United States Patent [191
`Licari et al.
`
`[11] Patent Number:
`[45) Date of Patent:
`
`5,002,818
`Mar. 26, 1991
`
`[S4] REWORKABLE EPOXY DIE-ATIACH
`ADHESIVE
`Inventors: James J. Licari, Whitter; Gabriel G.
`Bakbit, Huntington Beach, both of
`Calif.
`
`[75]
`
`[73) Assignee: Hughes Aircraft Company, Los
`Angeles, Calif.
`
`[21] Appl. No.: 408,864
`
`[22] Filed:
`
`Sep. 5, 1989
`
`Int. Cl.' ................................................ B32B 9/00
`[51]
`[52) u.s. Cl. .................................... 428/209; 428/323;
`428/413;428/414;428/901; 206/228; 206/230;
`528/422; 525/530; 525/531
`[58] Field of Search ................ 206/228, 230; 428/209,
`428/901,323,413, 414; 528/422; 525/530,531
`
`[56]
`
`Refereuces Cited
`PUBLICATIONS
`Resin-Phenoxy
`Resin
`Epoxy
`110(10):7705 1(s), 3M (Schenz).
`
`Blend,
`
`CA
`
`Polyimide Compsns Die Attach Adhesives, Mat Chern,
`EP Abs 89309229, Powell et al.
`High Temp Elec. Cond. Ads., Mactiner et al., Sampe
`Conf.
`Thermoplastic Films for Adh Bonding, Shore, 39th
`Electronic Comps. Proceedings.
`Primary Examiner-Patrick J. Ryan
`Attorney, Agent, or Firm-Terje Gudmestad; Jeannette
`M. Walder; W. K. Denson-Low
`[57)
`ABSTRACT
`An adhesive mixture reworkably adheres electronic
`integrated circuit dies to hybrid microcircuit substrates,
`and includes a thermosetting epoxy resin. A thermo(cid:173)
`plastic resin additive allows the mixture to retain the
`high adhesive strength of the epoxy resin up to approxi(cid:173)
`mately 150" C., or the upper limit of the operating and
`testing temperature range of the dies, and then soften
`sufficiently to enable defective dies to be removed at a
`temperature of preferably between 150" c. and zoo· c.
`without damage to the substrate or adjacent dies.
`
`23 Claims, 1 Drawing Sheet
`
`12b
`
`I&
`
`1
`
`Petitioner Samsung - SAM1008
`
`

`
`U.S. Patent
`
`Mar. 26, 1991
`
`Sheet 1 of 1
`
`5,002,818
`
`f6CL
`
`10
`\,
`
`12b
`
`12a
`
`2
`
`

`
`.~ 1
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`5,002,818
`
`2
`1. The larger devices have a larger surface area, and
`therefore require greater force to remove.
`2. The closer packed devices with small inter-chip
`spacings make the use of a heated tool to apply a shear
`5 force between devices difficult or impossible without
`damaging vicinal devices or wire bonds.
`3. The extra force and temperature necessary to re(cid:173)
`move a die can damage the underlying organic dielec(cid:173)
`tric, resulting in having to scrap a very expensive inter(cid:173)
`connect substrate.
`Thermoplastic films and paste adhesives are also
`being investigated because they soften or melt. How(cid:173)
`ever, these suffer from two limitations for electronic
`applications.
`1. Many of them must be dispensed from a solvent,
`and these solvents are difficult to completely remove,
`especially from underneath large area dies.
`2. Most require very high temperatures (300• C. or
`higher) to melt or soften and, depending on the extent of
`rework, such temperatures can be damaging to wire
`bonds and some devices.
`
`REWORKABLE EPOXY DIE-ATIACH ~HESIVE
`
`BACKGROUND OF THE INVENilON
`1. Field of the Invention
`~.
`The present invention generally relates to. tjte field of
`adhesives, and more particularly to an adhesive mixture
`which enables electronic integrated circy.it dies to be
`strongly attached to hybrid microcircuit substrates, and 10
`subsequently removed if necessary at a low enough
`temperature so as not to damage the subsJrates or adja(cid:173)
`cent dies.
`2. Description of the Related Art
`Thermosetting epoxies are widely used as adhesives 15
`for electronic applications. They are noted for their
`high adhesive strength to a wide variety of.s,rfaces and
`materials. Their adhesive strength derives. mainly from
`their highly polar molecular structure, whicit results in
`H-bonding and Vander Waals forces of attraction to 20
`surfaces such as alumina, ceramic, silicon, ather epox(cid:173)
`ies, and even gold. Epoxies are therefore widely used,
`for example, for integrated circuit die attachment in
`hybrid microcircuits, component attachment in printed
`wiring boards, heat sink attachment, and c~amic sub- 25
`strate attachment inside hermetically sealed packages.
`Epoxy resins are generally of the diglycidyl ether of
`bisphenol A (DGEBA) types cured with a variety of
`catalysts or hardeners as is well known in the art. The
`reworking of components, once bonded with t:poxy, has 30
`always presented a problem, and no i~ method has
`been heretofore discovered. Generally, belt' and pres(cid:173)
`sure are applied simultaneously to the tlevic~ to be re(cid:173)
`moved. When the epoxy reaches its"gifa/ transition
`temperature Ta, which is usually 200"-c. or higher, it 35
`softens, at which point a laterally appli¢ force effects
`its detachment. The entire circuit ofteh p~.4st be sub(cid:173)
`jected to high temperature (200" C. or higli$r), which
`risks damaging wire bonds or other temperature-sensi(cid:173)
`tive elements of the circuit. Nevertheless, with the use 40
`of small die (less than 100 mils square) and moderately
`spaced die (greater than 200 mils chip-to-chip); epoxy(cid:173)
`attached devices have been able to be reworked.
`With the emergence of a new generation of very high 45
`density circuits, also referred to as hybrid 'fafer scale
`integration, or multichip module, the combination of
`high strength and reworkability becomes a critical is(cid:173)
`sue. These new circuits utilize VLSIC (Very Large
`Scale Integrated circuits), VHSIC (Very High Speed 50
`Integrated Circuit), MIMIC (Millimeter Microwave
`Integrated Circuit), and other high-density dies that are
`approaching one inch square and are closely packed
`(less than 70 mils chip-to-chip) for high-speed applica(cid:173)
`tions. These large devices are also being assembled onto 55
`thin fllm multilayer interconnect substrates that have
`polyimide or another low-dielectric constant polymer
`as part of their structure.
`Die-attach adhesives which meet all current stan(cid:173)
`dards and must also be removable will be needed for 60
`hybrid circuits where gate-arrays of approximately 600
`mils square and chip-to-chip spacings of 70 mils are
`requirements. Other digital and analog circuits that will
`be fabricated using the HDMI (high density multichip
`interconnect substrate), also known as Hybrid Wafer 65
`Scale Integration, process will also require these adhe(cid:173)
`sives.
`Three problems arise in this environment.
`
`SUMMARY OF THE INVENTION
`The present invention fills the above discussed need
`by providing an adhesive mixture which includes a
`thermosetting epoxy resin. A thermoplastic additive
`allows the mixture to retain the high adhesive strength
`of the epoxy resin up to approximately 150• C., or the
`upper limit of the operating and testing temperature
`range of the dies, and then rapidly soften to enable
`selected dies to be removed at a temperature of prefera(cid:173)
`bly between tso· c. and zoo· c. without damage to the
`substrate or adjacent dies.
`A reworkable electronic component structure, ac(cid:173)
`cording to the invention, may comprise a substrate; an
`electronic circuit component; and an adhesive for re(cid:173)
`workably adhering the components to the substrate, the
`adhesive including: a curable thermosetting resin; and
`an additive selective to cause the adhesive, in its cured
`state, to soften at a temperature between approximately
`too• C. and 250• C. A reworkable, electronic, die-attach
`adhesive, according to the invention, may comprise a
`curable thermosetting resin; and an additive selected to
`cause the adhesive, in its cured state, to soften at a tem(cid:173)
`perature between approximately wo· c. and zso· c.
`The adhesive mixture may also include a filler to
`control shrinkage or expansion coefficients, enhance
`thermal conductivity, or render the adhesive electri(cid:173)
`cally conductive.
`The present invention further provides an electronic
`component structure including at least one electronic
`component, typically an integrated circuit die, which is
`reworkably adhered to a substrate by means of the pres(cid:173)
`ent adhesive.
`These and other features and advantages of the pres(cid:173)
`ent invention will be apparent to those skilled in the art
`from the following detailed description, taken together
`with the accompanying drawing.
`
`DESCRIPTION OF THE ORA WINGS
`The single Figure of drawing is a fragmentary side
`view, partially in section, of an electronic component
`structure including a reworkable die-attach adhesive
`according to the present invention.
`
`3
`
`

`
`5,002,818
`
`3
`DETAILED DESCRIPTION OF THE
`INVENTION
`Hybrid microcircuits include a number of component
`packages, or dies, which may be integrated circuits or 5
`discrete components, mounted on a substrate board.
`The present adhesive is especially intended for attach(cid:173)
`ing dies to substrates in the microelectronic art, in an
`environment in which the circuit may have to be re(cid:173)
`worked. Such reworking would include removal of 10
`particular dies from the substrate which prove to be
`defective in testing or which are subject to upgrading
`by replacement with improved components. However,
`the present adhesive is directly applicable to any field of
`technology in which two materials are to be strongly 15
`bonded together, subject to possible separation at a later
`time for any intended purpose.
`An electronic component structure embodying the
`present invention is illustrated in FIG. 1 and designated
`as 10. The structure 10 includes a hybrid microelec- 20
`tronic substrate 12 which has a conductive pad metalli(cid:173)
`zation layer 12b formed on top of a dielectric sheet 12a.
`Components 14 and 16, which may be microelectronic
`circuit dies which may be connected by wire bonding
`or tape automated bonding, are reworkably adhered to 25
`the substrate 12 by means of a reworkable, die-attach
`adhesive 18 of the invention. Although only two com(cid:173)
`ponents 14 and 16 are shown in the drawing for illustra(cid:173)
`tive purposes, a practical structure embodying the in(cid:173)
`vention will include a large number of components 30
`which are closely spaced together.
`The components 14 and 16 may range from discrete
`components such as power transistors or inductors, to
`integrated circuits with any desired high level of inte(cid:173)
`gration, and have leads 14a and 16a. The leads 14a and 35
`16a are ohmically connected to respective portions of
`the interconnect layer 12b by means such as soldering
`thermocompression or thermosonic bonding, although
`not illustrated in detail. The interconnect layer 12b may
`alternatively be formed on the top of the dielectric sheet 40
`l2a, or have a multi-layer configuration within the
`scope of the invention.
`The structure 10 is assembled by applying the adhe(cid:173)
`sive 18 to either or both of the substrate 12 and compo(cid:173)
`nents 14 and 16 in uncured form, pressing the compo- 45
`nents 14 and 16 onto the substrate 12 in proper align(cid:173)
`ment, and curing the adhesive 18 using an appropriate
`thermal or other curing mechanism. The structure 10
`may be reworked by removing selected components 14
`or 16. This is accomplished in one of two ways (a) by 50
`inserting a bladed tool heated to a temperature of 1 oo·
`C. to 250" C. (depending on the adhesive's softening or
`melt temperature) between the selected component 14
`or 16 and the substrate, and applying a shear force until
`the component detaches or (b) by applying heat selec- 55
`tively to the device, then removing it from the top of
`the device using a vacuum tool. The removal methods
`are given by way of illustration; other methods are
`possible. The leads 14a and 16a may be cut prior to
`component removal, or may be broken ofT as the com- 60
`ponent separates from the substrate.
`The present adhesive 18 includes two main compo(cid:173)
`nents which act in combination to enable strong adhe(cid:173)
`sive bonding up to a desired temperature, and sufficient
`softening of the adhesive to effect separation of the 65
`bonded surfaces at a slightly higher temperature. The
`primary component of the present adhesive is a thermo(cid:173)
`setting resin. Current state-of-the art epoxy adhesives,
`
`4
`although meeting all of the engineering requirements of
`current standards, are difficult to remove, and rework(cid:173)
`ing of high density circuits cannot be accomplished.
`More particularly, conventional epoxy resins, including
`those which may be utilized as the thermosetting resin
`in the present adhesive, begin to soften at their glass
`transition temperature T a, which is generally above
`zoo• C. Even in the softened state of the adhesive, con(cid:173)
`siderable mechanical force is required to remove a die
`from a substrate. Assembled electronic components
`usable in practical applications are generally designed
`to withstand processing and operating temperatures up
`to approximately 155• C. Heating of mounted dies to
`temperatures substantially in excess of zoo· c. will
`cause damage.
`In order to overcome this problem, the present adhe(cid:173)
`sive includes an additive which is mixed with the ther(cid:173)
`mosetting resin, and is selected such that the adhesive
`mixture, in its cured or hardened state, will soften suffi(cid:173)
`ciently in a temperature range between approximately
`150• C. and zoo• C. to enable die removal with a small
`enough amount of mechanical force to avoid damage to
`the substrate or adjacent dies.
`Although any thermosetting adhesive material may
`be utilized within the scope of the present invention, the
`preferred embodiment includes a thermosetting epoxy
`resin of the diglycidyl ether of bisphenol A (DGEBA)
`type. The epoxy resin may be supplied as a one-compo(cid:173)
`nent type or a two-component type, and includes a
`curing agent (hardener, activator, or catalyst) for hard(cid:173)
`ening or curing at room temperature with heat pro(cid:173)
`duced by exothermic reaction, or by the application of
`external heat or other radiation. The epoxy resin is
`selected such that the heat produced or applied during
`the curing process does not generate a temperature high
`enough to damage the microcircuit dies or substrate
`boards. Details of applicable epoxy resin systems are
`thoroughly covered in the available literature, for ex(cid:173)
`ample in a textbook entitled "Handbook of Epoxy Res(cid:173)
`ins", by H. Lee et al, McGraw-Hill, 1967. Thermoset(cid:173)
`ting epoxy formulations based, for example, on Epon
`8Z8 or other diglycidyl ether or bisphenol A and hard(cid:173)
`ened with amino compounds (primary, secondary, or
`tertiary) such as dicy, substituted ureas, etc. are suitable
`for use in the present invention.
`The second main component of the present adhesive
`is an additive which has a melting point or temperature
`T M selected to cause the adhesive mixture to retain
`strong adhesive bonding up to a desired temperature
`range of between approximately too• C. to 150• C., and
`then soften sufficiently to enable die removal. Although
`any additive which will produce the desired effect may
`be utilized within the scope of the invention, preferred
`additives include thermoplastic epoxy resins and other
`thermoplastic materials having melting points higher
`than approximately 150• C. Examples of applicable
`thermoplastic resins include polysulfones, polyetheri(cid:173)
`mides, block copolymers of chlorotrifluoroethylene
`with vinylidene fluoride, polycarbonates, polyamides,
`polyethyleneterephtalate, polyphenylene sulfides, fluo(cid:173)
`rinated polymers, and bismaleimides.
`The thermoplastic material may constitute I% to
`50% by weight of the adhesive, typically 10% to 30%.
`Examples of thermoplastic resins which may be utilized
`in the lower range of I% to 10% include polyphenylene
`sulfide (TM=Z85. C. to Z9o· C.), and crystalline or
`amorphous insulating compounds such as crystalline
`polyamide (TM=Z90• C.). As will become apparent
`
`4
`
`

`
`5,002,818
`
`5
`from subsequent description, the melting point of the
`thermoplastic material is not necessarily the same as the
`softening point of the cur~ adhesive mixture, due to
`interaction between the thermosetting and thermoplas(cid:173)
`tic components.
`In addition to the thermosetting and thermoplastic
`resins, the present adhesive may include one or more
`additional additives or fillers to control shrinkage or
`expansion coefficients, enhance thermal conductivity,
`or render the adhesive electrically conductive. Exam- 10
`pies of such fillers include, but are not limited to, alumi(cid:173)
`num oxide, silicon oxide, silicon dioxide, silicon nitride,
`titanium oxide, aluminum nitride, diamond, lithofrax,
`calcium carbonate, or other carbonates.
`The principles of the present invention will become 15
`better understood from the following· example, with
`reference being made to the Figure of drawing.
`
`6
`The shear strength of the 50% mixture began to devi(cid:173)
`ate from that of the pure epoxy material at approxi(cid:173)
`mately too• C., and decreased to a value of about 1000
`psi at tso• C. The shear strength further decreased by
`5 67% as the temperature was increased to 175• C.
`Ideally, the adhesive would retain full strength up to
`a value of approximately 155• C., and drop sharply to a
`very low level as the temperature is increased further.
`While several illustrative embodiments of the inven-
`tion have been shown and described, numerous varia(cid:173)
`tions and alternate embodiments will occur to those
`skilled in the art, without departing from the spirit and
`scope of the invention. Accordingly, it is intended that
`the present invention not be limited solely to the specifi-
`cally described illustrative embodiments. Various modi(cid:173)
`fications are contemplated and can be made without
`departing from the spirit and scope of the invention as
`defined by the appended claims.
`We claim:
`1. A reworkable electronic component structure,
`comprising:
`a substrate;
`an electronic circuit component; and
`an adhesive for reworkably adhering the component
`to the substrate, the adhesive including:
`a curable thermosetting resin; and
`an additive selected to cause the adhesive, in its
`cured state, to soften at a temperature between
`approximately too• C. and 250° C.
`2. A structure as in claim 1, in which the additive is
`selected to cause the adhesive to soften at a temperature
`between approximately 150° c. and 200° c.
`3. A structure as in claim 2, in which the additive has
`a melting temperature above approximately 150° C.
`4. A structure as in claim 3, in which the thermoset(cid:173)
`ting resin has a glass transition temperature above ap(cid:173)
`proximately 200° C.
`5. A structure as in claim 1, in which the thermoset(cid:173)
`ting resin comprises an epoxy resin.
`6. A structure as in claim 5, in which the epoxy resin
`comprises diglycidyl ether of bisphenol A.
`7. A structure as in claim 5, in which the additive
`constitutes between approximately l% and 50% by
`weight of the adhesive.
`8. A structure as in claim 5, in which the additive
`constitutes between approximately 10% and 30% by
`weight of the adhesive.
`9: A structure as in claim 5, in which the additive
`comprises a thermoplastic resin.
`10. A structure as in claim 9, in which the thermoplas(cid:173)
`tic resin comprises a thermoplastic epoxy resin.
`11. A structure as in claim 1, in which the component
`is an integrated circuit die.
`12. A structure as in claim 1, in which the adhesive
`further comprises a filler material.
`13. A reworkable electronic die-attach adhesive com(cid:173)
`prising:
`a curable thermosetting resin; and
`an additive selected to cause the adhesive, in its cured
`state, to soften at a temperature between approxi(cid:173)
`mately roo· c. and 250° c.
`14. An adhesive as in claim 13, in which the additive
`is selected to cause the adhesive to soften at a tempera(cid:173)
`ture between approximately 150• C. and 200° C.
`15. An adhesive as in claim 14, in which the additive
`has a melting temperature above approximately 150° c.
`
`25
`
`EXAMPLE
`A liquid thermoplastic material was blended in quan- 20
`tities of 10, 20, 30 and SO% by weight with a non-con(cid:173)
`ductive die-attach epoxy to evaluate its effect on die
`shear strengths. Silicon dies were mounted, cured, and
`shear tested at room temperature, tOO, tSO and 200• C.,
`using the adhesive mixtures in addition to the pure
`epoxy and pure thermoplastic materials. It was found
`that at too• C. and below, there was little degradation in
`the measured shear strength of the thermoplastic mix(cid:173)
`tures compared with the pure epoxy. However, at 1so• 30
`C., the shear force required for die removal was signifi(cid:173)
`cantly less (SO%) with the epoxy/thermoplastic mix(cid:173)
`tures than with the conventional die-attach epoxy.
`The thermosetting epoxy resin was an electrically
`nonconductive material designated as Ablebond 84-t, 35
`supplied by Ablestik Laboratories. The thermoplastic
`material was a thermoplastic epoxy resin designated as
`SM-933-60, supplied by Emerson & Cuming, Inc.
`The thermosetting and thermoplastic materials were
`mixed by hand in the above described ratios, and out- 40
`gassed prior to die attach. The dies were 0.08 inch
`square, and mounted on gold plated alumina substrates.
`All samples were outgassed again for S minutes after die
`attachment and then cured for one hour at t2s• C.
`The average shear strength for the samples contain- 45
`ing mixtures of thermoplastic material up to SO%
`showed a reduced amount of force in comparison to the
`pure epoxy adhesive required to remove -the dies from
`the substrate at temperatures of tso• C. to t1s• C.
`At too• C. anq below, the thermoplastic material 50
`appeared to have little effect on the measured shear
`strength of the epoxy. The pure thermoplastic material
`had very poor shear strength between 2s• C. and too•
`C. No tests at higher temperatures were performed.
`The conclusions of the tests are as follows.
`1. The addition of the thermoplastic material to the
`epoxy material lowered the shear force required for die
`removal at tso• C. and at t1s• C. by at least SO%.
`2. At and below too• C. the addition of the thermo(cid:173)
`plastic material appeared to have no effect on the mea- 60
`sured shear strength of the adhesive.
`The thermosetting epoxy resin without any additives
`retained a shear strength of approximately 2000 psi up
`to a temperature of about tss• C., above which the
`shear strength decreased to a value of about 1000 psi at 65
`t1s• C. The pure thermoplastic material had an initial
`shear strength of about t2SO psi, which dropped below
`soo psi as the temperature approached too· c.
`
`55
`
`5
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`

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`5,002,818
`
`7
`16. An adhesive as in claim 15, in which the thermo(cid:173)
`setting resin has a glass transition temperature above
`approximately 200" C.
`17. An adhesive as in claim 13, in which the thermo(cid:173)
`setting resin comprises an epoxy resin.
`18. An adhesive as in claim 17, in which the epoxy
`resin comprises diglycidyl ether of bisphenol A.
`19. An adhesive as in claim 17, in which the additive
`constitutes between approximately 1% and 50% by
`weight of the adhesive.
`
`8
`20. An adhesive as in claim 17, in which the additive
`constitutes between approximately 10% and 30% by
`weight of the adhesive.
`21. An adhesive as in claim 17, in which the additive
`5 comprises a thermoplastic resin.
`22. An adhesive as in claim 21, in which the thermo(cid:173)
`plastic resin comprises a thermoplastic epoxy resin.
`23. An adhesive as in claim 13, further comprising a
`filler material.
`• * * • •
`
`10
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`15
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`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
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`55
`
`60
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`65
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`6