Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-12-20
2004-08-31
Chang, Richard (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S846000, C029S848000, C029S852000, C438S123000, C438S125000, C438S126000, C438S127000, C438S111000, C438S112000, C216S014000, C257S676000, C205S125000
Reexamination Certificate
active
06782610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wiring substrate for mounting semiconductor elements and a fabricating method thereof.
2. Description of Related Art
As a wiring substrate for mounting semiconductor elements, which an LSI (Large Scale Integrated) chip is mounted, which is connected to a printed wiring, etc., and which is called, for example, an interposer for mounting semiconductor elements, there are two kinds of wiring substrates: one with a resin base such as a polyimide film, and the other with a metal base such as copper. Here, a description will be given on a fabricating method of the wiring substrate with a polyimide film base.
A polyimide film is prepared as a base, and a thin copper film having a thickness of about, for example, 0.2 &mgr;m is formed on each main surface of the base by, for example, sputtering. Next formed by drill machining or press working is a through hole for connecting to one another wiring films that are to be formed on those main surfaces. Thereafter, a copper film with a thickness of about, for example, 5 &mgr;m is formed by electroless plating on the surface of each of the above thin copper films. A resist film for patterning is further formed on this copper film placed on each surface of the base, and a copper wiring film (with a thickness of 30 &mgr;m, for example) is formed on each of the copper film by electroplating while using the resist films as masks. With this, one of the copper wiring films, which is the one formed on the front side of the base constitutes a normal circuit wiring and the other one formed on the back side of the base constitutes a ground line and a power source, line.
After removing the resist films used as masks, then removed by soft etching of copper are the thin copper films that have been formed over the entire surfaces as under films upon formation of the copper wiring films prior to the formation of the above resist films. By this removal, the copper wiring films are no longer electrically short-circuited with the thin copper films, resulting in copper wiring films that are independent of each other.
An insulating resin film is next applied to the front side surface and is patterned through exposure and development so that openings are formed in a portion where a solder ball is to be formed and a portion to be joined with an LSI. After that, the base is selectively etched from its back side to expose a portion of the copper wiring film, which is to be joined with the LSI, to enhance by, for example, gold electroless plating the connectivity of the copper wiring film surface to the ball electrode such as the solder ball or to the LSI. Thus fabricated is the wiring substrate called the interposer.
The above wiring substrate is bonded at its back surface side to an LSI chip through a buffer adhesive, and then lead ends of the copper wiring film are micro-joined with electrodes of the LSI chip. After sealing the micro-joined portion with a resin, one of the copper wiring films, which to on the opposite side of the base to the LSI chip, is plated, and the solder ball is mounted on a portion exposed to the opening of the insulating resin film. The solder ball is then shaped by reflowing.
Subsequently, a description will be made on a method of fabricating a conventional wiring substrate of the type with a metal base, such as copper. A base made of, for example, copper and having a thickness of about, for example, 100 to 200 &mgr;m is prepared. A resist film is formed so as to have a pattern negative to the pattern of a copper wiring film to be formed. Using this resist film as a mask, a thin gold film is formed by electroplating on one surface of the copper base, and the wiring film is then formed by electroplating of the copper film. On a region where this wiring film is formed, an insulating film having an opening in a portion to form a ball electrode and having a pattern that does not cover a portion of the wiring film, which is used as electrodes of an LSI chip, is formed. After the ball electrode is formed from, for example, nickel or gold by plating in the opening portion of this insulating film, the base, except for its periphery, is removed by selective etching from the back surface side to expose the back surface side of the wiring film. Thus, the wiring substrate called the interposer is fabricated. This wiring substrate is bonded at its back surface side to the LSI chip through a buffer adhesive, leads of the wiring film are connected to electrodes of the LSI chip, and the substrate is sealed with resin, thereby completing the mounting of the LSI chip.
Wiring substrates of the type with a polyimide film base generally have problems as follows. First, when circuits are formed on both surfaces of the base and are connected to each other to obtain a two-layer circuit with the aim of high integration, mechanical punching out is required. The hole punched out has to be minute as high integration and to downsizing of the substrate are taken, which is likely to make it difficult to connect to one another the circuits in the two-layer circuit. In addition, processing accuracy (about position and shape) of the hole, a final package outer shape and ball position accuracy are not easily be enhanced, and hence it is becoming more and more difficult to achieve the accuracy demanded.
Also, having the base of a polyimide film, the wiring substrate is hard to have sufficiently enhanced physical strength upon completion as a circuit board. Therefore, it can not avoid a problem of high likelihood of deformation, etc., at the packaging process.
Further, since the polyimide film forming the base is an insulating material and it is difficult to apply electric potential, it makes electroplating virtually impossible. This brings about another problem of the solder ball being required to be mounted in post-attachment at the packaging process. Namely, because of extremely poor adherence related to the joining strength between solder and copper, a very large area has to be saved for the solder ball mounting portion in the wiring film in order to prevent a defeat of ball falling off. This leads to a reduced number of wirings that are allowed to thread through (be formed between) the solder balls adjacent to one another in the arrangement pitch of the solder balls, which gives rise to still another problem by being a great cause for blocking high integration.
In order to improve reliability against falling off of the ball, which is judged in the temperature cycle test performed after the balls are mounted to the substrate, an area allotted for every solder ball needs to be large. For that reason, the number of wirings running through (formed between) the balls is restricted, presenting still another problem of putting limitation on design in which densification is intended by increasing the number of the balls.
The polyimide film forming the base also serves as a carrier in the fabrication. The film is thus not easy to form thinner, making it hard to lower the usage amount and cost of the material, and further brings about a problem of poor connectivity between the circuits on both surfaces. Still further, the thick polyimide film absorbs a lot of moisture, causing the package crack at the time of mounting.
Therefore, a wiring substrate using as the base a metal instead of the polyimide film has been developed. The wiring substrate as such does solve a portion of the above-described problems inherent in the wiring substrate using the polyimide film as the base, but that conventional wiring substrate is not yet free from problems. That is, since the base to formed from a metal, in order to form a circuit on its surface by electroplating, it is required to remove most of the metal forming its base by selective etching from the back surface. The substrate therefore should take a considerably complicated ma structure in the case of forming the two-layer wiring circuit. Further, when the whole base under the wiring film is removed, the substrate is weak to the deformation from the vie
Hirade Shigeo
Iijima Tomoo
Oosawa Masayuki
Armstrong Kratz Quintos Hanson & Brooks, LLP
Chang Richard
North Corporation
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