JP2008027588A - Conductive filler, and intermediate-temperature soldering material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conductive filler capable of fusing and jointing at lower temperature conditions (a peak temperature of 181°C or more) than reflow heat treatment conditions of Sn-37Pb eutectic solder and standing heat-resistant use at equivalent levels. <P>SOLUTION: The conductive filler consisting of a mixture body of first metal particles and second metal particles. The mixture body is provided with at least one metastable alloy phase at 110 to 130°C observed as a heat generation peak at the differential scanning calorimetry (DSC), and at least one melting point each at 120 to 140°C and 170 to 200°C observed as endothermic peaks. By putting the mixture body under heat treatment at 180 to 200°C, the conductive filler is obtained with a lowest melting point at 160 to 200°C after fusing and jointing the first metal particles and the second metal particles. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a conductive filler used as a bonding material for electric / electronic devices, and more particularly to a lead-free solder material and a conductive adhesive.

Solder is generally used for joining metal materials, and is made of an alloy material having a melting temperature range (solidus temperature to liquidus temperature range) of 450 ° C. or less. Conventionally, when an electronic component is mounted on a printed circuit board, Sn-37Pb eutectic solder having a melting point of 183 ° C. is used, and a temperature range of about 200 ° C. to 230 ° C. has been mainstream as reflow heat treatment. Generally, the reflow heat treatment conditions are set at a solder alloy melting point + 10 to 50 ° C.
However, in recent years, as in the EU environmental regulations (RoHS directive), the harmfulness of Pb has become a problem, and from the viewpoint of preventing environmental and human body contamination, the lead-free solder has been rapidly advanced. Under such circumstances, as a substitute for the Sn-37Pb eutectic solder, lead-free solder composed of Sn-3.0Ag-0.5Cu having a melting point of about 220 ° C. is currently used (see Patent Document 1). In general, reflow heat treatment is in the temperature range of about 240 ° C. to 260 ° C.
By the way, the lead-free solder containing Sn as a main component and having a melting point of about 220 ° C. has a higher melting point than that of Sn-37Pb eutectic solder. Therefore, recently, in order to suppress thermal damage of electrical and electronic equipment, soldering at a low temperature is required, and a reflow heat treatment condition equivalent to conventional Sn-37Pb eutectic solder and a heat-resistant bonding material. Is being considered.

As components that lower the melting point of lead-free solder alloys, the effects of Bi, In, Zn, etc. have been confirmed, but the melting point is insufficiently lowered by the quantity ratio, and Bi improves the wettability of the alloy to the base material. However, since it is easily segregated during solidification, its crystal structure is brittle, and ductility is poor, addition of a certain amount or more significantly impairs mechanical strength (see Patent Documents 2 and 3). Since In is a scarce resource and a very expensive material, if it is added in a large amount, the cost increases significantly (see Patent Document 4). Zn is inexpensive and has good mechanical properties, so it is expected to be put to practical use. However, it has very high activity, is easy to react, and is easy to oxidize. Bad and corrosion resistance is low. In bonding with Cu, not a Cu—Sn compound layer but a Cu—Zn compound layer is formed at the interface. The Cu—Zn-based compound layer has problems such as significant strength deterioration under high temperature and high humidity environment (see Patent Document 5).
The present inventors have previously proposed a lead-free conductive material that can be connected at a lower heat treatment temperature than Sn-37Pb eutectic solder (see Patent Documents 6, 7, and 8). However, this conductive material is characterized in that the minimum melting point rises to 300 ° C. or higher after connection by heat treatment and expresses connection stability, and has the same repair property as Sn-37Pb eutectic solder material. It was not a thing.

Japanese Patent Laid-Open No. 05-050286 Japanese Patent Laid-Open No. 05-228685 Japanese Patent Laid-Open No. 08-206874 Japanese Patent Application Laid-Open No. 08-187591 Japanese Patent Laid-Open No. 06-238479 JP 2004-223559 A JP 2004-363052 A JP 2005-005054 A

  The present invention has been made in view of the above circumstances, and can be melt-bonded at a temperature lower than the reflow heat treatment condition of the Sn-37Pb eutectic solder (peak temperature of 181 ° C. or higher), which is equivalent to the Sn-37Pb eutectic solder. An object of the present invention is to provide a conductive filler that can be used at heat resistance (heat resistance requirement 150 to 160 ° C.) and repair temperature. Another object of the present invention is to provide a solder paste using the conductive filler.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have reached the present invention.
That is, the first of the present invention is a conductive filler comprising a mixture of first metal particles and second metal particles, and the mixture is observed as an exothermic peak by differential scanning calorimetry (DSC). At least one metastable alloy phase at 110 to 130 ° C, and at least one melting point observed as an endothermic peak at two locations of 120 to 140 ° C and 170 to 200 ° C. A conductive filler having a minimum melting point of 160 to 200 ° C. after the first metal particles and the second metal particles are melt-bonded by heat treatment at −200 ° C.
The said mixture consists of 100 mass parts of 1st metal particles and 20-100 mass parts of 2nd metal particles, This 1st metal particle is Ag25-40 mass%, Cu 5-15 mass%, Bi2-8. The second metal particles are composed of an alloy having a composition of mass%, In2-8 mass%, and Sn29-66 mass%, and the second metal particles include In30-40 mass%, Ag5-15 mass%, Cu10-20 mass%, and It is preferable that it consists of an alloy which has a composition of Sn20-55 mass%.
The second of the present invention is a solder paste containing the above conductive filler.

  The conductive filler of the present invention can be melt-bonded at a temperature lower than the reflow heat treatment condition of Sn-37Pb eutectic solder (peak temperature of 181 ° C. or higher), and has the same heat resistance as that of Sn-37Pb eutectic solder (heat resistance requirement 150 to 160 [deg.] C.), and can be used as a bonding material at a repair temperature, so that there is an advantage that thermal damage to components, base materials, and peripheral devices during mounting can be reduced, and manufacturing costs and environmental loads can be reduced.

The conductive filler of the present invention comprises a mixture of first metal particles and second metal particles, and the mixture has a metastable alloy phase observed as an exothermic peak by differential scanning calorimetry (DSC). At least one at 110 to 130 ° C. and at least one melting point observed as an endothermic peak at 120 to 140 ° C. and 170 to 200 ° C., and the mixture is heat-treated at 180 to 200 ° C. Accordingly, the lowest melting point after the first metal particles and the second metal particles are melt-bonded is 160 to 200 ° C.
In addition, the measurement temperature range of differential scanning calorimetry (DSC) in the present invention is 30 to 600 ° C., and a calorific value or an endothermic amount of ± 1.5 J / g or more is quantified as a peak derived from the measurement object, Peaks less than that are excluded from the viewpoint of analysis accuracy.
In the present invention, the “melting point” refers to the melting start temperature and refers to the solidus temperature in differential scanning calorimetry (DSC).

When the mixture of the 1st metal particle and 2nd metal particle preferable as an electroconductive filler of this invention is illustrated, the metastable alloy phase observed as an exothermic peak by differential scanning calorimetry (DSC) will be 110-130 degreeC. And at least one first metal particle having at least one melting point observed at an endothermic peak at 170 ° C. to 200 ° C. and 320 to 380 ° C., and the endothermic peak without the exothermic peak. Examples thereof include a mixture with second metal particles having at least one observed melting point at 120 to 140 ° C.
When a heat history equal to or higher than the melting point of the second metal particles is given by the heat treatment, the second metal particles are melted and joined to the first metal particles. Thereby, the thermal diffusion reaction between the metal particles proceeds at an accelerated rate, the metastable alloy phase disappears, and a new stable alloy phase is formed. That is, the presence of a metastable alloy phase observed as an exothermic peak by DSC has an effect of promoting the thermal diffusion reaction. Since this newly formed stable alloy phase has a melting point of 160 to 200 ° C., the lowest melting point after heat treatment is 160 to 200 ° C.

As described above, the first metal particles used in the conductive filler of the present invention have at least one metastable alloy phase observed as an exothermic peak in differential scanning calorimetry (DSC) at 110 to 130 ° C., endothermic. Examples are metal particles having at least one melting point observed at the peak at two locations of 170 ° C. to 200 ° C. and 320 to 380 ° C.
As the metal particles exhibiting such thermal characteristics, a metal composed of an alloy having a composition of Ag 25 to 40% by mass, Cu 5 to 15% by mass, Bi 2 to 8% by mass, In 2 to 8% by mass, and Sn 29 to 66% by mass. Particles are preferable, and metal particles made of an alloy having a composition of Ag 30 to 35% by mass, Cu 8 to 12% by mass, Bi 2 to 8% by mass, In 2 to 8% by mass, and the balance Sn are more preferable.
As described above, the second metal particles do not have a metastable alloy phase observed as an exothermic peak by differential scanning calorimetry (DSC), and have a melting point observed at an endothermic peak of 120 to 140 ° C. The metal particle which has is illustrated.

As the metal particles exhibiting such thermal characteristics, metal particles made of an alloy having a composition of In 30 to 45% by mass, Ag 5 to 15% by mass, Cu 10 to 20% by mass, and Sn 20 to 55% by mass are preferable. Metal particles made of an alloy having a composition of 40% by mass, Ag8-12% by mass, Cu12-18% by mass, and the balance Sn are more preferable.
The mixing ratio of the first metal particles and the second metal particles is preferably 20 to 100 parts by mass of the second metal particles with respect to 100 parts by mass of the first metal particles.
The particle size of the metal particles can be determined according to the application. For example, in solder paste applications, it is preferable to use particles having a relatively high sphericity with an average particle diameter of 2 to 40 μm in consideration of printability. In addition, as a conductive adhesive application, it is preferable to use particles with relatively high sphericity in the filling of vias, with emphasis on hole filling properties. In surface mounting of parts, etc., irregular shaped particles are used to increase the contact area. It is preferable to use
Usually, fine metal particles are often surface oxidized. Therefore, in order to promote melting and thermal diffusion by the heat treatment in the above-mentioned application, it is preferable to add an activator for removing the oxide film or to perform at least one of pressurization.

As the method for producing the first and second metal particles constituting the conductive filler of the present invention, in order to form a metastable alloy phase or a stable alloy phase in the metal particles, an inert gas atomization which is a rapid solidification method is used. It is desirable to adopt the law. In the gas atomization method, an inert gas such as nitrogen gas, argon gas, or helium gas is usually used. However, in the present invention, it is preferable to use helium gas having a low specific gravity, and the cooling rate is 500 to 5000 ° C. / Seconds are preferred.
The solder paste of the present invention is composed of the conductive filler of the present invention and a flux composed of components such as rosin, a solvent, an activator, and a thixotropic agent. The content of the conductive filler in the solder paste is preferably 85 to 95% by mass. The flux is optimal for the surface treatment of the conductive filler made of metal particles, and promotes melting and thermal diffusion of the metal particles. A known material can be used as the flux, but it is more effective to add an organic amine as an oxide film removing agent. If necessary, a solvent may be added to a known flux to adjust the viscosity.

EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited at all by these Examples.
The differential scanning calorimetry was performed in a range of 30 to 600 ° C. using a “DSC-50” manufactured by Shimadzu Corporation under a nitrogen atmosphere under a temperature rising rate of 10 ° C./min.
[Examples 1 to 3]
(1) Production of first metal particles 1.0 kg of Cu particles (purity 99% by mass or more), 4.8 kg of Sn particles (99% by mass or more), 3.2 kg of Ag particles (purity 99% by mass or more), Bi 0.5 kg of particles (purity 99% by mass or more) and 0.5 kg of In particles (purity 99% by mass or more) are placed in a graphite crucible and heated and melted to 1400 ° C. with a high frequency induction heating device in a 99% by volume or more helium atmosphere did. Next, after this molten metal is introduced from the tip of the crucible into a spray tank in a helium gas atmosphere, helium gas (purity 99 vol% or more, oxygen concentration 0.1 vol%) is supplied from a gas nozzle provided near the crucible tip. Less than the pressure of 2.5 MPa), atomization was performed to produce first metal particles. The cooling rate at this time was 2600 ° C./second.

When the obtained 1st metal particle was observed with the scanning electron microscope (Hitachi, Ltd. product: S-2700), it was spherical. The metal particles were classified using an airflow classifier (Nisshin Engineering Co., Ltd .: TC-15N) at a setting of 5 μm, and then the overcut powder was classified again at a setting of 15 μm. Undercut powder was collected. The collected first metal particles had a volume average particle size of 4.9 μm.
Differential scanning calorimetry was performed using the first metal particles thus obtained as a sample. As a result, it was confirmed that the obtained first metal particles had endothermic peaks of 196 ° C., 359 ° C., and 415 ° C. and had a plurality of melting points. Moreover, the exothermic peak of 120 degreeC exists and it has confirmed having a metastable alloy phase.

(2) Production of second metal particles 1.5 kg of Cu particles (purity 99% by mass or more), 3.75 kg of Sn particles (purity 99% by mass or more), 1.0 kg of Ag particles (purity 99% by mass or more), In 3.75 kg (purity 99% by mass or more) of particles were placed in a graphite crucible and heated and melted to 1400 ° C. with a high-frequency induction heating apparatus in a helium atmosphere of 99% by volume or more. Next, after this molten metal is introduced from the tip of the crucible into a spray tank in a helium gas atmosphere, helium gas (purity 99 vol% or more, oxygen concentration 0.1 vol%) is supplied from a gas nozzle provided near the crucible tip. The second metal particles were produced by performing atomization by ejecting a pressure of less than 2.5 MPa. The cooling rate at this time was 2600 ° C./second.

When the obtained 2nd metal particle was observed with the scanning electron microscope (Hitachi Ltd. make: S-2700), it was spherical. The metal particles were obtained by classifying the overcut powder again at a setting of 15 μm after classifying the metal particles at a setting of 5 μm using an airflow classifier (manufactured by Nissin Engineering Co., Ltd .: TC-15N). Undercut powder was collected. The volume average particle diameter of the recovered second metal particles was 5.0 μm.
Differential scanning calorimetry was performed using the second metal particles thus obtained as a sample. As a result, it was confirmed that the obtained second metal particles had an endothermic peak at 136 ° C. There was no characteristic exothermic peak.

(3) Manufacture of metal particle mixture and solder paste Differential scanning using a conductive filler (average particle diameter of 4.9 μm) in which the first metal particles and second metal particles are mixed at a weight ratio of 100: 91. Calorimetry was performed. The DSC chart obtained by this measurement is shown in FIG. As shown in this figure, it was confirmed that endothermic peaks exist at 134 ° C. and 195 ° C. The 134 ° C. endothermic peak has a melting point of 128 ° C. (melting start temperature: solidus temperature). Also, an exothermic peak was present at 120 ° C. characteristically.
Next, the conductive filler 90.0% by mass, rosin-based flux 6.4% by mass, triethanolamine (oxide film removing agent) 1.6% by mass, stearic acid (activator) 0.4% by mass, and 1.6% by mass of ethylene glycol monohexyl ether (solvent) is mixed and sequentially applied to a solder softener (Malcom Co., Ltd .: SPS-1) and a defoaming kneader (Matsuo Sangyo Co., Ltd .: SNB-350). A solder paste was prepared.

(4) Confirmation of melting point and bonding strength The solder paste was placed on an alumina substrate and subjected to reflow heat treatment at a peak temperature of 181 ° C. in a nitrogen atmosphere.
The heat treatment apparatus used was a mesh belt type continuous heat treatment apparatus manufactured by Koyo Thermo System Co., Ltd. The temperature profile is 5 minutes for all processes, reaches 108 ° C. in 1 minute 30 seconds from the start of heat treatment, then gradually increases in temperature, reaches a peak temperature 181 ° C. in 3 minutes 15 seconds, and then gradually decreases in temperature. At the end of the heat treatment, a condition of 146 ° C. was adopted (hereinafter also referred to as “peak 181 ° C. heat treatment”).
Using the solder paste after the heat treatment as a sample, differential scanning calorimetry was performed. The DSC chart obtained by this measurement is shown in FIG. As shown in this figure, it was confirmed that endothermic peaks exist at 186 ° C. and 384 ° C. The 186 ° C. endothermic peak has a melting point of 164 ° C.

The solder paste was printed on a Cu substrate with a size of 2 mm × 3.5 mm, and after mounting the chip, a sample was prepared by heat treatment at a peak of 181 ° C. in the nitrogen atmosphere by the heat treatment method described above. For the printing pattern formation, a printing machine “MT-320TV” manufactured by Microtech Co., Ltd. was used, the mask was a metal mask, and the squeegee was made of urethane. The aperture of the mask is 2 mm × 3.5 mm and the thickness is 100 μm. The printing conditions were a printing speed of 10 mm / second, a printing pressure of 0.1 MPa, a squeegee pressure of 0.2 MPa, a back pressure of 0.1 MPa, an attack angle of 20 °, a clearance of 0 mm, and a printing frequency of once. In addition, a Cu chip having a thickness of 2 mm × 2 mm and a thickness of 0.5 mm was used.
Furthermore, when the chip bond strength in the shear direction of the produced sample was measured at a pushing speed of 10 mm / min with a push-pull gauge at room temperature (25 ° C.) and converted into a unit area, it was 13.7 MPa.

Next, the solder paste was placed on an alumina substrate and subjected to reflow heat treatment at a peak temperature of 204 ° C. in a nitrogen atmosphere. The heat treatment apparatus is the same as described above, and the temperature profile is 5 minutes for all processes, and reaches 111 ° C. in 1 minute 30 seconds from the start of heat treatment, and then gradually rises in temperature and reaches peak temperature 204 ° C. in 3 minutes 15 seconds. Thereafter, the temperature gradually decreased, and a condition of 162 ° C. was adopted when the heat treatment was completed (hereinafter also referred to as “peak 204 ° C. heat treatment”). This temperature profile assumes the reflow conditions used in the joining of general Sn-37Pb eutectic solder.
Using the solder paste after the heat treatment as a sample, differential scanning calorimetry was performed. As a result, it was confirmed that endothermic peaks exist at 184 ° C. and 384 ° C. The 184 ° C. endothermic peak has a melting point of 164 ° C.

In addition, the solder paste was printed on a Cu substrate at 2 mm × 3.5 mm by the same method as described above, and after mounting the chip, the sample was heat-treated at a peak of 204 ° C. by the heat treatment method in a nitrogen atmosphere.
Further, the chip bond strength in the shear direction of the prepared sample was measured at a pushing speed of 10 mm / min with a push-pull gauge at room temperature (25 ° C.), and converted to unit area, it was 19.4 MPa.
The above example was set as Example 3, conductive fillers having different mixing ratios of the first metal particles and the second metal particles were set as Examples 1 and 2, and these were pasted by the same method as described above. Table 1 shows the results of measuring the differential scanning calorific value and the chip bonding strength after heat treatment as Examples 1 and 2.

[Comparative Examples 1-5]
Table 1 also shows that, as a comparative example, when the first metal particles are alone (Comparative Example 1), when the mixing ratio of the second metal particles exceeds the upper limit (Comparative Example 2), and second metal particles. In the case of a single solder (Comparative Example 3), the result of measuring a conventional solder material is also shown. Comparative Example 4 is Sn-37Pb eutectic solder, and Comparative Example 5 is Sn-3.0Ag-0.5Cu lead-free solder.

As is apparent from the results in Table 1, in Example 3, the bonding strength exceeding the Sn-37Pb eutectic solder is shown in the heat treatment at the peak temperature of 204 ° C. Moreover, also in Example 1 and Example 2, it is a joint strength of the level which can fully be used practically.
Further, the minimum melting point after heat treatment is 160 ° C. or higher in Examples 1, 2, and 3, and the use of surface mounting with a heat resistance requirement of 150 to 160 ° C. in which Sn-37Pb eutectic solder is used. Then, it seems that it can be applied without problems.
As described above, by using the conductive filler of the present invention, it can be melt-bonded at a temperature lower than the reflow heat treatment condition of Sn-37Pb eutectic solder (peak temperature 181 ° C. or higher), and Sn-37Pb eutectic solder and It is possible to provide a bonding material that can be used in equivalent heat resistance applications (heat resistance requirement: 150 to 160 ° C.).

  The conductive filler of the present invention can be melt-bonded at a temperature lower than the reflow heat treatment condition of Sn-37Pb eutectic solder (peak temperature of 181 ° C. or higher) and bonded in an equivalent heat-resistant application (heat resistance requirement 150 to 160 ° C.). Use as a material can be expected.

It is the DSC chart obtained by the differential scanning calorimetry which used the conductive filler which mixed the 1st metal particle produced in Example 3 and the 2nd metal particle by weight ratio 100: 91 as a sample. It is the DSC chart obtained by the differential scanning calorimetry which used what reflow-heat-treated the solder paste produced in Example 3 at the peak temperature of 181 degreeC in nitrogen atmosphere.

Claims (3)

  A conductive filler comprising a mixture of first metal particles and second metal particles, the mixture having a metastable alloy phase of 110 to 130 observed as an exothermic peak by differential scanning calorimetry (DSC). Having at least one melting point observed as an endothermic peak at 120 ° C. and at least one melting point at 120 to 140 ° C. and 170 to 200 ° C., and heat-treating the mixture at 180 to 200 ° C., A conductive filler having a minimum melting point of 160 to 200 ° C. after the first metal particles and the second metal particles are melt-bonded.   The mixture is composed of 100 parts by mass of first metal particles and 20 to 100 parts by mass of second metal particles, and the first metal particles are composed of Ag 25 to 40% by mass, Cu 5 to 15% by mass, Bi 2 to 8% by mass. , In 2 to 8 mass%, and Sn having an alloy composition of 29 to 66 mass%, the second metal particles are In 30 to 45 mass%, Ag 5 to 15 mass%, Cu 10 to 20 mass%, and Sn20 to The conductive filler according to claim 1, comprising an alloy having a composition of 55% by mass.   A solder paste comprising the conductive filler according to claim 1.

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