Field of the invention

The present invention provides a curable liquid epoxy resin composition useful as underfill material for semiconductor devices, which comprises (a) an epoxy resin component, (b) an aromatic hydrocarbon formaldehyde resin, (c) a curing agent, and, (d) at least one filler having an average particle size of 5 nm to 100 μηη.

The aromatic hydrocarbon formaldehyde resin can lower the Tg of the curable liquid epoxy resin composition without affecting the CTE and/or modulus substantially once it has been cured.

Background

For the recent years, flip-chip bonding is widely used as a method for mounting semiconductor chips to meet the request for higher density and higher frequency wiring of electronic devices. In flip-chip bonding, the gap between a chip and a substrate is generally sealed with an encapsulant material called underfill. Today's highly integrated chips operating at full load can run at relatively high temperature. The underfill can improve the thermal conductivity of the chips, but the underfill will be heated in this process. Compared to eutectic and high lead solder, tin-silver-copper solder has lower CTE, higher elasticity and greater brittleness. In consideration of these properties and to prevent bump fatigue during reliability test, it's generally better to apply underfill with high Tg and low CTE (coefficient of thermal expansion). As a result of the brittleness of low-k dielectric layers, the destruction of low-k dielectric layers by the stress generated inside the flip chip packages has become a major issue. That is to say the underfills for low-k packages should have low stress and smaller warpage. It's expected that as the low-k trend expands, the underfill can provide less stress to protect the low-k dielectric layers. Low Tg underfill shows low stress to protect the low-k dielectric layers.

CPMT Symposium Japan, 2010 IEEE, Page 1 -4, which relates to an organic- inorganic hybrid polymer system showing low Tg, low CTE and moderate modulus. But the filler loading applied in this underfill is 65 wt%, which results in the increase of viscosity and compromises the fluidity of underfill. Summary of the invention

An object of the present invention is to provide a curable liquid epoxy resin composition, comprising:

(a) a liquid epoxy resin component,

(b) an aromatic hydrocarbon formaldehyde resin,

(c) a curing agent, and

(d) at least one filler having an average particle size of 5 nm to 100 μηη.

The curable liquid epoxy resin composition of the present invention shows low Tg after it is cured.

Another object of the present invention is to provide underfill material comprising the curable liquid epoxy resin composition of the present invention.

A further object of the present invention is to provide a method of encapsulating a semiconductor device with a semiconductor chip, preferably a flip-chip, electrically interconnected with a carrier substrate, comprising steps of:

(a) providing the underfill material of the present invention between the electrically interconnected surfaces of the semiconductor chip and the carrier substrate to form a semiconductor device assembly; and

(b) exposing the semiconductor device assembly to elevated temperature conditions sufficient to cure the underfill material.

Detailed description of the invention

The compositions according to the invention and the method according to the invention comprising the compositions according to the invention are described below by way of example, without the invention being limited to these exemplifying embodiments. References below to ranges, general formulae or classes of compound should be taken to encompass not only the corresponding ranges or groups of compounds that are explicitly mentioned, but also all sub-ranges and sub-groups of compounds that may be obtained by extracting individual values (ranges) or compounds. Where documents are cited in the context of the present description, it is intended that their content fully form part of the disclosure content of the present invention. Where percentages are given below, they are percentages in % by weight unless stated otherwise. In the case of compositions, the percentages, unless stated otherwise, are based on the overall composition. Where average values are reported below, the averages in question are mass averages (weight averages), unless otherwise indicated. Where measurement values are reported below, these measurement values, unless stated otherwise, have been determined under a pressure of 101 325 Pa and at a temperature at 25°C.

The epoxy resin component of the invention may be any of well-known epoxy resins as long as they have at least two epoxy groups per molecule and are liquid at room temperature (25°C).

Preferably the liquid epoxy resins are selected from the list of novolac type epoxy resins, such as phenol novolac type epoxy resins and cresol novolac type epoxy resins, bisphenol type epoxy resins, such as a bisphenol A type epoxy resins and bisphenol F type epoxy resins, bisphenol AD epoxy resins, aromatic glycidyl amine type epoxy resins, such as Ν,Ν-diglycidyl aniline, Ν,Ν-diglycidyl toluidine, diaminodiphenylmethane type glycidyl amine and aminophenol type glycidyl amine type epoxy resins, hydroquinone type epoxy resins, stilbene type epoxy resins, triphenol methane type epoxy resins, triphenol propane type epoxy resins, alkyl modified triphenol methane type epoxy resins, alkyl triphenol propane type epoxy resins, triazine-nucleus containing epoxy resins, dicyclopentadiene modified phenol type epoxy resins, naphthol type epoxy resins, aralkyl type epoxy resins, such as phenol aralkyl type epoxy resins or naphthol aralkyl type epoxy resin having a naphthalene, phenylene, and/or a biphenylene skeleton, aliphatic series epoxy resins, such as alicyclic epoxy type resins, such as vinylcyclohexene dioxide type epoxy resins.

As the epoxy resin component of the invention, the liquid epoxy resins may be used alone or in combination of two or more thereof.

As the epoxy resin component of the invention, epoxy resins in solid form at room temperature can also be used in combination with the liquid epoxy resins, as long as the mixtures are liquid at a room temperature.

In a further preferred embodiment of the present invention, the liquid epoxy resin is one or more selected from bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol AD epoxy resins, and naphthalene epoxy resins.

Although the content of the liquid epoxy resin component is not limited, it is preferable that the liquid epoxy resin component constitutes 5 to 30 wt%, and more preferably 5 to 20 wt%, based on the total weight of the curable liquid epoxy resin composition, for the reactivity, heat resistance, mechanical strength and flowability of the composition during the underfilling process. The curing agent used herein may be any of well-known agents and is not particularly limited. Preferred curing agents are selected from the list of amine compounds, phenol compounds, organic acid anhydrides, and carboxylic acids. Inter alia, aromatic amines, phenol compounds, and organic acid anhydrides are more preferred, particularly preferred are organic acid anhydrides.

From the working standpoint requiring that the liquid epoxy resin composition of the invention properly flow at room temperature, it is desirable to use a curing agent which is liquid at 25°C. When a curing agent which is solid at 25°C is used, it should preferably be dissolved in another curing agent which is liquid at 25°C so that the overall curing agent is liquid.

Preferred organic acid anhydride curing agents includes methyltetrahydrophthalic anhydride (mthpa), cis-1 ,2,3,6-tetrahydrophthalic anhydride (thpa), hexahydro-4- methylphthalic anhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, endo-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic anhydride, cyclobutane-1 ,2,3,4- tetracarboxylic dianhydride, benzophenone-3,3',4,4'-tetracarboxylic dianhydride, 4,4'- oxydiphthalic anhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy) bis(phthalic anhydride), trimellitic anhydride, pyromellitic anhydride, benzophenone tricarboxylic anhydride, ethylene glycol bistrimellitate, glycerol tristrimellitate, maleic anhydride, endomethylene tetrahydrophthalic anhydride, methylendomethylene tetrahydrophthalic anhydride, methylbutenyl tetrahydrophthalic anhydride, dodecenyl succinic anhydride, hexahydrophthalic anhydride, succinic anhydride, methylcyclohexene dicarboxylic anhydride, alkylstyrene-maleic anhydride copolymer, chlorendic anhydride, polyazelaic polyanhydride.

Although the content of the curing agent is not limited, it is preferable that the curing agent constitutes 5 to 35 wt%, and more preferably 5 to 30 wt%, based on the total weight of the curable liquid epoxy resin composition, for the reactivity, heat resistance, and mechanical strength of the composition.

The aromatic hydrocarbon formaldehyde resin is obtained by subjecting an aromatic hydrocarbon to reaction with formaldehyde under the presence of an acid catalyst under reflux for 2 to 8 hours. An aromatic hydrocarbon formaldehyde resin obtained by using xylene is referred to as a xylene formaldehyde resin, and that obtained by using mesitylene is referred to as a mesitylene formaldehyde resin.

Within the scope of the invention it is understood that the aromatic hydrocarbon component or the hydrocarbon formaldehyde resin is selected from toluene, ortho- or meta-xylene, mesitylene, pseudocumene, a monocyclic aromatic hydrocarbon compound having 10 or more carbon atoms, and a polycyclic aromatic hydrocarbon compound such as naphthalene or methylnaphthalene. And a mixture of two or more of the aromatic hydrocarbons can be used.

The aromatic hydrocarbon component is preferably selected from toluene, meta- xylene and mesitylene, more preferably meta-xylene.

The aromatic hydrocarbon formaldehyde resin is mainly composed of the aromatic hydrocarbon component consisting of 1 to 8 of aromatic nuclei (1 to 8 nuclides) in which the aromatic nuclei are bound via methylene moieties, dimethylene-ether moieties, and/or an acetal moieties, and an aromatic nucleus at the end of its molecule having a methylol group, an acetal group, and/or a methoxymethyl group. The aromatic hydrocarbon formaldehyde resin is known to react with a compound having a hydroxyl group, a carboxyl group, such as a phenol or a third component such as an aliphatic or aromatic carboxylic acid.

From the working standpoint requiring that the liquid epoxy resin composition of the invention properly flow at room temperature, it is desirable to use an aromatic hydrocarbon formaldehyde resin which is liquid at 25°C.

Preferred aromatic hydrocarbon formaldehyde resins are selected of ortho- or meta- xylene formaldehyde resins, mesitylene formaldehyde resins, and toluene formaldehyde resins.

In a further preferred embodiment of the present invention, the aromatic hydrocarbon formaldehyde resin is an ortho- or meta-xylene formaldehyde resin having hydroxyl and/or alkoxy groups. Preferrably the xylene formaldehyde resin is of the straight type, branch type, Novolac type, Resol type, Polyol type and/or EO (Ethylene oxide) added type.

More preferably, the ortho- or meta-xylene formaldehyde resin has an OH value of 10 to 250 mg KOH/g, further more preferably of 15 to 150, especially preferably of 20 to 100.

Preferred xylene formaldehyde resins are commercially available by Fudow Co., Ltd. under the following trademarks: NIKANOL ^® LLL, NIKANOL ^® LL, NIKANOL ^® L, NIKANOL ^® H, NIKANOL ^® HH, NIKANOL ^® H-80, NIKANOL ^® G, NIKANOL ^® Y-50, NIKANOL ^® Y-1000, NIKANOL ^® HP-30, NIKANOL ^® HP-70, NIKANOL ^® HP-100, NIKANOL ^® HP-120, NIKANOL ^® HP-150, NIKANOL ^® HP-210, NIKANOL ^® NP-100q, NIKANOL ^® GP-100, NIKANOL ^® GP-200, NIKANOL ^® GP-212, NIKANOL ^® PR-1440M, NIKANOL ^® PR-1440, NIKANOL ^® GRL, NIKANOL ^® K-100, NIKANOL ^® K-140, NIKANOL ^® K-1005, NIKANOL ^® L5.

Although the content of the aromatic hydrocarbon formaldehyde resin is not limited, it is preferable that the aromatic hydrocarbon formaldehyde resin constitutes 0.2 to 6 wt%, and more preferably 0.5 to 4 wt%, based on the total weight of the curable liquid epoxy resin composition, for the reactivity, heat resistance, and mechanical strength of the composition.

The at least one filler can be any of well-known fillers having an average particle size of 5 nm to 100 μηη used to reduce the coefficient of expansion of the composition. The filler can be organic or inorganic.

Preferred organic fillers are carbon black, graphite, and acrylate beads. More preferred inorganic fillers are selected from the list of clay, kaolin, talcum, mica, silica such as fumed silica and crystalline silica, calcium carbonate, sodium sulfate, magnesium sulfate, barium sulfate, alumina, titanium oxide, silica-titania, boron nitride, aluminum nitride, silicon nitride, magnesia, magnesium silicate, boron nitride. Furthermore preferred fillers are silica fillers. Especially preferred fillers are spherical fumed silica with an average particle size of 0.1 to 10 μηη, preferably 0.1 to 5 μηη. The fillers may be used alone or in admixture.

As used herein, the "average particle size" of the at least one filler can be determined by any method of the prior art, preferably the particle size distribution measuring instrument is based on the laser light diffraction method. The "average particle size" is a weight average value D ₅ o (particle diameter when the cumulative weight reaches 50%) or median diameter on particle size distribution measurement by the laser light diffraction method. Particular preferably the average particle size is measured by a Coulter LS and determined as median Diameter or D ₅ o.

Preferably at least one the fillers have previously been surface modified with coupling agents such as silane coupling agents and titanate coupling agents in order to enhance the bond strength between the resin and the filler. More preferably a surface modified inorganic filler is compounded in the composition.

Preferred coupling agents used herein are silanes including epoxysilanes such as - glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, v- glycidoxypropyltriethoxysilane and 3-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl silanes such as vinyltriethoxysilane; aminosilanes such as N- β -(aminoethyl)- γ -aminopropyltnmethoxysilane, γ-aminopropyltriethoxysilane, N-phenyl-v- aminopropyltrimethoxysilane; and mercaptosilanes such as γ -mercaptosilane. The amount of the coupling agent and the surface treatment technique are not particularly limited.

Particularly preferred fillers are spherical silica particles which have been surface modified with coupling agents selected from the above preferred silanes.

The surface modification ensures the compatibility of the filler materials with the epoxy resin and the flowability of the whole composition. An advantage of the compositions of the inventions is that they show no phase separation.

Preferably the term "phase separation" is understood as a macroscopical phase separation that means it is detectable optically by humans without the aid of any technical instruments, such as microscopes.

Preferably, nanoparticle fillers are further added, preferably silica and/or alumina nanoparticles, having an average particle size of 5 to 80 nm, preferably 10 to 50 nm. The nanoparticle filler preferably are provided in the form of colloidal dispersions, which are clear without any turbidity.

Preferably, the colloidal dispersion has a solid content of 40 to 50 wt%.

More preferably the nanoparticle fillers are colloidal silica nanoparticles having an average particle size of 10 to 50 nm.

Furthermore preferred silica nanoparticles are surface modified as disclosed in US2008/0306203 which is enclosed with its full disclosure herein by reference. Explicitly enclosed are the examples 12, 16, 17, 18 and 21 of the US2008/0306203.

Particularly preferably the silica nanoparticles are surface modified having an average particle size of 10 to 50 nm.

Preferred silica nanoparticles are commercially available from Evonik Hanse GmbH under the following trademarks NANOCRYL ^® , NANOPOX ^® and NANOPOL ^® . More preferred silica nanoparticles are NANOPOX ^® materials, particularly preferred is NANOPOX ^® E 470.

Preferably the fillers having the average particle size of 0.1 to 10 μηη, preferably 0.1 to 5 μηη are spherical fumed silica not surface modified.

The curable liquid epoxy resin composition according to the invention preferably comprises (a) a liquid epoxy resin which is one or more selected from bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol AD epoxy resins, and naphthalene epoxy resins,

(b) an aromatic hydrocarbon formaldehyde resin selected of ortho- or meta-xylene formaldehyde resins, mesitylene formaldehyde resins, and toluene formaldehyde resins,

(c) a curing agent, and

(d) at least one filler having an average particle size of 5 nm to 100 μηη.

Preferably the filler (d) consist of at least two different groups with at least one group having an average particle size of 0.1 to 10 μηη and at least one other group having an average particle size of 5 to 80 nm (nanoparticle filler). Preferably the nanoparticle fillers are surface modified.

Although the content of the at least one filler of the inventive composition is not limited, it is preferable that the filler constitutes 30 to 80 wt%, and more preferably 40 to 75 wt%, based on the total weight of the curable liquid epoxy resin composition.

The curable liquid epoxy resin composition according to the invention preferably comprises

(a) the liquid epoxy resin component constitutes 5 to 30 wt%, and preferably 5 to 20 wt%;

(b) the aromatic hydrocarbon formaldehyde resin constitutes 0.2 to 6 wt%, and preferably 0.5 to 4 wt%;

(c) the curing agent constitutes 5 to 35 wt%, and preferably 5 to 30 wt%;

(d) the at least one filler constitutes 30 to 80 wt%, and preferably 40 to 60 wt%, preferably at least one of the fillers is surface modified;

based on the total weight of the curable liquid epoxy resin composition.

The curable epoxy resin composition of the present invention can optionally comprise a catalyst, a diluter and/or an adhesive promoter.

As the catalyst, many different materials can be used depending upon the temperature at which cure is desired to occur. For instance, to achieve cure at a temperature in the about 100 to 180°C range, a variety of materials may be used. For instance, an imidazole or a metal salt such as copper or cobalt acetyl acetonate might be used.

Preferably the catalyst should be present in an amount with the range of about 0.05 to 1 .5 wt%, preferably 0.1 to 1 wt%, based on the total weight of the curable liquid epoxy resin composition.

Diluents can be used to modify the viscosity of resin composition, which is advantageous when the resin composition is used as underfill material applied for flip- chips with smaller gaps.

When a diluent is used, there may be used either a non-reactive diluent or a reactive diluent, and a reactive diluent is preferably used. In the present invention, the reactive diluent means a compound having an epoxy group and having a relatively low viscosity at a normal temperature, which may further have other polymerizable functional group(s) than the epoxy group, including an alkenyl group such as vinyl and allyl; unsaturated carboxylic acid residue such as acryloyl and methacryloyl. Preferred reactive diluents are mentioned a monoepoxide compound such as n-butylglycidyl ether, 2-ethylhexyl glycidyl ether, phenyl gylcidyl ether, cresyl glycidyl ether, p-s- butylphenyl glycidyl ether, styrene oxide and a-pinene oxide; other monoepoxide compound having other functional group(s) such as allyl glycidyl ether, glycidyl methacrylate, glycidyl acrylate and 1 -vinyl-3,4-epoxycyclohexane; a diepoxide compound such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, butanediol diglycidyl ether and neopentyl glycol diglycidyl ether; and a triepoxide compound such as trimethylolpropane triglycidyl ether and glycerin triglycidyl ether.

Adhesion promoters can improve the adhesion of the epoxy resin composition both to the substrate and the flip-chip. Preferred adhesion promoters include the above specified silanes used as coupling agents for the surface modification of the fillers.

In a preferred embodiment of the present invention, the curable liquid epoxy resin composition comprises:

(a) 10 to 20 wt% a liquid epoxy resin component;

(b) 1 .0 to 3.5 wt% an aromatic hydrocarbon formaldehyde resin;

(c) 15 to 30 wt% a curing agent;

(d) 50 to 65 wt% a spherical fumed silica having an average particle size of 0.1 to 10 μηι;

(e) 5 to 10 wt% a nano particle size silica having an average particle size of 5 to 80 nm, surface modified;

and optionally

(f) 0.1 to 1.5 wt% a catalyst, and

(g) 1 to 3 wt% an adhesion promoter,

based on the total weight of the curable liquid epoxy resin composition. Examples

The epoxy resin compositions of the examples and the comparative example were prepared by kneading the components with a three-roller mill according to the formulations indicated in Table 1 . The resulted mixture was degassed by using a planetary vacuum mixer, and then was cured at 190°C for 3 hours.

Materials

Epoxy EXA-850CRP Bisphenol A epoxy resin, commercially Resin available from DIC Corporation

Curing Hexahydro-4- agent methylphthalic anhydride

Catalyst 1 -Cyanoethyl-2-ethyl-4- methylimidazole

Xylene NIKANOL®Y-50 Straight type xylene resin, with a resin„Y" viscosity of 50 mPa- s (25°C), a specific gravity of 1.109 and a OH value of 20 mg KOH/g, commercially available from Fudow Co., Ltd

Xylene NIKANOLOH Straight type xylene resin, with a resin„H" viscosity of 630 mPa- s (75°C), a

specific gravity of 1.074 and a OH value of 33 mg KOH/g, commercially

available from Fudow Co., Ltd

Filler„E" NANOPOX® E 470 bisphenol A based epoxy resin

reinforced with nano silica particles, commercially available from Evonik Hanse GmbH

Filler„C" SO-C4 Spherical Silica with an average particle size of 1 μηη, commercially available from Admatechs Co., Ltd

Adhesion Dynasylan® GLYMO 3-Glycidyloxypropyltrimethoxysilane, promoter commercially available from Evonik

Industries AG

CTE was measured in a thermal mechanical analyzer (TMA Q400EM from TA Instruments) with compression mode under the following conditions: the system was stabilized at 25°C for 30 min and then the temperature was increased with the ramp of 10°C/min until 200°C, followed by cooling down with the ramp of 10°C/min until 25°C, and finally, the temperature was increased with the ramp of 10°C/min until 200°C. Tg was determined by the intersection point of the slop line of CTE.

Storage modulus was determined in dynamic mechanical analyzer (DMA Q800 from TA Instruments, single Cantilever Beam) with single cantilever beam under the following conditions: the system was stabilized at 25°C for 30 min and then the temperature was increased with the ramp of 3°C/min until 260°C, and the system vibration frequency was 1 Hz with 5μηι amplitude.

CTE and Tg as well as storage modulus of the examples and the comparative example were tested and the results are indicated in Table 2.

Table 1 : Formulations, Components as assigned in table materials

Component Comp Ex Ex 1 Ex 2 Ex 3 Ex 4

Epoxy Resin 14.54 14.54 14.54 14.54 14.54

Curing agent 19.5 19.5 19.5 19.5 19.5

Catalyst 0.16 0.16 0.16 0.16 0.16

Xylene resin„Y" — 2.0 2.5 3.0 —

Xylene resin„H" — — — — 3.0

Filler„E" 7.53 7.53 7.53 7.53 7.53

Filler„C" 49 51.5 52.2 52.8 52.8

Adhesion

1 .1 1 .1 1 .1 1 .1 1 .1 promoter

Filler Contained 56.6% 56.6% 56.6% 56.6% 56.6% (% by weight)

Table 2: Performance

Indicator Comp Ex Ex 1 Ex 2 Ex 3 Ex 4

Tg [°C], TMA 1 13.4 95.1 89.2 80.8 93

αι [ppm/°C] 29.4 28.39 31.9 29.66 31.02 a ₂ [ppm/°C] 1 12.3 108.6 1 12.2 106.8 1 12.7

Storage Modulus

6.3 6.6 7.0 6.6 6.6 (30 °C) [GPa]

Storage Modulus 0.26 0.25 0.22 0.16 0.13 (250 °C) [GPa]

It can be seen that the Tg of the examples is obviously lower than that of the comparative example, while the CTE (CM & 02) and/or modulus basically remains in the same level.