Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2001-12-11
2004-03-09
Stoner, Kiley (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S254000, C420S560000
Reexamination Certificate
active
06702176
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Sn—Ag—Cu based solder, a method for processing a surface of a printed wiring board and a method for mounting an electronic part, in particular, to a solder, a method for processing a surface of a printed wiring board and a method for mounting an electronic part, wherein the prevention of copper consuming is achieved at the time of soldering.
2. Description of the Related Art
Conventionally an alloy of 63% of Sn and 37% of Pb by mass is utilized as a solder for coating a copper circuit on a printed wiring board and for the connection between a footprint or through holes in a printed wiring board and leads of parts to be mounted. Recently, however, environmental pollution due to lead that has eluted from discarded electronic equipment has become a problem and solder that does not include Pb has been under intense development in the manufacturing of electronic parts.
As for lead-free solder that does not include Pb, a Sn—Cu based alloy, a Sn—Ag—Cu based alloy and a Sn—Zn based alloy are representative and alloys gained by adding Bi, In and/or Ge to these alloys have also been under examination.
As for the Sn—Cu based alloy, however, even an alloy of 99.2% of Sn and 0.8% of Cu by mass, which is of the eutectic composition, has a comparatively high melting point of 227° C. and, therefore, there is a shortcoming wherein in the case that the composition is changed for the purpose of preventing the below described copper consuming, the melting point becomes higher so that the printed wiring board and electronic parts to be mounted cannot withstand high temperature at the time of soldering. The heat-resistant temperature of printed wiring boards in general use is approximately 260° C.
As for the Sn—Zn based alloy, the solder melting point of 91% of Sn and 9% of Zn by mass, which is of the eutectic composition, is 199° C., which is close to the melting point, 183° C., of the alloy of 63% of Sn and 37% of Pb by mass, which is of eutectic composition. Accordingly, this is an alloy that is favorable from the viewpoint of the melting point. However, since Zn is an active element, there is a shortcoming in that the solder is significantly oxidized so that it is difficult to achieve a good soldering condition.
As for the Sn—Ag—Cu based alloy, the three element eutectic alloy of 95.8% of Sn, 3.5% of Ag and 0.8% of Cu by mass has a melting point of 217° C., which is higher than the melting point of the alloy of 63% of Sn and 37% of Pb by mass and the Sn—Zn based alloy and is still low enough for use as solder from the viewpoint of the heat resistance of printed wiring board and the like. In addition, even in a case where the processing temperature for coating a copper circuit on a printed wiring board and for the connection between a footprint or through holes in a printed wiring board and leads of parts to be mounted is assumed to be 250° C., an excellent soldered condition can be gained, of which the mechanical characteristics are also excellent, and, therefore, the Sn—Ag—Cu based alloy is the most suitable for practical application from among the above described lead-free solders.
Sn—Ag—Cu based alloys are disclosed in, for example, Japanese Unexamined Patent Publication No. Hei 2-34295, Japanese Unexamined Patent Publication No. Hei 2-179388, Japanese Unexamined Patent Publication No. Hei 4-333391, Japanese Unexamined Patent Publication No. Hei 6-269983 and Japanese Unexamined Patent Publication No. Hei 11-77366. The solder disclosed in Japanese Unexamined Patent Publication No. Hei 2-34295 is for the purpose of providing a lead-free solder while the solder disclosed in Japanese Unexamined Patent Publication No. Hei 2-179388 is for the purpose of improving corrosion resistance and electrical and thermal conductivity. In addition, the solder disclosed in Japanese Unexamined Patent Publication No. Hei 4-333391 is for the purpose of increasing creep characteristics, the solder described in Japanese Unexamined Patent Publication No. Hei 6-269983 is for the purpose of increasing wettability on the Ni based base material and the solder described in Japanese Unexamined Patent Publication No. Hei 11-77366 is for the purpose of increasing strength against thermal fatigue strength and joint characteristics.
In a case where a Sn—Ag—Cu based alloy is utilized, however, there is a problem wherein when the alloy is coated on a copper circuit of a printed wiring board by means of a hot air leveling method, the copper plating layer of the printed wiring board is consumed so as to be thinner and in the worst case wiring may be cut. In addition, in a case where parts are soldered through flow soldering, the copper plating layer of the printed wiring board is consumed and becomes thinner so as to cause soldering defects.
Therefore, a solder gained by adding 1% to 4% of Cu by mass to a Sn—Sb—Bi—In based alloy has been proposed (Japanese Unexamined Patent Publication No. Hei 11-77368). In addition, a solder gained by adding 1% to 3% of Cu by mass to a Sn—Zn—Ni based alloy, which is a Sn—Zn based alloy, has also been proposed (Japanese Unexamined Patent Publication No. Hei 9-94688).
Both of the solders disclosed in these gazettes are designed to prevent copper consuming by adding Cu. The melting point of the former, however, is too high because the solidus temperature is 208° C. while the liquidus temperature is 342° C. Since the latter is a Sn—Zn based alloy, there is a problem concerning oxidation as described above.
Moreover, in Japanese Unexamined Patent Publication No. Hei 11-216591 data gained by actually carrying out the measurement of the alloys of 96% of Sn, 3.5% of Ag and 0.5% of Co by mass as well as 98.8% of Sn, 0.7% of Cu and 0.5% of Co by mass and the like are described under the assumption that the addition of Co can prevent copper consuming. However, even in a case where copper consuming can be prevented such compositions allow the liquidus temperatures to significantly increase so that it is difficult to apply electronic parts to usable products due to safety concerns.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a solder that can prevent copper consuming and of which the melting point is controlled to the extent where no damage to electronic parts occurs, a method for processing a surface of a printed wiring board on which the solder is utilized and a method for mounting an electronic part by utilizing the solder.
According to one aspect of the present invention, a solder consists essentially of 1.0% to 4.0% of Ag by mass, 0.2% to 1.3% of Cu by mass, 0.02% to 0.06% of Co by mass, and the remaining of Sn and inevitable impurities.
According to another aspect of the present invention, a solder consists essentially of 1.0% to 4.0% of Ag by mass, 0.2% to 1.3% of Cu by mass, 0.02% to 0.06% of Co by mass, 0.02% to 0.06% of Ni by mass, and the remaining of Sn and inevitable impurities.
According to further another aspect of the present invention, a solder consists essentially of 1.0% to 4.0% of Ag by mass, 0.2% to 1.3% of Cu by mass, 0.02% to 0.06% of Co by mass, 0.02% to 0.06% of Fe by mass, and the remaining of Sn and inevitable impurities.
According to still further aspect of the present invention, a solder consists essentially of 1.0% to 4.0% of Ag by mass, 0.2% to 1.3% of Cu by mass, 0.02% to 0.06% of Co by mass, 0.02% to 0.06% of Ni by mass, 0.02% to 0.06% of Fe by mass, and the remaining of Sn and inevitable impurities.
According to the present invention a minute amount of Co prevents copper consuming at the time of soldering. In addition, since the content thereof is of a minute amount, the solder has a composition close to the eutectic of the Sn—Ag—Cu based solder and, thereby, the increase in the liquidus temperature can be controlled. Furthermore, since the solder is a Sn—Ag—Cu based solder, a high wettability and a high spreadability can be secured. In the case where not only Co but, also, a minute amount of Ni and/or Fe is contained, the melting rate of copper can be l
Hara Shiro
Ito Toshihide
NEC Toppan Circuit Solutions, INC
Stoner Kiley
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