Method of forming a solder layer on pads of a circuit and method of mounting an electric part on a circuit board

[Name of invention]

A method for forming a solder layer on a pad of a circuit board and a method for mounting an electronic component on a circuit board

[Brief Description of Drawings]

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view showing a part of a circuit board used in the present invention.

Fig. 2 is a cross-sectional view showing a pad having a semicircular cross section formed with a solder layer;

Fig. 3 is a diagram showing the relationship between the pad width and the thickness of the deposited solder layer;

[Detailed Description of the Invention]

The present invention relates to a method of forming a solder layer with a pad of a circuit board and a method for mounting an electronic component on a circuit board.

When electronic components are mounted on a circuit board, a thin solder layer is first formed on the pads to prevent oxidation of the pads on the circuit board and to improve solder wettability during component mounting. Conventional methods for forming this thin solder layer include a hot gas leveler method, an electroplating method, and the like.

In the hot gas leveler method, the circuit board is immersed in molten solder and pulled up, solder is attached to the pad array, and hot gas is sprayed while the solder is not solidified to blow off excess solder on the pad and between the pads. This is a method of forming a thin solder layer only on the pad.

However, in this method, in principle, it is difficult to uniformly control the thickness of the solder layer, and a large imbalance is likely to occur in the thickness of the solder layer. In particular, if the pad arrangement pitch becomes smaller, bridges (soldering between the pads) are more likely to occur, so it is necessary to reduce the thickness of the solder layer. Cu generated between solder₃ An intermetallic compound layer such as Sn is exposed on the surface, and the solder surface is oxidized. As a result, there is a problem in that solder wettability is significantly reduced when components are mounted in a subsequent step of the step.

In addition, although the electroplating method has the advantage of forming solder in a fine pattern, it is necessary to heat and melt it after electroplating because metal plating alone does not form an intermetallic compound that bonds copper foil and solder. Bridging is easy to occur. Also, the cost is about twice that of the hot gas leveler method.

On the other hand, when components are mounted on both sides of a circuit board, solder layers are first formed on the pads on both sides, and then components are mounted one side at a time (since components cannot be mounted on both sides at the same time). Since the side is heated to a high temperature in the reflow furnace when the component is mounted on the opposite side first, if there is a portion of the solder layer that is too thin at that time, oxidation will occur in that portion, and the wettability of the solder will become A lot of things get worse

Therefore, a method of forming a relatively thick solder layer with a uniform thickness is preferred.

In recent years, in response to the demand for lightness, thinness, and miniaturization of electronic devices, lead pitches of electronic components have been reduced to 0.8 mm, 0.65 mm, and 0.5 mm, and studies of 0.36 mm, 0.3 mm, 0.15 mm, etc. are also being conducted. In the conventional mounting method in which solder paste is printed on each pad, a lead pitch of up to about 0.5 mm can be accommodated once. However, when it is less than that, it is very difficult to supply an appropriate amount of solder paste onto each pad. Also, if the supply amount of the solder paste is too large, a paste bridge is likely to occur between adjacent pads. On the other hand, if the amount of solder paste supplied is too small, it becomes impossible to mount the components.

As described above, development of a mounting method capable of responding to a fine pitch has been demanded.

It is an object of the present invention to apply a relatively thick solder layer to the extent that the intermetallic compound is not exposed even when heated to the pad of the circuit board (the pad here is a generic term for the component mounting part, and includes, for example, the component mounting sulpaol. Hereinafter simply It is to provide a method of forming a solder layer that can be formed on the pad to have an almost uniform thickness.

Another object of the present invention is to provide a method for mounting an electronic component capable of responding to a fine pitch.

The present invention relates to a process of applying a paste composition containing organic acid Pb and Sn powder to a pad arrangement part of a circuit board, heating the composition to generate a solder alloy precipitation reaction, and selectively a Sn-Pb alloy solder layer on the pad The precipitation reaction proceeds in a state where a liquid atmosphere is formed on the pad arrangement portion and Sn powder is settled in this liquid when the paste-like composition becomes liquid by heating. A method for forming a solder layer in a furnace is provided.

Further, the present invention provides a step of selectively forming a Sn-Pb alloy preliminary solder on a pad of a circuit board, a step of applying a paste composition containing organic acid Pb and Sn powder on the preliminary solder, and the paste composition An electronic component to a circuit board having a step of placing an electronic component on it, heating the paste-like composition to generate a solder alloy precipitation reaction, and soldering the pad and the lead of the electronic component with the solder alloy A mounting method is provided.

In addition, the present invention relates to a step of applying a paste composition containing organic acid Pb and Sn powder to a pad arrangement portion of a circuit board, heating the paste composition to generate a solder alloy precipitation reaction, and mounting an electronic component on the pad A step of forming a solder layer of a Sn-Pb alloy in an amount required to A method for mounting an electronic component on a circuit board is provided, comprising: a step of heating the solder layer to solder the pad and a lead of the electronic component.

The present inventors first proposed a method of selectively depositing a solder layer of a Sn-Pb alloy on a pad by applying a paste-like composition containing organic acid Pb and Sn powder to a pad arrangement portion of a circuit board and heating it (Japanese Patent Application Laid-Open No. Hei 1). -157796 publication).

The solder layer forming method according to the present invention is characterized in that the solder layer is basically deposited by the method described in the above publication, and the state of the paste-like composition when the solder precipitation reaction proceeds is specified.

That is, in the solder layer forming method of the present invention, when the paste-like composition becomes liquid by the heating, a liquid atmosphere chamber in which Sn powder is settled is created on the pad arrangement portion, and Sn powder is settled in the liquid. That is, it is characterized in that the solder precipitation reaction proceeds in a state covered by the liquid.

In this way, it is possible to form a relatively thick solder layer with a uniform thickness without causing bridges in a fine pattern with a pad arrangement pitch of 0.5 mm or less. This was discovered for the first time as a result of the inventor's research.

In general, when a solder layer is formed by contacting a pad of a circuit board with molten solder, a compound layer of Cu and Sn (generally Cu₃ Sn, Cu₆ Sn₅(intermetallic compound) is formed to a thickness of about 1-2 μm. Therefore, in order not to expose the compound layer to the surface, it is preferable to attach the solder to a thickness of 5 μm or more.

While conventional solder layer formation is solder by re-melting Sn-Pb alloy, the solder layer formation method of the present invention is based on the difference in the ionization tendencies of Sn and Pb in a high-temperature solution containing Sn powder and organic acid Pb. The principle is Sn-Pb alloying by the precipitation of Pb into the system and melting of the precipitated Pb and Sn powder at the atomic level in the replacement of Sn and Pb. Re-melting of the solder occurs almost instantaneously when the temperature is raised in the case of conventional cream solder that is generally used, and the molten solder swells due to surface tension. However, in the present invention, the reaction takes time. It is suitable for forming a solder layer with a uniform thickness because the solder production is slow.

As a material of the solder precipitation composition, organic acid Pb and Sn powder for forming a solder alloy, one or more rosins and amines added to aid activation, and a viscosity modifier for maintaining a mixture thereof in a paste shape are the main components. and knead them and make a paste-shaped body at room temperature. These circuit boards are coated with an all over the pad arrangement portion, heated, and selectively deposited with solder on the pads.

What is important in this process is that when the paste-like composition becomes liquid by heating, as described above, a liquid waiting chamber in which Sn powder is deposited is created on the pad arrangement portion, and Sn powder is deposited into the liquid. In a state, that is, in a state covered with liquid, the solder precipitation reaction proceeds. This means that when the paste-like composition is heated, the viscosity decreases remarkably, and when the viscosity becomes too low, the liquid spreads widely around the pad arrangement, as a result, the amount of organic acid Pb relative to the Sn powder is insufficient and a sufficient amount of solder is produced. because it will be obtained. Although there is no problem with some spreading, the Sn powder mostly settles on the pad arrangement portion, and maintaining the state covered with the dissolved liquid is the point of forming a relatively thick solder layer of 5 μm or more with a uniform thickness.

In addition, since the mechanism of solder precipitation is that Pb separated by substitution with Sn from organic acid Pb collides with Sn powder and alloys, the chance of this collision becomes a rate-limiting condition for the precipitation amount. It is preferable to create a state in which the Sn powder settled on the pad arrangement portion is completely surrounded by the organic acid Pb and rosin in liquid form according to the reaction.

Next, an electronic component mounting method according to the present invention will be described.

This method also uses the solder deposition method described in Japanese Patent Laid-Open No. Hei 1-157796 described above, and generates the solder deposition reaction described in this publication in the soldering step for mounting electronic components.

In the first aspect of this mounting method, a step of selectively forming a Sn-Pb alloy pre-solder on a pad, a step of applying a paste composition containing organic acid Pb and Sn powder thereon again, and placing an electronic component thereon A method for mounting an electronic component is provided, comprising the steps of: depositing solder from a paste-like composition by heating, and thereby soldering a pad and a lead of an electronic component.

As described above, conventional soldering uses re-melting of solder, but since re-melting of the solder occurs almost instantaneously when the temperature is raised, the time for all of the solder to melt is shorter than the time for the solder to melt and spread over the pad. .

As described above, the solder molten on the pad swells due to the surface tension and relaxes beyond the limit of the surface tension. As a result, a bridging occurs easily with a fine pattern. Also, when the solvent component in the solder paste is heated, it turns into a liquid and melts between the pads. At this time, the solder particles that flowed together melt in an instant between the pads and cause a bridge.

On the other hand, in the electronic component mounting method according to the present invention, the precipitation of Pb into the system by substitution of Sn and Pb caused by the difference in the ionization tendency of Sn and Pb in a high-temperature solution containing Sn powder and organic acid Pb; Using the solder precipitation reaction based on the principle of Sn-Pb alloying by melting at the atomic level between the deposited Pb and Sn powder. Solder precipitation is performed very slowly compared to the case of the prior art.

Again, since this precipitation reaction is carried out under high temperature, the Sn-Pb alloy solder deposited on the pad surface reacts with Cu, which is the material of the pad, directly to form Cu.₃ Sn, Cu₆ Sn₅to form an intermetallic compound layer such as

In this case, the Sn-Pb alloy solder deposited on the intermetallic compound layer forms a solder layer while having a chemical bonding force with the intermetallic compound.

And, the major difference from the conventional method is as shown below. That is, in the case of the formation of the solder layer, in the conventional solder paste, as described above, each solder particle is melted in an instant and all at a uniform time to form the solder layer. On the other hand, in the present invention, as the solder, many fine solder particles with varying times to deposit on the pad are deposited on the pad. do. That is, the solder layer is formed by adding more time than in the case of the conventional method.

Incidentally, in the case of the present invention, even if solder particles are deposited between the pads, since the particles have various compositions, it is difficult to melt and integrate them in an instant as in the conventional solder paste.

In view of the above, according to the mounting method of the present invention, it is difficult to cause a bridge between the pads.

Also, Pb that collides with Sn powder near or between pads forms a Sn-Pb alloy there with the size of Sn powder, but it is sucked into the solder deposited on the pad and absorbed on the pad. In this way, soldering is possible without causing bridges even with a fine pitch of 0.5 mm or less.

Further, in the soldering step, the formation of the solder layer is preferably performed using the solder precipitation reaction in the method for forming the solder layer of the present invention described above. Thereby, since a relatively thick solder layer is formed with a uniform thickness, the lead pitch of the electronic component can be further refined.

As described above, the process of forming preliminary solder on the pad of the circuit board is preferably performed by applying a paste-like composition containing organic acid Pb and Sn powder to the pad arrangement portion of the circuit board and heating it. That is, even in the preliminary soldering step. The above-mentioned solder precipitation reaction is used. Thereby, it is possible to form a preliminary solder corresponding to the fine pattern.

In the first aspect of the mounting method according to the present invention, for example, if a relatively thick preliminary solder layer is formed on the pad with a uniform thickness as described above, a paste composition containing organic acid Pb and Sn powder is applied thereon, Again, as the electronic components are placed and heated, the leads and pads of the electronic components are soldered.

Also, in this preliminary solder formation step, an electroless plating method may be applied. The reason is that even in the electroless plating method, it is possible to perform preliminary soldering without generating a bridge between adjacent pads corresponding to a fine pattern. In the above method, a paste containing organic acid Pb and Sn powder is applied on the pad after preliminary solder formation on the pad, but this paste may be applied to the lead side of the electronic component to be mounted.

Again, at the same time as forming a preliminary solder on the pad of the circuit board, the paste applied to the lead of the electronic component is heated to form a preliminary solder layer on the surface of the lead, and then at least the pad of the circuit board or the lead of the electronic component. After applying a solvent to one side, it is also possible to solder both sides by heating.

Even when a preliminary solder is formed on the lead of an electronic component, it is preferable to apply the method using the above-described solder precipitation reaction or the electroless plating method.

In the second aspect, a step of applying a paste-like composition containing organic acid Pb and Sn powder to the pad arrangement portion of the circuit board, and heating it, a Sn-Pb alloy solder layer in an amount necessary for selectively mounting electronic components on the pad Provided is a method for mounting an electronic component comprising the steps of: forming an electronic component, applying a solvent thereon, placing the electronic component, melting the solder layer by heating, and soldering the pad and the lead of the electronic component; do.

In this case, the solvent may be applied to the lead surface of the electronic component, or at least the solder layer on the pad and the lead surface may be applied to either one.

In this method, the method for forming the solder layer is the same as the method described above, but the above-described preliminary paste is performed by first putting the required amount of paste on the pad and soldering the leads of the electronic component with the paste. In comparison, the process can be shortened.

In addition, if a halogen-free type solvent is used as the solvent at this time, cleaning after soldering is not necessary, or cleaning can be easily performed with a cleaning agent other than Freon at least. Therefore, it can contribute to the prevention of air pollution caused by Freon, which is a problem today.

In addition, in the case where a pad arrangement part having a fine pitch of 0.5 mm or less and a pad arrangement part having a pitch thicker than that are mixed on the same circuit board, the following mounting method can be obtained.

First, a solder layer with a thickness of, for example, several tens of μm required for soldering is formed on the pad arrangement part having a thin pitch, and a solder layer of about 5-10 µm is formed as solder in advance on the pad arrangement part having a large pitch. . Next, solder paste is printed on the pad arrangement portion subjected to this preliminary soldering, and electronic components are temporarily fixed by the adhesive force. After that, it is heated with a reflow furnace or the like to perform soldering. On the other hand, in the method described above, a solvent is applied to the pad portion on which a solder layer of several tens of μm is formed necessary for soldering, the electronic component is temporarily fixed with the adhesive force of the solvent, and soldered by heating with a reflow furnace, etc. .

Also, since the area and arrangement pitch of the pads on the same circuit board are different, the thickness of the solder layer required for each pad arrangement section may vary greatly, but in this case, the thickness required for each pad arrangement section is When different amounts of the paste composition are applied and heated, solder layers of different thicknesses can be formed on the respective pad arrays.

On the other hand, in some cases, the solder layer formed to a thickness of several tens of mu m has a semicircular cross-section as shown in FIG. 2 . In such a case, even if the lead of the electronic component is placed on the solder layer, it cannot be fixed accurately only by the adhesion force of the solvent. That is, there are cases where it is not possible to completely prevent misalignment between the pad and the lead. In the drawings, reference numeral 1 denotes an insulating substrate, 2 denotes a pad, and 4 denotes a solder layer.

In order to avoid this problem, a thick solder layer, for example, a solder layer with a thickness of several tens of μm, is deposited on a pitch with a small area or pitch, and then the solder layer is heated and pressed from the top surface to flatten the top surface of the solder layer. It is good to carry out In this case, a method of pressing while heating is also effective depending on the thickness or width of the solder layer. The heating temperature at this time differs depending on the composition of the precipitated solder, but it is preferable to set it to a temperature range 20-40°C lower than the deformation temperature of the precipitated solder. Further, the pressing force and holding time are appropriately selected depending on the thickness, area, and the like of the solder layer. With such a simple treatment, it is possible to prevent misalignment of the parts.

However, in the case of mounting the electronic component in the first aspect of the mounting method, it is possible to lower the soldering temperature than in the case of using the conventional solder paste. This is significant especially when soldering is performed using a high-melting-point solder rich in Pb.

For example, in a PGA (Pin Grid Array) or multi-chip module of a plastic-packaged type, when an IC chip mounted in a package is bonded to a circuit in the package by a bump formed with a Pb-rich solder, the IC Forming bumps on the circuit part side rather than forming bumps on the chip side enables manufacturing without affecting the product ratio to the raw material of the IC chip, so an advantage of low manufacturing cost can be expected. However, the method of forming bumps on the circuit board side requires that the circuit board must be capable of delivering a high-melting-point solder layer rich in Pb, so a substrate material with high heat resistance is required, and the total cost is high.

In the conventional method using a solder paste, when soldering is performed, it is necessary to make the temperature 30-60 DEG C higher than the melting point of the solder. This is because the solder must be melted and the solder must be diffused with the copper of the pad in a short time. On the other hand, in the method of the present invention, by appropriately selecting the composition ratio of the organic acid Pb and Sn powder in the paste molding composition, it is possible to carry out the solder analysis at substantially the same temperature regardless of the Sn/Pb ratio of the finished solder layer. For example, in the case of a high-melting-point solder containing a lot of Pb, the solder layer can be formed at a temperature considerably lower than the melting point of the solder formed by precipitation. Furthermore, since an intermetallic compound has already been formed by diffusion between the solder and copper, which is a pad material, at the time of forming the solder layer, when soldering is performed using the solder layer thereafter, in the conventional electroplating method, etc. It does not require energy to form an intermetallic compound by a method such as post-heating (eg Fusion), which is required. In other words, if the process can be shortened, it helps to reduce the cost.

Further, the solder layer forming method and the electronic component mounting method according to the present invention are applied to a circuit board as shown in FIG. 1, for example. In the first diagram, reference numeral 1 denotes an insulating substrate, on which a pad 2 for mounting electronic components is formed. Then, according to the components to be mounted, the pad arrangement part 3 in which the pads 2 are arranged is formed.

Example

Hereinafter, an embodiment of the present invention will be described.

Example 1

Here, an example of a method for forming a solder layer is shown.

Organic acid Pb 46%, activator 18%, Sn powder 25%, and viscosity modifier 11% (weight 5) were mixed to obtain a paste composition having a viscosity of 280,000-290,000 cps. This paste-like composition was applied with a thickness of 0.5 mm to a pad arrangement portion with a pad size of 026 x 2 mm, a pad arrangement pitch of 0.36 mm, and a 284-pin QFP mounting pad arrangement. Then, this circuit board was heated at 215 degreeC for 2 minutes. As a result of observing the situation at this time, the spread of the liquid paste composition is small, and the liquid collects in a layered form in and around the pad arrangement part, and the reaction proceeds in a state in which Sn powder is settling in the pad. It was confirmed that the solder was gradually formed on the top. The solder layer formed on the pad had an average of 9-10 μm, unbalance α=1-2 μm, and the solder composition was 64-69% at Sn wt%, and no bridges occurred.

As a result of forming the solder layer on the pad array portion with the pad array pitch of 0.15 mm in the same manner, it was found that the solder layer could be formed without causing a bridge.

On the other hand, in the conventional hot gas leveler method, a solder layer was formed to prevent bridges from occurring in the same pad array. The thickness of the obtained solder layer was 1-5 mu m, and the unevenness was large, and the thin portion of the thickness was exposed by the intermetallic compound layer. In addition, if the amount of solder applied to a thickness greater than this was increased, a large number of bridges occurred.

Next, assuming double-sided mounting, before heating (assuming the first mounting) and after heating (assuming the first mounting) of the spring water having a solder layer formed on the copper side (on the back side of the water following the first mounting) Wettability of the solder of the second installation in Korea was tested.

A sample is as follows.

A: A solder layer formed by the method of the present invention.

This is done by coating a paste-like composition (composition of organic acid Pb 37%, activator 10%, Sn powder 40%, viscosity modifier 13%) to a thickness of 300 µm on a 0.3 mm-thick copper plate, and then heating it on a 220°C hot paste for 2 minutes. A solder layer was formed.

B: A solder layer formed by a hot gas leveler method on the same copper plate.

C: A 0.3 mm thick copper plate on which no solder layer is formed.

For these three types of samples (5 pieces each), first, a halogen-free cream solder was applied before heating, and then the solder spreadability test according to JISZ31970 was performed by heating.

Next, the above three types of samples (5 each) were heated in a hot air dryer at 150 DEG C for 1 hour, then the same cream solder was applied and the same test was conducted.

The results were as shown in Table-1.

[Table 1]

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As described above, it was found that the solder layer formed by the present invention had good solder spreadability (wettability) even after heating the board assuming the second component mounting, and was excellent even as a pre-solder for double-sided mounting.

Example 2

Here, an embodiment of the electronic component mounting method according to the first aspect is shown.

A paste composition having a viscosity of 280,000-290,000 cps (centimeter poise) was obtained by mixing 46% of organic acid Pb, 18% of an activator, 25% of Sn powder, and 11% (wt%) of a kurtosis adjuster. This paste-like composition was applied to a thickness of 0.5 mm on a pad arrangement portion with a pad size of 0.26 x 2 mm, a pad arrangement pitch of 0.36 mm, and a 284-pin QFP mounting portion. Then, this circuit board was heated at 215 degreeC for 2 minutes. As a result, the preliminary solder formed on the pad had an average of 9-10 mu m, unbalance ? = 1-2 mu m, and the solder composition was 64-69 wt% of Sn, and no bridging occurred.

Next, a paste-like composition composed of 40% organic acid Pb, 33% activator, 18.4% Sn powder, and 85% viscosity modifier is applied on the pad arrangement portion subjected to this preliminary soldering, and a lead pitch of 0.36 mm, 284 pin QFP is applied thereon. decided This was heated through a reflow furnace under the conditions of 215° C.×30 seconds and 220° C.×2 minutes to precipitate solder, followed by soldering. As a result, it was possible to obtain a good soldering partner between the pad and the lead without a bridge.

Example 3

Here, an embodiment of the electronic component mounting method according to the second aspect is shown.

There are three types of parts to be mounted.

QFP: lead pitch 0.65mm 100 pins

Overall dimensions 20×13mm

Chip parts: total 42×7

SOP: lead perch 0.8mm 20 pins

Overall dimensions 9×8mm

The circuit board used is a 200×250 mm glass epoxy board, on which pads corresponding to the above components are formed. Pad dimensions for QFP are 0.5 x 2 mm, those for chip parts are 1.5 x 1.5 mm, and pad dimensions for SOP are 1.2 x 0.8 mm.

The paste composition used was a mixture of 40% organic acid Pb, 33% activator, 18.4% Sn powder, and 8.6% viscosity modifier.

This paste-like composition is applied over the entire pad at a ratio of 0.5-0.9 g for one pad pattern, and then heated with a reflow furnace using a far-infrared heater as a heating source under conditions of 215°C × 30 seconds and 220°C × 2 minutes. Then, solder was deposited. As a result, Sn 74% by weight. A solder layer having a thickness of 30 mu m was formed.

Next, a rosin-based solvent (Micro Solder-F-40: manufactured by Harima Chemical Co., Ltd.) with an aqueous solution resistance of 100,000 Ω cm or more is applied on it, the component is placed on it, heated through a reflow furnace, and solder was melted and the leads of each component were soldered to the pads.

As a result, the soldering state without the occurrence of bridges and solder balls was eliminated. After soldering, without washing, the presence or absence of ionic residues on the circuit board was measured using an omegameter (Model 600 manufactured by KENKO, USA), and the results were very good, 0.1-0.3 μg NaC1/sq.in.

In addition, this shape is different from the case of the conventional solder paste, and the amount of solder precipitation depends on the amount of Sn powder that has settled on the pad and its vicinity. Specifically, the thickness (H) of the solder deposited on the pad shape and the width (W) of the pad have a relationship as shown in FIG. where H = aW, 0.05<img file="KR0124924B1_D0002.tif" />a<img file="KR0124924B1_D0003.tif" />0.5 is required, preferably 0.1<img file="KR0124924B1_D0004.tif" />a<img file="KR0124924B1_D0005.tif" />is 0.3. At 0.05a, constriction (a remarkably small portion of solder) occurs in a part or a plurality of the deposited solder, which adversely affects the formation of a fillet required for solder bonding. On the other hand, in a0.5, a special characteristic occurs in a part or several places of the solder layer, and there is a possibility that the lead of the electronic component is disturbed by the projection and shifted.

Example 4

Here, an embodiment in which a Pb-rich solder layer is formed at the time of mounting an electronic component is shown.

(1) Organic acid Pb 67%. A paste-like composition comprising 10% of an activator, 13% of Sn powder, and 10% of a viscosity modifier was applied to a thickness of 400-500 μm on a copper (-) pad, and heated at 220° C. for 2 minutes to form a solder layer. This solder layer had a thickness of 8-10 mu m, a composition of Sn/Pb = 2/8, and a melting point of 293 DEG C. On the other hand, in the case of forming the solder layer using the conventional solder cream, it is not necessary to immerse the substrate in the molten solder (Sn/Pb=2/8) maintained at 293°C or higher, which is much higher than the 220°C of the present invention. have. As a result, the reliability of the substrate is greatly impaired.

(2) A paste-like composition composed of organic acid Pb 55%, activator 15%, Sn powder 13%, and viscosity modifier 17% is applied to a thickness of 300-400 μm on a copper pad and heated at 220° C. for 2 minutes to form a solder layer has formed This solder layer had a thickness of 4-8 µm, a composition of Sn/Pb = 3/7, and a melting point of 278°C. In the conventional method of using a solder paste, it is necessary to heat to about 320° C. in the case of this solder composition.

Moreover, as organic acid Pb of the said paste composition, Pb naphthenic acid, Pb rosin acid. Pb octylic acid, Pb oleate, Pb stearate, etc., rosin, organic acid, alkanolamine, amine acid, etc. as an activator, and castor wax, cellulose powder, butyl carbitol, hexyl carbitol, squalene etc. are used as viscosity modifiers, respectively. possible. Further, as the Sn powder, it is preferable to use a spherical powder produced by the atomization method having a particle size of 80 µm or less, preferably 25 µm or less, and having an oxygen content of 1500 ppm or less, preferably 1000 ppm or less.