JP3452503B2 - Molding method of base for semiconductor package - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a base for a semiconductor package. The base means a substrate on which a chip such as an IC chip or an LSI chip is mounted. The base is desired to have a good heat dissipation property and to have a metallographic structure in which the history of heat change during the production of a semiconductor package or the like and the dimensional change over time during operation as a semiconductor device are small. 2. Description of the Related Art In recent years, in ICs and LSIs used in computer devices and the like, the amount of heat generated by chips has increased with the progress of high integration of elements and high-speed operation. Temperature rise is not preferable for improving the reliability of semiconductor devices,
It is necessary to improve the heat radiation characteristics so as not to exceed a certain temperature. For this reason, various package structures for controlling the temperature rise have been proposed. For example, a hermetically sealed semiconductor container in which a package material is made of resin, ceramics, or copper, aluminum, or an alloy thereof having good thermal conductivity is disclosed in U.S. Pat. S. Pat. 4524238, JP-A-59-33
851 or Japanese Patent Application Laid-Open No. Hei 5-50138. Since this package uses a metal substrate, its heat radiation capability is improved as compared with the conventional structure. However, a main component for dissipating heat generated by the chip is a base (substrate) member. And a window frame member, and these are bonded to each other, so that the work at the time of assembling the semiconductor device is easy to perform, but the configuration becomes complicated and the heat radiation characteristics are not sufficiently considered. . As a semiconductor package having a metal heat radiator having an integral structure instead of a separated structure, a concave portion is provided at the center of a package base and a semiconductor device is fixed to this portion in Japanese Patent Application Laid-Open No. 8-501414 or U.S. Pat. S. P
at. 4,888,449. Regarding the processing method of the concave portion of the package base, which is a particularly important part among these, U.S. Pat. S. Pat. 4,939,316. The processing method mainly includes a milling, grinding, etching method or a deformation process as an alternative method. The method of forming the concave part of the package base by milling or grinding
Basically, because of cutting or removal, the strength of the thin portion (bottom) of the recess remains the same as the original material strength, and the flatness and surface roughness of the surface are determined by the tools, processing equipment and operating conditions used. That is, it is difficult to always maintain the processed surface in a constant surface state with good reproducibility. The etching method is similar in principle in forming a shape. According to the above-mentioned processing methods, the crystal grains of the metal structure are separated from the concave portions and the peripheral edge portions which are not processed. That is, among the above-mentioned conventional processing methods, the punching method is, as shown in FIG. 4, a method of punching a plate having a thickness to become an outer peripheral portion 6 and bonding a thin portion 7 to the hole to form a product. It is. On the other hand, as shown in FIG.
If the plate to be formed is semi-processed while leaving the thickness corresponding to the thin portion 7, the above-mentioned convex portion 9 is formed on the back surface of the plate to be the outer peripheral edge portion 6. The projection 9 is removed by cutting. The above method is superior in terms of surface flatness, surface roughness, and reproducibility of the quality of a molded product as compared with the above-described deletion methods such as the etching method, but no consideration is given to strengthening the concave portion. Not. Further, according to the conventional example, there is no description regarding the heat history at the interface between the semiconductor device and the package base due to the temperature history at the time of assembling and manufacturing the semiconductor package or the thermal displacement of the radiator due to the heat generated during the operation of the semiconductor device. No consideration is given, and no consideration is given to the structure of the crystal grains of the metallographic structure of the radiator for uniformly dispersing the heat generated at the center of the recess cavity to the entire package base including the outer periphery of the recess by heat conduction. . In order to efficiently radiate the heat generated in the semiconductor device to the outside, the bonding state of the interface between the semiconductor device and the substrate serving as a package base is improved, and And reducing the thickness of the concave portion for fixing the semiconductor device within a certain range. Factors that improve the bonding state at the interface include the flatness, surface roughness, bonding method, and the like of the bonding portion of the concave portion. Here, the flatness in particular must be maintained even after receiving a history of thermal change during assembly of the package or mounting and operation of the semiconductor device. On the other hand, as a substrate material to be used as a package base, copper, aluminum and alloys thereof are used because of their high thermal conductivity and cost. These materials have a higher linear expansion coefficient and lower material strength than silicon, which is a semiconductor device. The bonding portion between the semiconductor device and the package base is considered to be a kind of bimetal structure made of a combination of materials having different linear expansion coefficients and strengths. In this state, a history of heat change during package assembly or operation as a package is received. The semiconductor device,
In order to prevent the occurrence of defects that cause thermal resistance at the package base and at the interface, the concave member should be given a certain strength and have anisotropy with respect to heat conduction, so that it can be uniformly distributed in the plane direction. It is necessary to have a structure that disperses heat. In order to increase the strength, there is a method of increasing the thickness of this portion or using a material reinforced by adding an alloy element. In any case, the thermal conductivity is reduced. A phenomenon occurs and it is not effective. SUMMARY OF THE INVENTION It is an object of the present invention to increase the strength of a concave portion of a semiconductor package having a concave cavity made of a metal material, to enhance the history of heat change, and to further impart anisotropy to heat conduction, thereby improving heat dissipation characteristics and reliability. An object of the present invention is to provide a method of forming a package base excellent in the above. Another object of the present invention relates to a method in which the concave portions are strengthened only by plastic flow, and particularly to a method for forming both surfaces of the cavity portion of the package base without cutting. Still another object is to impart anisotropy to heat conduction in the cavity portion of the package base and to make crystal grains substantially continuous in the metal structure between the cavity portion and the thick outer peripheral portion, thereby forming a heat diffusion passage. It is an object of the present invention to provide a method of forming a package base having a more advantageous metal structure. If the interface between the semiconductor device and the package base is explained using oxygen-free copper as an example, the linear expansion is three times or more, so that the thermal expansion is repeated during the assembly and operation of the package. Generates stress. As a result, if the strength of the concave portion is low, defects such as vacancies and cracks that inhibit thermal diffusion are generated at the interface based on plastic deformation and the like, and the performance is reduced. On the other hand, the strength of oxygen-free copper is improved by the cold working rate as shown in FIG. Although the strength increases with the working ratio, in a usage such as a package base, it is desirable that the cold working ratio be uniform and the strength be clearly higher than 20%. That is, the metal material portion on which the semiconductor device is mounted is uniformly plastic-flowed to uniformly work-harden and strengthen it, and the crystal grains in this portion are plastically flowed in the plane direction to prevent heat conduction in the plane direction. It is desirable to have anisotropy to be advantageous. Furthermore, in order to improve the thermal diffusivity, the crystal grains of the metal structure between the plastic flow portion of the concave portion and the non-plastic flow portion of the outer peripheral edge are substantially continuous as shown in FIG. It is also important to form the structure and to dissipate the heat through the crystal grains having high thermal conductivity to the thick part. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a process chart for obtaining a product 4 as a package base by molding. The material 1 is a rectangular plate made of aluminum, copper, or the like, and is prepared for punching or cutting from a coil material or a fixed-length material. The raw material 1 is preformed by a mold and the preformed product 2
become. The preform 2 has a thin, flat bottom 2
a, a tapered portion 2b is formed. The dimensions and shape of the bottom 2a are maintained without further processing in the subsequent steps. The preformed product 2 is formed into a molded product 3 by a forming process. The molded product 3 has a bottom 3a, an edge 3
b, a surplus portion 3c is formed. The edge 3b corresponds to the tapered portion 2 of the preform 2
Therefore, the taper of the tapered portion 2b is an important factor in order to form the edge 3b into a regular size and shape. Unnecessary meat for the product 4 is collected in the surplus portion 3c, trimmed in the next trimming step, and scrap 5
Removed. When the above processing is performed, the product 4 is so-called work hardened, and therefore has high rigidity. Further, the product 4 is formed of continuous crystal grains. That is, when the product 4 is manufactured by cutting, the flow of the crystal grains is cut at the step. The above-described series of processing is performed by a so-called transfer processing method in view of the strength of the mold. That is, when the above-mentioned processing is performed by a so-called progressive processing method, there is a problem of the mold strength due to the arrangement of the mold, and it is necessary to provide an idle step or the like. According to the present invention, since the base for the semiconductor package is manufactured by plastic working, the material is hardened and hardened and hardly deformed. It is advantageous for conduction, it is advantageous for heat conduction because crystal grains are continuous because it does not involve processing such as cutting and shearing, and it is good for heat conduction at the interface because of good flatness and smooth surface. In addition, there is an effect that the productivity is good because of the transfer processing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 Process drawing FIG. 2 Explanatory drawing of work hardening FIG. 3 Explanatory drawing of continuous crystal grains FIG. 4 Explanatory drawing of conventional product FIG. DESCRIPTION OF THE PRIOR ART PRODUCTS [Description of References] 1 is a material, 2 is a preformed product, 2a is a bottom portion, 2b is a tapered portion, 3 is a molded product, 3a is a bottom portion, 3b is an edge, 3c is excess, 4 Is a product, 5 is a scrap, 6 is an outer peripheral portion, 7 is a thin portion, 8 is an adhesive portion, and 9 is a convex portion.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 23/12 H01L 23/50 B21D 22/02

Claims (1)

(57) Claims 1. A preforming step is provided in a step of forming a base for a semiconductor package, and in the preforming step, a flat bottom portion on which a semiconductor chip is mounted and a flat bottom portion are directed outward from the bottom portion. A method for forming a base for a semiconductor package.