Metal fusion bonding – Process – With condition responsive – program – or timing control
Reexamination Certificate
2000-07-12
2002-02-12
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
With condition responsive, program, or timing control
C228S042000, C219S388000
Reexamination Certificate
active
06345757
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a reflow soldering method and a reflow soldering furnace for heating a printed board on which solder paste is printed and electronic parts, such as surface-mounted devices (hereinafter referred to as “SMDs”), mounted on the printed board and soldering the electronic parts to the board.
Reflow soldering is known as a mounting technique for electrically connecting and mechanically fixing electronic parts, such as SMDs, to a printed board. In a reflow soldering process, various SMDs are mounted on a printed board, on which solder paste is printed in advance, in a manner such that their leads are in alignment with pads of a thin film circuit on the printed board. Thereafter, the printed board is introduced into a reflow soldering furnace (hereinafter sometimes referred to as “reflow furnace”) and heated, whereupon the solder paste is melted so that the SMDs are soldered to the printed board.
The reflow soldering furnace for carrying out this reflow soldering process comprises a furnace body that is provided with a conveyor for conveying the printed board. In the reflow soldering furnace body, preheating zones and a main heating zone (or reflow zone), which are defined by furnace walls, arranged in the conveying direction of the conveyor. The printed board and the SMDs thereon, as to-be-heated objects, are heated by means of heating means that are provided in the zones, individually. The heating means may be conventional heating devices, such as a hot-gas applier for blowing a hot gas against each to-be-heated object and a radiant-heat applier using a far infrared heater and the like.
In the preheating zones of the reflow soldering furnace, each to-be-heated object is heated to a temperature of 120 to 170° C. to ease thermal shocks on the SMDs. In the main heating zone that follows the preheating zones, the to-be-heated object is heated to a temperature of 210 to 230° C., which is higher than the melting point (180° C.) of solder by 30 to 50° C., whereby the solder is melted. The to-be-heated object delivered from the main heating zone is subjected to natural or forced cooling so that the solder solidifies, whereupon the reflow soldering is completed.
With the advance of diversification of electronic parts such as SMDs, there is an increasing demand for printed boards that are mounted with a large number of electronic parts of various types each. Accordingly, a large number of electronic parts with different sizes (or different heat capacities) are expected to be reflow-soldered to each printed board efficiently and securely. On the other hand, there are printed boards of various sizes. In some cases, electronic parts may be mounted on large-sized printed boards with large heat capacities. In consideration of these circumstances, electronic parts are expected to be reflow-soldered to various printed boards with high efficiency and reliability.
In the conventional reflow soldering process, the entire to-be-heated object is heated in the furnace in which the temperature is raised to a level higher than the melting point of solder by means of a hot gas or a combination of a hot gas and an infrared heater. If the heater output is not high enough for large-sized electronic parts with large heat capacities, however, the temperatures of the parts and their surroundings cannot be raised with ease. In some cases, therefore, joints (solder joints) between the printed board and leads of the electronic parts may not be able to be heated to a predetermined temperature, resulting in defective soldering.
The aforementioned underheating can be compensated with an increase of the hot gas temperature or the heater output. If this is done, however, those portions of the printed board which carry no electronic parts thereon or small-sized electronic parts with small heat capacities will overheat. In such a case, the thin film circuit on the printed board may be disconnected or cracked, and the small-sized parts may possibly be damaged or lowered in properties.
BRIEF SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a reflow soldering method and a reflow soldering furnace, capable of carrying out appropriate reflow soldering without entailing under- or overheating despite the differences in heat capacity between electronic parts mounted on a printed board.
In order to achieve the above object, according to the invention, there is provided a reflow soldering method for heating a to-be-heated object to a target temperature in one or more heating zones, comprising blowing a hot gas of a temperature lower than the target temperature against the to-be-heated object by using hot-gas applying means in the heating zones and applying radiant heat of a temperature higher than the target temperature to the to-be-heated object, thereby heating the to-be-heated object to the target temperature.
According to this reflow soldering method, electronic parts with a small heat capacity are cooled by means of the hot gas of the temperature lower than the target temperature for the heating zones, while electronic parts with a large heat capacity are heated to the target temperature by means of the radiant heat. By doing this, a plurality of electronic parts with different heat capacities can be soldered to a printed board when the temperature differences between the electronic parts are reduced.
In order to achieve the above object, according to the present invention, there is provided a reflow soldering method for preheating a to-be-heated object to a temperature lower than the melting point of solder in one or more preheating zones and then heating the to-be-heated object to the melting point of the solder in a main heating zone, comprising blowing a hot gas of a temperature lower than the target temperature for the preheating zones against the to-be-heated object by using hot-gas applying means in the preheating zones and applying radiant heat of a temperature higher than the target temperature to the to-be-heated object, thereby heating the to-be-heated object to the target temperature.
According to this reflow soldering method, electronic parts with a small heat capacity are cooled by means of the hot gas of the temperature lower than the target temperature for the preheating zones at least in the preheating zones, while electronic parts with a large heat capacity are heated to the target temperature by means of the radiant heat. By doing this, a plurality of electronic parts with different heat capacities can be soldered to a printed board in the main heating zone when the temperature differences between the electronic parts are reduced.
In the reflow soldering methods according to the invention described above, the heating of the to-be-heated object by means of the radiant heat includes joint use of far infrared rays with a wavelength of 2.5 to 5,000 &mgr;m and infrared rays including near infrared rays with a wavelength of 0.75 to 2.5 &mgr;m. In some cases, the joint use of the infrared rays and the far infrared rays may be an effective measure for further reduction of the temperature differences between the parts on the printed board. In general, the printed board easily absorbs infrared rays with a wavelength of 2.5 &mgr;m or more, while the electronic parts on the printed board easily absorb infrared rays with a wavelength of less than 2.5 &mgr;m. Thus, the printed board and the electronic parts thereon have their respective infrared absorption spectra. In consequence, the temperature differences between the printed board and the electronic parts can be further reduced by jointly using an infrared heater and a far infrared heater and controlling the ratio between the respective outputs of these heaters.
Preferably, the radiation spectra of the infrared heater should exhibit a maximum value within a wavelength region of less than 2.5 &mgr;m, and further preferably, within a region from 1 to 2.5 &mgr;m. On the other hand, the radiation spectra of the far infrared heater should preferably exhibit a maximum value with
Matsuoka Taro
Sakuyama Seiki
Dunn Tom
Edmondson L.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Fujitsu Limited
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