JP3169719B2 - Solder paste - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder paste used for mounting an electronic component on a circuit board.

[0002]

2. Description of the Related Art Conventionally, when electronic components are mounted on a circuit board, a solder paste formed by mixing a solder powder with a flux mainly composed of an adhesive and a viscosity modifier is formed into a paste. Printing is performed by a method such as screen printing according to the pattern of the pad, and the lead of the electronic component is placed thereon, and the lead is passed through a reflow furnace and heated to melt the solder paste and solder the lead and the pad. The method is adopted.

[0003]

In recent years, with the miniaturization of electronic devices and electronic components, the arrangement pitch of pads on a circuit board has become finer.

However, in the above mounting method,
It is necessary to print the solder paste accurately according to the pad pattern, but as the arrangement pitch of the pads becomes smaller, finer and more accurate printing technology is required,
Such printing becomes difficult.

[0005] In addition, the so-called one-character printing, in which the solder paste is solid-coated over the entire portion where a large number of pads are arranged or the band-like solder paste is printed only at the center of the length of the arranged pads, is also used. If the arrangement pitch is small, the solder straddles between the pads and short-circuits, and a bridge is likely to occur.

The inventors of the present invention have examined the phenomenon of bridging in detail. As a result, when the solder paste is heated, particles of the solder powder are melted, and if they are directly attached to the surface of the circuit board, the bridging is rarely generated. It was found that some of the molten solder particles aggregated in the solder paste to form giant particles.

When the giant solder particles settle on the surface of the circuit board, the individual pads are too small to absorb the large solder particles, and the solder protrudes from the pads and reaches adjacent pads to form a bridge.

[0008] Even when the solder particles settle between the pads, if the solder particles are fine, they are easily absorbed by any of the pads. Although it can be easily removed, if the diameter of the solder particles is large, a bridge is formed across the pads on both sides.

Conventionally, when the arrangement pitch of the pads is not so small, even if the solder particles are agglomerated, they are sufficiently small as compared with the size of the pads, and there has been almost no problem.
In recent years, as the arrangement pitch of the pads has become smaller, the arrangement pitch and the size of the solder particles have become closer, and if the solder particles have aggregated and become larger, the size of one of the particles cannot be ignored. .

Therefore, if the solder particles are prevented from aggregating in the solder paste and are prevented from settling on the circuit board even if they are agglomerated, the bridge is prevented even if the arrangement pitch of the pads is fine. be able to.

The present invention has been made in view of the above circumstances, and it is possible to roughly print on a circuit board having a small arrangement pitch of pads by solid coating or one-character printing on a pad portion, and furthermore, to form a bridge. It is an object of the present invention to provide a solder paste having no solder.

[0012]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a
-50% by weight, specific gravity of 2.5 or less and average particle diameter of 10
5 to 30% by weight of an organic or inorganic filler having a particle size of 0 μm or less, and the balance is a flux mainly composed of an adhesive and a viscosity modifier.

As the solder powder used in the present invention, eutectic solder is usually used. In addition, tin-rich solder, lead-rich solder, solder containing other metals, etc. may be used as necessary. Can also be used.

The solder particles are preferably as fine as possible with as small a particle size as possible, but in view of the effects and economics of the present invention, those having an average particle size of about 10 to 50 μm are suitable.

The content of the solder powder is 10-50.
Should be% by weight. If it is less than 10% by weight, the amount of solder as a solder paste is insufficient, and 50% by weight.
If it exceeds, a bridge is likely to occur.

The flux is mainly composed of a pressure-sensitive adhesive and a viscosity modifier, and the flux that has been conventionally used as the flux of a general solder paste can be used as it is.

Rosin or a derivative thereof or an organic acid instead thereof is used as the pressure-sensitive adhesive, and mineral spirit or a carbitol-based solvent can be used as the viscosity modifier.

In addition to these pressure-sensitive adhesives and viscosity modifiers, if necessary, a thixotropic agent such as caster wax may be added, or an activator such as an amine or halogen may be added.

The present invention is characterized in that an inorganic or organic filler is added.

As the filler, those having a specific gravity of 2.5 or less and an average particle diameter of 100 μm or less are used.

If the specific gravity of the filler exceeds 2.5, the filler sediments together with the solder particles in the solder paste at the time of heating, and there is no effect of preventing sedimentation of giant particles and preventing bridges.

When the average particle diameter of the filler exceeds 100 μm, the solder particles pass through the space between the filler particles, so that the effect of suppressing the movement of the solder particles and preventing the solder particles from uniting is reduced, and the generation of bridges is prevented. Can not.

The content of the filler is 5 to 30% by weight.
Should be. If the amount is less than 5% by weight, the effect of the present invention is not obtained. If the amount exceeds 30% by weight, the content of the solder paste is so large that the fluidity of the solder paste is poor, and printing on a circuit board becomes impossible.

[0024]

According to the present invention, since the solder particles and the filler particles are dispersed in the flux, the movement of the solder particles is suppressed by the filler particles, and the solder particles may be combined with each other to form giant particles. Absent.

Since the specific gravity of the solder particles is larger than the specific gravity of the filler particles, the small solder particles pass through between the filler particles and settle, and adhere to the pad surface. It cannot escape and cannot adhere to the pad.

[0026]

According to the present invention, therefore, the formation of giant particles due to the coalescence of the solder particles is suppressed, and many solder particles can be present as fine particles in the flux, and the giant particles can reach the surface of the circuit board. Without sedimentation, only fine particles settle and adhere to the pad surface, so that giant particles do not stick to the fine pad and protrude,
No bridging occurs.

Also, the solder particles settled in the space between the pads are easily absorbed on the surface of the pad due to the fineness of the particles, and the particles not absorbed remain as they are and are easily removed by washing. No bridging occurs between the pads on both sides.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. The composition of the solder paste used in the experiment is as follows.

Solder powder: Eutectic solder powder having an average particle diameter of 30 μm Filler: Flux Adhesive: Rosin Viscosity modifier: Hexyl carbitol Thixotropic agent: Custer wax Activator: Ethylamine hydrochloride

The method for measuring the bridge occurrence rate is as follows. Pattern pitch 0.3 on glass epoxy board
A pad pattern of mm (pad width: 0.2 mm, pad interval: 0.1 mm) is formed, a solder paste is solid-coated on the pattern with a thickness of 300 μm, and a hot plate at 270 ° C. (substrate temperature 220 ° C.) For 2 minutes.

After heating, the residue is washed with toluene, the pedestal state of the solder formed on the pads is observed, and the incidence of bridges (the number of bridges with respect to the total number of pads).
Was measured.

First, in order to examine the effect of the content of the solder powder, a solder paste having the composition shown in Table 1 was prepared, and the occurrence of bridges was examined.

As the filler, a Teflon resin powder (shape: amorphous, specific gravity: 2.3, average particle diameter: 30 μm) was used.

[0034]

[Table 1]

Next, the same experiment was conducted by changing the kind of the filler, and the composition and the result are shown in Table 2. Activated carbon (shape: amorphous, specific gravity: 2.5, average particle diameter: 10 μm) was used as a filler.

[0036]

[Table 2]

From the results shown in Tables 1 and 2, it can be seen that when the content of the solder powder is 50% by weight or less, any of the fillers of Teflon resin and activated carbon has an effect of preventing bridging.

It is considered that when the content of the solder powder exceeds 50% by weight, the frequency of coalescence of the solder particles sharply increases, and the function of suppressing coalescence by the filler and the sedimentation of the giant particles does not work sufficiently. Can be

Next, in order to examine the effect of the filler content, solder pastes were prepared with the formulations shown in Tables 3 and 4, and the occurrence of bridges was examined.

As the filler, Teflon resin powder was used in Table 3, and activated carbon was used in Table 4.

[0041]

[Table 3]

[0042]

[Table 4]

From the results shown in Tables 3 and 4, the addition of fillers prevents the formation of bridges in both Teflon powder and activated carbon. When the filler content is 5% by weight or more, the formation of bridges is almost completely suppressed. I can understand that

However, when the added amount of the filler is 40% by weight, the solder paste is almost in a state of no fluidity, and it is impossible to print a uniform thickness on the circuit board.

Next, in order to examine the effect of the type of filler, various paste types shown in Table 5 were used, and the amount of materials was fixed as follows to prepare a solder paste. Examined.

Solder powder: 20% by weight Filler: 10% by weight Adhesive: 41% by weight Viscosity adjusting agent: 24.5% by weight Thixotropic agent: 4.0% by weight Activator: 0.5% by weight

[0047]

[Table 5]

From the results shown in Table 5, the difference and the shape of the filler properties (organic and inorganic) have almost no effect on the occurrence of bridges, but the specific gravity and the average particle diameter have a large effect on the incidence of bridges. You can understand what you are giving.

That is, if the specific gravity of the filler is 2.5 or less, the occurrence of bridges is almost completely suppressed, but 2.
If a heavy object exceeding 5 is used, the incidence of bridging rises sharply.

Even with fillers made of the same material, if the average particle diameter is 100 μm or less, bridges hardly occur, but if it exceeds 100 μm, the generation of bridges cannot be suppressed.

From the above results, it can be understood that the intended object can be achieved by the present invention and the bridge between the pads by solder can be prevented.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Noriko Katayama 671 Mizuhashi, Noguchi-cho, Kakogawa-shi, Hyogo Pref. No. Furukawa Electric Co., Ltd. (72) Inventor Kazuto Hikasa 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd. (72) Inventor Hirokazu Shiroishi 2-6-1 Marunouchi, Chiyoda-ku, Tokyo (56) References JP-A-5-293688 (JP, A) JP-A-5-261586 (JP, A) JP-A-3-47693 (JP, A) JP-A-63-309390 (JP, A) JP-A-63-26293 (JP, A) JP-A-62-179889 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B23K 35/22-35 / 26 B23K 35/363

Claims (1)

(57) [Claims] 1. A composition comprising 10 to 50% by weight of a solder powder and 5 to 30% by weight of an organic or inorganic filler having a specific gravity of 2.5 or less and an average particle size of 100 μm or less, with the remaining being an adhesive and a viscosity. Solder paste characterized in that it is a flux containing an adjusting agent as a main component.