JP2011251332A - HIGH-TEMPERATURE Pb-FREE SOLDER PASTE USING Al POWDER - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-temperature Pb-free solder paste capable of securing excellent stress relaxation property and high joint reliability.SOLUTION: The high-temperature Pb-free solder paste includes: metal powders comprising Al powders having an average particle size of 1-100 μm or those obtained by coating at least a part of the metal powders with Au or the like to form a film having thickness of ≤1 μm; Zn-alloy solder powders comprising a binary or quaternary alloy containing a prescribed amount of Al or the like in Zn as a main component; and a flux. The metal powder accounts for 3-40 mass% with respect to 100 mass% of the total of the metal powder and Zn-alloy solder powder.

Description

  The present invention relates to a high-temperature Pb-free solder paste used for die bonding of a power semiconductor element.

  High-temperature solder typified by Pb-5 mass% Sn is used in die bonding of power semiconductor elements such as power transistors and assembly of various electronic components. In addition to the properties required for ordinary solder materials such as wettability to the material to be joined, the solder for these applications must be capable of being soldered at a temperature of about 380 ° C. or lower, and 2) soldered Solder does not remelt when mounting components on a printed circuit board, that is, does not remelt at a temperature of about 240 ° C. 3) Reliability of solder joints can be ensured, that is, bonding in a relatively high temperature environment Performance such as no deterioration of parts is required.

  By the way, in recent years, a movement to regulate the use of Pb as a solder material has been increasingly activated in consideration of prevention of environmental pollution. For example, with respect to Sn-40 mass% Pb solder used for mounting on a printed circuit board, so-called Pb-free solder represented by Sn-Ag-Cu solder has been developed, and substitution is already in progress. On the other hand, in the region of high-temperature solder described above, no suitable material that satisfies all the conditions has been found, and the current situation is that there has been little progress in replacing Pb-based solder with Pb-free solder. .

  As candidates for high-temperature Pb-free solder, use of a Zn alloy is proposed in Patent Document 1 and the like. However, when a Zn alloy solder is used, a chip joined with the solder may be cracked or peeled off after soldering. For example, Non-Patent Document 1 reports a chip cracking phenomenon when die-attaching Zn—Al-based Pb-free solder.

  The cause of this chip cracking is that the difference in thermal expansion coefficient between the substrate material Cu and the chip material Si is 5 times or more. That is, in the cooling after die attach, stress is generated due to the difference in the thermal expansion coefficient, and in the case of Pb-5 mass% Sn, the stress is relieved by plastic deformation, but the stress is relieved because the Zn-Al solder is hard. Due to not being.

  As a countermeasure against such chip cracking and peeling, it has been shown that the chip thickness is reduced. However, in reality, there is a limit to making the chip thinner, and it is unlikely that this measure can solve all cases. Patent Documents 1 and 2 also describe various compositions of Zn—Al solder in the same way, but it is speculated that the problem of chip cracking may basically occur as described above.

Patent No. 3850135 Japanese Patent No. 3945915

6th on “Microjoining and Assembly Technology in Electronics” February 3-4, 2000, Yokohama p.339-p.344

  The present inventors have found that the above-mentioned problems such as chip cracking that occur in a Zn-Al-based alloy are caused by the difference in thermal expansion coefficient between Cu and Si, as well as the shrinkage rate during solidification (+ indicates contraction,-indicates expansion). In the case of Zn, + 4.9% to + 6.9%, and in the case of Al, it is considered to be caused by + 6.4% to + 6.8%. Furthermore, the phase transformation around 280 ° C. in the Zn—Al phase diagram is considered as one of the causes.

  That is, during the cooling after die attachment, in addition to the shrinkage when the solder solidifies, phase transformation occurs near 280 ° C., which causes a volume change, which is considered to generate a high residual stress. As described above, in a Zn—Al-based alloy, especially in a region where Zn exceeds 70 mass%, it is considered that the problem of chip cracking or peeling is often difficult to solve only by adjusting the chip thickness. .

  In view of such problems, an object of the present invention is to provide a high-temperature Pb-free solder paste that is particularly suitable for die bonding of power semiconductor elements and can ensure high bonding reliability by excellent stress relaxation properties. When the solder paste of the present invention is used, the bonding temperature between the chip and the substrate is 450 ° C. or lower.

  In order to achieve the above object, the first high-temperature Pb-free solder paste of the present invention does not cover Al powder having an average particle diameter of 1 μm or more and 100 μm or less, or at least a part thereof. Metal powder obtained by applying a coating process for forming a film having a thickness of 1 μm or less using one or more of the group consisting of Au, Ag, Ni, and Cu, and Zn as a main component and Al as a second element It has a Zn alloy solder powder composed of a binary alloy and a flux, and when the total of the metal powder and the Zn alloy solder powder is 100 mass%, the metal powder is 3 mass% or more and 40 mass% or less. It is a feature.

  Further, the second high-temperature Pb-free solder paste of the present invention is not coated with Al powder having an average particle diameter of 1 μm or more and 100 μm or less, or at least a part thereof is Au, Ag, Ni, and A metal powder obtained by applying a coating treatment for forming a film having a thickness of 1 μm or less using one or more members selected from the group consisting of Cu, 0.5% by mass to 9% by mass of Al, and 0.5% Ge. Zn alloy solder powder of ternary or quaternary alloy containing 01% by mass to 9% by mass and / or Mg of 0.01% by mass to 0.5% by mass with the balance being Zn and inevitable impurities, and flux When the total of the metal powder and the Zn alloy solder powder is 100% by mass, the metal powder is 3% by mass to 40% by mass.

  ADVANTAGE OF THE INVENTION According to this invention, the high temperature Pb free solder paste which can ensure high joining reliability by the outstanding stress relaxation property can be provided. Such a solder paste can be particularly suitably used for die bonding of a power semiconductor element.

It is sectional drawing which shows the dummy chip on the lead frame made from Cu joined using the solder paste of an Example.

  The first high-temperature Pb-free solder paste of the present invention has problems such as chip cracking and chip peeling caused by solder solidification shrinkage and stress caused by thermal expansion difference between the chips and the substrate after bonding the chips and the substrate. In order to solve the above problem, a solder paste is obtained by uniformly mixing Zn alloy solder powder containing Zn as a main component and Al powder which is a soft metal. By uniformly dispersing the Al powder, generation of stress due to solidification shrinkage of the solder can be suppressed. That is, since Al is a metal that is very soft and easily plastically deformed, for example, stress due to a difference in thermal expansion between a Si chip and a Cu substrate can be absorbed.

  Furthermore, Al is previously contained in the Zn alloy solder powder as the second element, and when it is mixed with the Al powder, it is already a Zn—Al binary alloy solder. Therefore, when this Zn alloy solder is melted, good wettability is obtained with respect to the Al powder that exists in the solid state. In addition, uniform dispersion | distribution of Al powder and Zn alloy solder powder is industrially realizable by taking the form of a solder paste.

  The Al powder may be used as it is without being subjected to a coating treatment, but at least a part thereof is subjected to a coating treatment that forms a film using one or more elements of the group consisting of Ag, Au, Cu, and Ni. Also good. By forming a film of Ag or the like in this way, oxidation of the Al powder is prevented and good wettability with the Zn alloy is obtained, and the effect of uniformly dispersing the Al powder in the molten Zn alloy is obtained. can get. In the present invention, Al powder that has been coated and / or Al powder that has not been coated is also referred to as metal powder.

  The film thickness of Ag or the like is 1 μm or less. The reason is that if the thickness exceeds 1 μm, the above effect is saturated and the material cost increases. Although the specific method of the coating process which forms films | membranes, such as Ag, is not specifically limited, For example, after producing Al powder, it can form by performing electrolytic plating and electroless plating on the surface. As another method, for example, it is also possible to form a Ni film by adding and reducing a solution containing a Ni salt at the final stage of the production process of Al powder using a wet method.

  The content of the metal powder in the solder paste is 3% by mass or more and 40% by mass or less when the total of the metal powder and the Zn alloy solder powder is 100% by mass. If this amount is less than 3% by mass, the content of Al in the solder paste is too small, and an effect such as sufficient stress relaxation cannot be expected. On the other hand, when it contains more than 40 mass%, Zn alloy solder component will run short and sufficient joining will become difficult.

  It does not specifically limit about the purity and shape of Al powder to be used. As for purity, in order to make use of the flexible characteristics of Al, a material closer to pure metal is preferable to an intermetallic compound. Alternatively, an Al-based eutectic alloy that becomes a eutectic alloy with Al can also be used because of its excellent workability. On the other hand, the shape may be any of a spherical shape such as atomized powder, an irregular shape such as electrolytic powder, and a flat shape such as flake powder.

  The average particle diameter of Al powder shall be 1 micrometer or more and 100 micrometers or less. The reason is that if it is less than 1 μm, the aggregation of the Al powder is difficult to unravel, and it is difficult to uniformly disperse the Al powder in the Zn alloy. Conversely, if it exceeds 100 μm, the fracture starting point of the bonding layer made of the Zn alloy solder. This is because the reliability of the joint is lowered. In the present invention, the average particle diameter of the Al powder means an average value of the long diameter of the Al powder as a diameter.

  Zn alloy solder intervenes as a solder between a plurality of metal powders (Al powders) existing in a solid state and has a function of joining the metal powders together. The reason why Zn is selected as the main component of the alloy solder is that Zn has high thermal conductivity. That is, since Zn has a thermal conductivity approximately three times that of Pb, it is suitable as a solder material for a power semiconductor element in which a large current flows and becomes high temperature.

  The reason why Al is contained as the second element in the Zn alloy solder is that Al lowers the melting point of the solder and improves the workability and the oxidation resistance of the molten metal. An alloy containing Zn has a Zn—Al eutectic point at Zn = 95.0%, and the effect is particularly remarkable in this vicinity, the melting point is lowered, the crystal is refined, and the workability is remarkably improved.

  The Zn alloy solder in the first high-temperature Pb-free solder paste preferably contains Al in an amount of 0.5% by mass to 9.0% by mass. If the amount is less than 0.5% by mass, the effect of improving workability and the like cannot be expected, and if it exceeds 9.0% by mass, the melting point becomes too high and good bonding cannot be performed. The particle size of the Zn alloy solder powder is not particularly limited, but a powder of about 10 to 50 μm is often used for ease of manufacturing.

  A solder paste can be obtained by adding a liquid flux to the powder composed of the metal powder and the Zn alloy solder powder and kneading them. The type of flux is not particularly limited, and those used in Sn-based or Pb-based solder pastes may be used as they are, but more preferable are those for high temperatures. For example, a rosin compound, an organic acid, an organic amine compound or the like dissolved in a solvent such as alcohols, ethylene glycols, or glycerins can be used.

  Next, the second high-temperature Pb-free solder paste of the present invention will be described. In the second high-temperature Pb-free solder paste, the Zn alloy solder powder contains Al in an amount of 0.5 to 9% by mass, Ge in an amount of 0.01 to 9% by mass and / or Mg in an amount of 0%. 0.01% by mass or more and 0.5% by mass or less, with the balance being a ternary or quaternary alloy consisting of Zn and inevitable impurities, otherwise the same as the first high temperature Pb-free solder paste It is.

  Ge and Mg are both elements for the purpose of adjusting the melting point, and are added mainly for the purpose of lowering the melting point of Zn having a relatively high melting point of 419 ° C. in the solder field. Furthermore, since Ge is less oxidized than Zn, it has the effect of improving the wettability of the solder, while Mg has the effect of increasing the bonding strength and improving the reliability. As described above, the second high-temperature Pb-free solder paste of the present invention has excellent points as compared with the first high-temperature Pb-free solder paste, and can be used for a wider range of applications. .

  The reason why the Ge content is 0.01% by mass or more and 9.0% by mass or less and the Mg content is 0.01% by mass or more and 0.5% by mass or less is that the content is less than the above lower limit value. This is because the effect described above cannot be obtained due to too little. On the other hand, if the Ge content exceeds 9.0% by mass, the Ge content is greatly deviated from the eutectic point with Zn, and coarse grains are generated to reduce workability and the like. On the other hand, when the Mg content exceeds 0.5% by mass, a strong oxide film is formed on the solder surface due to the strong reducibility of Mg, which may cause voids.

  As described above, both the first and second high-temperature Pb-free solder pastes of the present invention can ensure high bonding reliability due to excellent stress relaxation properties, and thus are particularly suitable for die bonding of power semiconductor elements. Can be used. Further, it can also be suitably used as a Pb-free solder that replaces the Pb-based solder used in assembling various electronic components.

  Examples of the first and second high-temperature Pb-free solder pastes of the present invention will be specifically described below, but the present invention is not limited by these examples.

[Example 1]
<Production conditions>
The solder paste of samples 1-10 was produced with the following method. First, Zn and Al having a purity of 99.9% or more were prepared as metal raw materials, and these were weighed in predetermined amounts and melted in a high-frequency melting furnace. During dissolution, nitrogen was flowed at a rate of 700 ml / min or more per kg of raw material to prevent oxidation. The molten metal mixed by melting was taken out from the high-frequency melting furnace and cooled. As a result, ten types of Zn-based alloy ingots used for the solder pastes of Samples 1 to 10 were obtained. The results of measuring the compositions of these 10 types of Zn-based alloy ingots using an ICP emission spectroscopic analyzer (SHIMAZU S-8100) are shown in Table 1 below.

  2 kg of each of the 10 types of Zn-based alloy ingots shown in Table 1 above were weighed, pulverized using a stamp mill, and then sieved to obtain Zn alloy solder powder. When the average particle size of the obtained Zn alloy solder powder was measured using a laser diffraction particle size distribution meter (manufactured by Nippon Laser Co., Ltd.), all were about 28 μm. The average particle diameter of the Zn alloy solder powder is an arithmetic average of the major diameters of the Zn alloy solder powder as the diameter, similarly to the average particle diameter of the Al powder.

  Next, 10 types of Al powders respectively used for the solder pastes of Samples 1 to 10 were produced by a general atomizing method. In Example 1, the Al powder was not coated. The Al powder thus obtained and the above Zn alloy solder powder are mixed in the proportions shown in Table 2 below, and a flux composed of a commercially available alcohol and a reducing organic material (manufactured by Aoki Metal) is added at a predetermined proportion. A solder paste was prepared. At that time, a mixer (SR-500, manufactured by Shinky Corporation) was used. In Table 2, the average particle diameter of Al powder measured using a laser diffraction particle size distribution meter (manufactured by Nippon Laser Co., Ltd.) is also shown.

<Evaluation method>
The solder paste produced by the above method was evaluated from the viewpoints of die bonding properties, suitability for use of high-temperature solder, and bonding reliability as described below. In this connection reliability, it is possible to evaluate stress relaxation that is difficult to directly evaluate.

(Die bonding)
The die bonding property was evaluated using a solder die bonder (EDB-200 manufactured by dage). Specifically, as shown in FIG. 1, a Cu lead frame 1 in which an Ag plating layer 1a is formed and a dummy chip 2 in which an Ni vapor deposition layer 2a and an Ag vapor deposition layer 2b are formed are prepared. Were joined within a temperature range of 420 ± 3 ° C. via the solder paste 3 of each sample, and it was confirmed whether or not good joining could be obtained.

  The solder paste was supplied by dropping a suitable amount of solder paste in advance on the Ag plating of the lead frame. As for the quality of the bondability, that is, the die bondability, the void of the joint portion was observed using an X-ray transmission device (TUX-3000W manufactured by Token X-ray Inspection Co., Ltd.). 5% or more and less than 8% were evaluated as “Δ”, and 8% or more were evaluated as “x”. The void ratio was calculated by the following equation by observing the solder joint from a direction perpendicular to the joint surface with an X-ray transmission device, obtaining the void area and the joint area.

[Equation 1]
Void ratio (%) = void area / (void area + joint area) × 100

(Applicability of high-temperature solder)
The suitability for use as a high temperature solder was evaluated by a high temperature shear test according to JIS Z3198-7: 2003. That is, the sample after die bonding was set in a bond tester (manufactured by Techno Alpha Co., Ltd.), reheated to 240 ° C. while flowing nitrogen, and in that state, shear force was applied to the dummy chip and the solder joint. Specifically, the shear test jig was hooked on the dummy chip, and a shearing force of 90 (N) was applied in the horizontal direction with respect to the solder and dummy chip bonding surface. If the chip part is cracked and there is no crack or deformation in the joint part or solder part, “○” indicates that the chip is moved and displaced, or if the joint part or solder part is cracked or deformed, “×”. ".

(Joint reliability)
The reliability of the joint was evaluated from the following two viewpoints. That is, as reliability evaluation 1, Cu lead frames to which dummy chips similar to those used for the evaluation of die bonding described above were bonded using the solder paste of each sample were prepared, and these were transferred mold molds. Molded with an epoxy resin (EME-6300 manufactured by Sumitomo Bakelite Co., Ltd.) using a machine. These were held at 80 ° C. under a constant temperature and humidity condition of 80% for 1000 hours, and then the resin was opened and the joints were observed. As a result of this observation, a case where no crack was generated at the dummy chip or the bonding interface was evaluated as “◯”, and a case where there was a crack or peeling was evaluated as “X”.

  Furthermore, as reliability evaluation 2, a Cu lead frame to which a dummy chip similar to that used in the reliability evaluation 1 was bonded was prepared using the solder paste of each sample. And after repeating the cooling heat processing which consists of cooling to -50 degreeC and heating to 125 degreeC with respect to these as 400 cycles, resin was opened and the junction part was observed. As a result of this observation, a case where no crack was generated at the dummy chip or the bonding interface was evaluated as “◯”, and a case where there was a crack or peeling was evaluated as “X”. Table 3 below shows the evaluation results of the die bonding property, suitability for use of high-temperature solder, and bonding reliability.

  As is clear from the results in Table 3 above, the solder pastes of Samples 1 to 6 according to the present invention have good die bonding properties, suitability for use, and bonding reliability as high-temperature solder. On the other hand, the solder pastes of Samples 7 to 10 which are comparative examples have insufficient results in all the evaluations.

[Example 2]
Solder pastes of Samples 11 to 18 were produced in the same manner as in Example 1 except that Al powder having a surface of Ag, Au, Cu, or Ni formed thereon was used. An electroless plating method was used as a film processing method for forming a film. The composition of the Zn alloy ingot used in the solder pastes of these samples 11 to 18 is shown in Table 4 below, the average particle diameter of Al powder, the mixing ratio of Al powder and Zn alloy solder powder, the material and thickness of the film are shown in the following table. As shown in FIG. The thickness of the film is embedded in Al powder in a resin, polished with a polishing machine using finer ones in order, and finally buffed, followed by EPMA (device name: SHIMADZU EPMA-1600) The line analysis was performed using and measured.

  The solder pastes of these samples 11 to 18 were evaluated by the same evaluation method as in Example 1. The evaluation results are shown in Table 6 below.

  As is apparent from the results in Table 6, all of the solder pastes of Samples 11 to 18 according to the present invention have good die bonding properties, suitability for use, and bonding reliability as high-temperature solders.

[Example 3]
A sample was prepared in the same manner as in Example 1 except that Al powder having an Ag film formed on the surface was used, and a ternary or quaternary alloy containing Ge and / or Mg in addition to Al was used for the Zn alloy solder powder. 19 to 29 solder pastes were prepared. In addition, Mg used as the metal raw material had a purity of 99.9% or more and Ge had a purity of 99.95% or more. The composition of the Zn alloy ingot used for the solder paste of these samples 19 to 29 is shown in Table 7 below, the average particle diameter of Al powder, the mixing ratio of Al powder and Zn alloy solder powder, the type and thickness of the coating are shown in Table 7 below. It is shown in FIG.

  The solder pastes of these samples 19 to 29 were evaluated by the same evaluation method as in Example 1. The evaluation results are shown in Table 9 below.

  As is apparent from the results in Table 9, the solder pastes of Samples 19 to 25 according to the present invention have good die bonding properties, suitability for use, and bonding reliability as high-temperature solders. On the other hand, the solder pastes of Samples 26 to 29, which are comparative examples, have insufficient results in all the evaluations.

Claims (3)

  The Al powder having an average particle size of 1 μm or more and 100 μm or less is not coated, or at least a part of the Al powder has a thickness of 1 μm or less using at least one member of the group consisting of Au, Ag, Ni, and Cu. A high-temperature Pb-free solder paste having a metal powder obtained by performing a coating process for forming a film, a Zn alloy solder powder made of a binary alloy containing Zn as a main component and Al as a second element, and a flux. A high-temperature Pb-free solder paste, wherein the total amount of the metal powder and the Zn alloy solder powder is 100 mass%, the metal powder is 3 mass% or more and 40 mass% or less.   2. The high-temperature Pb-free solder paste according to claim 1, wherein the Zn alloy solder powder contains Al in an amount of 0.5% by mass to 9% by mass with the balance being Zn and inevitable impurities.   The Al powder having an average particle size of 1 μm or more and 100 μm or less is not coated, or at least a part of the Al powder has a thickness of 1 μm or less using at least one member of the group consisting of Au, Ag, Ni, and Cu. Metal powder obtained by applying a coating treatment to form a film, Al is contained in an amount of 0.5 mass% to 9 mass%, Ge is 0.01 mass% to 9 mass%, and / or Mg is 0.1 mass%. A high-temperature Pb-free solder paste having a ternary or quaternary alloy Zn alloy solder powder containing 01 mass% or more and 0.5 mass% or less, the balance being Zn and inevitable impurities, and a flux, A high-temperature Pb-free solder paste characterized in that the metal powder is 3% by mass or more and 40% by mass or less when the total of the Zn alloy solder powder is 100% by mass.

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