JP2004022709A - Soldering method and automatic soldering equipment of printed board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and equipment for soldering, which is free from a large shrinkage cavity even in the case of soldering with Pb-free solder which is free from Bi since a large and deep shrinkage cavity occurs at a soldered part in the case of soldering a printed board by a dipping method by using the Pb-free solder free from Bi and this weakens the mechanical strength of the soldering part. <P>SOLUTION: Pb-free solder free from Bi is previously melted in a jet solder tub of the automatic soldering equipment. After bringing the printed board into contact with the melt solder jetted from the jet solder tub, cold air at 0 to -30 °C is made to blow on the printed board by means of cold air jetting devices of a cooling apparatus to speedily cool it. In the cooling apparatus of the automatic soldering equipment, the cold air jetting devices are installed across a conveyer on the upper and lower sides of it, the cold air jetting devices are connected with a freezer, which is connected with a drier via a pipe. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of performing soldering by bringing a printed circuit board into contact with molten solder, particularly Pb-free solder containing no Bi, and an automatic soldering apparatus suitable for using Pb-free solder.
[0002]
[Prior art]
As a method for soldering a printed circuit board, there are a soldering method, a reflow method, and a dipping method. The ironing method has a problem in productivity because an operator or a robot has to solder the soldered portions one by one with a soldering iron. This ironing method is suitable for correcting soldering defects caused by other soldering methods, or for "post-installing" soldering electronic components that do not have heat resistance after soldering with other soldering methods . In the reflow method, mass production is possible because solder paste is applied to many soldering points and heated by a heating device such as a reflow furnace to perform soldering, but processing of powder solder used for solder paste and powder There is a disadvantage in that the material cost is high because the work of kneading the solder and the flux requires a great deal of work. Therefore, the reflow method is often used for high-end products requiring high reliability, such as communication devices and computers. In the immersion method, soldering is performed by processing with a fluxer, preheater, jet solder bath, and cooling device installed in an automatic soldering device.Since many soldering points can be soldered in one process In terms of productivity, it is most superior to other soldering methods. However, since the entire printed circuit board is brought into contact with the high-temperature molten solder, there is a problem of heat influence on electronic components and printed circuit boards having no heat resistance.
[0003]
Conventionally, a Pb-Sn solder alloy has been used for soldering a printed circuit board. If an electronic device incorporating a printed circuit board soldered with the solder breaks down or becomes old and becomes inconvenient to use, it has been discarded without repair or functional improvement. However, recently, resource saving has been recommended, and materials that can be reused from discarded electronic devices have been collected and reused. For example, plastic for electronic device cases, metal for frames, glass for monitors, etc. are collected because they can be reused, but printed circuit boards cannot be reused and have been discarded. This is because the printed circuit board has a copper foil adhered to a resin such as glass epoxy, and the copper foil is metallically adhered to the solder and cannot be separated until it can be reused. Therefore, the printed circuit board has been disposed of by being finely crushed and buried in the ground.
[0004]
When acid rain generated by the recent heavy use of fossil fuels penetrates and contacts the landfilled printed circuit board, the Pb component in the solder is melted out, and the acid rain containing the Pb component further increases. Penetrates deep into the ground and mixes with groundwater. When humans drink groundwater containing Pb components for many months as described above, Pb is accumulated in the body and eventually causes Pb poisoning. Therefore, recently, the use of Pb has been regulated on a worldwide scale, and so-called Pb-free solder containing no Pb has been used.
[0005]
Pb-free solder is mainly composed of Sn, and generally includes Sn-Ag, Sn-Cu, Sn-Zn, Sn-Sb, Sn-Bi, and Sn-In. In some cases, other metals are added to improve the characteristics.
[0006]
Generally, Pb-free solder has poor solderability compared to conventional Pb-Sn solder alloys, and has poor wettability for leads of electronic components, such as 42 alloy (42Ni-Fe) and Kovar (Co-Ni-Fe). Is used. That is, if a Pb-free solder having poor solderability is used for a lead having poor wettability, the solderability is further deteriorated. Therefore, the leads of the electronic component are plated with good wettability. The Pb-Sn alloy is used for the plating having good wettability despite the fact that Pb is regulated. By the way, Sn-Ag, Sn-Cu, Sn-Sb, and Sn-Ag-Cu Pb-free solders have a high melting point, and therefore Bi may be added for the purpose of lowering the melting point. However, when the Pb-free solder to which Bi is added is soldered to the Pb-Sn plated lead electronic component by an immersion method, a "lift-off" occurs in which the solder and the land of the printed circuit board are separated after soldering. Sometimes. The cause is that an alloy layer of Bi and Pb having a low melting point is formed in the soldered portion, and the alloy layer peels off. In order to suppress this lift-off, a method of quenching immediately after soldering has been adopted (JP-A-11-354919). This quenching is performed at a cooling rate of 10 to 20 ° C./s in order to prevent the formation of an alloy layer of Bi and Pb.
[0007]
[Problems to be solved by the invention]
Generally, metals other than Bi undergo solidification shrinkage when solidifying from a molten state to a solid. The state of solidification shrinkage varies depending on the composition of the metal, and ranges from a slightly dented metal surface after solidification to a deep hole with a large hole. In the conventional Pb-Sn solder alloy, the surface of the solder is slightly lowered after solidification, and hardly forms a shrinkage cavity. However, in the Sn-based Pb-free solder, shrinkage cavities are significantly generated, and the holes reach deep. Such a large and deeply closed shrinkage cavity causes a decrease in mechanical strength.
[0008]
In an electronic device, an electric circuit is energized during use. At this time, heat is generated from a power transistor, a coil, a resistor, and the like, and the inside of the case of the electronic device becomes high temperature. When the power is turned off to stop using the electronic device, the temperature in the case drops to room temperature. When a heat cycle occurs in which the high temperature during use and the low temperature during non-use are repeated many times, the heat cycle also affects the soldered part, and the difference in the thermal expansion coefficient between the solder and the printed circuit board soldered. Therefore, stress is applied to the solder, and eventually the solder is broken by metal fatigue. Moreover, if the solder has shrinkage cavities, the solder loses its resistance to metal fatigue and is easily broken.
[0009]
As described above, Bi is a metal that solidifies and expands. Therefore, when a Pb-free solder containing Bi is soldered by an immersion method, shrinkage cavities are not generated at all depending on the Bi content, or even if it is generated, it is extremely low. Or small. However, when Pb-free solder containing no Bi is put into a jet solder bath of an automatic soldering apparatus and soldered, a large shrinkage cavity is generated, and not only the mechanical strength of the soldered portion is reduced as described above, The appearance was bad. SUMMARY OF THE INVENTION An object of the present invention is to provide a soldering method that does not generate large shrinkage cavities in an immersion method using Pb-free solder that does not contain Bi, and an automatic soldering apparatus used for the method.
[0010]
[Means for Solving the Problems]
The present inventors have conducted intensive research on reducing shrinkage cavities in the immersion soldering using Pb-free solder containing no Bi. As a result, the printed circuit board was printed immediately after being brought into contact with the molten solder. It has been found that large shrinkage cavities do not occur if the substrate is cooled rapidly by blowing cold air, and the present invention has been completed.
[0011]
The present invention relates to a method in which a soldering portion of a printed circuit board is brought into contact with Pb-free solder containing Sn as a main component not containing Bi, which is jetted from a jet soldering bath, and then dried at a temperature of 0 to -30 ° C. A method of soldering a printed circuit board, characterized by rapidly cooling the printed circuit board by applying cold air.
[0012]
Another aspect of the present invention is an automatic soldering apparatus for soldering a printed circuit board by processing the printed circuit board with a fluxer, a preheater, a jet solder bath, and a cooling apparatus while transferring the printed circuit board by the transfer apparatus. The blower is installed on the upper and lower parts of the transfer device, and the cool air blower, refrigerator and dryer are connected by pipes. After the gas dried by the dryer is cooled by the refrigerator, the cool air blower An automatic soldering apparatus characterized in that the apparatus is ejected from the outside.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
The Pb-free solder in the printed circuit board soldering method of the present invention is a Sn-based Pb-free solder containing no Bi. Generally, when a metal transforms from a liquid to a solid, it undergoes coagulation shrinkage to generate shrinkage cavities, but only Bi is a metal that coagulates and expands when solidified. An object of the present invention is to prevent shrinkage cavities caused by solidification shrinkage, and Pb-free solder containing Bi that solidifies and expands during solidification is out of scope.
[0014]
In the method of soldering a printed circuit board according to the present invention, immediately after the printed circuit board is brought into contact with the Pb-free solder jetted from the jet solder bath, dry cold air at 0 to -30 ° C is sprayed on the front and back surfaces of the printed board. However, if this cold air is higher than 0 ° C., the effect of reducing shrinkage cavities is not exhibited. However, even if the temperature of the cold air is lower than −30 ° C., no further improvement in the effect can be expected, and the function of the cooling device is simply increased, and the cooling device becomes expensive. If the cool air is not dried, the moisture will adversely affect the printed circuit board.
[0015]
In the automatic soldering apparatus of the present invention, a cool air ejector of a cooling device is installed near a jet solder bath, and the cool air ejected from the cool air ejector hits a printed circuit board during soldering in the jet solder bath. In this case, the printed circuit board is cooled to cause a soldering failure. That is, if the printed circuit board is cooled by cold air while the printed circuit board is in contact with the solder in the jet solder bath, the molten solder solidifies before wetting the entire soldered portion, which causes unsoldering. Therefore, in order to prevent the cool air ejected from the cool air ejector from flowing toward the jet solder bath, a windshield may be installed between the cool air ejector and the solder bath. As the windshield, a curtain made of a heat-resistant resin suspended from above is suitable. If the windshield is installed not only on the upper cool air ejector but also on the lower cool air ejector, the flow of the cold seal to the jet solder bath can be further prevented. As the windshield installed in the lower cool air ejector, a flexible windshield such as polyimide, which can be erected, is suitable.
[0016]
【Example】
First, an automatic soldering apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a front sectional view of an automatic soldering apparatus according to the present invention, and FIG. 2 is a perspective view of a cold air blower partially cut away.
[0017]
In the automatic soldering apparatus 1, the transfer device 2 is running with a slight inclination (4 to 6 degrees) in the running direction. The conveying device of the automatic soldering device is a pair of rotating endless chain conveyors 3 and 3 installed at positions facing each other, and each of the inner chain conveyors runs in a direction of ascending. A large number of claws 4 are attached to the chain conveyor, and the chain conveyor is conveyed while holding the printed circuit board between the opposing claws.
[0018]
Below the transfer device 2, a fluxer 5, a preheater 6, a jet solder bath 7, and a cooling device 8 are installed in the running direction of the chain.
[0019]
The cooling device 8 installed in the automatic soldering apparatus of the present invention includes a pair of cool air ejectors 9a and 9b, a refrigerator 10 and a dryer 11, and the cool air ejectors 9a and 9b and the refrigerator 10 are connected by a heat retaining pipe 12. The refrigerator 10 is connected to a dryer 11 via a pipe 13, and the dryer 11 is connected to an air compressor (not shown) via a pipe 14. The heat retaining pipe 12 that connects the cool air blowers 9a and 9b and the refrigerator 10 is obtained by covering the outside of a flexible pipe 15 with a heat retaining material 16. In the refrigerator, cold air of 0 ° C or less is produced and sent to the cold air ejector, so if the pipe connecting the cold air ejector and the refrigerator touches the outside air directly, the temperature of the cold air in the pipe rises and the cold air The cooling effect of the ejector decreases. If the pipe is in direct contact with the outside air, the outside of the pipe will condense and form water droplets. If the water droplets hang down in the jet solder bath, the molten solder scatters around and harms the operator. If water drops droop on the printed circuit board after soldering, the electronic components may be broken by a cool shock or the water droplets may enter an electric circuit to impair the function of the electronic components. For this reason, a pipe connecting the cold air blower and the refrigerator uses a heat insulating pipe covered with a heat insulating material on the outside.
[0020]
The cool air blowers 9a and 9b of the cooling device 8 are installed at upper and lower portions of the transport device 2 with the transport device 2 interposed therebetween. The cool air blowers 9a and 9b are box-shaped. The cool air blower 9a installed on the upper part of the transfer device 2 has a large number of blowout holes 17 formed on the lower surface, and the cool air blower 9b installed on the lower part of the transfer device 2 has a large number of blowout holes 17 on the upper surface. … Is drilled. A windshield 18 is suspended from the outer wall of the cool air blower 9a installed at the upper part of the transfer device on the side of the jet solder bath. The windshield is a heat-resistant resin curtain that prevents the cool air blown out from the cool air blower from flowing toward the jet solder bath, and does not hinder the running of the printed circuit board even if the printed circuit board conveyed by the transfer device hits it. It is.
[0021]
Next, a method of soldering a printed circuit board in the automatic soldering apparatus of the present invention will be described.
[0022]
A liquid flux is applied by the fluxer 5 while the printed circuit board (not shown) is transferred by the transfer device 2, and then the printed circuit board is preheated to 100 to 150 ° C. by the preheater 6 and then jetted from the jet nozzle of the jet solder tank 7. The soldering surface of the printed circuit board is brought into contact with the molten solder, and the solder is adhered to the soldered portion. The Pb-free solder of Sn-3Ag-0.5Cu which solidifies and shrinks is put in the jet solder bath 7, and is in a molten state at 240C. The Pb-free solder has a solidus temperature of 217 ° C. and has a large shrinkage cavity upon solidification. The printed circuit board with the solder attached to the soldering portion is sent to the cooling device 8 by the transfer device 2. Cooling devices 8a and 9b are provided at the upper and lower portions of the cooling device 8 with the transporting device 2 interposed therebetween, and the cold air of −10 ° C. is blown out from the cold air blowing devices. It is quenched by cold air. No large and deep shrinkage cavities affecting the bonding strength were generated at the soldered portion of the printed circuit board thus soldered. As a comparison with the present invention, cooling after soldering was performed by blowing air using a conventional fan, and otherwise, the cooling was performed under the same conditions as the above-described method for soldering a printed circuit board of the present invention. As a result, in the air cooling by the fan, a deep and large shrinkage cavity was generated in the soldered portion.
[0023]
【The invention's effect】
As described above, according to the present invention, even in the case of a Pb-free solder in which shrinkage cavities increase during solidification, shrinkage cavities can be reduced as much as possible. The present invention has an unprecedented excellent effect that the mechanical strength is sufficiently increased with respect to the attachment portion and a reliable soldered portion is obtained.
[Brief description of the drawings]
FIG. 1 is a front sectional view of an automatic soldering apparatus according to the present invention. FIG. 2 is a partially cutaway perspective view of a cool air blower of a cooling apparatus.
REFERENCE SIGNS LIST 1 automatic soldering device 2 transport device 5 fluxer 6 preheater 7 jet solder bath 8 cooling devices 9a, 9b cool air ejector 10 refrigerator 11 dryer

Claims (3)

After the soldering part of the printed circuit board is brought into contact with the Sn-based Pb-free solder that does not contain Bi and is jetted from the jet solder bath, dry cold air having a temperature of 0 to −30 ° C. is applied to the front and back surfaces of the printed circuit board. A method of soldering a printed circuit board, wherein the printed circuit board is rapidly cooled. In an automatic soldering machine that carries out soldering of printed circuit boards by processing them with a fluxer, preheater, jet solder bath, and cooling device while transporting the printed circuit board by the transfer device, the cool air blower of the cooling device is located at the top and bottom of the transfer device It is installed, and a cool air blower, a refrigerator, and a dryer are connected by a pipe, and the gas dried by the dryer is cooled by the refrigerator and then blown out from the cool air blower. An automatic soldering apparatus, characterized in that: 4. The automatic soldering apparatus according to claim 3, wherein the cold air blower is provided with a windshield for preventing the cool air from flowing toward the jet solder bath.

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