive preforms30,30′ toward the outside edges of thelids40,40′ not only to avoid potential electrical bridging and other contamination problems, but also to avoid adhesive flowing into the interior of the space covered bylids40,40′ that can not be inspected. It is also noted that the temperature at which attachment and removal of thelids40,40′ of Examples 1-4 is performed is substantially lower than the temperature of about 220° C. at which soldering is performed, thereby reducing the likelihood that high temperature will disturb, damage or degrade the electronic devices proximate to such covers.
• Conventional isothermal curing or similar curing of thermosetting[0037]adhesive preforms30,30′ is generally undesirable because the time that thelids40,40′ andadhesive preforms30,30′ attached thereto are heated may be too long unless great care is exercised. If the time of pre-attachment heating to a temperature at or near the adhesive curing temperature is too long, the adhesive may gel too much or may partially cure and so not have sufficient strength to properly bond tosubstrate20,20′. Accordingly, it is desirable that the attachment bonding process employed with the adhesive selected foradhesive preforms30,30′ be improved over that of the prior art.
• In an improved cover attachment process,[0038]substrate20,20′ is preheated to a substantially higher temperature than arelids40,40′. For example,electronic circuit substrate20,20′ may be heated to about 150-200° C., i.e. a temperature sufficiently high to tack thermosettingadhesive preforms30,30′, whilelids40,40′ with thermosettingadhesive preforms30,30′ attached thereto are maintained at ambient temperature or an elevated temperature less than about 80°C. Lids40,40′ with pre-applied thermosettingadhesive preforms30,30′ attached thereto may be placed onto the preheatedelectronic circuit substrate20,20′ by a standard pick-and-place apparatus and, upon placement,lids40,40′ havingadhesive preforms30,30′ are heated by and become tacked tosubstrate20,20′. Thensubstrate20,20′ may be placed in a heating belt oven for about an additional 3-5 minutes at a temperature slightly below that of thesubstrate20,20′ preheat station. For example,substrate20,20′ may be preheated to about 175° C. and may be cured subsequent tolid40,40′ attachment in a belt-oven for an additional three minutes at about 150° C.
• In the case of thermoplastic[0039]adhesive preforms30,30′, post-attachment curing is not necessary and the only temperature requirement on the process for attachinglid40,40′ tosubstrate20,20′ is that the thermoplasticadhesive preform30,30′ be heated to the melt-flow temperature of the thermoplastic adhesive. The necessary heat can be provided by preheatinglids40,40′ or by the transfer of heat from thepreheated substrate20,20′ tolids40,40′. It is preferred to preheatlids40,40′ to a temperature substantially above the melt-flow temperature of the thermoplastic adhesive preforms30,30′ and to then presslids40,40′ against thewarm substrate20,20′ that may be at a temperature about 50-100° C. below the temperature oflids40,40′. The temperature differential causes rapid cooling of the thermoplastic adhesive preforms30,30′ immediately following pressing oflids40,40′ againstsubstrate20,20′, thereby promoting rapid setting of the thermoplastic adhesive.
• Thus, lids or covers[0040]40,40′ are attached to anelectronic circuit substrate20,20′ at a high rate, for example, one per second, and by employing automated assembly equipment of a kind presently available in most modern manufacturing facilities. This result is obtained with thermoplastic and thermosetting adhesives, and with electronic circuit modules, flip-chip modules, and printed wiring circuit boards whether receiving one or a large number of covers or lids attached thereto. The lids with adhesive preforms attached thereto applied in the foregoing manner may be of the same or different size and shape, may be of the same or different material, and may provide physical protection and/or electrostatic or electromagnetic protection.
• In addition, adhesive preforms and lids according to the present invention advantageously may be employed to avoid the so-called “blow-out” problem caused by gas trapped in the interior of a lid or cover that, when heated during the lid attachment process, ruptures the adhesive attachment between the lid and the package, thereby causing a failure in the[0041]adhesive seal30 between thecover40 and thesubstrate20. To this end, preforms30,30′ are formed having one or more gaps therein, as shown in FIG. 2, through which gas may bleed or flow. For example,adhesive preform30ahas onegap31 in one side thereof, whereasadhesive preform30bhas twogaps31, one in each of two opposing sides thereof.Adhesive preform30chas fourgaps31, one in each of the four sides thereof. Similarly,adhesive preforms30d,30eand30fhavegaps31 in one, two and four corners thereof, respectively. Each gap is narrow, being sufficient to permit entrapped gas molecules to pass, but is narrow enough to be closed by the flowing of the adhesive30 whencover40 is attached to asubstrate20 by heating and pressing againstsubstrate20. For example, in a squareadhesive preform30 formed of type ESP7450 adhesive that is about 0.35 inch long on each side, wherein the adhesive preform sides are about 40 mils wide and 6 mils thick, each of the four gaps is about 5 mils across. Segmented adhesive preforms30a,30b,30c,30d,30e,30fare easily fabricated and applied tocovers40,40′ by employing the method described herein because such preforms are deposited by accurate processes on arelease substrate32 and covers40,40′ are attached thereto while the preforms are still attached to therelease substrate32. Thereafter, thecovers40,40′ withadhesive preform30a,30b,30c,30d,30e,30fattached is easily handled by pick-and-place equipment. To attempt to form such gappedadhesive preform30a,30b,30c,30d,30e,30fby conventional methods which require handling of the preform would be extremely difficult, if not impossible, due to the small size and delicacy of the preform alone.
• While the present invention has been described in terms of the foregoing exemplary embodiments, variations within the scope and spirit of the present invention as defined by the claims following will be apparent to those skilled in the art. For example, the adhesives of which preforms[0042]30,30′ are formed may be filled with certain materials to tailor their characteristics to a particular application. Thermal conduction of the adhesive may be increased by the addition of particles of a high-thermal conductivity material, such as alumina (Al₂O₃), aluminum nitride (AlN), boron nitride (BN), silicon carbide (SiC), or diamond, which fillers may also be employed to modify the coefficient of thermal expansion thereof. The coefficient of thermal expansion thereof may also be reduced by the addition of particles of glass silicates, for example.

Claims (20)

What is claimed is:

1. A device cover for covering a device on an electronic substrate comprising:

a cover having a bonding surface defining a closed bonding pattern;

a bonding pattern of flowable adhesive on the bonding surface of said cover, wherein said bonding pattern of flowable adhesive is a flowable adhesive preform in the shape of the closed bonding pattern, and wherein said bonding pattern of flowable adhesive has at least one gap therein,

wherein the gap in said bonding pattern of flowable adhesive is sufficiently small as to be filled by said flowable adhesive when said flowable adhesive flows when the cover is adhered to the electronic substrate,

whereby a seal having a shape defined by the shape of the closed bonding pattern is formed between the cover and the electronic substrate by the flowed bonding pattern of flowable adhesive.

2. The device cover ofclaim 1 wherein said bonding pattern of flowable adhesive is formed by a method selected from the group consisting of mesh screening, stencil screening, contact screening, mask screening and ink-jet printing.

3. The device cover ofclaim 1 wherein said bonding pattern of flowable adhesive includes a dried flowable adhesive and/or a B-staged flowable adhesive.

4. The device cover ofclaim 1 wherein said flowable adhesive is selected from the group consisting of thermoplastic adhesives and thermosetting adhesives.

5. The device cover ofclaim 1 wherein the at least one gap is located at a corner of said bonding pattern of flowable adhesive and/or along a side of said bonding pattern of flowable adhesive.

6. The device cover ofclaim 1 wherein said cover is of a hollow rectangular shape having an open end, the edges at the open end thereof providing a bonding surface defining a rectangular bonding pattern.

7. The device cover ofclaim 1 wherein said flowable adhesive has a melting temperature at which its bond strength substantially decreases, which melting temperature is in a range of about 90° C. to about 180° C., which is substantially less than the melting temperature of solder.

8. A device cover for a device on an electronic substrate, said device cover having a gapped adhesive preform disposed on a bonding surface thereof and adapted to flow for attaching the cover to the electronic substrate comprising:

a cover having a bonding surface adapted for receiving adhesive and defining a closed bonding pattern;

an adhesive preform of flowable adhesive in the shape of the bonding pattern disposed on the bonding surface of said cover, wherein said flowable adhesive preform has at least one gap therein,

wherein the at least one gap is sufficiently small as to be filled by said flowable adhesive when said flowable adhesive flows for attaching the cover to the electronic substrate,

whereby a seal having a shape defined by the shape of the bonding pattern is formed between the cover and the electronic substrate by the flowed bonding pattern of flowable adhesive.

9. The device cover ofclaim 8 wherein said flowable adhesive preform is formed by a method selected from the group consisting of mesh screening, stencil screening, contact screening, mask screening and ink-jet printing.

10. The device cover ofclaim 8 wherein said flowable adhesive preform includes a dried flowable adhesive and/or a B-staged flowable adhesive.

11. The device cover ofclaim 8 wherein said flowable adhesive is selected from the group consisting of thermoplastic adhesives and thermosetting adhesives.

12. The device cover ofclaim 8 wherein the at least one gap is located at a corner of said adhesive preform and/or along a side of said adhesive preform.

13. The device cover ofclaim 8 wherein said cover is of a hollow rectangular shape having an open end, the edges at the open end thereof providing a bonding surface defining a rectangular bonding pattern.

14. The device cover ofclaim 8 wherein said flowable adhesive has a melting temperature at which its bond strength substantially decreases, which melting temperature is in a range of about 90° C. to about 180° C., which is substantially less than the melting temperature of solder.

15. A hollow device cover for covering a device on an electronic substrate, said hollow device cover having a gapped adhesive preform disposed on a bonding surface thereof and adapted to flow for attaching the hollow device cover to the electronic substrate, said hollow device cover comprising:

a hollow cover having an open end defining a bonding surface adapted for receiving adhesive and defining a closed bonding pattern;

a preform of melt-flowable adhesive formed in the shape of the bonding pattern and disposed on the bonding surface of said hollow cover, wherein said melt-flowable adhesive preform has two or more gaps therein,

wherein each of the two or more gaps is sufficiently small as to be filled by said melt-flowable adhesive when said melt-flowable adhesive flows for attaching the cover to the electronic substrate,

whereby a seal having a shape defined by the closed bonding pattern is formed between the hollow cover and the electronic substrate by the flowed bonding pattern of flowable adhesive.

16. The hollow device cover ofclaim 15 wherein said preform of melt-flowable adhesive is formed by a method selected from the group consisting of mesh screening, stencil screening, contact screening, mask screening and ink-jet printing.

17. The hollow device cover ofclaim 15 wherein said melt-flowable adhesive includes a dried thermoplastic adhesive, a B-staged thermoplastic adhesive, a dried thermosetting adhesive, and/or a B-staged thermosetting adhesive.

18. The hollow device cover ofclaim 15 wherein the two or more gaps of said preform are at one or more corners of the bonding pattern, along one or more sides of the bonding pattern and/or at one or more corners and along one or more sides of the bonding pattern.

19. The hollow device cover ofclaim 15 wherein said hollow cover is of a hollow rectangular shape defining a rectangular closed bonding pattern.

20. The hollow device cover ofclaim 15 wherein said melt-flowable adhesive has a melt-flow temperature at which its bond strength substantially decreases, which melt-flow temperature is in a range of about 90° C. to about 180° C., which is substantially less than the melting temperature of solder.

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