JP2000052082A - Solder paste for chip parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solder paste which does not cause chip rising even when micro chip parts are soldered. SOLUTION: A solder alloy wherein peaks of big heat absorption appear at three or more points between the start and the end of penetration in differential thermal analysis is powdered, so that a solder paste is manufactured from the powdered solder alloy. If the peaks of thermal absorption exist at three or more points, wetting force is divided into three or more and the surface tension is weak, whereby the chip rising does not occur.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder paste suitable for soldering fine chip components to a printed circuit board.

[0002]

2. Description of the Related Art In recent years, electronic components used in electronic devices,
In particular, chip resistors, chip capacitors, and the like have become extremely small. When such a minute chip component (hereinafter simply referred to as a chip component) is soldered with a solder paste of Sn-Pb eutectic solder, one electrode of the chip component rises away from the soldering portion of the printed circuit board. Standing ".

[0003] The cause of chip formation is that a strong surface tension of solder acts when the solder paste is melted. In other words, when the solder paste applied to both ends of the chip component melts, if the solder paste at both ends melts at the same time, both surface tensions will be balanced and the chip will not stand, but one of them will melt first. If this is the case, the upper part of the chip component is pulled by the surface tension of the previously melted one, causing the chip to stand.
The time difference between the melting of the solder paste at both ends is 1 /
Even a very small time difference of 10 seconds could not be balanced and would cause chip standing.

Conventionally, for a relatively large electronic component such as a QFP or SOIC, an adhesive is applied to an electronic component mounting portion of a printed circuit board in advance, the electronic component is temporarily fixed with the adhesive, and then the solder paste is applied. A method of melting is adopted. Even if a narrow chip component is temporarily fixed with an adhesive and soldered, chip standing will not occur, but if an adhesive is used for a fine chip component, the adhesive will adhere to the electrode of the chip component and In addition to impairing the conductivity of the printed circuit board, it is not possible to apply a small amount of adhesive to precise positions on narrow chip components, so adhesives can not be used for soldering chip components Was.

[0005] For this reason, attempts have been made to solve the chip formation with a solder paste. This is a method of soldering using a solder paste in which two kinds of powder solders having different melting temperatures are mixed. (Reference: JP-A-63-154288)

A solder paste in which two kinds of powder solders having different melting temperatures are mixed (hereinafter, referred to as a mixed solder paste) is composed of pure Sn powder (liquidus temperature 232 ° C.) and 95P.
b-Sn powder (liquidus temperature 315 ° C.) 59:
41, and when these are completely melted, 63
It has a eutectic composition of Sn-Pb.

[0007] This mixed solder paste is 95Pb-S.
Even if it is not heated above the liquidus temperature of the n-solder (315 ° C), it takes a little time to heat it to a temperature slightly higher than the eutectic temperature of Sn-Pb (183 ° C). It can be attached. This is because if two types of powder solder are in contact even at a low heating temperature, a molecular diffusion phenomenon occurs between the powder solders and the powder solder is partially melted.
Since the solder partially melted in this way has a higher degree of freedom, it is more liable to cause a diffusion phenomenon with other powdered solder, which spreads to spread all the powdered solder.

[0008] In this melting process, the molten solder becomes Sn
Since it is a binary component of -Pb, it has a semi-molten state in which an α phase in which Sn is dissolved in Pb and a β phase in which Pb is dissolved in Sn are mixed. Since the semi-molten solder has a lower surface tension than the completely molten liquid metal, when soldering chip components using mixed solder paste, the surface tension of the solder at both ends of the chip component when the solder paste melts is reduced. It is getting weaker. Therefore, the mixed solder paste makes it difficult for the chip component to rise even if there is a difference in the melting time between both ends.

[0009] However, the mixed solder paste is
When mixing the two types of powdered solder, no matter how long the stirrer is used for a long time, a locally uneven portion is formed, and the composition after melting differs from the initially intended eutectic composition. The temperature sometimes increased. When the liquidus temperature of the solder increases, the solder cannot be completely wetted on the printed circuit board at a predetermined soldering temperature, which causes a soldering failure.

Therefore, the present inventor considered that a powder of a solder alloy (hereinafter referred to as twin peak solder) in which thermal absorption peaks appear when melting starts in differential thermal analysis, and then peaks again when most of the melting occurs. It has been found that a solder paste mixed with a liquid or paste-like flux has an excellent effect of preventing chipping and has already been established as Japanese Patent No. 2682326.

[0011]

The chip component at the time when the inventor of the present invention invented the solder paste using the twin-peak solder alloy had a length of 1 mm in the vertical direction called "1005" and a length of 1 mm in the horizontal direction. Although the length was 0.5 mm, today's chip components are becoming even smaller, such as “0603”, that is, the vertical length is 0.6 mm and the horizontal length is 0.3 mm. ing. The chip component of “0603” is very likely to cause chip standing during soldering. In the case of the conventional "1005" chip component, chip standing could be almost completely prevented by using a solder paste using a twin-peak solder alloy. However, the "0603" chip component may occasionally have chip standing. Was. By the way, when a chip component of “0603” is soldered with a solder paste using 63Sn—Pb eutectic solder, the chip standing becomes an extremely high failure rate of 10% or more.

[0012]

The inventor of the present invention has proposed "060
As a result of intensive studies on a solder paste capable of completely preventing chip standing even in the soldering of the chip component of "3", when a solder alloy having three or more peaks of heat absorption in differential thermal analysis is used as the solder paste, "0"
The present inventors have found that chip standing can be prevented even for chip components of "603" and completed the present invention.

According to the present invention, there is provided a chip in which a powdery solder alloy and a liquid flux or a paste-like flux are mixed at three or more places where the peak of heat absorption in differential thermal analysis appears between the beginning and end of melting. It is a solder paste for parts.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION As the solder alloy used in the present invention, any solder alloy can be used as long as the peak of heat absorption in differential thermal analysis appears at three or more places between the start and end of melting. is there. The peak of the heat absorption referred to in the present invention means that the locus of the heat absorption point clearly appears in a mountain shape on the chart of the differential thermal analysis, and a peak that is slightly depressed is not recognized as a peak.

[0015] In a solder paste using a solder alloy in which thermal absorption peaks appear in three or more places in differential thermal analysis, the wetting force is divided into three parts at the time of soldering, so that the surface tension applied to each soldered part also becomes weak. Therefore, it is considered that the chip is unlikely to stand up. In other words, in the case of a twin-peak solder alloy, the wetting force is divided into two, such as one half at the first peak and two-half at the next peak. Since it is divided into three parts, first one third, then two thirds, and finally three thirds, the surface tension of each is even weaker than the twin-peak solder alloy, resulting in minute chip components But it doesn't cause a chip.

The liquid or paste-like flux used in the present invention is a flux which has been conventionally used for a general solder paste, for example, a resin obtained by dissolving rosin, an activator, a thixotropic agent and the like in a solvent. The mixing ratio of the solder alloy and the flux is preferably 8 to 12% as the flux amount.

[0017]

1 to 4 are differential thermal analysis charts of a solder alloy having three or more peaks used in the solder paste of the present invention.

The solder alloy shown in the differential thermal analysis chart of FIG. 1 is a solder alloy having a composition of Pb-10Sn-3.5Sb. In the trace of the differential thermal analysis of this solder alloy, 1
Melting starts at 85 ° C and ends at 288 ° C.
A tri (3) peak with a small first peak appearing at 8 ° C, followed by a large peak at 245 ° C, and a slightly larger peak at 278 ° C. This solder alloy is a so-called “high-temperature solder” whose liquidus temperature is higher than 232 ° C., which is the melting point of Sn. High-temperature solder is used for the first soldering when soldering is performed twice, so that the first soldered part does not melt during the second soldering, or the soldered part is always exposed to high temperature It is used for various places.

The solder alloy shown in the differential thermal analysis chart of FIG. 2 is a solder alloy having a composition of Sn-25Pb-20Bi-5In. In the trace of the differential thermal analysis of this solder alloy, melting started at 73 ° C. and ended at 168 ° C., during which the first peak was 80 ° C., the second peak was 108 ° C., and the third peak was 108 ° C.
Is a tetra (four) peak at 132 ° C. and a fourth peak at 163 ° C. This solder alloy is a low-temperature solder having a liquidus temperature lower than Sn 232 ° C. The low-temperature solder has a lower melting point than the first solder alloy so that when soldering is performed twice, the first soldered portion does not melt during the second soldering.

The solder alloy shown in the chart of the differential thermal analysis of FIG. 3 has a composition of Sn-27.7Pb-15Bi-2In.
Is a solder alloy. In the trajectory of the differential thermal analysis of this solder alloy, melting began at 121 ° C. and ended at 183 ° C., during which the first peak was 127 ° C. and the second peak was 153.
℃, the third peak appears at 170 ℃ (3
) Peak. This solder alloy also has a liquidus temperature of Sn
Low-temperature solder lower than 232 ° C.

The solder alloy shown in the differential thermal analysis chart of FIG. 4 has a composition of Sn-29.3Pb-20Bi-2In.
Is a solder alloy. In the trace of the differential thermal analysis of this solder alloy, the melting started at 111 ° C. and ended at 177 ° C., during which the first peak was 128 ° C. and the second peak was 143.
° C, the third peak appears at 167 ° C (3
) Peak. This solder alloy also has a liquidus temperature of Sn
Low-temperature solder lower than 232 ° C.

A solder paste was prepared using a Sn-27.7Pb-15Bi-2In tripeak solder alloy, a conventional 63Sn-Pb eutectic solder, and a twin peak solder alloy. These solder pastes were applied by printing using a metal mask, 100 chip components of “0603” were mounted on the applied portions, and then heated in a reflow furnace to perform soldering. As a result, the solder paste of the present invention using the tripeak solder alloy had no chips, but the solder paste using the twin peak solder alloy had three chips, and the solder paste using eutectic solder. The paste had as many as 16 chips standing.

[0023]

The solder paste of the present invention uses a solder alloy in which heat absorption peaks appear at three or more places between the beginning and end of melting, so that the wetting force at each peak becomes three or more. , The surface tension becomes very weak.
Therefore, the solder paste of the present invention is the same as “0603” of today.
It is possible to obtain a highly reliable soldered portion that never causes a chip to stand even for a very small and lightweight electronic component like the chip component of the above.

[Brief description of the drawings]

FIG. 1 is a differential thermal analysis chart of a Pb-10Sn-3.5Sb solder alloy

FIG. 2 is a differential thermal analysis chart of a Sn-25Pb-20Bi-5In solder alloy.

FIG. 3 is a differential thermal analysis chart of a Sn-27.7Pb-15Bi-2In solder alloy.

FIG. 4 is a differential thermal analysis chart of a Sn-29.3Pb-20Bi-2In solder alloy.

Claims (1)

[Claims] 1. A chip component characterized by mixing a powdery solder alloy and a liquid flux or a paste flux in which at least three peaks of heat absorption in differential thermal analysis appear between melting start and melting end. Solder paste.