JPS6012295A - Composition for solder paste - Google Patents

Abstract

PURPOSE:To obtain a solder paste which is hardly oxidizable after coating until soldering with the solder paste for a printed wiring board by limiting the particle size of solder powder. CONSTITUTION:Solder powder having a particle size larger than 5mu is used to form a compsn. for a solder paste. If the solder powder having the larger particle size is used in such a way, the compsn. is made stable to air oxidation. The effect of preventing the oxidation is remarkable perticularly when the coated solder paste is dried by hot wind. However if the particle size of the solder powder is too large, the coatability in the stage of, for example, printing the paste is poor and therefore the adequate partizle size is about 150mu in max. in said respect.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solder paste composition, and in particular, in a solder paste for printed wiring boards, the particle size of the solder powder is adjusted to 5μ so that the solder powder is less likely to be oxidized after application until soldering. Regarding something bigger.

Printed wiring boards are made by etching copper foil pasted onto the surface of a laminated board to form a circuit pattern according to an electrical circuit, and to which electrical components such as resistors and capacitors can be attached. To attach these electrical components, their lead terminals are soldered to the lands of the circuit pattern.

In this case, the circuit pattern is formed on the opposite side of the printed wiring board where the electrical components are attached and there is a land for soldering, and the circuit pattern is formed on the side of the printed wiring board where the electrical components are attached. For fff97 soldering, there is a so-called plane mount type that has a land. In the case of these chairs as well, molten solder is supplied to the parts and soldered, but in the former case, jet solder is supplied and soldered, and in the latter case, soldering is carried out by applying molten solder to the lands in advance. Solder paste is often applied and soldered by melting the solder paste.

Solder paste requires solder powder, a resin component such as rosin that provides coating properties and temporarily fixes the solder powder to the land until soldering, and a solvent that provides fluidity for application. The solder powder, which is further oxidized as necessary, and the soldering agent are composed of an activator that reduces the land surface and a solvent that imparts fluidity.
This is applied by silk screen printing or the like. Soldering using this solder paste is normally performed in a continuous process from applying the solder paste to melting the solder, but for example, when the production line is stopped at the end of the working time, the solder paste This leaves a number of printed wiring boards that have been coated with paint but have not yet been transferred to the melting process and further processing has been stopped. In such a case, for example, if there are holidays at the same time, the applied solder paste will be exposed to the air because the solder powder consists of lead and tin, and it will be exposed to air oxidation. I will be placed there. When a solder paste application containing such oxidized solder powder is transferred to the melting process by restarting the production line, the non-oxidized solder powder melts and aggregates, but the oxidized solder particles darken and do not completely melt. A problem arises in that the solder particles separate from the aggregated solder and remain at the application position, or they melt slowly and become scattered, forming solder balls and causing short circuits between conductors. This separation of the solder also reduces the strength of the solder joint and impairs the appearance of the solder joint finish.

Another example of using solder paste is pre-soldering. For example, it may take several months from the time the printed wiring board is manufactured until it is used, so in order to prevent oxidation of the lands during soldering, apply solder paste to the lands beforehand and let them fuse. , soldering is sometimes remelted during normal soldering operations. When performing this preliminary soldering, the same problem as above occurs because the solder paste is sometimes left to stand after being applied until it melts.

In order to eliminate the problems associated with the oxidation of solder powder, the solder paste can contain a large amount of activator that reduces the oxide, but since this activator is, for example, amine hydrochloride, it is often If it remains on the soldering part, it will accelerate the corrosion of the soldering land. If you try to avoid this, you will have to clean the soldering well after soldering, which reduces work efficiency and makes it difficult to ensure complete cleaning.Also, another method is to store printed wiring boards coated with solder paste in a refrigerator. Although it is possible to store the wiring board in the storage room, there is a problem that this process is troublesome and the wiring board absorbs water.

As described above, the present invention improves the problem that the conventional solder paste causes various problems because the solder is oxidized between the time it is applied to the printed wiring board and the time it is soldered. Therefore, it is an object of the present invention to provide a solder paste composition in which oxidation of the solder powder is reduced by making the particle size of the solder powder larger than 5 μm.

The solder powder used in the solder paste composition of the present invention has a particle size larger than 5 μm. Solder is made of lead and tin, and its powder has particle sizes distributed over a wide range. For example, the particle size distribution of solder powder manufactured by the atomization method is, for example, 30. μ
It has a peak at , and ranges from 5μ or less to 150μ or more before and after it. However, from the distribution of the number, the smaller the particle size, the larger the number, and the larger the particle size, the smaller the number gradually decreases. In the particle size distribution of the solder powder used in the solder paste manufactured by the atomization method as described above, the solder powder with a particle size of 5 μm or less accounts for 5 to 10% by weight of the total weight. For use in the present invention, 5 to 10% by weight of the solder powder, which is less than 5 microns, is removed. In this way, the effect of removing particles with a small particle size is that the smaller the particle size of the solder powder, the larger the sum of the surface areas of all the particles, so the more parts come in contact with air, the more likely oxidation will occur. Conceivable. From this point of view, the larger the particle size of the solder powder, the less likely it is to be oxidized, so it is preferable. It becomes stable and can prevent the formation of solder oxide for, for example, 10 days. The effect of preventing oxidation by increasing the particle size of the solder powder is particularly noticeable when the applied solder paste is dried with hot air, where the oxidation of the solder powder is accelerated. However, if the particle size of the solder powder is too large, the applicability of the solder paste will deteriorate, for example, when printing the solder paste.
0 μ is appropriate.

Methods for removing solder powder with small particle diameters include classification using a sieve and wind classification in which solder and powder are blown away by wind and those blown far away are removed.

In the solder paste of the present invention, in addition to the solder powder, a resin component such as rosin, an inorganic or organic activator, a solvent, and various additives which are normally used are mixed together with the solder powder.

The solder paste of the present invention is used, for example, for soldering the land of a flat mount type printed wiring board on which chip parts of electric elements are attached to the side on which a circuit pattern is formed. For soldering, it is applied to the land and dried, and then applied and dried on top of this. Immediately thereafter, the terminals of the chip components may be brought into contact with the applied solder paste and then heated and soldered. You can also solder in the same way as above after the applied solder paste is left as is when suspending the process, and in this case as well, since no solder oxide is formed in the applied solder paste, It is possible to perform soldering similar to solder paste immediately after application. In addition to the above, when there is a gap before soldering electrical components to a printed wiring board, pre-soldering is performed to prevent soldering lands from oxidizing, but the solder paste of the present invention can also be used in this case. used. Furthermore, it can be used not only in this but also in all applications where solder paste is used.

Next, an embodiment of the present invention will be described with reference to FIGS. 1 and 2.

Example 1 Solder alloy powder (Sn 63% by weight, Pb 37% by weight, particle size 20μ
A solder paste (viscosity of 2000 poise) was obtained by mixing 15 parts of vehicle (68% by weight of rosin with a softening point of 70°C, 2% by weight of aniline hydrochloride, and 30% by weight of butyl cellosolve).

Using a metal mask with a thickness of 0.1 mm, this solder paste was silk screen printed as shown in Figure 1(a) on an alumina substrate coated with Flux in advance, and after air-drying, the solder paste was heated to a thickness of 0.1 mm at room temperature. ,24.48.72,120,
Each was left for 240 hours (hr). The product after being left is left on a solder bath at 230° C. for 20 seconds to melt the solder paste, and after melting, it is allowed to cool naturally. This cooled product is shown in a diagram corresponding to each of the above-mentioned standing times (a
) The solder paste changed as shown in (b) to (h) in the figure. In the figure, the circles shown are the melted and agglomerated solder powder, and the parts surrounded by chain lines are the traces of fluff residue. For comparison, a solder paste in which 5% by weight of the total solder alloy powder of solder alloy powder with a particle size of 5 μm or less was added to the above solder paste was prepared in the same manner as above, and the result is shown in Fig. 2 (ao), which is the same as Fig. 1 (a). The results were printed in the same manner as in (a) to (h) above, and the results were shown in Figures 2 (b) to 2 (b). (h
Shown in ゛). In these figures, black dots indicate solder powder particles that have become black due to oxidation.

From these results, it can be seen that in the case of this example, no solder oxide was observed in the case shown in FIG. 1(a), which was melted immediately after printing for 0 hours, and the color of the Franks residue was transparent yellow. It showed a brown color. Similar to (a), no solder oxide was observed in the samples (b) to (h), which were left for 12 to 240 hours, and the color of the Franks residue was transparent yellowish brown. On the other hand, the comparative example (a゛), which was melted for 0 hours, i.e., immediately after sealing, was
Similar to ), no solder oxide was observed, and the color of the Franks residue was a transparent yellow-brown color, but in the case of ``b'', which melted after 12 hours of standing time, the Franks residue was black in the transparent yellow-brown color. Generation of solder oxide is observed.Leaving time 2
The amount of solder oxide generated increased in the case of (C) which was melted after 4 hours, and the color of the Franks residue became grayish and lost its transparency. As each melt increases (
In cases of do) to (h), the amount of solder oxide generated gradually increased, the color of the Franks residue changed from gray to black, and the transparency gradually changed to opacity.

Example 2 A solder paste was obtained in the same manner as in Example 1 except that the solder alloy powder was thicker than 10μ, and this was applied in the same manner as in Example 1 and left to melt. No solder oxide was observed.

Example 3 A solder paste coating was melted in the same manner as in Example 2 except that the solder alloy powder was made larger than 5μ in Example 1, and no solder oxide was observed even after 48 hours of standing.

As explained above, according to the present invention, since the solder powder of the solder paste has a particle diameter larger than 5 μm, the solder paste is applied to a printed wiring board, for example, and then left in the air. Even if the solder powder is heated, it is difficult to oxidize the solder powder.

Therefore, even if the line is stopped and the line is restarted a few days later, especially in a production line where the application and melting of solder paste is carried out in a continuous process, the solder paste remains in the air when the production line is stopped. For example, on a printed wiring board that has been left unattended, the applied solder paste will melt in the same way as the solder paste immediately after application, reducing the occurrence of unmelted solder and discrete solder balls caused by delayed melting. It is possible to avoid the problem of short-circuiting between conductors and to perform soldering with a good finished appearance and no reduction in strength. This makes it possible to avoid diminishing the benefits of an automatic production line.

[Brief explanation of drawings]

Figures 1(a) to <h> are diagrams showing the results of melting a coated product using a solder paste according to an embodiment of the present invention by varying the standing time, and Figures 2(a') to (h) 2) is a diagram showing the results corresponding to the above (a) to (h) of the comparative example. June 30, 1982 Patent applicant Ginya Ishii 582 (a) (b) (c) (d) (a) (B) (c') (d) (e) (f) (II) (h ) (en #:) #') (h') Procedure Neho formal text (method) % formula % 1. Indication of the case 1988 Patent Application No. 118878 2. Name of the invention Solder paste composition 3. Amendment Relationship with the case of a person who does “Brief explanation column of drawings in specification” “Drawing!F
7, Section 7” 7. Contents of amendment 1, page 13 of the specification, lines 7 and 8, r(a”) ~
(h”)” should be corrected to “(A)~(H)”. 2. In Shun Q3 r (a') J, r (A) J
, r (b') J, r (B) J, r (
c')'', r (C)J, r (d')
r (D) J, r(e”)”
(E) Jr (f') j, r (F)J
, ``(g')J and there, r (G)J, r (h
' ) J is corrected as r (H)J. (a) (b) (c) (d) and A no θwono (C) (D) t, no (f) (8) (h) (E) (r) (a) (H) Procedural amendment (Own ship November 22, 1981 Kazuo Wakasugi, Commissioner of the Patent Office1, Indication of the case, Patent Application No. 118878 of 19832, Name of the invention Solder paste composition 3, Relationship with the person making the amendment) Patent applicant: 3-13-8 Midoricho, Tokorozawa City, Saitama Prefecture Ginya Ishii 3-13 Kotesashicho, Tokorozawa City, Saitama Prefecture Kotesashi Heights M8
13 Ken Okutani 4, Agent ``Detailed explanation column of the invention in the specification'' 8. Contents of amendment 1, In the 7th line of page 10 of the specification, r (a') J is replaced with r (A) Correct it with J. 2. On page 10, line 10 of the specification, r(b'') ~ (h') J is replaced with "(B) ~ (H
)” is corrected. 3. In the first line of page 11 of the specification, the text r (a') J is corrected to r (A) J. 4. In the 4th line of page 11 of the specification, "r(b')J" is corrected to "r(B)J". 5. In the 6th line of page 11 of the specification, the text r (c') J is corrected to "(do)~(h')J, (D)~(H)".

Claims (1)

[Claims] (11) A solder paste composition having solder powder, characterized in that most of the solder powder is composed of powder particles having a diameter of more than 5 μm.