Method for manufacturing semiconductor device, thermally conductive sheet, and method for manufacturing thermally conductive sheet

Abstract

A method for manufacturing a semiconductor device includes a step for applying pressure in the thickness direction of a heat-conducting sheet to a heat-generating body and a heat-dissipating body between which the heat-conducting sheet is disposed, thereby bonding the heat-generating body and the heat-dissipating body via the heat-conducting sheet, and a step for manufacturing the heat-generating body and the heat-dissipating body by applying pressure in the thickness direction of the heat-conducting sheet. The thermal conductive sheet has a compressive elastic modulus of 1.40 MPa or less at a compressive stress of 0.10 MPa at 150 DEG C and an adhesive strength of 5.0 N.mm or more at 25 DEG C.

Description

Divisional Application Instructions

[0001] This invention is a divisional application of Chinese national patent application No. 201880096820.1, filed on August 23, 2018, entitled "Method for manufacturing a semiconductor device, a heat-conducting sheet, and a method for manufacturing a heat-conducting sheet". Technical Field

[0002] This disclosure relates to a method for manufacturing a semiconductor device, a heat-conducting sheet, and a method for manufacturing the heat-conducting sheet. Background Technology

[0003] In recent years, with the increasing density of wiring in multilayer wiring boards, the increasing density of wiring in semiconductor packaging, the increasing density of electronic components, and the increasing heat generation per unit area caused by the high integration of semiconductor components, there is a desire to improve the heat dissipation of semiconductor packaging.

[0004] In particular, semiconductor devices that generate a lot of heat, such as CPUs (Central Processing Units) and power devices, require excellent heat dissipation. These semiconductor devices have a structure that dissipates heat by sandwiching thermally conductive materials such as grease and thermal conductive sheets between a heat-generating element and a heat sink such as aluminum or copper, and making them closely connected (for example, see Patent Documents 1-4).

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 05-247268 Patent Document 2: Japanese Patent Application Publication No. 10-298433 Patent Document 3: Japanese Patent No. 4743344 Patent Document 4: Japanese Patent No. 5316254 Summary of the Invention

[0006] The problem that the invention aims to solve In recent years, with the increasing performance of semiconductor packaging, semiconductor chips and packages have become increasingly larger. This larger size makes it easier for grease to be pumped out during thermal cycling when using it as a thermal conductive material, making it difficult to ensure adequate heat dissipation. On the other hand, when using thermal pads as the thermal conductive material, the increased size of the heat-generating element leads to greater warpage of the semiconductor chip or package, making it easy for the thermal pad to detach from the heat-generating element or heat sink, further complicating heat dissipation.

[0007] In view of this situation, the objective of this disclosure is to provide a method for manufacturing a semiconductor device with excellent heat dissipation, a heat-conducting sheet capable of manufacturing a semiconductor device with excellent heat dissipation, and a method for manufacturing the heat-conducting sheet.

[0008] Methods used to solve problems The means to solve the above problems include the following methods.

[0009] <1> A method for manufacturing a semiconductor device, comprising the following steps: For a heat-generating element and a heat-dissipating element with heat-conducting sheets disposed between them, pressure is applied in the thickness direction of the heat-conducting sheets to bond the heat-generating element and the heat-dissipating element together via the heat-conducting sheets. The heat-conducting sheets have a compressive elastic modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C and an adhesive force of 5.0 N·mm or more at 25°C.

[0010] <2> The method for manufacturing a semiconductor device as described in <1>, wherein, The thermal conductivity of the heat-conducting sheet, determined by the thermal resistance measured using the steady-state method, is above 7 W / (m·K).

[0011] <3> A method for manufacturing a semiconductor device as described in <1> or <2>, wherein, The pressure is 0.05 MPa to 10.00 MPa.

[0012] <4> The method for manufacturing a semiconductor device as described in <3>, wherein, The pressure is 0.10 MPa to 0.50 MPa.

[0013] <5> A method for manufacturing a semiconductor device as described in any one of <1> to <4>, wherein, The heating element is a semiconductor chip, and the heat sink is a heatsink.

[0014] <6> A method for manufacturing a semiconductor device as described in any one of <1> to <5>, wherein, The area of ​​the surface of the heating element opposite the heat-conducting plate is 25mm². 2 above.

[0015] <7> A method for manufacturing a semiconductor device as described in any one of <1> to <4>, wherein, The heating element is a semiconductor package with a heat sink, and the heat sink is a heatsink.

[0016] <8> A method for manufacturing a semiconductor device as described in any one of <1> to <4>, wherein, The heating element is a semiconductor module.

[0017] <9> A method for manufacturing a semiconductor device as described in any one of <1> to <8>, wherein, The area of ​​the surface of the heating element opposite the heat-conducting plate is 100 mm². 2 above.

[0018] <10> A heat-conducting sheet having a compressive elastic modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C and an adhesive force of 5.0 N·mm or more at 25°C, disposed between a heating element and a heat sink of a semiconductor device, for bonding the heating element and the heat sink.

[0019] <11> The heat-conducting sheet as described in <10>, wherein, The thermal conductivity, determined by the thermal resistance measured by the steady-state method, is above 7 W / (m·K).

[0020] <12> The heat-conducting sheet as described in <10> or <11>, wherein, The heating element is a semiconductor chip, and the heat sink is a heat sink.

[0021] <13> The heat-conducting sheet as described in <10> or <11>, wherein, The heating element is a semiconductor package with a heat sink, and the heat sink is a heat sink fin.

[0022] <14> The heat-conducting sheet as described in <10> or <11>, wherein, The heating element is a semiconductor module.

[0023] <15> A method for manufacturing a heat-conducting sheet, comprising the following steps: In a heat-conducting sheet used to heat and apply pressure to a heat-generating body and a heat-dissipating body with heat-conducting sheets disposed between them, so that the heat-generating body and the heat-dissipating body are bonded together via the heat-conducting sheet, the compression ratio and thickness of the heat-conducting sheet are selected such that the compression amount of the heat-conducting sheet satisfies the following formula.

[0024] Formula: C > L2 - L1 L1: The predicted warpage (μm) of the heating element under heating and pressurization. L2: The predicted warpage (μm) of the heating element when cooled to 25°C after heating and pressurization have ended. C: The predicted compression (μm) of the heat-conducting sheet under heating and pressurization conditions. C = Thickness of the heat-conducting sheet before pressurization (μm) × Compression ratio under heating and pressurization conditions (%).

[0025] Invention Effects According to this disclosure, a method for manufacturing a semiconductor device with excellent heat dissipation, a heat-conducting sheet capable of manufacturing a semiconductor device with excellent heat dissipation, and a method for manufacturing the heat-conducting sheet can be provided. Attached Figure Description

[0026] Figure 1 This is a schematic cross-sectional view of the semiconductor device using a heat-conducting sheet as TIM1.

[0027] Figure 2 This diagram illustrates the warpage in a semiconductor device using a thermally conductive sheet as TIM1. Detailed Implementation

[0028] The following describes in detail the methods for implementing the present invention. However, the present invention is not limited to the following embodiments. In the following embodiments, unless specifically stated otherwise, the constituent elements (including element steps, etc.) are not essential. The same applies to numerical values ​​and their ranges, which do not limit the present invention.

[0029] In this disclosure, the term "process" includes not only processes that are independent of other processes, but also processes that can achieve their purpose, even if they cannot be clearly distinguished from other processes.

[0030] In this disclosure, within the numerical range represented by “~”, the values ​​recorded before and after “~” are respectively included as the minimum and maximum values.

[0031] In the numerical ranges described in this disclosure in stages, the upper or lower limit value of one numerical range can be replaced by the upper or lower limit value of other numerical ranges described in stages. Furthermore, the upper or lower limit value of the numerical ranges described in this disclosure can also be replaced by the values ​​shown in the embodiments.

[0032] In this disclosure, each component may also contain multiple corresponding substances. When multiple substances corresponding to each component are present in the composition, unless otherwise specified, the content or percentage of each component refers to the total content or percentage of the multiple substances present in the composition.

[0033] This disclosure may also include multiple particles belonging to each component. When multiple particles corresponding to each component are present in the composition, unless otherwise specified, the particle size of each component refers to the value of a mixture of the multiple particles present in the composition.

[0034] In this disclosure, the term "layer" is used to refer to both the entirety of the region and a portion of the region when observing the area where the layer exists.

[0035] In this disclosure, the term "layering" means stacking layers together, which can be combining two or more layers or being able to assemble and disassemble two or more layers.

[0036] In this disclosure, embodiments are described with reference to the accompanying drawings, but the configuration of these embodiments is not limited to the configuration shown in the drawings. Furthermore, the sizes of the components in the drawings are conceptual, and the relative sizes of the components are not limited thereto.

[0037] <Semiconductor Device Manufacturing Methods> The semiconductor device manufacturing method disclosed herein includes the following steps: applying pressure in the thickness direction of a heat-generating element and a heat-dissipating element, wherein the heat-conducting element and the heat-dissipating element are disposed between each other via a heat-conducting sheet, thereby bonding the heat-generating element and the heat-dissipating element through the heat-conducting sheet. The heat-conducting sheet has a compressive modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C, and an adhesion force of 5.0 N·mm or more at 25°C. According to the semiconductor device manufacturing method disclosed herein, it is possible to manufacture a semiconductor device in which heat-conducting sheet peeling is suppressed and heat dissipation is excellent.

[0038] [Fever body] The heat source in this disclosure is an object that dissipates heat by bonding a heat sink to a heat conductor via a thermally conductive sheet. Examples of heat sources include semiconductor chips, semiconductor packages, automotive power modules, and industrial power modules. Furthermore, a heat source does not necessarily mean that the component in contact with the thermally conductive sheet is capable of generating heat itself. For example, in the case of using the thermally conductive sheet for the TIM2 application described later, a heat sink mounted on a semiconductor package contacts the thermally conductive sheet, but in this case, the semiconductor package equipped with the heat sink is considered the "heat source".

[0039] There are no particular limitations on the size of the heating element. For example, when using the heat-conducting plate for the TIM1 application described later, the area of ​​the surface of the heating element facing the heat-conducting plate can be 25 mm². 2 The above can be 100mm 2 The above can be 200mm. 2 The above can also be 400mm. 2 The area of ​​the surface of the heating element opposite the heat-conducting plate can be, for example, 15000 mm². 2 Below, it can be 5000mm 2 The following can also be 2000mm 2 the following.

[0040] When the heat-conducting plate is used for the TIM2 application described later, the area of ​​the surface of the heating element facing the heat-conducting plate can be 100 mm². 2 The above can be 400mm 2The above can also be 1000mm. 2 The area of ​​the surface of the heating element opposite the heat-conducting plate can be, for example, 40,000 mm². 2 The following can be 20000mm 2 The following can also be 5000mm 2 the following.

[0041] When the heat-conducting plate is used in the power device applications described later, the area of ​​the surface of the heating element facing the heat-conducting plate can be 100 mm². 2 The above can be 400mm 2 The above can also be 1000mm. 2 The area of ​​the surface of the heating element opposite the heat-conducting plate can be, for example, 40,000 mm². 2 The following can be 20000mm 2 The following can also be 5000mm 2 the following.

[0042] [Heat sink] The heat sink in this disclosure is a component that dissipates heat from a heat-generating element via a heat-conducting fin. Examples of heat sinks include radiators, heat sinks, and water-cooling pipes.

[0043] [Heat-conducting sheet] The thermally conductive sheet disclosed herein is a sheet material disposed between a heat-generating element and a heat-dissipating element in a semiconductor device and used to bond the heat-generating element and the heat-dissipating element. In this disclosure, "sheet material" refers to a non-liquid sheet product, distinguished from liquid grease, etc. Here, "liquid" refers to a substance with a viscosity of 1000 Pa·s or less at 25°C. Viscosity is defined as the value measured at 25°C using a rheometer at a shear rate of 5.0 s⁻¹. The shear viscosity is measured at 25°C using a rotary shear viscometer equipped with a cone plate (40 mm in diameter, 0° cone angle).

[0044] When using grease as a thermally conductive material between the heating element and the heat sink, there is a possibility that the thermal resistance may increase due to pumping during thermal cycling. However, in the manufacturing method disclosed herein, pumping does not occur because a thermally conductive sheet is used.

[0045] Furthermore, when using sheet material as the thermal conductive material, the sheet may peel off as the warpage of the heat-generating element increases, thus failing to achieve the desired heat dissipation. However, the thermal conductive sheet used in this disclosure can maintain sufficient bonding area between the heat-generating element and the heat sink even in semiconductor packages with increased warpage. This ensures excellent heat dissipation characteristics. The reason may not be clear, but it is believed that when a thermal conductive sheet with the aforementioned specific compressive modulus and adhesive strength is stamped between the heat-generating element and the heat sink, the thermal conductive sheet is sufficiently flattened and sufficiently bonded to both the heat-generating element and the heat sink. Therefore, it is believed that even if the warpage changes after stamping, the bonding area can be maintained by following the warpage.

[0046] If the thermal conductive pad can maintain its bonding area without detaching from the heat source and heat sink, it can suppress the increase in contact thermal resistance and prevent the degradation of the heat dissipation characteristics of the semiconductor device. Therefore, it is desirable to maintain the bonding area between the thermal conductive pad and the heat source and heat sink even if the heat source warps.

[0047] The thermal conductive sheet used in this disclosure is not particularly limited in its application, as long as it is disposed between the heat-generating element and the heat-dissipating element of a semiconductor and used for bonding the heat-generating element and the heat-dissipating element. For example, the thermal conductive sheet may be a thermally conductive material disposed between a semiconductor chip serving as a heat-generating element and a heat sink serving as a heat sink (TIM1; Thermal Interface Material L1). Alternatively, it may be a thermally conductive material disposed between a semiconductor package with a heat sink serving as a heat-generating element and a heat sink serving as a heat sink (TIM2; Thermal Interface Material L2). Furthermore, it may be a thermally conductive material disposed between a semiconductor module serving as a heat-generating element and a heat sink (such as a thermally conductive material for power devices).

[0048] In the field of TIM1, which uses conventional grease, the heat-conducting sheet used in this disclosure is particularly useful as the heating element becomes larger and it becomes difficult to ensure sufficient heat dissipation with grease.

[0049] use Figure 1 A specific example of how the heat-conducting sheet is used as TIM1 will be explained. It is used such that one side of the heat-conducting sheet 1 is in close contact with the semiconductor chip 2 (heat-generating element), and the other side is in close contact with the heat sink 3 (heat sink). Figure 1In this design, the semiconductor chip 2 (heat generator) is fixed to the substrate 4 using a bottom filler 5, and the heat sink 3 (heat dissipation body) is fixedly adhered to the substrate 4 using a sealing material 6. Pressing is used to improve the seal between the heat-conducting sheet 1, the semiconductor chip 2, and the heat sink 3. By layering the heat generator and the heat sink with the heat-conducting sheet in between, heat from the heat generator can be efficiently conducted to the heat sink. Efficient heat conduction increases the lifespan of the semiconductor device, ensuring stable functioning even during long-term use.

[0050] The thermally conductive sheet used in this disclosure has a compressive modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C, and an adhesive force of 5.0 N·mm or more at 25°C. It is believed that by satisfying the above-mentioned ranges for the compressive modulus and adhesive force, even in semiconductor devices where the warpage of the heat-generating element increases, the thermally conductive sheet can maintain close contact with the heat-generating element and the heat-dissipating element, thus maintaining the bonding area. Therefore, heat dissipation is considered to be ensured.

[0051] It is believed that when the compressive elastic modulus is below 1.40 MPa at a compressive stress of 0.10 MPa at 150°C, the thermal conductive sheet exhibits excellent flexibility. Under pressure (stamping), it is easily flattened, making it easier to form a tight bond with the heating element and the heat sink. Furthermore, it is believed that even if the warpage of the heating element increases after stamping, the thermal conductive sheet can still maintain a stable tight bond with the heating element and the heat sink, suppressing the reduction of the bonding area.

[0052] The compressive modulus of the heat-conducting sheet at 150°C with a compressive stress of 0.10 MPa is 1.40 MPa or less, preferably 1.30 MPa or less, and more preferably 1.20 MPa or less. When the compressive modulus is 1.20 MPa or less, the sealing performance is further improved, and it is easier to follow warpage. There is no particular limitation on the lower limit of the compressive modulus at 150°C with a compressive stress of 0.10 MPa. The compressive modulus can be 0.50 MPa or more, or 0.70 MPa or more.

[0053] The compressive modulus of the thermal conductive sheet can be determined using a compression testing apparatus (e.g., INSTRON 5948 MicroTester (INSTRON)). A load is applied to the thermal conductive sheet at a displacement rate of 0.1 mm / min relative to its thickness direction, and the displacement (mm) and load (N) are measured. The strain (dimensionless) calculated from displacement (mm) / thickness (mm) is plotted on the horizontal axis, and the strain calculated from load (N) / area (mm²) is plotted on the horizontal axis. 2 The stress (MPa) obtained is shown on the vertical axis, and the slope at the specified stress is set as the compressive modulus (MPa). Specifically, for example, it can be measured using the method described in the examples.

[0054] The adhesive strength of the heat-conducting sheet at 25°C is 5.0 N·mm or more, preferably 6.0 N·mm or more, and more preferably 7.0 N·mm or more. If the adhesive strength is 5.0 N·mm or more, it can prevent the heat-conducting sheet from peeling off from both the heat-generating element and the heat-dissipating element when warping occurs and the distance between them increases. There is no particular upper limit to the adhesive strength. The aforementioned adhesive strength can be 20.0 N·mm or less, or 15.0 N·mm or less.

[0055] The adhesive strength of the thermal conductive sheet at 25°C can be measured using a universal physical property testing machine (e.g., a texture analyzer (Eiko Seiki Co., Ltd.)). At 25°C (room temperature), a 7mm diameter probe is pressed onto the thermal conductive sheet with a load of 40N and held for 10 seconds. The area obtained by integrating the load-displacement curve when the probe is lifted is taken as the adhesive strength (N·mm) at 25°C. Specifically, the method described in the examples can be used for measurement.

[0056] There are no particular limitations on the method for obtaining a thermally conductive sheet with a compressive elastic modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C and an adhesive strength of 5.0 N·mm or more at 25°C. For example, it can be obtained by adjusting the types and proportions of the thermally conductive fillers, resins, and other components used in the thermally conductive sheet.

[0057] There are no particular limitations on the thermal conductivity of the heatsink, but higher is preferred. The thermal conductivity of the heatsink, determined by the thermal resistance measured by the steady-state method, is preferably 7 W / (m·K) or higher, more preferably 10 W / (m·K) or higher, and even more preferably 15 W / (m·K) or higher. If the thermal conductivity is 7 W / (m·K) or higher, even if the thickness of the heatsink is increased to improve the warp tracking of the heating element, there is a tendency to easily suppress the increase in thermal resistance.

[0058] In this disclosure, the thermal conductivity of the heat-conducting sheet is specifically calculated as follows.

[0059] A heat-conducting sheet was cut into 10mm squares and sandwiched between a transistor (2SC2233) serving as a heat source and a copper block serving as a heat sink. While pressing the transistor at 80°C and 0.14MPa, the temperatures of the transistor (T1°C) and the copper block (T2°C) were measured when current was applied. Based on the measured values ​​and the applied power (W1W), the temperature per unit area (cm²) was calculated as follows. 2 Thermal resistance value X (K·cm) 2 / W).

[0060] X = (T1 - T2) × 1 / W1 Then, using the thickness t (μm), the thermal conductivity λ (W / (m·K)) is calculated as follows.

[0061] λ = (t × 10) －6 ) / (X×10 －4 ) The thickness of the heatsink is not particularly limited and can be appropriately selected according to the specifications of the semiconductor package used. There is a tendency for smaller thicknesses to result in lower thermal resistance, and a tendency for larger thicknesses to result in better warp follow-through. The average thickness of the heatsink can range from 50 μm to 3000 μm, and from the viewpoint of thermal conductivity and sealing, 100 μm to 500 μm is preferred, and more preferably 150 μm to 300 μm. Regarding the average thickness of the heatsink, the thickness at three locations is measured using a micrometer, and the arithmetic mean is given. The thickness of the heatsink can also be selected based on the warp of the heat-generating element, as described later.

[0062] There are no particular limitations on the compression of the heat-conducting sheet. For example, the compression at 150°C with a compressive stress of 0.10 MPa can be 20 μm to 1000 μm, 30 μm to 200 μm, or 40 μm to 100 μm. The compression at 150°C with a compressive stress of 0.15 MPa can also be the values ​​mentioned above.

[0063] The "compression" of a heatsink refers to the amount of compression of the heatsink when pressure is applied in the thickness direction of the heatsink. It is a value obtained by subtracting the thickness of the heatsink when pressure is being applied from the thickness of the heatsink before pressure is applied.

[0064] There are no particular limitations on the compressibility of the heat-conducting sheet. For example, the compressibility at 150°C with a compressive stress of 0.10 MPa can be 10%–60%, 15%–50%, or 15%–40%. The compressibility at 150°C with a compressive stress of 0.15 MPa can also be the values ​​mentioned above.

[0065] The “compression ratio” of a heat-conducting sheet refers to the ratio (%) of the compression amount (μm) to the thickness (μm) of the heat-conducting sheet before pressure was applied.

[0066] To protect the bonding surface, the thermally conductive sheet can also be prepared with a protective film on at least one side. In this case, the thermally conductive sheet with the protective film removed is used for bonding the heat-generating element and the heat-dissipating element. As the protective film, for example, resin films such as polyethylene, polyester, polypropylene, polyethylene terephthalate, polyimide, polyetherimide, polyethylene naphthalate, methylpentene, polytetrafluoroethylene, ethylene tetrafluoroethylene copolymer, and perfluoroalkoxyalkane, as well as coated paper, coated fabric, and metal foil such as aluminum can be used. These protective films can be used individually or in combination of two or more to form a multilayer film. The protective film is preferably surface-treated with release agents such as silicone or silica.

[0067] The composition of the thermal conductive sheet is not particularly limited as long as it meets the specific compressive modulus and adhesive strength requirements mentioned above. For example, thermal conductive sheets containing resin and thermally conductive fillers can be cited.

[0068] Examples of thermally conductive fillers include aluminum nitride, alumina, boron nitride, titanium dioxide, zinc oxide, silicon carbide, silicon, silica, glass, metal particles, carbon fiber, graphite, graphene, and carbon nanotubes. Thermally conductive fillers can also undergo surface treatment. A single thermally conductive filler can be used alone, or two or more can be used in combination.

[0069] There are no particular limitations on the shape of thermally conductive fillers; examples include spherical, ellipsoidal, scaly, granular, rod-shaped, needle-shaped, and fibrous fillers.

[0070] There is no particular limitation on the average particle size of the thermally conductive filler; it is preferably set according to the material of the thermally conductive filler.

[0071] There are no particular restrictions on the aspect ratio (major diameter / minor diameter) of the thermally conductive filler; it can range from 1 to 100, from 5 to 50, or from 10 to 40. The aspect ratio of the thermally conductive filler was determined using scanning electron microscopy (SEM) for 20 representative particles, and the arithmetic mean of the measured values ​​was used.

[0072] From the viewpoint of thermal conductivity, the thermally conductive filler is preferably oriented in the thickness direction of the thermally conductive sheet. In this disclosure, "oriented in the thickness direction" means that, in a thermally conductive filler having a major axis and a minor axis (i.e., an aspect ratio greater than 1), the angle (also called the "orientation angle") formed between the major axis direction of the thermally conductive filler and the surface (main surface) of the thermally conductive sheet is 60° or more. The orientation angle is preferably 80° or more, more preferably 85° or more, and even more preferably 88° or more.

[0073] From the perspective of balancing thermal conductivity and sealing, the content of thermally conductive filler in the heat-conducting sheet is preferably selected appropriately based on the material of the thermally conductive filler. For example, the content of thermally conductive filler relative to the total volume of the heat-conducting sheet can be 25% to 75% by volume, 30% to 60% by volume, or 35% to 50% by volume.

[0074] There are no particular restrictions on the resin contained in the thermal conductive sheet; for example, it can be a curable resin or a non-curable resin. Examples of resins include epoxy resin, silicone, acrylic resin, polyimide resin, bismaleimide resin, benzocyclobutene resin, phenolic resin, unsaturated polyester, diallyl phthalate resin, polyurethane, polyimide silicone, thermosetting polyphenylene ether, thermosetting modified polyphenylene ether, polybutene, polyisoprene, polysulfide, acrylonitrile rubber, silicone rubber, hydrocarbon resin, terpene resin, terpene phenol resin, and hydrogenated terpene phenol resin. One type of resin can be used alone, or two or more types can be used in combination.

[0075] The resin content in the thermal conductive sheet is preferably selected based on the type of resin and the desired flexibility, adhesion, sealing properties, sheet strength, and resistance to hydrolysis. For example, the resin content relative to the total volume of the thermal conductive sheet is preferably 25% to 75% by volume, more preferably 40% to 70% by volume, and even more preferably 50% to 65% by volume.

[0076] In addition to thermally conductive fillers and resins, thermally conductive sheets may also contain various additives such as flame retardants and antioxidants. There are no particular limitations on flame retardants; they can be appropriately selected from commonly used flame retardants. For example, red phosphorus flame retardants and phosphate ester flame retardants can be listed. Among these, phosphate ester flame retardants are preferred from the perspective of excellent safety and improved adhesion through plasticizing effects.

[0077] Regarding the manufacturing method of the heat-conducting sheet, there are no particular limitations as long as the method can obtain a heat-conducting sheet with the specific compressive elastic modulus and adhesive strength mentioned above. For example, a composition containing the components of the heat-conducting sheet can be prepared, and a sheet can be made by rolling, stamping, extrusion, coating, etc.

[0078] Alternatively, a composition containing the components of a thermally conductive sheet can be used to form a molded body, which can then be sliced ​​to produce a sheet. In this case, it is preferable to slice the molded body in such a way that the thermally conductive filler is oriented in the thickness direction.

[0079] In one embodiment, the heat-conducting sheet can also be manufactured by preparing a composition containing the components of the heat-conducting sheet, sheeting the composition to obtain a sheet, stacking the sheet to form a laminate, and slicing the side end faces of the laminate. By manufacturing the heat-conducting sheet using this method, an efficient heat conduction path is formed, tending to produce a heat-conducting sheet with excellent thermal conductivity and adhesion. Furthermore, the resulting heat-conducting sheet can also be laminated by attaching it to a protective film.

[0080] [Bonding methods for heating elements and heat sinks] In the semiconductor device manufacturing method of this disclosure, a heat-generating element and a heat-dissipating element, which are disposed between each other with heat-conducting sheets, are subjected to pressure in the thickness direction of the heat-conducting sheets, so that the heat-generating element and the heat-dissipating element are bonded together via the heat-conducting sheets.

[0081] In this disclosure, adhesion refers to a state in which multiple surfaces are joined together by chemical or physical force, or both. According to the semiconductor device manufacturing method of this disclosure, there is a tendency to maintain a good adhesion area between the heat-generating element and the heat-conducting sheet, the heat sink and the heat-conducting sheet, or both. When assembling a semiconductor device by bonding the heat-generating element and the heat sink via the heat-conducting sheet, the adhesion area is preferably 80% or more of the area of ​​the surface of the heat-generating element or heat sink facing the heat-conducting sheet, more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more.

[0082] As a method of placing a heat-conducting plate between a heat-generating element and a heat-dissipating element, the heat-conducting plate can be placed on the heat-generating element first, and then the heat-dissipating element can be placed via the heat-conducting plate; or the heat-conducting plate can be placed on the heat-dissipating element first, and then the heat-generating element can be placed via the heat-conducting plate.

[0083] Compared to a single heat-conducting sheet, the heat-generating element and the heat-dissipating element can be one or more, or one or both of them can be multiple.

[0084] For a heat-generating element and a heat-dissipating element disposed on a heat-conducting plate, pressure is applied in the thickness direction of the heat-conducting plate to bond the heat-generating element and the heat-dissipating element together via the heat-conducting plate. At this time, pressure can be applied from the heat-generating element side or from the heat-dissipating element side.

[0085] From the viewpoint of ensuring the tightness of the heat-conducting sheet and reducing the load on electronic components, the pressure applied in the thickness direction of the heat-conducting sheet is preferably 0.05 MPa to 10.00 MPa, more preferably 0.10 MPa to 5.00 MPa, and even more preferably 0.10 MPa to 1.00 MPa. From the viewpoint of reducing the load on electronic components, 0.10 MPa to 0.50 MPa is particularly preferred. From the viewpoint of ensuring the tightness of the heat-conducting sheet, the pressure can also be adjusted according to the thickness of the heat-conducting sheet. For example, when the thickness of the heat-conducting sheet is 200 μm or more, the pressure can be 0.20 MPa or less, and when the thickness of the heat-conducting sheet is less than 200 μm, the pressure can be greater than 0.20 MPa.

[0086] There are no particular restrictions on the temperature at which pressure is applied, but it is preferable to select a suitable temperature range based on the type of heat-conducting sheet. The temperature at which pressure is applied can be room temperature, but from the viewpoint of improving the compression ratio, the temperature after heating is preferred. For example, the temperature after heating can be 80°C to 200°C, 100°C to 190°C, or 120°C to 180°C.

[0087] Within a temperature range of 120°C to 180°C, a pressure of 0.10 MPa to 1.00 MPa is preferably applied. Setting the pressure to 0.10 MPa or higher, or the heating temperature to 120°C or higher, tends to result in excellent sealing performance. Furthermore, setting the pressure to 1.00 MPa or lower, or the heating temperature to 180°C or lower, tends to further improve the reliability of the seal. This is believed to be because it prevents the heat-conducting sheet from being excessively compressed and becoming thinner, or from developing excessive strain or residual stress in surrounding components.

[0088] When pressure is applied, for example, when a pressure of 0.10 MPa is applied at 150°C, the warpage of the heating element can be 10 μm or more, 20 μm or more, or 25 μm or more. Alternatively, when pressure is applied, for example, when a pressure of 0.10 MPa is applied at 150°C, the warpage of the heating element can be 80 μm or less, 70 μm or less, or 60 μm or less. The warpage of the heating element when a pressure of 0.15 MPa is applied at 150°C can also be within the above ranges.

[0089] The warpage of the heating element after pressure release can be, for example, 40 μm or more, 50 μm or more, or 60 μm or more. Alternatively, the warpage of the heating element after pressure release can be less than 150 μm, less than 140 μm, or less than 130 μm. When the warpage changes with temperature after pressure release, the aforementioned "warpage of the heating element after pressure release" refers to the warpage at 25°C.

[0090] In the process of bonding the heating element and the heat sink, the difference between the warpage of the heating element when pressure is applied and the warpage of the heating element after pressure is released can be 30 μm or more, 40 μm or more, or 45 μm or more. Alternatively, the upper limit of the aforementioned difference can be, for example, 120 μm or less.

[0091] The "warpage" of a heating element refers to the maximum deformation (μm) in the thickness direction of the heating element when it warps and deforms.

[0092] use Figure 2 An example of a method for measuring warpage when a heat-conducting sheet is used in a TIM1 application will be described. As follows, warpage can be measured based on the deformation of the substrate on which the heating element is mounted. The analysis range of warpage is the portion (heating element portion) a on which the heating element is mounted when viewed from the substrate side. In heating element portion a, the difference in displacement between the portion on which the substrate has the largest deformation in the thickness direction and the end of the heating element is defined as warpage b.

[0093] The greater the warpage, the easier it is for the thermal pad to detach from the heat source and heat sink when warpage occurs. The thermal pad used in this disclosure tends to maintain a good bonding area even when the warpage of the heat source is large, without detaching from the heat source and heat sink.

[0094] Regarding the specific method for bonding the heating element and the heat sink via the heat-conducting sheet, there are no particular limitations as long as the method can fix the heating element and the heat sink in a sufficiently tight contact state. For example, the following methods can be listed: placing the heat-conducting sheet between the heating element and the heat sink, fixing it with a clamp capable of applying pressure to about 0.05 MPa to 1.00 MPa, and then heating the heating element in this state, or heating it to about 80°C to 180°C using an oven or the like. Alternatively, the method of using a stamping press capable of heating and pressurizing at 80°C to 180°C and 0.05 MPa to 1.00 MPa can be listed.

[0095] In addition to clamps, screws, springs, and other clamps can also be used for fixing. However, to ensure a continuous and tight seal, it is preferable to further fix the joint using commonly used methods such as adhesives.

[0096] Semiconductor devices can be manufactured using heat-generating and heat-dissipating elements bonded together via thermal pads, as described above. There are no particular limitations on the types of semiconductor devices; examples include electronic materials incorporating integrated circuits (ICs) such as CPUs and memory. Additionally, examples include power devices such as bipolar transistors, power MOSFETs, and IGBTs.

[0097] Manufacturing Method of Heat Conductive Sheets In one embodiment of this disclosure, in a heat-conducting sheet used to heat and apply pressure to a heat-generating body and a heat-dissipating body disposed between them, thereby bonding the heat-generating body and the heat-dissipating body via the heat-conducting sheet, the compression ratio and thickness of the heat-conducting sheet can be selected such that the compression amount of the heat-conducting sheet satisfies the following formula.

[0098] Formula: C > L2 - L1 L1: The predicted warpage (μm) of the heating element under heating and pressurization. L2: The predicted warpage (μm) of the heating element when cooled to 25°C after heating and pressurization have ended. C: The predicted compression (μm) of the heat-conducting sheet under heating and pressurization conditions. C = Thickness of the heat-conducting sheet before pressurization (μm) × Compression ratio under heating and pressurization conditions (%) The predicted warpage (L1) of the heating element under heating and pressurization represents the predicted warpage of the heating element under the predicted pressure and temperature conditions. For example, in the case where the heating element and the heat sink are bonded together via a thermally conductive sheet under a pressure of 150°C and 0.10 MPa, it refers to the predicted warpage under the condition of applying pressure at 150°C and 0.10 MPa. Similarly, in the case where the heating element and the heat sink are bonded together via a thermally conductive sheet under a pressure of 150°C and 0.15 MPa, it refers to the predicted warpage under the condition of applying pressure at 150°C and 0.15 MPa.

[0099] The predicted warpage (L1) of the heating element during heating and pressurization can be, for example, 10 μm or more, 20 μm or more, or 25 μm or more. In addition, the predicted warpage (L1) of the heating element during heating and pressurization can be 80 μm or less, 70 μm or less, or 60 μm or less.

[0100] After heating and pressurization are completed, the predicted warpage (L2) of the heating element when cooled to 25°C can be, for example, 40 μm or more, 50 μm or more, or 60 μm or more. Alternatively, after heating and pressurization are completed, the predicted warpage (L2) of the heating element when cooled to 25°C can be 150 μm or less, 140 μm or less, or 130 μm or less.

[0101] The difference between L1 and L2 (L2 - L1) can be 30 μm or more, 40 μm or more, or 45 μm or more. Alternatively, the upper limit of this difference can be, for example, 120 μm or less.

[0102] The predicted compression (C) of the heat-conducting sheet under heating and pressurization conditions refers to the predicted compression of the heat-conducting sheet under the predicted pressure and temperature conditions. For example, when it is predicted that the heating element and the heat sink will be bonded together via the heat-conducting sheet under a pressure of 150°C and 0.10 MPa, it refers to the compression under the conditions of 150°C and 0.10 MPa. Similarly, when it is predicted that the heating element and the heat sink will be bonded together via the heat-conducting sheet under a pressure of 150°C and 0.15 MPa, it refers to the compression under the conditions of 150°C and 0.15 MPa.

[0103] The predicted compression (C) of the heat-conducting sheet under heating and pressurization conditions can be, for example, 20μm to 1000μm, 30μm to 200μm, or 40μm to 100μm.

[0104] The thickness of the heat-conducting sheet before pressurization refers to the average thickness of the heat-conducting sheet before pressurization. The preferred range of the thickness of the heat-conducting sheet is as described above.

[0105] The preferred range of the compression ratio of the heat-conducting sheet is as described above.

[0106] In the process of bonding the heat-conducting sheet to the heat-generating element and the heat-dissipating element, the heat-conducting sheet, manufactured by selecting the compression ratio and thickness as described above, has sufficient compression to keep up with the change in the warping of the heat-generating element, and therefore it is believed that the peeling of the heat-conducting sheet can be appropriately suppressed.

[0107]

Example

[0108] (Determination of compressive modulus and compression amount) In the measurements, a compression testing apparatus with a thermostatic bath (INSTRON 5948 Micro Tester (INSTRON)) was used. A heat-conducting sheet was cut into a circle with a diameter of 14 mm for testing. The heat-conducting sheet was sandwiched between 0.1 mm thick paper (release paper), and a load was applied to the thickness direction of the heat-conducting sheet at a displacement rate of 0.1 mm / min at a thermostatic bath temperature of 150°C. The displacement (mm) and load (N) were measured. The strain (dimensionless) calculated from displacement (mm) / thickness (mm) is plotted on the horizontal axis, and the strain calculated from load (N) / area (mm²) is plotted on the horizontal axis. 2 The calculated stress (MPa) is plotted on the vertical axis, with the slope at a stress of 0.10 MPa set as the compressive modulus (MPa). Additionally, the maximum displacement at arbitrary pressure is set as the compression (μm).

[0109] (Determination of adhesive strength) Using a universal physical property testing machine (texture tester (Eiko Seiki Co., Ltd.)), at 25°C (room temperature), a 7mm diameter probe is pressed onto a heat-conducting sheet with a load of 40N and held for 10 seconds. The area obtained by integrating the load and displacement curve when the probe is lifted is taken as the adhesive force (N·mm).

[0110] (Determination of thermal conductivity) A heat-conducting sheet was cut into 10mm squares and sandwiched between a transistor (2SC2233) serving as a heat source and a copper block serving as a heat sink. While pressing the transistor at 80°C and 0.14MPa, the transistor temperature T1 (°C) and the copper block temperature T2 (°C) were measured when current was applied. Based on the measured values ​​and the applied power W1 (W), the temperature per unit area (1cm²) was calculated as follows. 2 Thermal resistance value X (K·cm) 2 / W).

[0111] X = (T1 - T2) × 1 / W1 Furthermore, the thermal resistance value (K·cm) is used. 2 The thermal conductivity λ (W / (m·K)) was calculated as follows, along with the thickness t (μm).

[0112] λ = (t × 10) －6 ) / (X×10 －4 ) (Determination of warpage) The warpage was measured using a 3D heated surface shape measuring device (TherMoire PS200, AKROMETRIX). The warpage of the substrate corresponding to the chip area (20mm × 20mm) was measured.

[0113] The warpage of the substrate in the chip area at the assembly conditions of the package, i.e., 150°C, is 29 μm. Furthermore, the warpage of the substrate in the chip area at 25°C after assembly is 75 μm. Therefore, the difference in warpage is 46 μm.

[0114] (Bond area evaluation test) The bonding area was evaluated as follows. The bonding status was observed using an ultrasonic imaging diagnostic device (Insight-300, Insight Corporation) at 35 MHz using the reflection method. Furthermore, the image was binarized using image analysis software (ImageJ), and the proportion of the bonded area within a 20 mm square chip portion was calculated as the bonding area (%).

[0115] In the adhesive area evaluation test, a simple encapsulation was used as follows.

[0116] MCL-E-700G(R) (0.81 mm thick, Hitachi Chemical Co., Ltd.) was used in the substrate, CEL-C-3730N-2 (Hitachi Chemical Co., Ltd.) was used in the bottom filler, and silicone adhesive (SE4450, Dow Corning Toray Co., Ltd.) was used in the sealing material. Additionally, a 1 mm thick copper plate with nickel electroplating was used in the heat sink. The substrate and heat sink dimensions were set to 45 mm square, and the semiconductor chip dimensions were set to 20 mm square. At this point, the warpage of the heating element was 29 μm when a pressure of 0.15 MPa was applied at 150°C, and 75 μm when cooled to 25°C after pressure release.

[0117] The encapsulation assembly was performed as follows: A thermally conductive sheet of any thickness was cut into 23mm squares and attached to the heat sink. The semiconductor chip was placed on the heat sink with the thermally conductive sheet in between. Using a high-precision pressure-heat bonding device (HTB-MM, Alpha Design Co., Ltd.), pressure was applied to the thickness direction of the thermally conductive sheet at any temperature and pressure for 3 minutes. Afterward, it was treated in a constant temperature bath at 150°C for 2 hours to allow the sealant to fully cure.

[0118] <Example 1> A 0.3 mm thick thermal conductive sheet manufactured by Hitachi Chemicals Co., Ltd., with a compressive modulus of 1.16 MPa at 150°C and a compressive stress of 0.10 MPa, an adhesive strength of 7.6 N·mm at 25°C, and a thermal conductivity of 21 W / (m·K), was selected. The sheet was assembled and packaged using the method described above at 150°C and 0.15 MPa, bonding the thermal conductive sheet to a semiconductor chip (heat source) and a heat sink (heat sink). At this point, the bonding area was 99%. The bonding area, an indicator of warp follower performance, was over 90%, demonstrating excellent warp follower performance. The compression ratio of the thermal conductive sheet at 150°C and 0.15 MPa compression was 19%, and the compression amount was 57 μm.

[0119] <Example 2> A 0.2 mm thick thermal conductive sheet manufactured by Hitachi Chemicals Co., Ltd., with a compressive modulus of 1.16 MPa at 150°C and a compressive stress of 0.10 MPa, an adhesive strength of 7.6 N·mm at 25°C, and a thermal conductivity of 18 W / (m·K), was selected. The sheet was assembled and packaged using the method described above at 150°C and 0.15 MPa, bonding the thermal conductive sheet to a semiconductor chip (heat source) and a heat sink (heat sink). At this point, the bonding area was 95%. The bonding area, an indicator of warp follower performance, was over 90%, demonstrating excellent warp follower performance. The compression ratio of the thermal conductive sheet at 150°C and 0.15 MPa compression was 21%, and the compression amount was 47 μm.

[0120] <Example 3> A 0.15 mm thick thermal conductive sheet manufactured by Hitachi Chemicals Co., Ltd. was selected. This sheet had a compressive modulus of 1.16 MPa at 150°C with a compressive stress of 0.10 MPa, an adhesive strength of 7.6 N·mm at 25°C, and a thermal conductivity of 16 W / (m·K). The sheet was assembled and packaged using the method described above at 150°C and 0.31 MPa, bonding it to a semiconductor chip (heat source) and a heat sink (heat sink). At this point, the bonding area was 90%. The compression ratio of the thermal conductive sheet at 150°C and 0.31 MPa was 35%, and the compression amount was 52 μm.

[0121] <Comparative Example 1> As a thermally conductive material, a liquid silicone grease (manufactured by Sanwa Supply, TK-P3K) with a thermal conductivity of 2 W / (m·K) was selected. It was assembled and encapsulated using the method described above at 150°C and 0.03 MPa, and bonded to the semiconductor chip (heat-generating element) and the heat sink (heat-dissipating element). However, due to its liquid state, the compressive modulus and adhesive force could not be measured. The assembled silicone grease had a thickness of 40 μm and a bonding area of ​​63%.

[0122] <Comparative Example 2> A 0.3 mm thick thermal conductive sheet manufactured by Hitachi Chemicals Co., Ltd. was selected. It had a compressive modulus of 1.44 MPa at 150°C with a compressive stress of 0.10 MPa, an adhesive strength of 7.2 N·mm at 25°C, and a thermal conductivity of 20 W / (m·K). The sheet was assembled and packaged using the method described above at 150°C and 0.15 MPa, bonding it to a semiconductor chip (heat source) and a heat sink (heat sink). At this point, the bonding area was 72%. The compression ratio of the thermal conductive sheet at 150°C and 0.15 MPa was 13%, and the compression amount was 40 μm.

[0123] <Comparative Example 3> A 0.3 mm thick thermal conductive sheet manufactured by Hitachi Chemicals Co., Ltd. was selected. It has a compressive modulus of 1.73 MPa at 150°C with a compressive stress of 0.10 MPa, an adhesive strength of 1.8 N·mm at 25°C, and a thermal conductivity of 23 W / (m·K). The sheet was assembled and packaged using the method described above at 150°C and 0.15 MPa, bonding it to a semiconductor chip (heat source) and a heat sink (heat sink). At this point, the bonding area was 74%. The compression ratio of the thermal conductive sheet at 150°C and 0.15 MPa was 13%, and the compression amount was 44 μm.

[0124] <Comparative Example 4> A 0.3 mm thick thermal conductive sheet manufactured by Hitachi Chemicals Co., Ltd. was selected. It has a compressive modulus of 1.35 MPa at 150°C with a compressive stress of 0.10 MPa, an adhesive strength of 3.8 N·mm at 25°C, and a thermal conductivity of 18 W / (m·K). The sheet was assembled and packaged using the method described above at 150°C and 0.15 MPa, bonding it to a semiconductor chip (heat source) and a heat sink (heat sink). At this point, the bonding area was 78%. The compression ratio of the thermal conductive sheet at 150°C and 0.15 MPa was 16%, and the compression amount was 48 μm.

[0125] As described above, when the heating element and the heat sink are bonded using the method of the embodiment, the bonding area can be well maintained. Therefore, it is believed that a semiconductor device with excellent heat dissipation can be obtained.

[0126] As with individual documents, patent applications and technical standards, which are individually described and incorporated by reference, all documents, patent applications and technical standards described in this specification are incorporated by reference in this specification.

[0127] Explanation of reference numerals in the attached figures 1 heat-conducting sheet 2. Semiconductor chip (heating element) 3. Heat sink (heat dissipation unit) 4 substrates 5 Bottom packing 6. Sealing materials a. Heating element section (analytical range) b. Warpage amount.

Claims

1. A method for manufacturing a semiconductor device, comprising the following steps: For a heat-generating element and a heat-dissipating element with heat-conducting sheets disposed between them, pressure is applied in the thickness direction of the heat-conducting sheets to bond the heat-generating element and the heat-dissipating element together via the heat-conducting sheets. The heat-conducting sheets have a compressive elastic modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C and an adhesive force of 5.0 N·mm or more at 25°C.

2. The method for manufacturing a semiconductor device as described in claim 1, wherein, The thermal conductivity of the heat-conducting sheet, determined by the thermal resistance measured using the steady-state method, is above 7 W / (m·K).

3. The method for manufacturing a semiconductor device as described in claim 1 or 2, wherein, The pressure is 0.05 MPa to 10.00 MPa.

4. The method for manufacturing a semiconductor device as described in claim 3, wherein, The pressure is 0.10 MPa to 0.50 MPa.

5. The method for manufacturing a semiconductor device according to any one of claims 1 to 4, wherein, The heating element is a semiconductor chip, and the heat sink is a heat sink.

6. The method for manufacturing a semiconductor device according to any one of claims 1 to 5, wherein, The area of ​​the surface of the heating element opposite the heat-conducting plate is 25mm². 2 above.

7. The method for manufacturing a semiconductor device according to any one of claims 1 to 4, wherein, The heating element is a semiconductor package with a heat sink, and the heat sink is a heat sink fin.

8. The method for manufacturing a semiconductor device according to any one of claims 1 to 4, wherein, The heating element is a semiconductor module.

9. A method for manufacturing a semiconductor device according to any one of claims 1 to 8, wherein, The area of ​​the surface of the heating element opposite the heat-conducting plate is 100 mm². 2 above.

10. A heat-conducting sheet having a compressive elastic modulus of 1.40 MPa or less when the compressive stress is 0.10 MPa at 150°C and an adhesive force of 5.0 N·mm or more at 25°C, disposed between a heat-generating element and a heat-dissipating element of a semiconductor device, for bonding the heat-generating element and the heat-dissipating element.

11. The heat-conducting sheet as claimed in claim 10, wherein, The thermal conductivity, determined by the thermal resistance measured by the steady-state method, is above 7 W / (m·K).

12. The heat-conducting sheet as claimed in claim 10 or 11, wherein, The heating element is a semiconductor chip, and the heat sink is a heat sink.

13. The heat-conducting sheet as claimed in claim 10 or 11, wherein, The heating element is a semiconductor package with a heat sink, and the heat sink is a heat sink fin.

14. The heat-conducting sheet as claimed in claim 10 or 11, wherein, The heating element is a semiconductor module.

15. A method for manufacturing a heat-conducting sheet, comprising the following steps: The compression ratio and thickness of the heat-conducting sheet are selected such that the compression of the heat-conducting sheet satisfies the following formula. This heat-conducting sheet is used to heat and apply pressure to a heating element and a heat sink, which are disposed between each other, in the thickness direction of the heat-conducting sheet, thereby bonding the heating element and the heat sink together via the heat-conducting sheet. Formula: C > L2 - L1, L1: The predicted warpage (μm) of the heating element during heating and pressurization. L2: The predicted warpage (μm) of the heating element when cooled to 25°C after the heating and pressurization process has ended. C: The predicted compression (μm) of the heat-conducting sheet under heating and pressurization conditions. C = Thickness of the heat-conducting sheet before pressurization (μm) × Compression ratio under heating and pressurization conditions (%).

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