Ceramic package for a semiconductor device

Abstract: A ceramic package used for a semiconductor device comprises a plate-shaped base made of a material consisting essentially of ceramic. In plan view, the base is rectangular and has four corner portions (C₀) each located between two adjacent side walls. The corner portion (C₀) comprises a main bevel (23) and two auxiliary bevels (24₁, 24₂) located at respective sides of the main bevel and extending to the two side walls (W₁, W₂), respectively, so that two first corners (C₁₁,C₁₂) are defined between the main bevel (23) and the auxiliary bevels, respectively, and two second corners (25₁, 25₂) are defined between the auxiliary bevels (24₁, 24₂) and the side walls (W₁,W₂) of the base, respectively.

Description: This invention relates to a ceramic package for a semiconductor device, and more particularly, to such a ceramic package in which generation of cracks can be prevented. A ceramic package for mounting thereon a semiconductor chip or a similar electronic element is a package comprising a base made of ceramic, hereinafter referred to as a "ceramic base". As well known in the prior art, ceramic is generally has a strong compression stress resistance, and a weak for tension resistance. In a process for manufacturing semiconductor devices, a plurality of such ceramic packages are once stored in a storage or accommodation station, and picked up therefrom one by one and fed to the various work stations, such as a semiconductor chip mount station. In the accommodation station, the ceramic packages are guided by a suitable guide means, such as a rail, and fed in the forward direction. To smoothly feed these ceramic packages along the rail, it is usually necessary to provide gaps between side edges of the ceramic packages and side walls of the rail, and due to such gaps, the ceramic packages might slew transversely while being fed along the rail and come into touch with an adjacent ceramic package. This sometimes causes a generation of cracks or flaws in the ceramic package, particularly at corners or edge portions of the ceramic package. Also, a part of ceramic package is sometimes broken, i.e., edges of the ceramic package are sometimes nicked. To prevent a generation of such cracks or flaws, as shown in Fig. 12, it is well known in the prior art for, a rectangular ceramic package to be provided with bevel portions B at the respective corners of the ceramic base, to avoid a possible concentration of stress when an external force is exerted on the ceramic package. Such a ceramic base is known, for example, from US-A-4 862 247. In general, these bevel portions B effectively prevent the ceramic package form being broken at the corner portions, but do not completely avoid a generation of cracks or flaws in the ceramic package. An object of the present invention is to provide a ceramic package used for mounting an electric element thereon, such as a semiconductor chip or the like, in which generation of cracks or flaws in the ceramic package are prevented. According to the inventor's observations during experiments involving many samples, a generation of crack does not occur first at a corner which is formed by the bevel, but at a point inside of the ceramic base, and inward from the corner, as shown in Fig. 12. In particular, a laminated-type ceramic package generally has a via or through hole 10 which is filled with a conductive material for mutually connecting electrical patterns formed on the upper and lower surfaces of the ceramic package. In many samples of ceramic packages, the following has been observed; i.e. a crack extends toward the via, which generally has a low strength, from a point of edge of the package near to the via, as shown in Fig. 9. If such a large-sized crack is generated, a serious problem, for example, a disconnection of the electrical patterns in the vicinity of the via, arises. To solve the above problem, there is provided a ceramic package for mounting an electric element thereon, such as a semiconductor chip, comprising a plate-shaped base made of a material consisting essentially of ceramic, the base having, in a plan view, at least one corner portion located between two adjacent side walls, characterized in that the corner portion comprises a main bevel and at least two auxiliary bevels located at the respective sides of the main bevel and extending to the two side walls, respectively, so that two first corners are defined between the main bevel and the auxiliary bevels, respectively, and two second corners are defined between the auxiliary bevels and the side walls of the base, respectively. In the present invention, since a corner portion of the ceramic package is provided with a main bevel and two auxiliary bevels, if the ceramic package is moved suddenly, it first collides with a corner between the auxiliary bevel and the side wall of the package base. Then after one bound, the ceramic package collides with the opposite corner between the auxiliary bevel and the side wall. Thus, a distance between a position of the first collision to a position of the second collision is significantly reduced, with that of a ceramic package known in the prior art. Consequently, according to the present invention, both of the positions of the first and second collisions are located on the side wall inward of the corner portions of the ceramic package. Therefore, a rotational moment exerted on the ceramic package due to a reaction force of the first collision becomes smaller, and as a result, a rotational energy of the ceramic package is reduced and, therefore, a tension force exerted on the package at a second collision becomes significantly smaller, and thus a generation of cracks caused by the second collision is prevented. A particular embodiment of the present invention will now be described and contrasted with the prior art with reference to the accompanying drawings; in which:- Figure 1 is a view illustrating an experiment in which a sample of ceramic package slides down along a slope; Figure 2 shows a series of motions of the ceramic package after collision with a stopper and a bounce up at a corner thereof; Figure 3 is a partial plan view of a ceramic package having second or auxiliary bevels as well as the main bevels according to the present invention; Figure 4 is a partial perspective view of a ceramic package having additional bevels according to the present invention; Figure 5 is a plan view illustrating the distances between the corners defined by these bevels; Figure 6 is a partial plan view illustrating the dimensions of the corner provided with bevels; Figures 7A, 7B and 7C are partial plan views illustrating the respective bevels, in which 7A shows bevels of an embodiment; 7B, a round-shaped bevel; and 7C, a bevel of the prior art; Figure 8 shows experiments results for the samples of ceramic packages of a prior art example; Figure 9 shows experiments results for the samples of ceramic packages of an embodiment of this invention; Figure 10 is a view illustrating a sample of ceramic package which slews and is subject to a secondary collision; Figure 11 shows dimensions of a typical embodiment of a ceramic package according to the present invention; and Figure 12 is a partial plan view of a ceramic package having a bevel known in the prior art. As understood from TABLE I and Figs. 8 and 9, according to the embodiment of this invention, the crack generation rate was significantly reduced, compared with the prior art and the comparative example. First, a mechanism of the generation of cracks will be described with reference to Figs. 1 and 2. The inventors made a study of such a crack generating mechanism, and conducted experiments using an apparatus as shown in Fig. 1. In this experiment, samples of the ceramic package 20 were allowed to slide down an inclined rail 21, and the states of the sliding samples were visually recorded by a high speed video reorder so that a mechanism of the generation of cracks could be analyzed. As a result, it has been noted that, as shown in Fig. 2. the ceramic package 20 is usually inclined very slightly with respect to a stopper surface 22 while sliding down. Thus, the ceramic package 20 collides with the stopper surface 22 by a new corner C₁ which was defined by a bevel B₁, as shown in (b) of Fig. 2. After one bounce, as shown in (c), the ceramic package 20 again collides at an opposite corner C₂,as shown in (d), which was defined by a bevel B₂. As a result of this experiment, it was noted that the generation of cracks was not caused by the first collision, but by the second collision, apparently for the following reasons. At the time of the first collision, the ceramic package 20 maintains the same profile as it slides down the rail 21 and, therefore, only a compression force is exerted on the ceramic package 20. Such a mere compression force does not cause a generation of cracks. At the time of the second collision, however. the ceramic package 20 is subjected to a large rotational moment in a direction indicated by an arrow M about the position of first collision i.e., the corner C₁, due to a reaction force of the first collision, and therefore, the ceramic package 20 again collides with the stopper surface 22 at the opposite corner C₂, with a relatively large rotational energy. Such a second collision is such that it is as if a long, thin rod is held at one end thereof while the other end thereof collides with an object at a relatively small angle. Thus, the ceramic package 20 is subjected to a large tension force at the colliding surface thereof and, therefore, a generation of cracks is caused by the second collision. The inventors have created the present invention on the basis of the above-mentioned knowledge obtained from their experiments. Fig. 3 is a partial plan view of a ceramic package according to the present invention. The rectangular and plate-shaped ceramic package 20 comprises a base made of a material consisting essentially of ceramic and is provided at each corner portion C₀ thereof with a main bevel 23, which is the same as that of the prior art, and auxiliary bevels 24₁ and 24₂ positioned at the respective sides of the main bevel 23. Each auxiliary bevel 24₁,24₂ may be either a flat surface or a convex or rounded surface. Note, the surface of the main bevel 23 is not limited, and may be flat, round (R) or convex. New corners 25₁ and 25₂ defined between these two auxiliary bevels 24₁ and 24₂ and the side walls W₁ and W₂ of this ceramic package 20, respectively, are postioned in areas A₁ and A₂ defined between vias 26b₁ and 26b₂ (lines D₁ and D₂) located nearest to the corner C₀ and an adjacent via 26a (lines E₁ and E₂) parallel to the walls W₂ and W₁, respectively. Note, advantageously each corner 25₁ or 25₂ is located as far away as possible from the vias, i.e., is located at a position on the side wall W₁, W₂ and between two vias (26a; 26b₁, 26b₂) in the vicinity of a corner C₀ of the ceramic package 20. Here, "via" means any through hole, blind hole, cavity or the like at which stress is easily concentrated when the ceramic package 20 is subjected to an external force, such as a compression or tension force. Although Fig. 3 illustrates only a part of the rectangular ceramic package 20, i.e., a portion of one C₀ of the corners, each of the other three corners is also provided with two auxiliary bevels as well as one main bevel, in the same manner as mentioned above. In an embodiment shown in Fig. 4, in addition to the main bevel 23 and auxiliary bevels 24, sub bevels 26 may be formed between the front and rear surfaces S₁ and S₂ and the main bevel 23 and auxiliary bevels 24, respectively. According to this invention, as the ceramic package 20 is provided at each corner C₀ thereof with a main bevel 23 and two auxiliary bevels 24, if the ceramic package 20 slides down, it first collides with a stopper surface at a corner 25a in Fig. 5. After one bounce, the ceramic package 20 again collides with the stopper surface at the opposite corner 25b. As clearly understood from Fig. 5, a distance X from a positon of the first collision, i.e., a corner 25a, to a position of the second collision, i.e., a corner 25b, is significantly reduced, compared with a distance Y from a corner 23a to a corner 23b which corresponds to a distance between the first and second positions of a collision in a ceramic package known in the prior art. Consequently, according to the present invention, both of the positions of the first and second collisions 25a and 25b are located on the side wall at positions inward of the corner C₀ of the ceramic package 20. Therefore, a rotational moment exerted on the ceramic package 20 due to a reaction force of the first collision becomes smaller than that of the prior art. As a result, a rotational energy of the ceramic package 20 is reduced and, therefore, a tension force exerted on the package 20 at a second collision becomes significantly smaller, and thus a generation of cracks caused by the second collision is prevented. Also, according to the above-mentioned embodiment, since the corners 25₁ and 25₂ in Fig. 3, i. e., the positions of collision, are located far from the vias, the exertion of a very large stress the vias is prevented and, therefore, the generation of cracks at these vias is prevented or reduced. It should be appreciated that, although the areas around the vias are generally weak, such weak points can be compensated. Assuming that a single larger bevel was formed, as shown by a two-dot line in Fig. 6, to shift inside the positions of collision. i.e., corners 25, without an auxiliary bevel, an angle ϑ₂ (45 ° ) at the corners 25 would become much larger than the angle ϑ₁ at the corners 25 of the embodiment. Accordingly, stress would still be concentrated at the corners 25 of the angle ϑ₂, and such a solution would not completely prevent a generation of cracks. Assuming also that a relatively large round or R-shaped bevel was formed, as shown in Fig. 7B, so that a tangent line of the R-shaped bevel was smoothly continued to the side walls of the ceramic package 20 to shift inside the tangent point, without auxiliary bevels, such a solution would not completely prevent a generation of cracks, since a position of the first collision is not definitely determined under the conditions of sliding down. Sometimes, a position of first collision may be on the R-shape portion, so that a distance between the positions of first and second collisions would become longer, and thus a relatively large stress might be exerted on the ceramic package 20. The following table I shows experiments results, i.e., rate of crack generation number of samples with cracks, for samples of ceramic packages of an embodiment of this invention, a comparative example having a large R-shape bevel, and a prior art example, as shown in Figs. 7A, 7B, and 7C, respectively. The samples of ceramic package 20 in Fig. 1 were dropped and slid down along the slope of a rail 21 and, then, hit on a steel block 22. The distance L of the slope along which was 30 cm in condition A and 60 cm in condition B. The angle ϑ of the slope was 60° . The samples were all ceramic packages used for a pin-grid array type semiconductor device. The rail 21 was made of acrylic and the steel block was made of tungsten carbide. Fig. 8 shows experiments results for the samples of ceramic packages of a prior art example. On the other hand, Fig. 9 shows experiments results for the samples of ceramic packages of an embodiment of this invention. As understood from TABLE I and Figs. 8 and 9, according to the embodiment of this invention, the crack generation rate was significantly reduced, compared with the prior art and the comparative example. In the embodiment of a ceramic package as mentioned above, the angle of the main bevel 23 with respect to the side wall of the ceramic package 20 is approximately 45 ° . On the other hand, the angle (ϑ₁ in Fig. 6) of the auxiliary bevels 24 with respect to the side walls of the ceramic package 20 can be determined as follows. Assuming that, in an accommodation or storage station, the ceramic packages 20 are fed or slid downward in tandem along a guide means such as a rail having side walls which define small gaps with respect to the side edges of the ceramic package 20. Due to these gaps, the ceramic package swings transversely while sliding downward along the rail. Here, assuming that a maximum swing angle is α in Fig. 10, the angle ϑ₁ in Fig. 6 of the auxiliary bevels 24 with respect to the side walls should not be smaller than α, i.e., ϑ₁ ≧ α. In this case, as shown in Fig. 10, after the ceramic package 20 first collides with the stopper surface and then bounces away therefrom, it collides again with the stopper surface of the other corner. Under the above conditions the force of, such a second collision is taken by the corner 25 of the auxiliary bevel 24, or at least by the entire face of the auxiliary bevel 24. Assuming that such a second collision occurred at the corner 23a (Fig. 5), the crack generation rate would be increased, since the angle (approximately 45°) of the corner 23a is narrower than that of the corner 25. Therefore, according to the present invention, such a severe collision by the corner 23a (Fig. 5) can be avoided. For example, if a maximum swing angle (α) of the ceramic package in the accommodation station is 2°, the complementary angle ϑ₃ in Fig. 6 at the corner 25 is preferably 178° or slightly smaller. In addition, a subsequent ceramic package might be swung in the opposite direction with respect to a preceding ceramic package. Therefore, advantageously, from the viewpoint of safety, the complementary angle ϑ₃ in Fig. 6 at the corner 25 is 176° or slightly smaller Fig. 11 and the following TABLE II show dimensions of a typical embodiment of a ceramic package according to the present invention. Various changes and modifications may be made in the invention, for example, this invention can be applied to various types of packages having a ceramic base used for mounting a semiconductor chip, such as a pin-grid array type package, or chip-carrier type package.

Claims: A ceramic package for mounting thereon an electric element, such as a semiconductor chip, and comprising a plate-shaped base made of a material consisting essentially of ceramic, said base having, in a plan view, at least one corner portion (C₀) located between two adjacent side walls, wherein said corner portion (C₀) comprises a main bevel (23), characterized in that said corner portion further comprises: at least two auxiliary bevels (24₁, 24₂) located at respective sides of said main bevel and extending to said two side walls (W₁,W₂), respectively, so that two first corners (C₁₁, C₁₂) are defined between the said main bevel (23) and said auxiliary bevels, respectively, and two second corners (25₁, 25₂) are defined between said auxiliary bevels (24₁, 24₂) and said side walls (W₁,W₂) of the base, respectively. A ceramic package as claimed in claim 1, wherein said two adjacent side walls (W₁, W₂) form an angle of approximately 90° therebetween, and these side walls (W₁, W₂) and said main bevel (23) form an angle of approximately 135° therebetween, respectively, so that each of said first and second corners (C₁₁, C₁₂;25₁,25₂)have an angle of more than 135°. A ceramic package as claimed in claim 1 or 2, wherein said ceramic base has a plurality of vias (26a, 26b₁, 26b₂), such as through holes, blind holes, cavities or the like, including a first via (26a) located nearest to said corner portion (C₀) and at least one second via (26b₁, 26b₂) adjacent to said first via, and said second corners (25₁,25₂) are positioned in areas (A₁,A₂) defined between two parallel lines (D₁,E₁; D₂,E₂) which pass through said first and second vias (26a, 26b₁, 26b₂), respectively, and are in parallel to said side wall (W₁,W₂). A ceramic package as claimed in claim 3, wherein two second vias (26b₁, 26b₂) are provided, one via (26b₁) being positioned on a second straight line (D₂) passing through said first via (26a) and in parallel to one (W₁) of said side wall, and the other via (26b₂) being positioned on a first straight line (D₁) passing through said first via (26a) and in parallel to the other side wall (W₂). A ceramic package as claimed in claim 3 or 4, wherein one (25₁) of said second corners is positioned in an area (A₁) defined between said first straight line (D₁) and a line (E₁) passing through said one (26b₁) of the second vias and the other second corner (25₂) is positioned in an area (A₂) defined between said second straight line (D₂) and a line (E₂) passing through said other second vias (26b₁) and in parallel to the first side wall (W₁). A ceramic package as claimed in any preceding claim, wherein a complementary angle of said second corner (25₁, 25₂) defined between said auxiliary bevel (24₁, 24₂) and said side wall (W₁,W₂) of the base is equal to or a little more than a maximum swing angle (α), provided that the swing angle is determined as follows: said ceramic package swings transversely while being fed along guide means at said swing angle, assuming that a plurality of said ceramic packages are fed or slide downward in tandem along said guide means which define small gaps with respect to the side walls of the ceramic package. A ceramic package as claimed in any preceding claim, wherein said base has a front surface (S₁) and a rear surface (S₂) and sub bevels (26) are formed between said front surface (S₁) and said main bevel (23) and said auxiliary bevels (24), respectively, and between said rear surface (S₂) and said main bevel (23) and said auxiliary bevels (24), respectively. A ceramic package as claimed in any preceding claim, wherein each of said auxiliary bevels (24) is a flat surface. A ceramic package as claimed in any one of claims 1 to 7, wherein each of said auxiliary bevels (24) is a convex, rounded surface.

Inventors: IMAI KUNIHIKO, IMAI, KUNIHIKO
Applicants: SHINKO ELECTRIC IND CO, SHINKO ELECTRIC INDUSTRIES CO. LTD.
Application year: 1990
Main classification: H01L23
IPC codes: H01L23