Mold release film

Abstract

The present invention provides a mold release film which has a mold release layer and a base material layer, wherein the base material layer is a biaxially stretched polybutylene terephthalate film. When the mold release film is extended at an elongation of 10% to 50% in the longitudinal direction (MD) of the mold release film under the conditions of a temperature of 175°C, a sample width of 15 mm, an initial chuck-to-chuck distance of 50 mm, and a tensile speed of 50 mm / min, is held in the extended state for 2 minutes, and is subsequently moved in the direction opposite to the extending direction at a speed of 50 mm / min until the stress falls to 0, the recovery rate (%), which is obtained from an elongation length value ΔLa when extended from the initial value and a return value ΔLb of elongation from ΔLa when the stress is 0, is 40% to 80% inclusive. The mold release film, when used in a resin sealing process by a compression molding method, does not cause wrinkles and breakage even after expansion and contraction generated during the process, exhibits excellent releasability with respect to a sealing resin, and has excellent recyclability and recoverability after being tensioned in a high-temperature state.

Description

Release film

[0001] The present invention relates to a release film used during resin molding by compression molding, and more specifically to a release film that is used when resin-encapsulating a semiconductor chip by compression molding to protect the semiconductor element, and that has excellent peelability and does not wrinkle or tear. Technology background

[0002] In recent years, packaging technology for protecting and sealing semiconductor elements has changed from packages that seal elements individually to wafer level packages (WLP) and panel level packages (PLP) that seal larger areas, and mass production of these technologies is also progressing.

[0003] In the packaging process applied to large-area WLPs and PLPs, a semiconductor chip mounted on a wafer or substrate is conventionally placed in a mold, resin is injected, and the chip is encapsulated by compression molding. To facilitate the release of the encapsulated product from the mold, a release film is placed between the inner surface of the mold and the semiconductor chip, etc., and release films made of materials such as PTFE, ETFE, PET, FEP, polypropylene, polystyrene, and polyvinylidene chloride are used depending on the application (see Patent Document 1). When using these release films in compression molding, the release film placed on the mold surface is stretched by vacuum suction and then contracted when the mold is compressed, resulting in wrinkles. This problem becomes more pronounced as the mold cavity becomes larger and deeper, and wrinkles on the release film, especially at the periphery of the mold cavity, can become trapped in the encapsulating resin, preventing release and causing tearing.

[0004] To address this problem, proposed films that are less likely to wrinkle when stretched include an ethylene-tetrafluoroethylene (ETFE) copolymer film of a specific composition and manufacturing method (Patent Document 2), and a release film laminated with a 4-methyl-1-pentene copolymer (PMP) film as a release layer and a stretched film containing 50% by mass or more and 100% by mass or less of polybutylene terephthalate as a support layer (Patent Document 3).

[0005] JP 2017-35832 A International Publication No. 2018 / 008562 JP 2020-1328 A

[0006] However, the film using fluororesin proposed in Patent Document 2 has problems of being difficult to burn when incinerated after use, generating toxic gases, and being expensive as well as having a high environmental impact. In contrast, the release film proposed in Patent Document 3 generates almost no toxic gases when incinerated, but has various problems, such as poor recyclability because films of different materials are laminated together, and there is a difference in thermal properties between the support layer film and the release layer PMP film, which causes the release layer to partially peel off from the support during compression molding, resulting in wrinkles and becoming sandwiched between the sealing resin.

[0007] The present invention aims to provide a release film for use in a resin sealing process using a compression molding method, which does not wrinkle or tear even when subjected to expansion and contraction that occurs during the process, has excellent peelability from the sealing resin, is highly recyclable, and has recovery properties after being stretched at high temperatures.

[0008] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, they found that a film comprising a biaxially stretched polybutylene terephthalate film as a base layer and a release layer provided on at least one side of the base layer, wherein the film is stretched by 10% to 50% at a temperature of 175°C and a tensile speed of 50 mm / min, held in this state for 2 minutes, and then returned to the opposite direction from the stretching direction at a speed of 50 mm / min until the stress becomes zero, has a recovery rate of 40% to 80%, which is particularly suitable as a release film to be used in a resin sealing process in a compression molding method, and thus completed the present invention.

[0009] That is, the present invention relates to a release film having a base layer made of biaxially oriented polybutylene terephthalate, and the release films described below in [1] to [8] and the resin encapsulation method for semiconductor elements using the release films described below in [9] can be one form or one embodiment of the present invention.

[0010] [1] A release film having a release layer and a base material layer, wherein the base material layer is a biaxially stretched polybutylene terephthalate film, and under the conditions of a temperature of 175°C, a sample width of 15 mm, an initial chuck distance of 50 mm, and a tensile speed of 50 mm / min, when the release film is stretched in the longitudinal direction (MD) of the release film at an elongation of 10% to 50% and held in the stretched state for 2 minutes, and then returned at a speed of 50 mm / min in the direction opposite to the stretched direction until the stress becomes 0, the release film has a recovery rate (%) of 40% or more and 80% or less according to the following formula (1) from the elongation length value ΔLa from the initial value before elongation to the value during elongation and the elongation return value ΔLb from the elongation length value during elongation to the elongation length value at a stress of 0. Recovery rate (%) = [elongation return value ΔLb / elongation length value ΔLa] × 100... (1) [2] The biaxially stretched polybutylene terephthalate film of the base material layer has a tensile stress of 10 MPa to 40 MPa when stretched to an elongation of 10% to 50% under the conditions of a temperature of 175°C, a sample width of 15 mm, an initial chuck distance of 50 mm, and a tensile speed of 50 mm / min in its four directions (0° (MD), 45°, 90° (TD), 135°), and the ratio of the maximum value to the minimum value of the tensile stresses in the four directions is 1.5 or less. The release film according to [1]. [3] The release film has a thermal shrinkage rate of 1.5% or more after 30 minutes at 175°C in both its longitudinal direction (MD) and width direction (TD). The release film according to [1]. [4] The base material layer has a thickness of 15 μm or more and 25 μm or less, the release layer has a thickness of 0.1 μm or more and 5 μm or less, and the release film has a piercing strength of 9 N or more measured in accordance with JIS Z 1707. The release film according to [1]. [5] The release layer is a layer that does not contain a silicone component and is provided on at least one surface of the base material layer. The release film according to [1]. [6] Further having an antistatic layer, and the antistatic layer is provided between the release layer and the base material layer, or the release layer is provided on one surface side of the base material layer and the antistatic layer is provided on the other surface side. The release film according to [1]. [7] A release film for a process used in a resin sealing process by a compression molding method. The release film according to [1].[8] A process release film used in a resin encapsulation process by the compression molding method, the release film according to [7], which is stretched and compressed in the process. [9] A method for encapsulating a semiconductor element using the release film according to any one of [1] to [8].

[0011] The present invention can provide a release film that does not wrinkle or tear even when subjected to elongation-compression (expansion and contraction) in a resin encapsulation process using a compression molding method, has excellent peelability from encapsulating resin, and has recovery ability after being stretched at high temperatures.The present invention can also provide a method for encapsulating a semiconductor element using the release film.

[0012] Fig. 1 is a schematic cross-sectional view showing an example of a process for producing an encapsulated body (semiconductor element) by a compression molding method using the release film of the present invention. Fig. 2 is a conceptual diagram showing the measurement of the restoration rate of the release film of the present invention.

[0013] The present invention relates to a release film having a release layer and a base layer, wherein the base layer is a biaxially oriented polybutylene terephthalate film, and when the release film is stretched and held under predetermined conditions as described below and then released from the stretch, the recovery rate calculated from the elongated length of the film during stretching and the return value from the elongated length upon stretching and release is 40% or more and 80% or less.

[0014] <Substrate Layer: Biaxially Stretched Polybutylene Terephthalate Film> The release film of the present invention uses a biaxially oriented polybutylene terephthalate film as the substrate layer. Hereinafter, polybutylene terephthalate will also be referred to as "PBT," and biaxially oriented polybutylene phthalate film will also be referred to as "biaxially oriented PBT film." The biaxially oriented PBT film is a biaxially oriented film of PBT resin. In the present invention, the PBT resin is not particularly limited as long as it is a polyester having butylene terephthalate as the main repeating unit. Specifically, the PBT resin may be a homopolymer-type polyester (polybutylene terephthalate) obtained by condensing 1,4-butanediol or an ester-forming derivative thereof as the diol (glycol component) and terephthalic acid or an ester-forming derivative thereof as the dibasic acid component as the main components, or a copolymer-type polyester mainly composed of polybutylene terephthalate. In the present invention, in order to impart optimal mechanical strength properties, the polybutylene terephthalate resin preferably has a melting point of 200 to 250° C. and an intrinsic viscosity (IV value) in the range of 0.90 dl / g to 1.35 dl / g, and more preferably a melting point of 215 to 225° C. and an intrinsic viscosity (IV value) in the range of 1.15 dl / g to 1.30 dl / g. The intrinsic viscosity (IV value) of the polybutylene terephthalate resin is a solution viscosity measured at 30° C. using a mixed solvent of phenol / tetrachloroethane (1 / 1 by mass).

[0015] The above-mentioned copolymer-type polyesters (copolyesters) mainly composed of polybutylene terephthalate are polyesters obtained by condensing a portion of the terephthalic acid component as a dibasic acid component with another dibasic acid component such as isophthalic acid, phthalic acid, adipic acid, sebacic acid, etc., and / or a portion of the 1,4-butanediol component as a diol (glycol) component with another diol (glycol) component such as ethylene glycol, diethylene glycol, propylene glycol, neopentyl glycol, cyclohexanedimethanol, etc. Preferred examples of the above copolyesters include those containing 70% or more butylene terephthalate units.

[0016] The polybutylene terephthalate resin used in the biaxially stretched PBT film of the present invention may be mixed with other polyesters such as polyethylene terephthalate, polyethylene naphthalate, polyhexamethylene terephthalate, and poly(ethylene terephthalate / ethylene isophthalate), or other resins such as polycarbonate and polyamide, to the extent that the physical properties are not adversely affected, and then subjected to the stretching process described below. Furthermore, the polybutylene terephthalate resin may be laminated with the other polyesters or other resins and then stretched. Two or more of the above-mentioned other polyesters and other resins may be used. In the present invention, any of the following biaxially stretched PBT films can be used as the base layer of the release film of the present invention: a biaxially stretched film made only of PBT resin, a biaxially stretched film made of a mixed resin containing PBT resin, and a biaxially stretched film made of two or more layers of PBT resin and other resin.

[0017] Furthermore, when producing a biaxially stretched PBT film, in addition to the polybutylene terephthalate resin used in the present invention (or, in some cases, the other resins described above), additives such as lubricants, antiblocking agents, inorganic fillers, antioxidants, ultraviolet absorbers, antistatic agents, flame retardants, plasticizers, colorants, crystallization inhibitors, and crystallization accelerators may be added as needed. Furthermore, in order to avoid a decrease in viscosity due to hydrolysis during heat melting, the polybutylene terephthalate resin pellets used in film production are preferably sufficiently pre-dried to have a moisture content of 0.05% by mass or less, more preferably 0.02% by mass or less, before heat melting.

[0018] The biaxially stretched polybutylene terephthalate film serving as the base layer in the release film of the present invention has a tensile stress of 10 MPa to 40 MPa in all four directions (0° (MD), 45°, 90°, and 135° (TD)) when stretched to an elongation of 10% to 50% at a temperature of 175°C, a sample width of 15 mm, an initial chuck distance of 50 mm, and a tensile strength of 50 mm / min. The tensile stress in each of the four directions is 10 MPa to 40 MPa, and the ratio of the maximum to the minimum of the tensile stress in each of the four directions is 1.5 or less. The tensile stress in each of the four directions is more preferably in the range of 12 MPa to 35 MPa so that the release film can adhere to and conform to the mold without wrinkles when vacuum suctioned to the mold or when using a heat-based adsorption mechanism. Furthermore, the ratio of the maximum to the minimum of the tensile stress in each of the four directions is more preferably small, i.e., 1.2 or less, since isotropic conformability is required as the shape of the compression molding mold becomes more complex and larger.

[0019] The biaxially oriented polybutylene terephthalate film can be obtained by forming the polybutylene terephthalate resin into a film using an extruder and then stretching the resulting film. An example of a method for producing a biaxially oriented polybutylene phthalate film used as the base layer of the release film of the present invention is described below. First, the polybutylene terephthalate resin is melt-kneaded using an extruder set to a predetermined temperature (e.g., 210 to 280°C). Film formation then begins, but the film formation method is not particularly limited. For example, in the case of T-die film formation, the sheet-shaped molten resin is immersed in a water tank to directly water-cool both the inside and outside. On the other hand, in the case of annular film formation, the resin is extruded downward through an annular die attached to the extruder facing downward to form a molten tubular thin film. The film is then introduced into a cooling mandrel connected to the annular die, where cooling water introduced from each nozzle of the cooling mandrel directly contacts the inside of the molten tubular thin film to cool it. At the same time, cooling water is also flowed from an external cooling tank used in combination with the cooling mandrel, and the cooling water comes into direct contact with the outside of the molten tubular thin film to cool it. The temperatures of the internal and external water are preferably 30°C or less, and from the viewpoint of rapid film formation, 20°C or less is particularly preferred. If the temperature is higher than 30°C, whitening of the unstretched film and poor appearance of the unstretched film due to boiling of the cooling water may occur, and stretching may gradually become difficult.

[0020] In order to stably produce biaxially oriented polybutylene terephthalate films, it is necessary to minimize crystallization of the unstretched film (also referred to as the unstretched raw film). As described above, when the polybutylene terephthalate melt extruded by the extruder is cooled to form a film, the polybutylene terephthalate must be cooled at a rate above the crystallization temperature range, i.e., the cooling rate of the unstretched film is an important factor. The cooling rate of the unstretched film is 200°C / sec or higher, preferably 250°C / sec or higher, and particularly preferably 350°C / sec or higher. If the cooling rate is less than 200°C / sec, the resulting unstretched film may have high crystallinity and poor stretchability. The method for producing the unstretched film is not particularly limited as long as it satisfies the cooling rate of the unstretched film. However, from the viewpoint of rapid cooling film formation, the above-mentioned internal and external direct water cooling method is particularly preferred.

[0021] The method for biaxially stretching an unstretched film is not particularly limited and may be appropriately selected from, for example, simultaneous biaxial stretching, in which longitudinal and transverse stretching are performed simultaneously using a tubular or tenter system, or sequential biaxial stretching, in which longitudinal and transverse stretching are performed sequentially. From the viewpoint of the balance of longitudinal and transverse strength of the resulting biaxially stretched polybutylene terephthalate film, simultaneous biaxial stretching using a tubular system is particularly preferred. Taking into account the stretching stability and the strength properties and thickness uniformity of the resulting biaxially stretched polybutylene terephthalate film, the stretching ratio is preferably in the range of 2.7 to 4.0 times in both the longitudinal direction (hereinafter also referred to as "MD") and the transverse direction (hereinafter also referred to as "TD"). A stretching ratio of less than 2.7 times is undesirable because the tensile strength and impact strength of the resulting biaxially stretched polybutylene terephthalate film may be insufficient. Furthermore, if the stretching ratio exceeds 4.0, excessive molecular chain strain occurs due to stretching, resulting in frequent breakages and punctures during stretching, potentially hindering stable production. The stretching temperature is preferably in the range of 40°C to 80°C, and particularly preferably 45°C to 65°C. Unstretched films produced at the above-mentioned high cooling rates have low crystallinity and can therefore be stably stretched at relatively low stretching temperatures. High-temperature stretching above 80°C can cause the stretching bubble to oscillate violently, resulting in significant stretching unevenness and potentially hindering the production of films with good thickness accuracy. On the other hand, low-temperature stretching at temperatures below 40°C can cause excessive stretch-oriented crystallization, leading to whitening of the film and, in some cases, the stretching bubble may burst, making it difficult to continue stretching. By performing such biaxial stretching, a biaxially stretched polybutylene terephthalate film with low anisotropy can be obtained.

[0022] The obtained biaxially stretched polybutylene terephthalate film can be placed in a heat treatment facility using a hot roll system, a tenter system, or a combination of these for a desired period of time, and subjected to heat treatment at, for example, 180 to 240°C, particularly preferably 190 to 210°C, to obtain a biaxially stretched polybutylene terephthalate film with excellent thermal dimensional stability. If the heat treatment temperature is higher than 240°C, the bowing phenomenon may become too large, increasing anisotropy in the width direction. On the other hand, if the heat treatment temperature is lower than 180°C, the thermal dimensional stability of the film may be significantly reduced, potentially causing problems in the heat pressing process.

[0023] Furthermore, the surface of the obtained biaxially stretched polybutylene terephthalate film may be subjected to an adhesion-enhancing treatment using corona, plasma, ultraviolet light, or the like, or an in-line or offline adhesion-enhancing coating, in order to improve adhesion to a release layer. Furthermore, the surface may be provided with irregularities by embossing for purposes such as conforming to the irregularities of a mold during compression molding, imparting air release properties during adsorption, and controlling mold releasability.

[0024] In the release film of the present invention, the thickness of the substrate layer (biaxially oriented polybutylene terephthalate film) can be appropriately designed within a range that does not impair the effects of the present invention, and is usually 10 to 50 μm in consideration of transportability, winding property, etc. From the viewpoint of more reliably preventing tearing during use, it is preferably 15 μm or more, and from the viewpoint of facilitating conformability to the unevenness of a mold and reducing the environmental load during production and disposal, it is preferably 25 μm or less.

[0025] <Release Layer> The release layer constituting the release film of the present invention can be appropriately designed to match the resin to be compression-molded to which the release film is applied, as long as the effects of the present invention are not impaired. Regarding the resin used in compression molding, epoxy-based resins are primarily used as encapsulants for sealing electronic components such as semiconductor elements. Examples of release agent components that have good releasability relative to epoxy-based resins include silicone-based resins, fluorine-based resins, olefin-based resins, acrylic resins, and melamine-based resins, and these can be used alone or in combination in the release layer of the present invention. In particular, in the case of a release film used to seal electronic components such as semiconductor elements, it is preferable to select a release agent component that does not contain a silicone component, as this may cause problems such as poor insulation of the electronic components. Furthermore, to prevent cracks from occurring in the release layer when it is conformed to the mold, it is preferable to select a material with an elongation of 10% or more, more preferably 50% or more. Furthermore, in order to improve elongation, flexible resins, urethane-based resins, etc., may be added to the release agent component and used in the release layer, as long as the effects of the present invention are not impaired.

[0026] The thickness of the release layer of the release film of the present invention can be appropriately designed within a range that does not impair the effects of the present invention, and can be 0.1 μm to 5 μm, taking into consideration processability, recyclability, etc. In order to more reliably obtain releasability, the thickness of the release layer is preferably 0.3 μm or more, and in order to suppress the occurrence of cracks when the film is made to conform to a mold, it is preferably 2 μm or less. The release layer only needs to be provided on the surface that comes into contact with the above-mentioned sealing resin, etc., when the film is made into a release film. For example, it can be provided on at least one surface of the base layer, or on both surfaces of the base layer. Furthermore, another layer may be provided between the release layer and the base layer.

[0027] The release layer may contain organic or inorganic particles to enhance releasability, transportability, and design properties, as long as the effects of the present invention are not impaired. Furthermore, an antistatic agent and a conductive agent to suppress the generation of static electricity may be added.

[0028] <Method for Producing Release Film> The release film of the present invention can be produced by forming a layer of a release agent constituting a release layer on the biaxially oriented polybutylene terephthalate film, which is the base layer, using a known wet coating or dry coating method. Wet coating methods include reverse roll coating, dip coating, Mayer bar coating, knife coating, nozzle coating, die coating, spray coating, curtain coating, screen printing, and gravure coating. Dry coating methods include methods in which raw materials are heated and evaporated in a vacuum, such as heating with a point-shaped or thin slit nozzle, followed by evaporation and spraying, and examples of such methods include vacuum deposition, sputtering, and ion plating. The method for solidifying and fixing the release layer by curing or the like is not particularly limited, but can be achieved by high-temperature heating, ultraviolet irradiation, electron beam irradiation, plasma irradiation, or the like.

[0029] <Release Film: Recovery Rate> The release film of the present invention is characterized by being a film having a predetermined recovery rate. By having this predetermined recovery rate, even when the release film of the present invention is used in a resin encapsulation process using a compression molding method, even after the film is stretched and compressed during the process, the occurrence of wrinkles and tears can be suppressed and the film can be suppressed from getting caught in the encapsulation resin.

[0030] The restoration rate according to the present invention is defined and measured as follows. First, the release film is stretched in its longitudinal direction (MD) to an elongation of 10% to 50% under conditions of a temperature of 175°C, a sample width of 15 mm, an initial chuck distance of 50 mm, and a pulling speed of 50 mm / min. At this time, one end of the sample is fixed, and the other end is pulled and stretched in the longitudinal direction. The stretched state is then maintained for 2 minutes. After this maintenance, the other end is moved (returned) in the direction opposite to the stretched direction at a speed of 50 mm / min until the stress applied to the other end during the stretching reaches zero. When this series of operations is performed, the restoration rate (%) is calculated according to the following formula (1) from the elongation length value ΔLa from the initial value before stretching to the value at the time of stretching, and the return elongation value ΔLb from the elongation length value at the time of stretching to the elongation length value at zero stress: Recovery rate (%)=[recovery value ΔLb / elongation length value ΔLa]×100 (1) The recovery rate is more preferably 50% or more and 75% or less.

[0031] FIG. 2 shows a conceptual diagram of the recovery rate measurement according to the present invention. As shown in FIG. 2, a sample with an initial value ΔL0 (50 mm) in its initial state (elongation 0%) is stretched and maintained to an elongation of x% according to the above conditions. At this time, the elongation length value ΔLa is the length when stretched from the initial value ΔL0 to the elongation of x%. The ratio of the initial value ΔL0 to the elongation length value ΔLa is the elongation x% (elongation x (%) = (ΔLa / ΔL0) × 100). After maintaining the state of elongation x% for 2 minutes, the sample is returned to its original state according to the above conditions so that the load (stress) on the tensile portion becomes 0. Then, the return value ΔLb from the elongation length value ΔLa when stretched to elongation x% to the elongation length value at 0 stress is measured. The recovery rate is then calculated according to the above formula (1) using the above ΔLa and ΔLb. If the stretch length value ΔLa=the stretch-back value ΔLb, the restoration rate is 100%, and if the stretch-back value ΔLb=0, the restoration rate is 0%.

[0032] <Release Film: Heat Shrinkage Rate> The release film of the present invention can be one that has a heat shrinkage rate of 1.0% or more in its longitudinal direction (MD) and transverse direction (TD) after 30 minutes at a temperature of 175°C, more preferably 1.5% or more, and even more preferably 2.0% or more. By providing a film with a heat shrinkage rate of 1.0% or more, the film can conform well to the uneven shape of the compression molding mold, and the occurrence of wrinkles in the film on the mold surface can be suppressed. The heat shrinkage rate can be calculated, for example, by cutting the target film into 100 mm square samples in each of the MD and TD directions, treating (storing) this at 175°C for 30 minutes, measuring the lengths in each direction before and after treatment, and then calculating it according to the following formula (2): Heat shrinkage rate (%) = [(length before treatment - length after treatment) / length before treatment] x 100 (2)

[0033] <Release Film: Puncture Strength> The release film of the present invention can be one that has a puncture strength of 7 N or more, for example, 9 N or more, and more preferably 10 N or more, measured in accordance with JIS Z 1707. By using a film with a puncture strength of 7 N or more, holes are less likely to be formed in the release film during transport within a compression molding device, and the film is reliably adsorbed to and follows the uneven portions of the mold by vacuum suction, resulting in less loss during production.

[0034] The release film of the present invention may be provided with layers other than the release layer, such as an antistatic layer, as long as the effects of the present invention are not impaired. The antistatic layer suppresses charging of the release film, prevents the adsorption of dust and other particles caused by scraping of the encapsulating resin material, and prevents damage to electronic components and the like due to discharge of static electricity during compression molding. The antistatic layer can be provided, for example, between the release layer and the substrate layer. Furthermore, when the release layer is provided on one surface or one side of the substrate layer, the antistatic layer can be provided on the other surface or other side of the substrate layer.

[0035] In one embodiment, the release film of the present invention has a release layer that does not contain a silicone component, and has excellent mold conformability at high temperatures, recovery properties when compressed, releasability, and heat resistance, and therefore can be suitably used particularly as a release film for compression molding, for example, in a process of encapsulating electronic components such as semiconductor elements with a resin. That is, the release film of the present invention can be a process release film used in a resin encapsulation process by compression molding, and the release film of the present invention is a process release film used in a resin encapsulation process by the compression molding method, and is a release film that is stretched and compressed in the process.

[0036] <Method for Forming Resin Encapsulation> The release film according to the present invention described above can be used when resin-encapsulating materials such as semiconductors. The method for forming resin encapsulation using a release film according to the present invention, which performs resin encapsulation by compression molding, includes, for example, the following steps (i) to (iv), but is not limited to these steps: (i) a step of placing the release film in a mold having a recess so as to cover the opening of the recess; (ii) a step of stretching the placed release film so as to conform to the inner surface of the recess of the mold; (iii) a step of filling an encapsulating resin material onto the release film that has conformed to the recess; a step of placing the encapsulated surface of an object to be encapsulated at a predetermined position in the recess of the mold (iii) so as to face the encapsulating resin material on the release film; and (iv) a step of compression-molding the encapsulating resin material to form a resin encapsulation portion on the encapsulated surface.

[0037] <Method for producing sealed article> The release film according to the present invention can also be used in a method for producing a sealed article such as a semiconductor, etc. A method for producing a sealed article (such as a semiconductor) by compression molding includes, for example, the following steps (a) to (f), but is not limited to these steps. (a) a step of placing the release film according to the present invention on a lower mold of a compression molding mold having an upper mold and a lower mold having a recess formed by a bottom surface that can be moved up and down, so that the release film covers the opening of the recess of the lower mold; (b) a step of stretching the placed release film by vacuum suction so that it conforms to the inner surface of the recess of the mold; (c) a step of filling a sealing resin material onto the release film that has conformed to the recess; (d) a step of placing an object to be sealed having a surface to be sealed between the upper mold and the lower mold so that the surface to be sealed faces the sealing resin material on the release film, and clamping the upper mold and the lower mold; (e) a step of moving the bottom surface of the lower mold to compress the stretched release film and compression-molding the sealing resin material to form a resin sealing portion on the surface to be sealed; and (f) a step of opening the upper mold and the lower mold and removing the sealed object.

[0038] The release film according to the present invention can be used in a method for producing an encapsulated body (e.g., a semiconductor element) through the steps shown in the schematic cross-sectional views of Figures 1(A) to 1(D). Note that the production method is not limited to only the embodiment including the steps shown in Figure 1.

[0039] 1A is a schematic diagram showing a process of placing a release film to cover the opening (cavity) of a recess in a lower die of a compression molding die. In FIG. 1A, the compression molding die is composed of an upper die 2 and a lower die 3, which is composed of a lower side die 3-1 and a lower bottom die 3-2 that can be moved up and down. A semiconductor element 4 (object to be sealed) is placed on the lower surface of the upper die 2, with the lower bottom die 3-2 facing the sealed surface 4a of the semiconductor element 4 (object to be sealed). A release film 1 is then placed to cover the opening 3b of the recess 3a in the lower die 3.

[0040] 1(B) is a schematic diagram showing the process of stretching the release film by vacuum suction to conform to the inner surface of the recess of the mold, and the process of filling the release film conforming to the recess with an encapsulating resin material. In FIG. 1(B), the release film 1 in FIG. 1(A) is stretched downward toward FIG. 1 by a suction device (e.g., a vacuum suction device) not shown, so that it conforms to the inner surface of the recess 3a of the lower mold 3. The stretched release film is referred to as release film 1a. At this time, the lower bottom mold 3-2 can be heated in conjunction with suction to melt the lower surface of the release film 1a stretched to conform to the mold, resulting in an embodiment in which the release film 1a is adhered (adsorbed) to the lower bottom mold 3-2. Then, an uncured encapsulating resin material 5 is placed on the stretched release film 1a and filled. In addition, Figure 1 (B) shows the process of the upper mold 2 moving downward (descending) toward Figure 1, and the process of the molds closing so that the upper mold 2 and the lower mold 3 (lower side mold 3-1) come into contact with each other via the release film 1a.

[0041] 1C is a schematic diagram showing the steps of placing a sealed object having a sealed surface between an upper mold and a lower mold so that the sealed surface faces the sealing resin material on the release film, clamping the upper mold and the lower mold, and compressing the stretched release film while compressing the sealing resin material to form a resin sealing portion on the sealed surface. In FIG. 1C, the upper mold 2 and the lower side mold 3-1 are mated via the release film 1a, and after the upper and lower molds are clamped, the lower bottom mold 3-2 is moved upward toward FIG. 1. This action compresses the stretched release film 1a in FIG. 1B into a shrunk release film 1b, and compression-moldes the sealing resin material 5, i.e., hardens the sealing resin material 5, forming a resin sealing portion 6 on the sealed surface 4a of the semiconductor element (sealed object) 4 (i.e., resin sealing). The sealing resin material 5 is heated by the lower bottom surface mold 3-2 to a molten state, and is then compression molded (compression sealed) and hardened to form the resin sealing portion 6.

[0042] FIG. 1(D) shows a state in which, after the sealed surface 4 a of the semiconductor element (sealed object) 4 has been sealed with the sealing resin material 5 to form a resin sealing portion 6, the lower bottom surface mold 3-2 is moved downward (descended) toward FIG. 1(D) and the upper mold 2 is raised to open the molds; at this time, the release film 1 c is usually peeled off from the resin sealing portion 6 formed on the semiconductor element (sealed object) 4.

[0043] In the series of steps shown in FIG. 1 , the release film changes from the state of release film 1 in FIG. 1(A) to the state of release film 1a in FIG. 1(B) where it is stretched to fit the recessed portion 3a of the mold. Then, due to the operation of the mold, the release film changes from the state of release film 1a in FIG. 1(B) where it is stretched to the depth of the recessed portion 3a of the mold to the state of release film 1b in FIG. 1(C) where it is compressed to a depth of about 1 / 2 to 1 / 3 of the depth of the recessed portion 3a of the mold. That is, the stretched release film 1a becomes the release film 1b where it is shrunk by about 50% to 70%. In the present invention, by using a release film having a restoration rate of 40% or more as defined above, wrinkles are less likely to occur in the portion of the release film 1b-1 located on the side of the recessed portion 3a of the mold shown in FIG. 1(C) during compression molding. If the restoration rate of the release film is low, for example, excess film will be generated in the release film 1b-1 portion, which will easily wrinkle, and this wrinkled portion of the release film will easily become embedded in the encapsulating resin material 5. If the release film becomes embedded in the encapsulating resin material, the release film will be difficult to peel from the resin encapsulating portion 6 when the mold shown in FIG. 1(D) is opened. If the embedding is deep, the release film will tear and the release film will remain in the resin encapsulating portion 6. Therefore, to ensure good releasability and prevent defects due to embedding, it is more preferable that the restoration rate of the release film be 50% or higher. However, if the restoration rate of the release film exceeds 80%, the release film will shrink too much during compression molding (the state shown in FIG. 1(C)), which may result in the film excessively floating from the inner surface of the recess of the lower mold. To prevent this and achieve the designed encapsulating resin thickness, the upper limit of the restoration rate can be set to 80% or less. To achieve a better encapsulation state, a restoration rate of 70% or less is more preferable.

[0044] The present invention will be specifically described below using examples and comparative examples, but the present invention is not limited to the following examples.

[0045] Example 1 Polybutylene terephthalate resin pellets (homotype, melting point = 224°C, IV value = 1.26 dl / g) were dried in a hot air dryer at 140°C for 5 hours. The dried resin pellets were melt-kneaded in an extruder under conditions of cylinder and die temperatures of 210 to 260°C, and a molten tubular thin film was extruded downward from an annular die. Subsequently, the film was passed through the outer diameter of a cooling mandrel and folded with a carapace roll, after which it was taken up by a take-up nip roll at a speed of 1.2 m / min. The unstretched film produced under the above conditions was transported to a low-speed nip roll in an atmosphere of 20°C and subjected to simultaneous longitudinal and transverse biaxial stretching by a tubular method. The stretching ratios were 3.1 times in MD and 3.2 times in TD, and the stretching temperature was 60°C. Next, this biaxially stretched film was placed in a hot roll type heat treatment device and then a tenter type heat treatment device, and heat-treated at 210°C to obtain a biaxially stretched polybutylene terephthalate film. The thickness of this biaxially stretched polybutylene terephthalate film was 25 μm. Next, the biaxially stretched polybutylene terephthalate film was used as a substrate layer, and the following release agent composition 1 was applied to one side of the biaxially stretched polybutylene terephthalate film using a gravure coater with a line count of 180 and dried by heating at 120°C to form a release layer, thereby obtaining the release film of Example 1. The thickness of the release layer was 0.5 μm. (Release Agent Composition 1) - 5 parts by mass of melamine-based release agent (Arakawa Chemical Industries, Ltd., Aracoat (registered trademark) RL900) - 1 part by mass of crosslinking agent (Arakawa Chemical Industries, Ltd., Aracoat (registered trademark) RA2000) - 200 parts by mass of solvent: methyl ethyl ketone

[0046] Example 2 A release film of Example 2 was obtained in the same manner as in Example 1, except that the melt-kneading extrusion amount was changed and the thickness of the biaxially stretched polybutylene terephthalate film was set to 15 μm.

[0047] Example 3 The same procedure as in Example 1 was performed, except that when applying the release agent composition 1, the gravure roll was changed to a gravure roll with a wire size of 110, and the thickness of the release layer was set to 4.0 μm, to obtain a release film of Example 3.

[0048] Comparative Example 1 A release film of Comparative Example 1 was obtained in the same manner as in Example 1, except that the melt-kneading extrusion amount was changed and the thickness of the biaxially stretched polybutylene terephthalate film was set to 10 μm.

[0049] The pretreatment of the polybutylene terephthalate pellets and the extrusion temperature were the same as in Example 1, except that the polybutylene terephthalate was extruded from a T-die instead of a circular die and cooled to obtain an unstretched polybutylene terephthalate film having a thickness of 50 μm. This unstretched polybutylene terephthalate was used as a substrate layer, and a release agent was applied in the same manner as in Example 1 to obtain a release film of Comparative Example 2.

[0050] <Comparative Example 3> A release film of Comparative Example 3 was obtained in the same manner as in Example 1, except that the base layer was changed to a 25 μm biaxially stretched polyethylene terephthalate film (product name: Lumirror (registered trademark) T60, manufactured by Toray Industries, Inc.).

[0051] Comparative Example 4 A biaxially oriented polybutylene terephthalate film serving as a base layer was obtained in the same manner as in Example 1, except that the melt-kneading extrusion amount was changed and the thickness of the biaxially oriented polybutylene terephthalate film was set to 15 μm. Furthermore, a 50 μm-thick unstretched polymethylpentene film was obtained in the same manner as in Comparative Example 2, except that polybutylene terephthalate was replaced with polymethylpentene, and this was used as a release layer. Next, the base layer and the release layer were dry-laminated to obtain the release film of Comparative Example 4.

[0052] Comparative Example 5 The biaxially stretched polybutylene terephthalate film (thickness: 25 μm) obtained in Example 1 was used as the release film of Comparative Example 5, without forming a release layer.

[0053] Comparative Example 6 The biaxially stretched polybutylene terephthalate film (thickness: 15 μm) obtained in Example 2 was used as the release film of Comparative Example 6, without forming a release layer.

[0054] For the release films produced in Examples 1 to 3 and Comparative Examples 1 to 6, the recovery rate, heat shrinkage rate, tensile strength (base material layer) at high temperatures, and puncture strength of the release films were measured by the following methods, and a molding test by compression molding was also carried out. The results for the recovery rate, heat shrinkage rate, tensile strength, and puncture strength of the release films are the average values ​​of three tests each. The results obtained are shown in Table 1.

[0055] <Evaluation Items> (1) Recovery Rate of Release Film The recovery rate of the film at high temperatures was measured using a tensile tester with a thermostatic chamber (TENSILON RTM-100, manufactured by ORIENTEC CORPORATION). Each release film of Examples 1 to 3 and Comparative Examples 1 to 4 was cut into a size of 150 mm in the longitudinal direction of the film and 15 mm wide to prepare a test sample. This was stretched by 10% or 50% in the longitudinal direction of the release film under conditions of a chuck distance of 50 mm, an atmospheric temperature of 175°C, and a pulling speed of 50 mm / min (the stretch at this time was defined as the stretch length value ΔLa [mm]). After maintaining this stretched state for 2 minutes, the chuck was returned in the direction opposite to the stretching direction at a speed of 50 mm / min, and the amount of displacement (return) when the stress on the chuck became 0 (load 0) was measured and defined as the stretch return value ΔLb [mm]. The restoration rate was calculated from ΔLa and ΔLb according to the following formula (1): Film restoration rate (%) = [recovery value ΔLb / elongation length value ΔLa] × 100 (1)

[0056] (2) Heat Shrinkage of Release Film Each of the release films of Examples 1 to 3 and Comparative Examples 1 to 6 was cut into a size of 100 mm x 100 mm in the longitudinal direction (MD) x width direction (TD) to prepare a sample for measuring heat shrinkage. These samples were treated (stored) at 175°C for 30 minutes, and the lengths in the MD and TD before and after treatment were measured, and the heat shrinkage in both the MD and TD directions was calculated according to the following formula (2): Heat shrinkage (%) = [(length before treatment - length after treatment) / length before treatment] x 100 (2)

[0057] (3) Tensile strength at high temperature (substrate layer) The tensile strength at high temperature was measured using a tensile tester with a thermostatic chamber (TENSILON RTM-100 manufactured by ORIENTEC CORPORATION). The substrate layer film used in the above examples and comparative examples was cut into a sample width of 15 mm to prepare a test sample. This was then subjected to conditions of a chuck distance of 50 mm, an ambient temperature of 175 ° C, and a tensile speed of 50 mm / min, and the load was read when the sample was stretched by 10% or 50% in each of the four directions of 0 ° (MD) direction / 45 ° direction / 90 ° (TD) direction / 135 °. The tensile stress (MPa) was calculated by dividing this value by the cross-sectional area (thickness × 15 mm) of the sample before the test. The abbreviations for the substrate layer films are as follows: OPBT25: Example 1, Example 3, Comparative Example 5; OPBT15: Example 2, Comparative Example 4, Comparative Example 6; OPBT10: Comparative Example 1; CPBT50: Comparative Example 2; PET25: Comparative Example 3

[0058] (4) Puncture Strength (Release Film) The puncture strength was measured using a Tensilon universal testing machine (TENSILON RTG-1210, manufactured by A&D Co., Ltd.) under conditions of 23°C and 50% RH, by piercing the release films of Examples 1 to 3 or Comparative Examples 1 to 6 with a blunt needle (diameter 1 mmφ, R = 0.5 mm) at a puncture speed of 50 mm / min, and measuring the strength (N) when the needle penetrated.

[0059] (5) Compression Molding Test Using a compression molding machine manufactured by TOWA Corporation, encapsulating resin was molded onto the evaluation substrate. A roll of release film from Examples 1 to 3 or Comparative Examples 1 to 6 was attached to the molding machine and transported. The concave portion of a lower mold (adjusted to a length of 220 mm, a width of 54 mm, and a depth of 3.0 mm) heated to 175°C was covered so that the base layer of each release film faced the inner surface of the concave portion. This release film was then vacuum-suctioned to follow and adhere to the concave surface of the lower mold. Granular encapsulating resin (epoxy thermosetting resin, manufactured by Sumitomo Bakelite Co., Ltd.) was then placed on the release film (release layer side) on the concave portion of the lower mold, and the encapsulating resin was melted by the heat (175°C) of the lower mold. The upper mold with the evaluation substrate attached was then lowered to mate with the lower mold, and the bottom of the recess in the lower mold was then raised so that the package thickness would be 1.0 mm. The evaluation substrate was then sealed by compression molding at a mold temperature of 175°C for a sealing time of 2 minutes at 96 kN. After the sealing resin had hardened, the molds were opened, and the evaluation substrate molded with the sealing resin was separated from the release film.

[0060] (Mold Releasability) The releasability of the evaluation substrate from each release film during the above molding test was evaluated according to the following evaluation criteria. ∘: The release film peeled off naturally when the mold was opened after compression molding. ×: The release film did not peel off naturally when the mold was opened after compression molding. (Tears during Compression Molding) The occurrence of tears in each release film during the above molding test was evaluated according to the following evaluation criteria. ∘: The release film did not tear at the periphery of the mold cavity when the mold was opened after compression molding. ×: The release film broke at the periphery of the mold cavity when the mold was opened after compression molding. (Pinching and Wrinkles During Compression Molding) The occurrence of wrinkles and the like in each release film during the above molding test was evaluated according to the following evaluation criteria. ∘: There were no traces of the release film being pinched on the side of the periphery of the resin sealing part, and no wrinkle patterns were observed on the surface of the resin sealing part. △: There were traces of the release film sandwiched on the side of the peripheral edge of the resin sealed part, but no wrinkle patterns were observed on the other surface of the resin sealed part. ×: There were traces of the release film sandwiched on the side of the peripheral edge of the resin sealed part, and wrinkle patterns were observed on the other surface of the resin sealed part.

[0061]

[0062]

[0063] From the results shown in Tables 1 and 2, it is clear that the release films of the present invention, which have a recovery rate of 40% or more and 80% or less after 10% to 50% elongation at a temperature of 175°C, exhibit excellent releasability without tearing or wrinkles during compression molding (Examples 1 to 3). On the other hand, the release film of Comparative Example 1, which used a biaxially oriented polybutylene terephthalate film with a substrate thickness of 10 μm, broke during compression molding and was inferior. This is thought to be due to the thin substrate layer, which results in low strength at a molding temperature of 175°C. Furthermore, as shown in Comparative Examples 2 and 3, release films with low recovery rates were inferior, suffering from tearing and pinching. Note that the release film of Comparative Example 4 wrinkled on the flat portion of the mold during compression molding, and these wrinkles were transferred to the encapsulating resin, forming a wrinkled pattern on the surface of the encapsulating resin, resulting in impaired appearance quality in the resulting molded product (evaluation substrate). The reason for this is thought to be that there is a difference in the heat shrinkability of the film of the release layer and the heat shrinkability of the film of the base layer (the heat shrinkage rate (175 ° C) of the polymethylpentene of the release layer in the MD direction is 1% elongation and the same in the TD direction is 1%, while the heat shrinkage rate (175 ° C) of the biaxially oriented PBT film of the base layer in the MD direction is 3% or more shrinkage and the same in the TD direction is 1% or more shrinkage). Furthermore, in the release films of Comparative Examples 5 and 6, which did not form a release layer and used only biaxially oriented polybutylene terephthalate film, the film did not peel naturally when the mold was opened after compression molding, and did not reach the evaluation of tearing or wrinkles, resulting in them being unsuitable as release films.

[0064] According to the present invention, it is possible to provide a release film for use in a compression molding process, which is wrinkle-free, has excellent peelability and recyclability, and has recovery properties after being stretched at high temperatures.

[0065] REFERENCE SIGNS LIST 1 release film 2 upper mold 3 lower mold (3-1 lower side mold, 3-2 lower bottom mold) 3a recess 3b opening 4 semiconductor element (sealed object) 4a sealed surface 5 sealing resin material 6 resin sealing portion

Claims

1. A release film having a release layer and a base layer, the base layer being a biaxially stretched polybutylene terephthalate film, the release film being stretched in the longitudinal direction (MD) of the release film at an elongation of 10% to 50%, held in the stretched state for 2 minutes, and then returned to a stress of 0 at a speed of 50 mm / min in the opposite direction to the stretched direction, the recovery rate (%) obtained according to the following formula (1) is 40% or more and 80% or less, from the elongation length value ΔLa from the initial value before stretching to the value at the time of stretching and the return value ΔLb from the elongation length value at the time of stretching to the elongation length value at stress 0. Recovery rate (%) = [return value ΔLb / elongation length value ΔLa] x 100... (1) 2. The release film according to claim 1, wherein the biaxially stretched polybutylene terephthalate film of the base layer has a tensile stress of 10 MPa to 40 MPa when stretched to an elongation of 10% to 50% in four directions (0° (MD), 45°, 90° (TD), 135°) under conditions of a temperature of 175°C, a sample width of 15 mm, an initial chuck distance of 50 mm, and a tensile speed of 50 mm / min, and the ratio of the maximum value to the minimum value of the tensile stress in the four directions is 1.5 or less.

3. The release film according to claim 1, wherein the heat shrinkage rate of the release film after 30 minutes at 175°C is 1.5% or more in both the longitudinal direction (MD) and the transverse direction (TD).

4. The release film according to claim 1, wherein the base layer has a thickness of 15 μm or more and 25 μm or less, the release layer has a thickness of 0.1 μm or more and 5 μm or less, and the release film has a puncture strength of 9 N or more measured in accordance with JIS Z 1707.

5. The release film according to claim 1, wherein the release layer is a layer that does not contain a silicone component and is provided on at least one surface of the base layer.

6. The release film according to claim 1, further comprising an antistatic layer, the antistatic layer being provided between the release layer and the base layer, or the release layer being provided on one side of the base layer and the antistatic layer being provided on the other side.

7. The release film according to claim 1, which is a process release film used in a resin sealing process by compression molding.

8. The release film according to claim 7, which is used in a resin sealing process by the compression molding method, and is stretched and compressed in the process.

9. A method for resin encapsulation of a semiconductor element, comprising using the release film according to any one of claims 1 to 8.

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