JP7718064B2 - Buffer sheet, electronic component mounting method, and electronic component device manufacturing method - Google Patents

Description

This disclosure relates to a buffer sheet, a method for mounting electronic components, and a method for manufacturing an electronic component device.

Traditionally, displays using LEDs (Light Emitting Diodes) have mainly used LED packages (chip LEDs) measuring approximately 1 mm or larger, which are manufactured by mounting LEDs to a substrate using wire bonding. In recent years, in order to achieve higher pixel density and response speeds, a method has been adopted in which small LEDs, known as mini LEDs or micro LEDs, are directly mounted to a substrate using flip-chip bonding.

The size of electronic components mounted on boards is becoming smaller, not only for peripheral display components such as LEDs, but also with the spread of packaging formats such as FO-WLP (Fan Out Wafer Level Package) and FI-WLP (Fan In Wafer Level Package), as well as discrete semiconductors that perform a single function such as capacitors. Because mounting miniaturized electronic components individually on boards takes a significant amount of time, there is a trend toward methods that mount multiple components at once to improve productivity.

One method for mounting multiple electronic components simultaneously on a substrate is to arrange the components on the substrate via a connecting material such as solder, and then press the electronic components onto the substrate using a member heated to a temperature at which the connecting material melts (thermocompression bonding). With this method, if the shape, height, etc. of the electronic components are not uniform, or if the surface of the heated member that contacts the electronic components is tilted, pressure may not be applied evenly to the electronic components (particularly the connection with the substrate), potentially resulting in poor connections. Patent Document 1, for example, proposes thermocompression bonding using a sheet in which a cured silicone rubber composition layer is laminated on both sides of a heat-resistant resin film.

JP 2015-170690 A

As electronic components become increasingly smaller, the likelihood of connection failures caused by even slight variations in thermocompression bonding conditions increases. In addition, as electronic components become more densely packed, the spacing between electronic components placed on a circuit board tends to become narrower. As a result, existing sheets are unable to deform sufficiently to accommodate the narrower spacing between electronic components, and as a result, sufficient contact area with the electronic components is not obtained, resulting in insufficient pressure buffering.

In light of the above circumstances, the present disclosure aims to provide a buffer sheet that enables successful mounting of electronic components on a substrate, a method for mounting electronic components using this buffer sheet, and a method for manufacturing an electronic component device using this buffer sheet.

The means for solving the above problems include the following embodiments.
<1> A buffer sheet comprising a thermosetting layer containing a curable resin and a non-thermosetting layer containing a thermoplastic resin, wherein the thickness of the non-thermosetting layer is 10 μm or less.
<2> The buffer sheet according to <1>, which is used in a process of mounting electronic components on a substrate.
<3> The buffer sheet according to <2>, wherein the electronic component includes a micro LED.
<4> The buffer sheet according to <2> or <3>, wherein the non-thermosetting layer is disposed on the side facing the electronic component.
<5> The buffer sheet according to any one of <1> to <4>, wherein the thickness of the thermosetting layer is 10 μm to 100 μm.
<6> The buffer sheet according to any one of <1> to <5>, wherein the curable resin contains a (meth)acrylate compound.
<7> The buffer sheet according to any one of <1> to <6>, wherein the thermosetting layer contains a polymer component.
<8> The buffer sheet according to any one of <1> to <7>, comprising the non-thermosetting layer, the thermosetting layer, and a cover layer in this order.
<9> A method for mounting an electronic component, comprising a step of thermocompression bonding an electronic component and a substrate using a heating member, wherein the thermocompression bonding is performed in a state where the buffer sheet according to any one of <1> to <8> is disposed between the heating member and the electronic component.
<10> A method for manufacturing an electronic component device, comprising a step of thermocompression bonding an electronic component and a substrate using a heating member, wherein the thermocompression bonding is performed in a state where the buffer sheet according to any one of <1> to <8> is disposed between the heating member and the electronic component.

This disclosure provides a buffer sheet that enables successful mounting of electronic components on a substrate, a method for mounting electronic components using this buffer sheet, and a method for manufacturing an electronic component device using this buffer sheet.

FIG. 2 is a schematic diagram illustrating an example of the configuration of a buffer sheet. FIG. 1 is a schematic diagram showing an example of a thermocompression bonding process using a buffer sheet. FIG. 1 is a schematic diagram showing an example of a thermocompression bonding process using a buffer sheet. FIG. 1 is a schematic diagram showing an example of a thermocompression bonding process using a buffer sheet. FIG. 1 is a schematic diagram showing an example of a thermocompression bonding process using a buffer sheet.

Hereinafter, embodiments of the present disclosure will be described in detail, but the present disclosure is not limited to the following embodiments.
In the following embodiments, the components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values and their ranges, and they do not limit the present invention.
In the present disclosure, the term "process" includes not only a process that is independent of other processes, but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved.
In the present disclosure, numerical ranges indicated using "to" include the numerical values before and after "to" as the minimum and maximum values, respectively.
In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.
In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified.

<Buffer sheet>
The buffer sheet of the present disclosure is a buffer sheet comprising a thermosetting layer containing a curable resin and a non-thermosetting layer containing a thermoplastic resin, the non-thermosetting layer having a thickness of 10 μm or less.
In this disclosure, "cushion sheet" refers to a material used to reduce uneven pressure on one object when the object is pressed against another.
In the present disclosure, the term "thermosetting layer" refers to a layer that has the property of being hardened by heating.
In the present disclosure, the term "non-thermosetting layer" refers to a layer that does not have the property of being hardened by heating.

The buffer sheet of the present disclosure is used, for example, in the process of mounting electronic components on a substrate. Using a buffer sheet with the above configuration enables good mounting of electronic components on a substrate. In particular, it can effectively prevent connection failures even when mounting small electronic components at high density. The reasons for this are thought to be as follows.

The thermosetting layer included in the buffer sheet deforms to fit the shape of the electronic components placed on the substrate due to the pressure applied by a member that applies heat and pressure to the electronic components (hereinafter also referred to as the heating member), and hardens due to the heat applied by the heating member. In other words, the thermosetting layer hardens in a state where it has been deformed to fit the shape of the electronic components, and thereby maintains this deformed state.

The non-thermosetting layer included in the buffer sheet deforms in response to deformation of the thermosetting layer according to the shape of the electronic components. By setting the thickness of the non-thermosetting layer to 10 μm or less, the non-thermosetting layer can sufficiently follow the deformation of the thermosetting layer even when the spacing between the electronic components is narrow (for example, 100 μm or less).
As a result, the contact area of the buffer sheet with the electronic components can be sufficiently secured, and the effect of buffering the pressure applied to the electronic components can be satisfactorily exhibited.

The buffer sheet preferably has a non-thermosetting layer with a thickness of 10 μm or less on the side that comes into contact with the electronic component in the thermocompression bonding step.
By providing the buffer sheet with a non-thermosetting layer having a thickness of 10 μm or less on the side that contacts the electronic component during the thermocompression bonding process, the contact area of the buffer sheet with the electronic component during thermocompression bonding can be sufficiently secured, and the buffering effect of the pressure applied to the electronic component can be effectively exerted. Furthermore, after the thermocompression bonding process is completed, the buffer sheet can be removed from the electronic component without the thermosetting layer adhering to the electronic component.

From the perspective of ensuring a sufficient contact area between the buffer sheet and the electronic components, it is preferable that the thermosetting layer be temporarily softened by the heat applied during the thermocompression bonding process and then hardened.

From the viewpoint of preventing the non-thermosetting layer from adhering to the electronic component, the glass transition temperature of the thermoplastic resin contained in the non-thermosetting layer is preferably higher than the temperature of the heat applied in the thermocompression bonding step. For example, the glass transition temperature of the thermoplastic resin contained in the non-thermosetting layer is preferably 30°C or more higher, more preferably 50°C or more higher, and even more preferably 70°C or more higher than the temperature of the heat applied in the thermocompression bonding step.
In the present disclosure, the glass transition temperature of a thermoplastic resin is a value measured using a differential scanning calorimeter (DSC, manufactured by PerkinElmer, Model DSC-7) under the conditions of a sample amount of 10 mg, a heating rate of 5°C/min, and a measurement atmosphere of air.

The thickness (total thickness) of the buffer sheet is not particularly limited, and can be selected depending on the method of thermocompression bonding, the state of the object to be thermocompression bonded, and the like.
From the viewpoint of ensuring a sufficient pressure buffering effect, the thickness of the buffer sheet is preferably 20 μm or more, more preferably 40 μm or more, and even more preferably 60 μm or more.
From the viewpoint of ensuring sufficient thermal conductivity, the thickness of the buffer sheet is preferably 150 μm or less, more preferably 125 μm or less, and even more preferably 100 μm or less.

(Thermosetting layer)
The thermosetting layer contains a curable resin. Specific examples of the curable resin include (meth)acrylate compounds, epoxy resins, bismaleimide compounds, cyanate compounds, and phenol compounds. The curable resin may be a combination of a base resin and a curing agent.

From the viewpoint of the viscosity of the thermosetting layer before the curing reaction and the coefficient of thermal expansion after the curing reaction, the curable resin is preferably at least one selected from the group consisting of (meth)acrylate compounds, epoxy resins, bismaleimide compounds, and phenol compounds, more preferably at least one selected from the group consisting of (meth)acrylate compounds, epoxy resins, and bismaleimide compounds, and from the viewpoint of the curing rate, a (meth)acrylate compound is even more preferable. The thermosetting compound contained in the thermosetting layer may be used alone or in combination of two or more types.
In the present disclosure, "(meth)acrylate" means acrylate or methacrylate.

When the thermosetting layer contains a (meth)acrylate compound, the (meth)acrylate compound is not particularly limited and can be appropriately selected from commonly used (meth)acrylate compounds. The (meth)acrylate compound may be a monofunctional (meth)acrylate compound or a difunctional or higher functional (meth)acrylate compound. Specific examples of (meth)acrylate compounds include erythritol-type poly(meth)acrylate compounds, glycidyl ether-type (meth)acrylate compounds, bisphenol A-type di(meth)acrylate compounds, cyclodecane-type di(meth)acrylate compounds, methylol-type (meth)acrylate compounds, dioxane-type di(meth)acrylate compounds, bisphenol F-type (meth)acrylate compounds, dimethylol-type (meth)acrylate compounds, isocyanuric acid-type di(meth)acrylate compounds, isocyanuric acid-type tri(meth)acrylate compounds, and trimethylol-type tri(meth)acrylate compounds. Among these, at least one selected from the group consisting of trimethylol tri(meth)acrylate compounds, isocyanuric acid di(meth)acrylate compounds, isocyanuric acid tri(meth)acrylate compounds, bisphenol F (meth)acrylate compounds, cyclodecane di(meth)acrylate compounds, and glycidyl ether (meth)acrylate compounds is preferred.

From the viewpoint of the curing rate and the strength of the cured product after the curing reaction, the (meth)acrylate compound is preferably a di- or higher functional (meth)acrylate compound.
From the viewpoint of suppressing reaction inhibition (a reduction in which all functional groups do not react sufficiently) that occurs due to the large number of functional groups, the (meth)acrylate compound is preferably a difunctional (meth)acrylate compound or a trifunctional (meth)acrylate compound.
The thermosetting layer may contain one or more types of (meth)acrylate compounds.

When the thermosetting layer contains a (meth)acrylate compound, it may contain a polymerization initiator to promote the polymerization reaction of the (meth)acrylate compound. Examples of polymerization initiators include compounds that generate radicals when exposed to heat (thermal radical polymerization initiators).

Examples of thermal radical polymerization initiators include azo compounds and organic peroxides. From the standpoint of ease of handling and storage stability, organic peroxides are preferred.

Examples of thermal radical polymerization initiators include ketone peroxides, hydroperoxides, diacyl peroxides, dialkyl peroxides, peroxyketals, alkyl peresters (alkyl peroxy esters), and peroxycarbonates. A single type of thermal radical polymerization initiator may be used, or two or more types may be used in combination.

Specific examples of ketone peroxides include methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, acetylacetone peroxide, cyclohexanone peroxide, and methylcyclohexanone peroxide.

Specific examples of hydroperoxides include 1,1,3,3-tetramethylbutyl hydroperoxide, cumene hydroperoxide, t-butyl hydroperoxide, p-menthane hydroperoxide, and diisopropylbenzene hydroperoxide.

Specific examples of diacyl peroxides include diisobutyryl peroxide, bis-3,5,5-trimethylhexanoyl peroxide, dilauroyl peroxide, dibenzoyl peroxide, m-toluylbenzoyl peroxide, and succinic acid peroxide.

Specific examples of dialkyl peroxides include dicumyl peroxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, 1,3-bis(t-butylperoxyisopropyl)hexane, t-butylcumyl peroxide, di-t-butyl peroxide, di-t-hexyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, and di(t-butylperoxide)diisopropylbenzene.

Specific examples of peroxyketals include 1,1-bis(t-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-hexylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-2-methylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, 2,2-bis(t-butylperoxy)butane, and butyl 4,4-bis[(t-butyl)peroxy]pentanoate.

Specific examples of alkyl peresters (alkyl peroxyesters) include 1,1,3,3-tetramethylbutyl peroxyneodecanoate, α-cumyl peroxyneodecanoate, t-butyl peroxyneodecanoate, t-hexyl peroxyneodecanoate, t-butyl peroxyneoheptanoate, t-hexyl peroxypivalate, t-butyl peroxypivalate, 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, t-amyl peroxy-2-ethylhexanoate, t-butyl peroxy-2-ethylhexanoate, t-butyl peroxyisobutyrate, di-t-butylperoxyhexahydroterephthalate, and 1,1,3,3-tetramethylbutyl peroxy. 2,5-dimethyl-2,5-di-2-ethylhexanoylperoxyhexane, t-hexylperoxy-2-ethylhexanoate, t-hexylperoxyisopropyl monocarbonate, t-butylperoxylaurate, t-butylperoxyisopropyl monocarbonate, t-butylperoxy-2-ethylhexyl monocarbonate, and 2,5-dimethyl-2,5-di-benzoylperoxyhexane.

Specific examples of peroxycarbonates include di-n-propyl peroxydicarbonate, diisopropyl peroxycarbonate, di-4-t-butylcyclohexyl peroxycarbonate, di-2-ethylhexyl peroxycarbonate, di-sec-butyl peroxycarbonate, di-3-methoxybutyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, diisopropyl peroxydicarbonate, t-amyl peroxyisopropyl carbonate, t-butyl peroxyisopropyl carbonate, t-butyl peroxy-2-ethylhexyl carbonate, and 1,6-bis(t-butylperoxycarboxyloxy)hexane.

From the standpoint of reaction rate and storage stability, the following polymerization initiators are preferred: 1,1-bis(t-butylperoxy)cyclohexane, di(t-butylperoxide)diisopropylbenzene, dicumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, t-butylcumyl peroxide, and di-t-butyl peroxide.

When the thermosetting layer contains a polymerization initiator, the content thereof is preferably 0.1 to 20 parts by mass, more preferably 1 to 10 parts by mass, and even more preferably 2 to 5 parts by mass, per 100 parts by mass of the curable resin contained in the thermosetting layer.

The thermosetting layer may contain a polymer component to adjust the film-forming properties and viscosity before the curing reaction and the mechanical properties after the curing reaction.

The polymeric components contained in the thermosetting layer include thermoplastic resins such as acrylic resin, styrene resin, butadiene resin, imide resin, and amide resin. The thermosetting layer may contain only one type of thermoplastic resin, or two or more types.

The polymer component can be produced, for example, by radical polymerization of a polymerizable monomer. Examples of the polymerizable monomer include (meth)acrylic acid; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, and benzyl (meth)acrylate; (meth)acrylamides such as diacetone (meth)acrylamide; styrene or styrene derivatives such as styrene, vinyltoluene, and α-methylstyrene; ethers of vinyl alcohol such as vinyl-n-butyl ether; maleic acid; maleic acid monoesters such as monomethyl maleate and monoethyl maleate; fumaric acid; cinnamic acid; itaconic acid; and crotonic acid. These polymerizable monomers may be used alone or in combination of two or more. The polymer component may also be an acrylic block copolymer obtained by block copolymerization using at least one polymerizable component selected from the (meth)acrylate compounds described above as the curable resin.
In this disclosure, "(meth)acrylic acid" means acrylic acid or methacrylic acid, and "(meth)acrylamide" means acrylamide or methacrylamide.

The weight average molecular weight of the polymer component is preferably 5,000 to 1,000,000, and more preferably 20,000 to 500,000, from the viewpoint of film-forming properties and flowability.
The weight average molecular weight in the present disclosure is a value measured by gel permeation chromatography (GPC) and converted using a calibration curve prepared using standard polystyrene. The GPC conditions are shown below.
Pump: L-6000 type (product name, manufactured by Hitachi, Ltd.)
Column: Gelpack GL-R420 + Gelpack GL-R430 + Gelpack GL-R440 (total of 3 columns) (product name, manufactured by Showa Denko Materials Co., Ltd.)
Eluent: tetrahydrofuran (THF)
Measurement temperature: 40℃
Flow rate: 2.05 mL/min Detector: L-3300 RI (trade name, manufactured by Hitachi, Ltd.)

When the thermosetting layer contains a polymer component, the content of the polymer component is, for example, preferably 1 part by mass to 500 parts by mass, more preferably 10 parts by mass to 400 parts by mass, and even more preferably 100 parts by mass to 300 parts by mass, relative to 100 parts by mass of the curable resin.
When the content of the polymer component is 1 part by mass or more relative to 100 parts by mass of the curable resin, film-forming properties tend to be improved. When the content of the polymer component is 500 parts by mass or less relative to 100 parts by mass of the curable resin, the curability of the thermosetting layer is sufficiently ensured, and a sufficient pressure buffering effect tends to be obtained.

The thermosetting layer may contain an inorganic filler. Examples of inorganic fillers include particles of inorganic materials such as silica (e.g., fused silica, crystalline silica), calcium carbonate, clay, alumina, silicon nitride, silicon carbide, boron nitride, calcium silicate, potassium titanate, aluminum nitride, beryllia, zirconia, zircon, fosterite, steatite, spinel, mullite, and titania; beads obtained by spheronizing these particles; and glass fiber. These inorganic fillers may be used alone or in combination of two or more.

When the inorganic filler is particulate, its volume average particle size is preferably, for example, in the range of 0.01 μm to 15.0 μm, and more preferably in the range of 0.3 μm to 5.0 μm. When the volume average particle size of the inorganic filler is 0.01 μm or more, the effect of adjusting the viscosity of the thermosetting layer by adding the inorganic filler can be sufficiently obtained. When the volume average particle size of the inorganic filler is 15.0 μm or less, the curability can be adjusted and the elastic modulus of the cured product can be effectively controlled without impairing the shape conformability of the thermosetting layer to the electronic component.
In the present disclosure, the term "volume average particle size" refers to the particle size (D50) at which the cumulative volume distribution curve is 50% when plotted from the smallest diameter side using a laser diffraction particle size distribution analyzer.

When the thermosetting layer contains an inorganic filler, the content of the inorganic filler is, for example, preferably 5% by mass to 90% by mass of the total nonvolatile content contained in the thermosetting layer, more preferably 20% by mass to 80% by mass, and even more preferably 60% by mass to 75% by mass.
When the content of the inorganic filler is 5% by mass or more of the total nonvolatile content contained in the thermosetting layer, the effect of reducing the thermal expansion coefficient of the thermosetting layer tends to be large and the moisture resistance reliability tends to be improved.When the content of the inorganic filler is 90% by mass or less of the total nonvolatile content contained in the thermosetting layer, the effects of adding the inorganic filler, such as a decrease in the moldability of the thermosetting layer and powder falling, tend to be suppressed.

The thermosetting layer may contain components other than those mentioned above as needed. Examples of other components include polymerization inhibitors, curing accelerators, coupling agents, colorants, solvents, surfactants, and ion trapping agents.

To ensure smooth formation of the thermosetting layer, the material used to form the thermosetting layer may contain a solvent. For example, if the material used to form the thermosetting layer is in the form of a varnish containing a solvent, a thermosetting layer of the desired thickness can be formed. Examples of solvents include methyl ethyl ketone, xylene, toluene, acetone, ethylene glycol monoethyl ether, cyclohexanone, ethyl ethoxypropionate, N,N-dimethylformamide, and N,N-dimethylacetamide. These solvents may be used alone or in combination of two or more. The proportion of solvent contained in the material used to form the thermosetting layer is not particularly limited and can be adjusted according to the conditions for producing the thermosetting layer.

From the viewpoint of ensuring a sufficient pressure buffering effect, the thickness of the thermosetting layer is preferably 10 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more.
From the viewpoint of ensuring sufficient thermal conductivity, the thickness of the thermosetting layer is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less.

(Non-thermosetting layer)
The non-thermosetting layer contains a thermoplastic resin. The type of thermoplastic resin is not particularly limited and can be selected taking into consideration heat resistance during thermocompression bonding, peelability from the electronic component after the thermocompression bonding step, and the like.
Specific examples of thermoplastic resins include polyolefins such as polyethylene, polypropylene, and polyvinyl chloride; polyesters such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate; polyimide; ABS (acrylonitrile-butadiene-styrene) resin; AS (acrylonitrile-styrene) resin; acrylic resin; polyamide; and polyamideimide.

From the standpoint of heat resistance and processability (shape processing, thickness adjustment, etc.), polyethylene terephthalate and polyimide are preferred thermoplastic resins.

From the viewpoint of conformability to the shape of the electronic component, the thickness of the non-thermosetting layer is preferably 9 μm or less, more preferably 8 μm or less, and even more preferably 7 μm or less.
From the viewpoint of avoiding defects such as tearing during thermocompression bonding, the thickness of the non-thermosetting layer is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more.

(Cover layer)
Optionally, the buffer sheet may have a cover layer on the side of the thermoset layer opposite the non-thermoset layer, i.e., the buffer sheet may have a non-thermoset layer, a thermoset layer, and a cover layer in that order.
The cover layer serves, for example, to protect the surface of the thermosetting layer of the buffer sheet and to facilitate the separation of the buffer sheet from the heating member after the thermocompression bonding step.

The material of the cover layer is not particularly limited and can be selected depending on the desired function. For example, it may be a layer containing resin, metal such as copper or aluminum, inorganic oxide, etc. The resin may be a curable resin exemplified as the curable resin contained in the thermosetting layer, or a thermoplastic resin exemplified as the thermoplastic resin contained in the non-thermosetting layer, etc.

From the standpoint of durability and processability (shape processing, thickness adjustment, etc.), the cover layer preferably contains a resin, and more preferably contains a thermoplastic resin such as polyethylene terephthalate or polyimide.

The thickness of the cover layer is not particularly limited and can be selected depending on its function. For example, it may be in the range of 1 μm to 50 μm. From the perspective of ensuring sufficient strength, the thickness of the cover layer is preferably 10 μm or more. From the perspective of ensuring sufficient thermal conductivity, the thickness of the cover layer is preferably 50 μm or less.

Specific examples of the buffer sheet and thermocompression bonding process using the buffer sheet of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to these specific examples.

Figure 1 is a schematic diagram showing an example of the configuration of a buffer sheet. The buffer sheet 1 shown in Figure 1 has a cover layer 1-a, a thermosetting layer 1-b, and a non-thermosetting layer 1-c, in that order.

2 to 5 are schematic diagrams showing an example of a thermocompression bonding process using a buffer sheet.
2, the electronic component 2 is placed on the substrate 5 so that the bumps 3 of the electronic component 2 face the pads or wiring 4 on the substrate 5. The bumps 3 and the pads or wiring 4 are made of a metal material that melts with the heat of thermocompression bonding such as soldering.

Next, as shown in Figure 3, the heating member 6 is used to press the electronic component 2 toward the substrate 5. At this time, the buffer sheet 1 is placed between the heating member 6 and the electronic component 2. The buffer sheet 1 shown in Figure 3 is placed so that the non-thermosetting layer 1-c contacts the electronic component 2.

As shown in Figure 4, when the electronic component 2 is pressed toward the substrate 5 using the heating member 6, the thermosetting layer 1-b of the buffer sheet 1 deforms to fit the shape of the electronic component 2. At this time, the non-thermosetting layer 1-c also deforms to fit the shape of the thermosetting layer 1-b. In the buffer sheet of the present disclosure, the thickness of the non-thermosetting layer 1-c is 10 μm or less, so the non-thermosetting layer 1-c has excellent follow-up ability to the deformation of the thermosetting layer 1-b. This ensures a sufficient contact area between the buffer sheet 1 and the electronic component 2.
The thermosetting layer 1-b is hardened by the heat applied from the heating member 6, and the metal material contained in the bump 3 and the pad or wiring 4 melts to bond them together.

After the thermocompression bonding process is completed, the release sheet 1 is removed from the electronic component 2, as shown in Figure 5. Because the buffer sheet 1 has a non-thermosetting layer 1-c on the side that contacts the electronic component 2, the buffer sheet 1 can be removed from the electronic component 2 without the thermosetting layer 1-b adhering to the electronic component 2.

<Electronic component mounting method and electronic component device manufacturing method>
The mounting method for electronic components disclosed herein includes a step of thermocompression bonding an electronic component to a substrate using a heating member, and the thermocompression bonding is performed with the above-mentioned buffer sheet disposed between the heating member and the electronic component.

The method for manufacturing an electronic component device disclosed herein includes a step of thermocompression bonding an electronic component and a substrate using a heating member, and the thermocompression bonding is performed with the buffer sheet described above positioned between the heating member and the electronic component.

Hereinafter, the electronic component mounting method of the present disclosure and the electronic component device manufacturing method of the present disclosure may be collectively referred to as the "method of the present disclosure."
The method disclosed herein allows electronic components to be mounted on a substrate in a good condition. This method is particularly suitable for mounting multiple electronic components on a substrate. For example, it can effectively prevent connection failures even when small electronic components are mounted at high density.

The method of the present disclosure may be a method for mounting electronic components on a substrate at high density (i.e., with close spacing between adjacent electronic components).
For example, the interval between adjacent electronic components (or the minimum interval if the interval is not constant) may be 300 μm or less, 200 μm or less, 100 μm or less, or 50 μm or less. The lower limit of the interval between adjacent electronic components (or the minimum interval if the interval is not constant) is not particularly limited, but may be 10 μm or more.

In the method of the present disclosure, the conditions for carrying out thermocompression bonding are not particularly limited. For example, the temperature of the heating member may be in the range of 25°C to 400°C.
From the viewpoint of the heat resistance of the buffer sheet, the temperature of the heating member is preferably lower than the glass transition temperature of the thermoplastic resin contained in the buffer sheet.
The time during which the heating member is in contact with the electronic component (thermocompression bonding time) may be within the range of 1 second to 600 seconds.

The type of electronic component used in the method of the present disclosure is not particularly limited, and examples thereof include semiconductor elements such as diodes, transistors, and thyristors, as well as various components used in electronic device devices.
The type of semiconductor element is not particularly limited, and can be selected from LEDs, elemental semiconductors such as silicon and germanium, and compound semiconductors such as gallium arsenide and indium phosphide.

In one embodiment, the method of the present disclosure uses micro-LEDs as electronic components. Micro-LEDs are smaller than conventional LEDs (e.g., with a maximum diameter of less than 1000 μm) and are used in direct backlights for LCD screens, etc. To achieve higher image resolution, micro-LEDs tend to be mounted on a substrate at high density (i.e., with close spacing between micro-LEDs). The method of the present disclosure enables successful high-density mounting of micro-LEDs.

The material of the bumps of the electronic components used in the method of the present disclosure is not particularly limited. Examples include gold, silver, copper, solder, tin, nickel, indium tin oxide (ITO), indium, etc. Examples of solder include alloys primarily composed of tin-silver, tin-lead, tin-bismuth, tin-copper, etc. The bumps may be made of one material or a combination of two or more materials. The bumps may have a structure in which multiple types of metal are stacked.

The material of the substrate used in the method of the present disclosure is not particularly limited. Examples include glass, glass epoxy, polyester, ceramic, epoxy, bismaleimide triazine, polyimide, etc.

The substrate may have a wiring pattern on its surface, such as one formed by etching away unnecessary portions of a metal layer formed on the substrate surface, one formed by metal plating the substrate surface, or one formed by printing a conductive material on the substrate surface.
The material of the wiring pattern is not particularly limited, and examples thereof include gold, silver, copper, solder, tin, nickel, indium tin oxide (ITO), indium, etc. Examples of solder include alloys containing tin-silver, tin-lead, tin-bismuth, tin-copper, etc. as a main component. The wiring pattern may be made of one material or a combination of two or more materials. The wiring pattern may have a structure in which multiple types of metals are laminated.

The substrate may have connection portions called pads on its surface. Examples of pads include those formed by etching away unnecessary portions of a metal layer formed on the substrate surface, those formed by metal plating the substrate surface, and those formed by printing a conductive material on the substrate surface.
The material of the pad is not particularly limited, and examples thereof include gold, silver, copper, solder, tin, nickel, indium tin oxide (ITO), indium, etc. Examples of solder include alloys mainly composed of tin-silver, tin-lead, tin-bismuth, tin-copper, etc. The pad may be made of one material or a combination of two or more materials. The pad may have a structure in which multiple types of metals are laminated.

There are no particular limitations on the electronic component devices obtainable by the method of the present disclosure, and they may be, for example, electronic component devices with an image display function, such as computers, televisions, game consoles, mobile phones, and car navigation systems.
The method of the present disclosure can mount small electronic components on a substrate at high density, and therefore can be suitably used for manufacturing image display devices equipped with direct backlights.

The present disclosure will be explained in detail below based on examples, but the present disclosure is not limited to these examples.

(Preparation of Thermosetting Layer Composition)
A flask equipped with a stirrer was charged with 4 g of an acrylic block copolymer (manufactured by Kuraray Co., Ltd., trade name "LA4285") as a polymer component, 2 g of tris(2-acryloyloxyethyl)isocyanurate (manufactured by Showa Denko Materials K.K., trade name "FA731A") as a curable resin, 0.08 g of dicumyl peroxide (manufactured by NOF Corporation, trade name "Percumyl D") as a polymerization initiator, 16.7 g of silica particles (manufactured by Admatec Co., Ltd., trade name "SE2050", volume average particle size: 0.5 μm) as an inorganic filler, and 12.8 g of methyl ethyl ketone as a solvent. These were mixed with stirring to obtain a varnish-like composition for the thermosetting layer.

(Preparation of buffer sheet)
The obtained thermosetting layer composition was applied onto a polyimide film (manufactured by DuPont-Toray Co., Ltd., product name "Kapton 100H", thickness: 25 μm) as a cover layer, and dried by heating in a dryer at 70°C for 10 minutes to form a thermosetting layer with a thickness of 50 μm on the polyimide film.
A polyethylene terephthalate film (manufactured by Toray Industries, Inc., thickness: 6 μm) was superimposed on the thermosetting layer as a non-thermosetting layer, and the layers were laminated using a hot roll laminator under conditions of 100°C, 0.5 MPa, and 1.0 m/min, thereby obtaining a buffer sheet having a cover layer, a thermosetting layer, and a non-thermosetting layer in this order.

1: Buffer sheet, 1-a: Cover layer, 1-b: Thermosetting layer, 1-c: Non-thermosetting layer, 2: Electronic component, 3: Bump, 4: Pad or wiring, 5: Substrate, 6: Heating member

Claims (9)

A buffer sheet (excluding those used as sealing materials for electronic components) comprising, in this order, a cover layer, a thermosetting layer containing a curable resin, and a non-thermosetting layer containing a thermoplastic resin, wherein the thickness of the thermosetting layer is 20 μm or more and the thickness of the non-thermosetting layer is 10 μm or less. The buffer sheet according to claim 1, for use in a process for mounting electronic components on a substrate. The buffer sheet of claim 2, wherein the electronic components include micro LEDs. The buffer sheet according to claim 2 or claim 3, wherein the non-thermosetting layer is positioned on the side facing the electronic component. The buffer sheet according to any one of claims 1 to 4, wherein the thickness of the thermosetting layer is 20 μm to 100 μm. The buffer sheet according to any one of claims 1 to 5, wherein the curable resin includes a (meth)acrylate compound. The buffer sheet according to any one of claims 1 to 6, wherein the thermosetting layer contains a polymer component. A method for mounting electronic components, comprising a step of thermocompression bonding an electronic component to a substrate using a heating member, the thermocompression bonding being performed with the buffer sheet described in any one of claims 1 to 7 placed between the heating member and the electronic component. A method for manufacturing an electronic component device, comprising a step of thermocompression bonding an electronic component and a substrate using a heating member, the thermocompression bonding being performed with the buffer sheet described in any one of claims 1 to 7 positioned between the heating member and the electronic component.