JPH02211995A - Pasty solder - Google Patents

Abstract

PURPOSE:To prevent a Manhattan phenomenon by using the pasty solder which is formed by mixing at least two kinds of solder particles of different m. p. and selecting the average grain sizes of the solder particles having the high m.p. so as to be larger than the average grain size of the solder particles having the low m. p. CONSTITUTION:Surface-packaging parts 1 are joined by the pasty solder 10, 11 onto conductive layers 6, 7 formed on a printed circuit board 5, by which reflow soldering is executed. The solder formed by the low melting solder particles 10a and the high melting solder particles 10b heats up gradually and the surface packaging parts 1 are first lightly joined to the conductive layers 6, 7 by the low melting solder 10a of the small grain size. The effect of the surface tension is small and, therefore, the Manhattan phenomenon does not arise even if only the one side is heated up to a high temp. and is melted. The high melting solder particles 10b melts when the temp. rises higher. The other side is lightly joined by the low melting solder particles 11a and, therefore, the surface packaging parts 1 do not rise even if only the one side heats up to the high temp.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to paste-like solder suitable for use when attaching surface-mounted electronic components to a printed circuit board using a reflow soldering device or the like. .

[Prior Art] In hybrid type ICs and the like, in order to increase the packaging density, there is a tendency for electronic components that can be mounted on an IC board to be surface-mounted components that are light, thin, short, and small, such as chip-type capacitors.

In addition, there are various sizes of surface mount components, and the thickness of even the same electronic component varies widely.To mount such surface mount components, reflow soldering is usually used by equipment. It is often done.

As shown in FIG. 6, this is a state where the surface mount component 1 is temporarily fixed between the conductive layers 6 and 7 completely formed on a predetermined printed circuit board 5 using paste solder 8, 9, etc. Then, the printed circuit board 5 is completely transferred into a reflow oven (not shown).

As the paste solder 8.9, for example, 5n-pb eutectic solder (its melting point is 183° C.) can be used.

As the printed circuit board 5 passes through a high temperature atmosphere (183°C or higher) in the reflow oven, the paste solder 8
．． 9 is melted, and the printed circuit board 5 is cooled by subsequent feeding. The melted solder is now solidified, and the surface mount component 1 is bonded onto the printed circuit board 5.

[Problem M to be Solved by the Invention] By the way, as mentioned above, when reflow soldering uses a device to mount the surface mount component 1 on the printed circuit board S, reflow soldering is performed in a high temperature atmosphere inside the device. Not all surface-mounted components 1 passing through always exhibit a -like temperature distribution.

There are various shapes and sizes of electronic components that can be mounted, and small surface mount components adjacent to large surface mount components are affected by the large surface mount component, and the left and right sides of the same component This is because a temperature difference may occur even between the electrode portions 3 and 4.

If there is a difference in temperature like this, the paste solder 8
．． Naturally, the melting states of the pastes No. 9 are also different, and this temporarily causes a phenomenon in which even if the paste-like solder on the higher temperature side is completely melted, the paste-like solder on the other side is not completely melted.

In this way, for example, if the left paste solder 8 is completely melted and the right paste solder 9 is incompletely melted, the surface tension of the solder 8 itself will be higher on the completely melted solder 8 side than on the other side. The effect becomes greater.

The way this surface tension acts appears as a difference in the moment (vertical component of the rotational moment) in the height direction between the pole parts 3 and 4 of both molds, as shown by the arrows in Figure 6, and the complete melting side The moment applied to the electrode portion 3 is significantly larger than the moment applied to the incompletely melted electrode portion 4 side.

This causes the so-called Manhattan phenomenon in which the surface-mounted component 1 stands up as shown in FIG. 7.

The smaller the component, the greater the surface tension of the solder relative to the component's own weight, making the Manhattan phenomenon more likely to occur.

In today's electronic components, there is a tendency to make the components themselves smaller in order to improve the packaging density, and therefore the Manhattan phenomenon is likely to occur.

Further, in the vapor phase soldering method (VPS method) which has been developed in recent years, the Manhattan phenomenon is particularly likely to occur, which is a big problem.

Bonding defects due to the Manhattan phenomenon are a factor that significantly reduces the yield and reliability of reflow soldering.

In order to prevent the Manhattan phenomenon and eliminate soldering defects, it is conceivable to use an adhesive 14 to prevent the surface mount component 1 from rising as shown in FIG.

However, if this adhesive 14 is used, when the surface mount component 1 is misplaced as shown in FIG. 9, it will be soldered in a misaligned state.

This may lead to accidents such as contact with other electronic components, so it cannot be said to be a very good solution.

If adhesive is not used, even if the component is mounted misaligned, a horizontal moment (arrow) due to the molten solder will act on the surface mount component 1 as shown in FIG. 10A. A direction correction force (self-alignment) acts on the surface mount component 1. As a result, it is fixed at the normal position as shown in FIG.

If adhesive 14 is used, the self-alignment effect of the molten solder cannot be utilized.

In addition, in surface-mounted electronic components such as chip-type ceramic capacitors that have external electrodes on the side of the component, if the solder on the electrode on one side rises in temperature and melts first, the electrode on the side will get wet with the solder. , the moment due to the surface tension of the solder may act in the lateral direction, causing the parts to shift. Like the Manhattan phenomenon, this also causes poor bonding.

In addition, in an atmosphere heating method such as the Zaraani vPS method,
When reflow bonding ICs that have lead parts for surface mounting, the lead part, which has a smaller heat capacity than the printed circuit board part, rises in temperature first, so there is a temporary temperature difference between the printed board and the lead part. Can be done. In this case, the property of the molten solder is that it moves toward the higher temperature side, and the molten solder moves from the joint between the lead and the printed circuit board toward the lead, causing a joint failure. There is also. This phenomenon is called the wicking phenomenon.

Therefore, the present invention proposes a paste solder that does not cause the rise (Manhattan) phenomenon, lateral displacement, and wicking phenomenon of surface-mounted components, and which can be applied to the above-mentioned board device. This is extremely suitable.

[Means for Solving the Problems] In order to solve the above-mentioned problems, in this invention, a predetermined amount of solder particles (two types of solder grains and liquid flux, etc.) having different melting points are mixed to form a paste. The paste-like solder is characterized in that the average particle size of the solder particles on the higher melting point side is selected to be larger than the average particle size of the solder particles on the lower melting point side.

[Function] Low melting point solder grains 10a (lla) and high melting point solder grains 10
In the case of using solder mixed with B (lla), the temperature of the printed circuit board 5 is gradually raised in the conveyance path in the furnace, and the low melting point solder grains 10a (lla) of small particle size are first used.
The surface mount component 1 is then bonded to the conductive layer 6.7.

In this case, when there is a temperature difference between the left and right electrodes of the surface-mounted component 1 and the temperature on the conductive layer 6 side is higher, only the low melting point solder grains 10a of the conductive layer 6a melt.

However, in this case, the Manhattan phenomenon does not occur. This is because the effect of surface tension caused by the melting of the low melting point solder grains 10a is small, so that a moment sufficient to cause one side of the surface mount component 1 (the side of the conductive layer 7) cannot be obtained.

As time passes, the low melting point solder grains (small grain size) 11a on the other conductive layer 7 side also melt, thereby causing the surface mount component 1 to melt.
The electrodes 3.4 are both lightly bonded to the conductive layer 6.7.

In addition, in such a melting process, high melting point solder particles 1
Since 0b has a larger average particle diameter than the low melting point solder grains 10a, the surface of the high melting point solder grains 10b is partially diffused and melted under the influence of the melting of the low melting point solder. Even if it is completely high melting point solder grain 10
Since b does not diffuse and melt, the above-mentioned joining effect of the low melting point solder grains lOa is not impaired.

When the temperature of the printed circuit board 5 is gradually increased, the high melting point solder grains having a large grain size also melt. At this time, if there is a temperature difference between the left and right electrodes of the surface-mounted component and the conductive layer 6 side is higher, the high melting point solder grains 10b of the conductive layer 6 are melted first.

However, even in this case, since the conductive layer 7 side is lightly bonded by the low melting point solder grains 11a, the conductive layer 7 side
The surface mount component 1 does not rise due to the moment generated by the melting of the high melting point solder grains 10b on the side. Finally, the high melting point solder particles 11b on the side of the conductive layer 7 are also melted, and the surface mount component is bonded to the printed circuit board.

Therefore, the Manhattan phenomenon does not occur. Similarly, lateral displacement and wicking phenomena can also be prevented.

If the particle size of the low melting point solder particles to be mixed is made small, the particle size distribution of the solder particles in the paste can be widened, so that deterioration in printability can also be prevented.

[Example] Next, as an example of applying the paste solder according to the present invention to the soldering of the above-mentioned surface-mounted electronic components, a case will be described in which surface-mounted components are mounted on a printed circuit board using a reflow oven. This will be explained in detail with reference to FIG. 1 and subsequent figures.

Also in this invention, as shown in FIG. 1, surface-mounted components 1 placed on a predetermined conductive layer 6.7 formed on a printed circuit board 5 are bonded using paste solder 10.11. This constitutes a substrate device.

In this invention, a special solder as shown below is used as the paste solder for joining the surface-mounted components 1.

That is, in this invention, as shown in FIG.
Low melting point solder grain 10a (lla) and high melting point solder grain 1
Paste solder 10 (11) is used, which is a mixture of predetermined amounts of 0b (Ilb) and liquid flux 100.

In this case, the temperature difference between the two (difference in each melting point) is at least 5
A solder having a temperature of at least .degree. C., preferably 20 to 30.degree. C. or higher is selected.

For example, when solder grains with a melting point of 150°C (ternary alloy or quaternary alloy containing bismuth) are used as the low melting point solder grains 10a (lla), the high melting point solder grains 10a (lla) are
0b (llb) is 5n-P with a melting point of 183°C
B-based eutectic solder is used.

As the low melting point solder grains 10a (11a) actually used, solder grains having a melting point within the range of 130 to 170°C are suitable. Similarly, high melting point solder grains 10b (llb
) is preferably a solder grain whose melting point is within the range of 150 to 210°C.

If the diameter of the solder particles is about 10 μm or less, solder balls are likely to occur, and if the diameter is about 60 μm or more, the printability when placing paste solder by screen printing will deteriorate. - For boat fishing, the solder grains are selected to be 10 μm to 60 μm.

In the present invention, as shown in FIG. 3, the particle size distribution (particle size - weight %) of the high melting point solder particles 10b (1 lb) is larger than that of the low melting point solder particles 10a (llb). used. For example, the low melting point solder grains 10a (
10~5C) as shown by the solid line in Figure 3 as lla)
When using solder grains having a particle size of about am, the high melting point solder particles 10b (Llb) have a particle size of about 30 to 60 μm, as shown by the broken line in the figure.

If the particle size of the solder grains on the high melting point side is small, the solid high melting point solder grains will diffuse into the molten low melting point solder due to the influence of the melting of the low melting point solder grains, causing melt penetration. As a result, the behavior is the same as that of a single solder, so there is no advantage in mixing two types of solder grains.

Moreover, if only solder particles with large particle diameters are used, the printability when placing paste solder by screen printing will deteriorate.

For this reason, high melting point solder grains 10b (llb)
The particle size is set larger than that of the low melting point solder particles 10a (lla).

In this way, there are two types of particles with different melting points and different particle sizes.
Types of solder grains 10a (lla), 10b (llb)
are mixed as shown in FIG. 2, and a liquid flux 10c is added to form a paste.

When the surface mount component 1 is soldered as shown in FIG. 1 using the paste solder 10 (11) as described above, the surface mount component 1 can be bonded to the printed circuit board 5 through the following steps. Become.

First, when the glue passes through a flow furnace in a high temperature atmosphere, the low melting point solder grains 10a melt as shown in FIG. The paste solder is completely melted.

This melting step can be achieved by simply transporting the paste solder 10 through a reflow oven.

When using 183° C. solder grains as the high melting point solder grains 10b, the reflow furnace solder flow region (main heating region) is controlled to be 183° C. or higher.

FIG. 5 is a diagrammatic representation of the above-mentioned melting process. I will explain the order.

First, a predetermined conductive layer 6. is formed on the printed circuit board S.
A predetermined amount of the above-mentioned paste solder 10.11 is applied onto the solder 7, and the surface mount component 1 placed on the top surface thereof is temporarily fixed by the paste solder 10.11 (FIG. A).

In this state, it is transported into a reflow oven. Then, the temperature of the printed circuit board 5 is gradually raised in the preheat area of the conveyance path of the reflow oven, and first, the low melting point solder grains 10a,
lla begins to melt.

In this case, if there is a temperature difference between the electrodes of the surface-mounted component 1, the higher temperature side will melt. FIG. 5B shows a state in which the low melting point solder grains 10a are melted. Therefore, electricity
The iS side can be easily joined by the melted low melting point solder grains 10a, and this melting causes a slight surface tension to act on the electrode 3 side, and the paste solder 10 becomes as shown in the figure.
Its surface will lift slightly.

On the other hand, the low melting point solder grains 11a of the four electrodes have not yet melted.

However, as described above, the effect of surface tension due to the melting of the low melting point solder grains 10a is very small, and therefore the force is not enough to lift one side of the surface mount component 10. Therefore, the Manhattan phenomenon does not occur.

As the printed circuit board 5 is further heated, the other low melting point solder grains 11a also become molten, and the picture electrodes 3.4
Both can be easily joined (C in the same figure).

When the printed circuit board 5 is transported to the riff 0-furnace glue flow area (main heating area), the ambient temperature has risen to 190° C. or higher. Therefore, the high melting point solder grains 10b are also in a molten state, and the electrodes 3 are completely joined by the paste solder 10 (FIG. 3D). At this time, electrode 3.4
If there is a temperature difference between them, the high melting point solder grains 11b on the electrode 4 side will not reach a molten state for the same reason as described above.

However, since 1iti4 is completely bonded to the conductive layer 7 by the low melting point solder grains 11a, the Manhattan phenomenon does not occur in this case as well.

When the electrode 4 is transported to the center of the reflow oven, the temperature on the electrode 4 side also becomes quite high.
The solder paste 11 is also melted, and the electrode 4 is also completely joined by the paste solder 11 (see E in the same figure).

In this melting process, the two types of solder grains have different average particle diameters, so in the early stages of reflow processing, the solid high melting point solder grains do not completely diffuse into the melting low melting point solder. There is no chance of melting.

That is, the high melting point solder grains 10b are the low melting point solder grains 10a.
This is because the average particle diameter is larger than that of the high melting point solder particles, so it takes a considerable amount of time for the high melting point solder particles to diffuse into the molten low melting point solder.

Here, FIG. 4A shows an example in which a conventional paste solder is used in reflow processing in a vapor phase solder bath and a paste solder according to the present invention is used. Also, if this is graphed, it will look like Figure 4B.

Bi-containing solder with a melting point of 150°C was used as the low melting point solder, and 5n-P with a melting point of 183°C was used as the high melting point solder.
B-based eutectic solder was used. The vapor temperature of the vapor phase solder used was 215°C. Commonly known as 160 as a surface mount component
360 small ceramic capacitors called 8 types
I used it.

Of the amount of low melting point solder mixed in, 0% and 100% are respectively in the conventional example. As can be seen from this example, compared to the conventional method, the paste solder according to the present invention can significantly reduce the incidence of bonding defects such as Manhattan phenomenon by varying the amount of low melting point solder mixed in. This makes it possible to significantly improve the bonding reliability of surface-mounted electronic components.

[Effects of the Invention] As explained above, in the present invention, the paste-like solder is composed of at least two types of solder grains having different melting points, and thus have different melting temperatures. Therefore, the effect of surface tension caused by melting of low melting point solder grains in the initial stage of reflow is also reduced.

However, since the two types of solder grains have different average particle sizes, the low melting point solder grains melt first, and the surface mount components are lightly bonded.Then, when heated roughly, the high melting point solder grains melt. The particles melt and complete bonding of the surface mount component to the printed circuit board occurs.

Therefore, by using this paste-like solder on a mounting board for surface-mounted components as described above, it is possible to reliably eliminate the Manhattan phenomenon, lateral displacement, wicking, etc. of surface-mounted components.

Therefore, the paste-like solder according to the present invention is extremely suitable for use in board devices and the like on which surface-mounted components are mounted as described above.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the main parts of an example of a board device in which surface-mounted electronic components are fixed using the paste solder according to the present invention, and FIG. An explanatory diagram of the process, FIG. 3 is a diagram showing the particle size distribution (particle size - weight %) of solder particles constituting the paste solder according to the present invention, and FIG. 4A is a diagram showing the weight % of low melting point solder particles and bonding, respectively. A diagram showing the relationship with the defective rate, FIG. 4B is a characteristic diagram, and FIG. 5 is a diagram showing the bonding process of paste solder.
Fig. 6 is a diagram showing a conventional mounting state on a printed circuit board, Fig. 7 is a diagram showing the occurrence of the Manhattan phenomenon due to conventional paste solder, Fig. 8 is a diagram showing another conventional mounting state, and Fig. 9 is a diagram showing the conventional mounting state on a printed circuit board. The figure is a plan view at that time, and FIG. 10 is an explanatory diagram of the self-alignment effect. 10b, llb...High melting point solder grains 10c...Liquid flux 14...Adhesive

Claims (1)

[Claims] (1) A paste-like solder made by mixing at least two types of solder grains with different melting points and a predetermined amount of liquid flux, etc., in which the average particle diameter of the solder grains on the side with a higher melting point is set on the side with a lower melting point. Paste solder characterized by being selected to be larger than the average particle diameter of solder grains.