Metal fusion bonding – Process – Plural joints
Reexamination Certificate
1997-09-11
2001-01-30
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S103000
Reexamination Certificate
active
06179198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of soldering a bump of a bumped work to a pad of another work, and, more particularly, to a method of soldering a bump of a bumped electronic component on a pad of a circuit board.
2. Description of the Prior Arts
Soldering is conventionally performed between a bumped electronic component such as a flip chip and a circuit board such as a printed circuit board as follows. First, flux is applied on the surface of a bump formed on the bumped electronic component, or of a solder portion formed on the pad of the circuit board. Then, the bump is placed on the solder portion to mount the bumped electronic component on the circuit board. Subsequently, the circuit board is fed into a heating furnace to heat the solder portion to a temperature at its melting point (typically, about 200° C.) or higher for melting it, and then to cooling the solder portion so as to solidify it. In this procedure the bumped electronic component is mounted on the circuit board. Thereafter, a conductivity test is conducted for examining whether or not the bump of the bumped electronic component is properly soldered on the pad of the circuit board. The conductivity test is typically performed by attaching a probe of the tester to a terminal of the circuit board to check the conductivity. If the results of the conductivity test are not acceptable, a repair work is performed on the bumped electronic component. In the repair work, the circuit board mounted with the bumped electronic component is heated to remelt the solder portion, thereby removing the bumped electronic component from the circuit board. Once the bumped electronic component is removed, a part of the solder portion remains affixed to the pad. Then, the solder portion on the pad is scraped off with a tool or the like. After the solder portion is scraped off, a solder portion of a suitable amount is formed again on the pad. When the correction of the circuit board is completed with the above procedure, the bumped electronic component is mounted again on the circuit board.
However, a problem associated with the conventional method is that the remolten solder on the solder portion flows sidewise, and adjacent solder portions connect each other to cause the formation of a solder bridge, leading to short-circuiting. The reason therefor lies in the following.
Since the solder portion (alloy of low melting point metals such as Pb and Sn, i.e. lead and tin) formed on the pad of the circuit board is easily oxidized when it contacts air, an oxide film has been formed on the surface of the solder portion. The oxide film hinders wettability of the solder, and causes defective conductivity between the bump and the pad on the circuit board.
Then, the wettability of solder is conventionally improved by applying flux on the bump or solder portion. When the flux is used, the oxide film can be removed in heating and melting the solder portion, whereby the wettability of solder is improved to firmly solder the bump on the pad. However, if the oxide film is removed by using the flux, the fluidity of solder increases, so that adjacent solder portions connect each other to cause a solder bridge.
Since the flux has properties which deteriorate a circuit pattern on the circuit board, the conventional soldering method requires a step for washing and removing the flux after the bump is soldered on the pad. Thus, another problem associated with the conventional method is the number of steps required.
Furthermore, another problem associated with the conventional method for conducting a repair is that it is complicated, which requires much labor and a number of steps.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a method of soldering a bump of a bumped work on a pad of another work without using flux, so that good conductivity can be attained, yet without the formation of a solder bridge.
A second object of the present invention is to provide a method of mounting a bumped electronic component which facilitates the performance of a repair work if the quality of the soldering of the bump is not acceptable work.
The present invention can provide the method including the steps of pressing a projection projected downward from a bump of a bumped work against a solder portion formed on a pad of another work to partially break an oxide film on the solder portion with the projection, heating and melting the solder portion at a temperature equal to or lower than the melting point of the bump and the oxide film, and solidifying it.
In a constitution of the present invention, the projection of the bump breaks the oxide film on the surface of the solder portion, so that sufficient conductivity can be provided between the bump and the pad of the other work. In addition, since flux is not used, the oxide film on the surface of the solder portion is broken only at an area broken through by the projection of the bump, and the other area of the oxide film remains on the surface of the solder portion. Then, since the melting temperature of the oxide film is much higher than that of the solder portion, the oxide film is not melted even when the solder portion is heated and melted, and is in a state where the remolten and fluidized solder portion is coated by a hard shell. Therefore, there is no possibility that adjacent solder portions connect with each solder to cause a solder bridge.
The present invention can provide alternatively a method including the steps of placing a bump on a solder portion before heat treatment formed on a pad of a circuit board so as to press the bumped electronic component against a circuit board, thereby partially breaking an oxide film on the surface of the solder portion with the bump to contact the bump with the solder portion, attaching a probe of a tester to a terminal of the circuit board to check conductivity, and heating the bumped electronic component while pressing it against the circuit board to solder the bump to the pad.
In the alternate constitution of the invention, when the bumped electronic component is firmly pressed against the circuit board, the bump partially breaks the oxide film on the surface of the solder portion and contacts the solder portion. Therefore, the conductivity test can be conducted before the soldering process for heating the solder portion to melt and solidifying it, so that a repair work can be easily performed if the results of the test are unacceptable.
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Eifuku Hideki
Ozono Mitsuru
Sakai Tadahiko
Sakemi Shoji
Gamino Carlos J.
Matsushita Electric - Industrial Co., Ltd.
Ryan Patrick
Stevens Davis Miller & Mosher L.L.P.
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