Electric heating – Microwave heating
Reexamination Certificate
1999-06-01
2001-03-20
Walberg, Teresa (Department: 2811)
Electric heating
Microwave heating
C148S400000, C420S560000, C228S208000, C228S209000, C205S177000
Reexamination Certificate
active
06204490
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing an electronic circuit board on which LSIs, components, etc. to be used in an electronic circuit board are mounted, and particularly to a method of manufacturing an electronic circuit board on which these LSIs, components, etc., are mounted by means of a lead-free Bi-system solder. The solder alloy can be applied in bonding electronic components such as LSIs to a circuit board made of an organic material and is an alternative to conventional eutectic Pb—Sn alloy solders used in the soldering at a temperature of from 220° C. to 230° C.
2. Description of the Related Art
There have been problems of pollution of the global environment and adverse affects on living things by lead (Pb) in a eutectic Sn-Pb solder. It is thought that the pollution of the global environment occurs when lead is dissolved by rain, etc. from the dumped lead-containing electrical appliances exposed to sunlight and rain. The dissolution of lead tends to be accelerated by the recent acid rain. In order to reduce environmental pollution, therefore, it is necessary to use a lead-free solder alloy of low toxicity containing no lead as an alternative for the eutectic Sn—Pb alloy solder which is popular. This demand can be solved by using lead-free solder alloys which contain Bi, etc., such as an Sn—Ag—Bi alloy and an Sn—Ag—Bi—Cu alloy.
However, the lead-free solders containing Bi, etc. have had a problem of lacking reliability because of exfoliation at joint portions. Furthermore, the lead-free solders have had the problem that deterioration of strength and exfoliation occur due to heating of surrounding bonding portions (which do not require repairs) during repairing.
More specifically, a phenomenon called “lift-off” is known when a lead-free Bi-system solder is used in soldering a printed circuit board. The phenomenon is of exfoliation between a land of the board and the solder that occurs after soldering. It has been known that the lift-off occurs due to segregation of Bi in a Sn—Bi alloy solder (Collection of Lectures and Papers on Circuit Mounting at Grand Scientific Lecture Meeting, p. 67, Mar. 24, 1998). However, the mechanism of the lift-off is complex and has been uncertain. Further, it is shown in a News Paper of the Nikkan Kogyo Shinbun issued on of Apr. 27, 1998 that the lift-off can be prevented by rapid cooling using water or a steam spray.
Thus, a problem to be solved by the present invention is to prevent deterioration of strength and the lift-off phenomenon at bonding portions of lead-free Sn—Ag—Bi alloy solders having a low melting point, etc. and to prevent occurrence of deterioration of strength and the lift-off of surrounding bonding portions (which do not require repairs) during repairing. Namely, when soldering is performed in an ordinary flow process by means of a lead-free Bi-system solder, the lift-off occurs between the solder and a land. If the lift-off can be prevented, it is possible to perform soldering at a temperature comparable to the soldering temperature for the conventional eutectic Sn-Pb alloy solder. A problem of damage to the solder and components and an adverse effect due to a cooling liquid (especially, corrosion of contact portions) arises simply in the case of rapid cooling.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method and apparatus of manufacturing an electronic circuit board which can be free from fouling, corrosion, etc. of connector portions by relaxing thermal shocks to components, etc. without occurrence of the lift-off and without generation of cracks in the fillet portion of a solder joint.
Under the above object, according to the invention, there is provided a method of manufacturing an electronic circuit board which comprises bonding electronic components to the circuit board by means of a Bi-containing lead-free solder. The electronic components and circuit board are bonded together followed by cooling the solder at a cooling rate of from about 10 to 20° C./second.
According to one feature of the invention, in the method of manufacturing an electronic circuit board, which comprises bonding electronic components to a circuit board by means of a Bi-containing lead-free solder, the electronic components and circuit board are bonded together followed by cooling from a temperature close to the liquidus temperature to a temperature close to the solidus temperature of the solder at a cooling rate of from about 10 to 20° C./second.
According to another feature of the invention, in the method of manufacturing an electronic circuit board, which comprises bonding electronic components to a circuit board by means of a Bi-containing lead-free solder, the electronic components and circuit board are bonded together followed by cooling from a temperature close the liquidus temperature to a temperature close to the solidus temperature of the solder at a first cooling rate of from about 10 to 20° C./second and at a subsequent second cooling rate lower than the first cooling rate. The second cooling rate may be from 0.1 to 5° C./second.
According to still another feature of the invention, in the method of manufacturing an electronic circuit board, which comprises bonding electronic components to a circuit board by means of a Bi-containing lead-free solder, cooling is performed at a cooling rate of from 0.1 to 5° C. in the temperature range on or below a temperature close to the solidus temperature of the solder. The cooling may be performed with a cooling medium, such as air, inert gas, spray liquid, steam, liquid, liquid nitrogen and dry ice, at room temperature, at the highest, or below. The inert gas may be a mixture containing any one of liquid nitrogen and dry ice. Also, cooling may be performed by a flux cleaning agent, such as the Fluorinert liquid, at room temperature or less at the lowest in spray or shower form.
It was found by experiments that cracks in solder joints are prevented as stated above by rapid cooling at a cooling rate of from 10 to 20° C./second even if electronic components and a circuit board are bonded by a Bi-containing lead-free solder. Namely, it is possible to provide a method of manufacturing an electronic circuit board with few imperfections in soldering bonding portions by rapid cooling at a cooling rate of from 10 to 20° C./second.
It was also found that cracks in solder joints are prevented by slow cooling at a cooling rate of from 0.1 to 5° C./second in the temperature range on or below a temperature close the solidus temperature in the case where electronic components and a circuit board are bonded by means of a Bi-containing lead-free solder, because stress due to a temperature difference from the solidus temperature to room temperature is relieved. Namely, it is possible to provide a method of manufacturing an electronic circuit board with few imperfections in soldering-bonding portions by slow cooling in the temperature range on or below a temperature close to the solidus temperature.
It is possible to manufacture a printed electronic circuit board which is excellent in comprehensive respects by combining the two cooling ways of rapid and slow.
The above deterioration in strength and the lift-off of joints is caused by a difficulty in bonding between Cu and sulfur, which is due to segregation of Bi in the solder joint at the interface between the solder joint and the Cu land as below:
(1) When cooling after soldering, the substrate is cooled more slowly than the lead because of the higher heat capacity of the substrate than that of the lead.
(2) From the phase diagram of an Sn—Ag—Bi alloy, it is apparent that the difference between the liquidus temperature and the solidus temperature is as great as tens of degrees.
(3) Because of a high temperature of the Cu land of the substrate after soldering, the interface region finally soldifys and Bi segregates there.
(4) According to observations and analyses, it became apparent that in a joint where deterioration in strength occurs, Bi crystals segregate
Amano Yasuo
Ishida Toshiharu
Nakatsuka Tetsuya
Sakaguchi Suguru
Serizawa Koji
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Pwu Jeffrey
Walberg Teresa
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