1. Field of the Invention
The present invention relates to a connection method, a connection structure, and inspecting methods for the connection state when pad electrodes (component-side pad electrodes) formed on a component and pad electrodes (substrate-side pad electrodes) formed on a substrate are each connected by solder bumps.
2. Description of the Related Art
As an example of connection structure for connecting each of the component-side pad electrodes formed on a component and one of the substrate-side pad electrodes formed on a substrate by means of a solder bump, a connection structure as disclosed in Japanese Unexamined Patent Application Publication 9-82760 (see U.S. Pat. No. 5,914,536, an English language patent family member) is known.
As shown in FIGS. 7A and 7B herein, this connection structure is one wherein the semiconductor chip (surface-mount component) 111, in which a plurality of terminal electrodes (component-side pad electrodes) 112 is provided on the bottom surface thereof, is mounted onto a wiring substrate 113, and wherein each of the wiring patterns (substrate-side pad electrodes) 114 provided on the wiring substrate 113 and one of the component-side pad electrodes 112 are electrically connected by a bump (solder bump) 115. In this connection structure, each of bumps 115 is formed of solder 115a, and a metallic core 115b using Cu or the like. The portion (the outer peripheral portion) formed at the outside of the outer edge of the region corresponding to the plan view of the semiconductor chip 111, out of the solder 115a, makes contact with a solder resist 117 formed on the wiring patterns (substrate-side pad electrodes) 114.
In this connection structure, the reflow method is used for mounting a semiconductor chip onto the circuit board 113 of the semiconductor chip 111. Specifically, the connection is performed using the following method.
(1) First, a metallic core 115b is formed on each of the terminal electrodes 112 of the semiconductor chip 111, and a bump 115 is formed by covering the metallic core 115b by solder 115a. 
(2) Then, the semiconductor chip 111 is positioned and mounted on the wiring patterns (substrate-side pad electrodes) 114 on the wiring substrate 113, and after the solder 115a of each of the bumps 115 has been melted, the solder 115a is cooled down to solidify it.
In this manner, each of the terminal electrodes 112 and one of the wiring patterns (substrate-side pad electrodes) 114 is connected by a bump 115, and thus the semiconductor chip 111 is mounted onto the wiring substrate 113. Herein, the melted solder 115a flows and spreads over the wiring patterns (substrate-side pad electrodes) 114, which has a good solder wettability, and is stopped by the solder resist 117. Hence, by setting the amount of the solder 115a of the solder bump 115 and the area of the portion which is not covered with the solder resist 117 of the wiring patterns (substrate-side pad electrodes) 114 to an appropriate value, the contact area between the bumps 115 and the wiring patterns (substrate-side pad electrodes) 114 can be made constant, thereby ensuring a stable connection state.
In the above-described conventional connection structure, however, the substrate-side pad electrodes 114 are formed so as to be extended to the outside of the outer edge of the region corresponding to the plan view of the semiconductor chip 111. This makes it difficult to apply this connection structure to the case where surface-mount components such as semiconductor chips are mounted at a high density. In addition, this raises a problem in that the miniaturization of the product which is obtained by mounting semiconductor chips and the like is restricted.
The present invention has been made to solve the above-described problems. The object of the present invention to provide a connection method and a connection structure, using solder bumps, for component-side pad electrodes and substrate-side pad electrodes, and inspecting methods for the connection state thereof which are adaptable to high density mounting, and which allow the miniaturization of a product formed by mounting a surface-mount component onto a substrate.
In order to achieve the above-described object, the present invention provides a method for connecting each of component-side pad electrodes and one of substrate-side pad electrodes, when a surface-mount component wherein component-side pad electrodes are formed on the surface thereof opposed to a substrate, and wherein solder bumps are formed on the component-side pad electrodes, is mounted onto a substrate, wherein substrate-side pad electrodes are formed on the surface thereof. This method is characterized by comprising arranging the substrate-side pad electrodes inside the region corresponding to the plan view of the surface-mount component (hereinafter, referred to as the xe2x80x9ccomponent-corresponding regionxe2x80x9d); setting the size of each of the substrate-side pad electrodes, in the direction substantially perpendicular to the outer edge of the component-corresponding region (hereinafter, referred to as the xe2x80x9clength of substrate-side pad electrodesxe2x80x9d), larger than that of the corresponding component-side pad electrode, in the direction substantially perpendicular to the outer edge of the surface-mount component (hereinafter, referred to as the xe2x80x9clength of component-side pad electrodexe2x80x9d); and placing the surface-mount component on the substrate so that each of the solder bumps are opposed to a predetermined substrate-side pad electrode, and melting the solder bumps by heating, thereby connecting each of the component-side pad electrodes and one of the substrate-side pad electrodes through the solder.
In the connection method for the pad electrodes in accordance with the present invention, the substrate-side pad electrodes are arranged inside the component-corresponding region; the length of each of the substrate-side pad electrodes is set to be larger than that of the corresponding component-side pad electrode; and the surface-mount component is placed on the substrate so that each of the solder bumps are opposed to a predetermined substrate-side pad electrode, and heated to melt the solder bumping. Therefore, the solder which flows and spreads over each of the substrate-side pad electrodes, allows each of the component-side pad electrodes and one of the substrate-side pad electrodes to be connected with reliability.
Also, since each of the substrate-side pad electrodes is formed inside the component-corresponding region, a high-density mounting can be achieved without risk of generating short-circuiting, as well as the miniaturization of the products can be met. Furthermore, the interconnection between the substrate-side pad electrodes and the substrate side can be achieved by connecting to the wiring formed inside the substrate or on the back surface thereof through, for example, via holes or through holes. Thereby, a configuration wherein the substrate-side pad electrodes or wiring are not formed outside the component-corresponding area, can be obtained.
Moreover, since the length of each of the substrate-side pad electrodes is set to be larger than that of each of the component-side pad electrodes, it is possible to make a pass/fail discrimination, with ease and reliability, of the connection state of the pad electrodes by detecting the shapes of the solder (solder bumps) after they have been melted and have connected the pad electrodes, for example, in a nondestructive inspection by X-ray imaging. Specifically, if the solder bumps still have unchanged shapes, it will be recognized that the solder bumps have not yet been melted and flowed, while if the solder bumps have shapes other than the original shapes thereof, it will be recognized that the solder bumps have been melted and flowed, and consequently that each of the component-side pad electrodes and one of the substrate-side pad electrodes are connected by the solder with reliability.
In the present invention, as a substrate, a low-temperature sintered multilayer substrate, a resin substrate, an alumina package, or the like may be used.
When the low-temperature sintered multilayer substrate is used as a substrate, electrodes each formed by plating a thick-film copper electrode with nickel and gold may be used as substrate-side pad electrodes. When the resin substrate is used as a substrate, electrodes each formed by plating a copper foil with nickel and gold may be used as substrate-side pad electrodes. Also, when the alumina package is used as a substrate, electrodes each formed by plating a tungsten electrode with nickel and gold may be used as substrate-side pad electrodes.
However, other types of substrates and/or other types of substrate-side pad electrodes may be employed instead.
In the above-described connection method for the pad electrodes, preferably, the width of each of the component-side pad electrodes and that of the solder bump on each of the component-side pad electrodes are set to be larger than that of each of substrate-side pad electrodes.
By setting the width of each of the component-side pad electrodes larger than that of each of the substrate-side pad electrodes, and by setting the width of the solder bump on each of the component-side pad electrodes larger than that of the substrate-side pad electrode, it becomes possible to increase the flowing-in amount of solder in the longitudinal direction of each of the substrate-side pad electrodes, and thereby to improve the accuracy of a pass/fail discrimination of the connection state between each of the component-side pad electrodes and one of the substrate-side pad electrodes.
An inspecting method for the connection state of the pad electrodes in accordance with the present invention, is a method for inspecting the connection state of the pad electrodes connected by the above-described connection method. This inspecting method is characterized in that the shapes of the solder after each of the solder bumps has been melted and flowed on one of the substrate-side pad electrodes, are detected by a nondestructive inspection, and that thereby a pass/fail discrimination of the connection state between each of the component-side pad electrodes and one of the substrate-side pad electrodes is made.
By detecting the shapes of the solder after each of the solder bumps has been melted and flowed on one of the substrate-side pad electrodes, by a nondestructive inspection, in the above-described connection method for the pad electrodes in accordance with the present invention, it becomes possible to make a pass/fail discrimination, with ease and reliability, of the connection state between each of the component-side pad electrodes and one of the substrate-side pad electrodes by means of solder bumps. This allows the reliability of mounting of surface-mount components to be improved.
Another inspecting method for the connection state of the pad electrodes in accordance with the present invention, is a method for inspecting the connection state of the pad electrodes connected by the above-described connection method. This inspecting method is characterized by comprising the step of obtaining an X-ray transmission image by radiating X rays from the back surface side of the substrate; and the step of detecting the shape of solder after each the solder bumps has been melted and flowed on one of the substrate-side pad electrodes, from the obtained X-ray transmission image, and thereby making a pass/fail discrimination of the connection state between each the component-side pad electrodes and one of the substrate-side pad electrodes.
By obtaining an X-ray transmission image by radiating X-rays from the back surface side of the substrate, and by detecting, from the obtain X-ray transmission image, the shapes of the solder after each of the solder bumps has been melted and flowed, a pass/fail discrimination of the connection state between each of the component-side pad electrodes and one of the substrate-side pad electrodes by means of solder bumps, can be made with ease and reliability. This enables the reliability of mounting of surface-mount components to be improved.
The connection structure between the pad electrodes in accordance with the present invention is a connection structure between each of component-side pad electrodes formed on the surface of a surface-mount component opposed to the substrate and one of substrate-side pad electrodes formed on the surface of the substrate. This connection structure between the pad electrodes is characterized in that the substrate-side pad electrodes are arranged inside a component-corresponding region, and that the length of each of the substrate-side pad electrodes is set to be larger than that of the corresponding component-side pad electrode; and that each of the component-side pad electrodes is connected to the corresponding substrate-side pad electrode by the melted solder bump flowed therein.
In the above-described connection structure between the pad electrodes, since the substrate-side pad electrodes are formed inside the component-corresponding region, short-circuiting among electrodes of surface-mount components does not occur even when surface-mount components are mounted at a high density. Also, since the length of each of the substrate-side pad electrodes is set to be larger than that of the corresponding component-side pad electrode, it is possible to make a pass/fail discrimination of the connection state between the pad electrodes by detecting the shapes of solder after the solder bumps have been melted and flowed, for example, in a nondestructive inspection by X-ray imaging.
In the connection structure between the pad electrodes in accordance with the present invention, preferably, the width of each of the component-side pad electrodes is set to be larger than that of each of substrate-side pad electrodes.
By setting the width of each of the component-side pad electrodes larger than that of each of the substrate-side pad electrodes, it becomes possible to make the width of the solder bump of each of the of the component-side pad electrodes larger than that of the substrate-side pad electrode, when forming a solder bump on each of the component-side pad electrodes. This allows the flowing-in amount of solder in the longitudinal direction of each of the substrate-side pad electrode to be increased, and thereby enables the accuracy of a pass/fail discrimination of the connection state between each of the component-side pad electrodes and one of the substrate-side pad electrodes to be improved. Herein, the above-described connection structure between the pad electrodes can be obtained by the above-described connection method for the pad electrodes in accordance with the present invention.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.