Semiconductor device manufacturing method

Abstract

To provide a method of manufacturing a semiconductor device that simply determines and selects heat-conductive grease of high reliability with high pump-out suppression effect, and uses the heat-conductive grease of high reliability.SOLUTION: There is provided a method of manufacturing a semiconductor device that includes the processes of: obtaining shearing stress FT of heat-conductive grease as a candidate with which a loss elastic modulus / storage elastic modulus is 1 at temperature T; selecting heat-conductive grease which satisfies the FT of 100-200 Pa within a management temperature range of the semiconductor device; and attaching a resin sealed body 10 including a semiconductor element 1 mounted on a laminate substrate 2 to a cooler 9 through the heat-conductive grease 8 selected in the selecting process.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for manufacturing a semiconductor device which is excellent in the effect of suppressing pump-out of thermal conductive grease and has high long-term reliability. [Background technology]

[0002] Power modules used for power control switching applications generally have a structure in which semiconductor elements wired with metal wires, lead frames, and metal pins are soldered onto a laminated substrate and then sealed with resin. A cooler is attached to such a power module to cool the heat generated by the semiconductor elements. Normally, the contact surface between the power module and the cooler has unevenness due to surface roughness, resulting in the formation of an air layer with low thermal conductivity. Therefore, in order to improve heat dissipation at the contact surface, thermal conductive grease, which is a viscoelastic material with high thermal conductivity, is applied between the power module and the cooler.

[0003] When thermal conductive grease is applied to a power module, the power module warps and deforms due to heat generated by the semiconductor elements, causing the thermal conductive grease to be pushed out of the power module, resulting in a mechanical pump-out. As a result, the thermal resistance of the power module may increase. In addition, during high-temperature operation of the power module, the thermal resistance of the power module may also increase due to chemical factors of the thermal conductive grease, such as oil seepage, oil volatilization, and oxidative degradation.

[0004] As a method for evaluating the reliability of thermal conductive grease, ΔTj power cycle tests and ΔTc power cycle tests have generally been performed. However, these test methods require a large amount of test time. Therefore, as a simpler alternative method, an evaluation method for identifying thermal conductive greases with a high pump-out suppression effect has been proposed (see, for example, Patent Document 1). Patent Document 1 discloses a method for evaluating thermal conductive greases by obtaining a yield value from the y-axis intercept of a straight line plotting the 1 / 2 power of shear stress and the 1 / 2 power of shear rate based on the Casson equation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2012-4468 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, the method proposed in Patent Document 1 only evaluates the yield value of the thermal grease at 20° C., which is not sufficient for evaluation assuming the actual operating temperature of a power module. On the other hand, when evaluation is performed by a power cycle test simulating the actual operation, it takes a lot of time.

[0007] In view of the above problems, an object of the present invention is to provide a method for easily identifying and selecting highly reliable thermal conductive grease with a high pump-out suppression effect, and for manufacturing a highly reliable semiconductor device using the selected thermal conductive grease. [Means for solving the problem]

[0008] As a result of intensive research, the inventors have come up with the idea of ​​obtaining the pump-out characteristics of the thermal grease by evaluating the shear stress value when the storage modulus and loss modulus of the thermal grease become equal within the controlled temperature range of the semiconductor device, and have completed the present invention. That is, according to one embodiment, the present invention is a method for manufacturing a semiconductor device, comprising the steps of: The shear stress F at which the loss modulus / storage modulus = 1 at temperature T for a candidate thermal conductive grease T and Within the controlled temperature range of semiconductor equipment, F T selecting a thermal conductive grease having a thermal conductivity of 100 Pa or more and 200 Pa or less; a step of attaching the resin sealing body including the semiconductor element mounted on the laminated substrate to a cooler via the thermal conductive grease selected in the selecting step; Includes.

[0009] In the method for manufacturing a semiconductor device, the controlled temperature range is preferably from 25°C to 175°C.

[0010] The method for manufacturing a semiconductor device preferably further comprises the step of measuring the storage modulus and loss modulus of the candidate thermal conductive grease.

[0011] In the method for manufacturing a semiconductor device, the measuring step is preferably performed by vibration rheometer measurement.

[0012] In the method for manufacturing a semiconductor device, it is preferable that the resin encapsulant has a longitudinal dimension / transverse dimension ratio of 1.5 or more.

[0013] In the method for manufacturing a semiconductor device, the semiconductor element is preferably selected from Si or SiC. Effect of the Invention

[0014] According to the present invention, it is possible to evaluate the pump-out characteristics when thermally conductive grease is applied to an actual device, thereby enabling an efficient manufacturing method for semiconductor devices that reduces the development time required to consider the application of thermally conductive grease. [Brief description of the drawings]

[0015] [Figure 1]FIG. 1 is a conceptual cross-sectional view showing the configuration of a power module according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a photograph of the back surface of the conductive plate in a power module manufactured using Grease 1 of the example after a power cycle test was performed. [Diagram 3] FIG. 3 is a photograph of the back surface of the conductive plate in a power module manufactured using Grease 3 of the example after a power cycle test was performed. [Figure 4] Figure 4 shows the shear stress-strain curves of five types of thermal conductive grease at 25°C. [Diagram 5] Figure 5 shows the shear stress-strain curves of five types of thermal conductive grease at 125°C. [Figure 6] Figure 6 shows the shear stress-strain curves of five types of thermal conductive grease at 175°C. [Figure 7] FIG. 7 is a graph in which the shear stress (yield stress) at a strain of 300% in each of the graphs of FIGS. 4 to 6 is plotted with the temperature on the horizontal axis. [Figure 8] FIG. 8 is a graph showing the ratio of storage modulus GT' to loss modulus GT'' (tan δT) obtained when the shear stress was swept at T = 25°C for five types of thermal conductive grease. [Figure 9] FIG. 9 is a graph showing the ratio (tan δT) of the storage modulus GT' to the loss modulus GT'' obtained when the shear stress was swept at T = 125°C for five types of thermal conductive grease. [Figure 10] FIG. 10 is a graph showing the ratio of storage modulus GT' to loss modulus GT'' (tan δT) obtained when the shear stress was swept at T = 175°C for five types of thermal conductive grease. [Figure 11] FIG. 11 is a graph plotting FT obtained from FIGS. 8 to 10 against temperature T (25° C., 125° C., 175° C.) for five types of thermally conductive grease. [Figure 12]FIG. 12 is a graph plotting the ratio of storage modulus GT' to loss modulus GT'' (tan δT) versus temperature T for five types of thermal conductive grease. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiment described below.

[0017] According to one embodiment, the present invention relates to a method for manufacturing a semiconductor device. The method for manufacturing a semiconductor device according to the present embodiment includes the following steps. (1) The shear stress F at which the loss modulus / storage modulus = 1 at temperature T for the candidate thermal conductive grease T The process of obtaining (2) Within the controlled temperature range of the semiconductor device, F T A process to select thermal conductive grease that satisfies 100 Pa to 200 Pa. (3) A step of attaching a resin encapsulant including a semiconductor element mounted on a laminated substrate to a cooler via the thermal conductive grease selected in the selecting step.

[0018] A semiconductor device manufactured by the above manufacturing method will now be described. The semiconductor device may be a resin sealant including a semiconductor element mounted on a laminated substrate, which is attached to a cooler via a selected, predetermined thermally conductive grease.

[0019] 1 is a conceptual cross-sectional view of a power module, which is an example of a semiconductor device manufactured by the manufacturing method according to this embodiment. The illustrated power module has a resin sealing body 10 disposed on a cooler 9 via thermally conductive grease 8. The resin sealing body 10 is formed by sealing a semiconductor element 1 bonded to a laminated substrate 2 with a solder bonding layer 3, an implant pin 5 bonded to the semiconductor element 1 with a solder bonding layer 4, and a wiring substrate 6 connected to the implant pin 5 with a sealing resin 7.

[0020] The semiconductor element 1 is a power chip such as an IGBT (Insulated Gate Bipolar Transistor) or a diode chip, and various Si devices, SiC devices, GaN devices, etc. may be used. These devices may also be used in combination. For example, a hybrid module using a Si-IGBT and a SiC-SBD may be used. The number of semiconductor elements mounted on the laminated substrate may be one or more.

[0021] The laminated substrate 2 can be composed of an insulating substrate 22, a first conductive plate 21 formed on one of its main surfaces, and a second conductive plate 23 formed on the other main surface. The insulating substrate 22 can be made of a material having excellent electrical insulation and thermal conductivity. Examples of the material for the insulating substrate 22 include Al2O3, AlN, and SiN. In particular, for high-voltage applications, a material having both electrical insulation and thermal conductivity is preferable, and AlN and SiN can be used, but are not limited to these. The first conductive plate 21 and the second conductive plate 23 can be made of metal materials such as Cu and Al, which have excellent workability. Among such conductive plates made of Cu and the like, the second conductive plate 23 that is not in contact with the semiconductor element 1 is sometimes referred to as a back copper foil or a back conductive plate. The first conductive plate 21 and the second conductive plate 23 may be Cu or Al that has been treated with Ni plating or the like for the purpose of rust prevention or the like. The first conductive plate 21 and the second conductive plate 23 may be disposed on the insulating substrate 22 by a direct bonding method (direct copper bonding method) or an active metal brazing method. Although not shown, a modified example of the laminated substrate may be one in which two first conductive plates are discontinuously provided on the insulating substrate, one of which functions as an electrode bonded to a semiconductor element and the other as an electrode connected to a conductive connection member such as a lead frame.

[0022] The solder bonding layers 3 and 4 can be formed using lead-free solder. For example, Sn-Ag-Cu, Sn-Sb, Sn-Sb-Ag, Sn-Cu, Sn-Sb-Ag-Cu, Sn-Cu-Ni, Sn-Ag solder alloys can be used, but are not limited to these. The bonding layers can also be formed using a connecting material containing fine metal particles, such as a sintered body of nano silver particles.

[0023] A polyimide film substrate or an epoxy film substrate having a conductive layer of Cu, Al, or the like formed thereon can be used as the wiring board 6. A copper pin using copper can be used as the implant pin 5. Both the conductive layer of the wiring board 6 and the implant pin 5 may be Cu or Al that has been treated with Ni plating or the like for the purpose of rust prevention or the like.

[0024] The thermally conductive grease 8 is applied in a layer between the resin encapsulant 10 and the cooler 9. In particular, the thermally conductive grease 8 comes into contact with the second conductive plate 23 exposed from the resin encapsulant 10, and transmits the heat generated by the semiconductor element 1 to the cooler 9. The manufacturing method of the present invention includes a step of selecting the thermally conductive grease 8 having specific physical properties in a specific temperature range. Details will be described later.

[0025] The cooler 9 is made of a metal such as copper or aluminum, which has excellent thermal conductivity. The cooler 9 may be coated with Ni or a Ni alloy to prevent corrosion. The cooler 9 may be plate-shaped or may have cooling fins. The cooler 9 may be a water-cooled cooler configured so that a coolant such as water passes through it. It is preferable that the surface of the cooler 9, which faces the back surface of the resin sealing body 10 via thermal conductive grease (the upper surface of the cooler 9), is flat.

[0026] The sealing resin 7 may be any resin that is generally used for insulating and sealing semiconductor devices, and may include, but is not limited to, a thermosetting resin base and an inorganic filler, and may optionally further include a curing agent, a curing accelerator, and additives.

[0027] The thermosetting resin base is not particularly limited, and examples thereof include epoxy resin, phenolic resin, maleimide resin, etc. Among them, epoxy resin having at least two epoxy groups in one molecule is particularly preferred because of its high dimensional stability, water resistance, chemical resistance, and electrical insulation. Specifically, it is preferred to use aliphatic epoxy resin, alicyclic epoxy resin, or a mixture thereof.

[0028] Aliphatic epoxy resin refers to an epoxy compound in which the carbon to which the epoxy group is directly bonded is a carbon that constitutes an aliphatic hydrocarbon. Therefore, even if the compound contains an aromatic ring in the main skeleton, it is classified as an aliphatic epoxy resin if it satisfies the above conditions. Aliphatic epoxy resins can be used alone or in combination of two or more types. In addition, naphthalene-type epoxy resins and multifunctional epoxy resins with three or more functionalities have high glass transition temperatures, so the inclusion of these epoxy resins can improve heat resistance.

[0029] The term "alicyclic epoxy resin" refers to an epoxy compound in which two carbon atoms constituting an epoxy group constitute an alicyclic compound. Examples of alicyclic epoxy resins include monofunctional epoxy resins, bifunctional epoxy resins, and polyfunctional epoxy resins having three or more functional groups, but are not limited thereto. The alicyclic epoxy resin can be used alone or in combination with two or more different alicyclic epoxy resins. When an alicyclic epoxy resin is mixed with an acid anhydride curing agent and cured, the glass transition temperature increases, so that the aliphatic epoxy resin can be mixed with an alicyclic epoxy resin to achieve high heat resistance. The thermosetting resin base may be a mixture of the above-mentioned aliphatic epoxy resin and alicyclic epoxy resin, and the mixing ratio when mixed may be arbitrary.

[0030] The inorganic filler may be a metal oxide or a metal nitride, and examples thereof include, but are not limited to, fused silica, silica (silicon oxide), alumina, aluminum hydroxide, titania, zirconia, aluminum nitride, talc, clay, mica, glass fiber, etc. The inorganic filler may be used alone or in combination of two or more. In addition, these inorganic fillers may be microfillers or nanofillers, and two or more inorganic fillers with different particle sizes and / or types may be used in combination.

[0031] The thermosetting resin composition according to the present embodiment may contain a curing agent as an optional component. The curing agent is not particularly limited as long as it reacts with the thermosetting resin base, preferably the epoxy resin base, and can be cured, but it is preferable to use an acid anhydride-based curing agent, such as an aromatic acid anhydride, a cyclic aliphatic acid anhydride, or an aliphatic acid anhydride. The amount of the curing agent is preferably 50 parts by mass or more and 170 parts by mass or less, and more preferably 80 parts by mass or more and 150 parts by mass or less, relative to 100 parts by mass of the epoxy resin base.

[0032] The thermosetting resin composition may further contain a curing accelerator as an optional component. As the curing accelerator, imidazole or its derivatives, tertiary amines, boric acid esters, Lewis acids, organometallic compounds, organic acid metal salts, etc. may be appropriately blended. The amount of the curing accelerator added is preferably 0.01 parts by mass or more and 50 parts by mass or less, more preferably 0.1 parts by mass or more and 20 parts by mass or less, based on 100 parts by mass of the thermosetting resin base.

[0033] The thermosetting resin composition may also contain optional additives to the extent that the properties of the composition are not impaired. Examples of additives include, but are not limited to, flame retardants, pigments for coloring the resin, and plasticizers and silicone elastomers for improving crack resistance. These optional components and the amounts of the components added can be appropriately determined by those skilled in the art according to the specifications required for the semiconductor device and / or the encapsulant.

[0034] The semiconductor device manufactured in the present invention is not limited to the illustrated embodiment and shape, and various modifications are possible. The resin encapsulant may have a resin case, in which the laminated substrate and the semiconductor element are resin-encapsulated. The electrodes and output terminals of the semiconductor element and the laminated substrate may be connected by a lead frame, which is a conductive connecting member, and the semiconductor element and the external terminal may be connected by an aluminum wire. The wiring substrate may be encapsulated by the encapsulating resin, or may be provided outside the encapsulating resin. The encapsulating resin may have a single-layer structure made of one type of composition, or a two- or more-layer laminate structure made of two or more types of compositions. These modifications, not shown, are also included in the resin encapsulant according to the present invention.

[0035] In either embodiment, the second conductive plate and the sealing resin on the back surface of the laminated substrate are exposed on the back surface of the resin sealing body. The exposed surface (back surface) of the second conductive plate and the sealing resin surface may be approximately flush with each other, the second conductive plate may protrude from the sealing resin surface, or the sealing resin surface may protrude from the exposed surface of the second conductive plate. The embodiment in which the sealing resin surface protrudes from the back surface of the second conductive plate and the sealing resin is provided so as to surround the periphery of the second conductive plate is preferable because the heat conductive grease is less likely to be pumped out. The protrusion amount (step) of the sealing resin surface from the back surface of the second conductive plate is preferably 10 to 100 μm. Protrusion exceeding 100 μm is undesirable because it increases the thermal resistance. Furthermore, in order for the sealing resin surface to protrude downward from the second conductive plate surface even if the second conductive plate is warped due to thermal deformation of the module, the protrusion amount is preferably 10 μm or more. Moreover, since the average particle size of the filler that can be contained in the thermal conductive grease is usually about 1 μm to 100 μm, the protrusion amount is preferably 10 μm or more to suppress pump-out. Moreover, when the second conductive plate protrudes from the sealing resin surface, it is preferable that the protrusion amount is 100 μm or less. If the second conductive plate protrudes more than 100 μm, the thermal conductive grease may be easily pumped out toward the sealing resin surface, which may be undesirable.

[0036] The semiconductor device may further include a heat sink bonded to the resin encapsulant with a bonding material such as solder under the laminated substrate. The heat sink is made of metal or the like and serves to dissipate heat generated from the semiconductor element. In this way, when the heat sink is exposed on the back surface of the resin encapsulant, the heat sink is connected to the cooler via thermally conductive grease. In either case, the semiconductor device is manufactured in a manner that includes a step of attaching the resin encapsulant to the cooler via thermally conductive grease, which is one step of the present invention described below.

[0037] Next, specific steps in the method for manufacturing a semiconductor device according to the present invention will be described. (1)F T The process of obtaining In the first step, the shear stress at which the loss modulus / storage modulus = 1 at temperature T in the controlled temperature range is obtained for a candidate thermal conductive grease. In this specification, the shear stress at which the loss modulus / storage modulus = 1 at temperature T measured for the thermal conductive grease is referred to as F T It is defined as:

[0038] The candidate thermal conductive grease in this process may be a thermal conductive grease generally used in semiconductor devices, and may include a grease and a filler. The grease may be, for example, a silicone-based or non-silicone-based base oil, a thickener having a fibrous structure, and various additives. The thickener may be either one containing a metal soap such as lithium soap or one containing a non-soap component such as bentonite. The additive may include a general additive such as an antioxidant, a rust inhibitor, or a corrosion inhibitor. The filler may be alumina, pure aluminum, zinc oxide, or the like, which have high thermal conductivity. The filler can be added to the thermal conductive grease to improve thermal conductivity. The average particle size of the filler can be 1 μm to 100 μm, and more preferably 10 μm to 50 μm. In this range, the F T The particle size of the filler can be easily adjusted to the desired range. If the particle size is too small, the filler will be poorly dispersible in the grease and will be prone to aggregation, while if the particle size is too large, it may adversely affect the viscoelasticity of the thermal conductive grease. The particle size of the filler is related to the film thickness of the thermal conductive grease, and is preferably smaller than the film thickness of the desired thermal conductive grease. The preferred film thickness when applying the thermal conductive grease will be described later.

[0039] The candidate thermal conductive grease can be selected from commercially available products, or a composition containing the above-mentioned components can be appropriately prepared, and the selection range of the candidates is not particularly limited. A person skilled in the art can appropriately determine the candidate thermal conductive grease from the viewpoint of heat resistance, viscosity, thermal conductivity, etc. required in the operating temperature range of the semiconductor device, which is from about -40 ° C to about 175 ° C. In addition, when using a commercially available thermal conductive grease as a candidate, it is not preferable to select a thermal conductive grease that has been subjected to an operation to adjust the physical properties such as the viscosity of the commercially available product, such as heating, light irradiation, addition or removal of components, etc. as a candidate. This is because, in the manufacture of a semiconductor device, if the thermal conductive grease after the operation is used, the characteristics may vary greatly, process management may increase, and the quality of the product may vary.

[0040] In the present invention, the controlled temperature range of the semiconductor device refers to a temperature range from room temperature to the upper limit of the operating temperature of the semiconductor device, and can be expressed as t1≦T≦t2, where t1 is the lower limit and t2 is the upper limit. It is known that the effect of temperature on the pump-out characteristics of the thermal conductive grease is small in a temperature range lower than room temperature. Therefore, if the pump-out characteristics of the thermal conductive grease are controlled in a temperature range from room temperature to the upper limit of the operating temperature of the semiconductor device, the reliability within the operating temperature range of the semiconductor device can be ensured. The values ​​of the lower limit t1 and the upper limit t2 vary depending on the type and use of the semiconductor device to be manufactured, and are not limited in the present invention. In general, the lower limit t1 may be room temperature, for example, 20 to 25°C. The upper limit t2 may be, for example, about 150 to 200°C. As an example, the lower limit t1 may be 25°C, and the upper limit t2 may be 175°C.

[0041] The storage modulus and loss modulus are physical property values ​​of the thermal conductive grease, which is a viscoelastic body, and both values ​​can depend on temperature. The storage modulus is also expressed as G', and represents the elasticity (hardness) of the thermal conductive grease, which is a viscoelastic body. The loss modulus is also expressed as G'', and represents the viscosity of the thermal conductive grease. The loss modulus G'' / storage modulus G' obtained in this process is sometimes expressed as tan δ, also known as the loss tangent. Tan δ is a parameter that represents the contribution of viscosity in a viscoelastic body. In this specification, the storage modulus at a given temperature T is defined as G'. T and loss modulus G'' T Also, G'' T / G' T tan δ T It may be written as:

[0042] Storage modulus G T ' and loss modulus G T The values ​​of " and " can be obtained by vibration rheometer measurement at a specified temperature T (°C). The condition of the indentation thickness during measurement can be about 10 times the minimum possible grease application thickness (bond line thickness: BLT), and is preferably about 500 μm or more. If the application thickness is small, noise and errors caused by interactions between filler particles, such as collisions and contacts, can be prevented. In addition, the strain application The frequency of the measurement is preferably 1 Hz. The indentation thickness during measurement is the thickness of the grease film measured using a vibration rheometer. T = 1. T can be obtained by sweeping the shear stress in an oscillatory rheometer measurement at a given temperature T (°C). Specifically, in Figs. 8 to 10 of the examples described later, tan δ on the vertical axis T is the shear stress (horizontal axis) when tan δ is 1. T = 1 represents the stress required when the thermal conductive grease, which is a viscoelastic material, changes from elastic to viscous at the given temperature T.

[0043] tan δ TSince the value of depends on temperature T, in the temperature range satisfying t1≦T≦t2, tan δ at temperature T T , and tan δ T = 1. T We obtain the shear stress F T is preferably obtained at at least three different temperatures that satisfy the condition t1≦T≦t2. This is because it is preferable to perform measurements that take into account the case where elasticity and viscosity do not change monotonically with respect to temperature. In addition, in the temperature characteristics of tan δ demonstrated in the examples described later (FIG. 12), tan δ has a maximum value at 175°C, and it is possible to grasp the behavior by examining at least three points. For example, the shear stress F at T=t1 is t1 , shear stress F at T=t2 t2、 Temperature t that is approximately halfway between t1 and t2 m Shear stress F at tm More specifically, when t1 is 25° C. and t2 is 175° C., t m can be measured at three points T = 25°C, 125°C, and 175°C. However, the shear stress F T may be measured at four or more points.

[0044] (2) Process for selecting thermal conductive grease In the second process, the semiconductor device is heated to a controlled temperature range. T Select a thermal conductive grease with a thermal conductivity of 100 Pa or more and 200 Pa or less. In this process, F at multiple measurement temperatures T within the range of t1≦T≦t2 is T Candidate thermal greases that satisfy the condition are extracted, with the condition that the pressure drop is within the range of 100 Pa to 200 Pa. Thermal greases that satisfy the condition can be determined to have a pump-out suppression effect within the specified controlled temperature range, t1≦T≦t2, of the semiconductor device to be manufactured, and are selected as components of the semiconductor device to be used in the third process described below.

[0045] Here, pump-out will be explained. Pump-out refers to a phenomenon in which the power module itself, particularly the resin sealant, repeatedly deforms (warps) when it undergoes repeated temperature changes due to power cycles, etc., and the thermally conductive grease is discharged to the outside. The first and second steps of the manufacturing method according to this embodiment allow the selection of thermally conductive grease that is less likely to pump out. In general, even if the same thermally conductive grease is used, the more the power module warps, the more likely it is to cause pump-out, depending on the shape of the power module. Therefore, the selection of thermally conductive grease in the first and second steps is particularly effective in suppressing pump-out in a module in which multiple semiconductor elements are arranged in the longitudinal direction and in which the ratio of the longitudinal dimension to the lateral dimension is large. The longitudinal dimension and the lateral dimension referred to here refer to the dimensions of the resin sealant in a cross section parallel to the laminated substrate surface. A large ratio of the longitudinal dimension to the lateral dimension refers to, for example, a value of the longitudinal dimension / lateral dimension of 1.5 or more. The value of the longitudinal dimension / transverse dimension that is effective in suppressing pump-out may be, for example, about 1.5 to 4, and may also be about 2 to 3.5, or about 2.5 to 3. An example of a module having a large ratio of longitudinal dimension to transverse dimension is a 2-in-1 type having a half bridge composed of two laminated substrates in one module, as shown in Fig. 1. Similarly, pump-out can be effectively prevented by selecting the thermal conductive grease of the present invention in a 4-in-1 type having two 2-in-1 sets and a 6-in-1 type having three 2-in-1 sets.

[0046] The second step may optionally include a step of evaluating the heat resistance of the thermal grease in the controlled temperature range of the semiconductor device and selecting the thermal grease. The heat resistance is evaluated by measuring the 1% weight loss temperature of the oil contained in the thermal grease. If the 1% weight loss temperature is higher than the controlled temperature range defined above, the thermal grease can be determined to be heat resistant. If the 1% weight loss temperature is within the controlled temperature range or lower, the oil contained in the thermal grease is likely to volatilize, which may reduce the long-term reliability of the semiconductor device, and therefore the thermal grease can be determined to be undesirable.

[0047] The candidate thermal grease is F T If the condition above is not met, it can be determined that the thermal grease does not have sufficient pump-out suppression effect according to the criteria of this process and should not be selected as a component of the semiconductor device. Optionally, if the candidate thermal grease does not meet the condition of 1% weight loss temperature of oil, it can be determined that the long-term reliability is insufficient and should not be selected as a component of the semiconductor device.

[0048] The first and second steps can be said to be a method for evaluating thermal grease that can suppress pump-out at the controlled temperature of a semiconductor device. Therefore, the present invention also relates to a method for evaluating the reliability of thermal grease, which includes the above-mentioned first and second steps.

[0049] (3) Installation process The third step is a step of attaching the resin encapsulant 10 including the semiconductor element 1 mounted on the laminated substrate 2, as exemplified in FIG. 1, to the cooler 9 via the thermally conductive grease 8 selected in the selecting step. This step can also be said to be a step of assembling a semiconductor device using the selected thermally conductive grease 8. The manufacture of the resin encapsulant 10 can be carried out by a normal method suitable for the structure of the encapsulant, and is not particularly limited. For example, in the case of manufacturing the semiconductor device shown in the figure, the laminated substrate 2, the semiconductor element 1, the implant pin 5, and the wiring board 6 are joined by the solder bonding layers 3 and 4, and placed on an appropriate mold. Next, the encapsulating resin 7 is filled into the mold, and is molded by thermally curing under heating temperature and heating time conditions suitable for the encapsulating resin 7. The embodiment shown in the figure is not limited, and the resin encapsulant may be molded so that at least the back surface of the conductive plate is exposed. Examples of the molding method include vacuum casting, transfer molding, liquid transfer molding, potting, and the like, but are not limited to a specific molding method.

[0050] In this way, the resin sealing body 10 is manufactured, the thermal conductive grease 8 is applied, and the resin sealing body 10 is combined with the cooler 9 to form a semiconductor device. Specifically, the thermal conductive grease is applied to the back surface of the resin sealing body 10, the resin sealing body 10 is placed at a predetermined location on the cooler 9, and the resin sealing body 10 and the cooler 9 are fixed by a fastening member such as a screw. At this time, it is preferable to form the thermal conductive grease at least on the entire back surface of the second conductive plate 23. In order to improve the heat dissipation performance, it is more preferable to form the thermal conductive grease on the entire back surface of the resin sealing body 10 including the exposed surface of the second conductive plate 23. Alternatively, a layer of the thermal conductive grease 8 may be formed at a predetermined location on the cooler 9 using a mask or the like, and the resin sealing body 10 may be placed and fastened. The mask may be a metal mask or the like having a plurality of circular or polygonal openings. The thickness of the thermal conductive grease when applied can be about 70 to 150 μm. The thickness of the film when applied can be adjusted by the thickness of the mask. After application, the film thickness after fastening the cooler 9 to the resin sealing body 10 by a fastening member can be about 35 to 75 μm. From the viewpoint of thermal resistance, it is more preferable that the thickness is about 35 to 50 μm. If the thickness is thinner than 35 μm, the thickness will not be uniform and there is a risk of there being areas where the grease is not present.

[0051] According to the method for manufacturing a semiconductor device of the present embodiment, a thermally conductive grease can be easily selected without undergoing the power cycle test required by the conventional technology, and a highly reliable semiconductor device can be manufactured using the selected thermally conductive grease. EXAMPLES

[0052] The present invention will be described in more detail below with reference to examples of the present invention. However, the present invention is not limited to the scope of the following examples.

[0053] [Selection of candidate thermal grease] Five types of commercially available thermal grease were selected as candidate thermal greases. Table 1 shows the specifications of the candidate thermal greases. [Table 1]

[0054] [Evaluation of thermal resistance and pump-out of power modules] The pump-out resistance of thermal grease was evaluated in the actual operation of a power module. Five types of thermal grease candidates were mounted on the resin encapsulant shown in Figure 1 to manufacture a power module. In the power module shown in the figure, the value of the longitudinal dimension / transverse dimension of the encapsulant was 2.6. A ΔTj power cycle test was performed on these power modules with an ON time of 1 second and an OFF time of 2 seconds. Table 2 shows the conditions of the ΔTj power cycle test and the thermal resistance increase rate ΔRj (%) before and after the test. For the thermal resistance measurement, the temperature change ΔTj before and after the test was evaluated using a ΔTj power cycle device, and the thermal resistance was calculated by dividing it by the power consumption.

[0055] [Table 2]

[0056] The resin encapsulant after the power cycle test was peeled off from the cooler, and the conductive plate on the back side was observed. Figure 2 shows a photograph of the conductive plate back side after the power cycle test was performed on the power module manufactured using grease 1, and Figure 3 shows a photograph of the conductive plate back side after the power cycle test was performed on the power module manufactured using grease 1 and grease 3, respectively. In Figure 2, it was visually confirmed that grease 1 was retained on the conductive plate without any significant change from the initial mounting state (not shown). In Figure 2, white and gray grease with a mottled pattern was formed on the entire surface. In contrast, in Figure 3, it was confirmed that grease 3 did not remain on the conductive plate and was depleted (pumped out). In Figure 3, the grease had fallen out except for the left end. Although not shown, similar photographs were taken for greases 2, 4, and 5. The results of grease 2 were roughly the same as those of grease 1. Pumping out was confirmed for greases 4 and 5, just like grease 3. From the above, it is considered that the main cause of the thermal resistance increase ΔRj shown in Table 2 is the pumping out of the thermal conductive grease caused by the warping deformation of the module.

[0057] [Evaluation of viscoelasticity of thermal conductive grease] For five types of thermal conductive grease, an attempt was made to identify the thermal conductive grease with the highest pump-out suppression effect based on the shear stress-strain curves measured with an oscillatory rheometer, and the values ​​at which the storage modulus G' and loss modulus G'' obtained in response to the sweep of shear stress match (values ​​at which tan δ = 1). The rheometer measurement conditions were based on the assumption that an actual power module was used, and measurements were carried out at measurement temperatures T of 25°C, 125°C, and 175°C. An Anton Paar Physica MCR301 oscillatory rheometer was used, and strain was measured. application The frequency was 1 Hz, and the indentation thickness was all 500 μm.

[0058] FIG. 4 shows the shear stress-strain curves measured for five types of thermal conductive grease at 25°C, FIG. 5 shows the shear stress-strain curves measured at 125°C, and FIG. 6 shows the shear stress-strain curves measured for five types of thermal conductive grease at 175°C. FIG. 7 shows the shear stress (yield stress) at 300% strain plotted against temperature on the horizontal axis in each of the graphs in FIG. 4 to 6. From the shear stress-strain curves, it can be inferred that thermal conductive grease with small strain is less likely to be pushed out of the power module for a constant shear stress, and therefore has a high pump-out suppression effect. However, no clear correlation was observed between the shear stress-strain curves with respect to temperature and the thermal resistance rise ΔRj of each thermal conductive grease, and it was found that the pump-out suppression effect cannot be determined from the shear stress-strain curves.

[0059] Next, the shear stress value F when the storage modulus G' and loss modulus G'' obtained for the shear stress sweep are equal T The storage modulus G obtained when the shear stress was swept for each grease at T = 25°C in Figure 8, T = 125°C in Figure 9, and T = 175°C in Figure 10. T ' and loss modulus G T '' ratio (tan δ T ) is shown in Fig. 11. F obtained from Figs. 8 to 10 versus temperature T (25°C, 125°C, 175°C) T The graph shows the F at each temperature T. T The measured values ​​are shown.

[0060] [Table 3]

[0061] As shown in Figure 11, for Greases 1 and 2, which did not experience an increase in thermal resistance in the ΔTj power cycle test, F T was between 100 and 200 Pa. And in the temperature range from 100 to 175°C, F T It was found that the maximum value of F TA large F is thought to mean that a large force is required for the thermal grease to flow, and as a result, it is assumed that a high pump-out suppression effect can be obtained. T Thermally conductive grease having a viscosity of more than 200 Pa is difficult to apply in the manufacture of semiconductor devices, and is not suitable from the viewpoint of workability.

[0062] Based on the above investigation, it was possible to determine which thermal grease had a high pump-out suppression effect by evaluating the yield value using the above method.

[0063] [Correlation between viscosity and pump-out properties at room temperature] The shear viscosity of five types of thermal conductive grease was evaluated at room temperature using an oscillatory rheometer. The shear viscosity was plotted against the shear rate, and the shear viscosity was stable at a shear rate of 50 sec. -1 The shear viscosities of Grease 1 to Grease 5 were obtained at the above temperature. The results are shown in Table 4.

[0064] [Table 4] Table 4 shows that there is no correlation between viscosity at room temperature and pump-out properties.

[0065] [Thermal stability of thermal conductive grease] The temperature characteristics of tan δ were investigated for five types of thermal conductive grease to evaluate their stability against temperature. T The plotted results are an index of the temperature stability of the thermal conductive grease. As shown in FIG. 12 and Table 5 below, Greases 1, 2, 4, and 5 have a tan δ T The maximum value of was small and the fluctuation range was also small.

[0066] [Table 5]

[0067] [Heat resistance of thermal conductive grease] When selecting a thermal conductive grease, it is necessary that it has little weight loss, i.e., little deterioration, at temperatures higher than 100°C, which is the actual operating temperature of the power module, and that it also has high heat resistance. The heat resistance temperature was evaluated by thermogravimetric analysis. The measurement conditions for the thermogravimetric analysis were air, temperature range 30°C to 800°C, and heating rate 20°C / min.

[0068] Table 6 shows the heat resistance temperature (temperature at which the oil loses 1% weight) of the thermal conductive grease evaluated in the present invention, which was obtained from thermogravimetric analysis. [Table 6]

[0069] The heat resistance temperature of the thermal conductive grease is assumed to be the volatilization temperature of the oil, which is one of the components contained in the thermal conductive grease. Here, the heat resistance temperature was evaluated as the 1% weight loss temperature of the oil as the selection criterion for the thermal conductive grease. The 1% weight loss temperature of the oil is calculated by the method described below. Since the filler contained in the thermal conductive grease does not thermally decompose between 30°C and 800°C, which is the measurement temperature range of the thermogravimetric analysis, the weight loss can be considered to correspond to the proportion of the oil component. Therefore, the proportion of the oil component in the thermal conductive grease corresponds to the total weight of 100% minus the minimum weight percentage. The proportion of this oil component is converted to 100%, and the 1% weight loss temperature of the oil is calculated from the 1% weight loss temperature. In the present invention, this temperature is defined as the heat resistance temperature of the thermal conductive grease. Greases 1 and 2, which had little pump-out, also had good heat resistance. [Explanation of symbols]

[0070] 1. Semiconductor element 2 laminated substrate, 21 first conductive plate, 22 insulating substrate, 23 second conductive plate, 3, 4 solder joint layer, 5 implant pin, 6 wiring board, 7 sealing resin 8 thermal conductive grease, 9 cooler, 10 resin seal

Claims

1. For the candidate thermal conductive grease, a shear stress-strain curve at temperature T and frequency 1 Hz is obtained, and the shear stress is swept to obtain the shear stress F at which the loss modulus and storage modulus values ​​become loss modulus / storage modulus=1. Ｔ and Within the controlled temperature range of the semiconductor device, F Ｔ selecting a thermally conductive grease having a thermal conductivity of 100 Pa or more and 200 Pa or less; a step of attaching the resin sealing body including the semiconductor element mounted on the laminated substrate to a cooler via the thermal conductive grease selected in the selecting step; Including, the controlled temperature range is a temperature range from room temperature to an upper limit of the operating temperature of the semiconductor device, The method for manufacturing a semiconductor device, wherein the step of obtaining F T is performed at at least three different temperatures T within the controlled temperature range.

2. 2. The method of claim 1, wherein the controlled temperature range is from 25°C to 175°C, and the step of obtaining F T is carried out at least at three points where the temperature T is 25°C, 125°C, and 175°C.

3. 3. The method of claim 1 or 2, further comprising measuring the storage and loss moduli of the candidate thermal grease.

4. The method of claim 3 , wherein the measuring step is performed by oscillatory rheometry.

5. The method according to any one of claims 1 to 4, wherein the resin encapsulated body is an encapsulated body having a longitudinal dimension / transverse dimension ratio of 1.5 or more.

6. The method according to any one of claims 1 to 5, wherein the semiconductor element is selected from Si or SiC.

7. The method according to claim 1, wherein the selecting step includes a step of selecting a thermal conductive grease having F T of 100 Pa or more and 200 Pa or less, and having a 1% weight loss temperature of oil contained in the thermal conductive grease that is higher than the control temperature range.

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