Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1998-09-08
2001-08-07
Evans, Geoffrey S. (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S002100, C228S041000, C228S254000, C228S262200
Reexamination Certificate
active
06270002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for arranging balls on a mounting object. More particularly, the present invention relates to a method and an apparatus for arranging fine balls serving as ball bumps on an electrode on a semiconductor chip, an electrode on a semiconductor-mounting substrate, or on a semiconductor device electrode.
2. Description of the Related Art
Fine balls (bumps) are used as a medium for connecting an electrode on a semiconductor chip substrate and an external circuit. The technique of forming bumps for a ball grid array (BGA) by transferring fine balls onto a semiconductor substrate or a semiconductor chip is becoming more important. In this specification, a semiconductor chip for transferring balls and the like are collectively referred to as a mounting object.
The conventional technique for transferring balls onto a mounting object is as follows. A ball arrangement device has a ball suction holes pierced for sucking many balls. The positions of the ball suction holes correspond to positions at which the balls on a semiconductor chip or the like are to be transferred. All the balls are previously sucked in the suction holes, and the ball arrangement device is transferred to a mounting base. The mounting object onto which the balls are to be transferred is placed on the mounting base in advance. The balls can be transferred by pressing the balls sucked on the ball arrangement device onto the surface of the mounting object on the mounting base.
Along with the recent higher integration of semiconductor devices, there is an increasing number of electrodes for a single semiconductor. Usually, a semiconductor is provided with 100 to 300 electrodes, and more recently, a single semiconductor device has even 500 to 1,000 electrodes. As a result, the area of a semiconductor tends to increase, and the size of a ball is becoming smaller, and at the same time, there is a tendency of the arrangement interval between balls toward a narrower value. Furthermore, in the aforementioned technique of transferring the balls as a bump, it is becoming a common practice not only to transfer the balls in a number equal to that of electrodes for a semiconductor in a lump, but also to mount balls in a lump on a plurality of chips, thereby improving productivity.
In the above-mentioned conventional technique, many fine balls having a uniform size are provided in a tray. The surface sucking the balls of the ball arrangement device is brought closer to the fine balls in the tray. The ball suction holes are caused to suck the fine balls by sucking air through reduction of pressure on the back of the ball suction holes (FIG.
18
). Suction can more easily be accomplished by vibrating the tray upon suction to cause the ball to jump in the tray, or by causing the balls to float in the tray by air blow through the bottom of the tray.
The fine balls fed into the tray may contain balls having a diameter off the standard upper limit or off the standard lower limit mixed therein although in a slight amount. While balls having a larger diameter off the standard upper limit can easily be excluded in the manufacturing stage of balls, exclusion of balls having a smaller diameter off the standard lower limit may sometimes be insufficient. In the conventional ball arrangement apparatus, which does not have a function of excluding defective balls having a diameter smaller than the standard lower limit, mixture of defective balls would result in mounting of balls of an insufficient diameter on a product such as a ball grid array. When using solder balls, it is the usual practice to preliminarily attach the solder balls to the mounting object with bonding power of flux. small diameter of solder balls results in occurrence of such defects as some of solder balls left uncoated with flux or solder balls not coming into contact with the electrodes. When using gold balls, means for thermally pressing the gold ball against the object is employed. Gold balls having a small diameter lead to a defect in that sufficient pressing is unavailable, making it impossible to obtain pressure contact.
Because of feeding balls in a large quantity to the tray to cause the balls to jump therein, and as required to vibrate the tray, dust and dirt are inevitably entangled into the tray from surroundings. Since dirt and dust are mixed with the balls in a large quantity in the tray, it is impossible to remove them. If dust is sucked into the ball suction hole of the ball arrangement device in place of the balls, arrangement of the balls ends in failure.
In order to remove balls in excess adhering to the ball arrangement device, there are proposed a method for excluding balls in excess with an air blow or a brush after suction of the balls, and a method, disclosed in Japanese Unexamined Patent Publication No. 8-64,944(U.S. Pat. No. 5,657,528), of causing relative displacement of the ball arrangement device and the tray of the balls.
It has been the conventional practice to adopt a diameter of the ball suction hole of the ball arrangement device about 0.5 times as large as the diameter of the balls to e sucked. With a larger diameter of the ball suction hole, the balls cut into the ball suction holes under the pressure applied upon transfer of the balls onto the mounting object. When transferring gold balls requiring adoption of thermal pressure-contact, pressure is high, and when transferring solder balls, not requiring a high transferring pressure, the solder balls are susceptible easy deformation: it is therefore necessary to use a smaller diameter of the ball suction holes to prevent cut-in of the balls in all cases. As the sucking force of the ball is proportional to the cross-sectional area of the suction hole, a smaller diameter of the suction hole inevitably leads to a smaller sucking force.
When using solder balls, flux is usually transferred to the position of transfer on the mounting object or to the solder balls prior to transferring the balls to the mounting object. Transfer carried out without taking notice of adherence of excessive balls may a trouble of the flux adhering via the balls to the ball arrangement device. This not only results in a single defective semiconductor device to which excessive balls have been transferred, but also makes it impossible to start the next operation unless the ball arrangement device having the flux adhering thereto is washed, thus causing a serious decrease in equipment productivity. It is therefore necessary to prevent occurrence of an abnormality of adhering balls in excess.
A first object of the present invention is to exclude balls of a small diameter off the standard lower limit and entangled dust and dirt before sucking the balls into the ball arrangement device, thereby preventing failure of arrangement of the balls.
In the conventional method of sucking the balls jumping or floating in the ball tray into the ball suction holes of the ball arrangement device, each ball sucking hole must suck one ball. However, a plurality of balls may be sucked into a single ball suction hole, or balls may adhere to a portion other than the ball suction hole on the ball arrangement device, or there may be some ball suction holes not sucking the balls.
An attracting force may act between a ball and the ball arrangement device, so that a ball may be attracted at a portion other than the portions where vacuum attracted acts as at the ball suction holes. In order to exclude these excessive balls by means of an air blow or a brush, it is necessary to impart a force of a prescribed magnitude to these balls in excess. On the other hand, the suction force of the balls to the ball suction holes is proportional to the cross-sectional area of the suction hole and the suction pressure. Because the diameter of the ball suction hole is only about 0.5 times as large as the diameter of the ball as described above, the sucking force of the ball to the suction hole is not sufficient. If an air blow or brush cleaning with a force
Hayashi Nobuaki
Katoh Kouhei
Evans Geoffrey S.
Nippon Micrometal Co., Ltd.
Pittman Zidia T.
Wenderoth , Lind & Ponack, L.L.P.
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