The present invention relates to a method of measuring a thermal resistance of resin materials.
Industrial products using grease and resin at an interface between structural members are used over a wide range of industrial fields, for example, in the case in which a semiconductor device and a radiating plate are joined through a thermal-conductive resin, the case in which resin is molded or applied between members and then baked, the case in which gel resin is used in a state sandwiched between members, and the case in which a semiconductor device and a radiating plate are joined with an electrically conductive adhesive.
In such a product field in which heat flows through the resin, a value obtained by dividing, with an amount of passing heat, a temperature difference produced when the heat passes through the resin, that is, thermal resistance is extremely important for designing the products.
The thermal resistance of the resin comprises as its components the thermal resistance caused by conduction of heat passing through the resin itself and an interface thermal-resistance produced at interfaces between the resin and the members sandwiching the resin therebetween from both sides.
The thermal resistance caused by conduction of heat passing through the resin itself can be determined by the following equation if the flow of heat is one-dimensional.Rcond=L/(λ×A)  (1)where Rcond is the thermal resistance caused by conduction of heat, L is a length along which the heat passes, A is a cross-sectional area in which the heat passes, and λ is a thermal conductivity of the resin.
That is, if heat flows one-dimensionally, the thermal resistance Rcond caused by the conduction of heat through resin is proportional to the inverse of the thermal conductivity λ of the resin.
As a technique for individually determining the thermal conductivity λ of resin, for example, the simplest method is one usually called a stationary method, in which a test specimen having a constant sectional area A and a sufficiently long length L is made, a constant amount of heat Q is caused to flow through the test specimen, and temperature distribution of the test specimen in the direction of flowing heat is measured at a constant spacing, as shown in FIG. 2. This method itself is based on a very basic matter of heat, and a known method.
Assuming that there is a material having a thermal conductivity λ and a sectional area A, which each are constant, and that a constant amount of heat Q one-dimensionally passes through the material, and further assuming that the spacing between positions at which temperature is measured is ΔL and a temperature difference measured is ΔT, the following equation is established.Q=λ×A×ΔT/ΔL  (2)
From the equation (2), the thermal conductivity λ can be determined by the following equation.λ=Q×ΔL/(A×ΔT)  (3)
That is, if an amount of passing heat Q, a sectional area A in which heat passes, and a spacing ΔL between positions at which temperature is measured are known, the thermal conductivity λ can be experimentally determined by measuring the temperature difference ΔT.
It is also possible to measure the thermal resistance Rint of an interface by using the above described stationary method.
As shown in FIG. 3, two kinds of members each having known thermal conductivities are prepared, and a constant load is applied thereon, and heat is caused to flow through them. At that time, a temperature difference ΔT in the proximity of the interface between the members is determined from changes in the temperature in each of the members, and the resultant temperature difference divided with the amount of passing heat Q corresponds to the thermal resistance Rint of the interface. Further, with respect to FIG. 2 and FIG. 3, temperature is measured at two positions in each one member. However, in order to improve reliability of the measurement, there are some methods such as making the distance ΔL between the measurement points long, and increasing the number of the temperature measurement points.
In the case of a material having a small distance L passed through by heat, that is, in the case of a thin material, as a method of measuring thermal conductivity in the direction in which heat passes through the material, that is, in the direction of the thickness of the material, an alternating-current heating method and a method using laser are used, for example. Both methods are ones to determine the thermal conductivity by measuring the thermal diffusivity of the thin material and using a thermal capacity and density measured by using other methods.
As a document disclosing the method of measuring thermal diffusivity using the alternating-current heating method, JP-A-10-221279 is indicated, for example. Further, as a document disclosing the method of using laser, JP-A-2001-83113 is indicated.
On the other hand, as documents disclosing a method of measuring the thermal conductivity of resin in a state where a load is being applied to the resin, JP-A-8-136483, JP-A-2001-21512 and the like are cited.
The above described stationary method is characterized in that as ΔL is made larger, the reliability of measurement increases. Therefore, in case of measuring the thermal conductivity of materials of which distance L passed through by heat is very small relative to the sectional area A in which heat passes, that is, thin materials such as grease used for joining a heat generating member and a heat-diffusing member and a molding resin used in a semiconductor device, it is impossible to ensure a sufficient length of ΔL. For this reason, the stationary method has a problem that it is difficult to be applied to thin materials except for the case in which the thermal conductivity does not change even if the spacing ΔL between the temperature measurement positions is sufficiently increased by varying the thickness of a sample, that is, except for the case in which the thermal conductivity does not depend on the thickness of the sample. The thermal conductivity of resin is often dependent on the process of solidifying the resin and its thickness. In fact, it is needed to measure the thermal conductivity of the resin in its thin state.
Although the methods of measuring thermal diffusivity disclosed in the above described JP-A-10-221279 and JP-A-2001-83113 are characterized in that they can measure the thermal diffusivity of very thin materials, the methods are ones for measuring a temperature response when a non-stationary or transient signal of heat is provided for the materials, and require the additional measurement of specific heat and density.
In the method of measuring the thermal conductivity of resin disclosed in the above described JP-A-8-136483, there is a problem that shape of a test specimen, resin, is limited in dependent of the apparatus. That is, because a probe as a heat source is inserted in the resin, measurement is made on information including the thermal resistance of the interface between the probe and the resin. When this method is used in such a structure that the resin is sandwiched between other members from both sides in the direction of its thickness, it cannot measure the thermal resistance of the resin including the thermal resistance of its interface, except that the thermal resistance of the interface between the member and the resin is always equal to that of the interface between the probe and the resin.
Also in the method disclosed in the above described JP-A-2001-21512, it is difficult to measure the thermal resistance of resin including the thermal resistance of the interfaces between the resin and members sandwiching the resin therebetween.
On the other hand, in products using resin, there is a problem that the thermal resistance of the resin is strongly affected by manufacturing processes, information of material properties and surfaces of the members sandwiching the resin therebetween, and environments in which the resin is actually used.
For example, considering the case in which resin sandwiched between two sheets of members is baked and solidified, in general, the resin solidifies while various kinds of gas are being generated from the parent materials of the resin in the process of baking. What gas is generated depends on the material properties of the parent material of the resin and the baking process. Therefore, in order to measure thermal conductivity, when the parent material of the resin is baked with nothing present around it and a sample to be measured for thermal conductivity is cut out from the baked resin, gas is purged from the sample in each direction thereof, as shown in FIG. 4.
In contrast to this, in actual products using resin, since the parent material of resin is baked in the form in which the resin is sandwiched between other members, the direction in which gas is purged is limited to the direction in which the members sandwiching the resin therebetween are not present, as shown in FIG. 5. For this reason, the manner in which gas is purged in this case is different from the manner when there is nothing around the resin as described above, and therefore the composition and internal structure of the resin after baked are different from those of the above described sample. Thus, even if the thermal conductivity of the sample is accurately measured, it often does not conform to the thermal conductivity of the resin actually used in products.
Further, according to the temperature when products using resin are used and the temperature when the products are not used, the products may be used in the state in which the resin itself is melt, and there are cases in which the resin is re-solidified after melting and the resin repeats melting and solidifying. Thus, it is feared that due to such melting and solidification of the resin after it is mounted in products, the thermal conductivity of the resin and the thermal resistance of the interface may vary with time. In the above described case, when the variation with time of thermal resistance is not known in advance, even if only the thermal conductivity of a sample cut out from the resin without considering its mounting condition as shown in FIG. 4 is known, it often does not conform to the thermal conductivity of the resin actually used in the products, and the thermal resistance of the interface is not grasped, either.
In such a situation, when products have a structure in which heat escapes through resin, it is impossible to accurately predict a temperature rise of the products. However, in connection with such samples and actual products, conventionally, importance has not been attached to problems that a manufacturing process has an influence on the thermal resistance of the resin and that it is necessary to determine the inclusive thermal-resistance of resin including the thermal resistance of its interface. Further, in referring to data of the material properties of resin, and in legal persons and organizations or individuals, such as companies, manufacturing, selling, or distributing resin, there have been in general not found such databases arranged.
Therefore, in a product actually using resin, at the time of selecting one resin most suitable for the product concerned from among a plurality of choices of resin, users have to produce the product or a prototype close to the product and to actually measure a temperature rise of the products in which the above described choices of resin are used. Therefore, there has been a problem that a high prototyping cost and a long period are required to reach a selected resin.
The invention has an object to provide a method of measuring the thermal resistance of resin, allowing the thermal resistance of the resin to be measured in a state closest to the state in which the resin is incorporated in an actual product.