Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1997-03-03
2001-07-24
Gallagher, John J. (Department: 1733)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C156S307500, C524S440000
Reexamination Certificate
active
06265471
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention described herein relates to an adhesive paste for bonding higher power devices to a substrate with a minimum of thermal resistance through the adhesive bondline and the method of use.
The attachment of active devices, such as semiconductor dies, to a substrate or in an electronic package has historically been accomplished using organic or inorganic adhesives. Typically, a semiconductor die is bonded to a substrate that is part of the semiconductor package. The adhesives that accomplish this bonding or attachment are usually in the form of a wet paste which consists of (1) a binder or adhesive component, (2) a filler, and (3) an optional diluent.
The purpose of the adhesive is to create a bond between the semiconductor device and the substrate. This adhesive layer between the device and substrate is often referred to as the bondline. Inorganic and organic binders have both previously been used. The most common inorganic binders are glass, solder, or a eutectic of Si and Au. Generally, this type of binder is used in inorganic packages, such as ceramic or metal. They are not generally adaptable to bonding die in plastic packages because of the high temperature processing required. The lowest attach temperature, for example, with Ag filled glass is about 300° C. as described in U.S. Pat. No. 4,933,030. This processing temperature, however, is too high for use with a plastic laminate package. On the other hand, the thermal conductivity of these inorganic adhesive systems is high, ranging about from 40 to 65 W/mK.
Organic binders are the most common type of binders used for die attach in plastic packages because of their low processing temperatures. However, the thermal conductivities of prior organic adhesive systems have been limited to about 3-4 W/mK, thus limiting the amount of heat dissipation possible in plastic packages.
Organic binders can be further classified into thermoset and thermoplastic. Thermoplastic organic binders are described in detail in U.S. Pat. No. 5,391,604. Thermosetting binders, as the name implies, “set up” or “crosslink” during a thermal excursion. Typically, a thermoset adhesive contains an epoxy resin, a hardener, a filler, and in many cases a small amount of reactive solvent. The function of the hardener is to crosslink with the epoxy resin and change from a liquid to a solid. More recently, as described in U.S. Pat. No. 5,480,957, the hardeners can be “latent” i.e., they do not react with the resins until well above room temperature. The use of a latent hardener extends the pot life or time at room temperature before the viscosity increases to an unacceptable level for dispensing as a result of room temperature crosslinking.
Other ingredients sometimes used in thermoset adhesives in combination with an epoxy resin include a reactive diluent or solvent. The reactive solvent is typically an unsaturated low viscosity monomer capable of crosslinking with the epoxy and becoming part of the hardened structure. It is used to lower the viscosity of the epoxy resin to allow more loading of the filler. Thus, in the case of electro conductive adhesives, in which the filler usually is silver, higher electrical and thermal conductivities are realized. However, because of the lower molecular weight of the reactive solvent, only a limited amount can be used without sacrificing heat resistance and other functional properties. Thus, thermal conductivities have been limited to about 3-4 W/mK in prior art organic adhesives.
In some cases, a fugitive non-reactive solvent is used with an epoxy resin. For example, U.S. Pat. No. 3,716,489 describes a volatile solvent used to dissolve a photochromic material in an epoxy resin to form a transparent epoxy photochromic filter. Unlike the invention described herein, the above material is not used as an adhesive nor is it loaded with a particulate conductive filler.
U.S. Pat. Nos. 5,011,627 and 4,564,563 further describe screen printable electro conductive pastes that include fugitive, non-reactive solvents that are used to dissolve the thermoplastic and epoxy resins. These examples, however, are used as conductive traces, such as on membrane keyboards, rather than adhesives and hence the solvents are easily evaporated before entrapment in the structure by the crosslinking. Thus, they could not be employed as adhesives wherein the solvents must be volatized from the bondline between the die and substrate before crosslinking occurs to avoid entrapment of the solvent and voids.
SUMMARY OF THE INVENTION
The instant invention provides an organic adhesive with a thermal conductivity an order of magnitude higher than those previously available, and, in addition, is easily dispensed and processed at low temperatures, provides a strong adhesive bond between the die and substrate, and good storage characteristics.
Specifically, the present invention provides a thermosetting adhesive paste comprising by volume:
a) about 15-75% of one part thermosetting liquid resin,
b) up to about 45% solvent, and
c) about 20-45% conductive filler,
the paste, when cured, having an electrical resistivity less than about 50×10−6 ohm-cm and a thermal conductivity greater than about 5 W/mK.
There is also provided a method of using the adhesive paste to bond microcircuit components to substrates and the assembled article that is useful in the electronic industry.
The present invention further provides a method of attaching a component to a substrate which comprises applying a paste as defined above to the substrate at a thickness of at least about 3 mils, placing the component on top of the applied adhesive and within 3 mils of the substrate, and preferably within about 1 mil, with the adhesive filling the bondline, and heating the assembly to a temperature of at least about 150° C. for at least about 1 hour.
DETAILED DESCRIPTION OF THE INVENTION
Each of the principal components of the adhesive pastes of the present invention is described below and as an integral part of the unique paste system which provides unexpectedly high thermal properties.
Thermoset Resin/Hardener
A wide variety of known thermoset resins can be used in the present invention, and will be selected depending on the functional requirements of the cured material. Preferred are cycloaliphatic epoxy resins having a molecular weight of less than about 1000 and preferably less than about 500. Resins found to be particularly satisfactory are those manufactured by Ciba Geigy and available under product codes LMB5414 and LMB6136. The low viscosity of the resin is attractive to allow a high percent loading of the filler at a reasonable viscosity without the use of a large amount of fugitive solvent.
The hardener which is incorporated in the liquid resin should be latent, that is, is activated at temperatures greater than about 100° C. The specific hardener is selected to be effective at a temperature which is compatible with the fugitive solvent properties, as more fully discussed below. The LMB5414 resin with hardener system delays the start of crosslinking to above 100° C., and has been found to be particularly satisfactory.
The latency of the curing or crosslinking is a key parameter of the system. Crosslinking must be delayed sufficiently in temperature to allow a sufficient amount of the fugitive solvent to be extracted or dried from the adhesive matrix. If too much crosslinking occurs before the solvent evolves from the adhesive, the solvent can become entrapped under the die in the three dimensional network that results from crosslinking. Thus, the relationship between the vapor pressure of any solvent used and the latency of the crosslinking is important.
It is preferable that the resin be substantially free of alkali and halide ions. If such ions are present in the cured adhesive product in a non-hermetic environment, they can migrate to the bond areas, resulting in undesirable conditions including corrosion.
Solvent
The solvent preferably used in this invention is characterized by the following general properties:
1) non-reactive with
Diemat, Inc.
Gallagher John J.
Huntley & Associates
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