1. Field of the Invention
This invention relates to minute copper balls suitable for use as copper cores of copper core solder balls which are used for forming bumps on semiconductor electronic parts (referred to below simply as electronic parts) such as BGA (ball grid array) devices, CSP""s (chip scale packages), and MCM""s (multichip modules) and to a method for their manufacture.
2. Description of the Related Art
Minute solder balls having a diameter in the range of 50-1000 xcexcm are used for forming bumps on electronic parts. After solder balls are distributed on the electrodes of an electronic part, the solder balls are melted to form bumps. When the electronic part is mounted on a printed circuit board, the electronic part is disposed on the printed circuit board so that the solder bumps on the electronic part coincide with the portions to be soldered on the printed circuit board, and then the solder bumps are melted to solder the electronic part to the printed circuit board.
A typical solder ball is comprised solely of solder. When solder bumps made from such solder balls are formed on an electronic part and melted in order to solder the electronic part to a printed circuit board, the molten solder bumps are flattened by the weight of the electronic part which lies thereon. As a result, adjoining solder bumps may be fused to each other, or the molten solder may spread to the exterior of the electronic parts, thereby causing short circuits and other operational problems.
In light of these problems, xe2x80x9ccopper core solder ballsxe2x80x9d for forming solder bumps on electronic parts have been developed. A copper core solder ball has a core of a copper ball, and the core is coated with solder by plating. If copper core solder balls are used to form solder bumps on an electronic part, when the electronic part is soldered to a printed circuit board, only the solder coating of the copper core solder balls melts. Thus, although the weight of the electronic part is applied to the bumps during soldering, the copper balls forming the cores can support the weight, and the solder balls are not flattened. Accordingly, copper core solder balls enable highly reliable connection of electronic parts to printed circuit boards.
Electronic parts such as BGA devices, CSP""s, and MCM""s have a large number of electrodes, and the distance between electrodes is extremely small. Copper balls for copper core solder balls used with such electronic parts must have a prescribed size with a high dimensional accuracy, and they also need to have good sphericity. As used herein, sphericity of a ball is defined by the following equation:
Sphericity (%)=[(Dmaxxe2x88x92Dmin)÷(Dmax+Dmin)/2]xc3x97100xe2x80x83xe2x80x83(1)
where Dmax is the largest diameter of a solder ball and Dmin is the smallest diameter thereof. It is desirable for copper balls for use in copper core solder balls to have an average sphericity (for 100 samples) of 2% or lower.
Copper balls used as cores for copper core solder balls have conventionally been manufactured by a method which comprises cutting a thin copper wire to make copper chips having fixed lengths and heating the copper chips to a temperature above the melting point of copper to form them into spheres. However, with such a conventional method, no matter how the manufacturing conditions are varied, it is difficult to obtain copper balls having an average sphericity of at most 2%.
Since copper balls manufactured by a conventional method have poor sphericity, copper core solder balls produced from these copper balls by plating with solder likewise have poor sphericity. When copper core solder balls having poor sphericity are being distributed on an electronic part, they cannot be held reliably by a suction device used to grasp solder balls in a solder ball distributing apparatus, and the solder balls are sometimes not distributed on all the electrodes of the electronic part, i.e., some electrodes may not have a solder ball thereon.
Another problem with conventional copper core solder balls is that the bonding strength of solder bumps formed therefrom on an electronic part is lower than the bonding strength of solder bumps formed from solder balls comprised solely of solder.
This invention provides minute copper balls which have good sphericity and which have a sufficient bonding strength when plated with solder to form copper core solder balls and then used to form solder bumps on electronic parts.
This invention also provides a method for the manufacture of such copper balls.
The present inventors found that instead of forming minute copper balls from pure copper, it is effective to form copper balls by melting small pieces comprising a Cuxe2x80x94Zn alloy to form the pieces into spheres. Since Zn has a much higher vapor pressure than Cu, most of Zn is removed by vaporization at the high temperature at which copper melts, and only a slight amount of Zn remains in the balls formed by melting. Although the resulting minute balls contain a slight amount of Zn, they can be effectively used as minute copper balls in the manufacture of copper core solder balls. Compared to Cu, a Cuxe2x80x94Zn alloy has a high surface tension when melted, and it forms minute copper balls having a high sphericity when formed into spheres by melting. In addition, it has been found that when bumps are formed on an electronic part using copper core solder balls having cores of copper balls which contain a slight amount of Zn, a decrease in bonding strength encountered with conventional copper core balls can be significantly suppressed.
According to one aspect of the present invention, minute copper balls have a composition consisting essentially of 0.01-0.5 wt % of Zn and a remainder of Cu and having a spherical shape with a diameter in the range of 50-1000 xcexcm.
According to another aspect of the present invention, a method of manufacturing minute copper balls comprises heating small pieces comprising a Cuxe2x80x94Zn alloy, the alloy consisting essentially of 0.5-40 wt % of Zn and a remainder of Cu, to a temperature which is at least the melting point of copper without contacting the pieces with each other to form the pieces into spheres having a Zn content of 0.01-0.5 wt %, and then cooling the spheres to form solid balls.