JP2004221326A - Soldering method using lead-free solder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a soldering method using a lead-free solder. <P>SOLUTION: The automatic soldering method uses a lead-free solder to apply primary soldering to the lower surface of a printed board to be transferred and finishing soldering successively. Furthermore, the temperature around the lower surface of the printed board is kept at 200°C or more in the entire section between one point where the contact with a primary solder jet starts and another point where the contact with a finishing solder jet starts. Preferably, the temperature around the lower surface thereof is kept at 220-250°C in the entire section. Therefore, such a method can effectively prevent solidification of solder, resulting in defective soldering at a low temperature where the temperature of a lead-free solder adhered to the lower surface of the printed board during the primary soldering is sharply reduced before the following secondary soldering starts. As a result, an appropriate soldering can be conducted using a high-melting-point lead-free solder, which is difficult to be used in the conventional soldering method. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a better soldering method using lead-free solder.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, automatic soldering of a printed circuit board and a chip component includes a primary soldering and a subsequent soldering, and a device used for the soldering is a primary soldering blower tower for the primary soldering and finishing. It consists of a finish soldering tower for soldering and an outer tank in which they are juxtaposed. Then, the molten solder blows out from the solder outlet at the top of the primary solder outlet tower to form a primary solder jet, and then the molten solder blows out from the solder outlet at the upper part of the secondary solder outlet tower to form a finished solder jet. Then, the printed circuit board and the chip component are soldered. As an example, there is a soldering method using an apparatus as shown in FIGS. 2 and 3 of JP-A-5-69124.
[0003]
By the way, as the solder, lead-tin solder has been generally used, but since lead contained therein may cause environmental pollution, from the viewpoint of environmental problems, it is used as an alternative to lead-tin solder. The development of lead-free, so-called lead-free solder has been promoted and has already been put into practical use.
[0004]
[Patent Document]
JP-A-5-69124
[Problems to be solved by the invention]
Therefore, when the conventional automatic soldering method described above is used as it is and soldering is performed on the printed board with lead-free solder, a problem that soldering failure easily occurs on the printed board after finishing soldering has occurred. I have.
The applicant has determined to observe the temperature change on the lower surface of the printed printed board when soldering is performed using lead-free solder in order to find out one cause of the problem. For soldering, a lead-free solder (melting point 220 ° C.) was used for a conventional soldering device using lead-tin solder, the melting temperature of the solder was 260 ° C., and the transfer speed of the printed circuit board was 1 m / min. Was performed under the following conditions. The measurement of the temperature (s ′) of the lower surface of the printed board was observed with a temperature sensor attached to the lower face of the printed board.
The results are shown in the temperature change curve in the graph of FIG. The horizontal axis in FIG. 4 represents the elapsed time from the start of transport of the printed circuit board, and the vertical axis represents the temperature of the lower surface of the printed circuit board.
[0006]
As shown in FIG. 4, the temperature (s ′) of the lower surface of the printed substrate conveyed on the conveyance path rapidly rises from a point (P1 ′) at which contact with the primary solder jet starts and reaches approximately 250 ° C. After that, the temperature is maintained at the same temperature until the point where the contact with the primary solder jet ends (P2 ′), but after the printed board passes through P2 ′, the temperature drops rapidly, and the final solder jet becomes The temperature drops to about 180 ° C. until the point (R1 ′) at which the contact starts. After the contact with the finishing solder jet has started, the temperature s ′ on the lower surface of the substrate rises again, and after reaching approximately 250 ° C., the same temperature is maintained until the point (R2 ′) at which the contact with the finishing solder jet ends. Has been maintained.
[0007]
Here, the applicant has noticed that the time (t ′) maintained at about 250 ° C. in the time from R1 ′ to R2 ′, that is, the time in contact with the finished solder jet, is extremely short. Inferred as follows. The lead-free solder adhered to the printed circuit board by the primary soldering solidifies before contact with the subsequent finished solder jet starts, but the solder is sufficiently melted through the contact with the finished solder jet and adheres to the lower surface of the substrate. It is considered that the condition is that it is necessary to make contact with the finishing solder jet maintained at about 250 ° C. for a certain period of time or more. However, since the contact with the finishing solder jet is limited to a certain time, in order to satisfy the condition, the temperature of the lower surface of the substrate needs to reach approximately 250 ° C. from the temperature at R1 ′ in as short a time as possible. I think that the. As a result, of the contact time between the printed board and the finish solder jet, the time maintained at about 250 ° C. becomes longer, and the lead-free solder that is primarily soldered to the printed board should melt sufficiently.
[0008]
However, in the conventional automatic soldering, the difference between the temperature at R1 ′ (about 180 ° C.) and the temperature at R2 ′ (about 250 ° C.) is large, and it takes longer to raise the temperature from R1 ′ to the temperature at R2 ′. Time is needed. As a result, of the contact time between the printed board and the finished solder jet, the time (t ′) maintained at 250 ° C. is shortened, and the lead-free solder that is primarily soldered to the printed board is not sufficiently melted. That is, it is considered that the soldering failure was caused by the fact that the finish soldering was not sufficiently performed.
From these assumptions, the applicant has devised the above problem by devising a temperature control from the point where the contact with the primary solder jet ends (P2 ′) to the point where the contact with the finished solder jet starts (R1 ′). With the idea of solving the above, the present invention has been completed.
[0009]
[Means for Solving the Problems]
That is, in detail, the present invention relates to an automatic soldering method for performing primary soldering and subsequent finish soldering on the lower surface of a printed circuit board to be conveyed using lead-free solder. Wherein the temperature near the lower surface of the printed circuit board is maintained at a temperature of 200 ° C. or more over the entire area from the point where the contact with the primary solder jet ends to the point where the contact with the finish solder jet starts. About the attachment method. A preferred embodiment thereof relates to the above-mentioned soldering method, wherein the temperature near the lower surface of the printed board is maintained at a temperature in the range of 220 ° C. to 250 ° C. over the entire area.
[0010]
The method of the present invention is a method particularly suitable for automatic soldering using lead-free solder, and on the transfer path of the printed board, the lower surface of the printed board is in contact with the primary solder jet in the primary soldering. The temperature near the lower surface of the printed circuit board is maintained at 200 ° C. or more over the entire area from the end point to the point where the contact with the finish solder jet in the finish soldering starts.
[0011]
As means for maintaining the temperature near the lower surface of the conveyed printed circuit board at a temperature of 200 ° C. or higher, there is a means for heating the lower surface near the surface by, for example, hot air or a heater. However, as a more effective means, the distance A from the point where the contact between the printed board and the primary solder jet ends to the point where the contact with the finish solder jet starts is shortened, and the primary soldering is performed on the lower surface of the printed board. Means for adjusting the temperature of the attached lead-free solder so that it comes into contact with the finish soldering jet in a shorter time before the temperature of the lead-free solder does not decrease to such a low temperature that soldering failure is likely to occur. When the substrate transport speed is 1 m / min, the distance A is preferably 20 mm or less. Alternatively, a means for increasing the transport speed of the printed board and adjusting the printed board to be brought into contact with the finish soldering jet in a short time may be used.
[0012]
As described above, the soldering method according to the present invention increases the temperature in the vicinity of the lower surface of the printed circuit board by 200 degrees over the entire area from the point where the contact with the primary solder jet ends to the point where the contact with the finish solder jet starts. ° C or higher, so that the temperature of the lead-free solder primary soldered to the lower surface in the area is effectively reduced to a temperature at which the solder causes soldering failure and solidified. Can be suppressed. Therefore, even in soldering using lead-free solder having a particularly high melting point, better soldering can be performed.
[0013]
【Example】
The present invention will be described in more detail with reference to the drawings. The following examples are not intended to limit the invention.
[0014]
Embodiment 1 FIG.
The soldering device 1 will be described as a device specifically illustrating the soldering method of the present invention. As shown in FIGS. 1 and 2 (a cross-sectional view taken along line AA in FIG. 1), a soldering apparatus 1 includes an outer tank 2 made of cast iron filled with molten lead-free solder, and heats the solder. And a solder feeder 6 and 6 ′ for feeding the molten lead-free solder in the outer tank 2 to the primary solder outlet tower 4 and the finished solder outlet tower 5. Prepare. The primary solder port tower 4 and the finished solder port tower 5 on which the surface of stainless steel has been subjected to chromium nitride treatment are juxtaposed so as to be forward and backward in the transport direction of the printed board 7 (arrow direction P in FIG. 2). . In the upper part of the primary solder outlet tower 4, a solder outlet part 9 composed of a perforated flat plate 8 is provided. On the other hand, in the upper solder outlet part 9 ′ of the finish solder outlet tower 5, a plurality of diaphragms 10 are provided. , Are arranged at regular intervals from each other by a shaft 11 connected to a vibrator.
[0015]
The printed circuit board 7 conveyed in the P direction contacts the primary solder jet formed by the lead-free solder in the outer tank 2 being fed to the primary solder port tower 4 and passing through the solder blowout section 9. Thus, the printed circuit board 7 is soldered by being brought into contact with a finished solder jet formed from the outer tank 2 through the solder blowing section 9 ′. In the present embodiment, a lead-free solder having a melting point of about 220 ° C. is used, the melting temperature of the solder is 260 ° C., the transfer speed of the printed circuit board 7 is 0.9 m / min, and the point at which contact with the primary solder jet is completed. The distance (L in FIG. 2) from to the point where contact with the finished solder jet started was set to 15 mm. The temperature (s) of the lower surface of the printed board was measured by a temperature sensor attached to the lower surface of the board. The results are shown in the temperature change curve in the graph of FIG. The horizontal axis in FIG. 3 represents the elapsed time from the start of transport of the printed circuit board, and the vertical axis represents the temperature of the lower surface of the printed circuit board.
[0016]
According to FIG. 3, the temperature s on the lower surface of the printed board rises sharply from the point (P1) where the contact with the primary solder jet starts, and after reaching about 250 ° C., the point where the contact with the primary solder jet ends. The temperature is maintained until (P2). Then, after the printed circuit board 7 has passed through P2, the temperature s on the lower surface of the printed circuit board decreases to about 220 ° C., but rises from a point (R1) where contact with the finishing solder jet starts, rises again, and rises to 250 ° C. After reaching, the same temperature is maintained until a point (R2) where the contact with the finishing solder jet ends.
Here, although the temperature s of the lower surface of the printed circuit board is lower than the temperature at P2, the distance L between P2 and R1 is set as short as 15 mm. The time is further shortened, and as a result, the decrease in the temperature s of the lower surface of the printed circuit board is suppressed to about 220 ° C., which is lower than the corresponding temperature (about 180 ° C.) in FIG. Results in a smaller degree of reduction. For this reason, the temperature s of the lower surface of the printed circuit board rises in contact with the finishing solder jet, and the time to reach 250 ° C. is earlier than that of the conventional method, and therefore, the time for the printed circuit board 7 to reach from R1 to R2, The time (t) at which the printed board 7 is kept at 250 ° C. in contact with the finishing solder jet is longer than the time t ′ in FIG. Therefore, the lead-free solder adhered to the printed circuit board 7 by the primary soldering is heated and melted for a sufficient time by contact with the finishing solder jet, so that the possibility of causing poor soldering is lower. It is possible to perform better soldering using lead-free solder than in the case of (1).
[0017]
【The invention's effect】
The soldering method of the present invention is the automatic soldering method, wherein the temperature near the lower surface of the printed substrate conveyed over the entire area from the point where the contact with the primary solder jet ends to the point where the contact with the finish solder jet starts. Is maintained at a temperature of 200 ° C. or higher. Therefore, the temperature of the lead-free solder attached to the lower surface of the printed circuit board due to the primary soldering rapidly drops before the subsequent secondary soldering starts, and solidifies to a low temperature enough to cause soldering failure. Can be effectively prevented. Therefore, it is possible to perform better soldering using lead-free solder having a high melting point, which has been difficult to cope with the conventional soldering method.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a soldering device 1 used in a soldering method of the present invention.
FIG. 2 is a cross-sectional view taken along line AA of FIG.
FIG. 3 is a graph showing a change in temperature (s) of a lower surface of a printed circuit board conveyed in the soldering method of the present invention.
FIG. 4 is a graph showing a change in temperature (s ′) of a lower surface of a printed substrate conveyed in a conventional soldering method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Solder 2 Outer tank 3 Heater 4 Primary solder port tower 5 Finish solder port tower 6 Solder feeding section 7 Printed circuit board 8 Perforated flat plate 9 Solder blowing section 10 Vibration plate 11 Shaft P1, P1 'With primary solder jet Contact point P2, P2 'Point where contact with primary solder jet ends R1, R1' Point where contact with finish solder jet starts R2, R2 'Point where contact with finish solder jet ends

Claims (2)

Using lead-free solder, in the automatic soldering method of performing primary soldering and subsequent soldering to the lower surface of the printed circuit board to be conveyed, the contact with the primary solder jet on the conveying path of the printed circuit board is reduced. A soldering method, comprising: maintaining the temperature near the lower surface of the printed board at a temperature of 200 ° C. or more over the entire area from the end point to the point where the contact with the finishing solder jet starts. The method according to claim 1, wherein a temperature near a lower surface of the printed circuit board is maintained at a temperature in a range of 220C to 250C over the entire area.

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